Where does energy come from?

Daniel and Kelly talk about what energy is, how it flows, and whether it has to come from anywhere! See omnystudio.com/listener https://omnystudio.com/listener for privacy information.

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

• 0:00Daniel Whiteson?

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• 2:09Daniel Whiteson?

  One of my favorite things about physics is that it lets us think concretely and carefully about the basic nature of our experience. What is space? What is matter? What is time? What's a particle? Why are there waves everywhere? Physics helps us convert these squishy conversations on rooftops over smoked banana peels into robust mathematics. The math doesn't require persuasion or a clever choice of words. It's either right or it's wrong. So instead of answering a question about the nature of the squishy universe, we get to poke and prod our mathematical description of it. Sometimes that gives us real insight, like when relativity tells us that distance and velocity have to be relative rather than absolute, because that's the only mathematical model that describes the universe we see.

• 2:54Daniel Whiteson?

  Other times, it surprises us by telling us that we might just be asking the wrong question. Maybe the things we thought were important just reveal our fundamental misunderstandings of the universe and how it works. Today, we'll be asking what physics can tell us about energy? What is it? And more importantly, where does it come from? Welcome to Daniel and Kelly's Extraordinary Energetic Universe.

• 3:33Kelly Weinersmith?

  Hello, I'm Kelly Wiener-Smith. I study space and parasites, and I just got over having the flu for a week, and I really could have used energy.

• 3:42Daniel Whiteson?

  Hi, I'm Daniel. I'm a particle physicist, and I love how Kelly opens the podcast with so much energy every time.

• 3:49Kelly Weinersmith?

  Yeah, well, we both have a lot of energy. Speaking of energy, do you rely on caffeine? What is your favorite form of liquid energy?

• 3:57Daniel Whiteson?

  I do rely on caffeine. I'm drinking coffee right now. Here you go.

• 4:01Kelly Weinersmith?

  Oh, oh.

• 4:04Daniel Whiteson?

  For you ASMR folks.

• 4:06Kelly Weinersmith?

  Yeah.

• 4:08Daniel Whiteson?

  Yes, I have caffeine in the mornings, but I can't have any really after lunch or interferes with my sleep. I'm at that age where sleep is not necessarily a given.

• 4:17Kelly Weinersmith?

  Yeah, same. I used to drink coffee all day long. I like didn't drink water in college.

• 4:22Daniel Whiteson?

  That doesn't sound like a good idea.

• 4:24Kelly Weinersmith?

  You know, I got a lot done. I got a whole lot done, but I could still fall asleep. But now I also, if it's after lunch, forget it. I can't have coffee anymore either.

• 4:32Daniel Whiteson?

  Remember, folks, this is not a health podcast. Do not take our advice.

• 4:37Kelly Weinersmith?

  Yeah, no, please don't. Don't follow our lead. When you were a kid, did you ever try Jolt?

• 4:43Daniel Whiteson?

  Jolt was the kind of thing that was so forbidden in my family. Like, my mom was very controlling about the kind of food we could eat. You know, she like counted potato chips and this kind of thing. So junk food, definitely awful. This Jolt, she saw this like battery acid, like there was no way she would allow us. Did you try Jolt?

• 5:01Kelly Weinersmith?

  Yeah, I had, do you remember Pixie Sticks?

• 5:03Daniel Whiteson?

  Yeah.

• 5:04Kelly Weinersmith?

  So there used to be like a mega Pixie Stick. It was like a giant plastic tube, like the thickness of my thumb, and it was just filled with sugar and food coloring. And we had like a party once where we just like had those Pixie Sticks with Jolt.

• 5:19Kelly Weinersmith?

  And I got pretty sick. And Jolt made me feel horrible. The way too much caffeine makes me feel now. But I tried it once, and it was the one thing as a kid where I was like, I shouldn't do that again. Anyway, I had a lot more fun, it sounds like.

• 5:31Daniel Whiteson?

  It does sound like you had more fun, but you also got sick. But I have a basic biology question for you about caffeine. I mean, I understand if you eat a Pixie Stick, it's sugar. Sugar has energy. You could potentially get a high and a rush and feel energetic from that. But caffeine, how does that give me energy or make me alert? Is it really energy from the caffeine that my body is using? Is it releasing some energy stores in my body? Or is it some much more complicated neuro answer? Dot, dot, dot. It depends.

• 6:00Kelly Weinersmith?

  Probably it depends. I don't actually know. I didn't prepare the answer for that coming in here. I believe it binds to certain receptors in your brain. My job in college was barista. I worked at like four different coffee shops. And so I've had like shirts with the caffeine molecule on the front. And so, yeah, I think it binds with some receptors in your brain. And I don't know how that results in you feeling awake, but it works.

• 6:20Daniel Whiteson?

  It pulls on some levers and ropes of the Rube Goldberg machine that is my mind.

• 6:24Kelly Weinersmith?

  That's right. That's right. And thank God for it. I don't know how I would have gotten through college without it. At some point, I was working at a coffee shop because I couldn't afford. All the coffee I wanted to drink, but if I worked at the coffee shop, I could afford it and pay my rent. So anyway, I had a problem maybe, but I also had fun.

• 6:40Daniel Whiteson?

  Well, energy is a really fun topic because it's something that is colloquial and we talk about in our lives and whether you feel energetic and whether you drink caffeine, whatever. And it's also something deeply important to physics and the universe. So it's one of these topics that people have like a personal feeling about, like where energy is, where it comes from, what it means. And it's also really pervasive in popular science, you know, E equals mc squared. Energy is mass. People tell us there's a lot of nonsense out there about energy. So I thought it'd be good to have an episode where we dig into some details about what energy is and where it comes from and what we know and don't know about it.

