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If we’re right about the Everett interpretation being the right way to read quantum mechanics, then during the 20th century we learned something about the universe and our place in it that’s at least as striking as our discovery that the stars were other suns, and that there were other planets and other galaxies.

Our place in the universe has been changed at least as radically by that discovery as by anything else.

David Wallace

Quantum mechanics — our best theory of atoms, molecules, and the subatomic particles that make them up — underpins most of modern physics. But there are varying interpretations of what it means, all of them controversial in their own way.

Famously, quantum theory predicts that with the right setup, a cat can be made to be alive and dead at the same time. On the face of it, that sounds either meaningless or ridiculous.

According to today’s guest, David Wallace — professor at the University of Pittsburgh and one of the world’s leading philosophers of physics — there are three broad ways experts react to this apparent dilemma:

  1. The theory must be wrong, and we need to change our philosophy to fix it.
  2. The theory must be wrong, and we need to change our physics to fix it.
  3. The theory is OK, and cats really can in some way be alive and dead simultaneously.

Physicists tend to want to change the philosophy, and philosophers want to change the physics.

In 1955, physicist Hugh Everett bit the bullet on Option 3 and proposed Wallace’s preferred solution to the puzzle: each time it’s faced with a ‘quantum choice,’ the universe ‘splits’ into different worlds. Anything that has a probability greater than zero (from the perspective of quantum theory) happens in some branch — though more probable things happen in far more branches.

This explanation of quantum physics, called the ‘Everettian interpretation’ or ‘many-worlds theory,’ does seem a little crazy. But quantum physics already seems crazy, and that doesn’t make it wrong. While not a consensus position, the many-worlds approach is one of the top three most popular ways to make sense of what’s going on, according to surveys of relevant experts.

Setting aside whether it’s correct for a moment, one thing that’s not often spelled out is what this many-worlds approach would concretely imply if it were right.

Is there a world where Rob (the show’s host) can roll a die a million times, and it comes up 6 every time?

As David explains in this episode: absolutely, that’s completely possible — and if Rob rolled a die a million times, there would be a world like that.

Is there a world where Rob can fly like Superman?

No, that’s physically impossible and quantum randomness doesn’t change that.

Is there a world where Rob becomes president of the US?

David thinks probably not. The things stopping Rob from becoming US president don’t seem down to random chance at the quantum level.

Is there a world where Rob deliberately murdered someone this morning?

Only if he’s already predisposed to murder — becoming a different person in that way probably isn’t a matter of random fluctuations in our brains.

Is there a world where a horse-version of Rob hosts the 80,000 Horses Podcast?

Well, due to the chance involved in evolution, it’s plausible that there are worlds where humans didn’t evolve, and intelligent horses have in some sense taken their place. And somewhere, fantastically distantly across the vast multiverse, there might even be a horse named Rob Wiblin who hosts a podcast, and who sounds remarkably like Rob. Though even then — it wouldn’t actually be Rob in the way we normally think of personal identity.

OK. So if the many-worlds interpretation is right, should that change how we live our lives?

Despite it revolutionising our understanding of what the universe is, David’s view is that it mostly shouldn’t change our actions.

Maybe you now think of a time you drove home drunk without incident as being worse — because there are branches where you actually killed someone. But David thinks that if you’d thought clearly enough about low-probability/high-consequence events, you should already have been very worried about them.

In addition to the above, Rob asks a bunch of burning questions he had about what all this might mean for ethics, including:

  • Are our actions getting more (or less) important as the universe splits into finer and finer threads?
  • If the branching of the universe creates more goodness by there being more stuff, then should we want to do the unpleasant things earlier and pleasant things later on?
  • Is there any way that we could conceivably influence other branches of the multiverse?

David and Rob do their best to introduce quantum mechanics in the first 35 minutes of the episode, but it isn’t the easiest thing to explain via audio alone. So if you need a refresher before jumping in, we recommend this YouTube video.

