59 | Adam Becker on the Curious History of Quantum Mechanics

0:00:00 Sean Carroll: Hello everyone and welcome to the Mindscape Podcast. I'm your host, Sean Carroll, and today we have another episode of quantum mechanics. I know this makes everyone very excited. Today, I'm talking to Adam Becker, who is a PhD in Physics, but has gone the route of becoming a freelance science writer, and most interestingly, for our current purposes, he's the author of a book called What is Real? This is a really wonderful book, I've been recommending it right and left. It's about the history of quantum mechanics in the 20th century, but it's not the typical history. Most histories of quantum mechanics focus on from 1900 to 1935, and what Adam really does is look at what happens after 1935. He does a little bit of the early stuff, but the point is that, as I talk about it in my own book, Something Deeply Hidden, there's this weird thing where quantum mechanics is invented, it comes with this giant problem called the measurement problem, what really happens when you measure a quantum system, why do the rules seem to be different? Etcetera. And then after a very brief flurry of interest in talking about the measurement problem with people like Bohr and Einstein and Schrodinger going back and forth, it's forgotten. It's just dropped. In fact, it becomes disreputable to even think about the measurement problem.

0:01:16 SC: So Adam really digs into the history of the people who refused to be quieted when talking about this huge problem in physics, the plucky band of rebels who really said, "No, actually, this is an important thing. We need to think about it." So he talks about Hugh Everett, of course, but also David Bohm, John Bell, a lot of the experimentalists, like Clauser and Aspect and others, who really tackled this problem and tried to remind the physics community that despite what their advisors told them, the measurement problem in quantum mechanics is really, really important. So it's not about, here, solving the measurement problem, it's about the history of how physicists talked about that problem. And it's a fascinating history with colorful characters, so you'll both learn a little bit about quantum mechanics, but also learn a little bit about the history and sociology of physics, which is really much more fascinating than it has any right to be, so let's go.

[music]

0:02:27 SC: Adam Becker, welcome to the Mindscape Podcast.

0:02:29 Adam Becker: Well, thanks for having me. It's great to be here.

0:02:32 SC: Of course. This is a slightly unusual show just because... They're all slightly unusual, I don't know why I keep saying that.

0:02:37 AB: Sure.

0:02:37 SC: But quantum mechanics, I should say this is a slightly usual show. It's the single topic I do the most, 'cause I'm coming out with a book on quantum mechanics. Now, I do quantum mechanics for living, but you've written a book that's already out about quantum mechanics, so for the purposes of this interview, I'm gonna pretend that I don't know much about quantum mechanics, if that's okay. I'm gonna try to get it out from you.

0:02:58 AB: Sure.

0:03:00 SC: And specifically, is it okay to characterize your book as a history of quantum mechanics?

0:03:05 AB: Yeah, I think that's what it is. Yeah, it's definitely a history of quantum mechanics. It is, I would say, a somewhat unusual history of quantum mechanics, and I think it's a good history of quantum mechanics, but I'm biased.

0:03:19 SC: Well, it's unusual, I guess, you're gonna say that in a sense that history, there's a set of things that happen, but you have an angle, right? You have a theme.

0:03:29 AB: I have an angle. That's one of the things that makes it unusual, in that I think that the dominant take on what quantum mechanics says, this thing that's called the Copenhagen interpretation but also goes by other names, like the Orthodox interpretation, or whatever, I am not a fan, to put it mildly. The other thing, though, that I think makes my book somewhat unusual as a history of quantum mechanics is... There are a lot of other popular histories of quantum mechanics out there, and most of them start in, say, 1900 with Max Plank, and then they say, "Max Plank found the black body radiation law, and that was the beginning of the quantum revolution," and then they moved forward from that with Einstein and the photoelectric effect, and Bohr and his model of the atom, and then they end with, "And then Heisenberg and Schrodinger developed fully-fledged modern quantum mechanics, and everybody lived happily ever after and then maybe Einstein and Bohr got into a fight, but Bohr won, and then 30 years later, John Bell did a thing, and that's the end of the book, right?" My book inverts that structure. I only have a chapter or two at the beginning about the very early days of quantum mechanics and most of the book focuses on stuff that happened from, say, 1945 onward, after World War II.

0:04:53 SC: Which is fascinating. Just so... If there's any people out there in the audience who think quantum mechanics is fun, but history is not, it's really, really illuminating to go through this particular kind of history because you see where people got certain attitudes, and quantum mechanics is hard and it shows us a lot about physics and the physics community, right?

0:05:16 AB: Yeah. Yeah, I mean, I think that there is general attitude or idea among some physicists and some scientists of various stripes, that history and personalities and culture don't really matter that much in science and aren't really reflected in science, and that's just wrong.

0:05:38 SC: It's just the objective truth, Adam. What are you talking about? We just find the truth and we lay it out there for people.

0:05:43 AB: Yeah, that's clearly true. Science is a body of facts, it's not a human process. Yeah, but science is a human process, right? And it's affected by all other human processes. Which is not to say that science is just bullshit that people make up.

0:05:56 SC: Well, we certainly make choices along the way about how to do science, right? What's important, what questions we're gonna tackle and so forth, and quantum mechanics is a great example. So why don't you give your version of what is quantum mechanics?

0:06:11 AB: Oh boy. Quantum mechanics is a phenomenally successful physical theory. It is the best theory we have to account for the behavior and predict the behavior of very small objects, but also things that are constituted of very small objects, which is basically everything. And so quantum mechanics... So one definition of quantum mechanics is it is... Well, hold on, quantum physics, right?

0:06:46 SC: I don't care. Quantum theory, quantum physics, quantum mechanics, those mean the same thing.

0:06:49 AB: Quantum mechanics, we usually use that for the non-relativistic theory, but anyway, yeah, quantum physics, because it's this theory for tiny things and things made of tiny things, which is everything. It's basically our best physical theory of everything, with a little asterisk on it, and the asterisk is general relativity, which is a whole other thing. But basically anything that... Where gravity doesn't matter that much, where that much means matters less than it does, I don't know, inside of the heart of a dense star or near a black hole. Quantum mechanics is fantastic.

0:07:33 SC: Okay, but what does it actually say? Let's imagine someone is listening, who just doesn't know about uncertainty or wave functions or anything like that.

0:07:42 AB: Yeah. Well, that's the question. Quantum mechanics is... So one way of answering the question is, quantum mechanics is this really good mathematical tool, for predicting the outcomes of experiments and predicting how various physical phenomena will play out. As for what it tells us is going on in the world, that's a question of interpreting the mathematical structure and the results of the experiments that we do. And that's exactly what's at stake in the story that I'm telling in my book, the question of how to interpret this incredibly successful theory, to figure out what it's telling us about the world is very controversial, and has been since the early days of quantum mechanics, back in the 1920s and even before, I mean that's why... I don't think we've actually said the name of my book yet, but that's why the title of my book is What is real? It's not a finished story, and that question is the question at hand.

0:08:45 SC: Yeah, you do notice this, whenever you're trying to talk about... Whenever I'm trying to talk about quantum mechanics, is that you can't say anything about what the theory says without choosing a formulation of it, without taking sides in these interpretation wars.

0:08:58 AB: Exactly, yeah, I could say something that would make you as a many worlds person, very happy, and say, "Well, quantum mechanics tells us that things can be in more than one place at once in different universes, and very small systems composed of a very small number of things can interfere with their counterparts in other universes, but then when you get larger aggregate number of things, that doesn't happen as much unless you perform special tricks." That would make a lot of people very angry if I said that that was what quantum mechanics said, without qualification.

0:09:35 SC: Don't read into it, Adam, you gotta be able to make people angry.

0:09:37 AB: [chuckle] Oh, don't worry, I have no trouble making people angry. There are a lot of people angry with me.

0:09:43 SC: Why don't we just be a little bit historical and imagine that we are there at the Solvay Conference, right? The Fifth International, for those of you who don't know, this was this... 1927, I think?

0:09:53 AB: Yes.

0:09:54 SC: This big... It was the moment when quantum mechanics matured, right?

0:10:00 AB: Yeah.

0:10:00 SC: And all the great, bright people in the physics world were there, and they chatted it out, and tempers flared.

0:10:06 AB: Exactly. Yeah. No, this was... I don't know, we're in LA. It was the Oscars of quantum mechanics, right? So yeah, this was a major, major conference in 1927 in Belgium, and all of the key players in the formulation of quantum mechanics, or almost all of them were there.

0:10:27 SC: So for example.

0:10:28 AB: Yeah, so for example, Albert Einstein was there, Niels Bohr, the great Danish physicist was there, Louis de Broglie was there, Werner Heisenberg was there, Erwin Schrodinger was there, Max Born.

0:10:42 SC: Planck was there.

0:10:43 AB: Yeah, Planck was there.

0:10:45 SC: Pauli.

0:10:45 AB: Pauli, Marie Curie.

0:10:46 SC: Dirac. Everyone.

0:10:46 AB: Dirac. Yeah, everybody was there... I think Rutherford was there, everyone was there. And this was a great coming-out party for the version of quantum mechanics or the way of thinking about quantum mechanics that Heisenberg and Born and Bohr and Pauli had cobbled together. Which is not to say that those four were the only ones involved nor is it to say that those four all agreed with each other. But they agreed enough that they were able to put together a presentation that they gave, and specifically a presentation that Born and Heisenberg, I believe, gave, in which they said, "Okay, here's the theory, here's how it works, it's done, it's a complete theory, and you can't add anything to it, and it doesn't let you say anything about what's going on before you take a measurement."

0:11:47 SC: And this is what we call the Copenhagen interpretation?

0:11:49 AB: Yes, exactly... Or a version of the Copenhagen interpretation.

0:11:53 SC: No one agrees on what the Copenhagen interpretation is.

0:11:54 AB: Yeah, no one agrees. That's the other thing.

0:11:55 SC: But it's still what we teach our students today.

0:11:57 AB: We teach our students something that plausibly goes by that name, yeah.

0:12:00 SC: That's right.