• 7:16Kelly Weinersmith?

  Yes. And you always do a great job of getting us back on track. That was fantastic. And the other people who keep us on track are our amazing audience. And so we asked them, where does energy come from? And if you want to answer questions for us, you can write us at questions at danielandkelly.org. But let's go ahead and hear from our audience. Where does energy come from?

• 7:35Speaker 06

  Energy comes from heat, which comes from photon. I have no idea. Is it a mathematical trick? Is it just an accounting system? Who knows? Nobody knows where energy comes from, and nobody's ever going to know. It is

• 7:51Speaker 04

  the magic of the universe. I don't think energy comes from anywhere because you can't have not energy. Well, it comes from fuels that permeate the entire universe. The sun's energy comes from hydrogen fusion caused by gravity. So I'd have to say gravity.

• 8:08Speaker 07

  Guess it comes from the big bang and has just been circulating in different forms ever since. Solar powered. Now, if you're talking about the energy that powered the big bang or energy required to keep all the quantum fields fluctuating alive, I imagine a physicist is better suited to provide insight. Hint, hint. So I guess that energies comes from symmetries in nature. Obviously, the big bang has a big role in it, but it might not be the whole story.

• 8:37Daniel Whiteson?

  Since you can't create energy, I assume it originates from the Big Bang? And before then, who knows? I guess it's always been around. It just changes form after form. Perhaps what we're calling spacetime is actually energy. Oh, wow. I really am stumped by this one. I really don't know.

• 9:03Kelly Weinersmith?

  All the energy in the universe today came from the storehouse of energy released in the Big Bang.

• 9:09Speaker 02

  Some energy comes from the Big Bang and some comes from dark energy. But most energy in this universe comes from eating your Wheaties. Have we not always had the same amount of energy?

• 9:23Speaker 07

  I would say all energy originated from the Big Bang and everything since then has just been conversions of different forms. But then you can ask, where did that come from? And where does dark energy come from? And I don't know.

• 9:37Daniel Whiteson?

  There's some really philosophical answers in here. I loved listening to these.

• 9:41Kelly Weinersmith?

  Well, I mean, that's what I've come to expect from the audience. It's like always a nice combination of like philosophical, some people who get like exactly the right answer or get really close. And then some people who know they don't know, and so they just go all in on being hilarious. I always love listening to the answers.

• 9:56Daniel Whiteson?

  This comment that you can't have not energy? I had to pause that and think about it for a while. I was like, wow, that sounds really profound, but I'm not actually even sure what it means. Like a lot of philosophy.

• 10:06Kelly Weinersmith?

  I thought that they were trying to say if there wasn't energy, none of us would be here, which doesn't really the answer to the question where it comes from, but just something had to have made it exist. I don't know. What do you think?

• 10:19Daniel Whiteson?

  I didn't know. Yeah. What is not energy? Is that energy equals zero? It's true, you can't have zero energy in the universe because of quantum mechanics? Yeah. Fascinating.

• 10:30Kelly Weinersmith?

  Yeah.

• 10:30Daniel Whiteson?

  Really fun answers here. And a lot of assumptions you hear that energy doesn't come from anywhere because we've had the same amount all the whole time. Because people have this conception, which we're going to take apart and debunk a little bit, that energy is conserved. The energy has to come from somewhere because you can neither create nor destroy it, and therefore it can only flow.

• 10:52Kelly Weinersmith?

  So, yeah, when I was looking through your outline and I saw energy is not conserved, I was like, what? I'm sure I've heard that a lot of times. And so, you know, I think a lot of us are looking forward to having our preconceptions about energy sort of cleared up.

• 11:07Daniel Whiteson?

  Yeah, exactly. But before we get into the mind blowing revelations about the nature of energy, we have to be clear and careful about what is it we're talking about anyway? Are we talking about how the earth keeps warm? Are we talking about what happens when you eat your Wheaties? Are we talking about philosophical revelations about the fundamental nature of the universe? So the first thing we have to do, of course, is define what do we mean when we say energy?

• 11:34Kelly Weinersmith?

  Yeah, so as a biologist, I'm thinking ATP, I'm thinking at it at a chemistry level. But where as a physicist, do you think about it like that as well? Or this is just a totally different topic?

• 11:45Daniel Whiteson?

  No, ATP is like a chemical store of energy. So it's completely related to the topic at hand, but it's an example, right? It's like if somebody says to you, hey, what is a vegetable? And you're like, an eggplant is a vegetable. You're like, yeah, that's true. But what makes it a vegetable? Why do we have them? Where do vegetables come from? Right? Showing you an eggplant doesn't answer that question.

• 12:06Kelly Weinersmith?

  Right. And then I'll go on a long rant about how vegetable is not a phylogenetic category and it's something humans made up. And so, anyway, go ahead.

• 12:13Daniel Whiteson?

  Oh, my gosh. Wow. Ooh, I love when we accidentally stumble upon a hidden Kelly rant. Love that. Wow. Okay.

• 12:21Kelly Weinersmith?

  Let's go deeper, but not on vegetables. So what is energy? How does a physicist start to answer this question?

• 12:27Daniel Whiteson?

  I like to answer this question by pointing out that we can invent any kind of concept. I mean, you can just make up a word, call it, you know, bliblian or whatever, and then define it, and it could be a thing. And the question is like, does it reveal anything about the universe? Is it related to the physical universe in some insightful way? Or is it just some nonsense that you made up? And so from that point of view, like, everything in physics is something we made up. And a lot of it is just nonsense we then ignored because it wasn't useful. And some of it actually seems to reflect something that's happening in the universe and is therefore useful and we think maybe insightful about the fundamental nature of reality. Right? And so, energy. Is that way. It is something we invented. It's just like a concept we came up with.

