Many-Worlds Interpretation Explained: Do Parallel Universes Really Exist?

<v Speaker 1>Welcome to the quart Side Quantum Physics Podcast, an exploration
<v Speaker 1>of the fundamental structure of reality, where quantum laws govern matter, energy,
<v Speaker 1>and information. Here, uncertainty is a feature, not a flaw,
<v Speaker 1>and understanding begins at the smallest scales.
<v Speaker 2>I think everyone has that moment eventually. Oh yeah, you're
<v Speaker 2>sitting in traffic maybe, or you're just staring at a
<v Speaker 2>ceiling fan at three in the morning, and your brain
<v Speaker 2>just starts playing the what if game?
<v Speaker 3>Right, the what if game?
<v Speaker 2>What if I'd taken that job in Chicago, what if
<v Speaker 2>I hadn't ordered the shellfish?
<v Speaker 4>What if I'd turn left instead of right ten years ago.
<v Speaker 3>It's the road not taken exactly.
<v Speaker 2>It's the road not taken. And it's I think it's
<v Speaker 2>fundamental to how we process regret or relief or just
<v Speaker 2>you know, simple curiosity.
<v Speaker 4>It's part of the human condition, it is, and usually we.
<v Speaker 2>Treat these as fantasies, their mental exercises. In fiction, they're
<v Speaker 2>plot devices, you know, the mirror universe where everyone is
<v Speaker 2>evil and has a goateee.
<v Speaker 4>Of course, the goatee is the classic signifier.
<v Speaker 2>But the material we are looking at today suggests something, well,
<v Speaker 2>something pretty unsettling. It suggests that those what ifs aren't fantasies.
<v Speaker 3>At all, not even close.
<v Speaker 2>They might be the literal, mathematical architecture of our reality.
<v Speaker 3>It's a concept that You're right.
<v Speaker 4>It sounds like it belongs in a comic book, but
<v Speaker 4>it's actually one of the most rigorous, I would even
<v Speaker 4>say conservative, and definitely hotly debated ideas in modern physics.
<v Speaker 2>We are talking, of course, about the many world's interpretation
<v Speaker 2>of quantum mechanics. MWI and I want to set the
<v Speaker 2>table properly here because we've gone through a massive stack
<v Speaker 2>of sources, from Hugh Everett's original nineteen fifty seven thesis
<v Speaker 2>all the way to modern analysis by philosophers of science
<v Speaker 2>like David Wallace.
<v Speaker 3>It's a huge body of work.
<v Speaker 2>It is. And the goal today isn't to just say
<v Speaker 2>whoa parallel universes are cool?
<v Speaker 4>Right, that's the popside version. We aren't here for the
<v Speaker 4>sci fi tropes. Yeah, We're here to understand why why
<v Speaker 4>a serious sober physicist would look at the fundamental math
<v Speaker 4>of the universe and conclude that the only logical explanation
<v Speaker 4>is that everything that can happen does happen.
<v Speaker 2>And that is the crux of it, This theory wasn't
<v Speaker 2>invented to write a good story.
<v Speaker 3>No, not at all.
<v Speaker 2>It was invented to fix a problem, a massive glaring
<v Speaker 2>hole in our understanding of how the world actually works.
<v Speaker 3>That's the most important context. You have to start there.
<v Speaker 4>Many world creates a lot of let's say, philosophical headaches,
<v Speaker 4>a few, Yeah, but it solves a physics crisis, a
<v Speaker 4>genuine textbook breaking crisis.
<v Speaker 2>So let's get right into that. I want to start
<v Speaker 2>with the big headline claim, just to get my own
<v Speaker 2>head around it. Again. When we talk about many worlds
<v Speaker 2>or MWI, we are effectively saying that if I had
<v Speaker 2>a choice between coffee and tea this morning, and I
<v Speaker 2>picked coffee, Okay, there's a version of me physically just
<v Speaker 2>as real as I am right now, who picked.
<v Speaker 3>T Yes, that's the claim.
<v Speaker 2>And that version of me is existing right now, in
<v Speaker 2>this moment.
<v Speaker 4>Existing right now in a branch of what's called the
<v Speaker 4>universal wave function that is, for all intents and purposes,
<v Speaker 4>causally disconnected from the one you and I are currently
<v Speaker 4>recording it.
<v Speaker 2>Okay, that is the claim, and it's a big one.
<v Speaker 2>So to understand how we could possibly get there, you're right.
<v Speaker 2>We have to start with the villain of the story.
<v Speaker 2>The villain, yes, the thing that many worlds is trying
<v Speaker 2>to defeat. And in our notes this is referred to
<v Speaker 2>as the quantum measurement problem.
<v Speaker 3>Correct.
<v Speaker 4>You really can't appreciate the elegance of the solution, Everett's
<v Speaker 4>solution unless you understand just how broken and frankly, how
<v Speaker 4>cluggy the standard explanation was.
<v Speaker 2>So let's go back to basics quantum mechanics. We know
<v Speaker 2>that at the subatomic level, electrons photons, the absolute building
<v Speaker 2>blocks of matter. Things do not behave like billiard balls.
<v Speaker 4>Right, It's a completely different rule book. In our macroscopic world,
<v Speaker 4>a billiard ball has a definite position and a definite speed.
<v Speaker 2>It is either here or it is there.
<v Speaker 3>You can point at it, you can point at it.
<v Speaker 4>But in the quantum world, before you measure a particle,
<v Speaker 4>it doesn't have those definite properties. It exists in something
<v Speaker 4>called a superposition.
<v Speaker 2>I've always visualized this as a haze or a cloud
<v Speaker 2>of possibility. The electron isn't a single point, it's a
<v Speaker 2>smear of probability.
<v Speaker 4>That's a good heuristic, It's a really good way to
<v Speaker 4>think about it Mathematically, it's described by something called a
<v Speaker 4>wave function, right, and this wave function assigns a number
<v Speaker 4>technically a complex number and amplitude to every possible location
<v Speaker 4>the electron could be found in.
<v Speaker 2>So the electron is effectively everywhere within that wave at.
<v Speaker 4>Once in a very real physical sense. Yes, it's not
<v Speaker 4>that we don't know where it is. Is that it
<v Speaker 4>doesn't have a single wear.
<v Speaker 2>Okay. So we have this wave, this cloud of possibility,
<v Speaker 2>and we have a law that governs how that wave
<v Speaker 2>moves and changes over time.
