#497 – Biggest Mysteries in Physics: Antimatter, Dark Energy & ToE – Don Lincoln

Don Lincoln is a particle physicist at Fermilab who has spent decades working at the frontiers of high energy physics. Thank you for listening ❤ Check out our sponsors: https://lexfridman.com/sponsors/ep497-sc See below for timestamps, and to give feedback, submit questions, contact Lex, etc. CONTACT LEX: Feedback – give feedback to Lex: https://lexfridman.com/survey AMA – submit questions, videos or call-in: https://lexfridman.com/ama Hiring – join our team: https://lexfridman.com/hiring Other – other ways to get in touch: https://lexfridman.com/contact EPISODE LINKS: Don’s Facebook: https://facebook.com/Dr. Don. Lincoln/ Don’s Website: https://drdonlincoln.com/ Don’s LinkedIn: https://bit.ly/4nHeNiF Don’s YouTube Playlist: https://bit.ly/3PCIW67 Don’s X: https://x.com/DrDonLincoln Don’s Books: https://amzn.to/4uYbkOZ Don’s Great Courses: https://shop.thegreatcourses.com/don-lincoln Don’s Audible: https://adbl.co/4wGioRV Fermilab’s YouTube: https://www.youtube.com/fermilab Fermilab’s Website: https://www.fnal.gov/ Fermilab’s X: https://x.com/fermilab SPONSORS: To support this podcast, check out our sponsors & get discounts: Upwork: Platform for hiring freelancers. Go to https://upwork.com/lex https://lexfridman.com/s/upwork-ep497-sc Larridin: Measure AI adoption in your business. Go to https://larridin.com https://lexfridman.com/s/larridin-ep497-sc Fin: AI agent for customer service. Go to https://fin.ai/lex https://lexfridman.com/s/fin-ep497-sc LMNT: Zero-sugar electrolyte drink mix. Go to https://drinkLMNT.com/lex https://lexfridman.com/s/lmnt-ep497-sc Shopify: Sell stuff online. Go to https://shopify.com/lex https://lexfridman.com/s/shopify-ep497-sc Perplexity: AI-powered answer engine. Go to https://perplexity.ai/ https://lexfridman.com/s/perplexity-ep497-sc OUTLINE: (00:00) – Introduction (00:34) – Sponsors, Comments, and Reflections (08:52) – Unifying the laws of nature (23:23) – Einstein, special relativity, and general relativity (40:31) – Electroweak force (52:13) – How particle colliders work (1:10:16) – Higgs boson discovery (1:20:35) – Theory of everything (1:50:20) – Physics of empty space (1:57:45) – Antimatter (2:18:35) – Dark energy (2:22:23) – Dark matter (2:50:59) – Future of physics PODCAST LINKS: – Podcast Website: https://lexfridman.com/podcast – Apple Podcasts: https://apple.co/2lwqZIr – Spotify: https://spoti.fi/2nEwCF8 – RSS: https://lexfridman.com/feed/podcast/ – Podcast Playlist: https://www.youtube.com/playlist?list=PLrAXtmErZgOdP_8GztsuKi9nrraNbKKp4 – Clips Channel: https://www.youtube.com/lexclips

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Lex Fridman?

  The following is a conversation with Don Lincoln, a particle physicist at Fermilab, who has spent decades working at the frontier of high-energy physics. This was a mind-blowing and inspiring conversation. Don turned out to be one of my favorite people to talk to about physics. Truly a unique mind with that Richard Feynman ability of taking very complicated ideas and explaining them simply, without losing any of the essential brilliant insights at the core of those ideas.

• 0:34Lex Fridman?

  And now, a quick few-second mention of each sponsor. Check them out in the description or at lexfridman.com/sponsors. It is, in fact, the best way to support this podcast. We've got Upwork for hiring quality freelancers, Larridan for understanding how AI is used in your business, Finn for customer service AI agents, Element for electrolytes, Shopify for selling stuff online, and Perplexity for curiosity-driven knowledge exploration. Choose wisely, my friends. And now, on to the full ad reads. I try to make them interesting, but if you skip, please still check out the sponsors. I enjoy their stuff. Maybe you will too. To get in touch with me, for whatever reason, go to lexfridman.com/contact,

• 1:18Lex Fridman?

  including for something I recently tweeted about, which is for travel recommendations. If you want to help me out and help me figure out where next I should go in the world, and also help me figure out where I'm in the world right now and what is going on and why we're here and all the big mysteries of life. Anyway, I'm supposed to be talking about our first sponsor, Upwork. This episode is brought to you by them. I think it's a new sponsor, but I've been using Upwork for so many years. It's all blending together. I'm clearly sleep deprived. I've been a huge fan of Upwork for a long time. I've used them over and over and over and over to accomplish all kinds of incredible tasks.

• 1:59Lex Fridman?

  It is a great platform that helps you find expert freelancers to help you build, design and scale your business to help make your life easier. Really an incredible site. Now, of course, I think pretty much everybody knows they're extremely useful on the consumer side. What not everybody knows, but they still are incredibly useful on the business side. So if you're a company trying to hire, they make it super easy for you to access the top 1% of talent on Upwork. They have all kinds of really nice AI-powered shortlisting tools to help you narrow down the search. You can match the right freelancer in under six hours. It just makes me happy to watch that.

• 2:40Lex Fridman?

  And it makes me happy that platforms like Upwork exist to make the whole process super easy. Visit upwork.com/lex right now and post your job for free. That's U P W O R K dot com slash lex. This episode is also brought to you by Leredin, a platform that helps organizations understand how AI is being used across their business and what it's doing for productivity and performance. AI is clearly revolutionizing the way we, each individual, many of us, are living our individual lives. But when multiple people come together and they use AI tooling, agentic engineering, together with multiple other people, especially at scale, to accomplish the mission, the

• 3:25Lex Fridman?

  task, the big overarching moonshot task that a company chases, that is a fascinating puzzle, fascinating optimization problem. But it starts with data. A company needs to understand how the AI tools are being used. That's what Leredin does. It tracks the usage data and outcomes, allowing businesses to figure out how to maximize productivity and how to connect all the different use cases and activity around AI to actual results for the business. If AI is part of your organization, now's the moment to start optimizing its utilization. Head to leredin.com to book a demo and start maximizing impact from AI.

• 4:10Lex Fridman?

  This episode, I'm not sure why I'm so hyped, but I am, is brought to you by Finn, the number one AI agent for customer service. Very few things are as important for business as taking care of the customer. This is the thing, when I had that conversation with Jeff Bezos, wonderful conversation on many levels, especially on the human level, really, really, really a conversation I enjoyed very much. We'll probably do another one. Maybe we'll do one with Jeff and Elon together. Me, as a person who loves space, who loves the ambitious, literal moonshot of humanity reaching out toward the stars, that makes me dream, that gives me hope about the future.

• 4:51Lex Fridman?

  So the two of them that are leading the way on those efforts being together at the table, I think would inspire a lot of people. Anyway, I say all that because Jeff has talked a lot about the value of the customer, about putting customer first. So getting the customer service piece right is incredibly difficult, but is incredibly important. And what Finn provides is tooling around that. It's trusted by over 6,000 customer service leaders, top companies, including AI companies. You know, when AI companies trust your AI agent that is focused on customer service, you know, then it's legit. And Finn is indeed legit. Go to fin.ai/lex

• 5:34Lex Fridman?

  to learn more about transforming your customer service and scaling your support team. That's fin.ai/lex. I swear I'm sober. I don't know why I'm excited about this. I am in the middle of nowhere. I can't say where, nor do I know where I am. Let's keep going. This episode is brought to you by Element, my daily zero sugar and delicious electrolyte mix, a packet of which I just consumed. I'm heading to bed soon. It was a very tough day, lots of moving around, putting, I don't even know what this is, but it's hotel water, dumping a packet of Element into it and sipping on it, even though it's warm, it still makes me feel like

• 6:19Lex Fridman?

  home. It makes me feel like I have my life together. You get a little sodium, a little potassium, a little magnesium, and altogether, a little bit of love into my life. Watermelon salt flavor makes me feel like I'm in Austin, Texas, in that summer heat, just happy to be alive. Get a free account sample pack with any purchase. Try it to drink element.com/lex. This episode is also brought to you by Shopify, the very mention of which makes me happy because I know how much they value engineering and the whole endeavor of engineering just makes me happy. Anyway, Shopify is a platform designed for anyone to sell anywhere with a great looking online store.

• 7:02Lex Fridman?

  Whenever I do a not read about Shopify, I keep talking about engineering. I never talk about what Shopify actually does. I use it to sell shirts. There's businesses that do much more sophisticated stuff, but I will keep talking about the engineering because bringing the infrastructure to life that makes it all happen. Toby is a magician, but also is a legit nerd, a legit engineer. His tweets are some of my favorite. I love it when great CEOs and great leaders are also engineers at heart and never stop tinkering, never stop building, never stop being at the low level of figuring out how

• 7:44Lex Fridman?

  systems work, how to innovate, how systems work and how to do difficult things in ways that's never been done before. That is the beauty of the tinkerer mindset that Toby embodies and their whole team. I met them at New Reps, the Shopify engineering team, great human beings, love what they do, but also of course Shopify itself, the platform, love what they do. Sign up for a $1 per month trial period at Shopify.com/lex. That's all lowercase. Go to Shopify.com/lex to take your business to the next level today. This is the Lex Fridman Podcast. To support it, please check out our sponsors in the description where you can also find

• 8:28Lex Fridman?

  ways to contact me, ask questions, give feedback and so on. And now, dear friends, here's Don Lincoln.

• 8:53Lex Fridman?

  In describing the search for a theory of everything in physics, you describe the history of physics can be told effectively as a kind of history of unifications. There's this centuries-long quest to show that these distinct phenomena are actually linked by some unified underlying principles, even starting with Newton, that you can think of the effort of physics as well as trying to unify the laws of nature. So I was wondering if we could talk through the history of unification, that lens of physics.

• 9:31Speaker 01

  There are, of course, lots of different ways to do physics, but the way that I would say that particle physicists, cosmologists do is they are trying to really find, basically, the underlying principles that govern the laws of nature. If we go back, say, to the, I don't know, 1650s or so, you're the most brilliant person around and you've noticed two things. One you've noticed is that when you trip, you fall. That is the nature of gravity that we all experience day to day. But then there's sort of astronomy where you look out at the heavens and you see the stars march across the sky. You see the planets move through the stars.

• 10:14Speaker 01

  And there, that seems to have absolutely nothing to do with what happens when you drop your sandwich and the dog grabs it from you. So the brilliant thing was when Newton looked at that and he thought about, maybe the moon is falling, but it's missing the earth. So what we had is that in maybe 1650, you had what we might call the laws of celestial gravity, the gravity that governs the heavens, and terrestrial gravity, the gravity that is here on earth. Now, we don't think of it that way anymore. We think of it as just gravity. But at that time, that wasn't at all obvious. And in fact, if you look in the books, Newton's theory is Newton's law of universal gravity. The universal is there.

• 10:59Speaker 01

  And the reason is, is because he realized these two things that seem to have nothing to do with one another were indeed one and the same. I mean, this is absolutely brilliant. I mean, Newton is... Arguably, one of the most brilliant humans I of which I'm ever aware. But at any rate, it is the first sort of easily to describe unification of physics that you can state in a way that sort of makes sense to to modern humans. I mean, you can go back farther than that where people are talking about chemistry, the nature of atoms. You go back to Democritus, who was wrong about very many things. But the idea that there was a smallest particulate form of matter is right. So it's kind of funny, you talk, you read the chemistry books and they say that the idea

• 11:43Speaker 01

  of atoms goes back to Democritus. And, you know, he, his idea was that like, um, there was a smallest atom of oil, which was smooth and it was smooth, of course, because, well, oil is smooth. There was a smallest atom of vinegar because vinegar is tart and it pricks your tongue. So therefore atoms were little sharp, pointy things. And so he was wrong about a lot, but he was right about the idea that there was a small particle. And, and we now know a very, we have a very different concept than he did. So you can go back farther than that. But getting the unification, there are more examples. For instance, if you go back to, say, eighteen thirty or so, scientists were trying to understand

• 12:27Speaker 01

  electricity, for instance. And there was a lot going on. People really understood things, but at the time you would have two phenomena that are familiar to us now. One is a magnet, which, you know, at the time mostly magnets were, were simply little pieces of iron that had been magnetized and they could stick to steel. And then you had electricity, which was at the time they were generating little sparks that they could play and have fun with, or more broadly, a lightning bolt blazing across the sky. And so when you think about this, that lightning bolt and that little magnet seemed to be really unrelated.

• 13:06Speaker 01

  But over the eighteen hundreds, a number of scientists were exploring little aspects of it. What happens when you run electricity through a wire? It seems to make a magnetic field. You know, they, they, it was a whole bunch of experiments and there were a lot of names. But in about the eighteen sixties or so, James Clark Maxwell took all of those ideas that had been percolating around for the previous fifty years and wrote his laws of electromagnetism. And they're really fascinating. If you look at the laws of electromagnetism, they are. Their differential equations or integral equations. But basically, what they say is on one side, you have a bunch of terms that have electricity in them.

• 13:49Speaker 01

  And then you have equals on the other side, a magnetism thing. So forgetting all of the mathematical symbols, you have electricity side equals a magnetism side. Electricity equals magnetism. And that is a staggering concept. The fact that these two things, a lightning bolt and the magnet that holds your kid's art to the refrigerator are one and the same. And this was another case where electricity and magnetism became unified into electromagnetism. So now we have two examples. One, gravity being unified, terrestrial and celestial gravity, and then electricity and magnetism. So I'll tell you about some more in a moment. But one thing that's kind of important, because the goal is of course to unify everything.

• 14:34Speaker 01

  If I could do what I want to do, I would have some unified theory that would explain all the behavior of all energy, matter, space and time, which is a grand goal.

• 14:43Lex Fridman?

  And we should say that maybe one of the goals of science more broadly, outside of physics even, is to construct models that can generalize the world. So if you look at Darwinian evolution, that was a very beautiful theory that captures another layer of reality of like how this particular thing that we see here on Earth happens.

• 15:11Speaker 01

  Right.

• 15:11Lex Fridman?

  So when we talk about theory of everything in physics, that's capturing a different layer of abstraction about the functioning of the universe.

• 15:19Speaker 01

  Right. The whole Darwinian evolution, the fact that our genetics has a significant overlap with the genetics of a banana is pretty staggering. It's astonishing that that works. So that is amazing. But for at least the class of scientists that I am, what we think of is, well, sure, biology is interesting and all, but when you get right down to it, it's caused, whatever happens in biology is caused by the movement of. Molecules. And then you say, well, that's great and all, but molecules, they do what they do because they're made of atoms. And then the next step is, well, you know, atoms, that's great, but atoms work the way they do because of the nucleus and the electrons. And then

• 16:03Speaker 01

  the nucleus is protons and neutrons. And so there are those of us, myself included, who want to dig down to the very, very bottom and find out what is the smallest building block of nature from which all of these other far more complex and interesting and abstract things are. But what is at the very, very bottom? And also, that's great. But if you know what the smallest building blocks are, that doesn't tell you the story. That's like having a whole bunch of Legos, but not knowing how to put them together. You also need to know how they interact, how they work. And so that's what we study forces. So there are the various subatomic forces of which we're familiar. And for instance, electricity and

• 16:45Speaker 01

  magnetism are components of electromagnetism, which then governs the behavior of things like, this is amazing, electromagnetism explains, of course, electricity magnetism, but it explains how light works. It explains how much of chemistry works. So electromagnetism, 1860 or 70. The wonderful thing about that is if you take Maxwell's equations and you apply a little bit of calculus, it's very easy to see that the laws of electricity and the laws of magnetism combined together make what's called a wave equation, which that shows that these electric and magnetic fields oscillate, they vary. And if you have something

• 17:30Speaker 01

  that's very, that's a wave, and the wave then moves. And if you do the math, you find out that the speed at which these waves move is the speed of light. And so People said, wow, the speed of light comes out of those equations. And that had to be, I think, very persuasive. And of course, electromagnetism also plays a really significant role in chemistry. Because after all, atoms are held together by electromagnetic forces. There's more to how atoms work. There is all the quantum mechanics stuff. But if you did not have electromagnetism, or if electromagnetism was very different, then atoms would be very different. So it plays a very big role in, in holding us together.

• 18:13Speaker 01

  So it's a staggering advance in science to have a good behavior in that. And of course, being able to, to tame electromagnetism is why people can hear you when you do your podcast. Because through the miracles of the internet, just, or just electricity running the computers. I mean, this is a case, if I can get on a small soapbox, where people back then said, well, why are you messing around with magnets and sparks? And who cares? Well, that very fundamental digging into the laws of nature has spinoffs. And it has spinoffs. One of the big spinoffs is our entire technological society.

• 18:58Speaker 01

  Without being able to govern electricity, we'd still be farmers and shoemakers in cities. But we certainly would not have everything that we do. So off my soapbox. But it's really a lovely thing to show how this, this digging into deep, fundamental, not understood, mysterious things can a hundred or 200 years later transform the world. And the type of science I do now, people often ask, well, what good is knowing about how the inside of atoms work, how the inside of quarks work? And I don't know the answer to that, but just being a little more pragmatic. If I go back, say, a hundred years, where people were trying to understand how the protons

• 19:43Speaker 01

  and neutrons inside atoms held together, how they split, how they, they, how you could combine them and so forth. This has led to nuclear power. Now, whatever you think about nuclear power, and some people like it and some people don't, but it is powerful. It will generate energy for humanity. And it may be that is the path that that we take as we move away from digging fossil fuels out of the ground. Humanity is going to need power, no matter what. Nobody is going to go back to the way things were in the 1700s. And one enormous source of energy that is there for us to take if we so choose is the modification of the nucleus of atoms.

