#414 — Strange Truths

Sam Harris speaks with David Deutsch about quantum physics and current events. They discuss the "many-worlds" interpretation of QM, Schrödinger's cat, constructor theory, quantum computing and whether it will ever be practically possible, recent developments in AI, the prospects of artificial super-intelligence, the alignment problem, antisemitism and the historical persecution of Jews, misconceptions about Israel, the future of the Jews in Israel and the West, and other topics. If the Making Sense podcast logo in your player is BLACK, you can SUBSCRIBE to gain access to all full-length episodes at samharris.org/subscribe http://samharris.org/subscribe. Learning how to train your mind is the single greatest investment you can make in life. That's why Sam Harris created the Waking Up https://www.wakingup.com/makingsense app. From rational mindfulness practice to lessons on some of life's most important topics, join Sam as he demystifies the practice of meditation and explores the theory behind it.

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

• 0:06Sam Harris?

  Welcome to the Making Sense Podcast. This is Sam Harris. Just a note to say that if you're hearing this, you're not currently on our subscriber feed, and will only be hearing the first part of this conversation. In order to access full episodes of the Making Sense Podcast, you'll need to subscribe at samharris.org. We don't run ads on the podcast, and therefore it's made possible entirely through the support of our subscribers. So if you enjoy what we're doing here, please consider becoming one.

• 0:36Sam Harris?

  I am here with David Deutsch. David, it's great to see you again.

• 0:39Speaker 01

  Nice to be here again.

• 0:41Sam Harris?

  It's been a long time. I didn't actually check to see when our last conversation was, but I think it was probably about five years ago. It has to be.

• 0:48Speaker 01

  Wow, is it that long? Yeah, well, a lot has happened since.

• 0:52Sam Harris?

  Yeah, yeah, it's, uh, history has been eventful. So I'm going to take us on a tour through many topics about which you are well qualified to have strong opinions. The first will seem intimately related as they relate to science, about which you have thought much. But at the end, I want to talk about world events and the explosion of antisemitism we've witnessed since October 7th, 2023. I know you're connected to those topics as well. So we, you know, you and I've had at least two very long podcast conversations where we dealt with mostly the topics in your second book, The Beginning of Infinity, and then tried to bridge a conversation between that

• 1:34Sam Harris?

  and the foundations of human knowledge, its prospects for the future, and also just how that relates to human values and morality. And so people can go back and listen to those conversations. We have at least four hours, if not five, on those topics. But here, I realize we've neglected to talk, I think, at all about the topic covered in your first book, The Fabric of Reality, which was the many worlds interpretation of quantum mechanics, among other things, and also your more recent work, Constructor Theory, your contributions to quantum computing, and also just how you view the state of that. You and I have spoken about artificial intelligence before, and I'm going to want to just hear about your recent thoughts on that

• 2:18Sam Harris?

  and your experience of the developments in the technology. And then again, then we'll talk about the tractor beam of a very ugly history that seems to be pulling us all back into the stream of things that would be best left behind us. So let's talk about quantum mechanics and your favorite interpretation of it, the many worlds thesis. What is that?

• 2:40Speaker 01

  So first of all, I've long ago gone off calling it an interpretation. I think an interpretation, calling it an interpretation is, is part of the thing that went wrong with physics in the mid 20th century, where because people didn't like what quantum theory was saying about reality, they invented, well, I guess it was invented by philosophers, but physicists latched on to this idea that the scientific theory consists of a mathematical formalism, which doesn't have a meaning, and then an interpretation which assigns a meaning to each of the mathematical objects.

• 3:24Speaker 01

  And so that may, and by definition, then, the, neither of those by itself is testable.

• 3:31Sam Harris?

  Right.

• 3:32Speaker 01

  So only the two together are testable. Now, I think that's, that's.

• 3:36Sam Harris?

  Actually, David, can I, can I just pass over that ground one more time, just to make sure people understand what you're saying? So you're saying that it was in fashion for many generations to view quantum mechanics as a calculation device that produced absurdly accurate answers, but the picture of reality that it, that it was giving us was open to many quite discordant interpretations. I mean, they were just completely irreconcilable. They, they looked like very different pictures of reality, and we were left with physicists kind of picking their favorite interpretation, or just declining to do that at all and just go on calculating. And so it was just not clear what picture of reality quantum mechanics was giving us.

• 4:20Sam Harris?

  Is that accurate to say?