While exploring what David calls our “best theory of pretty much everything,” they also cover:

  • Why quantum mechanics needs an interpretation at all
  • Alternatives to the many-worlds interpretation and what they have going for them
  • Whether we can count the number of ‘worlds’ that would exist
  • The debate around what quantum mechanics is, and why a consensus answer hasn’t emerged
  • Progress in physics over the last 50 years, and the practical value of physics today
  • The peculiar philosophy of time
  • And much more

Get this episode by subscribing to our podcast on the world’s most pressing problems and how to solve them: type 80,000 Hours into your podcasting app. Or read the transcript below.

Producer: Keiran Harris
Audio mastering: Ryan Kessler
Transcriptions: Sofia Davis-Fogel and Katy Moore

Highlights

What is quantum mechanics?

David Wallace: Quantum mechanics is our best theory of the very small — of atoms and molecules, of the subatomic particles that make them up. But because big things are made of small things, then quantum theory is really our best theory of pretty much everything. So it underpins most of modern physics, from scales right down to the scale of the Higgs boson that gives master particles that we try to look at in the Large Hadron Collider in CERN, all the way up to the quantum fluctuations in the early universe that give rise to the structure of galaxies on the largest scales. And it’s relied on by physics at every scale in between that. Computers will do as just one example where we need quantum mechanics to understand how their components work.

David Wallace: In quantum mechanics, if any object can have one or other property, then it can somehow have both properties at the same time. So, particles aren’t just on the left or on the right. They’re on the left and on the right at the same time. They’re not just spinning this way or spinning that way. They’re spinning this way and that way at the same time.

David Wallace: And then if that was just confined to the microscopic, then that might be okay. Maybe we could just say, “Look, we’re evolved plains apes. Natural selection didn’t suit us to intuit the very small. Maybe we just need a new language to talk about it.” But anytime you engage with this indefiniteness, this “two things at the same time” — what physicists call “superpositions” — then the multiplicity kind of infects the system that’s engaging with it. So if I’ve got one particle that’s in two places at the same time, and I scatter another particle off it, now the other particle would bounce differently if the particle is in one place than if it was in another place. So now, suddenly the scattering particle is doing two things at the same time.

David Wallace: So there’s a temptation to try to think about this “two things at the same time” as just uncertainty or lack of information, that somehow this is just a fancy way of saying, “Well, it might be one thing. It might be another thing. We don’t know which one.” The short answer is that doesn’t work. To do all the explaining work quantum mechanics does, it needs a phenomenon called “interference,” where the two possibilities can reinforce or cancel out in ways that wouldn’t make sense if these were just probabilities. And you’re absolutely right that what really matters is the extent to which this stuff doesn’t stay microscopic.

David Wallace: So, at least according to the theory, if a scientist makes a measurement to say, “Where’s the particle?” and the particle is in two places at the same time, the theory tells us that the measurement device predicts two results at the same time. If the measurement device was an old-fashioned pointer, and it was supposed to point to the left if the particle was on the left and point to the right if the particle was on the right, then according to the theory, if the particle was on the left and on the right at the same time, then the pointer is pointing to the left and to the right at the same time. And that doesn’t just sound unintuitive — that sounds ridiculous. Maybe we can’t even understand what it would be, but at any rate, that’s not what we see pointers doing.

Rob Wiblin: What couldn’t we possibly explain otherwise?

David Wallace: Transistors, DNA, most of modern chemistry, all of particle physics, why nuclear weapons work, why the sun shines, why the galaxies are where they are. I mean, name me a phenomenon in physics you’ve heard of, and I’ll tell you that quantum mechanics is probably needed to explain it.

Many-worlds theory

David Wallace: Quantum theory predicts that cats are alive and dead at the same time. And our immediate response is, “That couldn’t possibly be true.” Why not? Well, I’ve seen lots of cats. You’ve seen lots of cats. We’ve never seen them alive and dead at the same time. What would it look like to see a cat that was alive and dead at the same time? I think your general impression is that it would be sort of like being really drunk, seeing double or something.

David Wallace: That’s your intuition as to what it would look like if you saw a cat that was alive and dead at the same time. But intuition’s a lousy way to predict what you’ll see in a physical theory. There’s a lovely, almost certainly apocryphal, too-good-to-be-true story about Wittgenstein, the philosopher. So supposedly he’s crossing the court in Cambridge with a colleague, and he sort of stops suddenly, as Wittgenstein is wont to do, and says, “Why was everyone so resistant, so surprised by the idea that the earth went around the sun?” And his colleague said, “Well, because it looks as if the sun goes around the earth.” And supposedly Wittgenstein thinks for a minute and says, “Well, what would it have looked like if the earth went around the sun?”

