#414 — Strange Truths
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Summary
In this episode of the Making Sense Podcast, Dr. David Deutsch joins me to talk about anti-Semitism, quantum mechanics, artificial intelligence, and the "Many Worlds" thesis, among other things. We also talk about what it means to be Jewish in the 21st century.
Transcript
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Welcome to the Making Sense Podcast. This is Sam Harris. Just a note to say that if you're
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I am here with David Deutsch. David, it's great to see you again.
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It's been a long time. I didn't actually check to see when our last conversation was, but I think it
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was probably about five years ago. It has to be...
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Well, is it that long? Yeah, well, a lot has happened since.
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Yeah, yeah. History has been eventful. So I'm going to take us on a tour through many topics
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about which you are well-qualified to have strong opinions. The first will seem intimately related
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as they relate to science, about which you have thought much. But at the end, I want to talk about
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world events and the explosion of anti-Semitism we've witnessed since October 7th, 2023. I know
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you're connected to those topics as well. So we, you know, you and I've had at least two very long
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podcast conversations where we dealt with mostly the topics in your second book, The Beginning of
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Infinity, and then tried to bridge a conversation between that and the foundations of human knowledge,
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its prospects for the future, and also just how that relates to human values and morality. And so
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people can go back and listen to those conversations. We have at least four hours,
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if not five, on those topics. But here, I realize we've neglected to talk, I think, at all about
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the topic covered in your first book, The Fabric of Reality, which was the many worlds interpretation
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of quantum mechanics, among other things, and also your more recent work, Constructor Theory,
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your contributions to quantum computing, and also just how you view the state of that.
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You and I have spoken about artificial intelligence before, and I'm going to want to just hear about
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your recent thoughts on that and your experience of the developments in the technology. And then
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again, we'll talk about the tractor beam of a very ugly history that seems to be pulling us all back
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into the stream of things that would be best left behind us. So let's talk about quantum mechanics
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and your favorite interpretation of it, the many worlds thesis. What is that?
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So, first of all, I've long ago gone off calling it an interpretation. I think calling it an
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interpretation is part of the thing that went wrong with physics in the mid-20th century, where,
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because people didn't like what quantum theory was saying about reality, they invented, well,
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I guess it was invented by philosophers, but physicists latched on to this idea that the
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scientific theory consists of a mathematical formalism, which doesn't have a meaning, and then
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an interpretation, which assigns a meaning to each of the mathematical objects. And so that means,
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and by definition, then, neither of those by itself is testable.
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So only the two together are testable. Now, I think that's terrible.
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Actually, David, can I just pass over that ground one more time just to make sure people understand
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what you're saying? So you're saying that it was in fashion for many generations to view quantum
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mechanics as a calculation device that produced absurdly accurate answers, but the picture of
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reality that it was giving us was open to many quite discordant interpretations. I mean, they were
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just completely irreconcilable. They looked like very different pictures of reality. And we were left
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with physicists kind of picking their favorite interpretation or just declining to do that at
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all and just go on calculating. And so it was just not clear what picture of reality quantum mechanics
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That's what the consensus view has been and probably still is. But I disagree with that. I think there's
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only ever been one interpretation of quantum theory. And it was invented in the 1950s by Schrodinger
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and then Everett, who developed it properly. And it says, among other things, that the reality
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described by quantum theory is one of many universes. By universe, I mean the thing we see around us,
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the thing we see with telescopes. It's an enormously vast thing. But quantum theory says that the true
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reality consists of reality consists of that and many copies of it and a lot more. And we call that the
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multiverse. And so these interpretations of it, for example, things like nothing exists except what we
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observe. And therefore, when the calculational tools say that there are many of us observing many instances
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of the thing, that doesn't say anything about reality. That's just a calculational tool.
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And what really happens is that when we finally observe something, all the various versions of us
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collapse, as it's called. And only one of them remains, which is much like the universe of classical physics,
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which my favorite analogy for this is, well, first of all, note that nowhere else in science does anyone do this.
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It's only in quantum theory that one splits the predictive part of the theory and the explanatory part
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like that. So I like to say that this is exactly like if the controversy, if there was a controversy
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about whether dinosaurs existed, whether, you know, the creationist account of the origin of fossils
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or the biologist's account. And people said, well, they're just interpretations. I mean, the universe
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was formed 6,000 years ago. And whoever created it, the creator, put fossils into the ground to test
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And so, therefore, the idea that there actually were dinosaurs, which no one has ever seen and no
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one will see. And there's no evidence that any times before 6,000 years ago existed, because all
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that evidence is just, according to our theory, artificial. And by the way, we don't see dinosaurs.
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People say we see dinosaurs because we see their fossils. But fossils are actually stones.