<v Speaker 3>We do the Schrodinger equation.
<v Speaker 2>The Shroding your equations.
<v Speaker 4>This is the bedrock, This is the absolute foundation. It
<v Speaker 4>is to quantum mechanics what Newton's laws of motion are
<v Speaker 4>to you throwing a baseball.
<v Speaker 2>And what's the key thing to understand about it?
<v Speaker 4>The key thing, the absolutely critical thing, is that the
<v Speaker 4>Schrodinger equation is deterministic and it is linear.
<v Speaker 2>Okay, let's break that down. Deterministic meaning if I know
<v Speaker 2>the state of the wave right now, I can predict
<v Speaker 2>exactly what it will look like in five.
<v Speaker 3>Minutes, exactly to the letter.
<v Speaker 4>It evolves smoothly, the wave ripples out. It might interfere
<v Speaker 4>with itself. It's changes shape, but it does so in
<v Speaker 4>a perfectly predictable, continuous way.
<v Speaker 2>It never jumps, it never snaps, never.
<v Speaker 3>The math is smooth as glass.
<v Speaker 2>And this is the big butt.
<v Speaker 3>Here's the problem, here's the crisis.
<v Speaker 2>When we actually look at an electron, when we hit
<v Speaker 2>it with a photon, when we measure it with a detector,
<v Speaker 2>we don't see a wave. We don't see a smear.
<v Speaker 2>We see a dot. We see it at one specific location.
<v Speaker 4>And that is a measurement problem in a nutshell. How
<v Speaker 4>do you get from smear to dot? The math says
<v Speaker 4>it should all be a smear our. I say it's
<v Speaker 4>always a dot.
<v Speaker 2>The math is smooth, but reality looks chunky.
<v Speaker 4>Chunky is a great word for it quantized.
<v Speaker 2>So the standard answer, the one that's been taught in
<v Speaker 2>textbooks for nearly a century now, is called the Copenhagen interpretation.
<v Speaker 4>Developed by the giants Nils Bohr, Werner Heisenberg and their colleagues.
<v Speaker 3>In the nineteen twenties.
<v Speaker 2>And what was their solution.
<v Speaker 4>Their solution was to introduce a new rule, a second rule,
<v Speaker 4>completely separate from the Schrodinger equation. They said that when
<v Speaker 4>a measurement is made by an observer, the wave function collapses.
<v Speaker 2>Collapses. It's such a violent word it is.
<v Speaker 4>It goes instantly from that spread out wave of possibilities
<v Speaker 4>to a single definite spike at one location, and all
<v Speaker 4>the other possibility they vanished.
<v Speaker 3>They vanished from reality instantly.
<v Speaker 2>This has always felt incredibly clumsy to me. It's just
<v Speaker 2>it's not elegant. It basically says the laws of physics
<v Speaker 2>work one way when no one is looking, this beautiful
<v Speaker 2>smooth wave like evolution, right, and then they work in
<v Speaker 2>a completely different, random, instantaneous way the moment I opened
<v Speaker 2>my eyes.
<v Speaker 4>It is clumsy. It's what physicists call ad hoc. It's
<v Speaker 4>a rule tacked onto the theory for the sole purpose
<v Speaker 4>of explaining why we don't see superpositions in our daily lives.
<v Speaker 2>But it's not in the math.
<v Speaker 3>It is absolutely not in the math.
<v Speaker 4>The Schrodinger equation doesn't have a collapse term. There's no
<v Speaker 4>variable for observers looking now.
<v Speaker 2>It effectively requires magic.
<v Speaker 3>It does.
<v Speaker 2>It requires the act of observation, maybe even consciousness, to
<v Speaker 2>fundamentally alter physical reality instantaneously and in a way that
<v Speaker 2>literally violates the primary equation of the theory.
<v Speaker 4>And it creates this infamous Heisenberg cut problem, which is
<v Speaker 4>what it's a question of where you draw the line,
<v Speaker 4>where does the quantum world stop and the classical world begin.
<v Speaker 4>Is an atom quantum, yes, obviously, is a molecule made
<v Speaker 4>of atoms? Quantum is a virus, is a bacterium? Is
<v Speaker 4>a cat? Is a human observer? At what point does
<v Speaker 4>an object stop being a wave and become a thing
<v Speaker 4>that causes a wave to collapse?
<v Speaker 2>And Copenhagen can't answer that.
<v Speaker 4>It has no answer. It just says the cut is
<v Speaker 4>somewhere between the small thing and the big thing. It's
<v Speaker 4>completely arbitrary.
<v Speaker 2>So we have this situation where the core math is
<v Speaker 2>beautiful and continuous and predictive, but our interpretation of it
<v Speaker 2>requires this ugly, unexplained, arbitrary snap. Every time we decide
<v Speaker 2>to measure something exactly.
<v Speaker 3>It's a patch.
<v Speaker 4>It's a software fix for a bug in our understanding
<v Speaker 4>of reality, and a lot of physicists, even back then,
<v Speaker 4>we're deeply unhappy with it.
<v Speaker 2>So enter Hugh Everett the Third. It's nineteen fifty seven.
<v Speaker 2>He's a graduate student at Princeton studying under the famous
<v Speaker 2>John Wheeler. He looks at this whole mass.
<v Speaker 3>This whole Frankenstein's monster of a theory.
<v Speaker 2>And he proposes something just breathtakingly radical.
<v Speaker 4>He asks a question that is almost childlike in its simplicity,
<v Speaker 4>it's so powerful. He just asks, what if the Schrodinger
<v Speaker 4>equation is just true all the time, all the time,
<v Speaker 4>for everything, no exceptions.
<v Speaker 2>Meaning, what if the wave function never collapses?
<v Speaker 4>Right? What if we just take that second role, the
<v Speaker 4>collapse postulate, and we just delete it, We throw it
<v Speaker 4>in the trash. What if we assume that the wave
<v Speaker 4>evolves smoothly forever, even during a measurement.
<v Speaker 2>Okay, but that's the whole problem, right, Yeah, if the
<v Speaker 2>waves never collapses, then the superposition never goes away.
<v Speaker 4>It doesn't.
<v Speaker 2>The electron is still in multiple places at once, but
<v Speaker 2>I see it in one place. My detector gives me
<v Speaker 2>one click. How did Everett reconcile the math? Says it's
<v Speaker 2>everywhere with my eyes say it's here.