• 20:27Speaker 01

  Seem to have absolutely nothing to do with anything, and yet it provides humanity with an opportunity, which of course requires that we think carefully of how we do that and and if we want to, but it gives us something that we didn't have before.

• 20:41Lex Fridman?

  Yeah, it's very clear that nuclear fusion and nuclear fission will unlock huge amount of energy that's required for civilization to flourish, but that's almost like near term.

• 20:54Speaker 01

  Mm hmm.

• 20:55Lex Fridman?

  Longer term, you can think about things like, we'll talk about dark energy crisis and antimatter. Maybe if you figure out some of the mysteries around antimatter, that too would lead to energy sources, how to produce energy, that too might lead to counterintuitive propulsion systems for us humans to travel through through the universe. Now, right now, it seems farfetched, too expensive, too complicated, too difficult, but breakthroughs in the fundamental theoretical physics might lead us to unlock some incredible energy sources, incredible technologies that will allow humans to explore the universe.

• 21:39Lex Fridman?

  And of course, we should also mention that as always with technologies, the double edged sword, it will most likely lead to the development of more dangerous weapons or other sources of harm. And then we, as a civilization, kind of have to walk that line.

• 21:58Lex Fridman?

  And hope we figure out how to do more good than bad with the technologies we build.

• 22:03Speaker 01

  Right, but but we have to really remember while people worry about nuclear weapons, which are admittedly very dangerous and even nuclear power, which has waste that has to be dealt with. What science is doing is working out, finding power that nature has presented to us. This is not new. Fire is like that, too. Fire can burn down your house or it can cook your steak. Power is like that, and that's just something that we have to understand as humanity. And that's why this needs to be, you know, when we talk about science, it has to be a broad conversation by all of society because what scientists can do is figure out how the world works.

• 22:48Speaker 01

  Society has to figure out how we wish to apply that or not apply that.

• 22:52Lex Fridman?

  Also, solving the mysteries and the puzzles of the universe in itself is effing awesome.

• 23:00Speaker 01

  It is, it is.

• 23:01Lex Fridman?

  So, I mean, that's the thing that makes us human in part is looking at a thing and saying, how does this work? And then together, a bunch of apes get together, like poke the thing, kind of shake the thing. And then over time, you have rockets going all into space. You build roads and bridges. You build the internet. Anyway, so we talked about, and we talked about Maxwell. That takes us in the 20th century in terms of unification. There's a guy named Einstein, and we wrote a book, who did quite a lot of progress on the effort of unification.

• 23:37Speaker 01

  Sure. So Einstein, he's a pretty amazing guy. In 1905, he had his miracle year where he wrote multiple papers. The one that most people know about is special relativity, where he showed something that makes no sense to anybody who's not really dug into it very hard. And that is that two people experience time differently.

• 24:03Speaker 01

  Time is a fascinating thing. We don't really understand what time is, which is weird. You think that that'd be something we'd understand very well, but we really don't. We know a lot about it, but really understanding it, not so much. But Newton thought that time was just universal for everyone. So my time, your time, some person's time on Mars or on Alpha Centauri. Everybody experienced time the same. What Einstein showed was that that wasn't the case, that different people moving at different speeds with respect to one another experienced time differently, which is absolutely a mind blowing concept. Now, most people think that Einstein then said, well, he invented spacetime,

• 24:46Speaker 01

  that that space and time are the same thing. And he was behind that, but that actual insight came from one of his teachers, a guy by the name of Minkowski, who looked at Einstein's equations. Minkowski was a little bit more mathematically inclined than Einstein. And he saw that if you look at the equations, you have basically one person's space and time equals some numbers times this person's space and time. And so that's kind of a staggering thing. So, so that is where Einstein and Minkowski really did this unbelievable concept that that space and time are actually pretty much the same thing.

• 25:30Speaker 01

  That runs afoul of our understanding of how the world works because time just moves. It's continuous. We, we know what it is at a visceral level.

• 25:41Speaker 01

  And an experiential level, we might not understand it on a formal level, but we know what time is. It's what keeps, makes today today and not yesterday or tomorrow. Space is a little different. You can walk somewhere. You can walk back. You can move around. You have more freedom to move in space than you have to move in time. You can always move forward in time. It's just moving backwards. It turns out to be a little more difficult. But yeah, Einstein's understanding that that is the case. It caused everybody. To think about the world very, very differently. And that was in 1908, when Minkowski really laid it out in a strict spacetime. And that also led to the work on special relativity led to the speed limit, the speed of light. Well, it was a premise.

• 26:26Speaker 01

  He had two premises. One was that the laws of nature are the same for everybody. So if you're moving at some speed, or if I'm moving at some speed, I can say I'm not moving and saying you're moving at some speed. That's not controversial. That is what we call Galilei in relativity. It's from hundreds of years ago. But what Einstein said that was controversial was that everybody measures that the speed of light is the same. Irrespective of how we're moving with respect to each other. You'll measure the speed of light to be a number. I'll measure the speed of light to a number. And that's very, very different from what Newton would have said, or Galilei, or any of the old guys.

• 27:07Speaker 01

  And it was taking those two things together that caused all of the weirdnesses of special relativity. Now, you could then very easily say, well, that second premise that everybody measures the speed of light to the same is just dumb. And that, you know, you could test that. So that's where testing relativity comes in. And Einstein's equations, which include those two assumptions, it predicts the behavior of everything perfectly well. Now, we've actually measured, done experiments where we can say that the speed of light is the same for everybody. That's not how that's been in the beginning. It was really that assumption leads to predictions.

• 27:52Speaker 01

  The predictions are true, so the assumption is true. Now, there is a, for those people, for your viewers who want to say, well, how do you measure that the speed of light is the same for everyone? The particle physicists do this. And the way you do this is the following. There are some subatomic particles that when they decay, they emit light. That's their decay product. And so you collide two things together. So you know when the particle was created. Then you have surrounded your collision point by a detector. And you measure how long it takes for light to get to your detector. And by God, it's the speed of light, which it should be.

• 28:35Speaker 01

  However, sometimes in these collisions, some of these subatomic particles you make are coming out at very high speed. They might be coming out at 95 or 97 or very large fraction of the speed of light. And then they decay into photons. And so you measure how long it takes for the proton to get to your detector. And it says it's light travels at the speed of light. Now, if it were that if Einstein's conjecture was incorrect, you'd have a particle coming out at near the speed of light. It would be decaying into a particle traveling at the speed of light. Then that particle should have traveled at, say, two times the speed of light or something like that. So it should have taken half as much time to get to the detector, but it doesn't.

• 29:20Speaker 01

  So this is a hard, serious measurement that shows that something, you know, we can measure the speed at which light comes out of this stationary created particle. And it's the speed of light. Then we can measure what the speed is of it coming out of something that's moving. And it's still the speed of light. So that is an actual measurement, but that is not something that was possible in Einstein's day, but it is now.

• 29:45Lex Fridman?

  Just to take a small tangent, how weird is it? In the full ranking of weirdness that is physics, how weird is it that there's that speed limit of this speed of light?

• 29:56Speaker 01

  Well, I have to tell you, when I first encountered this, it's pretty freaking weird. It's like pegs the weird meter. But as you become more familiar with it, as you become more, more comfortable with the idea. The thing to remember is the speed of light.

• 30:16Speaker 01

  It's the speed of light through spacetime. Once you embrace that, that makes a whole ton of sense. It all of a sudden makes everything fall much more into place. I think that there is an ultimate speed. Isn't that shocking? It just simply says that it's a property of space in the same way that there is. You know, space can can transmit a certain strength, electric field. It can trans it can support a certain things. Whatever space is, and we don't know what space is, but whatever it is, it has the capability of transmitting these things at that one speed through space or time. And everything else comes from our insisting that we keep space and time different.

• 31:01Speaker 01

  That's that's how I view it. And at least for me, that once I accepted that, it all became very comfortable.

• 31:10Lex Fridman?

  So the nature of my question actually here that will apply over and over. It's trying to empathize, trying to put ourselves in the shoes of the people before space and time are unified.

• 31:20Speaker 01

  Yes.

• 31:21Lex Fridman?

  Into spacetime and and really experience and think through how difficult of a leap is that?

• 31:27Speaker 01

  Huge.

• 31:28Lex Fridman?

  The reason I I sort of say that is we are now in the modern day in the 21st century. And of course, we're going to have to make leaps like that in our future. So what are the unifications we're not seeing in front of our eyes? So for example, there's so many examples through through your work, through your lectures of

• 31:51Lex Fridman?

  Paul Dirac taking antimatter seriously. Looking at what the math shows and saying, I really think this thing exists.

• 32:00Speaker 01

  Right.

• 32:00Lex Fridman?

  I mean, it just sounds insane.

• 32:03Speaker 01

  It does.

• 32:03Lex Fridman?

  And so I think this is a good warm up. The spacetime unification is a good warm up as we march through the 20th century. Cause it gets, in my view, at least weirder and weirder, even with Einstein himself.

• 32:17Speaker 01

  Well, let me give you an even more basic example, sodium and chloride. Sodium is an explosive metal. You put it in water and it's kind of neat. You put it in water and it just, it doesn't quite explode, but it gets hot and it pops around. Chlorine, it's a gas. It's going to kill you. So these two things are deadly. They're awful. And yet, when you mix them, you put it on your food. It might salt, right? And so this is a case where, where this whole A unification and B this deeper understanding, in this case of chemistry, of how two things that are dangerous can be brought together and turned into something not only innocuous, but necessary for human life.

• 33:02Speaker 01

  And so this is not unusual, what you're describing. I mean, when you think about it, forget about everything else, just the fact that, you know, we tell little kids, little kids, that the world is made of atoms. Now, that's crazy. Most people have never seen atoms and yet, nobody really doubts it anymore. And I think it's just a case of familiarity and then the culture slowly accepts it and it's then, it's real even without the evidence. In fact, one of the courses you described there, how we know what we know. I think that's a valid question. How do we know there are atoms? And of course, there are ways we do.

• 33:39Lex Fridman?

  And by the way, on that front, I would love to go through how we know the building blocks in the universe as we march towards quarks. That, in the course that you mentioned, is one of the most fascinating things of this philosophy of atoms being around for a very long time. Then you concretize and you actually can prove or have strong observations that indicate that there is atoms and then there is a nucleus, there is electrons, there is photons, there is quarks and all that. I mean, it gets weirder and weirder. And now we're facing the mystery, is there building blocks even smaller than that? But anyway, Einstein turns out didn't just do special relativity. By the way, I, I really think he deserves three Nobel Prizes.

• 34:24Lex Fridman?

  He got it for photoelectric effect. The fact that he didn't get it for general relativity is a crying assucy. I don't understand. Obviously should have gotten it for general relativity and, and special relativity. I mean, I think special relativity is separate for general relativity in terms, as far as Nobel Prizes go. So general relativity is another unification.

• 34:46Speaker 01

  Yes, that's right. What Einstein realized was that if you were in a rocket ship and the rocket ship was a very quiet rocket ship and it was accelerating, you would feel like you were experiencing gravity. And so, as, as you say, it's one of his happiest moments when he realized that acceleration and gravity feel very much the same. What I'm impressed by is that idea, which is already a pretty neat idea, somehow led him to take his spacetime idea, take this acceleration-gravity idea and realize that he could describe gravity as the bending of spacetime.

• 35:27Speaker 01

  Spacetime being constant, like east, west, north, south, that's already hard enough. But now he's saying, well, you know, take your, your map and crinkle it and bend it and so forth. And that's gravity. That is a staggering, mind-blowing idea.

• 35:44Lex Fridman?

  I guess, I wonder if you can comment on what do you think is the idea generation process that leads to that? So probably an Einstein case has to start with, what if gravity is itself spacetime geometry? You, you have to have a thought like that, right?

• 36:02Speaker 01

  Yes, I think so. There's a lot about science. There's of course, knowing what went before. There is knowing the mathematics that allows you to figure out the implications of your theory. There is the discipline to argue with yourself and other people because most ideas are wrong. But then there's what you just described, that intuitive spark. And that is something that is very, very difficult to, to create. There's a reason that we venerate these people is because it is an unusual.

• 36:36Speaker 01

  Feature and most people only have that aha moment once in their lifetime, if they have it at all. And then there's a tricky business because I'm sure you do. And I get a lot of letters from, from creative thinkers who don't have all of the, the history and the mathematical discipline and the, the self, you know, self critique that's necessary. And so they come up with these ideas and often it's easy to see where they just don't play out. So in order to be that person who changes the way we see the world, ideas themselves are not enough. These, these creative ideas, that's not enough.

• 37:18Speaker 01

  You need it with the discipline and the critique. And it's that amalgam of those things that, you know, make you a genius that, that history remembers.

• 37:30Lex Fridman?

  But it's hard to know in a, in a field of people, you might be tempted to call crazy. There could be geniuses there and it's hard to know which is which. We should mention that Einstein himself

• 37:44Lex Fridman?

  couldn't see the genius in quantum mechanics initially. Couldn't see the, the correctness, I should say. So he could see them, the insanity of gravity bending spacetime, but quantum mechanics was too weird for Einstein.

• 38:01Speaker 01

  In all fairness, it's weird for me too. But, um, but the thing is even while that is true and Einstein maybe spent the last few years of his life trying to, to blend electricity and magnetism, gravity in a, a single thing. And he was unsuccessful, but he still was a very, very valuable critic of quantum mechanics. It's not that he didn't understand it. Because he did understand that he thought about the implications and all this quantum entanglement business, not all of it, but he was responsible for saying, well, if you're right, then this. And of course, then people went out and found out that Einstein's implication of quantum mechanics was real. And so they could say, see, quantum mechanics is real.

• 38:47Speaker 01

  So, you know, he was thinking deeply about it and he was doing exactly that thing I said. There's that spark idea, but there's that critique idea. And if you're able to critique an idea, you might kill it. And that is, it's always depressing when I have this brilliant idea and it gets killed. But it's better to be killed than to keep it around and waste time on it.

• 39:13Speaker 01

  And so he was, in that case, not generating the aha. But he was saying is, all right, let's take your aha, let's see it's right. What does it mean? It means this, that allows people to go test it. And so he was contributing very crucially to that other part of scientific advancement, which is not just the aha moment, but the beat it to death, test it, critique it and make sure it's real. And it's only after all of that has been done, that you really assure you're right. And that's why science is such a powerful tool.

• 39:52Speaker 01

  It is that, that combative, just downright kind of jerky critique that most people don't like. They don't like people saying, ah, your ideas, you know, might be wrong. But that is, it is crucial, is crucial part of the scientific process.

• 40:08Lex Fridman?

  Plus, there's that quote on the other side of it that I've heard you mention, which is,

• 40:15Lex Fridman?

  you know, I believe your idea is crazy, but is it crazy enough?

• 40:19Speaker 01

  I love that one. Yes, yes. We all agree that your idea is crazy, but is it crazy enough?

• 40:24Lex Fridman?

  And there is some degree of taking those leaps of crazy, but it has to be backed with rigor.

• 40:30Speaker 01

  Right.

• 40:31Lex Fridman?

  And the unifications continue that as we take steps towards the standard model, which is such an incredible part of physics in the 20th century. So can you describe that unification?

• 40:42Speaker 01

  So, you know, we're sort of jumping forward here now to the 1930s or thereabout. And by that time, people had realized that there are

• 40:53Speaker 01

  Four distinct forces that do not seem to be connected. One is gravity, two is electromagnetism, and those are things people are relatively familiar with. But there are two other forces that only have any real importance inside the nucleus of atoms, which is why most people have no experience with them. One is the strong nuclear force, which holds the nucleus of the atoms together. And the other one is what we call the weak nuclear force, which is responsible for some types of of radioactivity. And since most people don't play around with nuclei, and most people don't play around with radioactivity, they don't know what that is.

• 41:33Speaker 01

  But by the 30s, scientists had done enough experiments, done enough theorizing to to say that there were these four forces. And that was already a triumph. I mean, we, in our goal for a theory of everything, we'd like to think that there is one force, which is what we're talking about, the unification. Maybe these four forces are are just different ways of looking at a single underlying force. But in the 30s, that's where we were. There were the four forces. So we move ahead. And in the late 50s and early 60s, some people were thinking that maybe the weak nuclear

• 42:15Speaker 01

  force and electromagnetism actually were the same. So they were working on trying to bring together these two forces to show that they're connected. And it came true. They were able to show that electricity and magnetism were actually two different facets of a single force that we now call the electroweak force. Now, the story that you're told in in articles about this, about what people have called the Higgs boson or the God particle, the story is very, very simplified.

• 42:54Speaker 01

  Because in 1964, the, um, there were three groups with six individuals who came up with important papers talking about what's called the Higgs field. And I'll get to back, get to what that is in a minute, but the Higgs field is important. But it wasn't until 1967, so three years later, that Steven Weinberg and some others actually unified electromagnetism in the weak force.

• 43:21Lex Fridman?

  Sheldon Glashaw, Abdussalam, and Steven Weinberg successfully unified electromagnetism in the weak nuclear force that showing that at high energies, uh, these two forces were merged into a single electro weak force.