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All the dinosaur has disappeared long ago. And fossils are just stones which take on the shape
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So you're saying that as scientists, it should matter to us which of these pictures of reality
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is in fact true, and only one of them can be true. And the fact that you can cook up a highly
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implausible, but nonetheless unfalsifiable variant does not mean that we should consider it as an
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In fact, the dinosaur theory, the creationist dinosaur theory, is actually much more specific
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and detailed than the so-called other interpretations of quantum theory, because they always have
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a thing like, and then a miracle occurred, which is a bit like, and then God created it. But the thing
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is, they are willing to say what God created, whereas the other interpretations of quantum
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theory do not say that. In fact, they say, you're not allowed to ask what happened, what
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happens between the setting up of a quantum experiment and viewing the outcome.
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Well, let's remind people of just kind of the heart of this controversy, because I think
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people will have heard of specific things like the double slit experiment or Schrodinger's
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cat, right? And one of these, I mean, maybe Schrodinger's cat is the best place to illustrate
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the variant here. Just what has been the standard view of the seeming paradoxes here, and what
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has been the temptation to follow Hugh Everett into the many worlds theory?
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By now, I think the standard or prevailing theory is just the shut up and calculate,
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as you mentioned at the beginning, because people have got really tired and feel vulnerable
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using things like the collapse interpretation, which has got so many holes in it by now.
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So I think most physicists just say, well, you know, I'm not interested in that. I'm only
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And the collapse interpretation, again, I just feel like we need to double-click on that.
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on some of these things so that people who haven't recently read one of your books or
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another book on physics will follow us. So it has long been thought that observation plays a key
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role in reality becoming what it is at the quantum level. So Schrodinger cooked up a thought experiment,
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which I believe sought to illustrate how deeply counterintuitive and perhaps unacceptable the
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state of affairs was, the idea of the cat in the box, you know, waiting to be killed or not based
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on the decay of some radioisotope. And it was then left to the greatest physicists of the time to
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imagine that their physics was telling them that the cat was neither alive nor dead. It was in some
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so-called superposition of those two states until one of the physicists opened the box and observed
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whether it was alive or dead. And this is true, you know, so, I mean, this begets many questions,
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but starting there, what is unacceptable to you about that? And then what does many worlds tell us
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is in fact true? Right. Well, what's unacceptable to me and to Schrodinger was that this state,
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not that this state could exist of half alive and half dead or whatever, a superposition of alive and
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dead, but that when you open it, this resolves into one of those two. So what is it that resolves it?
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Well, being observed, some people said it's being conscious of the, another, some people said it's
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when the outcome becomes known to the scientific community. And meanwhile, it gets peer-reviewed.
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Yeah. Yes, exactly. And, and, uh, and originally, um, Niels Bohr, who kind of set off this, this
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silliness said, you're not allowed to ask that quantum theory does not answer that question of, of what
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was happening in between when the state was prepared and when it's observed. You're not allowed
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to answer that. Physicists don't like that. So they, they produce these, these various interpretations
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that make even less sense than that. Because the, uh, by the way, Niels Bohr also said that it's no good
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looking for a theory of what happens in between those two times. Quantum theory is a complete theory.
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And therefore what it says can't be asked really can't be asked. It's, it's some, something one is
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not allowed to theorize about. So Einstein and, and some others, um, couldn't accept that because
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they thought that physics in particular, uh, was about how the world actually is not about what we
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are allowed to say or think. Right. So, yeah. So I'd, I've forgotten what the question was there.
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So the question is, so if it's not consciousness that's collapsing the way of function and resolving
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this superposition, um, and one could wonder whether the consciousness of the cat, if, if it has
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consciousness, why that would be insufficient. How does many worlds come to the rescue? What is said to be
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true now? Yes. By the way, the, the, you have to say the consciousness of the cat does not affect it
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because in principle, we could reconstruct the interference. So, as long as it hasn't, as long as
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the wave function hasn't collapsed, you can restore the initial state and have the cat and the, and the
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poison, uh, not be affected yet. So it has to be something that the experimenter does, somebody who is
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somehow connected with the laws of physics. Now, what, what happens according to Everett's, Everettian
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quantum theory is that when we do an experiment, countless numbers of us, or if you like, a thick, uh,
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layer of us, a stratum in the multiverse with many identical copies, all of them setting up the experiment
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and starting the, the apparatus going, all of those remain identical to each other until they look
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at the cat. And at that moment, the interaction of the outside world with the cat and with the,
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with the, with the version of us makes us differentiate into two copies or actually millions of
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copies, but let's just say two copies. Okay. But wait a minute, where, where did these
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copies of us come from? They were already there in the modern interpretation of the, uh, in the
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modern version of the, of the theory, uh, the way that Everett originally framed it, or rather the way
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that DeWitt originally, uh, Everett himself didn't have an opinion about this, but DeWitt called it
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splitting into two. But since then, the consensus among Everettian physicists is that the, the multiple
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copies are always there. Some of them perform this experiment. And from then on, we, we can talk
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about them, what happens to them. And then that continuum of observers or of experimenters
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differentiates that they're no longer identical. The interaction with the cat makes them no longer
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identical, just as the cat itself is no longer identical across the swathe of universes in which it
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exists. In half of them, it is alive. In half of them is dead. When we interact with that, the same
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thing happens to us. We, we see alive and we see dead in different universes. But so the, the copies
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that were already present before the experiment was performed, these presumably were made in prior
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split-ins based on prior quantum events, correct? So there are parts of the multiverse in which we
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don't exist at all because the planet that hit the earth, uh, billions of years ago destroyed all life.