[laughter]

0:12:04 AB: 'Cause there's... Here's another way of answering your earlier question of what quantum mechanics is. And I'll get back to the Solvay conference. Quantum mechanics is a pair of rules for predicting what's going to happen in the world, one of the rules is this thing called the Schrodinger equation which is exactly the kinda thing that we love to have as a law of nature, it is deterministic, it's a beautiful partial differential equation, it's very, very pretty. It says that there are these things in the world... Or that there are things, whether they're in the world is controversial.

[laughter]

0:12:41 SC: It's really hard to make a sentence here.

0:12:43 AB: It's so hard. [chuckle]

0:12:44 SC: But there are things.

0:12:45 AB: There are things called wave functions, and as the name implies, they wave and they ungulate in a way that is perfectly predictable through the beautiful mathematics of the Schrodinger equation. And then there is this other thing called the Born rule, and the Born rule doesn't look like a normal law of nature nature, it doesn't look like the laws of nature that physicists were accustomed to in the 1920s, which is not, in and of itself, a strike against it, that's fine.

0:13:20 SC: Sure. We could be dramatic and new, that's implied.

0:13:22 AB: Exactly, that's good... Or can be good. But the Born rule also...

0:13:26 SC: Named after Max Born, not the concept of being born.

0:13:28 AB: That's true, yes, named after Max Born, and the same with Schrodinger equation is named after Erwin Schrodinger, and to give credit where credit is due, Heisenberg came up with something like the Schrodinger equation, it was something that was mathematically equivalent. But Max Born, he said, "Okay, these wave functions, there is another way that they behave, they... When you look at something that has a wave function associated with it, or when you take a measurement of something that has a wave function associated with it, which is supposed to be everything in quantum mechanics, that wave function collapses. It goes to zero everywhere, except in one spot or in one spot... In spot or in one way...

0:14:15 SC: One value.

0:14:16 AB: Yeah, one value. Everything but one value goes to zero, and which one it's going to be is probabilistic. Which one is going to not be zero is probabilistic, and you can determine that by looking at the values of the wave function the moment before you make the measurement. And the problem is that these two rules for how wave functions work contradict each other.

0:14:40 SC: Right.

0:14:41 AB: And so there's the question, When do you use one and when do you use the other one?

0:14:46 SC: So one rule for when you're not looking at it, and another rule for when you are.

0:14:51 AB: Yeah, something like that. And so the question is which one do you use when? And the traditional answer, the Copenhagen answer, is to say, "Oh, you use the Born rule, this collapse rule, when you're looking at things, when you're making a measurement, and use the Schrodinger equation for when you're not looking." What do we mean by looking? What's a measurement? What counts as a measurement? Who has to do it, right? Do you need a PhD to do a measurement?

0:15:19 SC: What if you just glance at it?

0:15:20 AB: What if you just glance at it? What if a monkey does it? What if a bacterium does in? All these questions. So this assorted host of questions that go along with this one core question of, which rule do we use when? That's the measurement problem.

0:15:37 SC: Right, the measurement problem of quantum mechanics.

0:15:39 AB: Yeah, and the Copenhagen interpretation has... Claims to have a solution to this, claims to have many solutions to this, because the Copenhagen interpretation isn't any one thing. But at the Solvay conference, the solution that was proposed was it's not right, it's not good scientific practice to talk about what's going on when you're not looking.

0:16:04 SC: So the philosophical maneuver that they made?

0:16:07 AB: Yes, it is something along those lines, and not everyone was happy with this. In particular, Albert Einstein was very unhappy with this, and so he starts getting into it with Niels Born in particular, but really the rest of them as well. He proposes a couple of thought experiments to try to get around this to show why this can't quite be right, or at least can't be the whole story. He is misunderstood basically right off the bat, and this goes on for years.

0:16:40 SC: Well, there's this popular idea that Einstein just couldn't handle or couldn't keep up with quantum mechanics.

0:16:45 AB: Yeah, and that's... It's such a strange myth because...

0:16:50 SC: It's the opposite of true.

0:16:51 SC: Yeah, it's the opposite of true, which usually we call that false but, I mean, Einstein, first of all, was one of the founders of quantum mechanics. His 1905 paper on the photoelectric effect postulated the existence of quantized packets of light photons, and that's hardly the only work he did on quantum mechanics throughout his career. He did a lot of very, very important work on quantum mechanics both before and after... Though probably less after, the full modern formulation of quantum mechanics shows up in the mid-1920s. There is no evidence whatsoever that he couldn't keep up with it, he definitely understood it.

0:17:32 SC: Yeah.

0:17:34 SC: He spent a lot of time thinking about it. He once said that he spent more time thinking about quantum mechanics, or I think the phrase was that he used up more brain grease on the quantum mechanics than he did on relativity, which is what he's more known for. I mean, hell, he got the Nobel Prize for his work on quantum mechanics, not for his work on relativity, which is a weird historical quirk, and Nobel Prizes aren't that important, anyway. But yeah.

0:18:04 SC: So he was upset. And I think it makes perfect sense if you don't know a lot about quantum mechanics, I think you should sympathize with why he was upset at the Solvay conference, both because there are these two different rules that you use under different circumstances, and just because it all seemed so ill-posed, right?

0:18:21 AB: Yeah.

0:18:22 SC: The whole point of physics is to be precise, and beautiful, and exact, and this hand wavy thing about making measurements somehow sneaked in.

0:18:30 AB: Yeah, yeah. I mean, he... Einstein had, what was at the time, a fairly common-sense view of what physics was about. He said, "Physics is about describing what's in the world, and now you're telling me, 'No, physics is about predicting the outcomes of experiments. What's an experiment? What... If it's really only about predicting the outcomes of experiments, then how are we supposed to use it to explain things that happen in the natural world when we're not around?'" which people were doing already at the time. Quantum mechanics has always been used to do more than just predict the outcomes of experiments. It has also been used to explain all kinds of very interesting natural phenomena, ranging from stuff as basic as, why does the sun shine? To why is it that silicon behaves in this funny way, where it's not really an insulator and it's not really a conductor, it's a semi-conductor, right?

0:19:35 SC: Yeah.

0:19:35 AB: And that lets us build computers. So everything from basic fundamental facts about the world around us, like why I can't pass my hand through the table to all sorts of strange and wonderful modern technology like LEDs, or most of this recording equipment.

0:19:54 SC: And I thought... You mentioned this very quickly, but I wanna dwell on this philosophical leap made by the Copenhagen interpretation.

0:20:02 AB: Yeah.

0:20:02 SC: So if you really say that the job of your physical theory, in this case, is to simply predict the outcomes of experiments. They said this... The advocates of Copenhagen, as far as I can tell, were sometimes more explicit about this than others, they were never completely on the same page, but there's a version of it which really says that and it's almost solipsistic, idealistic, and if you really push it, you're saying it's all in the mind.

0:20:29 AB: Yeah.

0:20:29 SC: You're saying the world, as something that exists external to me, is irrelevant to physics. What is important to physics is the predictions that I can make and the observations that I can make.

0:20:41 AB: Yeah, yeah, I think that that is definitely one way to take what they were saying. I don't think that there is any single Copenhagen interpretation. And while Niels Bohr and Max Born and Pauli, and Heisenberg and the others may have each had their own individual positions. I don't think that you can combine all of those to make something coherent. And in some cases, like with Niels Bohr, it's not even clear what each individual's position was. There's no consensus on what...

0:21:20 SC: Even within one paper...

[overlapping conversation]

0:21:21 SC: Exactly, yeah, yeah. I... Speaking of people being mad at me, this is something that some people are mad at me for, they say, "But you said the Niels Bohr had this position?" I'm like, "No, I didn't, I didn't say that Niels Bohr had any position. I don't know what position he had and neither this anybody else."

0:21:35 SC: I just quoted him a few times in my book because if you tried to translate what he says or paraphrase, people are gonna be mad at you.

0:21:42 AB: Yeah, exactly. He was a brilliant physicist, and a very interesting man, who did lots of wonderful things and he just was not a particularly clear writer or speaker.

0:21:54 SC: Well, David Albert, when I interviewed him for the podcast, he had a wonderful take on Niels Bohr. He said that Niels Bohr was the person in history who he would most like to have met Personally.

0:22:03 AB: Yeah.

0:22:04 SC: And his reason why is because people would meet him, and they would come back after talking to Niels Bohr. And number one, they would say, "This was the most brilliant, wonderful, transcendentally wise human being." And they would start spouting complete nonsense about the foundation of quantum mechanics.

[laughter]

0:22:23 SC: So how can one person have this effect?

0:22:25 AB: Yeah, no, it's a real question. I mean, I don't know. I don't know if he's the person in history I'd most like to invite to dinner, but I'm certainly very curious about himself.

0:22:36 SC: But they did win the PR battle.

0:22:37 AB: Yes, they did.

0:22:39 SC: After Solvay, it more or less became standard conventional wisdom that the Copenhagen interpretation was how we think about quantum mechanics. And there were a few people who didn't like that, like Einstein, also Schrodinger.

0:22:51 AB: Yeah, that's right. Schrodinger did not like it.

0:22:53 SC: But they became increasingly marginalized at least in terms of how we think about quantum mechanics.

0:22:57 AB: Yeah, that's exactly right, yeah, people thought Einstein and Schrodinger were outta touch and kooky. And maybe they were kooky, but they were not wrong, I think, to have doubts about this, but their work was increasingly outside of the mainstream of the work that the rest of the physics community was engaging in, not in the sense that, "It was wrong or fringe." but in that the rest of the physics community was just not thinking about these things, they were working on other things. And part of that was for really obvious historical reasons, this debate was going on in the early to mid 1930s, mostly in Europe, there were some things, also going on in the 1930's in Europe, into the '40s that were somewhat distracting to a lot of people.

0:23:51 SC: And physicists, in particular, had other things to worry about.