• 13:12Daniel Whiteson?

  We talked recently in the podcast about entropy. And the history of energy is similar, predates entropy, because entropy was defined as moving of energy. Remember, the E N in entropy comes from energy. But this is something people have been wondering about for a long time, like, how do you get a machine to work? And what's going on with heat and temperature? And so people came up with this concept of energy hundreds of years ago. It was actually Leibniz first identified energy of motion, like things in motion have energy. And he called it this vis viva, this living force, just energy of motion. He recognized that that was like a thing.

• 13:49Kelly Weinersmith?

  Hmm, but like an asteroid hurtling towards the earth would have energy. But in what way is that a living force? Was he like specifically thinking about living organisms?

• 13:58Daniel Whiteson?

  No, he was just thinking about animation, not necessarily like something is alive, but it's in motion. And so I think he was thinking more about the motion than the actual life force. So the definition of life in some sort of biological way. But that's something we still recognize, right? We call that kinetic energy today. And kinetic energy is definitely a form of energy, like photons have energy, right? They zoom around the universe, they're moving really, really fast. So it's a type of energy. But again, this is like quoting the eggplant, right? It's not a definition of the concept, it's an example of the kind of thing. And it comes from history. And something that's especially fascinating and revealing about kinetic energy is that it's not conserved. Right? For example, you throw a ball up into the air. It slows down, right?

• 14:42Daniel Whiteson?

  Because of gravity, if you throw it straight up, it slows down. And eventually, when it reaches the top of its arc, it's stopped. So kinetic energy on its own is not something that's conserved in the universe. Like, the universe doesn't seem to have a special relationship with kinetic energy. Just the way you could make up any other thing like, hey, how many ice cream cones are there currently in the universe? That's a number that we don't expect to stay constant, right? I eat one, it's gone, I decrease the number. I make one, it goes up. Kelly makes 5,000 ice cream cones, it goes up a lot, right? It's not anything meaningful or specific. And kinetic energy is sort of the same way, like when the ball gets thrown up into the air, it loses that kinetic energy. Where does the kinetic energy go?

• 15:27Daniel Whiteson?

  Well, kinetic energy can just go up and down. In this case, we know it actually transforms into another kind of energy we call potential energy. Because now that ball has gained altitude, which means it's moved up in the gravitational field. And so it's imbued now with something we call potential energy. And so now we have two kinds of energy. Kinetic energy, energy of motion, and potential energy, which is energy of position or arrangement.

• 15:53Kelly Weinersmith?

  Okay, do all different kinds of energy have the same units?

• 15:57Daniel Whiteson?

  Yes.

• 15:57Kelly Weinersmith?

  Units help me think about scientific phenomena.

• 16:00Daniel Whiteson?

  Uh huh, yes, all energy has the same units. You can use joules, for example, as a unit of energy. And kinetic energy can be calculated in joules and potential energy also calculated in joules. And you can go back and forth, right? If you're standing on the top of a building, you have a lot of potential energy because you're high up. You have a lot of altitude. And you jump off that building, you turn that potential energy into kinetic energy. Because by the time you get to the bottom of the building, you no longer have the altitude. So you have no potential energy and you have a lot of velocity, right? So you've turned potential energy into kinetic energy. And you roll a ball up a hill or you throw a ball up in the air, kinetic energy turns into potential energy. And the same thing is true, for example, like a spring. A spring is a way to store potential energy.

• 16:44Daniel Whiteson?

  And if you push on a spring and you let it go, it'll oscillate back and forth and back and forth. And what's happening there is it's sloshing kinetic energy into potential energy, back into kinetic energy, back into potential energy. So you know, like a simple model of you have a block of stone on a spring. It'll just sit there oscillating back and forth if there's no friction, turning kinetic and potential energy back and forth into each other. Which is also a really helpful way of thinking about waves in general. Like whenever we're talking about fields oscillating, even quantum fields, that's what's happening is that they're sloshing back and forth between different kinds of energy. And so now we go from having just kinetic energy, energy of motion, to having two kinds of energy, which if there's no friction, do appear to conserve the total energy.

• 17:29Daniel Whiteson?

  So kinetic energy not conserved, potential energy not conserved, but their combination in simple systems with no friction or air resistance or anything does seem to be conserved, which suggests that it's like something maybe more important to the universe.

• 17:44Kelly Weinersmith?

  Okay, all right. So for the examples that we've talked about so far, you've got like a ball rolling down a hill, or a ball being thrown up in the air or something, that it seems like the energy for those activities came from the energy of the person who's making it happen. Is that an okay way to think about it? Both of those examples seem like the energy is coming from the person who set the ball in motion.

• 18:05Daniel Whiteson?

  Or carried it originally to the top of the building and dropped it, yeah.

• 18:09Kelly Weinersmith?

  Right, right. Okay, so we haven't really gotten to where energy came from, because energy seems like it's coming from the person who started it. Is that a fair way to think about it?

• 18:16Daniel Whiteson?