• 43:38Speaker 01

  Right. And that was in 67. All right. Um, everybody talks about this thing happening in 64, but it really wasn't. It happened over quite a few years, actually. But, all right. So now let's, what you said is true. So, um, uh, Weinberg, Glashaw, and Salam showed that electromagnetism in the weak force at high energies were the same. There was a problem, however, and the problem is that electromagnetism has an infinite range. Um, and we know that because we can see stars that are millions of light years away. I mean, that shows you that the range of that force is essentially infinite. The weak force,

• 44:19Speaker 01

  however, um, basically becomes non-existent on distances much smaller than the size of a proton. So that, you know, to say, oh, they're the same. And yet, one can reach across the universe and one can't reach out of an atom. Well, that's just dumb. I mean, the obvious thought here is, well, we just proved that that whole idea is stupid. So throw it away. Ridiculous. And that is where these ideas from 1964 came in and saved the day. So how can it be true?

• 44:56Speaker 01

  That the electro weak force is real and electromagnetism and the weak force acts so differently. The way that could happen is if these forces were transmitted by a particle moving from one subatomic particle to the other. In the case of electromagnetism, it's the photon. In the case of the weak force, we call them now the W and Z particles. So the idea is that that Higgs and his colleagues came up with is saying, all right, electro weak force is real.

• 45:39Speaker 01

  The way we make it so that there is now an electromagnetic force and a weak force is the force carrying particle of electromagnetism has no mass. The force carrying particle of the weak force has a mass. And so what was done is a field was postulated that there was this additional field that was kind of distinct from this electro weak field and we call it the Higgs field. And the Higgs field permeates all of space. And here's the kicker, some particles interact with a field and some particles don't interact

• 46:24Speaker 01

  with a field. The ones that interact with the field get mass and the ones that don't interact with the field don't have mass. And so that's the idea is that the Higgs field gives the weak force particles mass. However, the photon lasts at the Higgs field, doesn't see it, and it has no mass.

• 46:44Lex Fridman?

  And I should say here, going to perplexity. The big picture view, the Higgs field is a quantum field that fills all of space and gives many elementary particles, just as you're saying, their mass through their interaction with it. The Higgs boson is the particle associated with ripples or excitations of this field. In modern particle physics, every type of particle corresponds to a field that exists everywhere. The Higgs field is one such scalar field, meaning at each point in space, it has a single numerical value rather than a direction. The Higgs field differs from most other fields because even in empty space, empty in quotes, by the way,

• 47:26Lex Fridman?

  empty space, its average value is not zero. This non-zero vacuum value is what enable it to indulge particles with mass.

• 47:35Speaker 01

  Right. So let's talk about something a little more familiar, just to try and hang some intuition on those words. All right. So right in front of us, there is a gravitational field. Now, you can't see it, but right there, right there, check it out.

• 47:49Lex Fridman?

  Yep.

• 47:50Speaker 01

  If I were to take something, a pen or whatever, and put it there, it feels a force and it falls. Very insightful, I know. So we have the gravity field and we have the pen that has a mass and the mass and the gravity field interact and it drops. Now, if we had another.

• 48:11Lex Fridman?

  I have an object for demonstration purposes.

• 48:15Speaker 01

  Performance art. Here we go. This thing has mass and we drop it. How remarkable. It falls. But when we step back and think about what really happens, it's the mass of this thing and the interaction with this invisible field we see here, that's what gives this weight. Now, I have this particle here that you can't see, but it's there. It has no mass and I leave it there. Well, since it has no mass, it doesn't feel gravity. It's still floating there. And that is really all the Higgs field is. Some particles have effectively what you could call the Higgs charge that interacts and sees the field and other particles don't.

• 48:58Speaker 01

  And that is really what, what, what you read just basically means. Now it's kind of neat because in the ordinary day, there is a Higgs field right there and the Higgs field is not zero, just like gravity is not zero and things will get mass. But it's super high energies. The Higgs field, the strength of the Higgs field goes to zero. So whether things have mass or whether they have a Higgs charge or not, they have no char or they have Higgs charge, Higgs field zero. They don't interact. It has no mass. So that's kind of what Weinberg and Salama Glassow said is that very high energies, the Higgs field is zero. Since the Higgs field is zero, the weak force particles don't feel mass and therefore they

• 49:43Speaker 01

  can travel at the speed of light, just like the photon does and everything's happy. It is when the universe cooled down after the big bang, it was very hot, very high energy, nothing had mass. The universe cooled and at a certain temperature, what happened is the Higgs field turned on. And at the moment it turned on, it gave mass to the weak force particles, did not give mass to the photon. So that's what we call electro weak symmetry breaking. So it's a mouthful, but all it says is there was a moment in time, early in the history of the universe at 10 to the minus 12 seconds after the big bang, the Higgs field turned on and particles got mass. So that's the whole idea.

• 50:28Speaker 01

  So this is another really neat thing. So the electro weak symmetry theory doesn't need Higgs because that only really applies at very, very high energies. But in order to make it work at low energies, you need to fix the theory and you need to fix the theory by effectively putting a Band-Aid on the theory. Higgs theory is just a Band-Aid on top of electro weak symmetry theory. And that is the Band-Aid that fixes it because it gives mass to particles. At low energy.

• 51:00Lex Fridman?

  But how does then this Band-Aid, the field and the Higgs boson come into play on the experimental front, on the evidence

• 51:10Speaker 01

  Okay.

• 51:11Lex Fridman?

  discovery front. So what is this Higgs boson?

• 51:13Speaker 01

  Okay, excellent. So we have never seen the Higgs field. Higgs field is a hypothetical theoretical thing. But that is true of of most of our fields. We've never seen the electromagnetic field. We've never seen the gravity field. We've seen the effect of the field. And so all of these theories are now what we call quantum field theories. And that the whole idea of quantum fields, if you have a quantum field, but that field can vibrate like a drumhead. And so it doesn't vibrate just exactly like a drumhead, but it vibrates locally. So you can have specific localized vibrations. And those specific localized vibrations are the particles. In the electromagnetic field, the vibration is the photon. In the

• 51:58Speaker 01

  Higgs field, the vibration is the Higgs boson. And so what we can do is not see the field, but we can actually excite the field, make it vibrate, and detect the vibrations. So the Higgs boson idea was predicted in 64, it became useful in 67, and then scientists started looking for it. So in the early 2000s, people were starting to think that we had built particle accelerators more powerful or powerful enough to actually to be able to create these vibrations and detect them. So the accelerator that was working at the

• 52:41Speaker 01

  time was a large particle accelerator outside Chicago at Fermilab called the Tevatron. And we were colliding protons and antimatter protons at near the speed of light at very high energy. And that was the accelerator at which the top quark was discovered in 95. But we had upgraded our apparatus. We had 10 times the number of collisions per second. We had slightly more energy and we were banging the protons and the antimatter protons together, hoping that we would actually find the Higgs boson.

• 53:13Lex Fridman?

  Can you actually back up a little bit and look at the bigger picture? So Fermilab has this legendary accelerator that there's also a personal story with you connected to it. Because I mean, there's a million questions I want to ask you and we'll ask you about some aspects of that. So this idea of an accelerator, the design and the physics of an accelerator, how is that productive for understanding and discovering different aspects of particle physics?

• 53:40Speaker 01

  Well, I'm so glad you asked. I mean, this is fascinating. All right. Everybody has heard Einstein's equation E equals mc squared. Nobody knows what it means. Maybe they heard that energy equals mass and mass equals energy. I don't know, you know, but they heard the equation, the most famous equation in all of science. But buried inside that equation is a really thoroughly fascinating concept that energy and matter are equivalent. And you can, in fact, convert movement energy into mass. And so this is something that we've known for a long time.

• 54:20Speaker 01

  This was predicted back in basically 1928, so a long time ago, actually almost 100 years ago. And it is not in the slightest bit controversial. We can do this all the time. So the simplest thing is to take two particles that have no, no structure. So, you know, the closest thing you could have to BBs that are just true mathematical BBs. If you smash those two things together, it's coming in with a huge amount of energy from one direction, a huge amount of energy from the other direction. The directions cancel. So the net momentum, the net energy of this has no motion. So you have these two things coming in with a, you know, exactly balanced energy.

• 55:05Speaker 01

  And if they collide, they could stop. Well, that energy has to go somewhere. And that energy can literally create mass, create particles. Now, there are special rules about what happens. If you have two things coming together and it creates a particle, it has to create an antimatter particle to balance it. That's just kind of the rules of the laws of nature. Why is that the case? Well, we have some ideas, but in many respects, the answer is because those are the laws of the universe. And that's the things that we try to understand. But this is absolutely true. So what what particle accelerators do, among other things, is simply transform energy into particles.

• 55:49Speaker 01

  And so basically, any particle that doesn't exist in nature, we can make in this way. You can make the antimatter electron by taking two particles, smashing them together. The energy sits there and it will make an electron and an antimatter electron, and it just does. And we know that the antimatter electron was discovered in 1932. This is all pretty easy. The antimatter proton was discovered in 1955 at the Berkeley Bevatron. And so this is just what you do. You can convert energy into a matter antimatter particle. Now, the converse goes true, and that's something we might talk about. You can take matter and antimatter and bring it together,

• 56:34Speaker 01

  and it'll make energy. It's the the process can go both ways. Energy can make matter and antimatter. Matter and antimatter can make energy. And this is just true. We do it all the time. There's no question that this is the case.

• 56:48Lex Fridman?

  And we should also mention that this is the reason why Fermilab had a nice stash of antimatter particles. So as a side effect, you can also collect antimatter in this kind of way. You can produce antimatter, but is it extremely costly?

• 57:03Speaker 01

  Oh, very, very costly. In order at the Fermilab machine, we would have to smash 100,000 protons into something.

• 57:13Speaker 01

  To make one antimatter proton. So I mean, it took some work.

• 57:18Lex Fridman?

  Is there some extremely precise recipe of of being able to produce particular kinds of particles and all this kind of stuff when you smash two things together? Is there like, how can you control accurately which kind of particles you're trying to produce?

• 57:35Speaker 01

  If you want to make antimatter electrons, you smash together energy at a certain, it's just easier with electrons because the electrons, to the best of our knowledge, have nothing inside them. So they're simple. They have a certain mass and that's that. So if you smash particles together with the right energy, you can make them very, very easily because you can, it's like a old style radio back in the day where you had to dial it in. You could get right on the station and you could hear the the signal. And if you were off a little, it didn't work. The problem for things like protons and so forth is they're not point like particles. They're kind of like garbage cans full of stuff. And so it's

• 58:13Speaker 01

  very difficult to make antimatter protons. Now you can get more of them by increasing the energy at which you collide two particles together. If you're at below a certain energy, the, and you collide, say you collide two protons together at kind of low energy, you just don't have enough energy to make an anti proton. And so it doesn't happen. You get to a certain energy and you can just barely make them. The more energy you collide them together, the more you make. So that is just sort of how it works. More is better.

• 58:48Lex Fridman?

  And then with with CERN, if you compare maybe CERN and Fermilab, going to perplexity here, CERN's accelerator, the Large Hadron Collider, LHC, is the world's highest energy proton collider, while Fermilab's current and plant accelerators focus on intense proton beams for neutrino physics, rather than pushing the absolute energy frontier, absolute energy frontier, meaning highest possible energy.

• 59:17Lex Fridman?

  Smashing of protons together.

• 59:19Speaker 01

  Correct. So, one, we were talking about accumulating antimatter.

• 59:24Lex Fridman?

  Yes.

• 59:24Speaker 01

  All right. And so there, that is typically making antiprotons, as opposed to making all particles in general. So let's focus on the antiproton side to begin with. All right. So Fermilab doesn't make antiprotons anymore. We stopped making them in 2011. And it's because we shut our big accelerator down to concentrate on a different facet of particle physics. However, at the time, we would smash protons with an energy of 120 GeV. And in that, we would make antiprotons. So that's a ton of energy.

• 1:00:04Speaker 01

  It's true that the CERN accelerator, the big accelerator, is now much higher energy than the Fermilab accelerator was. No problem. But that's not how they make antiprotons. The, all of these big beam, big laboratories, it's not one accelerator. At Fermilab, there were five distinct accelerators. And it was basically like shifting an old standard car because you couldn't just go zero to super speed in one accelerator. You had to go from one to another, getting higher and higher. Well, at CERN, they use one of the, basically their second gear in their very big accelerator complex to make antimatter protons.

• 1:00:44Speaker 01

  And their accelerator is only 26 GeV compared to the 120 GeV at Fermilab. Fermilab's not operating, but when it was operating, it operated at an energy of about four times higher than what CERN is doing now. And why is that? Well, it's because what CERN needs to do is to not make as many antiprotons as Fermilab did. They are doing a very different current experimental program. They're doing a fascinating experimental program, including trying to figure out, does anti-gravity fall up or down? Which is kind of neat. And we sort of know the answer to that. Different, that's separate. But anyways, so getting back to the antiproton business.

• 1:01:24Lex Fridman?

  Yeah.

• 1:01:24Speaker 01

  Well, Fermilab doesn't do it now. It was top dog. It's not anymore. The only really big antiproton accelerator creator is a small accelerator at CERN. Okay. So that's the antiproton thing. And if we get back to antimatter, we could talk about that because that is way cool.

• 1:01:40Lex Fridman?

  Yeah.

• 1:01:41Speaker 01

  Now, the other side of your thing about making high-energy unknown particles, bigger is better. And it is true that the LHC is a very high-energy machine, is about seven times more powerful in terms of energy per collision. It is also about 100 times more collisions per second than the Fermilab machine. So it is true that the LHC can make bigger, heavier particles that the old Fermilab, the Tevatron, ever couldn't. And that is true. So if you want to look at high-energy stuff, yeah, you go to CERN now, which is why many

• 1:02:20Speaker 01

  of my colleagues, including myself, once we had measured all of the sort of frontier measurements we thought we could make with the Fermilab accelerator, we saw this bigger, more powerful machine with seven times the energy and 100 times more collisions per second. We said, heck yeah, let's go work on that. And to give you a sense of scale, the top quark, which is the heaviest particle ever discovered, discovered at Fermilab in 1995, there were two discovery papers, and the one on which I was a co-author, we had worked for a good chunk of, between six months and a year, of collecting collisions, and there were a lot

• 1:03:00Speaker 01

  of collisions. And our paper had 38 top quark candidates, 38. And we knew that half of them were crap, because when you make a detector like that, there's what you call background, so you have the background and the good stuff, and we know it was about 50/50. So we had maybe 19 top quarks after working for between six months and a year of collecting data. But now, at the LHC, we make a top quark every second.

• 1:03:30Speaker 01

  And that's what higher energy and more collisions per second will do for you. That extra energy, you're above threshold, you make a ton of them. And when you compare the 1995 Fermilab accelerator to the current

• 1:03:44Speaker 01

  CERN accelerator is probably a thousand times of collisions per second. So it went from painstaking, pulling teeth to, yeah, now top quarks are a background. We try to get rid of them. There's just too many of them. They get in the way of searching for the stuff we really want to search. They are like so thirty years ago.

• 1:04:02Lex Fridman?

  By the way, is there something to be said about the, the kind of signal processing here? How you remove the noise, how you remove the background? How you determine which particles, which there's probably some like incredible nuance there. Even outside of the scope of this conversation.

• 1:04:18Speaker 01

  So let me just throw some numbers out. All right. So at the CERN accelerator, when it's operating, the collisions occur at a prodigious rate. We get about a billion with a B collisions per second. Now, each one of those, yeah, yeah, that's what I said.

• 1:04:34Lex Fridman?

  Wow.

• 1:04:35Speaker 01

  Now, it turns out some of them are happening at the same time. So there's about of order forty million moments in time per second. Where you would take a shot and inside that moment, there might be twenty collisions. So that's why where we get to the billion.

• 1:04:51Lex Fridman?

  Yeah, but can you individually pinpoint the collisions?

• 1:04:55Speaker 01

  Sort of to a, to a degree. So the beams are, you know, the, when people think of beams, they think of like laser beams, but that's not really what particle beams look like. Particle beams look like little tiny sticks of spaghetti, except they're much thinner. They're not as fat as a stick of spaghetti. And they're at the LHC, different accelerators are different. They're about this long. And so you have one of them going one way, full of protons and another one going the other way, full of protons, and they pass through each other. And as they pass through each other, you should think of this as like a swarm of bees.

• 1:05:29Lex Fridman?

  Yep.

• 1:05:30Speaker 01

  And this is like a swarm of bees. And mostly the bees pass through each other and don't do anything. But every so often, some of the bees hit nose on and. Stripes and wings and everything everywhere.

• 1:05:42Lex Fridman?

  That's awesome.

• 1:05:42Speaker 01

  And so as they collide through each other, you know, one collision's here and one's here and one's here. And you can't tell too much side to side because the beams are really small. They're sort of the thickness of a human here. But you can see along the direction and there, you know, this is about the right size. And so we have detectors around them and we can actually see, oh, particles came from here and particles came from here. And that's amazing. All right. So at any one crossing, there's maybe 20 collisions. Now, most collisions are absolutely boring. They're boring because they are, they, they exemplify physics that we know a great deal about already. We've tested it for, for decades. We know all about it.

• 1:06:27Speaker 01

  We don't care. I mean, it's kind of blasé that we can say, oh, yeah, yeah. We're making a billion particles, subatomic particles every second. But who cares? Because, you know, but that's just the way of, you know, frontier scientists. So what you need is you need to pick out the cool ones, the weird ones, the ones that nobody's seen before. And so what happens is these things, these beams collide and we surround the collision point within the enormous detector. There are two absolutely ginormous detectors at the LHC. One of them called CMS, which is the one I'm on. And the other one is called ATLAS, which is the other one. And we don't speak of because, no, they're both really amazing.

• 1:07:12Lex Fridman?

  Okay, it's good to know that there's friendly competition even inside. That's awesome.

• 1:07:17Speaker 01

  The fact is they are both amazing, absolutely amazing detectors.

