Science Friday
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This year marks the 100th anniversary of two papers that sparked the field of quantum mechanics.
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Science Friday is supported by MathWorks, creator of MATLAB and Simulink software for technical computing and model-based design. MathWorks, accelerating the pace of discovery in engineering and science. Learn more at mathworks.com. Hi, I'm Ira Flatow, and this is Science Friday. Today on the podcast, 100 years into quantum science, and it's still really weird. But is the weirdness true?
It's not that they're really particles or really waves. They're really a third kind of thing that has some characteristics of each. And that's not like anything we experience in the world around us, which is why the whole theory seems strange.
100 years ago this summer, physicist Werner Heisenberg wrote a letter to Wolfgang Pauli, and in it, he revealed his new ideas about what would eventually be known as quantum theory. And 100 years later, that theory has been expanded into a field of science that has revolutionized the way we look at the world around us, from the ultra small to the ultra big, from subatomic particles to the makeup of the universe. But it's still really weird and often counterintuitive. Here to help us celebrate 100 years of quantum science and separate quantum fact from science fiction is Dr. Chad Rosell. He's the R. Gordon Gould Associate Professor
of Physics and Astronomy, also Chair of the Department at Union College in good old Schenectady, New York. Welcome to the program.
Thank you for having me on.
Okay, 100 years, but what actually are we observing and celebrating in detail?
So Heisenberg is really kind of a late stage in the early development of quantum. Really, it's, in some senses, the 125th anniversary of the kickoff, which was Max Planck in 1900, who was desperately trying to explain the color of light from a hot object. And he had to resort to this weird mathematical trick to make the equations work out, where he essentially assigned an energy to the frequency of light. And a few years later Einstein picked that up and ran with it. Planck always thought it was an ugly mathematical trick. Einstein took it seriously, ran with it.
And then around 1913, Niels Bohr proposed a quantum atom. Sort of the cartoon solar system atom that that you see in children's books and that sort of thing. With electrons going around the nucleus, which works brilliantly for hydrogen. But it's kind of hard to make work in detail for for anything else.
Well, if this all happened 125 years ago, why are we celebrating 100 years ago?
So by the mid 1920s, it had gotten sort of untenable. And people realized that that what they were trying to do with the Bohr picture of the atom wasn't really working. And and Heisenberg is one of the people who really sat down and said, OK, let's go back to basics and think about what really can we say about this? And realized that it didn't make sense to talk about the electron following a definite orbit. But he had to talk about just what you could measure. And that is the first complete mathematical formulation of quantum mechanics.
So what are the basic principles that make up quantum theory? I mean, I want you to sum it up in 25 words or less.
So the the basic idea is you can think of everything in the universe as having both particle-like character. You can count things, you can detect things at a particular instant at a particular position. And also some wave-like characters. They're they're sort of distributed over space. They're delocalized a little bit. They interfere with each other if you if you allow them to take two different paths. And that gives you this weird combination of of properties. It's not that they're really particles or really waves. They're really a third kind of thing that has some characteristics of each. And that's not like anything we experience in the the world around us, which is why the whole theory seems strange.
You know, so many of the ideas that come out of quantum theory just seem so weird. Stuff can pop out of empty space. You have two particles that can be linked across space. Entanglement, do we know how real these effects are?
The fascinating thing is all of these things are absolutely and verifiably real. Right? The idea of entanglement, that's Einstein's last really great contribution to this debate. This notion of it being problematic that these particles could be correlated across distance in ways that seem to violate relativity. A guy named John Bell in the 1960s showed that there's an experiment you can do to test whether that non-locality is real. And people subsequently went and did those experiments. And every time this has been tested, the result is very clear that the sort of theory Einstein preferred is ruled out. And the quantum theory where these things are correlated across space and time in ways
that seem to defy our intuition is absolutely true.
When Einstein called it spooky action at a distance, was that pejorative? Was he making fun of the idea?
He was talking about the notion that things could change instantaneously over a wide area of space. And he found that idea really ridiculous and outlandish, something that simply couldn't be allowed, in large part because he had spent so much time with the development of relativity, working out exactly the restrictions on sending information through space and the relationship between space and time. And something that goes against that was really offensive to him.