0:23:54 AB: Indeed. Yes, they did, yeah. So by the 1940s, people were working on other things, such as the Manhattan Project. And then after the Manhattan Project, after the war, suddenly there was this realization due to the... I hesitate to use the word "success" for the Manhattan Project, but due to the achievements of the Manhattan project, there was recognition by governments and industry and the military that there was a lot of value to be had in funding massive amounts of applied physics. And so rather than worrying about the foundations of quantum mechanics, this influx of money and the people who came along with that were concerned with working out implications of quantum physics, which is a good thing to do. There was a lot of really, really great science done in that period. And I do think that if the attitude had been, "Okay, you know what? There is a problem here with the foundations, we'll come back to it later. There's something that we need to figure out, but in the meantime, we have a lot of really interesting work to do, and maybe that'll help us understand the theory better. And that will shed more light on things." If that had been the attitude, I wouldn't have had a book to write.

0:25:18 SC: Yeah.

0:25:20 SC: And I think that's a very sensible attitude, there have been many other theories in the history of science that have worked that way. Newton himself talked about his own theories in this way sometimes. But that's not what happened. Sorry. That's not what happened, instead it was, "Oh, those problems are solved, we don't have to worry about the foundations, now we can work on these applications of the theory." And...

0:25:50 SC: Sorry, the foundations, we mean, is the correct attitude towards this measurement problem really just to only ask about measurement outcomes or is there some other version of quantum mechanics? Some other way of thinking about things that is more traditionally physically scientific?

0:26:08 AB: Exactly, yeah. The... With this influx of money and people that came after World War II, questions like, "But what does the theory mean?" were swept under the rug. And there's been a lot of really good work on how the changes in physics after World War II led to changes in the way that quantum mechanics was taught. That's been done by the historian of science, David Kaiser at MIT, who has gone back through physics curricula and textbooks from the time and found that the larger the classroom the less time spent on the meaning and foundations of the theory and the more time spent on applications. And I think that makes sense, it's easier to teach people how to solve a partial differential equation than it is to talk about these thorny philosophical issues at the heart of the theory. So...

0:27:08 SC: I do think that we're gonna get... We'll get to the plucky band of [0:27:13] ____ did wanna keep thinking about the foundations...

0:27:15 AB: And blew up the death star.

0:27:17 SC: But let me try... Since my usual spiel is what a huge mistake it was to ignore the foundations, let's try to be charitable to the people who did ignore the foundation.

0:27:26 AB: Absolutely.

0:27:26 SC: As you said, there's a lot of applied physics to do. There was even non-applied physics, meaning when Murray Gell-Mann is working out the SU3 symmetry group, the Eightfold Way... There's still applications of this to the military or anything like that. But so what would someone back then have said if you said, "Why aren't you thinking about quantum mechanics as a foundational theory?"

0:27:49 AB: Well, I think that answers would have varied, right? One answer that someone could have given was, "But we are." Take a look at, I think this is the late 1940s, early 1950s, speaking of non-applied foundational work of a different kind. Take a look at what happened with... How do I describe this without using jargon? There was this little shift in the way that atoms emitted light called the Lamb shift.

0:28:34 SC: The Lamb shift, right. So a slight miscalculation between what you predicted and what you saw.

0:28:38 AB: Exactly, yeah, and at first, people just thought it would go away, and then, as experimental techniques and theory got better and better, the mismatch got more and more obvious until it became clear there was a real problem. And this led to the development of a new foundational theory of quantum physics called quantum electrodynamics, and this is what the single piece of work that Richard Feynman is most well known for, as well as Julian Schwinger...

0:32:38 SC: Tomonaga.

0:32:38 AB: Tomonaga, and...

0:32:38 SC: Freeman Dyson.

0:32:38 AB: Freeman Dyson, yeah, exactly, and...

0:32:38 SC: But, okay, to be fair, that could be classified as a particular theory within the quantum mechanical umbrella rather than the foundations of quantum mechanics.

0:32:38 AB: Sure, that's true, but if you would asked these people...

0:32:38 SC: It had nothing with the measurement.

0:32:38 AB: That's right, it had nothing to do with a measurement problem, but it certainly wasn't really applied, right? So if you asked those people, "Why aren't you paying attention to the measurement problem?" They might have said something like, "The theory works very well, this has all been worked out, and we're focusing on real problems, not chasing phantoms." So that's one possible answer. Another possible answer would be to say, "Well, what do you mean?" Rather than to say, "The problem's been solved." They would say, "No, there was never a problem there," right? It's not a problem.

0:32:38 SC: Right.

0:32:38 AB: They might take some version of the Copenhagen interpretation and really swallow it entirely and say, "No, it really is all about these observations that we make, that's really all that matters." We're very good at doing physics, physicists are very good at doing physics, we're not always good at describing how we're doing physics.

0:32:38 SC: Oh, we're terrible.

0:32:38 AB: Yeah, exactly.

0:32:38 SC: That's okay. Those who cannot do, teach. Sometimes those who can do don't know how to teach.

0:32:38 AB: Right, exactly, but yeah. I mean, just the same way that we deny, "Oh, things outside of the realm of science have no influence on science itself." You can see somebody saying, "Oh, no, no, no, no, physics is definitely all about the outcomes of measurements," and then they'd go off and do something that's clearly not about the outcomes.

0:32:38 SC: But no one ever said that before quantum mechanics, right? I mean, I guess Mok might have said that. They were philosophers of old.

0:32:38 AB: Yeah, Mok said things like that. There were philosophers. Yeah, and Mock was both a philosopher and a physicist. Yeah. So yeah, people definitely did say things like that beforehand. In fact, some people thought that that's what Einstein had been saying in 1905 when he came up with special relativity, and that was a big inspiration for them. Though, if you then go back and take a look at Einstein's other work from 1905, it's very clear that actually, that's not really the kind of thing that he thought.

0:32:38 SC: Okay, speaking of which, Einstein, and also Schrodinger who was on his side, they didn't give up post-Solvay conference. And so their not giving up was a huge boon to human kind in the sense that they basically invented entanglement, right?

0:32:38 AB: Yeah, I mean. Yeah, it forced the rest of the physics community to understand that entanglement was maybe the defining trait of quantum mechanics.

0:32:38 AB: You should tell us what entanglement is.

0:32:38 AB: Yes, that... Well, so that's another thing where no matter how I describe it, it's going to be picking some interpretation. So let me see what I can do.

0:32:38 SC: So pick the right one.

0:32:38 AB: Yeah, pick the right one, clearly. Yeah. The joke here, of course, is that you have a preferred interpretation of quantum mechanics, and I just have an anti-preferred interpretation of quantum mechanics. [chuckle] But yeah, so entanglement, quantum mechanics makes it very clear that under really a wide variety of circumstances, you can have two things that are widely separated.

0:32:55 SC: Particles or whatever.

0:32:56 AB: Particles or whatever really, that... Despite the fact that they are very far apart, they could be on other sides of the solar system, and despite the fact that there's no obvious connection between them, there are instantaneous correlations in experiments that you conduct on each of the two of them. And you can account for that in several different ways, [chuckle] and that's another one of the things that's at stake in this interpretation debate.

0:33:29 SC: Right. So the reason why Einstein... So, yeah, you see two particles far away, quantum mechanics says that when we measure them, all we do is predictive probability for getting certain outcomes. But what Einstein and Podolsky and Rosen say, that the probability of getting one thing is deeply affected by doing a measurement on the other one, even if they're really far away.

0:33:49 AB: Exactly.

0:33:50 SC: And, this seems to violate at least the spirit of Einstein's Theory of Special Relativity.

0:33:56 S?2: Yes.

0:33:56 SC: So you can see why he'd be upset about it. But it doesn't actually let you send signals faster than the speed of light or anything like that.

0:34:03 AB: Yeah, you can prove that you can't use it to send signals like that. It looks like an instantaneous influence, it doesn't look like something that you could use for instantaneous communication.

0:34:15 SC: And so why did Einstein think this was a big deal?

0:34:17 AB: Einstein thought this was a big deal because he saw no reason to believe in any kind of instantaneous influence. He said, "Well, if we really wanna say that we can't talk about what's going on before we make measurements, then we have to say it's an instantaneous influence, because you make a measurement, way over here and you make a measurement way over there, and they have correlated outcomes all the time. So if it really, if there was really no fact of the matter before you made the measurement, then they must have been conspiring immediately across great distances." "Or," he said, "You could do the reasonable thing and say that, "No, when you first sent them off in different directions, they already had a agreed-upon set of properties. And so that lets you get around this without saying that the two objects have the long distance conspiracy going on."

0:35:14 SC: So even though we couldn't predict what our measurement outcomes were, the particles knew what their measurement outcomes were going to be.

0:35:19 AB: Exactly, so Einstein, says, given that the two options are something totally normal like that or this long distance what he called "spooky action at a distance," he chose door number one.

0:35:33 SC: Yeah.

0:35:34 AB: And given that those were apparently the two options on the table at the time, that seemed like a reasonable thing for him to do. And the response that he received was not great. Niels Bohr wrote a famously impenetrable response, which he later apologized for how poorly written it was, but then didn't really proceed to elaborate on what he had meant.

0:36:00 SC: I was told that part of it was even printed in the wrong order, is that a true story?

0:36:03 AB: Okay, so right, so the printing in the wrong order, this is really inside baseball. So it was not originally printed in the wrong order. However, in the dark days before the Internet, if you wanted to find Bohr's reply to the EPR paper, you... The Einstein, Podolsky, Rosen paper, you had to go find it in a book. And the book where it was most famously and widely available was a book from the early 1980s called, I think, Quantum Theory and Measurement, it's a big, red book. And in the first... I think, in all of the first edition printings of that book, two pages of Bohr's reply were swapped and it's only a four or five per reply. So that's a pretty big difference. And the story which is, I think, impossible to verify, is that nobody noticed for years.

0:37:00 SC: Yeah, 'cause they couldn't understand it anyway, what difference does it make what order the pages are in?