  Yeah, I think that's fair. And there's something cool you can do there, which is sort of trace the path of the energy, right? Like, say the person threw the ball into the air. So you're right, it has potential energy, which came from its kinetic energy, which came from the energy stored in that person's muscles, right? Which converted some sort of chemical energy storage into the contraction of those muscle cells, which accelerated the ball. And that chemical energy storage came from something that person ate, right? And that thing the person ate probably grew, and maybe it was an animal and it ate plants, or maybe it was a plant directly, and that grew somehow by drinking light from the sun, right? And so you can play this game where you trace the energy back. And one of our listeners

• 18:58Daniel Whiteson?

  commented that a lot of the energy on Earth seems to come from the sun, which is true, right? There's a little bit of a source of energy from inside the Earth, but most of the energy does come from the sun. And playing this game, tracing the energy back, has inside of it an implicit assumption that you can trace energy, because again, it has to come from somewhere because it's conserved. It doesn't just disappear, and it doesn't appear. And again, I'm not saying that energy is conserved. I'm saying that if energy were conserved, then you can play this game, which is really fascinating, because it allows you to like, you know, sort of rewind the accounting of the universe. And it is kind of mind blowing to realize that like, wow, almost all the energy that we use here on Earth was originally created in the sun by fusion, and then gobbled up by plants.

• 19:45Kelly Weinersmith?

  All right, let's take a break and when we come back, we are going to dig into whether or not mass could be considered a kind of energy.

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• 20:57Speaker 06

  Hi, I'm Jenna Oshkowitz. You might know me from Glee. This is At the Table, the food allergy podcast, a show about food, connection, and what it really looks like to live with food allergies. Each episode, we hear stories from real people like Daniel. People would always think I was lactose intolerant, so I just kind of shortened my explanation to I'm deathly allergic in a big way that defined a lot of my childhood. And while there are challenges, there are moments that remind you you're not alone.

• 21:28Kelly Weinersmith?

  One of the best guys that I dated, he made a point to memorize all my food allergies.

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• 22:03Kelly Weinersmith?

  All right, we're back. So before the break, we were talking about how a lot of energy on Earth comes from the sun. And so I'm thinking about, you know, plants growing, putting on mass as, you know, their chloroplasts are turning energy from the sun into energy for them. And then I eat them and put on a little bit of mass, just a little, and use that energy to make a lot of ice cream cones, which makes people very happy. Should I think about mass as a third kind of energy?

• 22:28Daniel Whiteson?

  It's tempting, right, to think about mass as another kind of energy because we talked about stored energy, chemical energy and ATP, energy stored in protons as they get fused together in the sun. And you hear a lot that mass is another kind of energy. People say E equals mc squared, right? And so therefore, mass is energy and energy is mass. Yeah, so that's not really true. And the deeper answer is that mass is just an indicator of internal stored energy. So think about some object and you can categorize all of its energy into two different boxes. One is the motion of that object, like is the whole thing moving? If so, it's got some kinetic energy, right? Cool.

• 23:11Daniel Whiteson?

  The other kind of energy can have is internal energy. Like maybe there's a bunch of particles and they have bonds or there's chemistry or something. All that internal stored energy is what we call mass. Mass is not a new form of energy. It's not a special kind of energy on its own. It's an indicator of how much energy is stored inside something.

• 23:34Kelly Weinersmith?

  Okay, so like if you've got a big truck and a little truck and you set both of them going down the hill, the big truck is going to have more energy because it has more mass. And it's an indicator that it has more energy, but it's

• 23:51Daniel Whiteson?

  Yeah, exactly. If you measure the inertia of something, right? By giving it a push, you give something a certain amount of force and you measure its acceleration. That tells you it's inertia because F equals ma. Then what you're really measuring is how much energy is stored inside something. You can make this very concrete, like take a rock that has a certain mass. Shoot a photon at it. What happens? The photon is absorbed by the rock. And where does that energy go? It like goes into the bond somehow. It makes some rock molecule vibrate more. Or if it's a gas that you're hitting, then it makes those particles move faster. Some sort of internal energy, right? It doesn't matter. The mass of that rock goes up when it eats that photon.

• 24:32Daniel Whiteson?

  Which means like you lie in the sun absorbing photons, your mass goes up. Or you know, you have an electric car parked in your garage and you've plugged it in. And now you're adding energy to the battery. Its mass is going up, right? Because that mass is an indicator of how much energy is stored inside of whatever kind. As long as it's internal stored energy, it contributes to the mass. So the mass is not on its own, a new kind of energy. It's just an indicator of how much energy is stored inside. And that's what E equals mc squared means. E there doesn't mean total energy. It's not saying that all mass is energy and all energy is mass. E really means just internal stored energy. And there's a fuller equation that captures the full energy of the object. It has a term for momentum as well.

• 25:17Daniel Whiteson?

  Wait, so it really tells you the energy is internal stored energy as revealed by mass. And another term for energy of motion, energy of momentum.

• 25:27Kelly Weinersmith?

  All right, so mass is just a reflection of kinetic and potential energy.

• 25:31Daniel Whiteson?

  Internal stored energy, which could be kinetic or potential. Yes, absolutely.

• 25:36Kelly Weinersmith?

  Okay, are there other forms of energy? Or are they all essentially just different versions of kinetic and potential energy? Period.

• 25:43Daniel Whiteson?

  It's a great question and the answer is yes. Kinetic and potential energy are the two categories of energy. And there's nothing else, right? So everything can be either kinetic or potential energy. And if it's internal stored energy, it could be kinetic or potential, then we call it mass. If it's just motion of the whole object, then that's kinetic energy of the whole shebang. And this helps you understand things like, well, how can photons have energy if they don't have mass? People write to me a lot and say like, photons have energy, right? Well, equals mc squared, therefore photons have mass, right? Hmm, not true. Photons don't have mass. Because again, that equation equals mc squared, that E is just internal stored energy. Photons have no internal stored energy because they have no mass. But the full equation has a term for momentum in it.

• 26:28Daniel Whiteson?

