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Many-Worlds Interpretation Explained: Do Parallel Universes Really Exist? · The Quark Side - Quantum Physics Podcast

• 0:01
  Speaker 1: Welcome to the quart Side Quantum Physics Podcast, an exploration
• 0:05
  Speaker 1: of the fundamental structure of reality, where quantum laws govern matter, energy,
• 0:11
  Speaker 1: and information. Here, uncertainty is a feature, not a flaw,
• 0:16
  Speaker 1: and understanding begins at the smallest scales.
• 0:27
  Speaker 2: I think everyone has that moment eventually. Oh yeah, you're
• 0:30
  Speaker 2: sitting in traffic maybe, or you're just staring at a
• 0:33
  Speaker 2: ceiling fan at three in the morning, and your brain
• 0:35
  Speaker 2: just starts playing the what if game?
• 0:37
  Speaker 3: Right, the what if game?
• 0:38
  Speaker 2: What if I'd taken that job in Chicago, what if
• 0:40
  Speaker 2: I hadn't ordered the shellfish?
• 0:42
  Speaker 4: What if I'd turn left instead of right ten years ago.
• 0:45
  Speaker 3: It's the road not taken exactly.
• 0:47
  Speaker 2: It's the road not taken. And it's I think it's
• 0:49
  Speaker 2: fundamental to how we process regret or relief or just
• 0:56
  Speaker 2: you know, simple curiosity.
• 0:57
  Speaker 4: It's part of the human condition, it is, and usually we.
• 1:00
  Speaker 2: Treat these as fantasies, their mental exercises. In fiction, they're
• 1:04
  Speaker 2: plot devices, you know, the mirror universe where everyone is
• 1:07
  Speaker 2: evil and has a goateee.
• 1:09
  Speaker 4: Of course, the goatee is the classic signifier.
• 1:12
  Speaker 2: But the material we are looking at today suggests something, well,
• 1:16
  Speaker 2: something pretty unsettling. It suggests that those what ifs aren't fantasies.
• 1:21
  Speaker 3: At all, not even close.
• 1:23
  Speaker 2: They might be the literal, mathematical architecture of our reality.
• 1:27
  Speaker 3: It's a concept that You're right.
• 1:28
  Speaker 4: It sounds like it belongs in a comic book, but
• 1:30
  Speaker 4: it's actually one of the most rigorous, I would even
• 1:33
  Speaker 4: say conservative, and definitely hotly debated ideas in modern physics.
• 1:38
  Speaker 2: We are talking, of course, about the many world's interpretation
• 1:41
  Speaker 2: of quantum mechanics. MWI and I want to set the
• 1:43
  Speaker 2: table properly here because we've gone through a massive stack
• 1:47
  Speaker 2: of sources, from Hugh Everett's original nineteen fifty seven thesis
• 1:52
  Speaker 2: all the way to modern analysis by philosophers of science
• 1:55
  Speaker 2: like David Wallace.
• 1:56
  Speaker 3: It's a huge body of work.
• 1:58
  Speaker 2: It is. And the goal today isn't to just say
• 2:01
  Speaker 2: whoa parallel universes are cool?
• 2:03
  Speaker 4: Right, that's the popside version. We aren't here for the
• 2:06
  Speaker 4: sci fi tropes. Yeah, We're here to understand why why
• 2:10
  Speaker 4: a serious sober physicist would look at the fundamental math
• 2:13
  Speaker 4: of the universe and conclude that the only logical explanation
• 2:17
  Speaker 4: is that everything that can happen does happen.
• 2:19
  Speaker 2: And that is the crux of it, This theory wasn't
• 2:21
  Speaker 2: invented to write a good story.
• 2:23
  Speaker 3: No, not at all.
• 2:23
  Speaker 2: It was invented to fix a problem, a massive glaring
• 2:27
  Speaker 2: hole in our understanding of how the world actually works.
• 2:30
  Speaker 3: That's the most important context. You have to start there.
• 2:34
  Speaker 4: Many world creates a lot of let's say, philosophical headaches,
• 2:37
  Speaker 4: a few, Yeah, but it solves a physics crisis, a
• 2:41
  Speaker 4: genuine textbook breaking crisis.