0:37:02 AB: And also Bohr's reply served this very important social function just by existing, because people could say, "Oh, you don't have to worry about that EPR thing, Bohr worked it out, it's on that page." And so people didn't actually read it, it was just there. But yeah, you can now, of course, download it online and whatnot. But a story that I know is true, 'cause I don't know if nobody noticed it, it definitely was printed in the wrong order. When I first read it, I first read Bohr's reply, I guess it was 2005, so I did not have to get it from that book, I did get it from the Internet, I have the pages in the right order, I printed them out, I read it. And then I went to the professor that I was working with at the time and I said, "This is a really bad translation, is there a better translation available into English?" 'cause I thought, "Well, Bohr wasn't a native English speaker, maybe this was written in Danish."

0:38:00 SC: I see where this is going.

0:38:01 AB: Yeah, and then the professor told me, "No, no, Adam, he wrote this in English."

0:38:07 SC: Yeah, this is the original.

0:38:08 AB: Yeah, so yeah, it's just a fabulously impenetrable reply.

0:38:14 SC: But like you say, it let the rest of the community say, "Oh yeah, Einstein's objection... " And by the way, Einstein's objection was not "Quantum mechanics is wrong." he thought that it was just a stepping-stone towards a bigger theory that we would someday have.

0:38:26 AB: Exactly, yeah, he said that it was simply incomplete. And in fact, I think that was the title of the paper, "Can quantum mechanical description of reality be considered complete? Not correct." So yeah... No, people thought that Einstein was wrong. Einstein actually wrote a letter to Schrodinger shortly after the EPR paper came out saying that he had gotten letters from a couple dozen different physicists from around the world explaining why he was wrong, and none of them agreed with each other. So yeah, Einstein and Schrodinger laughing at everybody else behind their backs. Schrodinger meanwhile writes a paper in support of the EPR position saying, "Look, this is this long-distance connection, this is a fundamental property of quantum mechanics, it's everywhere, and you can't really get around it, and to show you another example of the same thing, let's talk about a cat in a box with radiation and a vital sign." And this is where the famous Schrodinger's cat thought experiment comes from.

0:39:34 SC: That's right.

0:39:34 AB: And so Schrodinger points this out as he comes up with this thought experiment to explain why there must be some deeper fact about the world, or so he thought, that quantum mechanics doesn't capture. Because otherwise, maybe you can have a particle that's not in any particular state before you look, but the Schrodinger's cat thought experiment says, "Well if you have a particle in that state, you can set up an experiment where a cat is neither dead nor alive before you look." And saying that a particle is in... A subatomic particle that we have no direct experience with is in neither one state or the other, that's one thing, but cats are either dead or alive. There are cats in this apartment right now, I haven't seen them. I am sure that they are either dead or alive.

0:40:28 SC: Yes. In fact, they're alive.

0:40:30 AB: Yes.

0:40:30 SC: Okay, yes.

0:40:30 AB: Yeah.

0:40:31 SC: But okay, just get the philosophy on the table here.

0:40:34 SC: Yeah.

0:40:35 SC: Einstein and Podolsky and Rosen make this point about spooky action at a distance. But they didn't just say, "And that's obviously crazy." Like Schrodinger with his cat, his argument was really at the level of, "That's obviously crazy, two of you don't believe that.

0:40:49 AB: That's true, that's true. No, the EPR thought experiment in that respect is more rigorous.

0:40:53 SC: EPR was a little more rigorous. They wanted to make the case that if you believe that there really is something actually happening at every location in space in ways that... They tried to make carefully defined and so forth, then quantum mechanics couldn't be complete. And this is gonna skip ahead a little bit, but then John Bell comes along and says that, "Okay, Einstein is basically sketching out an aspiration. Some day we'll have a better theory that explains all this without spooky action at a distance, and Bell basically proved that no such theory can ever reproduce the predictions of quantum mechanics."

0:41:28 AB: Yeah, Bell... People have called Bell the person who proved Einstein wrong. People have also called Bell the person proved Einstein right. I think that neither of those are really correct, though I'm more sympathetic to the second one, because what Bell proved was that Einstein was right to be worried, but that his proposed solution couldn't work. Because you can modify the EPR thought experiment in this very subtle and brilliant way that will basically give you a real experiment that you could build, and the results of that experiment, if they conform with the predictions of quantum mechanics, can't be accounted for with the pre-existing answers that Einstein had in mind for how these particles were arranging to have these long distance correlations.

0:42:29 SC: Right. And Bell, so is an example of how it was to try to do foundations of quantum mechanics. We're talking about '60s and '70s now. And he was a perfectly respectable particle theorist at CERN, at the laboratory, where we discovered the Higgs Boson a few years ago. And correct if I'm wrong, but he basically hid the fact that he was working on the foundations of quantum mechanics from his colleagues at CERN.

0:42:53 AB: He didn't... I don't know, "Hide" is a little bit strong.

0:42:57 SC: He didn't advertise it.

0:42:57 AB: He certainly didn't advertise it. Yeah, there is this nasty... Well, "nasty" is probably a little strong. There was certainly whispers about John Bell at CERN that, "Oh, he did something important in quantum foundations, but don't worry about it because quantum mechanics works anyway."

0:43:15 SC: Yeah. [chuckle] Don't hold it against them.

0:43:17 AB: Yeah, exactly, yeah. One of Bell's good friends, who he worked with on the other work he did, not quantum foundations. Bell's everyday physics work was in quantum field theory, and one of Bell's good friends was Martinus Veltman. And the story is that Veltman one day said to Bell, "You did this thing in quantum foundations, do I need to worry about this? Will it affect my work in quantum field theory at all?" And Veltman is a very good quantum field theorist.

0:44:01 SC: He won the Nobel prize.

0:44:02 AB: Yes, he did win the Nobel prize, yes. And so Veltman asks Bell this question, and Bell says, "No, don't worry about it, it's not gonna affect your work at all." which is true. And so Bell, yeah, he certainly didn't advertise that he was working on this stuff. He also didn't work on it in a serious way at great length until well after his career was established and safe. He thought about this stuff in college, and then he thought about it a little bit in early... In his early graduate work, and then he was dissuaded from working on it and so he put it off to the side and came back to it later.

0:44:47 SC: And a lot of people... Even to this day, there are physicists who will claim that what Bell proved was that you can't have what are called hidden variables.

0:44:56 SC: Yeah.

0:44:57 SC: That you can't have secret new values... Secret new parameters of physics that as Einstein had hoped, would fix the outcomes of future experiments even if we human beings don't know about it.

0:45:10 AB: Yeah.

0:45:11 SC: But that's not what Bell proved. In fact, we have to talk about David Bohm, who is certainly one of the more interesting characters in the story.

0:45:19 AB: Yes, he is. Yeah. So no, Bell certainly didn't prove that you can't have these hidden variables like Einstein wanted. Bell proved that you can't have a particular kind of hidden variables.

0:45:34 SC: Ones without spooky action?

0:45:35 AB: Ones without spooky action at a distance. And really what he proved was not really about hidden variables at all, it was really saying that you can't have a theory without spooky action at a distance of some kind unless you break something even more fundamental.

0:45:55 SC: Which we'll get to later.

0:45:56 AB: Which we'll get to later. Yeah, exactly.

0:45:57 SC: My man whoever, broke all the day.

0:46:01 AB: Yeah, no, I think the way I describe it in my book is he showed that you either have to have spooky action at a distance or something even weirder.

0:46:07 SC: Yeah.

0:46:11 AB: But yeah, so David Bohm, right? So David Bohm was, in a lot of ways, the inspiration for Bell's work, and someone whose work Bell admired, very clearly. David Bohm was a student of Robert Oppenheimer, the guy who was in charge of the Manhattan Project, though he himself only... He didn't follow Oppenheimer to Los Alamos.

0:46:41 SC: Why is that? [chuckle]

0:46:42 AB: Well, he didn't follow Oppenheimer to Los Alamos because David Bohm had been briefly a member of the Communist Party in Berkeley, where he was doing his PhD work, and so when he applied for security clearance to go to Los Alamos, he was denied that clearance. They did not tell him that this is why they denied him the clearance, they lied to him and told him it was because he had relatives in Europe who could be held as hostage against him, which while true in theory, was definitely not the real reason as documents uncovered well after Bohm died proved. But yeah, Bohm did do some important work that was relevant to the Manhattan Project, for his PhD, which was promptly classified and thus removed from his apartment forcibly by military police because he didn't have clearance to have his own work. And so...

0:47:40 AB: It was a crazy time. We're talking about the early '40s.

0:47:42 AB: Yeah, yeah, this is '41 or '42, something like that... Maybe more like '43. It doesn't matter. The point is it's during World War II, it was a crazy time, like you said. And yeah... No, he didn't have any of his research notes, Oppenheimer had to go to the UC Berkeley administration and say, "You have to give him a PHD anyway, just trust me." Which they did, which was good. Bohm had no problem with the Copenhagen interpretation at this time. Oppenheimer was a great admirer of Bohr, Bohm was a great admirer of Oppenheimer, and so he didn't really think too critically about it, and it seemed to work for him. Then he started teaching quantum mechanics classes, so teaching is what's gonna get you in trouble, clearly.

0:48:32 SC: Well, clearly not usually, since most physicists seem to have no trouble ignoring the foundations even when they teach quantum mechanics.

0:48:38 AB: That's fair, that's fair.

0:48:39 SC: But Bohm was a thoughtful guy.

0:48:40 AB: Yes, Bohm was a thoughtful guy. He was teaching quantum mechanics classes at Berkeley out of Oppenheimer's research notes... Or, sorry, lecture notes. And then he got a position at Princeton and continued to teach out of a mix of his own notes and Oppenheimer's notes, and started turning that into a textbook. And that textbook he wrote was trying to give the best version of the Copenhagen interpretation that he could, and make it as clear as he could make it. And in the process of trying to do that Bohm's faith in the Copenhagen interpretation just plummeted, until by the end of the process, he was just completely plagued with doubt.

0:49:23 SC: The more you think about it, the less sense it makes.