• 1:07:21Lex Fridman?

  But CMS is just a little cooler than.

• 1:07:23Speaker 01

  Oh, yeah, yeah. I mean, you know, in, in, in particle physics, we really absolutely want our competitors to do extremely well. It's just not quite as well as we do.

• 1:07:33Lex Fridman?

  Got it. All right. So these two giant detectors.

• 1:07:35Speaker 01

  Right. So, but one of them are detected, the CMS detector. It's the small one. It is 70 feet long, 50 feet high, 50 feet wide. It's five stories tall. It weighs 14,000 tons.

• 1:07:47Lex Fridman?

  Small one.

• 1:07:48Speaker 01

  Small. Atlas experiment is one hundred and fifty feet long, eighty feet across. It weighs only seven thousand tons, just a piece of cake. You could take the atlas detector and you could put four of them on a soccer or football field and it would fill the field up with just enough room on the sidelines for the cheerleaders and the water boy and the coaches and stuff. That's how big they are. So these are absolutely ginormous detectors and basically they're cameras. And what they can do is they can take pictures forty million times per second. Now, all the data comes streaming off that detector and we can't record it all. It would just fill up all of our tapes and it would be full of all these boring things we don't care about. So what we do is as the beams pass through one another,

• 1:08:29Speaker 01

  we teach our detectors to say, we only want one where there are certain configurations that might be interesting, like there's gobs of energy in the detector, or there's a gob of energy on one side and nothing on the other side, or there's four gobs of energy or whatever. These are called triggers. And so what we have is fast electronics that take the forty million possible pictures per second. And it says, you know, about a hundred thousand of those are really cool. We should think about them. And then it passes not all of the forty million, but those hundred thousand to the next level, which are commercial processors

• 1:09:12Speaker 01

  that have basically our final analysis code, but optimized to run very, very quickly. And what they do is they do a really quick and dirty analysis to say, to further refine what's good and what's not. And that, that computer farm then accepts about a thousand collisions per second. And we record those for further analysis. So that's what's really happening. Of the fifty million possible collisions per second, the fast electronics and then the computers pick the thousand. And then we pass those through analysis software and hand them to the graduate students and they pick through them, looking and finding the handful that are the next Nobel Prize. So that's how that works.

• 1:09:57Speaker 01

  And that, that is truly astonishing. I'm hats off to the accelerator builders, the detector builders, the, the, the people who make the software work, the people who make the not, not gigabytes, not terabytes, but petabytes of data flow around the world seamlessly. It's really amazing. I'm very grateful.

• 1:10:15Lex Fridman?

  So take me to July 4th, 2012, the discovery of Higgs boson.

• 1:10:22Speaker 01

  So this is really fun because the people searching for the Higgs, it's a, it's a community. And the entire community knew that the LHC was coming online. So even though many of us had been working on the Fermilab accelerators, a lot of us were transitioning to the CERN accelerator. So we were in the very funny business of wearing our Fermilab detector people hats, trying desperately to find the Higgs boson at Fermilab, while simultaneously wearing our CERN hats, knowing that CERN was going to be able to find it if existed. And so, you know, we were a little neurotic, kind of wanted, you know, our old stuff to work.

• 1:11:08Speaker 01

  And, and there was an awful lot of people on both experiments.

• 1:11:11Lex Fridman?

  Did you have a sense that one of the two places would be able to find the Higgs? First, did you think the Higgs boson existed? And second, did you think that these accelerators have the chance to find them?

• 1:11:25Speaker 01

  So I was cognizant of the fact that the Higgs boson might not exist, but there was a lot of evidence pointing in the direction that it might be. I knew that both experiments, the Fermilab accelerator and the CERN accelerator, would either find or rule out the Higgs if it existed.

• 1:11:45Lex Fridman?

  Rule out?

• 1:11:46Speaker 01

  Well, that's a possibility. I mean, maybe the Higgs theory was wrong.

• 1:11:49Lex Fridman?

  Yeah.

• 1:11:50Speaker 01

  Right? Until you, until you know it's there, it might be wrong. It's like dark matter. People talk about dark matter. It might not be real. I mean, I, it probably is, but it might not be.

• 1:11:58Lex Fridman?

  So you knew at these energy levels, you would be, you should be able to find the Higgs boson?

• 1:12:02Speaker 01

  Yes. So that's the nice thing about this kind of physics, because there was a theory, that theory made predictions. Now, there were parameters in the theory. We didn't know if the mass was this, we'd get this thing. If the mass was this, we'd get this thing. But we could do the, the calculation for every conceivable Higgs mass. And so then we could search, look, well, let's say the Higgs mass is 100 in some units. Did we see it there? No. Then it's not 100. Well, let's look at 103. Is it there? No, so we could do that. Both accelerators could either find it or definitively rule out the predictions

• 1:12:43Speaker 01

  of simple Higgs theory, 100% guaranteed. However, the CERN accelerator had 10 times the collisions per second and three and a half times the energy. So remember when I said with the top quarks, it was like six months for 19 versus one a second? There's no question, the writing is on the wall. The LHC was going to have an easier time of it if it was real. However, so, but you know, I'm a Fermilab scientist and we wanted Fermilab, you know, come on, we want Fermilab to win. Yeah, of course. So we were busting our butt and we had done what I said. We had ruled out this region. We had certain mass ranges.

• 1:13:28Speaker 01

  We knew it wasn't there. And we finally said, if there was a Higgs boson, if it existed, which we didn't know at the time, its mass was somewhere between, if I recall, between like 120 and 145. All right, we'd ruled out all the other stuff. And so, wearing our CERN hat, we said, okay, we're going to find that. But we were really, really, really trying to do it. Now, if we had had another two years or maybe three years of running the Fermi accelerator, Fermilab would have discovered or ruled out, or in this case, turning out, discovered the Higgs boson because it's a real thing. We would have found it without a question.

• 1:14:09Speaker 01

  But we didn't have enough data in July of 2012. We needed a couple more years. Unfortunately, in 2008, fortunately, the LHC had turned on. It broke, they had to fix it, turned on again in 2010. It ran poorly in 2011. In 2012, they pushed up their sleeves and said, let's do this. And it turned on. And so, you know, there was this, Fermilab knew if it didn't have it now, it wasn't, it was too late. Anyways, 2012 rolls around and.

• 1:14:47Speaker 01

  Like two days before the announcement at CERN was July 4th. So two days before that Fermilab made a measurement and said, we can rule out certain regions, but certain regions we can't rule out. But what we know, and this is important, if the Higgs boson exists, it must be in this region for which we are not capable yet of ruling out. So that's where we were two days before the LHC said, we got it. It was July 4th, 2012.

• 1:15:23Lex Fridman?

  So detecting the Higgs boson confirmed the existence of the Higgs field, the mechanism through which fundamental particles like electrons and quarks acquire mass in the standard model.

• 1:15:34Speaker 01

  Correct, although let's be very specific of what we did then. We found a particle consistent with the existence of the Higgs boson. There were alternative theories at the time that predicted not one, but multiple Higgs bosons. So there's a theory called supersymmetry, which said that there was not one, but five Higgs bosons. The standard original 1964 Higgs theory says there were one. And so all we really knew at the time was we found a theory. We did not necessarily confirm that Higgs was right. We found data that said that it looked like Higgs was right.

• 1:16:14Speaker 01

  But until we ran for longer, we were unable to rule out other alternative theories. So that's the, the deal. Now, in the fullness of time, it is after all, what 14 years now later, we have been able to basically rule out some of those other things. And by now, we have validated a things. We found the mass of the particle. We know the spin of the Higgs boson that has a spin of zero. We have discovered the Higgs boson decays. It preferentially decays into the heaviest particles it can through energy conservation. Can't decay in the top quarks. It's too light to decay into top quarks, but it can decay.

• 1:16:57Speaker 01

  Into bottom quarks, it could decay into W and Z particles, can decay into a weird way into photons. And we have looked for all of the hypothesized decays of the original Higgs theory. And we have validated that it decays in those ways at the rates that theory predicted. And so now, in the fullness of time, I'm pretty comfortable saying Peter Higgs and Robert Braut and Francois Anglare and his colleagues, they were right back in the 60s. But we weren't sure on July 4th. All we knew is we found a particle consistent. But the thing is with these discoveries, they're often just barely discoveries.

• 1:17:38Speaker 01

  It takes a while to go and do the more complex, detailed measurements, and that's what we've done.

• 1:17:44Lex Fridman?

  So at the time, I remember it being called, referred to as the God particle.

• 1:17:51Lex Fridman?

  You also had a minor in theology. So throwing that all together, so calling it the God particle is speaking to the importance, the potential importance of discovering this particle. Do you think that is in some degree justified? Like, if we look at the big impact of it on the, on the history of physics, how important was it to find and show that the Higgs field is real?

• 1:18:17Speaker 01

  Well, I don't think it is as important as, for instance, some of Einstein's stuff. I mean, it was, it's an important prediction. Like, the prediction of quarks was very important and interesting in validating this. The Higgs was kind of like validating that quarks existed. It's an important stepping stone, and I, I do not wish to denigrate it in any way. But there's ones that changed the way we thought about the world, like Einstein did. It wasn't that sort of thing. And there is a funny story. So the reason they call it the God particle is this book by Leon Letterman.

• 1:18:54Speaker 01

  And if you read his book, he, uh, he says, well, you know, we, we call it the God particle, but, but we should call it the goddamn particle because it's been causing us so much trouble trying to find it. And Leon ran Fermilab. So, and he wrote a foreword for one of my books. And, you know, I talked to him. He said, he was a really funny guy. And the real truth was the book was called The God Particle because his publisher thought it would sell more copies. But, but, you know, then that got into the mindset of the reporters and so forth. And we called it The God Particle. Leon never really thought of it as anything to do with a religious or even, I mean, he was an incredible jokester, The Goddamn Particle.

• 1:19:34Lex Fridman?

  It is a really important part of our model of the universe.

• 1:19:37Speaker 01

  It is.

• 1:19:37Lex Fridman?

  That there's this field that it gives mass to some particles and not others. That's...

• 1:19:41Speaker 01

  Right. It, it, it's a huge thing, but it was part of the standard model. The standard model had known forces. It had known particles. It had all that. The, the Higgs boson, the one thing that is true is it was the last unvalidated piece of the standard model. The standard model does not answer all questions, which is why we have unanswered questions in physics. But it was a punctuation point, end of about 50 years of discovery and searching, where we finally were able to say, the standard model, while, while incomplete, it's mostly right as far as it goes.

• 1:20:20Lex Fridman?

  Quick 10 second thank you to our sponsors. Check them out in the description. It really is the best way to support this podcast. Go to lexfridman.com/sponsors. And now, dear friends, back to my conversation with Don Lincoln. We did a whirlwind tour of the history of physics and took a little tangent on this incredible discovery of the Higgs boson.

• 1:20:45Lex Fridman?

  Uh, but we didn't go all the way yet. There's this dream of the grand unified theory, the gut, that is a step towards the toe, theory of everything. So can we talk about the gut first? So what's, what's entailed in the gut?

• 1:21:01Speaker 01

  So the gut is short for grand unified theory. We talked about that. There were four known subatomic forces, the electromagnetic force, gravity, the strong force and the weak force, and electro-weak symmetry unification merged to the weak force and electromagnetism into the electro-weak force. So what gut hopes to do is to merge the electro-weak force and the strong force into one grand unified force. Now that leaves gravity outside because gravity is seemingly fundamentally significantly different. Then subsequently, it is hoped that a higher energy, we will be able to blend the theory of everything together with all of the known

• 1:21:44Speaker 01

  subatomic forces, the strong, weak, and electromagnetic forces, and then gravity. But so as you say, gut is sort of a way station along the way. That's the goal. And at this point, I would have to say that I do not see a fast progress in the immediate future. I think we're a ways away from that at this point.

• 1:22:06Lex Fridman?

  You mean on the gravity front?

• 1:22:08Speaker 01

  Maybe we'll come up with something really cool. We certainly had some ideas back in the early eighties that we tested and they didn't pan out.

• 1:22:15Lex Fridman?

  Uh, speaking of which, string theory is the thing you're referring to. Uh, so string theory posits that particles are tiny vibrating strings, and by tiny would mean extremely tiny at the scale of a plank length. Uh, then there's, there's other leading candidates like loop quantum gravity. Uh, maybe there's some alternate theories.

• 1:22:39Lex Fridman?

  In the works, so can you link on that a little bit more? Do you think a theory of everything exists?

• 1:22:46Speaker 01

  So I hold personally that there are rules that govern matter and energy, space, time, and they probably are rules that I don't know. There are probably phenomena I'm not aware of. But I do believe that something, there are, is a rule that governs reality. And so in that sense, once we understand the rules that govern reality, the fundamental rules, that would be a theory of everything. You know, there are things that are unknowable, like, for instance, inside black holes, we don't know what's inside there. But that doesn't mean that there's not something inside there. So there's a distinction between what we can know and truth. So I do believe that there are the rules.

• 1:23:29Speaker 01

  And I do believe that with sufficient time, technology, effort, we will be able to figure this all out. Now, this isn't a thing in my lifetime. It's not a thing in my grandchildren's lifetime, or even their grandchildren's lifetime.

• 1:23:41Lex Fridman?

  Whoa, whoa, whoa, that's a pretty strong statement, right? That's a pretty strong statement, saying we're, we're 50 to 100 years out from finding a theory of everything.

• 1:23:52Speaker 01

  It took 200 years to go from unifying gravity to unifying electromagnetism. It took 100 years to go from unifying electromagnetism to unifying the electroweak force. Now, you could say, well, gee, that's went from 200 to 100, so it's getting faster. But it's also getting harder, because the unification scale is of order 10 to the 15, which we can do the math, that's a quadrillion times higher than the highest energy accelerator we can build today. And it was one thing to, you know, we are reaching diminishing returns. We get something like a factor of seven increase in particle accelerator energy every 20 years.

• 1:24:36Speaker 01

  And so we have to get to a quadrillion times. Now, you know, if you really did believe a factor of seven every 20 years, then that's, we're talking like 500 years. But, you know, this is like Moore's law, that it doesn't continue forever. We're not going to every seven, I mean, every 20 years, get another factor of seven. So yes, I think it's a very long time. That's my prediction. You know, some people are far more optimistic, and we can talk about that.

• 1:25:04Lex Fridman?

  We should also actually mention. That I guess your intuition behind that is not just the part where you come up with a theory that's beautiful and seems to be internally consistent, but you have to have a theory that's making falsifiable, testable predictions.

• 1:25:23Speaker 01

  Correct.

• 1:25:23Lex Fridman?

  And you have to have a feasible engineering construction, a methodology for creating an experiment that tests that prediction. So I think a lot of your, this is 50, 100, 200 years from now, intuition is maybe about the second part of that, which is like you need to have an experiment.

• 1:25:45Speaker 01

  Yeah, yes, yes, but you know, let's say, I mean,

• 1:25:50Speaker 01

  you alluded to superstrings. I haven't answered that question. I'll table that for a moment. Superstrings is a fascinating idea. I don't believe it, but I love it. I hope it's true. And there's a real, you know, aphorism, and it says you should absolutely never believe what you think. So even if you think superstrings is true, you shouldn't believe it because it hasn't been tested. Now, let's say superstring is correct. I mean hypothesis, it's correct, 100% correct. I don't know it's correct. So I don't care. I mean, you know, it could be correct, but I don't, you know, until it's validated, it's just a wild ass guess. You know, so I, we have to have a way of validating it. So yes, the, the empirical side of it is important.

• 1:26:35Speaker 01

  I mean, you could wake up tomorrow and have the theory that is the perfect theory. But if I can't prove it, I don't care.

• 1:26:42Lex Fridman?

  If we were to think, this going back to the great courses on the evidence for modern physics, we're talking about energy levels and tiny particles to the degree where the kind of prediction we would be making is not accelerator type predictions. So it's probably going to be impossible to build an accelerator that detects something like a string. So you have to make predictions about macro scale behaviors.

• 1:27:16Speaker 01

  That's another alternative.

• 1:27:18Lex Fridman?

  It's a different kind of prediction.

• 1:27:21Speaker 01

  Sure.

• 1:27:21Lex Fridman?

  Do we even have intuitions about what kind of predictions there would be? So one, of course, one of the lines of intuitions has to do with black holes. We're in the singularity. Physics of black holes combine certain elements of general relativity and quantum mechanics. So there you could see some kind of predictions you could make. But you can't really mess with a black hole. It's not like you can create a black hole in the lab.

• 1:27:47Speaker 01

  And the energies that we're talking about, the sizes we're talking about are inside a black hole, which you can intrinsically never see.

• 1:27:53Lex Fridman?

  Yeah.

• 1:27:54Speaker 01

  So, you know, you can only see the outside of a black hole, not the inside of a black hole. So what you said, you did say something that was incredibly important and incredibly correct and probably won't happen, but that's still good. Okay, so we have two choices. When you talk about superstrings, either superstrings are correct and they're making predictions up at the Planck energy scale, at which point we have to somehow build facilities that can generate Planck energies. That's possibility one. Possibility two is this theory, which is currently only applicable at Planck energy scales, someone figures out a way to take those equations and solve them in a way

• 1:28:38Speaker 01

  that say predicts the mass of the electron. Right? And that is a tricky business. I am not a string theorist, so I can't tell you that that's likely, but I can tell you that they've been working on it since the eighties and they haven't gotten very far. Furthermore, if I, I think it's fair to characterize that string theory is still a vague idea and that's unfair, but let me tell you why I say that. Because what they have are approximate solutions to approximate equations.