Rob Wiblin: …the same.

David Wallace: Exactly. Yeah. Because this is how it looks, and the earth does go around the sun. And so what was really going on in that kind of, “Well, it doesn’t look like it” is something like our intuition of what it would look like if the earth went around the sun is not the same as how it actually looks. So our intuition is that the sun would be essentially whizzing past, and we would seem to be flung backwards onto the earth by the force of our acceleration, or something. But if you actually ask the physics what it would look like, you realize those things wouldn’t happen. Those are bad intuitions about what being on a moving planet is like. And so similarly, in the quantum case, ask the theory what it would be like to see a cat that’s alive and dead at the same time. You don’t get the seeing double answer; you don’t get the being drunk answer. You want to think something like this: If I saw a live cat, I’d go into a state that you might describe as seeing a live cat.

David Wallace: And if I saw a dead cat, then I go into a state you might call seeing a dead cat. So if I see a cat that’s alive and dead at the same time, according to the equations of quantum mechanics, then I go into a state which is seeing a live cat and seeing a dead cat at the same time. And if I tell you about it on this podcast, then you go into a state of hearing David say the cat’s alive, and hearing David saying the cat’s dead at the same time. And when people listen to the podcast, everyone who hears it goes into this mixture of hearing you reporting the cat’s alive, and hearing you reporting the cat’s dead at the same time. And in a pretty short order, the whole planet knows the cat is alive, and everyone knows the cat is dead at the same time. And those two bits of the theory aren’t talking to each other anymore, these are sort of separate strands of reality inside the quantum state.

David Wallace: I don’t think “splitting” does a bad job of describing it, but you have to understand that all of that is non-fundamental. It’s not that there’s some new fundamental law of physics that says, “Suddenly the world is split.” It’s rather that if you look at what the actual laws of physics tell you, you started off with the world being structured to represent one set of goings on, and then it changes in a way that now it’s structured to represent two sets of goings on. If you shine a light through a partial mirror and you originally had one part of light, and when it hits the mirror one part of light goes off in one direction one goes off in the other direction, did something split? Yeah. But not as a matter of fundamental law. It’s just that the natural way to describe the underlying goings on is that I have two parts of light rather than one part of light.

David Wallace: And so similarly, in the quantum case, the natural way of describing what’s going on is, before the measurement, things were structured in the way of there being one classical world. And after the measurement, there were two parts of disconnected bits of structure in the world. And one of them describes the live cat world, and one of them describes the dead cat world. And then you can layer various bits of metaphor. And then if you want to say, “Well, actually there was a vast number of worlds and they differentiated one from another,” David Deutsch has that way of talking, for instance, you can do that. If you want to say, “Well, the world split,” you can do that too. But the physics doesn’t care. It’s just a way of talking about a higher level of differentiation appearing in the underlying equations. And none of it is fundamental; there’s no completely sharp notion of how many worlds there are, for instance.

What stuff actually happens

David Wallace: I mean the boring quick answer is anything that you thought had a probability greater than zero, according to quantum theory, happens in some branch. But if you then want to interrogate that and ask, what does that mean? Well, are there branches in which you can fly like Superman? No, flying like Superman is physically impossible. Are there branches in which you roll a dice a million times and you get a six every time, yeah, absolutely. That’s highly improbable, but there’s nothing stopping that from happening. Is there a branch in which the charge of the electron is different from what it is? We don’t know. Because our current physics doesn’t tell us whether the charge of the electron is a fundamental thing that’s just written into the laws of physics, or whether it’s actually something a bit more parochial that will come out of some deeper physics. If it’s a bit more parochial, there’ll be some branches where there’s one charge of the electron and some branches will be another charge. If it’s fundamental, then the charge of the electron will be the same everywhere.

Rob Wiblin: Is there a path where I’m U.S. president, constitutional requirements notwithstanding?