0:49:25 AB: Yes, which... I certainly agree. So then Bohm went and met with Einstein, who had looked at Bohm's textbook once it came out, and called Bohm into his office and said, "Look, you wrote this book, and how do you feel about it?" And Bohm said, "I'm plagued with doubt." And Einstein said, "Look, that's because you're trying to defend an indefensible position. You did the best job that you could do, but no one can do it because it doesn't work." And so Bohm walked out of that meeting thinking, "Can I find another way to look at this? Can I find another way to think about quantum mechanics?" And he did. He independently rediscovered a set of ideas that Louis de Broglie, one of the other founders of quantum mechanics, had come up with back in the late 1920s, and then finished the work that de Broglie started, and put together this theory which goes by a bunch of different names, Bohmian mechanics, de Broglie-Bohm theory, pilot wave theory. I like calling it pilot wave theory 'cause that's descriptive of the content of the theory. But...

0:50:39 SC: Also, by the way, when de Broglie actually tried to present his theory at the Solvay Conference, and he was just hectored out his own theory for no good reason.

0:50:49 AB: Yeah, basically, yeah. I mean, it is true that the job wasn't done, but he could have finished it and he didn't, yeah.

0:50:58 SC: So Bohm, okay. Bohm finished that, took up that mantle.

0:51:00 AB: Exactly, Bohm took up that mantle, puts this thing together relatively quickly, and sends it off for publication. The problem is that at the same time that all of this was happening, Bohm's past was catching up with him. He got called up in front of the House on American Activities Committee, and testified in front of it to a committee including Richard Nixon, who was a congressman at the time, and he was asked to name names and tell people who the other people in the Communist Party in Berkeley during World War II were. He wouldn't do it, he was held in contempt of congress, he was arrested in his office in Princeton, and by the time he got back to campus after his friends posted bail, he had been suspended and banned from the Princeton campus. And he was working on all of these ideas during that suspension, and...

0:52:03 SC: What year are we talking now?

0:52:04 AB: This is 1951. Yeah, yeah, yeah. This is 1951. So while he's suspended, he works on these ideas about quantum mechanics and then has his day in court, he's cleared on all charges because it turns out that you don't have to name names, there's... He had pled the First and the Fifth Amendments, there is a First and a Fifth Amendment.

0:52:32 SC: Federal constitution.

0:52:32 AB: Yeah, exactly. Yeah. So he was cleared on all accounts, but Princeton effectively fired him after that, and he couldn't get a job anywhere else, even though he had recommendation letters from Einstein and Oppenheimer. He couldn't get work doing academic physics anywhere in the US or Europe much to his disappointment. Finally he gets a position in Brazil, and so toward the end of 1951, he goes down to Brazil, and shortly after arriving he's summoned to the US consulate where they illegally confiscate his passport and tell him that he can only have it back if he goes back to the United States, which he doesn't wanna do 'cause he's worried that he's gonna be arrested again and maybe brought up on false charges or something. So he's trapped.

0:53:24 SC: But now it's the height of the Red Scare.

0:53:25 AB: Exactly, it's the height of the Red Scare and the McCarthy Era, so Bohm is trapped in Brazil. He can't get out. He was planning to give a bunch of talks in support of his ideas because his papers were due to be published imminently, just a couple months later. So when they are finally published in very early 1952, Bohm can't defend his ideas through anything other than writing letters to people. And so mostly, he's just ridiculed. Someone... One of the quotes that I found in my research for my book, which I liked the best, was someone said that Bohm had... Something to the effect of "had a very illustrious set of people sticking knives in his back all year long" in 1952, which yeah, it's absolutely true. Wolfgang Pauli, Werner Heisenberg.

0:54:25 SC: Yeah, these are not the political knives of being a communist, these are physics knives of like, "You're a crazy person who doesn't understand quantum mechanics."

0:54:32 AB: Exactly, yeah, but the fact that he was a communist also didn't help, not only because he couldn't leave Brazil, but because at that time, most physics funding was coming from the military, and it was the height of the Red Scare. And so if someone in your physics department was suspected of being a communist, that could turn off the faucet of funding, and that's a scary thing. So yeah.

0:54:58 SC: And basically, to make it very, very brief, his theory, the pilot wave theory was the hidden variable theory that Einstein wanted, except rather than avoiding spooky action at a distance, it... So spooky action is doing all the work here.

0:55:14 AB: Yeah, it's riddled with spooky action at a distance.

0:55:16 SC: There was a great quote from Bell, saying that he resolved the EPR paradox in the way that Einstein would have liked the least.

0:55:22 AB: Yeah, something like that, yeah, it's a really... Another quote from Bell, actually, about Bohm's theory, was that if you shook a magnet right here, it would instantaneously affect the position of every single electron anywhere in the universe, which is not a pleasant thing to consider as a physicist, doesn't mean it's wrong.

0:55:55 SC: Isaac Newton would have been perfectly happy.

0:55:57 AB: Exactly. Yeah. Newton would have been fine with it. Obviously, the influence is very, very small.

0:56:03 SC: And by the way, this hasn't gone away, this Bohmian mechanics, pilot wave theory is still one of the leading contenders for a sensible interpretation of quantum mechanics, not so much among physicists but among philosophers and people who work on foundations. And how would you characterize the state of play with reconciling these kinds of ideas with quantum field theory and relativity?

0:56:26 AB: Yeah, I mean the fundamental problem with pilot wave theory, Bohmian mechanics, whatever you wanna call it, is that it doesn't seem to play nice with relativity for exactly these reasons. Relativity doesn't have a preferred frame of reference, it doesn't let you have things go faster than light, Bohmian mechanics seems to violate that. I would have a much better answer to your question about the state of play in about three hours. I'm getting dinner with Chip.

0:56:58 SC: Oh Stevens? My collaborator, yes.

0:57:01 AB: Yeah, but...

0:57:05 SC: We rule out the pro-causality here in this podcast, so that's not a version of quantum mechanics we like. So information you will get in the future cannot propagate back to us here.

0:57:12 AB: Yeah, it can't be used here.

0:57:14 SC: So the way that I like to put it, I'm just trying to check... Reality check myself. I'm not an expert on Bohmian mechanics.

0:57:19 AB: Yeah.

0:57:20 SC: People have tried to make it compatible with relativity and quantum field theory. There have been yeoman-like efforts.

0:57:27 AB: Yes.

0:57:27 SC: I don't think there's been anything completely convincing.

0:57:29 AB: I think that's correct. If you ask them why the problem is there, they will say something like, or at least some of them will say something like, "Well, quantum field theory actually has its own foundational issues independently of the measurement problem in quantum mechanics." which is true. And that makes it very, very difficult to come up with a Bohmian formulation of quantum field theory because it's not clear what quantum field theory you want that can account for all the different phenomena that quantum field theory accounts for. I don't quite know what to make of that.

0:58:17 SC: I think it's a great example of how non-algorithmic physics really is.

0:58:22 AB: Yes.

0:58:22 SC: Not the results you get from physics with the process of doing it, right? You have all of these sorts of puzzles, things we don't yet know the answer to, and some of them, you have to say, "Oh don't worry, that'll be figured out." Whereas others, you have to say "This is really important, we should really focus on solving this." And different people are gonna make different choices.

0:58:41 AB: Exactly, yeah. No, physics is definitely subject to contingency and personality and random fate.

0:58:51 SC: For 50 years, the physics as a field, made the choice that the foundations were not worth worrying about.

0:58:56 AB: Exactly. And even now, I'd say that the standard position is, "Don't worry about it." It's not as rabid as it was before. It's not, "You shouldn't worry about it." It's "Don't worry about it."

0:59:11 SC: I think it's still pretty rabid, but we don't need to talk too much about Everett because it's my podcast, the audience will get plenty of Everett over the years. But interestingly, he was there at Princeton in the early '50s, did they overlap? Everett was a student and Bohm was an assistant professor.

0:59:28 AB: Yes, that's correct. They didn't overlap, they just missed each other by something like two years. So I think Everett shows up at Princeton as a grad student in, I want to say, '53 or '54, and Bohm is gone by the end of '51.

0:59:46 SC: I wonder if all of this work on quantum foundations at Princeton in the early '50s wasn't ultimately Einstein's fault. Einstein certainly influenced Bohm. He apparently taught a class like a mini lecture.

1:00:04 AB: He gave a lecture that Everett was at, though Everett doesn't...

1:00:10 SC: Doesn't remember? [chuckle]

1:00:10 AB: Yeah, he didn't remember later in life. On the other hand, other people say that Everett was there. It makes sense that Everett was there, and also, I don't know, Everett was a funny guy.

1:00:20 AB: Yeah, he was a funny guy.

1:00:21 AB: And people forget things but, yeah... And, certainly...

1:00:25 AB: And there was a lot of sherry consumed. [chuckle]

1:00:26 AB: Yes, there was a lot of alcohol of various kinds. It's absolutely true. But yeah, Everett certainly read a great deal of Einstein. Everett also talked with Eugene Wigner, who was someone who was at Princeton at the time, who harbored his own kinds of doubts about the Copenhagen interpretation. And so I think he was definitely an influence on Everett and probably vice versa. And yeah, Einstein was there. And the other thing is Princeton, at that time, was one of the best places to be doing physics in the world.

1:01:07 SC: Smart people would end up there.

1:01:08 AB: Exactly, yeah. And I think that smart people are more likely to question these truths that they've just been handed on a platter. Which is not to say that if you question those truths that makes you a smart person.

[laughter]

1:01:26 SC: There's a lot of physics to get through here, so I don't wanna dwell to much on Everett 'cause we'll get there otherwise, but Bohm suggested an answer to the measurement problem, mainly, they were hidden variables, we just don't know their values. And so measuring is revealing some truth about nature that we hadn't known before. And Everett suggested a completely different one, he gives up on... And so what Bohm gives up on is locality in a sense, he allows for spooky action at a distance.

1:01:54 AB: That's right. Yeah, it's certainly a little bit more complicated than measurements simply reveal things that were already there because measurements can still influence things but it's a much less mysterious process.