• 1:29:13Speaker 01

  And that is already saying that we're a ways away from really getting a handle on that. So, yes, there could be some bright young lad or lass out there, someone listening to this podcast right now, who figures out a way to take superstring theory and solve them in tractable ways that makes predictions from the scale at which they currently apply down to measurable scale today. And if that happens, well, then I might retract my question or my concept. There's a reason why I think that probably isn't true. That's probably not valid. So let me, I love this. All right. So let's back up. I'm going to pitch. I wrote

• 1:29:55Speaker 01

  this book for Oxford, Einstein's Unfinished Dream. And Einstein's Unfinished Dream was to come up with a theory of everything. It was unfinished because, well, it's unfinished. And so, the second part, the tagline of that book is practical progress towards the theory of everything with the emphasis on practical. Because when you read books about theories of everything, when you see podcasts, when you listen to YouTube videos or whatever, they are often written by theorists. And theorists are, they're big idea people. They're very, very smart. But, but there's a pragmatism that is often missing, in the sense that they say, well, superstrings, look, they have these little vibrating things and wouldn't

• 1:30:38Speaker 01

  it be cool, you know, but you got to get to the, do you know it? So, let's pretend superstring theory or something like it is correct. The energy scale at which that should occur is of order 10 to the 15 times higher, 10 to the 19 GeV. We can currently do things at 10 to the fourth GeV, give or take. So that is 10 to the 15. That's a quadrillion times higher energy. So what we are doing now is we are looking at the world with our very best measurements and we are trying to project out a quadrillion times higher and figure out a theory that explains everything. Now, I have this, I have a couple of analogies,

• 1:31:23Speaker 01

  but I like this. Suppose that you were some, you know, Joe Australopithecus 2 million years ago or something in Africa, wandering around somewhere in Kenya. Mm hmm. Right? You're about a meter in size. So you can walk a meter. Meter scale is like your scale. You can walk 10 meters in every direction. That's 30 feet. No problem. You can work 100 meters, 300 feet. You can work 1,000 meters. That's half a mile. You can work 10,000 meters. That's 60 miles. 100,000 meters is 10 to the fifth. And that's unlikely. But the distance that we need to go from what we can see to the plank scale, it's not 10 to the fifth, it's 10 to the 15th. So that means, in my analogy, think about

• 1:32:07Speaker 01

  this guy who's walking around Africa. Now, if he walks, you know, 100 feet or something, it looks a lot like what it is now. He can make a prediction about what he sees. And when he goes to that new place, it's probably going to be okay. But if he starts walking 500 miles east, well, he, walking around the center of Africa, has no concept of, for instance, the Indian Ocean. He would never predict sperm whales or kraken. He would never predict what it's like the bottom of the ocean is. Going north, he's, he's in Africa. He would never, ever have a clue about the Alps or Antarctica. Going even smaller distances, going a mile up, things wouldn't be very different. But if he goes 10 miles up, he wouldn't breathe and he'd freeze. If he goes 100 miles up, he would

• 1:32:52Speaker 01

  die. If he goes two miles down, he would roast. The point being is

• 1:33:00Speaker 01

  We are like that Australopithecus. We have a realm that we can study and we can even predict to some validity, what would happen if we go some distance away. But the farther away we go, the less and less our local prediction really represents the reality of those more distant times. And so basically, his theory about the world would be totally bogus. So even if he had the best theory, his theory would not have anticipated the Alps. It would not have anticipated penguins, flamingos, not there, you know? And that is just the case. So now, that's what we're

• 1:33:43Speaker 01

  doing. We are taking something and we have reason to understand what we know and we can predict a factor of 10 or 100. But I think it is the absolute, the pinnacle of arrogance to think that what we can do, given the understanding that we have from what we've measured now and predicted out a quadrillion times higher than we can see now. So my opinion, and this is partly because I'm an experimentalist, the correct way to make progress, practical progress towards the theory of everything, is to look around at the things that we don't have answers to right now. For instance, are there something smaller than quarks? I don't know. Is dark matter real?

• 1:34:27Speaker 01

  I don't know. If it's real, what is it? I don't know. Is dark energy real? Yes, probably, but I don't know. What is the nature of space and time? I don't know. But these are questions we can explore. And I would expect, and this is my prediction, that, all right, we're going to figure things out at a factor of 10 or 100 times better than we can do now. And we might be able to do that in my kid's lifetime or something like that. But in order for us to predict a quadrillion times higher, I'm pretty sure superstring theory is wrong, not because people aren't smart, but because something new is going to happen. I mean, if you were talking about chemistry, you would have never predicted nuclear physics. And that's a small increase in energy,

• 1:35:11Speaker 01

  right? The idea that there is something in the nucleus of atoms that causes the sun to burn, there's a reason why people didn't believe, you know, they calculated how old the sun should be, and it should only be 10 million years old, because otherwise it would burn out. Well, that's clearly wrong and it's wrong because of nuclear physics. That is why I feel fairly confident to say, well, someone could think, well, superstring might be right or something. And maybe it's right and I hope it's right. It would be awesome if it's right. But what are the odds when you're making something with that tiny lever arm predicting it out a quadrillion away and say, oh yeah, we got it right. What are the odds? And my answer is you've got to be kidding me. Now, I could be wrong and I

• 1:35:56Speaker 01

  admit that I could be wrong. But that's why I think the real issue is not the brilliance of humanity. It's the stuff we haven't found. We don't know. I mean, the simple one, I'm saying it's simple and it's not, but what is dark matter? We don't have a bleeping clue. Not a clue. We know a lot of what it isn't, but we don't know what it is. And so, you know, talk about superstrings. All right. Well, maybe dark matter fits in superstrings or maybe dark matter is governed by a physics that is completely diametrically opposed to the superstring concept.

• 1:36:30Lex Fridman?

  And allow me a bit of a thought experiment here, a brief thought. My intuition says that when you propose a theory of everything, the kind of prediction you want to make involves a kind of leap of conceptual understanding that Einstein did. So for example, you want to come up with something like spacetime and then gravity bends spacetime. So it's not merely that you have this beautiful mathematical framework, but that framework allows you to rethink how you see reality enough to make a prediction that's about the macro world.

• 1:37:14Speaker 01

  I mean, to come up with something like spacetime, you know, there's one idea, for instance, to say the space and time aren't real. They emerge from entropy. Yeah, that's a way of, a new way of thinking and maybe there's some validity and I want people to think about it. But in the end, it's just an idea. And that's the real key thing. And as you say, it has to tie to a macro world. You have to validate. If you don't validate, it's a crazy idea. Theorists are

• 1:37:44Speaker 01

  Incredibly creative, smart, wonderfully interesting people, but I don't care. I want a measurement that validates the idea because there are so many. I mean, if you read the journals, there are so many theoretical papers with all these nifty ideas that die. You know, one that was recently I liked and might still be true was that dark matter that, you know, our simple model of dark matter is that there's a subatomic particle out there that's heavy and it's floating around and it's causing gravity. But someone said, well, you know, maybe there's complex dark matter, which means there's a whole dark sector. So there are dark atoms and they interact with one another. And that is a nifty idea and I love it.

• 1:38:29Speaker 01

  And that was all the rage for a while and we looked at it and it may still be true, but the simple ideas have been mostly invalidated because we've tested it and it doesn't work. Same thing, there was a talk about large extra dimensions. The reason that gravity is so much weaker than the other forces was, well, maybe gravity can sneak into more dimensions than the other forces.

• 1:38:50Lex Fridman?

  It leaks into those dimensions.

• 1:38:52Speaker 01

  That was a cool idea. I mean, but that's the point is you have these lovely, cool, interesting ideas that constantly die. And so, you know, I would love for a new nifty idea to be the idea, but I don't know how to pick it out of the hurricane of wrong ideas.

• 1:39:12Lex Fridman?

  I mean, that's the real beauty of science. It really is. I mean, theories kind of get some of the glory sometimes, but the real beauty emerges from the experiment and the demonstration that the theory is correct.

• 1:39:26Speaker 01

  And there are two directions. You're talking a top down. Someone comes up with this big idea that's testable, but you also have the other way that science advances and it's not. With the theory that is then tested, it's with the huh, that's weird. For instance, either in in the 1930s with Fritz Wiki or in the 1970s with Vera Rubin. She did a simple thing. She said, how fast are galaxies rotating? Because it's an easy thing to calculate. You could literally calculate that with high school physics. And you get an answer and then you measure it and it's wrong. And so that's the wow, huh, I don't know what that is. And that led to the hypothesis of dark matter. Now, dark matter is not a theory of everything, but it's a clue.

• 1:40:11Speaker 01

  It's a powerful clue. We should pull tug at that thread. Maybe our entire theoretical edifice unravels. Or maybe it doesn't. Maybe it's just a snag and we can fix what we have now. I'm not sure. So that's another option is to simply look at many measurements that are very precise. And find ones where the outcome and the prediction with established theory disagree. And that is a clue.

• 1:40:36Lex Fridman?

  Before we leave the topic, we've got to talk about string theory. In your view, is it basically dead? As I understand, one of the primary flaws of string theory. Outside of the testable experiments that we're talking about is because it's relies on these unobserved extra dimensions. There is a hope that it uniquely could explain our universe. But it turns out this quote landscape. There's an enormous so-called landscape of possibilities that it leads to. And so it renders the theory basically unpredictable. Because it can describe all kinds of universes and therefore you can just select.

• 1:41:19Lex Fridman?

  Tune in to describe ours.

• 1:41:22Speaker 01

  I agree to a degree, but I bring it back to my prior objection. It is absolutely true that superstring theory. In its current manifestation, aside from the extra dimensions, which are at some level small potatoes. It allows for an extremely large number of possible universes. But if we were able to take.

• 1:41:55Speaker 01

  Those predictions and somehow connect it to a physical measurement. Then what we would do is we'd lop off those alternatives. We'd throw them away as saying, well, you know, those are like an equation. You know, X plus five, I can put in any number I want in there. It doesn't matter, but if X plus five equals nine, then I've ruled out a whole bunch of numbers except four. And so this is a case of string theory does allow for many predictions. But if we could rule them out by connection to a measurement, then it would no longer do it. We would modify string theory and we would retain the vibrating string concept, which I really, really like.

• 1:42:37Speaker 01

  I mean, I really like it, but until we can can validate this, it's it's I we can't. So now you ask, is it dead or not? In my opinion, it is very difficult to kill such a theory. I mean, really truly kill it because kill it means make a prediction and it fails. But what can happen and what is happening is people have been working on it since the seventies. So we're talking of order 50 years. People have been working on it and it has not solved the problem. And so I think what's happening is people are looking at that and saying,

• 1:43:17Speaker 01

  I do I want to spend my life working in this direction with a very likely possibility that 30 years from now will be not much farther along than we are now. It's a lot like back in the 1940s when people started thinking about the meaning of quantum mechanics. And I wanted to do that when I was a kid in the seventies. But then when I went to grad school, I realized that people, very smart people, people smarter than me, had been working on that for most of their lives and made no definitive progress. And so you have to decide as a scientist who wants to answer questions, do I really want to take on a question that is so hard that it will not be answered in my lifetime? And I think that's what's happening with a lot of superstring theories.

• 1:44:01Speaker 01

  People are saying it's really neat. It might be right, but I don't want to devote my life to something that I might not see progress forward in my lifetime. What do you think about the alternate theories? Do you think there's anything interesting in those? You know, many of those theories are espoused by passionate people. They have fans. People love them, but they don't do what science needs to do, which is make predictions. Now, loop quantum gravity is a little different. That one is better developed. And that one is not a theory of everything, so we should make that clear. Loop quantum gravity is not a theory of everything. It is simply a theory of quantum gravity, period.

• 1:44:44Speaker 01

  It does not aspire to include all of the known forces. It simply tries to take gravity, which is currently intrinsically. It treats space as smooth and continuous. For those of your viewers who are mathematically inclined. In Einstein's theory of general relativity, gravity is infinitely divisible. There is no smallest bit, and so essentially the laws of calculus apply. However, it is possible that at a small enough scale, space is no longer divisible in the same way that you can, you know, take a cup of water out of a swimming pool and then a quarter a cup and so forth.

• 1:45:28Speaker 01

  But eventually, once you've taken out a single molecule of water, you can no longer take out a smaller thing. So loop quantum gravity attempts to quantize gravity. So that's what it does. And so this is unlike string theory, which attempts to bring gravity in with the other forces. And in fact, the reason, the fundamental, the one reason why string theory became so interesting to the theoretical community is string theory was not being developed as a theory of everything. It was being developed as a theory of the strong force.

• 1:46:09Speaker 01

  And it was in competition with QCD, which is the currently accepted theory of the strong force. And it turned out the two groups, the string theory groups and the QCD groups competed for a while. And string theory basically failed the race. And people paid attention to quantum chromodynamics, QCD. But then somebody noticed in string theory that one of the things it predicted was a zero mass spin two particle. And you can prove that any zero mass spin two particle is the graviton. And so if you see a theory that has a zero mass spin two particle, you are now have a candidate for dragging gravity in.

• 1:46:52Speaker 01

  And then, oh, my gosh, people got terribly excited because now this theory, which was working in the direction of the other quantum forces, brought in gravity. And now it was a candidate theory for everything. But, um, but that's not what loop quantum gravity is. Loop quantum gravity is simply trying to understand the nature of space itself, which is already a fantastic thing. And I talked to Ravelli every so often. I write about his theory. And I point out some of the issues with the theory. But I'm usually about like two months behind as he and his colleagues are developing and so forth. Because one of the things is originally loop quantum gravity predicted that the speed of light would not be universal.

• 1:47:36Speaker 01

  The speed of light would depend on the frequency of the light. So high frequency would travel at one speed and low frequency would travel at a different speed. And it had to do with the wavelength of light basically interacting with the structure of space. You know? And so that was an issue with loop quantum gravity. And so if you look at gamma ray bursters, which are, you know, super explosions of astronomical events that are billion light years away or more. And they spit out light in all wavelengths. And if that loop quantum gravity prediction were correct, when you saw a, um, one of these gamma ray bursters, you would see the wavelength of one light appearing on Earth at a different time than another wavelength because of the different speeds.

• 1:48:21Speaker 01

  And that wasn't the case. They appear at the same time. And so I went on to say, well, this pretty much killed loop quantum gravity, only to get a prickly note from Mr. Or Dr. Ravelli saying, you know, we've disproved that. We've changed the theory. That's no longer true. And now that that prediction, that old prediction of loop quantum gravity is no longer valid. And so that observation of the sun, the uniformity of the speed of light no longer kills the new loop quantum gravity. It would have killed the old one, but it didn't kill the new one.

• 1:48:55Lex Fridman?

  By the way, that example of different, uh, speeds of light, uh, based on wavelength. That's a beautiful thing that a theory, that's a testable thing.

• 1:49:05Speaker 01

  It is.

• 1:49:06Lex Fridman?

  Right? So like those kinds of things. And if it in fact did explain a phenomenon of that sort, that's a good sign for the theory, right? If it correctly predicted.

• 1:49:15Speaker 01

  There was another brilliant observation recently. Love it. The observation of gravity waves. And it was from two neutron stars orbiting and coalescing. And so they made gravitational waves. Fantastic. But they also, because they were not black holes, they were neutron stars. They hit and exploded, gave off a tremendous bright flash of light. And so astronomers saw the flash. Gravitational wave astronomers saw the ripples of spacetime. It was 140 million light years away, which means light would have traveled 140 million years to get here.

• 1:49:58Speaker 01

  And the two incidents, light and gravity, both arrived within 1.7 seconds of one another. And that tells you that gravity travels at the speed of light. That was a brilliant, fantastic measurement. Now, we thought gravity traveled at the speed of light, but now we have a measurement. We proved it and Emmett, I am impressed.

• 1:50:18Lex Fridman?

  Our universe is so fascinating. Speaking of which, since we brought up antimatter, we'll have to talk about it. You've talked about it in several of your lectures from different angles, including the dark energy crisis and including empty space and vacuum and so on. So let's look at the empty space angle. So, you know, it turns out that empty space is not empty.

• 1:50:43Speaker 01

  It's true, which is kind of bizarre.

• 1:50:46Lex Fridman?

  Can you, can you speak about what do we know about what makes up empty space?

• 1:50:50Speaker 01

  That's a hard, hard question because we don't know what space is, but let's start out. Let's just start out with something simple. We'll assume that space is not quantized. Okay. Now it probably is, I don't know, but you know, we've got to start with somewhere. So let's start out with sort of the space of calculus, the space that you can divide forever. The modern version of quantum mechanics is called quantum field theory. And it postulates that A, space exists. Then it postulates that within space, there exist fields for every known subatomic particle. So there is a photon field. There's an electron field. There's an up quark field. There's a down quark field.

• 1:51:35Speaker 01

  There's a, all the fields.

• 1:51:39Speaker 01

  And those fields can vibrate. And when they vibrate, those are the subatomic particles. So an electron field vibrating in a characteristic way is an electron. Now, it's also possible for the electron field to vibrate, not in the characteristic way, but in a way that's still vibrating, but it's not an exact electron. So this is what we call virtual particles. Now, virtual particles, there are lots of ways to talk about them. And the way I'm talking about now is the most correct and the most sophisticated way that we can talk about them.

• 1:52:20Speaker 01

  I will talk about them briefly in a simpler way to help. But right now, that's the important thing is that there are these fields. Specific vibrations are the known particles. Vibrations that are a little different are these virtual particles. They're particles that don't truly exist. And so that is what we think space is. There is all of these fields. They're all vibrating a little. If you insert the right amount of energy, you can get it to vibrate in the characteristic way and make that subatomic particle. But even when you don't, there is, the particles, I mean, the fields are there and they are vibrating.