  And photons definitely do have momentum, which is why photons also have energy. So you can have energy without having any mass, right? Because photons, again, are pure motion energy. And then you can play all sorts of fun games to confuse yourself by what this means. Like, you know, take a box, for example, and shoot a photon in it. And don't let the photon get absorbed. Put like mirrors inside the box, the photon just bounces around. You still just have a photon in the box. Then what you've done is you've added to the mass of the box by adding a massless photon. People have this intuition that mass is like the amount of stuff in something, but it really isn't. It really just captures the internal stored energy.

• 27:08Daniel Whiteson?

  Even if you add massless things to your box, then you're increasing the mass of the box. It's really kind of mind bending. It really makes you change your understanding of what mass is.

• 27:19Kelly Weinersmith?

  I need more coffee. Yeah. But I'm following you. Okay, so let's dig in more to this question about, you know, when, if ever, is energy conserved? So you were talking about throwing a ball in the air and it goes from kinetic energy to potential energy. But it doesn't always get conserved. Can we talk about the history of why we used to think energy was conserved?

• 27:43Daniel Whiteson?

  It's a really fascinating story. And, you know, for a long time, we just sort of noticed that energy seemed to be conserved. Like if you created a new quantity, number of ice cream sandwiches or something, and then you notice that weirdly, no matter what you did, the number didn't change. Like every time Kelly ate an ice cream sandwich, Daniel made one. There was this weird cosmic connection or vice versa. Then you'd be like, something is going on here, right? Like the universe respects ice cream sandwiches or something.

• 28:12Kelly Weinersmith?

  Thank goodness. I want to live in that universe.

• 28:16Daniel Whiteson?

  And that's sort of the case for energy, that in lots of situations that we studied, energy seems to be conserved. And so we assumed that it was. Same is also true for mass. For a long time, people assumed that you couldn't create or destroy mass. A lot of people still out there repeat that, even though now we know that it's not true. We know that you can convert mass into energy of motion or into potential energy that's not internal. I do that all the time at my day job, right? When we smash protons together, we are turning their mass into new forms of energy, returning their energy into mass all the time, back and forth. And every time you absorb photons from the sun, you're turning their energy into mass. But that's a hard thing to notice unless you have control of tiny particles and you can see these things

• 28:59Daniel Whiteson?

  happen, which is why chemists for a long time reasonably believed that mass was conserved, because they would measure the stuff going into a reaction and the stuff coming out of a reaction. And they never saw it change to within their measuring uncertainties. Now, of course, we know that if they had measured more accurately, they would have noticed very tiny changes in mass, because mass is very, very dense stuff, right? The c squared there tells you that a big change in energy is required to make even a small change in the mass. Anyway, the same thing was long true for energy. People are like, okay, well, mass is not conserved, we're going to give that up. But energy, still, energy definitely has got to be conserved, right? Because people made all these measurements, and it seemed like the more accurately you measured it, the more you discovered energy had to go somewhere.

• 29:42Daniel Whiteson?

  So in realistic studies, for example, say you throw that ball up into the air, what's going to happen? Well, it's not going to convert all of its kinetic energy into potential, because some of it's going to get lost to air resistance. Or if you roll a ball down a hill, right? Some of it's going to get lost to friction. That energy seems to creep out of those systems. But as long as you define the system to include those things, you account for friction and you account for air resistance, you're able to account for all of that energy. The ball's flying up into the air, and it loses energy to air resistance, but that energy then just goes into heating the air. Or if you're sliding down a hill and you feel friction between your pants and the grass, then you're heating up the grass and your pants. That energy is still there. And so for a long time, people

• 30:25Daniel Whiteson?

  just noticed, even with more and more careful accounting, the energy did seem to be conserved.

• 30:30Kelly Weinersmith?

  So I remember that we were talking about Emmy Noether's, oh, and I worked so hard to not say her name during the episode because I knew I'd get it wrong, but here I went and put myself in that position. But anyway, that she had come up with a really great law or theory that explained when things are symmetrical and when they're not. And so it sounds to me like we're going to determine here that energy is not symmetrical and that Emmy could tell us that if she were here.

• 30:55Daniel Whiteson?

  Yeah, it's really cool that we can do more than just measure the number of ice cream sandwiches and notice that they do or do not change and then conclude something. It's not just empirical. We actually now have theoretical ways to argue whether something should be conserved or not. And as you say, that's due to Emmy Noether, whose name I'm sure I'm also mispronouncing, and the number of mispronunciations is not conserved. Like, it just seems to grow, go up and up and up. Every time I hear this name.

• 31:23Kelly Weinersmith?

  Exponential.

• 31:24Daniel Whiteson?

  You hear different pronunciations of it. So, German listeners, please, don't be shy, write in and correct our pronunciations. We want to get this stuff right. Anyway, Emmy Noether told us that every time we have a conservation law in the universe, you're right, it's connected to some symmetry of the universe. And symmetry has a sort of special meaning there. It says that you can transform your experiment, you can move it or rotate it in some way, and you should get the same answer. And so a famous and powerful example is momentum conservation. You know, alongside energy, we have this concept of momentum, which is similar to energy, right? Momentum contributes to your energy. We said there's internal stored energy, and then there's energy of momentum, which come together to make your total energy. And a famous and important example of that

• 32:09Daniel Whiteson?

  is momentum, which is something that is actually conserved in our universe. And when we say symmetry there, what we mean is that you can do an experiment and then transform the experiment according to some symmetry and do the experiment again, you should get the same answer. So if you build a physics experiment and you do it in space, you know, you collide billiard balls or whatever, and then you do it 10 meters to the left, or 100 meters to the right, or forward 1,000 meters, you should get the same answer. So that's the symmetry of the universe. Doesn't care where you do your experiment. As long as you set up the conditions the same way. Nearby masses or whatever, you should get the same answer. That's a fundamental symmetry of the universe. And that symmetry gives you conservation of momentum.