1:02:09 SC: And Bohmian mechanics hearkens back to the classical paradigm, where there are definite values of things and we measure them.

1:02:17 AB: Yeah, in some ways.

1:02:18 SC: In some hearkening, reaching academics.

1:02:18 AB: Yes, there's definitely some hearkening. Yeah, it's true.

1:02:22 SC: Whereas, Everett takes completely the opposite point of view. He says, "There's no hidden variables, there's just this wave function. We should take it seriously. There's not two different ways of evolving, there's only one way of evolving." And the price he pays is that when you measure something, the universe branches into multiple copies.

1:02:39 AB: Yes, yeah, which is this radical and deeply strange solution to the problem but, again, deeply strange is not a strike against things.

1:02:49 SC: Exactly. And Bohm was chased out of the country and then stabbed in the back.

1:02:56 AB: Yes.

1:02:57 SC: Everett took his golden parachute. He didn't even try to get a job as a physics professor.

1:03:03 AB: Yeah, he was certainly not happy with the way that his work was received. Everett's mentor and advisor at Princeton was the great physicist, John Wheeler, and Wheeler was one of Bohr's most devoted students. And so when he saw Everett's work, he really liked Everett's work at first but he wanted it to get the blessing from the master, to get the blessing from Bohr. And Bohr and...

1:03:36 SC: That was never gonna happen.

1:03:37 AB: Yeah, that was never gonna happen. Bohr and his inner circle never gave it that blessing because, of course, they didn't. And the whole experience was definitely a traumatizing one for Everett, but on the other hand, Everett was never gonna stay in academia.

1:03:52 SC: That's my impression.

1:03:54 AB: Yeah, it wasn't where his passions were.

1:03:55 SC: He wasn't chased out.

1:03:56 AB: Yeah, he wasn't chased out. If he had wanted to stay, he might have had problems. I don't know. Wheeler was still willing to go to bat for him but Wheeler made Everett make a lot of revisions to his ideas in his PhD thesis to try to make Bohr happy. It didn't work, of course. And then Everett did what he was always going to do, which is, he finished his PhD and went and got a job working for the military industrial complex, doing essentially war gaming.

1:04:26 SC: Simulations for nuclear bomb falling.

1:04:30 AB: Exactly, yeah.

1:04:32 SC: So there is this brief moment in the 1950s when a couple of people at Princeton thought deeply about the foundations of quantum mechanics. And it's fairly safe to say that their efforts had zero impact for the next few decades.

1:04:48 AB: Didn't have big impacts for a decade or two. Yeah, that's true. And I'm also... These are not the only people who thought deeply about the foundations of quantum mechanics. They're just the ones whose work ended up being the most influential in the coming years. But yeah... No, people didn't... There wasn't a lot of widespread open talking about the foundations of quantum mechanics, there wasn't a lot of research done on it. The work that Bohm did and that Everett did was probably the most notable work done in the quantum foundations at that time. And then Bell shows up, Bell is one of these people who had never been happy with the story that he'd been presented with about quantum mechanics. And then he got into fights with his college professors about it.

1:05:40 AB: And then shortly before finishing his university studies in 1949, shortly before finishing college, and he's in Northern Ireland, he's at Queens College in Belfast, I think. The point is, he's at college, he's almost done, and then he comes across a book by Max Born, in which Born describes, among other things, a proof from the great and mighty physicist, John von Neumann. And John von Neumann is one of the giants of 20th century math and physics, and so if there's a proof of something by John von Neumann, it's almost certainly correct. And Born says that von Neumann proved that this way of understanding quantum physics, as fundamentally driven by chance, that happens... Like these weird random chance events that happen when you make measurements is the only way to think about quantum mechanics.

1:06:45 SC: Yeah, so it can't really be deterministic hidden variables.

1:06:47 AB: Right, exactly, there's no other way to think about it. And so Bell is very impressed by this and he wants to go look at the proof but he can't because at the time it's only available in German. Bell doesn't speak German but he thinks, "Okay, I'd better put this down because von Neumann is right, and if I keep thinking about this, I'm just gonna fall down a well and I'm never gonna come out." So he goes off and does some work in accelerator physics, and then while he's working that job in 1952, he sees the papers from David Bohm, and he reads these papers, and he immediately realizes there's nothing wrong with what Bohm said. Bohm may not be right, but it's certainly a perfectly legitimate way of looking at quantum mechanics. And so then Bell realizes, "Well, von Neumann must have been wrong."

1:07:38 SC: "How can you prove something can't exist when here's an example of it existing?"

1:07:41 AB: Right, exactly. So here's a counter-example. So he, again wants to go look at the proof, it's still only available in German. So he goes off and works on other things for a few years, he goes to graduate school, he suggests to his PhD advisor that he could either one day give a talk about either Bohm's work and the quantum measurement problem, or about Accelerator Physics, and his PhD advisor... I'm never gonna... I never pronounce this name correctly... Rudolph Peierls?

1:08:17 SC: Yeah, who cares?

1:08:17 AB: Yeah, it doesn't matter. The point is, well known physicist, another student of Bohr, or a colleague of Bohr, student of Heisenberg, said... Basically gave him a look and said, "Why don't you give a talk about accelerator physics?" And so he just left quantum foundations aside for a while. And then finally, in the mid-1960s, he gets a chance to think about these things. And he sits down and looks at von Neumann's proof, which is finally available in English, along with a couple of other proofs that are related, and he said, "I looked at von Neumann's proof and it fell apart in my hands. It's not just wrong, it's silly, It clearly doesn't work." And so he writes...

1:09:04 SC: But was it that he didn't... He made a mistake in the proof or just didn't prove the thing that people were saying...

1:09:10 AB: He didn't prove the thing that people said he proved.

1:09:12 SC: Yeah, he proved something else.

1:09:13 AB: And he also didn't prove the thing that he thought he'd proven, right? This proof shows up in von Neumann's textbook, quantum mechanics book, which is a really incredible intellectual work with this one flaw in it, but in that book, von Neumann makes it clear that this is how he's thinking about it. And it's not correct, he hasn't proven what he said.

1:09:36 SC: My impression was... Again, I forget where, maybe from your book, that Einstein, who did speak German, had seen von Neumann's book and was at least skeptical of this claimed proof.

1:09:46 AB: You know, I didn't...

1:09:47 SC: And he talked about it to Bohm.

1:09:48 AB: Yeah, there are stories about that, and I'm not sure... Like the stories about Einstein's awareness of von Neumann's proof are not particularly well-sourced.

1:10:01 SC: Okay.

1:10:01 AB: So I didn't put them in my book because I didn't want to get...

1:10:04 SC: Yeah, exactly. Good.

1:10:05 AB: Yeah, and then of course, people attacked me for not putting them in my book because no one's ever happy. But the point is, yes, Einstein may have been aware of it, he may not have been aware of it, but even if he was aware of it, he was skeptical of it. But the point is von Neumann's proof didn't prove what von Neumann, and especially a lot of other people, thought it proved. And so Bell works on it and looks at that proof and these other proofs that are like it and shows, "No, they don't rule out hidden variables, that's not what they do, they do this other thing."

1:10:38 SC: And Grete Hermann knew it.

1:10:40 AB: Yes, that's right, Grete Hermann, a mathematician and philosopher and student of Emmy Noether, pointed out the problem with von Neumann's proof just a few years after he published it in 1935, and nobody listened to her.

1:11:00 SC: No one cared.

1:11:00 AB: Yeah, nobody cared, yeah. A variety of reasons, probably including the fact that she was a woman.

1:11:06 SC: There you go.

1:11:07 AB: But Bell independently rediscovers these problems, also discovers the problems in similar proofs that have been published since, publishes a paper basically saying, "Look, these proofs don't do what you think they do, hidden variables are still on the table, they just need to meet these requirements." And then at the end of the paper, he says, "But there is still the question about theories, hidden variable theories, whether they need to be like Bohm's in that they need to have this non-locality. That's an open question, which somebody should answer." And so then he starts working on that immediately afterward.

1:11:46 SC: Yeah.

[overlapping conversation]

1:11:47 AB: Right. And then he answers it and he says, "No, this non-locality, it's a fundamental feature not just of hidden variables, but of any theory that's going to reproduce the results of quantum mechanics, with the exception of theories that break something even more fundamental."

1:12:02 SC: Like many worlds.

1:12:03 AB: Like many worlds.

1:12:04 SC: Yeah, so basically if experiments have definite outcomes...

1:12:08 AB: Yes, if experiments have definite outcomes, and there's not some vast conspiracy going back to the beginning of time...

1:12:17 SC: Super determinism.

1:12:18 AB: Yes, exactly.

1:12:19 SC: Just in case the audience does know the buzzwords, we'll let them in on that.

1:12:22 AB: Yes, exactly. So those are pretty much the only two ways out. Now, people are definitely not gonna be happy with me for saying that, but yeah, you'll sometimes hear people say, "No, you can save locality if you get rid of the hidden variables." That's not true because then you're still left with Einstein's EPR argument.

1:12:43 SC: Right.

1:12:43 AB: Then you'll hear people...

1:12:45 SC: Like you already said, Bell's theorem in his argument wasn't really about hidden variables.

1:12:50 SC: That's right.

1:12:51 SC: It was about, "There's no way to reproduce quantum mechanics without spooky action at a distance."

1:12:54 AB: Yeah, exactly, that's exactly right. And you'll hear people say, "Well, you can save locality if you give up realism." I have yet to hear a definition of realism that satisfies that, unless you're giving up the idea of things, in which case, sure, you can have locality.

1:13:14 SC: But people do.

1:13:15 AB: People do, but then what does locality even mean? So, yeah.

1:13:22 SC: So let's put ourselves where we are now. So it's like the '60s or '70s, most of the physics community has been ignoring the measurement problem of quantum mechanics. It was Bohmian effort in the '50s, but even Bohm didn't keep talking about it a lot. He did other things. I remember there's this quote from Yakir Aharonov, who was one of his students. And together they invented the Aharonov-Bohm effect, which is really, really important.