• 4:22Speaker 01

  That's what the consensus view has been, and probably still is. But I disagree with that. I think there's only ever been one interpretation of quantum theory, and it was invented in the 1950s by Schrödinger and then Everett. Who developed it properly. And it says, among other things, that quantum, the reality described by quantum theory is one of many universes. Shall we, by universe, I mean, the thing we see around us, the thing we see with telescopes, you know, it's enormously vast thing. But quantum theory says that the true reality consists of that and many copies of it and a lot more.

• 5:05Speaker 01

  So, and that's, that we call that the multiverse.

• 5:08Sam Harris?

  Right.

• 5:08Speaker 01

  And so, these interpretations of it, for example, things like the, the, nothing exists except what we observe. And therefore, when the calculational tools say that there are many of us observing many instances of the thing, that doesn't say anything about reality, that's just a calculational tool. And what really happens is that when we finally observe something, all the various versions of us collapse, as it's called, and only one of them remains, which is much like the universe of classical physics, which my, my favorite analogy for this is, is, well, first

• 5:51Speaker 01

  of all, note that nowhere else in science does anyone do this. It's, it's only, it's only in quantum theory that one splits the predictive part of the theory and the explanatory part like that. So I, I like to say that this is exactly like if the controversy, if there was a controversy about whether dinosaurs existed, whether, you know, the, the creationist account of, of the origin of fossils or the biologist's account. And people said, well, they're just interpretations. I mean, the, the, the universe was formed 6,000 years ago and

• 6:33Speaker 01

  Whoever created it, the creator put fossils into the ground to test our faith or whatever.

• 6:41Sam Harris?

  Right.

• 6:41Speaker 01

  And so therefore, the the idea that there actually were dinosaurs, which no one has ever seen, and no one will see, and there's no evidence that it that any times before 6,000 years ago existed, because all that evidence is just, according to our theory, artificial. And by the way, we don't see dinosaurs. People say we see dinosaurs because we see their fossils, but fossils are actually stones. All the dinosaur has disappeared long ago. And fossils are just stones which take on the shape of the dinosaur in reality.

• 7:20Sam Harris?

  So you're saying that as scientists, it should matter to us which of these pictures of reality is in fact true, and only one of them can be true. And the fact that you can cook up a highly implausible, but nonetheless unfalsifiable variant, does not mean that we should consider it as an equally serious candidate for being true.

• 7:41Speaker 01

  That's right.

• 7:42Sam Harris?

  Right.

• 7:43Speaker 01

  In fact, the dinosaur theory, the creationist dinosaur theory, is actually much more specific and detailed than the so-called other interpretations of quantum theory, because they always have a thing like, and then a miracle occurred, which is a bit like, and then God created it. But the thing is, they are willing to say what God created. Whereas the other interpretations of quantum theory do not say that. They, in fact, they say, you're not allowed to ask what happened, what happens between the setting up of a quantum experiment and the viewing the outcome.

• 8:21Sam Harris?

  Well, let's remind people of just the kind of the heart of this controversy, because I think people will have heard of specific things like, you know, the double slit experiment, or Schrodinger's cat, right? And one of these, I mean, maybe Schrodinger's cat is the best place to illustrate. The variant here, just what has been the standard view of the seeming paradoxes here and what has been the temptation to follow Hugh Everett into the many worlds theory?

• 8:53Speaker 01

  By now, I think the the standard or prevailing theory is just to shut up and calculate, as you mentioned at the beginning, because people have got really tired and feel feel vulnerable using things like the collapse interpretation, which has got so many holes in it by now. So they they I think most physicists just say, well, you know, I'm not interested in that. I'm only interested in getting results.

• 9:19Sam Harris?

  And the collapse interpretation, again, I just feel like we need to double click on some of these things so that

• 9:23Speaker 01

  Yeah.

• 9:24Sam Harris?

  People who are not haven't recently read one of your books or another book on physics will follow us. So it has it has long been thought that observation plays a key role in reality becoming what it is at the quantum level. So Schrodinger cooked up a a thought experiment, which I believe sought to illustrate how deeply counterintuitive and perhaps unacceptable the state of affairs was the idea of the cat in the box, you know, waiting to be killed or not based on the decay of some radioisotope. And it was then left to the the greatest physicists of the time to imagine that their physics

• 10:07Sam Harris?

  was telling them that the cat was neither alive nor dead. It was in some so called superposition of those two states until one of the physicists opened the box and observed whether it was alive or dead. And this is true, you know. So, I mean, this begets many questions, but starting there, what is unacceptable to you about that? And then what does many worlds tell us is in fact true?