• 32:55Daniel Whiteson?

  So Emmy Noether has this amazing conceptual bridge where she tells you what symmetry generates a conservation law. So conservation momentum comes from this symmetry of translation. Where you don't care about where you are in space. Where space is relative. There's no like official zero. There's no like golden numbers glowing in the universe. It doesn't matter where you do your experiment. And so now more than just saying, hey, look, I noticed momentum is conserved. You can say why it's concerned. And you can inspect that reason, say, does that make sense? You know, we can say momentum is conserved because the universe has no preferred location. You can do your experiment anywhere. And we're like, all right, that makes sense. And that's a good reason for momentum to be conserved. Okay, so what about energy?

• 33:38Daniel Whiteson?

  Emmy Noether tells us that energy is conserved in the universe if the universe's laws don't change with time. Right, so momentum is conserved if the universe's laws don't depend on location. If you don't have like different laws of physics in different places. Energy is conserved if the laws don't change with time. If like the same rules of electromagnetism apply today and tomorrow and in a thousand years.

• 34:03Kelly Weinersmith?

  It feels like I want that to be the case. But I think we've decided that energy is not conserved. Can you give me an example of how it's not conserved?

• 34:10Daniel Whiteson?

  Yeah, it's fascinating because we assume that the laws of physics are constant, like all the time. Think about just how simple it is to read about an experiment in a journal and say, I'm going to reproduce that experiment and see if it's true. And I'm going to do the same thing and see if I get the same answer. And reproducing it later and assuming you get the same answer assumes the laws of physics shouldn't change, right? And it's really important to our whole process of science that the laws of physics we're uncovering now are the same as they were a thousand years ago, or even a billion years ago. Like when we look at galaxies deep out into space in early times, we assume we know what the speed of light was back then and how mass worked and all this kind of stuff. So A universe without any constant laws is very difficult universe to do science in,

• 34:55Daniel Whiteson?

  right? And so it's not true that the laws of physics are just willy nilly changing with time. That's not something we've seen. But there are a couple of aspects of the laws of physics which do seem to be changing with time. And one of them is that the universe is expanding. So space itself, the frame in which we're doing all of this stuff is not static. And when Emily Nuthr says the universe's laws have to be constant in time, that includes space, right? It means space itself needs to be constant. The way like conservation momentum requires you to be the same location in space. It's put in the context of the space. In that same way, energy conservation requires you to have basically the same amount of space.

• 35:38Daniel Whiteson?

  It's not changing the laws of electromagnetism or, you know, the speed of light or anything. It's just saying, hey, the conditions of your experiment are not constant because your universe is expanding. And we've known for like 20 or so years that the universe is expanding and not just expanding, but accelerating in its expansion. Like every year we are making more space between galaxy clusters and between ice cream sandwiches and everywhere in the universe, space is expanding. And we actually have a not so terrible explanation for why space expanding would mean that energy isn't concerned.

• 36:12Kelly Weinersmith?

  Ooh, I want to hear about that after the break.

• 36:25Speaker 01

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  People would always think I was lactose intolerant. So I just kind of shortened my explanation to I'm deathly allergic in a big way that defined a lot of my childhood.

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• 38:29Kelly Weinersmith?

  All right, we're back. So you blew our minds a second ago by explaining that the expanding universe is why energy can't be conserved. It's crazy to me that something that's happening so far away, but I guess also happening everywhere is impacting my day to day experience of energy. But go ahead and tell us the crazy thing you were planning.

• 38:46Daniel Whiteson?

  Well, you might think, well, that sounds kind of abstract, right? Like, you're telling me the universe is expanding. And so in some sense, the context of our experiments is changing. But why does that mean that energy is not conserved? Like, is energy being created somewhere or destroyed somewhere? So let's dig into the details there. Like, energy actually is being created by the expansion of the universe. We talk about the expansion of space as creating more space, right? Like, the distance between us and some cluster of galaxy is growing. And not because we're applying some force to accelerate away from. This is just like general relativity, space is expanding. Right? We're not measuring any acceleration. They're not measuring any acceleration with their accelerometers.

• 39:26Daniel Whiteson?

  But the distance between our galaxies is increasing and faster and faster every year. So new space is being made. Well, what does that mean for new space to be made? Space is filled with quantum fields, right? There's the electron field, there's electromagnetic field, there's fields for all the quarks, all these fields in space. And all those fields have non-zero energy. Like, these are quantum fields, which means they can never go down to total zero. They always have some energy in them. So you pop out a new cube of space, boom, it comes with quantum fields that have energy in them, right? So new space means new energy. So right there, making more space means you're creating energy.

• 40:09Kelly Weinersmith?

  But doesn't it just mean the energy gets spread more thin across this new space?

• 40:13Daniel Whiteson?

  Because every chunk of space is the same, and the rules of quantum mechanics have a minimum requirement of that energy. It's not something we deeply understand, but it's something that's required for the accelerated expansion of the universe. We know that dark energy behaves this way. We see, for example, as the universe expands, the density of dark energy doesn't change in the universe. Like, the density of protons goes down. You have a certain number of protons, you know, some huge number of protons in the universe. As the volume of the universe increases, the density of those protons drops exactly the way you would expect, right? You double the volume, the density goes down by a factor of two. But dark energy's density does not decrease. You double the volume, the density stays the same, right?