• 1:53:05Speaker 01

  So those vibrating vibrations are what we called virtual particles. Now, your viewers may have heard of virtual particles in other ways, in which case it says that space is just empty. And what happens is matter and antimatter particles briefly appear for a very short period of time before they coalesce back again and disappear and reemerge back into the field. And so that, these are both correct. So what happens is, is that's what quantum field theory says, is it says that these ripples are hearing or these particles are appearing and disappearing. And so that just sounds nuts. You look at empty space, you're not seeing anything happening, but they're happening

• 1:53:46Speaker 01

  fast enough that they can't be seen, but they do have consequences. And there are two experimental measurements that I can think of that validate that this thing that sounds crazy is really happening. And one is called the Casimir effect. So in the Casimir effect, you take two metal plates, that parallel plates, and you put them near one another, very, very close. Now, if this is the case, if these virtual particles exist, then in between the plates, these particles appearing and disappearing, and outside the plates, the particles are disappearing and disappearing. However, because these plates are close to one another, this puts a constraint on the

• 1:54:31Speaker 01

  wavelength of the particles that can occur between the two plates, because the particles cannot extend outside the plates. So the short wavelength particles can exist inside the, between the plates, but the longer ones cannot. However, outside the plates, there is no constraint. So short wavelength and long wavelength particles can exist there. And the net effect is there are more particle, virtual particles outside and less particles inside, and therefore you have a net pressure, which would then push those two plates together. That is a prediction we've been talking about, and guess what? It happens. Those plates push together.

• 1:55:13Speaker 01

  So that is a validation for the existence of these particles in empty space. Now, there is another measurement, and this changes the magnetic properties of particles like the electron, the muon, and so forth. And so this was discovered in 1948. So if you take old school, standard quantum mechanics, you know the spin of an electron, you know its charge, you can calculate its magnetic moment, and it comes out to a number. If you do the measurement, what you find is the measurement disagrees with the quantum mechanics, the 1930s quantum mechanical prediction by 0.1%. And that was measured 1948.

• 1:55:58Speaker 01

  And people went huh. So this happened at the Shelter Island Conference in New York. And on the way home, someone who saw the this measurement thought about it. And they invented what we now call quantum electrodynamics. So old quantum mechanics quantizes matter. The second quantization quantizes both matter and the fields. In this case, quantized the electric fields. And so in this quantized field, it predicts that surrounding a bare, say, electron, which is spinning and has a has a charge.

• 1:56:39Speaker 01

  There is this this bath of particles, virtual particles appearing and disappearing all around it. And the ensemble of all of those particles appearing and disappearing will alter the magnetic properties that you can measure for the subatomic particle. And it changes it by 0.1%. And we have measured this and we have not measured this imprecisely. We have measured the magnetic properties of both the electron and the muon to 12. Count them 12 significant figures. And the theory and the data agree number for number for 10 places.

• 1:57:21Speaker 01

  And then, once you get out to the very end where both the theory and the data have some imprecision, they then disagree. And so maybe there's some interesting stuff going on there. But 10 figures, it's just staggering.

• 1:57:35Lex Fridman?

  So virtual particles refer to matter and antimatter particles coming to life.

• 1:57:41Speaker 01

  Correct.

• 1:57:41Lex Fridman?

  Can we just talk about the the antimatter part of that? So it's starting with Paul Dirac, one of the most legendary examples of. Math leading to physics. So the math suggesting that something like an antimatter should exist and Paul Dirac taking it seriously and then eventually showing that it does exist. So what evidence do we have for antimatter?

• 1:58:05Speaker 01

  So antimatter was predicted in 1928. Paul Dirac was trying to merge quantum mechanics and relativity because the original Schrodinger equation did not, was not relativistic. And in doing so, he basically, the equations were complex, but in the end, it came down to something like equation squared equals one. You take the square root of both sides, you get equation equals plus one or minus one. Plus one was the electron, minus one was something. You didn't know what it was. There was some conversation for a while, thought maybe it might be the proton, but that didn't seem to work out. And so he insisted that his equations were right and that there was an antimatter, he didn't call it an antimatter, but a positively charged sibling

• 1:58:51Speaker 01

  of the electron, what we now call the positron, the antimatter electron. So it was predicted. It was discovered in 1932 by Carl Anderson and his student Seth Nedermaier. They saw an antimatter electron. And that was pretty cool. So that right there, they knew it was real. Antimatter was predicted, it was observed, that's that. In 1956, the antimatter proton was created and that required a large particle accelerator, high enough energy to, to, to make it. And that was done at Berkeley. And a year later, the antimatter neutron was discovered.

• 1:59:29Speaker 01

  So at this point, and now jumping head to now, we can make, using the energy by smashing particles together, we can make antimatter protons, we can make antimatter electrons. We have gone so far to make antimatter helium nuclei. So we have made two anti-protons and two anti-neutrons, combine them together to make an antimatter helium nuclei. This has been done, been observed, no question. At CERN, they have gone so far as to make antimatter hydrogen.

• 2:00:06Speaker 01

  They take a beam off one of their lower energy accelerators. They make antimatter protons. They collect them. They slow them down. They cool them to almost absolute zero. They take sodium 22, which makes antimatter electrons. They slow them down. They bring them together. They coalesce them and they make literal antimatter hydrogen atoms with an antimatter proton surrounded by an antimatter electron. And they have done incredible measurements. They have agitated the atoms and caused it to emit light. They have looked at the light that comes out of antimatter atoms. And the question is, is does the light coming out of antimatter hydrogen atoms have exactly

• 2:00:51Speaker 01

  the same spectral characteristics as ordinary hydrogen, which we predict that it does? And the answer is it does. So the tests have been staggering. We now know a great deal about antimatter hydrogen. Recently, recently, like 2023, I believe it was one of the experiments called Alpha at CERN made antimatter hydrogen, put it in a bottle and released it and watched which way it would go. Did it fall up or did it fall down? Because while it kind of makes sense maybe to think that maybe antimatter falls up in the same way that we have Coulomb's law, you've got electric charges and they might attract or repel.

• 2:01:35Speaker 01

  However, there was lots of ample theoretical reasons to believe that antimatter also would fall down. So they did this fantastic measurement and they first they put in hydrogen and they calculated that some if they did this, something like 80% of the hydrogen atoms would fall through the bottom of the bottle and 20% would go through the top just because gravity is very weak and the atoms will escape wherever they do, but there will be a bias pulling hydrogen atoms down. So they did the exactly the same thing. And what did they find? They find that antimatter falls down. Now, they do not have

• 2:02:16Speaker 01

  a good enough measurement at this time to say that the gravity that antimatter experiences is 100% that of matter. What they have measured is that antimatter fell down with 75% the strength of regular matter, but there were big uncertainties. There was plus or minus 0.13 due to the experiment, which was good but imperfect, and plus or minus 0.16 due to their their theoretical model. So it's like 0.75 plus or minus something like 0.29.

• 2:02:56Speaker 01

  And that means there's a good chance it's between 0.5 and 1, which means it's consistent with 1. So they are improving their measurements.

• 2:03:06Lex Fridman?

  Well, if I can, I would love to take a bit of a tangent on that topic because I went down a rabbit hole watching some of your videos on antimatter. And I mean Fermilab was the hub for the production of antimatter for quite a while.

• 2:03:21Speaker 01

  It was.

• 2:03:21Lex Fridman?

  I saw that NASA said that the global estimate for the current rate of production of antimatter is 1 nanogram per year. Can you speak to how hard was it to make antimatter? And also, you did mention in a video that, you know, if matter and antimatter meet, they produce a lot of energy.

• 2:03:41Speaker 01

  Mm hmm.

• 2:03:42Lex Fridman?

  I think 20 grams of antimatter is equivalent to a one megaton nuclear warhead in terms of explosive energy. Yeah. So all of those questions together. So how hard is it to produce antimatter?

• 2:03:55Speaker 01

  It's freaking hard. Okay. All right. So here's the deal. So at the time, until 2011, Fermilab was the most powerful antiproton production facility on the planet. Every 2.3 seconds, we would smash 10 to the 13 protons into a target, and we would get out 10 to the eighth antiproton. So basically, in order to get a single antiproton, we needed to smash 100,000 protons into material.

• 2:04:30Speaker 01

  So every 2.3 seconds, we would get of order 10 to the eighth antiprotons. And what we would do is we would collect them over the course of 12 hours or so. And we would get in the end, we would have to collect them and cool them down and so forth. Of order 10 to the 12th antiprotons every 12 to 24 hours. So 10 to the 12th sounds like a lot. It really does. That is a trillion. But you need to remember that a gram of antimatter is 10 to the 23 antiprotons. So that means over the course of a day, we were able to create something like 100 billionth

• 2:05:14Speaker 01

  of a gram. And so if we did that for a year, then that would be about a nanogram. So about a nanogram a year, give or take. That's, that's a reasonable estimate. So a nanogram, one billionth of a gram. So that means at that rate with that facility, it would take a billion years running with very little downtime to make a single gram of antimatter. If you combine one gram of antimatter and one gram of matter together, the energy release is equivalent to the combined Hiroshima and Nagasaki explosions. So that tells you, if you wanted a megaton, you need about 25 times more.

• 2:05:59Speaker 01

  So you would have to run for 25 billion years to get a megaton of explosive power.

• 2:06:04Lex Fridman?

  Let me lay it all out because I think it's pretty interesting, actually. This is a NASA estimate of how much it costs to produce antimatter. So looking at all the, the cost of the accelerator, all, everything combined together, to do enough for a one megaton antimatter bomb is such a thing, it would be even possible on the order of 25 grams, like we mentioned, will cost about, based on the NASA estimate. 1.5 quadrillion dollars. By the way, NASA wasn't talking about a bomb. It's just me adding. NASA was talking about the estimate, the cost of 62 to 63 trillion dollars per gram of anti-hydrogen, actually is what they're referring to.

• 2:06:49Lex Fridman?

  So compared, I was looking at estimates, the current best estimate is how much it takes to produce a one megaton nuclear warhead. Everything combined is about 10 to 50 million dollars in the United States. So you're talking about difference in terms of a weapon with equal power, 50 million dollars versus 1.5 quadrillion dollars. To me, what's interesting, weapons is just one indication of this. One other possibility, and NASA also writes about this, is the use of antimatter and propulsion systems.

• 2:07:23Speaker 01

  Right.

• 2:07:24Lex Fridman?

  Just like you can use nuclear fission and maybe even nuclear fusion down the line. In propulsion systems, I saw that one gram can help get us to Alpha Centauri star system. If we can get to point two times the speed of light in 20 years, meaning it would take us 20 years to get to Alpha Centauri. Is any of this a possible future? The use of antimatter for generation of energy? Because we should mention that it's extremely compact. It has the obvious downside that it's extremely costly to produce. We don't know how to do that kind of scale.

• 2:08:02Speaker 01

  Right.

• 2:08:02Lex Fridman?

  The upside is it's compact.

• 2:08:04Speaker 01

  Very powerful. So the short answer is it is not a physics problem. It's an engineering problem. So I have people for that. Okay, but no, no.

• 2:08:15Lex Fridman?

  Yeah.

• 2:08:16Speaker 01

  The truth is that antimatter, if you are able to assemble it and store it, sure, it would be able to take that antimatter, heat up matter and shoot it out the back of a rocket and it would. You know, do what rockets do and it would make us go quick and that would be fine.

• 2:08:35Lex Fridman?

  And we should mention the thing that you just mentioned is correct. One of the hugest challenges is the containment.

• 2:08:40Speaker 01

  Oh, 100%.

• 2:08:41Lex Fridman?

  Anti-matter when it comes in contact with matter is a problem.

• 2:08:47Speaker 01

  Right. So if you were unable to, uh, to contain your trip to Alpha Centauri for even a millionth of a second, boom, that would not be good. You know, it reminds me of the, uh, the Star Trek where Scotty saying Captain, you know, the anti-matter pods are about, you know, look, losing containment, gonna blow. And that's exactly what would happen.

• 2:09:10Speaker 01

  So the short answer is yes, anti-matter as in principle, we could make and use as a, a source of energy, but there are probably far less expensive sources of energy.

• 2:09:25Speaker 01

  You know, it depends on what you need to do. The Voyager probes are still chugging along with plutonium. Now they're running out of energy at this point, but we could, you know, presumably do a somewhat better job if we needed to. So I, I like the idea of anti-matter, you know, but the reality is the danger, not the obvious danger of weapons, but the danger of if you wanted to be in a ship run by anti-matter, if it ever got loose, well, you, you would never know it. That would be that.

• 2:09:54Lex Fridman?

  The reason I, I find this kind of inspiring is anti-matter is in the space of physics that has a lot of mysteries. There's a lot of exploration to be done. And so this kind of connection to energy means that, if we have a bunch of breakthroughs on the anti-matter side, that might lead to a better propulsion system, better energy generation systems.

• 2:10:20Speaker 01

  In principle.

• 2:10:20Lex Fridman?

  There's some combination of engineering here, but there's some combination of understanding the fundamental physics.

• 2:10:26Speaker 01

  I mean, we know how to do this. You know, we, we know you take energy, you make anti-matter. You have to contain it, you have to store it, you have to do all the hard things. But I, I would be shocked if there was some like new addition to the theory that made anti-matter production easier.

• 2:10:45Lex Fridman?

  Interesting. So we know how to produce anti-matter with accelerators. You're saying there's not. Breakthroughs in physics that could lead to different mechanisms for the generation of antimatter.

• 2:10:58Speaker 01

  You have to concentrate energy, that's it. If there's another way to concentrate energy, that would work too.

• 2:11:04Lex Fridman?

  And our best knowledge of how to concentrate energy is the accelerator.

• 2:11:07Speaker 01

  And remember, we're talking concentrating it into volumes the size of a proton. I mean, if you concentrate it to the size of your thumb, well, then, you know, it's really the density that matters, the local density. And so when you smash two protons together, all of that's occurring in a tiny, tiny volume. So it's the local density of energy that matters. If you had a lot of energy in a thimble or something, it's probably not dense enough. You know, it really has to be in close proximity for that to happen. And then when it does, it's, it's okay. So, so if there's another way, we know how to do it, to make that, that density thing with, with accelerators.

• 2:11:50Speaker 01

  If someone has a bright idea on how to make highly dense energy, then yeah, making antimatter is a piece of cake. But that's the crux, concentrated energy.

• 2:12:02Lex Fridman?

  Yeah, and how to do so in a cost efficient manner, not trillions of dollars.

• 2:12:08Speaker 01

  Well, yeah.

• 2:12:09Lex Fridman?

  So one of the big mysteries with antimatter is the bigger why. Where is the antimatter that should kind of be there? If the whole idea is that anytime you generate matter, you generate the same amount of antimatter. And yet when we look out into the observable universe, it seems like there's not antimatter for the most part there. So what do we understand about this mystery? What are the possible explanations as to why?

• 2:12:36Speaker 01

  So there's this thing called baryogenesis. And as you say, so reiterating a little bit what you just said, these are both Einstein things. Einstein says that when you take energy, you make matter and antimatter in equal quantities. And Einstein says, after the big bang, there was a lot of energy in the universe, which should have made matter and antimatter. We only see matter. Where'd the antimatter go? And the answer is, we don't know. However, there are some ideas and there's a lot of thinking on it. And in fact, for me, it's doing an experiment right now with neutrinos, trying to, to better understand what it was that made the matter

• 2:13:20Speaker 01

  and antimatter not be the same. Now, we do have a measurement of how much different it should be. And it's kind of neat. We can do this by counting the number of protons in the universe, just looking at galaxies and so forth. And then we can look at the cosmic microwave background, which is sort of the aftermath of the big bang. And we can count the number of photons from the cosmic microwave background. And with a little bit of math, what we can do is we can then say that somehow in the early universe, something made a very, very tiny asymmetry. So that for every billion, billion with a B,

• 2:14:05Speaker 01

  antimatter particles that existed in the universe, there were a billion and one matter particles. The billions canceled, annihilated, destroyed each other. And that extra one that's left over is us. And so what physics mechanism made that ever so slight asymmetry is not understood. There are some thoughts. One thought is that, well, that's just how it was. When the universe was formed, there wasn't asymmetry. It was not made by matter and antimatter. Another possibility is there are various numbers of theories

• 2:14:49Speaker 01

  all under the word baryogenesis. And bario coming from word baryon, which basically means protons and genesis, meaning the creation of. And we'd say that simply because the protons are the heaviest particles. And so baryogenesis is just the creation of matter. And there are just a number of theories in quantum mechanics that say that matter and antimatter can Can oscillate back and forth into one another and there is a slight asymmetry in how that happens. And we know that this is true to a degree. We've measured it in the 1960s with a different form of matter.

• 2:15:30Speaker 01

  I mean, you know, not protons, but a type of ephemeral matter that only exists in particle accelerators. And so we know that there is a slight difference between matter and antimatter, but it's not enough. It doesn't explain that. We're not sure. So at Fermilab, we have this idea, which kind of turns things on its head and it's not baryogenesis, it's leptogenesis. So leptons are the electrons. And because Fermilab is currently the world's most powerful neutrino accelerator, and neutrinos are leptons, there is this idea. Now leptogenesis is incredibly complicated, but the idea is that it is possible.

• 2:16:15Speaker 01

  We know that neutrinos actually change their identity. There are three different types of neutrinos, like, I don't know, cats and jaguars and tigers. And if you have a beam of just cats, if you go along a little while, you find there's cats and jaguars and then tigers and then they'll be back to all cats again. And so this oscillation thing is called neutrino oscillation. We've known it's been true since 1998. And what we are studying is we're going to make a beam of neutrinos and another beam of antimatter neutrinos. And we're going to study the oscillation behavior of the two of them. And it is possible, it is unlikely, but it is possible that the two of them will oscillate at slightly different rates.