1:13:47 AB: Yes, it is.

1:13:47 SC: And someone asked him, "Aharonov, did you ever talk to Bohm about his theory?" And Aharonov says, "No, we only ever talked about physics."

1:13:57 AB: [chuckle] I hadn't heard that quote, but that seems pretty plausible to me. Yeah, he definitely... He told me that when he started working with Bohm, they had an agreement to not work on that stuff. I think in part because Bohm was worried about what impact that would have on Aharonov's career, and quite understandable, yeah.

1:14:15 SC: And there was even this infamous memo from the editor of the Physical Review saying, "We won't even look at papers in the foundations of quantum mechanics."

1:14:25 AB: Yeah, yeah, which someone later pointed out... He said, "We're not gonna look at papers in the foundations of quantum mechanics unless they propose a new experimental result, or a new experiment that you can do." And someone pointed out, "Well, if we had that policy 30 years ago, that would've forced you to reject Bohr's reply to EPR.

1:14:47 AB: Yeah. The sign of the times, though.

1:14:49 SC: Exactly, yeah... No, it was really bad. So yeah, in the '50s we had Baumann-Everett, in the '60s we had Bell. Bell's paper is published, he doesn't hear anything about it for years. Part of this is because there's this weird story about where that paper and the other paper he did before that were published, which is a whole thing that we're not gonna get into, but yeah, people just weren't paying attention. And then at the very end of the 1960s, Bell gets correspondence about this paper for the very first time from a graduate student named John Clauser at Columbia University, who wants to conduct an experiment to test this particular set of outcomes of quantum mechanics to see if Bell's proof holds about the world. Because basically what Bell showed was, "You can do an experiment, and if the experiment agrees... If the outcome of the experiment agrees with quantum mechanics, then you don't have locality. And if it does... And if it doesn't agree with quantum mechanics, then you can have locality, but also you've broken quantum mechanics," which is important.

1:15:58 SC: Yeah, exactly. So Bell... The experiment couldn't show that Bell was wrong, Bell gave us horns of a dilemma, right?

1:16:02 AB: Exactly, yes.

1:16:03 SC: Either there's spooky action at a distance, or quantum mechanics is wrong, or even worse things, like many worlds.

1:16:09 AB: Yeah.

1:16:09 SC: So you can experimentally figure out which one world.

1:16:12 AB: Yeah, either quantum mechanics is correct in this outcome or we can have locality. So you have to actually do that experiment, and Clauser wanted to do it, and so did a couple of other people. And so Clauser got together with a guy named Abner Shimony at Boston University, and a couple of other guys named Horne and Holt, and they wrote a paper massaging Bell's result into a form that could actually be tested, called the CHSH paper, after their initials.

1:16:55 SC: This is always an underappreciated part of the process of physics.

1:16:58 AB: Yes.

1:16:58 SC: Turning the crazy things that the years do into something that can be experimentally probed.

1:17:01 AB: Or at least something that can be computed, right?

1:17:03 SC: Yeah.

1:17:04 AB: But yeah, yeah, exactly. So they do this, and then in the very early 70s, I think '71 or '72, Clauser who at this point is a postdoc at Berkeley, along with another guy named Stuart Freedman, actually do an experiment to test the predictions of quantum mechanics in this situation, and that they find that quantum mechanics works.

1:17:36 SC: Surprise! [chuckle]

1:17:37 AB: Yeah. Surprise! Exactly. Pretty much everybody thought that it would, Clauser was not sure. Clauser really thought that it might turn out the other way, and if it had, he would be even more well known than he already is. But yeah... But no, quantum mechanics survived, which meant that we couldn't have locality with again the...

1:18:02 SC: Not in one world.

[overlapping conversation]

1:18:03 AB: Yes, not in one world, yes.

1:18:06 SC: And that's been that idea of testing these things has been upgraded and carried forward to the present day.

1:18:12 AB: Yeah.

1:18:12 SC: All sorts of more elaborate tests of Bell's inequalities.

1:18:15 AB: Exactly, yeah. So Clauser was first, and then in the late 70s, this guy named Alain Aspect did a more detailed experiment, testing again the predictions of quantum mechanics in these conditions, and found again that quantum mechanics worked. And Clauser and Aspect really had very different career trajectories from that point on. Clauser, when he did this, he was a postdoc, he didn't have a permanent position, and he had a lot of trouble getting a permanent position and ultimately never really did, even though he'd done this really impressive experimental work, so his career suffered as a result of doing this work. Aspect had a permanent position before he did the work.

1:19:13 SC: Wise.

1:19:14 AB: Yes. And in fact, when he'd gone to John Bell to talk with him about doing this work. Aspect met with Bell before setting out on this experimental journey, Bell wouldn't even talk to him about it until Aspect assured him that he had a permanent position, because Bell was so worried about damaging the careers of young physicists. But Aspect did this experiment and then went out and gave a lot of talks about it. And Aspect is very good at giving talks.

1:19:50 SC: Makes a difference, right?

1:19:51 AB: Yeah, it makes a very big difference. So he's very good at...

1:19:53 SC: Niels Bohr was very good at convincing people of his point of view, and Einstein, for all his genius, just expected people to go along with him, he was not that good at the sales pitches.

1:20:03 AB: No, he really wasn't, he was not a people person. Yeah. And whereas Bohr almost had to work with people to do his work. Einstein almost never worked with people, or he worked with a very small group of people.

1:20:16 SC: He worked with people on the EPR paper, but then afterward he never talked to Podolski again.

1:20:20 AB: Yeah, he did not like how that paper turned out. And I really think that the clearest versions of the EPR paradox, as Einstein thought about it, are in Einstein's later writing, where he really explained it much more clearly, but yeah.

1:20:39 SC: So Aspect was a big force in changing how the field thought about the foundations of quantum mechanics?

1:20:44 AB: Exactly, because he showed, "Hey... " He made people aware that there was at least one really nice experiment to be done in quantum foundations, and that, in turn, caused people take a look at Bell's work. And then when they did that, all sorts of interesting things happened. Not only was there renewed interest in quantum foundations, but this also drove the new field of quantum information and quantum computing.

1:21:18 SC: And to be fair, I do give people a hard time... The physics community, I give them a hard time for ignoring quantum foundations. But there is not only a feeling that there's more interesting things to be done, but there is a feeling that it's impossible to make progress on that kind of question because it's more a philosophy question, there's no experimental input, etcetera, etcetera, and that is one way of changing that is to do an experiment that has a big impact on this field.

1:21:41 AB: Exactly, yeah. Exactly yeah, that's exactly right. I think, as I was saying before, if the attitude of the physics community was, "It's very hard to make progress and so I'm not going to work on it because it's really hard." That'd be one thing, that be fine. The problem is when you change that to this normative statement, when you say, "Oh it's very hard and you shouldn't work on it, it's a bad idea to work on it, not just because you'll suffer professionally, but because that's not the thing that we do as physicists."

1:22:14 SC: And that's really endemic.

1:22:16 AB: Yes...

1:22:16 SC: So in fact let's... I guess there's two big things I still have to... I still wanna ask you about.

1:22:21 AB: Sure.

1:22:21 SC: One is despite all the bad mouthing we've done with the Copenhagen interpretation.

1:22:27 AB: Yes.

1:22:27 SC: It's come back, or at least versions of it have come back in the form of these epistemic approaches to quantum mechanics. There's a whole subset of people who really get behind the idea that all we're supposed to be doing, all quantum mechanics purports to do is to make predictions for experimental outcomes.

1:22:48 AB: Well, there are definitely people who believe that, and there are definitely people who support these epistemic interpretations. Epistemic, meaning that the wave function is about our knowledge about the world rather than a thing in the world. But I wouldn't say that the people who support these epistemic interpretations are all people who say "No, quantum mechanics is really only about the outcomes of experiments, and we shouldn't be doing more than that as physicists." Someone we both know, Matt Leifer, at Chapman University, he is certainly a supporter of these epistemic interpretations, but he is a scientific realist. He thinks that there is a world and there's stuff in the world, and the job of physics is to go after that stuff. He just thinks that the quantum wave function isn't one of the things in the world. It's a statement about our knowledge.

1:23:40 SC: That's right, it's a very good certification. Yeah. So we have this quantum wave function, it's the thing that you can't get away without having in any version of quantum mechanics.

1:23:48 AB: Yes, and I think every... That's one of the few things everyone would agree on.

1:23:51 SC: There is a wave function.

1:23:52 AB: Yes.

1:23:52 SC: Schrodinger equation talks about the evolution of something.

1:23:55 AB: Yeah, but at least there is a quantum state.

1:23:58 SC: There's a quantum state, right, but amazingly, we can't say, in our best understanding of the world, whether that thing is real or just a tool.

1:24:08 AB: Yeah.

1:24:08 SC: And so the epistemic folks wanna say, "It's just a tool, it's just like a probability distribution." When you say that, I've flipped a coin, that I haven't looked at it yet, so there's a 50/50 chance it's heads or tails, I can give a probability distribution to that, but there is a reality beneath it, and they wanna make the wave function like that.

1:24:28 AB: Yeah, I think that's right. There are people who think that... I don't know, there are positions that I... I don't know. Now we're getting back to the subject of people being angry with me. There are people who hold positions that are extremely Copenhagen-like, who will say, "No, the wave function is a statement of our knowledge, and there isn't a thing underneath it."

1:24:55 SC: Well and proudly so, right? There are people who identify themselves as Copenhagen supporters, not just because there's nothing better but because that's the right answer.

1:25:04 AB: Exactly, yeah. And when you push them on, "What do you mean by Copenhagen?" you get a variety of interesting answers. And one of the things that happens is you get them jumping between mutually contradictory positions, which, if you can pull that off is a really effective rhetorical move 'cause it turns out that anything follows from a contradiction. So you can always answer any question that you've been given, you just have to contradict yourself.