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Okay. So now this, um, I must admit this at first glance and perhaps even at second glance
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seems like the least parsimonious theory ever proffered. Uh, and we, we, we seem to be, at least we imagine
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we're in the parsimony business in science. How is it that this is acceptable? This idea that,
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I mean, it is, can this be summarized by saying that everything that can happen does happen?
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Well, everything that can happen according to the laws of physics.
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Right. But like, so, but like literally, so, so everything that's possible,
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is it the same as saying that everything that's possible is in fact actual somewhere?
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Everything that's possible according to the laws of physics is actual somewhere, but you have to add
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that most of those things that, that those things happen in widely differing proportions of the
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multiverse. So, uh, if I toss a coin, roughly 50% of the worlds will see heads 50, roughly 50% tells,
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but a few of them will see the coin melt and, and, uh, dribble off the table onto the floor.
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Okay. So this is now kind of amounted to a bit of a sidebar conversation. I'll get us back to the
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main topic, but this interests me. So what this does to the notion of possibility is that it
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conserves it in the sense that, well, there's no such thing really as possibility. Everything,
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everything that can happen does happen. It doesn't happen the same number of times. So there's kind
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of like a frequency difference across the multiverse.
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Yes. Uh, it turns out that frequency is not good enough to, to support the notion of probability
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that we need in physics and in everyday life. And I inaugurated a research program called the
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decision theoretic approach to probability in quantum theory. Others took that and ran with it. And now we
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have a really watertight version that's not based on frequency, but it's based on what a rational person
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would do if they thought that there are Everettian universes and that the future is going to
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differentiate. So it comes to the right answer. So there isn't any dramatic conclusion from this,
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except the probability works as it was postulated to work from the beginning of quantum theory.
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But probability in this case is a useful fiction.
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Okay. So we're in a kind of actualist universe. There is only the actual.
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And it just happens a countable number of times. And those number of times are different,
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Yes. A measurable number of times, but a countable number of different times. Yes.
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Right. Okay. Well, the nerds can thank us for that little detour. Again, back to this question
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of parsimony. This just does seem on its face to be multiplying things quite literally way too much
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to seem plausible. Before we dig into more of the details, what was your psychological,
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give me your psychobiography with respect to this theory. How long did it take you to accept it?
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What was that process like? And what do you recommend to those who are hitting stumbling
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Well, I was a graduate student at the time, a first year graduate student. And I'd heard of
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this theory before. And I thought it was, like you said, I thought it was one of a number of
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different interpretations that one could use for the formalism. And I was just learning the theory.
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And I was obsessed with physics at the time, not metaphysics. So I didn't much think about it
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until, well, two things happened. One is that I met Bryce DeWitt in a pizza place in Oxford,
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and we were having lunch and a bunch of us. And I happened to know that DeWitt had been active
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in promoting Everett's theory. So I asked him, I forgot what I asked him. It was very kind of him
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to answer, because he's probably been asked this question hundreds of times. But I asked him
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something like, well, if there are many copies of me, which one am I? And he very gently said,
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well, you are actually all of them. And they are all asking that question. And so he explained it to
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me. We went into the details of the theory as well. I was going to say mathematical details,
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but it was the physical details that we went into. And by the end of that conversation,
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I was convinced. But I was still not very interested.
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Well, some of you were convinced. There have to be parts of the multiverse where you were not
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convinced, and you walked away astounded that a fellow Oxfordian could believe such a thing.
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You're omitting the existence of error correction. And error correction is a very important part.
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It's a very fundamental part of human thinking, especially rational thinking. So you might think
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that there's a 50-50 chance that I would get up from that table convinced and not convinced.
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Well, not 50-50, but maybe even just one in a million, right? There's some universe where you
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died, you were struck dead mid-sentence by the power of his words, right?
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That has got nothing to do with being persuaded and not persuaded. Let's consider the universes in
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which I survived. Okay. Then I think the ones in which I was not convinced are a tiny,
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tiny proportion. It's like if someone wanted to say to me that electrons are actually as massive
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as bowling balls. And there it's the other way around. In most universes, he wouldn't have persuaded
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me. It would take something like a cosmic ray strike to undo the error correction that would have
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happened. So then what do you make of the failures of error correction, at least on your account,
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demonstrated by your fellow physicists who don't accept this theory? I mean, presumably there are people
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even contemporaries of DeWitt and Everett. At the time, you have people who you greatly respect.
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