• 2:43
  Speaker 2: So let's get right into that. I want to start
• 2:45
  Speaker 2: with the big headline claim, just to get my own
• 2:48
  Speaker 2: head around it. Again. When we talk about many worlds
• 2:52
  Speaker 2: or MWI, we are effectively saying that if I had
• 2:55
  Speaker 2: a choice between coffee and tea this morning, and I
• 2:58
  Speaker 2: picked coffee, Okay, there's a version of me physically just
• 3:02
  Speaker 2: as real as I am right now, who picked.
• 3:04
  Speaker 3: T Yes, that's the claim.
• 3:06
  Speaker 2: And that version of me is existing right now, in
• 3:09
  Speaker 2: this moment.
• 3:10
  Speaker 4: Existing right now in a branch of what's called the
• 3:13
  Speaker 4: universal wave function that is, for all intents and purposes,
• 3:16
  Speaker 4: causally disconnected from the one you and I are currently
• 3:18
  Speaker 4: recording it.
• 3:19
  Speaker 2: Okay, that is the claim, and it's a big one.
• 3:21
  Speaker 2: So to understand how we could possibly get there, you're right.
• 3:24
  Speaker 2: We have to start with the villain of the story.
• 3:26
  Speaker 2: The villain, yes, the thing that many worlds is trying
• 3:28
  Speaker 2: to defeat. And in our notes this is referred to
• 3:31
  Speaker 2: as the quantum measurement problem.
• 3:33
  Speaker 3: Correct.
• 3:33
  Speaker 4: You really can't appreciate the elegance of the solution, Everett's
• 3:37
  Speaker 4: solution unless you understand just how broken and frankly, how
• 3:41
  Speaker 4: cluggy the standard explanation was.
• 3:44
  Speaker 2: So let's go back to basics quantum mechanics. We know
• 3:47
  Speaker 2: that at the subatomic level, electrons photons, the absolute building
• 3:51
  Speaker 2: blocks of matter. Things do not behave like billiard balls.
• 3:55
  Speaker 4: Right, It's a completely different rule book. In our macroscopic world,
• 3:59
  Speaker 4: a billiard ball has a definite position and a definite speed.
• 4:02
  Speaker 2: It is either here or it is there.
• 4:05
  Speaker 3: You can point at it, you can point at it.
• 4:06
  Speaker 4: But in the quantum world, before you measure a particle,
• 4:09
  Speaker 4: it doesn't have those definite properties. It exists in something
• 4:12
  Speaker 4: called a superposition.
• 4:14
  Speaker 2: I've always visualized this as a haze or a cloud
• 4:16
  Speaker 2: of possibility. The electron isn't a single point, it's a
• 4:20
  Speaker 2: smear of probability.
• 4:22
  Speaker 4: That's a good heuristic, It's a really good way to
• 4:24
  Speaker 4: think about it Mathematically, it's described by something called a
• 4:27
  Speaker 4: wave function, right, and this wave function assigns a number
• 4:31
  Speaker 4: technically a complex number and amplitude to every possible location
• 4:35
  Speaker 4: the electron could be found in.
• 4:36
  Speaker 2: So the electron is effectively everywhere within that wave at.
• 4:40
  Speaker 4: Once in a very real physical sense. Yes, it's not
• 4:43
  Speaker 4: that we don't know where it is. Is that it
• 4:45
  Speaker 4: doesn't have a single wear.
• 4:46
  Speaker 2: Okay. So we have this wave, this cloud of possibility,
• 4:50
  Speaker 2: and we have a law that governs how that wave
• 4:53
  Speaker 2: moves and changes over time.
• 4:55
  Speaker 3: We do the Schrodinger equation.
• 4:57
  Speaker 2: The Shroding your equations.
• 4:58
  Speaker 4: This is the bedrock, This is the absolute foundation. It
• 5:02
  Speaker 4: is to quantum mechanics what Newton's laws of motion are
• 5:06
  Speaker 4: to you throwing a baseball.
• 5:08
  Speaker 2: And what's the key thing to understand about it?
• 5:10
  Speaker 4: The key thing, the absolutely critical thing, is that the
• 5:13
  Speaker 4: Schrodinger equation is deterministic and it is linear.
• 5:17
  Speaker 2: Okay, let's break that down. Deterministic meaning if I know
• 5:20
  Speaker 2: the state of the wave right now, I can predict
• 5:22
  Speaker 2: exactly what it will look like in five.
• 5:24
  Speaker 3: Minutes, exactly to the letter.