David Wallace: My quick guess is probably no. But it’s slightly delicate, because there are probably configurations of incredibly implausible worlds that have the same shape as the configuration in which you became U.S. president. Because of some ridiculously unlikely but not completely impossible series of little fluctuations or disturbances. But is there a history of things happening in which you became U.S. president? I’d be pretty surprised, because I don’t think the events that caused you not to become us president are well characterized as pieces of random chance. I mean, try this as a slightly more mundane example. I mean, it’s a mundane but kind of morally charged example. Is there a branch in which I decided to shoot someone this morning? I don’t own a gun, let’s pretend I own a gun. I hope the answer’s no, because I’m pretty sure I’m not the kind of person that will randomly shoot someone.

David Wallace: And I’m pretty sure that’s not a matter of random fluctuations in my brain. I don’t think it’s like if I walk past someone there’s then a random quantum chance that I shoot someone. There are people out there who when they walk past somebody, have a random chance of killing them. They’re psychopaths with very severe illnesses of various kinds. Ordinary people basically are not in that kind of category. That’s something we’d normally call physically possible, there’s no law of physics that prevents me from shooting someone, but equally the fact that I didn’t shoot someone is not a matter of random chance. So there isn’t a branch in which I shot that person.

David Wallace: I mean, there’s probably a non-zero chance of some amazingly unlikely series of fluctuations in my neurons, such that they all fire in such a way that my arm does move and pull the trigger. But I wouldn’t call that kind of thing me deciding to shoot someone. That’s more like a free muscular spasm or something. Yeah. I mean, this is in some ways as much a philosophy of mind point as a physics point. I mean, to decide to do something is to have reasons and there to be the kind of high-level processes in your brain that count as forming reasons and intentions and acting on them. You can plausibly believe that some of the process of doing that is chancy. I mean, the fact that I decided to wear a blue shirt rather than a white shirt this morning was whimsical.

David Wallace: And maybe that whim is explainable in some deep deterministic way, or maybe it’s genuine quantum chance. But one’s reasoned decisions aren’t whims. It’s particularly easy to say, “Shall I kill someone?” It doesn’t seem very plausible that those decisions for reasons are things that are chancy. I mean, it’s a little bit of a guess about how the philosophy of mind and how the psychology will turn out here. But on reasonable guesses, I don’t think there are going to be these branches where you do weird, awful things or something.

Rob Wiblin: So in order to answer these questions, it seems like you have to go through some process of thinking about what things can be changed through quantum fluctuations. And it sounds like we don’t have a totally unambiguous answer to that.

David Wallace: Yeah. We don’t have a completely unambiguous answer, but we’ve got quite a good answer to it. And the answer basically goes, quantum fluctuations… There’s three big sources of that. One of them is explicit stuff we do in the lab. We actually do a quantum experiment intentionally. That’s a very rare special case. The second is where random quantum fluctuations get magnified up by some natural process. So here’s a mundane example. If you’ve ever seen a flickering fluorescent light tube, that flickering process is a quantum-mechanically random process. So it flickers differently in different worlds. Here’s a slightly more morbid example: whether a given cosmic ray causes a mutation that triggers cancer in you, that is a quantum-mechanically random process. The third category, and I think the most important for working out which of these worlds happen, is that anything that’s classically chaotic becomes quantum mechanically indeterminate relatively quickly.

David Wallace: The brain does not seem to be a randomizing device of that kind. But the weather is, for instance, chaotic. The butterfly flaps its wings or not, then the weather will turn out differently. So if the butterfly’s in a superposition of flapping its wings or not, then the weather will end up in a superposition of different states. So the weather, we can be pretty certain, is different in different branches of the multiverse. And much more dramatic things like the contingencies of chance that lead to one evolutionary process happening and another one not, gives us reason for thinking there were probably sentient horses and sentient velociraptors, it’s again because there’s enough chaotic processes. And that will just get magnified up to quantum chance.

If the many-worlds theory is right, does that change the impact of any of our actions?

David Wallace: I think it mostly doesn’t. That’s a little bit subtle, though. I mean, I’m guessing a lot of the people asking that question are sympathetic to, or at least understand something like a utilitarian picture of ethics, and nothing in the decision theoretic calculus of doing ethics particularly forces you towards a particular utility or disutility. Rational behavior in this framework can be maximized, including rational ethical behavior, can be maximized in expected utility, but the mere principle that that’s what’s rational doesn’t tell you what the utility function is.