<v Speaker 3>This is his genius.
<v Speaker 4>This is where we have to talk about entanglement, right,
<v Speaker 4>This is the engine that drives the whole many world's idea.
<v Speaker 2>Usually when we hear entanglement, we think of that spooky
<v Speaker 2>action at a distance between two particles on our part.
<v Speaker 3>Einstein's famous phrase.
<v Speaker 2>Yeah, but ever it applied it in a new way, right.
<v Speaker 2>He applied it to the observer.
<v Speaker 3>He did.
<v Speaker 4>He took the Copenhagen interpretation's biggest problem, the special status
<v Speaker 4>of the observer, and turn it into his solution. He
<v Speaker 4>just treated the observer, the scientist, the camera as just
<v Speaker 4>another quantum system.
<v Speaker 2>Which I mean that just makes sense. We are made
<v Speaker 2>of atoms. Of course, atoms obey the laws of quantum mechanics.
<v Speaker 2>Therefore we obey the laws of quantum mechanics exactly.
<v Speaker 3>There is no cut, there is no classical world.
<v Speaker 4>It's all just quantum mechanics from the bottom all the
<v Speaker 4>way to the top.
<v Speaker 2>So let's trace the chain of events in Everetts view.
<v Speaker 2>Walk me through a measurement.
<v Speaker 4>Okay, you have an electron in a superposition. Let's say
<v Speaker 4>it's fifty percent chance of being at point A and
<v Speaker 4>fifty percent chance.
<v Speaker 3>Of being at point B.
<v Speaker 2>A simple two state system.
<v Speaker 3>Simple two state system.
<v Speaker 4>Yeah, Now, you the observer, want to find out where
<v Speaker 4>it is. So you set up a detector. Maybe it's
<v Speaker 4>a device that flashes a red light if the electron
<v Speaker 4>is at A and a green light if it's at bay.
<v Speaker 4>The electron interacts with your detector at that moment they
<v Speaker 4>become entangled. Now the detector is also in a superposition.
<v Speaker 2>So it's not just the electron anymore. The detector is
<v Speaker 2>now in a state of gonna flash red, gonna flash green.
<v Speaker 4>Precisely, position has grown to include the detector. Now the
<v Speaker 4>light from that flash travels to your eye.
<v Speaker 2>The photon hits my retina.
<v Speaker 4>Now your eye is entangled with the detector, which is
<v Speaker 4>entangled with the electron. Your retina is in a superposition
<v Speaker 4>of registered a red flash and registered a green flash, and.
<v Speaker 2>That signal goes to my brain.
<v Speaker 4>And now you are entangled your brain state. The specific
<v Speaker 4>pattern of neurons firing enters a superposition. There is a
<v Speaker 4>state of you that saw the red light and thinks ah,
<v Speaker 4>the electron was at A in a state of U
<v Speaker 4>that saw the green light and thinks ah, it was
<v Speaker 4>at B.
<v Speaker 2>So the superposition didn't disappear, It didn't collapse, It just grew,
<v Speaker 2>It spread, it.
<v Speaker 4>Spread, it engulfed you. The wave function of the entire
<v Speaker 4>universe now contains two distinct components, two branches, and.
<v Speaker 2>In one component there is the U who saw a
<v Speaker 2>and in the other a U who.
<v Speaker 3>Saw B both physically real.
<v Speaker 2>And because the schroding Your equation is linear, these two components,
<v Speaker 2>these two branches, they just they continue to evolve independently
<v Speaker 2>from that point on.
<v Speaker 3>That's the key.
<v Speaker 4>Once they split, for all practical purposes, they don't interact anymore.
<v Speaker 4>This is why it's called the many world's interpretation. It's
<v Speaker 4>not that the universe physically splits apart like a log
<v Speaker 4>being chopped by an axe.
<v Speaker 2>That's the sci fi image it is.
<v Speaker 4>It's more like the universal wave function differentiates into non
<v Speaker 4>interacting branches, each of which contains a consistent version of reality,
<v Speaker 4>each of which feels from the inside like a complete universe.
<v Speaker 2>This brings us inevitably to the cat. I feel like
<v Speaker 2>we are legally required by the laws of physics podcasting
<v Speaker 2>to discuss Schrodinger's cat.
<v Speaker 4>We can't avoid the cat. It's the most famous thought
<v Speaker 4>experiment in history.
<v Speaker 2>So the classic setup a cat in a box. There's
<v Speaker 2>a single radioactive atom, there's a Geiger counter. If the
<v Speaker 2>atom decays, it triggers the.
<v Speaker 4>Counter, which releases a hammer, which smashes a vial of poison,
<v Speaker 4>and the.
<v Speaker 2>Cat dies A bit morbid but okay, if the atom
<v Speaker 2>doesn't decay, nothing happens.
<v Speaker 4>Cat lives, right, and you leave it in there for
<v Speaker 4>exactly one half life, So there's a perfect fifty to
<v Speaker 4>fifty chance of decay.
<v Speaker 2>And since the atom is a quantum system, it's not
<v Speaker 2>decayed or not decayed. It's in a quantum superposition of
<v Speaker 2>decayed ind not decayed.
<v Speaker 4>Which means the entire system in the box becomes entangled.
<v Speaker 4>The Geiger counter is in a superposition of triggered and
<v Speaker 4>not triggered. The hammers fallen and not fallen, the poison
<v Speaker 4>is released and not released.
<v Speaker 2>And therefore the cat ends up in a superposition of
<v Speaker 2>dead and.
<v Speaker 4>Alive, which is the absurd conclusion Schrodinger was trying to
<v Speaker 4>point out. He was trying to show how ridiculous the
<v Speaker 4>Copenhagen view was when you scaled it up right.
<v Speaker 2>He was saying, look, if you take your math seriously,
<v Speaker 2>you get this nonsense. A cat that's both dead and alive.
<v Speaker 4>And in the Copenhagen view, the absurdity is resolved.
<v Speaker 3>When you open the box.
<v Speaker 4>The active observation forces the universe to choose snap, the
<v Speaker 4>wave function collapses, the cat is definitively dead. Or are
<v Speaker 4>definitively alive.
<v Speaker 2>But in everts view.
<v Speaker 4>In everts view, the universe makes no choice. The math
<v Speaker 4>is never absurd. The math is the reality. The atom
<v Speaker 4>is both. The poison is released and not released.