Well, that was the idea that light is a constant, nothing can go faster than it. How could spooky action at a distance, how could something so far away react instantaneously?
Exactly. And that's the...
Was there an answer to that?
The the modern understanding is that this this very definitely does happen. And then there's questions about how you interpret that. What do you say goes on inside the box? Right? We can do these experiments and we see that, you know, you have a particle in in Vienna and another particle in in London. And you measure them very close together in time and their properties turn out to be extremely well correlated. But why that is, is open to debate. Some people say it's it's really all just about information. It's all we're doing is we're learning things about what we know about the outcomes of measurements.
Other people say that that there is some weird sort of collapse that happens that doesn't respect the boundaries of space in the way that that we normally think of and that we just have to deal with.
After the break, trying to test and extend the rules of quantum to the macroscopic world, does it work?
One of the open questions is, you know, how far can you push that? Can you get to, you know, things the size of red blood cells? Can you get to bigger than that?
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If these early papers that were celebrating 100 years of now, if they were the introduction to the field, and even going back, as you say, 1900, 1910, 1920s, if they were the first chapters of a quantum physics book, let's say, what chapter would you say we are in now?
I would say sort of your Planck-Einstein is kind of chapter one. The Bohr old quantum theory is two. What we're celebrating is really the start of chapter three. And then I would say there's probably three more after that. There's sort of the solidification of quantum theory and the development of QED in the 1950s. Kind of 60s and 70s, you get the particle physics revolution and the development of the standard model, which is really closed off with the discovery of the Higgs boson in 2012. But the basic framework is really pretty well nailed down by the mid 80s.
And then kind of 80s, 90s up into today, you get these experiments where we have the ability to control the states of atoms and light with sufficient precision that we can start to really probe these bizarre quantum things. And that leads to things like quantum information science and quantum computing that are some of the most exciting things going now. And I think that's kind of its own chapter, the ending of which hasn't been written yet.
So that's the frontier now, is what you're saying?
Where we're at now is sort of a combination of getting these applications of the weirdness, the things that allow you to do kinds of calculations quickly that you wouldn't be able to do with a standard classical computer. That's really one of the great frontiers of this. Another that kind of brings in general relativity is the notion of trying to reconcile quantum mechanics with general relativity, which is an incredibly hard problem.
Are there limits to how big a thing quantum rules apply to? Can it govern big things above X, let's say?
So that's an open question, and there are people working very hard on those kinds of problems. There are some groups that have made very, very nice careers for themselves out of demonstrating the wave nature of progressively larger molecules. They're up to these big, like, almost borderline viruses that they can do interference experiments with, send them through multiple slits and see that they interfere like waves afterwards. And one of the open questions is, you know, how far can you push that? Can you get to, you know, things the size of red blood cells? Can you get to bigger than that?
And there are some of these models of how to combine quantum with gravity or how to address these problems of what's going on with quantum measurements. Some of them say that there should be an upper limit, that if you get to a certain size, that other effects should kick in and sort of destroy the quantumness of things. And so that's an open question that people are working very hard to probe.
You brought up the magic word slits, because that's where a lot of this all started, right? And I want to bring that up because recently MIT researchers did an experiment that was a version of the famous double slit experiment, and some of the interpretations of their experiment says that Einstein was slightly wrong. Can you tell us about that, what was going on there?
Yeah, so this is a famous experiment where you send light through, or an electron, because it behaves like a wave, you send it through a slit, and that light will diffract, and it will behave in a wave-like manner, and there are certain places where you'll see a high probability of observing the electron later, and certain places where you'll see zero probability.
Right.
And Einstein's claim was, well, you could put that slit on some kind of spring that let it move back and forth, and that would, you know, measuring how much the slit moved would tell you which direction the particle was kicked and how hard, and then you would know both the momentum of this thing and the position. And Mills Boer pointed out that now if you do that, then you're sort of smearing out where the slit is in a way that that destroys that interference pattern. You no longer have these places where there's zero probability of finding the electron. What this MIT experiment does is they scattered light off an array of atoms that were held
in position and they could hold the hold them in position very loosely or very tightly. If they hold them very tightly, they show that there's this clear wave behavior. There are these places where there's zero probability of seeing any light and that if instead they hold them very loosely, the atoms are able to recoil in a way that that in principle could give you information about which way the light was deflected. And when that happens, your interference goes away and you start to see the disappearance of these regions where where there's interference and and no probability of seeing any light. And it agrees beautifully with the quantum prediction and so is goes against what Einstein was
hoping to be able to do with that apparatus.