1:25:31 SC: So QBism is an example of an epistemic theory.

1:25:34 AB: It is.

1:25:35 SC: Do you understand QBism well enough to talk about it for the audience. This is capital Q, capital B-ism. So, quantum Bism, quantum Bayesianism.

1:25:45 AB: Yeah, although they now say that that's not what it stands for, and it's not clear what it does stand for, it's just a name.

1:25:49 SC: Just a name all by itself.

1:25:50 AB: Yeah.

1:25:50 SC: So I think that this is where I'm going to get people mad at me for my book because I tried to be... I'm very pro-Everett, pro-many worlds.

1:25:58 SC: Yeah.

1:25:58 SC: I tried, as I said in the book, to be fair, but not balanced. So I certainly gave all of the good lines to Everett in quantum mechanics, but I tried to say correct things about the other interpretations. And QBism or epistemic approaches, more generally, are where the chances are greatest that the proponents of those theories will not think I'm being fair. I try to be fair but I just can't figure out what they're saying.

1:26:24 AB: So speaking of people being mad, I was originally planning to have a chapter in my book about epistemic theories, and then my editor started yelling at me for being way over word count, which I was. And I ultimately decided, "This is a book about the history of quantum foundations and how we got from where we were to where we are." And these epistemic interpretations, or as I call them in my book, information-based interpretations, because I try to use less jargon.

1:27:03 SC: No, that's good. You probably... Smart choice.

1:27:05 AB: Yeah. I don't know that they like it, but whatever. They're new-ish, right? They certainly have Copenhagen-ish DNA, but they are mostly things from the 1990s and later.

1:27:19 SC: They're young and being developed, we should be fair to them.

1:27:22 AB: Yeah, exactly. And so I decided, "Oh, that means that I can get away with not discussing them a great deal because we haven't seen how they're gonna play out yet." And if you're writing a history book that tries to go up to the present day, the hardest part is always gonna be the last 15 to 20 years.

1:27:42 SC: Yeah.

1:27:42 AB: And so I just...

1:27:44 SC: "We haven't yet decided what history says."

1:27:46 AB: Yes. Exactly, yeah. So I do mention them in my book but I don't go into it in a great deal of detail. And I talked with Matt about this, and he said, "Yeah, that seems like a perfectly reasonable move." However, some other side epistemic people and some Q-B-ists or QBists, whatever, I'm bad at pronouncing things, are not happy about that, and they're also not happy with how I present Bohr in my book, and whatever. But I have to say, I don't think that I really understand their position terribly well either, and that could be me, but I'm not sure that it is.

1:28:32 AB: I have tried quite a bit to understand it, although once I realized that I wasn't gonna be going over it in great detail on my book, I stopped trying quite as hard.

1:28:39 SC: Don't try as hard.

1:28:41 AB: Yeah, because I was working on the book. But I have since tried some more, and I'm actually... I'm going to have a chance, I hope, later this year to sit down and talk with David Mermin, one of the QBists, again. But I did talk with him, I had two long conversations with him while working on my book. He's someone I know from when I was in college, 'cause he's at Cornell, and that's where I went for college. And he's the aforementioned professor who told me... Gave me the bad news that, "No, there's not a different translation of Bohr's paper." So yeah, David Mermin is a lovely guy, he and I disagree about quantum mechanics and I'm not sure that I understand his position. I am not sure what to make of QBism.

1:29:31 SC: Okay, that's fine.

1:29:32 AB: Yeah.

1:29:33 SC: That's perfectly fair. Probably the most prudent thing you can say.

1:29:36 AB: I think that's right.

1:29:37 SC: Yeah.

1:29:37 AB: Yeah.

1:29:38 SC: But is it a... The other big point I wanted to give us a chance to talk about, is the very fact of the existence, the resurgence, the renaissance of people trying to make something like Copenhagen respectable. Is it a reflection of the fact that quantum foundations more broadly are becoming slightly more respectable in physics? I mean, you still don't see... Look, Caltech and Harvard and Princeton are not gonna hire physicists who study the foundations of quantum mechanics as senior faculty members. But maybe with not only the experiments in quantum optics and so forth, but also interest in quantum computing, quantum information. It's become a little bit less objectionable to worry about the foundations of quantum mechanics.

1:30:31 AB: Yeah, I mean it definitely has become less objectionable. You've talked with David Albert about this. Things are certainly not the way that they were when he was in graduate school, right? When I was an undergrad, I was curious about these things and I had some professors saying, "Why are you asking these questions?" But there was also David Mermin, who said, "Yes, ask these questions." And my PhD advisor is certainly not someone who spends time thinking about these things, even though he's a very, very, very good physicist, and very smart man. But he doesn't mind that I think about these things. He just would've minded if I had spent time doing that rather than doing my research when I was in graduate school, and I didn't do that. So yeah, I think that part of it is just people look for new options, right? People come into a new field or come into a newly resurgent field and want to do something new, which is completely understandable. I also think some of it comes from an understanding on the part of people who are sympathetic to the Copenhagen interpretation, that if they want it to survive, they can no longer ignore the competition. So there... For many, many years, the strategy was, what alternatives, right? There are no alternatives.

1:32:06 SC: I did at one point, I'm on the Colloquium Committee at Caltech, and I suggested getting a talk on Bohmian mechanics, and I was swiftly slapped down with a withering gaze, if that's a mixed metaphor that I can get away with. So we're not quite there yet.

1:32:20 AB: Yeah.

1:32:20 SC: Situation might be improving a little bit.

1:32:22 AB: Yeah, yeah. I think that's right. I mean, on the one hand, sure that's true. On the other hand, people have been inviting me to give physics colloquia at some pretty good places, like I gave talks... It wasn't a colloquium, but it was a public talk sponsored by the Harvard Physics Department. Cornell's invited me back, Michigan's invited me back, granted that's 'cause I went to both of those places. Michigan, for my PhD, but still those are good schools, they're good physics departments, and other departments that I have no affiliation with have also invited me.

1:32:57 SC: And you did not get tomatoes thrown at you?

1:33:00 AB: I did not get tomatoes thrown at me. I thought I was going to get tomatoes thrown at me, and I didn't. I gave a talk at Berkeley Lab, which is almost in my backyard, and nothing bad happened. And they knew what I was coming to give a talk about. I'd made it very, very clear in the title in abstract. I was not saying, "I'm not a partisan for any particular interpretation." I was just saying, "Hey the measurement problem is a thing, and it's an important open problem in physics. That doesn't mean that everybody has to work on it anymore than anybody... Than everybody has to work on any other open problem. We wouldn't want that but you should be aware that it's an open problem the same way that everyone's aware that we don't have a theory, we don't have a definite well-accepted theory of quantum gravity, right? That's a well known open problem, even if you don't work on that."

1:33:50 SC: And relatedly, maybe in fact, part of the reason why people have not been too much in favor of things about quantum foundations, is that it has the aura of philosophy about it.

1:34:01 AB: Yes.

1:34:01 SC: And for those of us who think it's an important problem, that's a good thing.

1:34:06 AB: Yeah.

1:34:06 SC: It's been probably that in statistical mechanics, arrow of time type stuff. The foundations of quantum mechanics, the area in which the philosophers have in some sense carried the torch for a while as physicists have been ignoring things.

1:34:19 AB: Yeah, yeah, I think that's right. The whole disciplinary split between physics and philosophy is a pretty modern invention. We used to call scientists "natural philosophers," right?

1:34:32 SC: Einstein and Bohr would not have understood it.

1:34:35 AB: Exactly. Yeah, it used to be that physicists generally had a pretty good schooling in philosophy and certainly didn't have outright contempt for it. And in less than a century, we've got to a point where many well known scientists and physicists do you have open public contempt for philosophy. And I do talk about that a little bit toward the end of my book. I'm really not quite sure why that happens.

1:35:10 SC: I've heard it suggested that it coincided with the shift in the center of physics from Europe to the United States.

1:35:15 AB: I think that that's correct. And I think that there's certainly.

1:35:18 SC: Americans have always been more down-to-earth, practical folk.

1:35:21 AB: Yes.

1:35:21 SC: "None of this abstract nonsense for us, we're gonna build things."

1:35:24 AB: That is absolutely true, but it's also not true that there are no American physicists who have philosophical inclinations, there always have been some, right? I think there's a lot of different sociological things that went into it. World War II not only shifted the center of physics to the US, it also created the era of big science, and that certainly had something to do with it. But I'm still puzzled, you get physicists saying these things about philosophy, and I don't think... Sure, we physicists have a deserved reputation for being arrogant when it comes to looking at other fields, but I don't think that you would get a physicist just saying, "I don't understand why anybody does, I don't know, sheet metal production," right? "That just seems like a silly thing. And anybody who studies best practices in that is just wasting their time," right? It's, "I don't know anything about sheet metal production, but I do know that it's probably good to do it in a way that people don't get hurt, right? Because when you're manufacturing anything, people can get hurt. That's why I'm not an experimental physicist." But...

1:36:45 SC: I think you said, I forget whether it's in your voice, in your book, or whether you were quoting someone, but there was this wonderful idea that part of the reason... Might have been Bell, who said, "Part of the reason for the rejection of philosophy and foundational questions more generally was that most physicists think that if they would just spend 20 minutes thinking about it, they could figure it all out, they just haven't found the time yet."

1:37:06 AB: That is indeed Bell. Yes, yeah, that's exactly right.

1:37:10 SC: Quite incredible.

1:37:11 AB: Yeah... No, he was a really, really good writer and speaker.

1:37:15 SC: Well, I hope that things are getting better. Well, we're on the same side in terms of what would qualify as better, but I do think that slowly, gradually, it's getting there. Your book, I think, definitely had a salutary effect, hopefully mine does.

1:37:28 AB: I hope so too.

1:37:30 SC: And we're looking forward to whatever book you end up writing next.

1:37:33 AB: Thank you. Yeah, so am I.

1:37:34 SC: Adam Becker, thanks so much for being on the podcast.

1:37:36 AB: Thanks for having me.

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