<v Speaker 3>The cat is dead and alive.
<v Speaker 2>And when I open the lid.
<v Speaker 4>You become part of the system. You become entangled with
<v Speaker 4>the contents of the box. The superposition just expands one
<v Speaker 4>final time to include you.
<v Speaker 2>So the universe now contains a branch.
<v Speaker 4>A branch with a dead cat and a very sad
<v Speaker 4>observer who is now looking at.
<v Speaker 2>A dead cat in another branch, and.
<v Speaker 4>A completely separate branch with a live cat and a
<v Speaker 4>very happy, relieved observer.
<v Speaker 2>And the crucial point here, the thing that's so hard
<v Speaker 2>to grasp, is that both of those observers are me.
<v Speaker 4>Yes, they're both you. They both share your exact memories
<v Speaker 4>up to the very moment of the split. They both
<v Speaker 4>feel like the one and only real you.
<v Speaker 2>The you looking at the live cat feels like the
<v Speaker 2>only you. He has no idea his counterpart.
<v Speaker 4>Exists, and the you looking at the dead cat feels
<v Speaker 4>like the only you. He is now living in a
<v Speaker 4>world where the cat died and he has no access
<v Speaker 4>to the reality where it lived.
<v Speaker 2>This solves the measurement problem because there is no measurement problem.
<v Speaker 4>There's no collapse. The math just keeps running.
<v Speaker 3>It's seamless.
<v Speaker 2>It creates a perfectly seamless deterministic physics. The entire wave
<v Speaker 2>function of the universe evolves according to one single rule.
<v Speaker 4>But the cost, the cost is that we have to
<v Speaker 4>accept that all these other outcomes, all these other branches,
<v Speaker 4>actually exist. They are not possibilities, they are realities, which.
<v Speaker 2>Brings up the immediate subjective objection, the thing everyone asks.
<v Speaker 3>First, I know exactly what you're.
<v Speaker 2>Going to say. I don't feel like I'm splitting. If
<v Speaker 2>I'm constantly branching into literally billions of copies every second,
<v Speaker 2>shouldn't I feel I don't know, stretched or diluted or something.
<v Speaker 4>It's a very common intuition, a very natural one. But
<v Speaker 4>the physics actually explains precisely why you don't feel it.
<v Speaker 4>It's a phenomenon called decoherence.
<v Speaker 2>We see this term a lot in the sources. It's
<v Speaker 2>a more modern addition to Everett's original idea. Right, he
<v Speaker 2>didn't have the full picture.
<v Speaker 4>That's right, the full mathematical theory of decoherence was really
<v Speaker 4>developed in the seventies and eighties, and it's what makes
<v Speaker 4>MWI a truly robust modern theory. It's the wall between
<v Speaker 4>the worlds.
<v Speaker 2>So what is it? How does it work?
<v Speaker 4>Think of it this way. Why do we see weird
<v Speaker 4>quantum effects like superposition and interference in tiny isolated atoms
<v Speaker 4>but not in big things like bowling balls, Because they're
<v Speaker 4>it's not just the size, it's the interaction. Big things
<v Speaker 4>are constantly interacting with the environment, right.
<v Speaker 2>Air molecules, photons from the lights, thermal radiation. They're constantly
<v Speaker 2>bouncing off us.
<v Speaker 4>Billions and billions of them every nanosecond, and all those
<v Speaker 4>tiny interactions carry away information. They scramble the phases of
<v Speaker 4>the different parts of the wave function. This process is decoherence.
<v Speaker 2>Okay, scramble the phases. I like the analogy of the
<v Speaker 2>radio stations that was used in one of the papers.
<v Speaker 2>Can we unpack that?
<v Speaker 4>It's a perfect analogy. Imagine you have two radio stations.
<v Speaker 4>If they are broadcasting on frequencies that are very very
<v Speaker 4>close together, say one on one point five FM and
<v Speaker 4>one on one point.
<v Speaker 2>Five to one FM, You'll get in inference.
<v Speaker 4>You'll get interference you'll hear a staticky mess. You'll hear
<v Speaker 4>bits of both at once. That's like a quantum superposition
<v Speaker 4>where the branches are still coherent. They can still interact
<v Speaker 4>and interfere with each other, like.
<v Speaker 2>In the famous double flit experiment, where an electron wave interferes.
<v Speaker 3>With itself exactly.
<v Speaker 4>But as the system you the cat, the detector interacts
<v Speaker 4>with the environment, the branches get kicked by all those
<v Speaker 4>air molecules and photons, and it causes them to drift
<v Speaker 4>apart in frequency.
<v Speaker 2>They become mathematically orthogonal. Orthogonal is the technical term. It's
<v Speaker 2>a fancy word from geometry that just means they're at
<v Speaker 2>right angles to each other in this abstract mathematical space.
<v Speaker 3>But they have zero overlap.
<v Speaker 2>Zero overlap. Yeah. Once decoherence happens, which for a macroscopic
<v Speaker 2>object like a cat or a person, takes an unthinkably
<v Speaker 2>tiny fraction of a second, the live cat branch and
<v Speaker 2>the dead cat branch are completely out of phase with
<v Speaker 2>each other. They can no longer affect each other in
<v Speaker 2>any way. So if I am the live cat observer,
<v Speaker 2>the reason I don't see that goes to the dead
<v Speaker 2>cat is this reason I don't hear ninety eight point
<v Speaker 2>five FM when my radio is tuned to one on
<v Speaker 2>one point five FM.
<v Speaker 4>It's a perfect analogy. The radio waves for that other
<v Speaker 4>station are in the room. They're passing through your body
<v Speaker 4>right now. They're physically present.
<v Speaker 2>But my receiver, my reality isn't in tuned to them.
<v Speaker 4>You are dynamically isolated from them. They can't interfere, they
<v Speaker 4>can't make your speaker vibrate. They are, for all intents
<v Speaker 4>and purposes, in a separate universe.
<v Speaker 2>So the worlds aren't floating in bubbles in outer space. No,
<v Speaker 2>they are superimposed right here, right now, in this room.
<v Speaker 4>They occupy the same space. They just don't interact.
<v Speaker 2>That is I mean, it's logically sound, but it is
<v Speaker 2>emotionally terrifying.