David Wallace: So you could say, for instance, maybe in the many-worlds setup, I now realize that my lucky actions where I did something that could have been bad but in fact it wasn’t, I drove drunk or something, but not nothing bad came of it, I could be more aware that, of course, there’ll be branches in which something terrible came of it, and those branches are no less real than my branch, and the suffering in that branch is no less real than the suffering in my branch, therefore maybe I should be much more risk averse in an Everettian framework. Maybe I should put a much higher disutility on bad things, such that even quite low probabilities of bad things shouldn’t deter me from avoiding them. Maybe that’s true. If you thought that, then maybe learning that Everett was true would cause you to adjust your utility function quite sharply. Psychologically, I can’t report that that’s happened to me. My inclination is to think that even without Everett, if you’d thought clearly enough about low-probability/high-consequence events, you should already have been very worried about them. But that needn’t hold in general.

Rob Wiblin: I’ve heard from other people who’ve thought about this a little bit that they also lean towards thinking that it shouldn’t really impact how we evaluate the goodness and badness of different decisions. And it seems like that mostly stems from the fact that before Everett, we thought, say, that there was a 50% chance of outcome A and a 50% chance of outcome B, and then we do some expected value calculation where we weight them by the probability and then goodness. After many worlds, we say half of the worlds are A and half of the worlds are B, and then we weight them by the fraction of the worlds that are in each one, and then you do an expected value calculation across that, and it just looks the same. The math looks the same as long as you decide to use the fraction of the worlds and the probability the same way within your moral framework.

David Wallace: That’s basically right. The thing I’d add to that is that if Everett’s true, it’s been true all along. So when you originally thought that you were deciding what to do based on the probability, what probability really meant all along was the fraction of branches, you just didn’t know it. So the thing you were doing all along was already the Everettian thing. At some level this comes down to how you think about your metaethics. I mean, there’s a certain very pure style in some corners of philosophy, and probably in some of your readers, that says something like, “The way I should think about my ethics is I should just reason from the beginning as to what the virtuous person would do with no external world input, and then I should do it.” And if that’s your basis, then of course, if you were really badly wrong about the metaphysics of the world, like you didn’t know it was branching, maybe learning that fact would cause you to completely change your ethical assessment.

David Wallace: But if you’ve got a bit more naturalistic take on ethics, ethics are what they are because of how they’ve developed, and you’re not going to be able to find a view from nowhere that justifies them, but nonetheless, we’re in the situation we’re in, well then again, the situation we’re in has always been a quantum mechanical situation. To go back to the Wittgenstein example from earlier, should the discovery of the fact that the lights in the sky were stars have changed our ethics? I think the answer is no, in the short run at least. It’s not that cheating on your partner or refusing to give money to save the starving child somehow changes its character because the lights in the sky are other suns.

David Wallace: Of course at some level that transformed our worldview, and in the long run that had big impacts on our ethics and our whole way of thinking about life, and maybe Everett will do that too. But the immediate questions about should I do this or that thing weren’t much changed because we understood how we were situated in the world. The basic mundane things around us were still the same mundane things around us.

Getting our heads around indefinite branches

David Wallace: You might imagine that we lived in a two-dimensional universe. Maybe I’m a two-dimensional fish swimming around in a two-dimensional ocean, and then you might imagine that there could be lots of two-dimensional universes, and they’re stacked on top of each other, and they can interact a little bit. So, I interact a little bit with fish a little bit below me or above me but not at all with fish a long way from me.

David Wallace: If that was true, actually, then probably it wouldn’t make sense to say that I was a fish just in one layer of this big stack of two-dimensional universes, because the processes that made me up might kind of do a certain amount of cohering from one layer to another. So what you’re going to get there is a world of rather thin beings and a world in which entities don’t really interact very far through the stack of two-dimensional universes, but where the sort of autonomous chunks of this are not going to be single slices — they’re going to be slightly indefinitely defined chunks of slices.

