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