<v Speaker 4>It's what makes the theory so robust. Though it's not
<v Speaker 4>an extra assumption, it's a direct consequence of the Schrewdinger
<v Speaker 4>equation itself. It explains why the world looks classical while
<v Speaker 4>we see one definite thing, even if the underlying reality
<v Speaker 4>is this massive, branching quantum wave.
<v Speaker 2>But let's play Devil's advocate for a bit, because this
<v Speaker 2>theory is still very contras It's not like every physicist
<v Speaker 2>is signed on. The sources identify two big persistent attacks
<v Speaker 2>on MWI, right, the classics, the probability problem and the
<v Speaker 2>extravagance complain, let's.
<v Speaker 4>Tackle extravagance first, because it's the more philosophical one. This
<v Speaker 4>is basically the Oukham's razor argument.
<v Speaker 2>Ouckham's razor says, the simplest explanation is usually the best one.
<v Speaker 4>Don't multiply entities beyond necessity, And.
<v Speaker 2>Critics look at MWI and say, hold on, you're proposing
<v Speaker 2>an infinite or near infinite number of universes branching off
<v Speaker 2>every microsecond, just to explain where a single electron is.
<v Speaker 2>That is the opposite of simple, That is ontologically bloated.
<v Speaker 4>It's often called ontological extravagance. Yeah, things wasteful. Why create
<v Speaker 4>all these worlds?
<v Speaker 2>But the MWI defenders and evert yourself, even back in
<v Speaker 2>the fifties have a very clever rebuttal to.
<v Speaker 3>This they do.
<v Speaker 4>They argue that you have to distinguish between the complexity
<v Speaker 4>of the theory and the complexity of the world the
<v Speaker 4>theory describes.
<v Speaker 2>Okay, unpack that difference for us.
<v Speaker 4>The theory of many worlds is incre ardibly simple. You
<v Speaker 4>could write it on a napkin. It consists of one equation,
<v Speaker 4>the Schrodinger equation.
<v Speaker 3>That's it.
<v Speaker 4>No collapse postulate, no hidden variables, no measurement rules, no
<v Speaker 4>arbitrary cut between quantum and classical. Just one beautiful linear
<v Speaker 4>law that always applies.
<v Speaker 2>So the input the theoretical machinery is as simple as
<v Speaker 2>it could possibly be.
<v Speaker 4>But that simple input, when you let it run, generates
<v Speaker 4>a fantastically complex output the multiverse.
<v Speaker 2>Okay, Now compare that to the Copenhagen interpretation.
<v Speaker 4>Copenhagen gives you a simple output one world the one we.
<v Speaker 2>See, which feels less extravagant.
<v Speaker 4>But to get that simple output, the theory has to
<v Speaker 4>be a mess. You need the Schrodinger equation plus the
<v Speaker 4>collapse postulate, plus a definition of measurement, plus a rule
<v Speaker 4>for where to place the Heisenberg cut.
<v Speaker 2>So the argument is which is more parsimonious simple laws
<v Speaker 2>that generate a complex universe or complex messy ad hoc
<v Speaker 2>laws to force the universe to stay simple?
<v Speaker 4>Exactly, and Everett argues that the entire history of science
<v Speaker 4>has been about finding the simplest possible laws, even if
<v Speaker 4>they reveal the universe to be larger and stranger than
<v Speaker 4>we were comfortable with.
<v Speaker 2>That's a great point when we realized the Earth wasn't
<v Speaker 2>the center of the solar.
<v Speaker 3>System, a perfect example.
<v Speaker 2>The math game much simpler. Heliocentrism is mathematically cleaner than
<v Speaker 2>the epicycles of geocentrism, but the universe suddenly got way
<v Speaker 2>way bigger, and we feeled a lot less important.
<v Speaker 4>Precisely, MWI is just the ultimate extension of that Copernican principle.
<v Speaker 4>We aren't special. Our outcome isn't the only one. We
<v Speaker 4>aren't the only show in town.
<v Speaker 2>Okay, I think that's a very compelling defense for the
<v Speaker 2>extravagance charge, But the probability problem seems stickier. This one
<v Speaker 2>actually gave me a headache while I was reading about it.
<v Speaker 4>It's the technical achilles heel of the theory, and it's
<v Speaker 4>where most of the modern debate happens.
<v Speaker 2>So here's my understanding of it. In standard quantum mechanics,
<v Speaker 2>we use something called the Born rule to calculate probability
<v Speaker 2>correct It says that the probability of an outcome is
<v Speaker 2>related to the amplitude of it. It's part of the
<v Speaker 2>wave function. If the wave amplitude is big, the probability
<v Speaker 2>is high.
<v Speaker 4>Probability equals amplitude squared to be precise, and it works perfectly.
<v Speaker 4>It's one of the most well tested rules in all
<v Speaker 4>of science.
<v Speaker 2>Right, But in many worlds everything happens. If I flip
<v Speaker 2>a quantum coin in one branch, it's heads, in another
<v Speaker 2>its tails. Both outcomes exist with one hundred percent certainty
<v Speaker 2>in the multiverse as a whole. Yes, So how can
<v Speaker 2>I say there's a fifty percent chance of heads. The
<v Speaker 2>concept of chance seems to lose all meaning if every
<v Speaker 2>outcome is guaranteed to occur somewhere, or thirty percent chance
<v Speaker 2>of rain.
<v Speaker 3>This is a deep, deep problem.
<v Speaker 4>If all branches are equally real, why do we experience
<v Speaker 4>the world as if they aren't. Why do we experience
<v Speaker 4>the statistics of the born rule?
<v Speaker 2>Right? If I buy a lottery ticket, there is a
<v Speaker 2>branch where I win. I know that for a fact
<v Speaker 2>in mwy So why do I feel correctly that I
<v Speaker 2>am almost certainly going to find myself in a branch
<v Speaker 2>where I lose?
<v Speaker 4>The modern defense, and this is largely from philosophers. A
<v Speaker 4>physic like David Wallace and Simon Saunders relies on something
<v Speaker 4>called self locating uncertainty and the concept of branch weight.
<v Speaker 2>Branch weight. Okay, this sounds like we're literally weighing the universes.
<v Speaker 4>In a sense, you are go back to the wave function.
<v Speaker 4>It's not just a flat list of possibilities.
<v Speaker 3>It has a shape.
<v Speaker 4>Some parts of the wave are thicker than others, they
<v Speaker 4>have a higher amplitude.
<v Speaker 2>Is this the loaf of bread analogy?