• 2:16:59Speaker 01

  And if the neutrinos oscillate at slightly different rates, then that, along with several other highly improbable things, can tie together and might explain why there is more matter in the universe. So if I was going to bet the farm, I'll bet that they oscillate at the same rate. But I don't know, and you don't know till you do the measurements. So that's what we're doing. There are some other experiments trying to measure it right now. So there's a big race between the Fermilab group and another group in Japan to see who gets there first and make this measurement. And we will find out. If it turns out, though, that there is a difference in this oscillation rate between matter and antimatter,

• 2:17:41Speaker 01

  it will be a huge clue in this very, very difficult puzzle. I wish I could tell you I knew what the answer is, but but literally, nobody knows. I mean, and that's the thing of being a research scientist like me is if you're not confused, you're not doing your job.

• 2:17:59Lex Fridman?

  So there is this desperate, or not desperate, exciting search for this tiny asymmetry.

• 2:18:05Speaker 01

  Yes.

• 2:18:05Lex Fridman?

  It's so, so crazy to think that everything we see around us is a result of this tiny asymmetry, that there was this gigantic annihilation of matter and antimatter in the early universe. And this is just some little accident.

• 2:18:23Speaker 01

  Yeah, yeah, that's crazy.

• 2:18:24Lex Fridman?

  It's a happy accident.

• 2:18:26Speaker 01

  That is, I mean, it's totally crazy.

• 2:18:28Lex Fridman?

  This is one of the areas of physics where there's a lot of mystery.

• 2:18:32Speaker 01

  Mm hmm.

• 2:18:35Lex Fridman?

  Okay, so can we pull at that thread a little further? Let's talk about our intuition of what is dark energy as it connects to empty space and everything we've been talking about. What's, what's the cleanest definition of dark energy?

• 2:18:50Speaker 01

  So dark energy is either energy of space or energy in space. The most common statement is the energy of space. And it is essentially a repulsive form of gravity. And we believe this is real. And the reason we believe this is real is from observation. This is one of those things where we talked about a while ago, where I said that, you know, you can think about things, this theoretical stuff, and try to come up with a measurement, or you can make measurements and see where they disagree with predictions and lead that in a direction. So back in the late 1990s,

• 2:19:33Speaker 01

  some astronomers were looking at the expansion rate of the universe. So the big bang occurred, the universe is expanding. The universe is full of matter, matter attracts. So the gravity due to the matter of the universe should slow the expansion of the universe. And the only question was, how much? There were three possibilities. The possibilities were there was so much gravitational force that the expansion of the universe would slow, stop and be pulled back together in a big crunch. Number two was that the universe would continue expanding, slowing down, but never really stopping. And then the third possibility was the exact critical case

• 2:20:15Speaker 01

  where expansion would slow forever and approach zero only at infinity, never quite stopping or reversing. So those were the possibilities, door number one, two, or three. So they did the measurement and what did they found? It was door number four. The universe was not only expanding, but the expansion was speeding up. And the only way that could happen, given that gravity slows it down, is there was a repulsive force. And the name we give to that repulsive force is dark energy. This is something that Einstein postulated early on in his his development of general relativity. But then, because at the time, he knew that his theory predicted that the universe would collapse.

• 2:20:59Speaker 01

  But he believed the universe was eternal and not unchanging. And so he needed something to counterbalance the that collapse. And so he invented dark energy. He didn't call it that, called it the cosmological constant. But then a few years later, Edwin Hubble discovered that the universe was indeed expanding. And so since the universe was no longer static, Einstein said, no need for cosmological constant, took it back out. Thought it was a dumb idea that he put it in and was embarrassed. However, in 1998, it became clear that his original idea that there should be some sort of repulsive form of gravity was real and it's put back in the theory. And so that's what it is.

• 2:21:42Speaker 01

  We are pretty confident at this point that the expansion of the universe is speeding up. And the thing driving it is dark energy. Now, what is dark energy? I don't know. As I said, the most common thought is that it is the energy of space itself. But it is at least conceivable that there is a field in space where space exists. And that field is pushing space apart. That's another conceivability that I'm not sure that we have the instrumentation to distinguish, but that's not what normally people think. People think it is literally a property of space.

• 2:22:23Lex Fridman?

  But but there is the, what you call the worst prediction in physics, which is

• 2:22:30Speaker 01

  Oh, yeah, that's another one.

• 2:22:31Lex Fridman?

  A nice little insight about the complicated nature of dark energy. So the observations, as you described, say that empty space has a tiny energy density that accelerates expansion of the universe. But quantum field theory's prediction for what vacuum energy should be when coupled with gravity is much larger.

• 2:22:56Lex Fridman?

  So this is what makes for the quote, you have a video on this worst prediction in physics. Can you can you explain this crisis?

• 2:23:04Speaker 01

  Well, there's a measurement and you can measure how fast the universe is expanding. And from that you get a measurement of dark energy. However, if you then say, well, suppose the dark energy is due to fields in space. So that's quantum field theory. Hey, I know a lot about quantum field theory. And so we can take the quantum field theory and we can calculate what the density of energy is due to quantum field theory. And basically what you do is you take within a volume, the all of the wavelengths, the longer wavelength, the shorter wavelength, the shorter, shorter and shorter, and you can add them all up.

• 2:23:46Speaker 01

  And each wavelength adds a certain amount of energy. And if you add that all up, then you get a number and that number is the rather embarrassing 10 to the 120 power times, that's a one with 120 zeros after it, bigger than the measurement of dark energy.

• 2:24:11Lex Fridman?

  Yeah.

• 2:24:12Speaker 01

  So you go, yuck, that is not fun at all. And that is because the equation comes to the highest energy or the smallest wavelength particle that you can imagine to the fourth power, since anything to the fourth power is a big deal. So that's where you get that awful number. Now, if it turns out that there is some new physics that's just about at the energy scale we can measure using our biggest particle accelerators. Remember, I told you that that was a factor. The maximum energy scale Planck scale is 10 to the 15 times bigger than what we can measure now.

• 2:24:54Speaker 01

  So let's say that we don't have to calculate up to the Planck scale because something happened, something changes at the energy that we know right now. Well, then that means we don't have to integrate to Planck scale. We integrate to 10 to the 15th less of the Planck scale. And this thing is to the fourth power. So 10 to the 15th to the fourth power is 60. So now, even if we say, you know, Don, he's brilliant. He's going to find something at the LHC tomorrow. It's going to solve all this problem. Now we've solved it. It's much better. It's only different by 10 to the 60 power, which is still pretty bleeding big. So the short answer is there is very clearly something going on, something wrong, very

• 2:25:38Speaker 01

  badly wrong in the quantum field theory. You know, we have to have, maybe there's another field that balances out the energy that cancels it down. And even that, you know, that's not so, so outrageous. You know, you could imagine that there's another, you know, like we have matter and antimatter, they balance pretty well. Okay, maybe there's something going on. You could cancel that out. That'd be perfect.

• 2:26:04Speaker 01

  Canceling something to zero is easy because, you know, plus one and minus one, zero, plus two, minus two, zero. But we still have dark energy. Dark energy is a little bit. So if it cancels, it doesn't cancel exactly because it left over that little bit of dark energy. So that is its own curiosity. Perfect cancellation, pretty easy. Theorists do that, you know, eight times before breakfast. Imperfect cancellation, much harder.

• 2:26:31Lex Fridman?

  Just to elaborate a little bit, what do you think solving in quotes, solving dark energy would look like?

• 2:26:37Speaker 01

  Well, you could, what you would do is you would hypothesize that there existed some other field that had the, the, the reverse effect of existing quantum fields.

• 2:26:47Lex Fridman?

  But not to zero.

• 2:26:48Speaker 01

  But not to zero. So, but if you had it to go to zero. You know, sure, maybe there's a field that that exists at really high energies that we haven't seen yet. I don't know, but it cancels things out and we're cool.

• 2:27:01Lex Fridman?

  How would we then demonstrate the existence of that field?

• 2:27:04Speaker 01

  Uh, well, that would depend on the the prediction.

• 2:27:06Lex Fridman?

  How do you even come up with a new field?

• 2:27:09Speaker 01

  Like all theorists do. Well, let's add something to my equation and see what happens. I mean, and that's okay. I mean, I I'm being glib about that, but that is precisely what you do. You say, what change? We we have this thing that works quite beautifully, except it fails here. What is the addition that we need to make that changes very little in this realm that we measured? And yet fixes this hard thing. And so you literally just go da, da, da. Okay, what do I need? Plus six or something. And as long as it makes no changes where it would hurt our measurements and fixes the big thing, then that is at least a candidate theory.

• 2:27:50Speaker 01

  Now, that doesn't mean it's right, but it at least gives you an understanding of what the right answer should look like. And so that's the first step is what should the real answer look like? Or what is a possible real answer? And then, once you kind of know that, then other people can look and say, well, let me think about a theory that kind of has the required properties to do what we needed to do. So it's a multi-step process, but the first step is how do we tame this problem? Without coming up with really terrible predictions that we've already ruled out.

• 2:28:34Speaker 01

  And and so that's what you do. And then, you know, that that is literally a sensible, viable theoretical thing, you know, because you have to explore cool ideas.

• 2:28:45Lex Fridman?

  I mean, one of the reasons dark energy is super interesting is it kind of gives us a mechanism by which we can talk about the deep future of the universe. I mean, it's making, we have observations about the expansion of the universe. But it's also giving us the mechanism of that, right? So we can talk about.

• 2:29:07Lex Fridman?

  Any weirdness, any good model we have that to capture some of the weirdness of dark energy might give us insights about how this thing ends, how the universe, about the deep future of the universe, right?

• 2:29:19Speaker 01

  Absolutely. As it stands right now, if dark energy is real and who knows, you know, if it's real exactly as we've measured it, then as the universe gets bigger and bigger, dark energy becomes a bigger and bigger component of the energy balance of the universe and it takes over and it drives the continued accelerated expansion of the universe. And if dark energy gets lower, you know, for some reason that we don't understand, maybe it changes over time, gets smaller, that could change things. If it gets bigger, it could change things.

• 2:29:53Lex Fridman?

  That is one of the big open questions, whether it's constant over time or not.

• 2:29:57Speaker 01

  Right. And there has been a recent measurement that suggests that dark energy is getting smaller. However, that is a new measurement, not confirmed, blah, blah, blah, blah, blah. Nobody should believe it, but it's a hint that maybe it's changing, which is kind of cool in itself because the current bias until recently is that dark energy is constant. Now, I want to be super careful because it's misleading. People say dark energy is constant. Dark energy is a density. Now that, think about that. You have a certain density, let's start with that. Then the universe expands. So, energy is volume times density.

• 2:30:38Speaker 01

  If the universe gets bigger and the density is constant, that means dark energy is increasing. It's not just increasing as a fraction and overwhelming ordinary matter, but ordinary matter, as the universe expands, its density decreases because it's constant and the volume gets bigger, the density drops. Dark energy, until recently, is thought to be constant density.

• 2:31:05Lex Fridman?

  So that's what's implied when you say constant, you say constant density, which means it's actually increasing because space is increasing. The size of space is increasing.

• 2:31:13Speaker 01

  Interesting. So that's a weirdness. And that then ties in to the nature of space. Why does that tie into the nature of space? Well, because if dark energy is a field in space, if you increase the volume, you would think the energy density would drop. But if space is increasing and space is quantized, and I don't know if it is, then maybe what's happening is space isn't stretching, but like little space particles are appearing as the space. You know, there's like bubbles of space appearing and each bubble contains a certain amount of dark energy. And so therefore that would give you a sense that dark energy is a property of space rather than a field in space.

• 2:31:56Speaker 01

  But that's all very hand wavy guessworky stuff.

• 2:32:01Lex Fridman?

  So if you had bet all your money, is dark energy like a real physical? What does that even mean? Thing that exists versus is this just a renaming of the cosmological constant?

• 2:32:16Speaker 01

  Unfortunately, I think it's both.

• 2:32:19Speaker 01

  Well, I mean, it is, it's describing a reality, but it's also maybe telling us something about space.

• 2:32:27Lex Fridman?

  Literally a property of space.

• 2:32:29Speaker 01

  Yeah, I mean, that's kind of what it looks like, given that it seems to be constant density. That seems to me, now, this is not something anybody should believe, please, nobody believe this. But it seems to me that this is leaning towards the idea that A, it's a property of space. B, space is quantized. C, as space is expanding, little quantum of space are appearing. And D, each one of those quanta has a certain amount of energy associated with it. And that would kind of explain the constant density. Now, please, that's not, anybody should, nobody accepts that. This is just nonsense.

• 2:33:13Lex Fridman?

  But a lot of the stuff that you just said is experimentally, probably experimentally testable. You can probably construct the bubbles of.

• 2:33:23Speaker 01

  Finding out the bubbles of space, but those quanta conceivably are plank sized bubbles.

• 2:33:28Lex Fridman?

  Yeah, the quanta.

• 2:33:30Speaker 01

  Well, they'd be quantum of space. I mean, the idea is, you know, you look at a sand dune and it looks smooth and continuous, but you can see individual grains of sand, right? And so what this is saying is as this dune expands, new grains of sand are appearing and each one of them is a quantum of space.

• 2:33:47Lex Fridman?

  So what kind of experiments can we do in the coming decades? Or centuries to understand dark energy better?

• 2:33:54Speaker 01

  I mean, people have been talking about quantum entanglement of gravity. In standard quantum mechanics, a particle can be in two places at the same time. All right. So now you have two particles. So this particle can be in two places in the same time. This particle be two places at the same time. You put them near one another. Well, if they're close to each other, there's a certain gravitational force. If they're far apart, they're certain. And if they, one is closer than one is far, you have another one. You can calculate the effects of gravity having to do with quantum entangled particles being in two places. And people are talking about doing this and trying to see

• 2:34:36Speaker 01

  if in doing such a measurement, they might be able to definitively determine whether gravity is a quantum phenomena or a continuous phenomena. And that is potentially a measurement that could be done soon-ish because the technologies of inherent in all of this recent work on quantum mechanics is allowing people to be able to make instrumentation that might be precise enough to do this measurement. Now, this will not tell us what quantum gravity is. It will not tell us anything, but it will tell us that gravity is quantized.

• 2:35:20Speaker 01

  And just knowing that, well, for one thing, it shuts out a whole realm of continuous gravity and the theoretical community will then turn its attention, forget this stuff and think over here. Now, that doesn't tell you that space is quantized, but it tells you that gravity is quantized if it bears out. So, and if gravity is quantized, then people will start thinking more about space being quantized.

• 2:35:45Lex Fridman?

  I have to ask, because you mentioned dark matter is perhaps even more mysterious than dark energy.

• 2:35:51Speaker 01

  Okay.

• 2:35:51Lex Fridman?

  Can you, can you, can you build up the intuition why it's more mysterious? What is dark matter?

• 2:35:56Speaker 01

  Oh, gosh, what is dark matter? A, I don't know. B, it's terribly fascinating.

• 2:36:02Lex Fridman?

  Yeah.

• 2:36:02Speaker 01

  All right. So, first thing, the most important thing, because I'm an experimentalist by God. The first thing is, why do we believe there's dark matter? And the reason is that astronomical measurements do not agree with predictions by Newtonian or relativity theory. Galaxies spin too fast, clusters of galaxies move too quickly, and the distortion of very distant galaxies due to the gravitational field of near galaxies disagrees with the prediction from what we see from the observed matter. So there are three very distinct reasons why we are predicting that that something is wrong

• 2:36:44Speaker 01

  in our understanding of either philosophy, physics, or the matter budget of the universe. The easiest one to talk about is the spinning galaxies. Now, this is, what I'm saying is not unique to spinning galaxies, just easiest to talk about. So galaxies are observed to spin more quickly than they should if we add up the gravity we see. By all rights, galaxies spinning that fast should blow themselves apart, and they don't. So what can be the answer? Well, you have the force required for a star to orbit, to move in a circle. You have the force due to gravity, and they're connected by an equal sign. And the prediction is wrong.

• 2:37:29Speaker 01

  So either the force due to gravity is wrong, the force needed to move in a circle is wrong, or the equal sign is wrong. I mean, this is really simple. One of those things is wrong. So one possibility is simply that Newton's law of gravity, mass times the mass over r squared times a constant, that's just wrong. Another possibility is Newton's F equals ma that we are taught in introductory physics is wrong. Both of those are eminently possible. Over here, maybe we don't understand gravity. Or maybe there's more mass than we can see. So these, you know, I mean, it's nice that you can look at this really simply and come up with a list, you know, cookbook of things we can test.

• 2:38:15Speaker 01

  And, um, and so we've done that. We've gone and said, what are the possibilities? Well, the most obvious possibility is that there is more mass than we can see. There's black holes, there's hydrogen gas that we can't see, whatever. There's something out there. So that was the first thing. So you go and you look and there's no hydrogen gas because we can see that through the radio waves. That's not it. In the nineties, we went looking for black holes, rogue planets, things like that. Those exist, but not enough of them. That's not it. And so now we're left with, there's some sort of matter that we can't see, or we don't understand gravity, or we don't understand inertia.

• 2:38:58Speaker 01

  Now, I personally, if you ask me this, oh, I don't know, 25 years ago, I would have said the most likely answer is that we don't understand inertia or gravity. You know, if 20 years ago, 25 years ago, that's what I would have said. No problem. However, there have been a couple of observations that, um, that have caused me to change my thinking. And I think that dark matter is more likely. One of them is called the bullet cluster. So the bullet cluster, there are two large clusters of galaxies. In these large clusters of galaxies, well, any galaxy consists of a couple of components. There are the galaxies themselves.