• 10:32Speaker 01

  Right. Well, what's unacceptable to me and to Schrodinger was that this state, not that this state could exist of half alive and half dead or whatever, a superposition of alive and dead, but that when you open it, this resolves into one of those two. So what is it that resolves it? Well, being observed, some people said it's being conscious of the another. Some people said it's when the outcome becomes known to the scientific community.

• 11:05Speaker 01

  And meanwhile,

• 11:06Sam Harris?

  It gets peer reviewed?

• 11:09Speaker 01

  Yeah. Yes, exactly. And, and, and originally, Niels Bohr, who kind of set off this silliness, said, you're not allowed to ask that quantum theory does not answer that question of what was happening in between when the state was prepared and when it's observed. You're not allowed to answer that. Physicists don't like that. So they, they produce these, these various interpretations that make even less sense than that, because the, by the way, Niels Bohr also said that it's no good looking for a theory of what happens in between those two times. Quantum theory is a complete

• 11:51Speaker 01

  theory. And therefore, what it says can't be asked, really can't be asked. It's something one is not allowed to theorize about. So Einstein and, and some others couldn't accept that because they thought that physics in particular was about how the world actually is, not about what we are allowed to say or think.

• 12:16Sam Harris?

  Right.

• 12:17Speaker 01

  So, yeah, so I'd forgotten what the question was there.

• 12:20Sam Harris?

  Well, so the question is, so if it's not consciousness that's collapsing the wave function and resolving this superposition, and one could wonder whether the consciousness of the cat, if it has consciousness, why that would be insufficient. How does many worlds come to the rescue? What is said to be true now?

• 12:39Speaker 01

  Yes. By the way, the, you have to say the consciousness of the cat does not affect it because in principle, we could reconstruct the interference. So as long as it hasn't, as long as the wave function hasn't collapsed, you can restore the initial state and have the cat and the, and the poison not be affected yet. So it has to be something that the experimenter does, somebody who is somehow connected with the laws of physics. Now, what, what happens, according to Everett's Everettian quantum theory, is that when we do an experiment, countless numbers of us, or if you

• 13:19Speaker 01

  like, a thick layer of us, a stratum in the multiverse with many identical copies, all of them setting up the experiment and starting the, the apparatus going. All of those remain identical to each other until they look at the cat. And at that moment, the interaction of the outside world with the cat and with the with the version of us makes us differentiate into two copies, or actually millions of copies. But let's just say two copies.

• 13:58Sam Harris?

  Okay, but wait a minute, where did these copies of us come from?

• 14:01Speaker 01

  They were already there in the modern interpretation of the in the modern version of the of the theory. The way that Everett originally framed it, or rather, the way that Dewitt originally Everett himself didn't have an opinion about this, but Dewitt called it splitting into two. But since then, the consensus among Everettian physicists is that the multiple copies are always there. Some of them perform this experiment, and from then on, we can talk about them, what happens to them.

• 14:37Speaker 01

  And then that continuum of observers or of experimenters differentiates that they're no longer identical. The interaction with the cat makes them no longer identical, just as the cat itself is no longer identical across the swathe of universes in which it exists. In half of them, it is alive, in half of them is dead. When we interact with that, the same thing happens to us. We see alive and we see dead in different universes.

• 15:09Sam Harris?

  But so the copies that were already present before the experiment was performed, these presumably were made in prior splittings based on prior quantum events, correct?

• 15:20Speaker 01

  So there are parts of the multiverse in which we don't exist at all because the planet that hit the Earth billions of years ago destroyed all life.

• 15:30Sam Harris?

  Okay, so now this, I must admit, this, at first glance, and perhaps even at second glance, seems like the least parsimonious theory ever proffered. And we seem to be, at least we imagine we're in the parsimony business in science. How is it that this is acceptable, this idea that, I mean, is, can this be summarized by saying that everything that can happen does happen?

• 16:00Speaker 01

  Well, everything that can happen according to the laws of physics.

• 16:03Sam Harris?

  Right, but like, so but like, literally, so, so everything that's possible, is it the same as saying that everything that's possible is in fact actual somewhere?

• 16:13Speaker 01

  Everything that's possible according to the laws of physics is actual somewhere, but you have to add that most of those things, that those things happen in widely differing proportions of the multiverse.

• 16:26Sam Harris?

  Mm-hmm.

• 16:27Speaker 01

  So if I toss a coin, roughly 50% of the worlds will see heads, roughly 50% tells, but a few of them will see the coin melt and dribble off the table onto the floor.