<v Speaker 4>I saw, Yes, I think that's a great one. Imagine
<v Speaker 4>the total wave function of the universe. After you buy
<v Speaker 4>a lottery ticket is a giant loaf of bread. After
<v Speaker 4>the draw, you slice it. One slice, the lost the
<v Speaker 4>lottery slice is ninety nine point nine nine nine nine
<v Speaker 4>nine percent of.
<v Speaker 2>The loaf, and the one the lottery slice is a microscopics.
<v Speaker 4>Liver, a tiny crumb. Now, both slices exist, both are real.
<v Speaker 2>But the argument is that you, the rational agent who
<v Speaker 2>is about to experience one of these outcomes, should care
<v Speaker 2>about the measure the weight of the brand you are
<v Speaker 2>ending up in.
<v Speaker 4>It's about where the bulk of U flows. The probability
<v Speaker 4>isn't about if an outcome happened, It's about what fraction
<v Speaker 4>of the total unice ends up in that outcome.
<v Speaker 2>So it's not about if it happens, it's about where
<v Speaker 2>the bulk of reality flows exactly.
<v Speaker 4>Philosophers like David Wallace argue this using decision theory. They
<v Speaker 4>basically prove that if you want to be a rational
<v Speaker 4>actor in a branching universe, if you want to make
<v Speaker 4>good bets, the only logical strategy is to bet according
<v Speaker 4>to the weight of the branches, which just happens to
<v Speaker 4>match the Born rule.
<v Speaker 2>So when I say there was a one percent chance
<v Speaker 2>of this happening, what I'm really saying is after this event,
<v Speaker 2>one percent of the total stuff of the multiverse, including me,
<v Speaker 2>will be in branches where this happened.
<v Speaker 4>Yes, and therefore you should expect to find yourself to
<v Speaker 4>wake up in the next moment in the thicker branch.
<v Speaker 4>It's a very subtle argument, and frankly, not everyone buys it.
<v Speaker 4>It's still hotly debated, but it's the best and most
<v Speaker 4>rigorous answer MWI has.
<v Speaker 2>It redefines probability from how often does this happen to
<v Speaker 2>how much me ends up there.
<v Speaker 3>That's a perfect summary.
<v Speaker 2>So okay, let's briefly look at the alternatives, because is
<v Speaker 2>not the only game in town. If we don't like
<v Speaker 2>it and we hate the magic of Copenhagen, what's left
<v Speaker 2>on the menu.
<v Speaker 4>Well, the sources we looked at highlight two main contenders
<v Speaker 4>that try to offer a different way out, pilot wave
<v Speaker 4>theory and objective Clacks theories.
<v Speaker 2>Let's start with pilotwave. This is also called Bohemian mechanics.
<v Speaker 4>Right after David Bone. This theory tries to restore the
<v Speaker 4>classical particle. It says, look, the electron is a real
<v Speaker 4>particle and it is in one definite place at all times,
<v Speaker 4>so no superposition of location, no superpositional location. But it
<v Speaker 4>says the particle is guided by a pilot wave that
<v Speaker 4>is also real and tells it where to go.
<v Speaker 2>So the wave is real and the particle is like
<v Speaker 2>a tiny surfer riding on it.
<v Speaker 4>That's a great way to put it. This gets rid
<v Speaker 4>of the multiverse and the measurement problem. There is only
<v Speaker 4>one world and the particle is always in it. But
<v Speaker 4>the price you.
<v Speaker 3>Pay is steep.
<v Speaker 2>What's the catch?
<v Speaker 4>It requires extreme non locality. The pilot wave has to
<v Speaker 4>instantly know the position of every other particle in the
<v Speaker 4>U S universe to correctly guide your one particle that.
<v Speaker 2>Sounds like it clashes pretty hard with Einstein's relativity, which
<v Speaker 2>says nothing can travel faster than light.
<v Speaker 3>It does.
<v Speaker 4>It's very, very hard to reconcile with Einstein, and to
<v Speaker 4>many physicists it feels a bit artificial, like you're forcing
<v Speaker 4>the universe to be classical just because you want it
<v Speaker 4>to be by inventing this extra pilot wave entity.
<v Speaker 2>Okay, so that's one and the other was objective collapse.
<v Speaker 4>These are theories with names like GRW. They take a
<v Speaker 4>different approach. They say, maybe the Schrodinger equation is just
<v Speaker 4>slightly wrong.
<v Speaker 2>WHOA, Okay, that is a bold move changing the fundamental
<v Speaker 2>equation of quantum mechanics.
<v Speaker 3>It is.
<v Speaker 4>They add a tiny new mathematical term to the equation,
<v Speaker 4>and this new term basically says, every once in a while,
<v Speaker 4>completely on its own, a superposition just collapses randomly, no
<v Speaker 4>observer needed.
<v Speaker 2>So it's a built in self destruct mechanism for superpositions.
<v Speaker 4>Yes, and it's set up so that for a single particle,
<v Speaker 4>this spontaneous collapse would almost never happen, but for a
<v Speaker 4>big object with trillions of particles, a collapse is guaranteed
<v Speaker 4>to happen almost instantly.
<v Speaker 2>So that's how it explains why we don't see cats
<v Speaker 2>and superpositions exactly.
<v Speaker 4>And the real beauty of this idea is that it's testable.
<v Speaker 4>If the math is different, we should be able to
<v Speaker 4>design an experiment to measure it. Have we we have,
<v Speaker 4>And that's the problem for these theories. We keep doing
<v Speaker 4>experiments making larger and larger objects into superpositions and keeping
<v Speaker 4>them isolated, and we never see them collapse spontaneously.
<v Speaker 2>So we're pushing the boundary.
<v Speaker 4>We've done it with massive molecules, things with thousands of atoms.
<v Speaker 4>They still act like perfect quantum waves. So the experimental
<v Speaker 4>evidence is actually squeezing the objective collapse theories out of existence.
<v Speaker 2>The more we confirm that the schroding your equation is
<v Speaker 2>correct for bigger and bigger things, the more many worlds
<v Speaker 2>just looks like the default truth you're left with if
<v Speaker 2>you take the math series.
<v Speaker 4>That's exactly the modern argument for it. It's the simplest
<v Speaker 4>theory whose predictions haven't been falsified.
<v Speaker 2>This leads us to the legacy of the theory. Touch
<v Speaker 2>on the personal story of Hugh Everett again, because it's
<v Speaker 2>honestly tragic.