David Wallace: Once you’ve got that reality, you might imagine I can really take away the definite slices at all, and I can just say my universe is three-dimensional, but the interactions are very strongly confined to the plane, and they only go a little bit up and down. That’s a situation in which you’ve clearly, in some sense, got a multiverse — the things going on very much deeper into the stack or higher in the stack are not interacting with things at this level, but the world doesn’t really have a sharply, a distinctly discrete breaking down into slices…

David Wallace: On any sensible way of thinking about the branches, there are going to be vast numbers of me who are psychologically indistinguishable from one another. Let’s say somebody in a lab in China is currently looking at a Geiger counter. Well, that Geiger counter is constantly causing the world to branch, but not in any way that’s remotely salient to me.

David Wallace: So uncontentiously, if something like the many-worlds theory is true, there are just lots and lots and lots of branches in which I’m having the same experiences, so it’s not as if… Maybe there’s even wilder metaethics where what I care about is the number of versions of me that are sufficiently psychologically different that they’re having different experiences. But if we’re just talking about a mere count, then again, it’s not obvious that the way you’d want to define that means that the counterversions of me is actually going up. Maybe it’s just that the level of variety across versions of me is going up. Again, the mathematics doesn’t care about this.

Practical value of physics today

Rob Wiblin: Maybe we should kick the can down the road a bit on some of this physics stuff, and focus on solving practical problems. And then leave this as something that future generations can solve with their hopefully vastly superior analytical capabilities.

David Wallace: I don’t think that’s an indefensible position to adopt. Let me give the counter case without necessarily saying the counter case is compelling. The main counter case I’d make is that physics absolutely has the potential to be making a whole bunch of transformative contributions to the world. And the divide between fundamental physics and non-fundamental physics is very blurry in terms of methods.

David Wallace: I’ll give you a concrete example. So at a formal mathematical level, the way we understand the process by which the Higgs boson gives mass to particles is pretty much exactly the same as the way in which we understand how superconductivity is possible. And superconductivity really matters technologically. A room-temperature superconductor would be epically transformative in vast amounts of our infrastructure.

David Wallace: So a whole bunch of things in physics have a lot of potential to be really important to how we develop as a society in the relatively near term. And you really can’t hive off the community of people doing fundamental physics from the community doing those kinds of applicable physics.

David Wallace: If you try the strategy you were discussing, which would have been defensible on the same grounds 50 years ago, you’d materially have harmed the development of our solid-state understanding of superconductivity, because you’d have closed off the important back and forth that was happening between the solid state physicists and the particle physicists.

David Wallace: So I think there’s at least a live argument that that kind of back and forth of techniques and ideas and applications and concepts really means that doing deep theoretical physics is important and contributory, and you can’t really do it in a way that artificially says, “Only do this part of it.”

David Wallace: There are lots and lots of things that could come out of physics that are really important for the way our world would be in the short, medium, and longer terms. I mean, the ones you can immediately think of tend to be things that have probably got a bit too applied to be directly connected to theoretical physics. So something like battery technology… Even probably these days, how you want to make the latest superconductor, then it’s probably true that that development can be seen as a genuine piece of applied physics, but now we’re not talking about distant future million-year, 1,000-year time horizons, now we’re talking about a matter of a few decades.

David Wallace: Techniques like renormalization group theory, which I won’t go into in detail, but it’s a really important analytical tool in huge amounts of physics and even in bits of science beyond physics, is again, something that was developed out of very theoretical considerations in physics. Part of it is about developing mathematical tools and technology. Part of it is about drawing certain analogies. Part of it’s just to do with the general principle that smart people in a particular discipline are not generally helped in their development in that discipline by being artificially corralled.

Articles, books, and other media discussed in the show

David’s work

Introduction to quantum mechanics

More on the many-worlds interpretation

Related episodes

About the show

The 80,000 Hours Podcast features unusually in-depth conversations about the world's most pressing problems and how you can use your career to solve them. We invite guests pursuing a wide range of career paths — from academics and activists to entrepreneurs and policymakers — to analyse the case for and against working on different issues and which approaches are best for solving them.

The 80,000 Hours Podcast is produced and edited by Keiran Harris. Get in touch with feedback or guest suggestions by emailing [email protected].

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