• 40:58Daniel Whiteson?

  So dark energy behaves in this weird way that when you create more space, you also create more dark energy.

• 41:04Kelly Weinersmith?

  Where does it come from?

• 41:05Daniel Whiteson?

  Where does it come from? Yeah.

• 41:06Kelly Weinersmith?

  How does that keep happening?

• 41:07Daniel Whiteson?

  You ask where it comes from, and that's a question that only makes sense if it's conserved, if it has to come from somewhere, right? Like, if I make an ice cream sandwich, do you say, hey, where did that ice cream sandwich come from? That seems to violate the laws of physics. You're like, no.

• 41:20Kelly Weinersmith?

  I wouldn't ask questions about a new ice cream sandwich, Daniel. I would just enjoy it. But go on.

• 41:25Daniel Whiteson?

  You know, it's true. You had to assemble it out of bits, but like the assembly itself didn't violate some law of physics. You didn't have to destroy an ice cream sandwich somewhere else to make this one, right? There's no limit on the number of ice cream sandwiches. Thank the Lord.

• 41:37Kelly Weinersmith?

  Hallelujah.

• 41:38Daniel Whiteson?

  Yeah, I know. We're all relieved. And so that question, where does the energy come from, only makes sense if things have to flow. If you can just change the total energy in the universe, or change the number of ice cream sandwiches, then the question itself isn't actually as meaningful, right?

• 41:53Kelly Weinersmith?

  So, energy comes from nowhere. Energy can just like, boop, pop up.

• 41:57Daniel Whiteson?

  Energy can just pop up. Yeah, absolutely. And it can also disappear, right? We talked about how the density of protons changes as the universe expands and the density of dark energy. So, protons dilute in a way that makes a lot of sense, right? Like, you double the volume, the density goes down by a factor of two. Interestingly, side note, same thing is true of dark matter. Dark matter, as you double the volume of the universe, dark matter density drops by two, which is one reason why we think of dark matter as matter, because it dilutes the way matter does. Because radiation doesn't. Photons, as you double the volume of the universe, their energy density drops by more than two. Like, you have a certain number of photons in the universe. Now, you increase the volume

• 42:41Daniel Whiteson?

  of the universe, you have the same number of photons and more volume. So, you might think, oh, the energy density just decreases because you've increased the volume. Not true, because the expansion of space also redshifts those photons. It stretches their wavelength to redder colors or longer wavelengths. Longer wavelength means lower energy. So, you have a universe filled with photons, you expand that universe, their energy density drops faster than the energy density of matter or dark matter. Where does that energy go? It doesn't have to go anywhere. It just goes.

• 43:14Kelly Weinersmith?

  Moves into the space that was just created. I've solved it. Where's my Nobel Prize?

• 43:20Daniel Whiteson?

  I love that idea. People write to me about that a lot. They're like, okay, energy is created when you expand space and also destroyed. Do these two numbers add up? And it would be amazing if they did, right? And that would be a beautiful explanation, you know, plus 7,000, minus 7,000, boom, I think we figured it out. These two numbers are vastly different scales. Like, the energy in photons in the universe is tiny. Like, much less than 1% of all the energy in the universe is in photons. But the dark energy of the universe is like 2/3 of all the energy. So, energy is increasing in dark energy much, much more rapidly than energy is decreasing by redshift of photons. So, we don't think that photons are like getting turned into dark energy or anything.

• 44:01Kelly Weinersmith?

  Okay, so, bottom line here, we don't know where energy comes from. We know it's not conserved. Physicists have job security.

• 44:12Daniel Whiteson?

  Yeah, we don't even really still have an answer to the original question of like, what is energy, right? It's still a description of something we've seen. And we have a little bit more theoretical footing now because we could, in principle, define energy as the thing that would be conserved if the laws of physics were perfectly symmetric in time. Right? You can use Noether's theorem as a definition of energy and say, "Oh, energy is that thing which would be conserved." And that's kind of cool. I like that. It's very theoretically grounded. But it doesn't give you like a conceptual sense for like, "What is this thing?" But unfortunately, that sort of turns you back to the philosophy of it like, "Well, what do you mean? What is this thing? What kind of answer are you looking for when you ask, 'What is energy?'"

• 44:55Daniel Whiteson?

  What part of the question is not satisfied by like, "Well, here's a description of the kinds of energy. Here are the rules that it follows. Here's how it's created. Here's how it's destroyed. Here's how it's not conserved." What part of the question is not answered by that description, do you think?

• 45:10Kelly Weinersmith?

  I have no idea.

• 45:12Daniel Whiteson?

  Yeah, I feel like there's something about it that's still unsatisfied. It's still like, "Mm, yeah, but what is it, you know? Like, why do we have it? Where does it come from?" It's a deep human question, but like, the human questions we ask aren't always the appropriate questions. There aren't always the ones that answer the question. Sometimes, we discover that something is different from how we expected it to behave. And that means the kind of questions we should ask about it are changing. Because energy isn't something that comes from somewhere. It's a feature of the universe, and it does seem to be important because under lots of contexts, it is. conserved, but it's not fundamental in that way. And so it could just be something the humans notice, something the humans like to calculate, something that connects with our everyday

• 45:54Daniel Whiteson?

  experience in a way that's important to us, but isn't to like alien physicists, for example.

• 46:00Kelly Weinersmith?

  It only appears to be conserved on human scales because it's being lost at such low numbers, it's hard to measure, but it's not even conserved on small scales. It just looks that way.

• 46:10Daniel Whiteson?