<v Speaker 3>It really is.
<v Speaker 4>Here's a guy who, in his mid twenties as a
<v Speaker 4>grad student, solves what is arguably the biggest conceptual problem
<v Speaker 4>in all of physics. He writes this brilliant, revolutionary thesis,
<v Speaker 4>and the reaction from the physics community crickets, crickets, or worse,
<v Speaker 4>open mockery. Neils Bohr, who was basically the pope of
<v Speaker 4>quantum physics at the time, completely dismissed it without even
<v Speaker 4>really engaging with it.
<v Speaker 2>His advisor, the famous John Wheeler, he tried to protect
<v Speaker 2>him right, but he was also caught in the middle.
<v Speaker 4>Wheeler was a diplomat. He saw the genius in Everett's idea,
<v Speaker 4>but he also wanted to stay on good terms with
<v Speaker 4>Bohr and the Copenhagen establishment, so he made effort rewrite
<v Speaker 4>the thesis. They cut out all the most radical and
<v Speaker 4>most exciting language about splitting worlds and observers, and they
<v Speaker 4>called it the relative state formulation. They made it as
<v Speaker 4>dry and obscure and mathematically dense as possible.
<v Speaker 2>It buried the lead.
<v Speaker 4>They completely buried it, and Everett, who was by all
<v Speaker 4>accounts a proud and sensitive man, was just discouraged. He
<v Speaker 4>basically said, fine, if you don't want my idea, and
<v Speaker 4>he walked away from academic physics.
<v Speaker 2>He went to work for the Pentagon doing Cold war analysis.
<v Speaker 4>He did he became a defense analyst. He used his
<v Speaker 4>incredible mathematical genius to model nuclear war strategies and weapons systems.
<v Speaker 4>He spent the rest of his life in a suit
<v Speaker 4>chainsmoking calculating megadeths, while his theory about the fundamental nature
<v Speaker 4>of reality gathered dust on a shelf.
<v Speaker 2>And he died young, didn't he before the theory really
<v Speaker 2>started to get popular.
<v Speaker 4>He died of a heart attack in nineteen eighty two
<v Speaker 4>at age fifty one, just as the theory was starting
<v Speaker 4>to be rediscovered and championed by people like Bryce DeWitt,
<v Speaker 4>who coined the name Many Worlds, and a young David Deutsch.
<v Speaker 4>Everett never saw his vindication.
<v Speaker 2>Speaking of David Deutsch, this is a connection I hadn't
<v Speaker 2>made before going through the sources, the really strong link
<v Speaker 2>between Many Worlds and the field of quantum computing.
<v Speaker 4>This is a crucial modern piece of the puzzle. David
<v Speaker 4>Deutsch's one of the founding fathers of quantum computing, and
<v Speaker 4>he is a die hard, unapologetic proponent of Many Worlds.
<v Speaker 2>Why what's the connection? I mean, it seems like two
<v Speaker 2>separate fields.
<v Speaker 4>Think about how a quantum computer actually works. It's not
<v Speaker 4>just a faster classical computer. It doesn't just check one
<v Speaker 4>answer at a time. It puts its bits, its quibits,
<v Speaker 4>into a massive superposition to check millions or billions of
<v Speaker 4>possible answers simultaneously.
<v Speaker 2>Right, it's parallel processing on an exponential scale.
<v Speaker 4>So Deutsch asks a very simple, very practical, almost engineering
<v Speaker 4>level question, where are those calculations actually happening?
<v Speaker 2>I see where this is going.
<v Speaker 4>If you have a quantum computer chip with just three
<v Speaker 4>hundred interacting quibits, it can perform more simultaneous calculations than
<v Speaker 4>there are atoms in the entire visible universe.
<v Speaker 2>That's a staggering number.
<v Speaker 4>It is, so Deutsch asks, if there is only one universe,
<v Speaker 4>how can that one little silicon chip, which is made
<v Speaker 4>of a finite number of atoms, hold and process a
<v Speaker 4>quantity of information larger than the universe itself.
<v Speaker 2>It doesn't fit. The numbers don't add up.
<v Speaker 4>Deutsch argues that the only logical explanation is that the
<v Speaker 4>computer is outsourcing the work. It's running the calculations and
<v Speaker 4>the other parallel branches of the multiverse, and then using
<v Speaker 4>quantum interference to bring the results back together in our
<v Speaker 4>branch to give us the final answer.
<v Speaker 2>So, in his view, a working quantum computer is functional
<v Speaker 2>experimental proof of the multiverse exactly.
<v Speaker 4>He says, you ask where the computation happened, I can
<v Speaker 4>tell you where the answer came from. It came from
<v Speaker 4>those other worlds that you can't see. It's a very bold,
<v Speaker 4>but very concrete claim.
<v Speaker 2>That creates a tangible bridge between this high concept, almost
<v Speaker 2>metaphysical idea and the technology we are building in labs
<v Speaker 2>right now.
<v Speaker 4>It takes it out of the realm of philosophy and
<v Speaker 4>puts it into the realm of engineering.
<v Speaker 2>I want to spend our last section talking about the implications,
<v Speaker 2>because if we accept this, if we just provisionally accept
<v Speaker 2>that the math is real and this is how the
<v Speaker 2>universe works, it changes our understanding of everything time identity debt, meaning.
<v Speaker 4>It rewrites the human condition from the ground up.
<v Speaker 2>Let's start with the arrow of time. We all have
<v Speaker 2>this incredibly strong perception of time as flowing forward. The
<v Speaker 2>past is fixed and singular, the future is open and
<v Speaker 2>full of possibilities. How does MWI explain that in a.
<v Speaker 3>Really beautiful way.
<v Speaker 4>Actually in MWY, the branching structure of the universe is.
<v Speaker 3>The arrow of time.
<v Speaker 2>Explain that.
<v Speaker 4>Think of the multiverse as a tree. At the bottom,
<v Speaker 4>you have a single trunk. That's the big bang, that's
<v Speaker 4>the past. It's a state of low entropy, high order,
<v Speaker 4>a single history. As you move up the trunk, as
<v Speaker 4>time moves forward, it splits into branches, and those branches
<v Speaker 4>split into smaller branches, and they split again and again
<v Speaker 4>into a vast complex canopy.
<v Speaker 2>So the future is the canopy.