• 16:41Sam Harris?

  Okay, so this is now kind of amounted to a bit of a sidebar conversation. I'll get us back to the main topic, but this interests me. So what this does to the notion of possibility is that it conserves it in the sense that, well, there's no such thing really as possibility. Everything, everything that can't happen does happen. It doesn't happen the same number of times. So there's kind of like a frequency difference across the multiverse.

• 17:06Speaker 01

  Yes, it turns out that frequency is not good enough to, to support the notion of probability that we need in physics and in everyday life. And I inaugurated a research program called the Decision Theoretic Approach to Probability in Quantum Theory. Others took that and ran with it, and now we have a really watertight version that's not based on frequency, but it's based on what a rational person would do if they thought that there are Everettian universes and that the future is going to differentiate. So it comes to the right answer.

• 17:47Speaker 01

  So there isn't any dramatic conclusion from this, except that probability works as it was postulated to work from the beginning of quantum theory.

• 17:58Sam Harris?

  But probability, in this case, is a useful fiction?

• 18:02Speaker 01

  Yes.

• 18:03Sam Harris?

  Okay, so we're in a kind of actualist universe. There is only the actual.

• 18:08Speaker 01

  Yes.

• 18:08Sam Harris?

  And it just happens a countable number of times. And those number of times are different depending on what we're talking about.

• 18:15Speaker 01

  Yes, a measurable number of times, but a countable number of different times, yes.

• 18:19Sam Harris?

  Right. Okay, well, that's, uh, the nerds can thank us for that little detour. Again, back to this question of parsimony. This just does seem on its face to be multiplying things quite literally way too much to seem plausible. Before we dig into more of the details, what was your psychological, give me your psycho biography with respect to this theory? How long did it take you to accept it? What was that process like? And what do you recommend to those who are hitting stumbling blocks on it?

• 18:54Speaker 01

  Well, I was a graduate student at the time, a first year graduate student. And I'd heard of this theory before. And I thought it was, like you said, I thought it was one of a number of different interpretations that one could use for the formalism. And I was just learning the theory. And I was obsessed with physics at the time, not metaphysics. So I didn't much think about it until, well, two things happened. One is that I met Bryce DeWitt in a pizza place in Oxford. And we were having lunch and a bunch of us.

• 19:36Speaker 01

  And I happened to know that DeWitt had been active in promoting Everett's theory. So I asked him, I forgot what I asked him. It was very kind of him to answer, because he's probably been asked this question hundreds of times. But I asked him something like, well, if there are many copies of me, which one am I? And he very gently said, well, you are actually all of them. And they are all asking that question. And so he explained it to me. We went into the details of the theory as well. I was going to say mathematical details, but it was the physical details that we went into.

• 20:18Speaker 01

  And by the end of that conversation, I was convinced. But I was still not very interested.

• 20:24Sam Harris?

  Well, some of you were convinced. There are, there have to be parts of the multiverse where you were not convinced and you walked away astounded that a fellow Oxfordian could believe such a thing.

• 20:35Speaker 01

  You're omitting the existence of error correction. And error correction is a very important part. It's a very fundamental part of human thinking, especially rational thinking. So you might think that there's a 50/50 chance that I would get up from that table convinced and not convinced.

• 20:55Sam Harris?

  Well, not 50/50, but maybe, maybe even just one in a million, right? There's some, there's some universe where you died, you were struck dead mid-sentence by the power of his words, right?

• 21:05Speaker 01

  That, that, that has got nothing to do with being persuaded and not persuaded. Let's consider the, the universes in which I survived.

• 21:15Sam Harris?

  Okay.

• 21:15Speaker 01

  Then I, I think the, the ones in which I was not convinced are a tiny, tiny proportion. It's, it's like if someone wanted to, to say to me that electrons are actually as massive as bowling balls. And there it's the other way around. He wouldn't, in, in, in most universes, he wouldn't have persuaded me. It would take something like a cosmic ray strike to undo the error correction that would, that would have happened.

• 21:44Sam Harris?

  So then what do you make of the failures of error correction, at least on your account, demonstrated by your fellow physicists who don't accept this theory? I mean, presumably there are people, even contemporaries of DeWitt and Everett at the time, people who you really respect. If you'd like to continue listening to this conversation, you'll need to subscribe at samharris.org. Once you do, you'll get access to all full-length episodes of the Making Sense podcast. The Making Sense podcast is ad-free and relies entirely on listener support. And you can subscribe now at samharris.org.