You know, on this 100th anniversary, so to speak, of quantum mechanics, if you ask the average person if they know anything about quantum mechanics, they they'll probably say if they do Schrodinger's cat.
Yep.
Right? What year was that? And why was that so important?
Schrodinger's cat, he first puts that out in, I think, 1935. He and Einstein at about that same time were both really dissatisfied with quantum mechanics on a philosophical level and kind of leaving the field. Einstein's parting shot was this paper with Podolsky and Rosen that introduced the idea of entanglement. Schrodinger wrote this this article about how again, he also thought that quantum mechanics was fundamentally incomplete. And one of his illustrations is this this cat thought experiment. One of the big points he was making was that you could take something that everybody agreed was quantum, like an atom, and put it in this superposition state where it's either decayed or not decayed.
And then you can tie that inextricably to the state of something very big that everybody agreed was not quantum, like a cat, and the cat is either alive or dead. But the state of the cat depends wholly on the state of the atom. And he was saying that this division between quantum and not quantum just doesn't make any sense because you can do things like that. And that's really philosophically troubling. And also enormously productive because people have been arguing about that for decades now.
Yeah, it's still, still kind of mind numbing when you think about it.
Yeah, it's amazing, but, but it's, you know, inspired a huge number of, of really cool experiments over, over the years. So it's, it's great stuff.
Yeah, let me move from there to one of my favorite topics, which is the dark universe. We are in a universe that we don't know what most of it is made out of, right? Like 90, 95% is dark energy, dark matter. Do we need new physics to make all of this work or just better understand the rules we already have?
Pretty definitely, we need, we need new physics for, for that. The dark matter, you can show there are some good arguments that, that whatever dark matter is, it can't be made up of particles that we already know about. And the, the idea of dark energy is you can make some plausible, you know, guesses as to what would be the cause of, of this sort of pressure that's causing the universe expansion to accelerate. But, you know, the getting the numbers to work out requires some really implausible hand waving unless you have some other new physics acting. So, you know, the best current guesses is that the dark matter is probably made up
of some kinds of particles that we haven't yet been able to detect. And so there's a, a huge variety of experiments out there looking in different ranges of, you know, different types of new particles, different masses, different interaction strengths, and, and so on, and really trying to, to search as many places as we can to find what the, what the thing is that's accounting for the dark matter.
I'm going to ask you to pull out your crystal ball. Uh, I have my own right here and look 100 years into the future for quantum science. Tell me what you see.
Hundred years into the future for quantum science, I think, you know, I'm, I'm hopeful that somebody will have thought of an experiment that distinguishes between these different interpretations that allows us to say once and for all, you know, okay, are we really in a world where we have this, this complicated branching wave function in this massive superposition state that sort of effectively looks like different universes? Or is there something going on that, that causes a real collapse of the wave function and, and, you know, changes reality in a fundamental way?
These are ultimately questions of philosophy that somebody needs to think very hard about, you know, what are the premises that underlie these different approaches? And, you know, is there a way to push those sort of to a limit where we see different answers to the same question and contest that experimentally? That took 30 years with the EPR experiment, this Einstein Podolsky Rosen entanglement paper, took 30 years before John Bell came up with an experiment you could do that, that would distinguish between these. So, you know, another hundred is probably good for this, this much harder question of, of what's going on and you'd make these measures.
Well, let's all meet back here, 100 years from now and see if you are right.
I would love to.
Thank you, Dr. Rozell, for taking the time to be with us today. It's fascinating.
Thank you very much. This was fun.
Dr. Chad Rozell is the R. Gordon Gould Associate Professor of Physics and Astronomy and also Chair of the Department at Union College in Schenectady, New York. Hey, thanks for listening. This episode was produced by Charles Berkwhist. See you next time. I'm Arif Leidh.
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