  Yeah, that's exactly right. And I think it's useful to think about this because it impacts other questions we have. Like, a lot of listeners who wrote in thought about the conservation of energy and they talked about how the Big Bang had energy and maybe all the energy in the universe just came from the Big Bang, right? And so I hope the answer today reveals that like, no, there's some energy in the universe which was created after the Big Bang, right? This expansion of the universe is making energy and that's post-Big Bang energy. And also, some of the energy of the Big Bang is gone now. Like, you have the Big Bang, you have matter and antimatter created, they annihilate, you have a universe mostly filled briefly with photons, so photons did dominate the universe's energy budget initially, but a lot of those photons

• 46:54Daniel Whiteson?

  are now red-shifted and their energy is gone. Like, we talk on the podcast a lot about the cosmic microwave background radiation, this energy from the early universe that reveals so much about how the universe came together and what it means and how it rippled. But those photons are very, very red. They're at a temperature of like 2.7 Kelvin, very, very cold. The plasma that made them was like 300,000 degrees Kelvin, right? They were a glow of a very, very hot plasma, very energetic photons originally. That energy is just gone now, right? It didn't go anywhere. So, a lot of the energy in the universe didn't come from the Big Bang and a lot of the energy of the Big Bang is now gone. It's fizzled out.

• 47:36Kelly Weinersmith?

  Do we have less energy over time, more energy over time, or do we not know how this balance is working out?

• 47:41Daniel Whiteson?

  We definitely have more energy over time because dark energy is much more dramatic. And so all the energy created by dark energy, the expansion of the universe vastly outweighs the energy lost due to redshifts of photons.

• 47:55Kelly Weinersmith?

  So you had said that earlier, but is that the only way that energy is lost? Or is energy lost while humans are doing all of these reactions down here? And is that impacting the like mass balance equation for the universe?

• 48:09Daniel Whiteson?

  To my knowledge, the expansion of the universe is the only source of energy gain or loss, right? You notice that it's the underlying mechanism for both of these things because it's the place that Noether's theorem is violated, right? It's the thing that doesn't respect the time symmetry. So the expansion of the universe redshifts those photons, breaking conservation of energy, and creates new space, breaking conservation of energy. So anything that's sensitive to the expansion of the universe can be a source of the violation of conservation of energy. And I want to make another comment about the Big Bang, which is it seems like a little bit of a loss of power to explain and explore. Like if energy was concerned, then we could do this cool thing of tracing it back into the sun and then where did that came from? Where that came from, all the way back to the Big Bang.

• 48:54Daniel Whiteson?

  It's like we have a ledger, you know, that tells us how energy sloshes around and maybe by tracing that we could learn something. And that seems really cool and feels like, oh, maybe we've lost that. Maybe we can no longer learn about the Big Bang by studying energy. Well, just now we can ask different questions. You know, we can ask how the energy for the Big Bang came about. Like, we know that the universe was filled with hot frothing energy early on. Where did that come from? We no longer have to just answer that by saying, oh, it came from this other thing with equal energy. Now we have more ideas about how you can create all that energy from some earlier denser states. So if anything, it opens up the examination to think differently about the origin of the universe. And, you know, that's what we want.

• 49:37Daniel Whiteson?

  We want our understanding of physics to evolve and to give us new ways to think about the whole context of reality and where it all came from. And so to me, that's exciting. It like removes blinders a little bit and gives us a broader sense for how the universe works and maybe how it all started.

• 49:55Kelly Weinersmith?

  I bet you are great at writing grants. Okay, we were wrong about that, but that's okay, because it's the fascinating thing is actually revealed now.

• 50:03Daniel Whiteson?

  That's right. So please send me a million dollars to make a lot of ice cream sandwiches, because I have a lot to learn, you know?

• 50:09Kelly Weinersmith?

  Please share those ice cream sandwiches, Daniel. Yes, Daniel needs this money. It's important, everybody.

• 50:14Daniel Whiteson?

  Exactly. My Cookies and Cream project, really, top priority for national security.

• 50:20Kelly Weinersmith?

  That's right. That's right.

• 50:23Kelly Weinersmith?

  All right, well, I learned a ton today, actually, and can't wait to tell my daughter, actually, about some of this stuff and blow her mind. Somehow, I feel like she's going to find this really interesting.

• 50:32Daniel Whiteson?

  Yeah, so energy is not conserved, which means it doesn't have to come from anywhere. But our understanding does seem to be growing, and it seems to emanate from these deep studies of how the universe works. So let's keep doing that.

• 50:45Kelly Weinersmith?

  Let's hope our understanding is expanding faster than the information we're losing.

• 50:51Daniel Whiteson?

  All right, go off and enjoy an ice cream sandwich, everyone. On us.

• 50:54Kelly Weinersmith?

  Oh, not on us. Not on us, Daniel.

• 50:56Daniel Whiteson?

  Virtually on us. Spiritually on us.

• 50:59Kelly Weinersmith?

  Don't send us the receipts. That's right.

• 51:01Daniel Whiteson?

  Emotionally on us. There you go.

• 51:03Kelly Weinersmith?

  Okay.

• 51:11Kelly Weinersmith?

  Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. We would love to hear from you.

• 51:16Daniel Whiteson?

  We really would. We want to know what questions you have about this extraordinary universe.

• 51:22Kelly Weinersmith?

  We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you.

• 51:29Daniel Whiteson?

  We really mean it. We answer every message. Email us at [email protected].

• 51:35Kelly Weinersmith?

  Or you can find us on social media. We have accounts on X, Instagram, Blue Sky, and on all of those platforms, you can find us at D and K Universe.

• 51:45Daniel Whiteson?

  Don't be shy. Write to us.

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