• 2:39:44Speaker 01

  There is the hydrogen gas that surrounds the galaxies and maybe there is dark matter. And if dark matter is real, or dark matter is not real, you will get different answers if those two galaxies pass through one another. The galaxies themselves should pass through one another, basically not interacting. But the big thing is the gas clouds. So if there's big clouds of gas, as the galaxies pass through one another, the clouds should interact and the gas cloud should stop in the middle and be really, really hot. So then you would see, if there were no dark matter, you would see a cluster of galaxies, cluster of galaxies, a big gas cloud in the middle. And because the big gas cloud in the middle is much more massive than the galaxies themselves,

• 2:40:29Speaker 01

  you would expect to see distortions that we call dark matter distortions in the middle. If however, dark matter is real, the galaxies pass through one another, the cloud stops. Dark matter doesn't interact with the clouds, so it passes through. In that case, you would expect to see the distortions where the galaxies are. And that's what we see. So that is a strong evidence in my mind. The bullet cluster is strong evidence that dark matter is a real thing. And there is another example, which is much more recent, dark bullet cluster was a while ago, called the dragonfly galaxies. There's dragonfly two and dragonfly four. These are galaxies that rotate exactly according to Newton's laws.

• 2:41:11Speaker 01

  And so the fact that they rotate exactly according to Newton's laws says that whatever's causing galaxies to rotate too fast is not a property of matter. But if you had a galaxy where there was no dark matter, for whatever reason, it got stripped off or something. This is one of those lovely ironies that the existence of a galaxy with no dark matter is very strong evidence that dark matter is real. Because you can take the dark matter out. So the DF2 and DF4 also suggests to me that dark matter is real. So now, while it remains possible that we need to modify the laws of inertia, or we need to modify the laws of gravity, those are possible still.

• 2:41:55Speaker 01

  In my opinion, and now this is Don's opinion, but it's probably the opinion of most of the scientific community. Dark matter is likely a real thing. Now, that's great. I've taken you all the way to dark matter. So now you're going to ask me, you're going to say, Don, what is dark matter? I'm going to, I don't know.

• 2:42:16Speaker 01

  But I know what it isn't. Okay, I know that it is not black holes. I know that it is not rogue planets. I know that we've done the measurements. We've looked across nearly every mass range for compact objects and ruled them out. So if dark matter is real, it can't be made of those. So then you're left with the idea that dark matter is a particle. And that's what we've thought about. The name for the the dark matter particle that we've called for a long time is a WIMP for a weakly interacting massive particle. And we have spent the last, God, thirty years looking for them in the various ways. There are three ways that we might see dark matter. The direct way, which says that dark matter exists literally everywhere in this room, in our laboratory.

• 2:43:00Speaker 01

  And the dark matter is passing through the earth like a wind, and we put up detectors trying to see those. We have done that, and we've seen nothing.

• 2:43:09Lex Fridman?

  So she said we have done that for neutrinos.

• 2:43:11Speaker 01

  We've done that for many different types of dark matter. We just simply put detectors in labs, deep underground. And we can see neutrinos in them. It's true. But dark matter would have especially heavy dark matter, what these these WIMPs. They have a different signature, and we've seen no evidence of dark matter interaction in these detectors.

• 2:43:33Lex Fridman?

  So neutrinos are also weakly interacting and also have mass.

• 2:43:37Speaker 01

  They are.

• 2:43:37Lex Fridman?

  But not enough mass. So WIMPs are

• 2:43:41Speaker 01

  Right.

• 2:43:41Lex Fridman?

  heavy on the um.

• 2:43:42Speaker 01

  Right. Neutrinos are indeed WIMPs of a sort. Now, we have to be careful what we mean by WIMPs. They are weakly interacting massive particles. But we can calculate, and there's just not enough mass in them. It's not it.

• 2:43:54Lex Fridman?

  Got it.

• 2:43:54Speaker 01

  So we need another form. And we have seen zero evidence of this wind of dark matter through the the earth. Another possibility is you look where you think dark matter might be concentrated at the center of galaxies. And if dark matter exists and there's antimatter, dark matter, maybe they annihilate and make photons. And so we look for gamma rays and various other signatures of annihilating dark matter. And there are always constantly.

• 2:44:26Speaker 01

  Announcements of, oh, we saw it, or we didn't. Oh, you know, the problem is that way of looking for dark matter is hard because there are other ways of making, for instance, gamma rays like neutron stars and stuff. And you really need to understand the details of galaxies really, really well to believe that. And then the final option is what I do where we smash particles together at high math or high energy, we try to make dark matter particles. If you make dark matter particles, because they don't interact except via gravity, they escape with your detector. So what you're seeing, what you hope to see is an event where you collide particles, a dark matter particle escapes and you don't see it. But the recoil you see on the other side, because momentum is conserved. So you see a blob of energy on this side, nothing on the other side.

• 2:45:12Speaker 01

  Maybe that's dark matter. And that also happens with neutrinos. So you need to understand everything about neutrinos and calculate how many of those you see, and then hope you see more. And then that might be dark matter. Again, that hasn't worked. So we've ruled out some dark matter particles. But the problem is the range of space of possible mass, if dark matter is of a particulate form. The range of viable dark matter ranges from something like the mass of an asteroid to far lighter than an electron and everywhere in between. And we have looked, we've ruled out some little spots in that phase space, but that's a big range. Is it really possible to miss a particle the size of an asteroid?

• 2:45:58Speaker 01

  The astronomical searches were not sensitive to that level of of dark matter. But, you know, then you would expect that there would be some of those in the solar system. And if they're what we think, like asteroids or something, then we'd heat them up and we'd eventually see them. But if they're really like truly dark matter, doesn't interact with matter, which means they wouldn't absorb energy from the sun. So they'd be really dark. I don't know, maybe they're out there. But the only way we have, we searched for them was a thing called microlensing. So if a massive object, you have a distant star.

• 2:46:37Speaker 01

  And a massive object pass between that star and your eye, that star will momentarily brighten. And so you just look for these, what they call micro lensing events and you count them. And you see some, and we did see some, you know, black holes pass in front of stars and we, and we've seen them. But we just haven't seen enough. And for very low mass particles like asteroids, they, um, they just wouldn't make enough a brightening effect to see. So there's like a minimum sensitivity of brightening. And that about a third of the mass of a moon, our moon is about the sensitivity that we had. So,

• 2:47:19Speaker 01

  you know, that, you know, nobody, I think, really thought that these low mass guys were likely. What they thought was more likely, they were just unseen black holes, which I thought, you know, I think is completely reasonable. Then when that got ruled out, I thought, okay, modified gravity or, uh, or inertia. Well, now the bullet cluster and dragonfly seems to have ruled that out. So I'm stuck in my head with dark matter seems to be real and it, we don't know what it is. And it makes up a giant percentage of matter in the universe. It is five times more prevalent than ordinary matter. This is incredible. This is so fascinating. And that's why it's cool. So if someone out there is, you know, a young person who wants to get into this,

• 2:48:02Speaker 01

  understanding dark matter is a big deal. I mean, it's five times more prevalent. The problem is, is as I told you, if the mass is ranging from an asteroid to far lighter than an electron, if you get on an experiment that looks at one little range of mass, maybe you weren't the lucky guy that measured the right place, you know? And that's one of the reasons why, as fascinating as I think it is, I'm not doing dark matter experiments because, you know, if you make an experiment that searches one mass range, it'll be blind to another mass range. So what you need is you need many groups doing all sorts of radically different experiments, exploring all sorts of parameter space.

• 2:48:44Speaker 01

  And with all that said, until you see it, there still is the possibility that maybe we don't understand gravity or inertia right, you know, you can't rule that out.

• 2:48:55Lex Fridman?

  If there is dark matter out there, you're hoping is actually somehow detectable.

• 2:49:00Speaker 01

  I mean, I don't know what it is. I think it's cool. It's very, very fascinating. That is one thing I really do hope in my lifetime is is understood because I'd like to know the answer to that.

• 2:49:10Lex Fridman?

  And that's the thing that you could, legitimately, you can see a discovery of.

• 2:49:16Speaker 01

  You got to get lucky, though. I mean, you got to look in the right place, whatever it is.

• 2:49:19Lex Fridman?

  Just imagine.

• 2:49:20Speaker 01

  Or you have to come up with that really cool theoretical idea that everybody's overlooked, which is another possibility. And there are people who are really, really religiously hating dark matter, largely because we've looked so hard for so many years. And the experiments in today's world are a million times more sensitive than when I was a starting student and they still haven't seen anything. And that's why people really hate dark matter. I mean, some of them because they think we should have seen it by now, but you know, I don't know.

• 2:49:52Lex Fridman?

  I mean, I'm a sucker for direct observation. Indirect is obviously also really great, but direct, just imagine pointing your telescope in a certain direction. And because of some artifact of cosmology, being able to directly detect a giant amount of a thing that you could say is dark matter.

• 2:50:11Speaker 01

  You would see it orbit, things orbit it.

• 2:50:13Lex Fridman?

  Yeah.

• 2:50:13Speaker 01

  Or it would eclipse things in front of it or.

• 2:50:16Lex Fridman?

  Yeah, like in an obvious way because some of the stuff you mentioned with DF2 and DF4, those are like brilliant indirect deductions that there should be something like dark matter. But some obvious, yeah, blocking, occluding this kind of thing.

• 2:50:36Speaker 01

  We did that in the 90s with experiments called Macho Ogul and some others. They looked for a black hole that you just can't see, you know, black hole you can't see. It's perfect. It's a perfect candidate for dark matter. And if there's enough of them out there, then remember there's five times the number of stars, which means there's a whole lot of freaking black holes out there. We should have seen them and we didn't.

• 2:51:00Lex Fridman?

  What a grand mystery. We covered so many of them. I could talk to you for a thousand more hours, Don. Let me, if I can, ask you about a little bit more of a, on the personal side. You have a really inspiring life story. Your folks didn't go to college. Can you just tell me about your childhood and where you found the love for physics and science and maybe how you found your journey to, to, to become a physicist, given the, the context of where you came from?

• 2:51:34Speaker 01

  Well, uh, you know, I grew up a poor kid in the Boondocks. Great parents, but not ones that could guide me terribly academically, but very, uh, very nurturing. You know, my mom would laugh that she could stop helping me math after like sixth or seventh grade, you know? Um, but they were supportive. And there were a couple of things that, a couple, three things, I think, that folded into it. One is I was a voracious reader as a kid. I loved science fiction. I would read a book a day. It drove my mother nuts because she would try to be nice. She'd buy me a book and I'd say, thank you. And the next day it'd be done.

• 2:52:13Speaker 01

  You know, it just drove her completely nuts. But anyways, but science fiction is good for fostering imagination. And so that's precisely what it did. In addition, and this is where the more serious science came along, there were lovely science communicators that were popular in the 1970s. Isaac Asimov, Carl Sagan, a guy by the name of George Gamow. They wrote books about science aimed at a layperson. You know, as a kid, I surely couldn't read a, a textbook and understand it, but I could read and, and, you know, get a hint of what science was. And on top of that,

• 2:52:58Speaker 01

  You know, I was, as most scientists, people who became scientists, irrepressibly curious about everything. And I had sort of a quasi philosophical mind. I mean, I was interested in things that, questions that have in the past been theological and then philosophical, and now are more scientific, questions about how did the universe come into existence? Why is the universe the way it is? Why are the laws of the universe what we see them to be? How will the universe, was it created? How will it be destroyed? These are, you know, big questions that have bothered humanity for, well, thousands of years. And so, you know, I did, you said I had, you know, philosophy and religion minors in college,

• 2:53:43Speaker 01

  and I did because I was curious about that. I was hoping that learning that history might help me understand these questions. And it was in college where I came to realize that the answers that I were searching for were not to be found in those directions, but I still learned about how those questions have been asked in the past. And so I became a scientist. And the only question was, was I going to be a cosmologist slash astrophysicist or a particle physicist? And then when I had to make that decision, it was the mid eighties. And at the time, there were a lot fewer cosmology measurements.

• 2:54:25Speaker 01

  There was an awful lot of thinking about the universe and not enough measuring. Whereas with particle physics, by God, you could do experiments. And so the, what attracted me was the ability to actually get an answer and not just mull over what an answer might be. And so I became a particle physicist. It was difficult without having, you know, family mentors or anything like that. But, but, you know, I managed. And that actually is why, well, I'm here and why I have spent a fair bit of my time writing books and so forth, because I figure that there has to be some other kid

• 2:55:08Speaker 01

  out there in Iowa, Kansas, Montana, or somewhere out in some little town without a lot of access to the kinds of thing that people, you know, who have highly educated parents do. And I'm hoping that, you know, some of them will have read some of the things I've written and will find their own path forward because I found it very rewarding over the years. And, um, you know, I've been doing this long enough that I'm sure this is true. I've had kids come up to me at the lab and say, hey, I'm a summer intern because I saw your video or read your book or, you know, whatever. So I know that at least I've made a small impact.

• 2:55:50Speaker 01

  I mean, I always would like to do more and, you know, I appreciate the opportunity that your, uh, audience affords me, um, cause I, I think it's important to talk about these things. These are really cool, fascinating questions. They are unanswered and they are just waiting for youngsters to come and spend some time thinking about them. Cause one of your viewers might be one of the people who answer these questions that have stymied very smart people for decades.

• 2:56:21Lex Fridman?

  And we should also say that you're a legit scientist. So we'll mention Sean Carroll, who's a legit scientist, legit physicist, but is also a good science communicator. Anyway, I did want to mention, I don't know if this is true, but I, I kind of heard you talk about this, that when you first showed up to Fermilab, you were like working crazy hours, working extremely hard, eight AM to midnight.

• 2:56:47Speaker 01

  I did.

• 2:56:48Lex Fridman?

  First of all, I love that.

• 2:56:52Lex Fridman?

  Can you speak to what drove you, maybe the value of hard work in those contexts? And you, in the early career, when you discover a thing you're passionate about?

• 2:57:03Speaker 01

  Well, yeah, I mean, obviously being smart, you know, if you're Einstein, then maybe you can slack, I guess. Although even he didn't do that, but I'm not Einstein. But the fact is, when I was young and I was unencumbered, no, no family, no kids or something. I couldn't imagine anything I wanted to do more. I mean, some people, they want to go out to the club. They want to, I don't know, play soccer or something, but I wanted to make measurements and I wanted to understand and, and, and learn. And that was fantastic. And so as a graduate student, and this isn't for everybody, but I worked outrageously. I would, from Monday through Saturday, I would be at the lab voluntarily because I wanted to be from

• 2:57:46Speaker 01

  8:00 AM to midnight. And on Sunday, I would work from eight until about five. And that's because from five to midnight, I had to wash clothes and buy groceries and things like that. And I loved it, you know? And I still love it. I can't do that anymore. But, but that's simply because I have other obligations. But had I been rich, I would have done the same thing. You know, it's, it's something I truly, truly loved. And the, I mean, there is absolutely nothing more fascinating to me than having a hard problem and figuring it out. And that,

• 2:58:27Speaker 01

  you know, that work ethic. Well, there's a couple of things that separate smart people from no kidding scientists, because all scientists are smart. But the thing that, that separates that, that many scientists have is a, a drive and a real grit. The, for me and for so many scientists that I know, trying to measure something and having it not work just kind of ticks me off. And I am not going to let the universe in my lab or whatever, beat me. And, you know, some people, they, you know, if the thing breaks, it's like, oh, man, that didn't work. And a lot of people, well, I'm gonna go home. I'm fed up. No, it would just

• 2:59:12Speaker 01

  kind of make me mad. And I'd put more effort into it. And, you know, not every, I mean, okay, I was crazy. I worked long hours. But, but I think the people who are really good at this will do maybe not that much, you know? Some people have to have a better life than that. But, but a lot, because it's just, you can't imagine not knowing the answer. And that, if, when you see that, as an older guy, you don't, maybe not to that degree, but when you see that kind of drive, that, that, that- Intensity of trying to get the answers, you know that person's a winner. And so if, you know, some student out there, if it doesn't, you know, bring you joy is,

• 2:59:56Speaker 01

  what's her name, the Japanese girl says, if it doesn't bring you joy, and it might not be for you. And then you could be a person who reads about it and, you know, is involved, but if you want to be a real scientist, it has to be just part of what you are here.

• 3:00:09Lex Fridman?

  And by the way, it is a hard life, but it is also a very fulfilling one. So, working hard towards the thing you love is a really fulfilling way to be.

• 3:00:22Speaker 01

  I think that's true for an artist or something, you know, anybody, a musician, you know, musician, they just keep practicing because it is who they are.

• 3:00:32Lex Fridman?

  Well, I'm glad there's people like you at a place I admire, like Fermilab, one of the many places in the United States and the world that is carrying the beacon of great science and great engineering forward. Don, thank you so much for everything you do, for all the teaching you do online, for all the incredible physics works that you do at Fermilab. And thank you so much for talking today.

• 3:01:03Speaker 01

  Thank you for having me.

• 3:01:05Lex Fridman?

  Thanks for listening to this conversation with Don Lincoln. To support this podcast, please check out our sponsors in the description, where you can also find links to contact me, ask questions, give feedback, and so on. And now let me leave you with some words from Marie Curie, a two-time Nobel Prize winner, first in physics, second in chemistry. Nothing in life is to be feared. It is only to be understood. Thank you for listening. I hope to see you next time.