<v Speaker 4>The future is high entropy precisely because it is highly branched.
<v Speaker 4>There are vastly more ways for the universe to be
<v Speaker 4>than there were in the past. And you can't go
<v Speaker 4>back in time for the same reason you can't climb
<v Speaker 4>down a tree and expect all all the branches to
<v Speaker 4>magically merge back into one single twig.
<v Speaker 2>The structure of the multiverse itself defines the direction of time.
<v Speaker 4>It connects thermodynamics and the second law with quantum mechanics
<v Speaker 4>in a really profound way.
<v Speaker 2>That's beautiful, it really is. But then we get to identity,
<v Speaker 2>and this is the part that I think really freaks
<v Speaker 2>people out.
<v Speaker 3>Oh yeah.
<v Speaker 2>The question which one is the real me?
<v Speaker 4>And the simple and very unsettling answer is they are.
<v Speaker 2>All you, but they're so different. In one branch, I'm
<v Speaker 2>a millionaire because I bought the right stock. In another,
<v Speaker 2>I'm homeless. In one, I died in a car crash
<v Speaker 2>this morning. How can all of those be the same person?
<v Speaker 4>Because you share a common causal past up until the
<v Speaker 4>moment of the split, the stock purchase, getting in the car,
<v Speaker 4>you're the exact same physical entity. You share every memory,
<v Speaker 4>every scar, every childhood trauma, every thought.
<v Speaker 2>So I'm not a single person moving through time. I'm
<v Speaker 2>more like a colony or a process that's constantly diversifying.
<v Speaker 4>You are a branching system. That's the best way to
<v Speaker 4>think about it. It completely dissolves the class classical idea
<v Speaker 4>of a singular, indivisible soul or self that persists unchanged
<v Speaker 4>through time.
<v Speaker 2>It reminds me of that old philosophical question about the
<v Speaker 2>transporter and Star.
<v Speaker 3>Trek, the teleporter problem.
<v Speaker 2>Yet, if it scans you, destroys the original, and reassembles
<v Speaker 2>you somewhere else, is it still you? MWA says, it
<v Speaker 2>copies you, doesn't destroy the original, and both are you?
<v Speaker 4>And this leads to some very heavy ethical questions that
<v Speaker 4>philosophers are grappling with. How so, let's say I do
<v Speaker 4>something reckless, like I decide to drive home drunk. In
<v Speaker 4>a single universe world, I might get lucky, I might
<v Speaker 4>get home safe, and I think few got away with it.
<v Speaker 4>I count that as a win. But in the multiverse,
<v Speaker 4>in the multiverse, because there's a quantum randomness in the
<v Speaker 4>swerve of my hands or the path of another car,
<v Speaker 4>you have guaranteed that in some branches you killed someone,
<v Speaker 4>you have created worlds of tragedy.
<v Speaker 2>So moral responsibility becomes about branch weight. You have a
<v Speaker 2>responsibility to act in a way that minimizes the measure
<v Speaker 2>the total amount of the suffering worlds exactly.
<v Speaker 4>You want to act in a way that maximizes the
<v Speaker 4>good branches.
<v Speaker 3>So to speak.
<v Speaker 4>It forces you to think about consequences not as maybe,
<v Speaker 4>but as somewhere.
<v Speaker 2>That is a heavy, heavy burden.
<v Speaker 4>It is, But on the other side of that coin,
<v Speaker 4>it also offers a strange comfort also regret. We started
<v Speaker 4>this whole conversation talking about the road not taken. What
<v Speaker 4>if I had done X? MWI says you did do
<v Speaker 4>X is not here, not in this branch. Right, you
<v Speaker 4>don't have to mourn the lost potential as something that
<v Speaker 4>vanished from existence. That potential was realized. The girl you
<v Speaker 4>didn't ask out in another branch. You asked her out,
<v Speaker 4>and maybe you're happily married. The book you didn't write,
<v Speaker 4>you wrote it and it was a bestseller.
<v Speaker 2>It's all happening. The symphony is playing every possible note.
<v Speaker 4>We just only get to hear one melody line at
<v Speaker 4>a time.
<v Speaker 2>As we wrap up, I want to leave the listener
<v Speaker 2>with a final thought that I found in the notes,
<v Speaker 2>which really hammered this home for me. It's about the
<v Speaker 2>location of these other worlds.
<v Speaker 4>This is the thing people get wrong most often thanks
<v Speaker 4>to science fiction.
<v Speaker 2>We watch movies and we think parallel universes are somewhere else,
<v Speaker 2>in another dimension through glowing portal, a galaxy, far far away.
<v Speaker 3>Right, spatially distant.
<v Speaker 2>But if Many Worlds is correct, they aren't far away
<v Speaker 2>at all. They are here.
<v Speaker 4>They are less than an atom's width away from you.
<v Speaker 4>They are right where you are.
<v Speaker 2>So the version of me that ordered the tea instead
<v Speaker 2>of the coffee this morning, huh, he is sitting in
<v Speaker 2>this exact chair right now, he.
<v Speaker 4>Is in the same position in space, occupying the same
<v Speaker 4>volume of space. The wave function of his body completely
<v Speaker 4>overlaps with the wave function of your body.
<v Speaker 2>But because we have decohered.
<v Speaker 4>Because your atoms and his atoms are massively out of
<v Speaker 4>phase with each other, you pass right through each other
<v Speaker 4>like ghosts. The radio waves of his reality are washing
<v Speaker 4>over you this very second, but you can't tune in.
<v Speaker 2>It makes the room feel very crowded.
<v Speaker 3>All of a sudden, it is crowded.
<v Speaker 4>We are surrounded by infinite variations of ourselves, enacting every
<v Speaker 4>possible tree, tragedy and triumph silently, invisibly, right under our noses.
<v Speaker 2>And every time a neuron fires in your brain to
<v Speaker 2>make a decision, every time a cosmic raise zigs instead
<v Speaker 2>of zags, you are adding another ghost to the room.
<v Speaker 4>You are building the multiverse moment by moment.
<v Speaker 2>Well, on that particularly existential note, I think I'm gonna
<v Speaker 2>go flip a coin and consciously split the universe.
<v Speaker 3>Into make it a good one.
<v Speaker 2>This has been mind bending. Thanks for walking through it
<v Speaker 2>with me.
<v Speaker 3>Always is my pleasure.
<v Speaker 2>Catch him, that's branch