• 5:26
  Speaker 4: It evolves smoothly, the wave ripples out. It might interfere
• 5:29
  Speaker 4: with itself. It's changes shape, but it does so in
• 5:32
  Speaker 4: a perfectly predictable, continuous way.
• 5:34
  Speaker 2: It never jumps, it never snaps, never.
• 5:37
  Speaker 3: The math is smooth as glass.
• 5:39
  Speaker 2: And this is the big butt.
• 5:40
  Speaker 3: Here's the problem, here's the crisis.
• 5:42
  Speaker 2: When we actually look at an electron, when we hit
• 5:45
  Speaker 2: it with a photon, when we measure it with a detector,
• 5:48
  Speaker 2: we don't see a wave. We don't see a smear.
• 5:51
  Speaker 2: We see a dot. We see it at one specific location.
• 5:54
  Speaker 4: And that is a measurement problem in a nutshell. How
• 5:56
  Speaker 4: do you get from smear to dot? The math says
• 5:59
  Speaker 4: it should all be a smear our. I say it's
• 6:02
  Speaker 4: always a dot.
• 6:02
  Speaker 2: The math is smooth, but reality looks chunky.
• 6:07
  Speaker 4: Chunky is a great word for it quantized.
• 6:10
  Speaker 2: So the standard answer, the one that's been taught in
• 6:12
  Speaker 2: textbooks for nearly a century now, is called the Copenhagen interpretation.
• 6:17
  Speaker 4: Developed by the giants Nils Bohr, Werner Heisenberg and their colleagues.
• 6:22
  Speaker 3: In the nineteen twenties.
• 6:23
  Speaker 2: And what was their solution.
• 6:24
  Speaker 4: Their solution was to introduce a new rule, a second rule,
• 6:27
  Speaker 4: completely separate from the Schrodinger equation. They said that when
• 6:31
  Speaker 4: a measurement is made by an observer, the wave function collapses.
• 6:35
  Speaker 2: Collapses. It's such a violent word it is.
• 6:37
  Speaker 4: It goes instantly from that spread out wave of possibilities
• 6:40
  Speaker 4: to a single definite spike at one location, and all
• 6:43
  Speaker 4: the other possibility they vanished.
• 6:45
  Speaker 3: They vanished from reality instantly.
• 6:47
  Speaker 2: This has always felt incredibly clumsy to me. It's just
• 6:51
  Speaker 2: it's not elegant. It basically says the laws of physics
• 6:55
  Speaker 2: work one way when no one is looking, this beautiful
• 6:58
  Speaker 2: smooth wave like evolution, right, and then they work in
• 7:02
  Speaker 2: a completely different, random, instantaneous way the moment I opened
• 7:06
  Speaker 2: my eyes.
• 7:07
  Speaker 4: It is clumsy. It's what physicists call ad hoc. It's
• 7:11
  Speaker 4: a rule tacked onto the theory for the sole purpose
• 7:13
  Speaker 4: of explaining why we don't see superpositions in our daily lives.
• 7:18
  Speaker 2: But it's not in the math.
• 7:19
  Speaker 3: It is absolutely not in the math.
• 7:20
  Speaker 4: The Schrodinger equation doesn't have a collapse term. There's no
• 7:23
  Speaker 4: variable for observers looking now.
• 7:26
  Speaker 2: It effectively requires magic.
• 7:28
  Speaker 3: It does.
• 7:28
  Speaker 2: It requires the act of observation, maybe even consciousness, to
• 7:32
  Speaker 2: fundamentally alter physical reality instantaneously and in a way that
• 7:37
  Speaker 2: literally violates the primary equation of the theory.
• 7:39
  Speaker 4: And it creates this infamous Heisenberg cut problem, which is
• 7:43
  Speaker 4: what it's a question of where you draw the line,
• 7:45
  Speaker 4: where does the quantum world stop and the classical world begin.
• 7:48
  Speaker 4: Is an atom quantum, yes, obviously, is a molecule made
• 7:51
  Speaker 4: of atoms? Quantum is a virus, is a bacterium? Is
• 7:54
  Speaker 4: a cat? Is a human observer? At what point does
• 7:57
  Speaker 4: an object stop being a wave and become a thing
• 8:00
  Speaker 4: that causes a wave to collapse?
• 8:02
  Speaker 2: And Copenhagen can't answer that.
• 8:04
  Speaker 4: It has no answer. It just says the cut is
• 8:05
  Speaker 4: somewhere between the small thing and the big thing. It's
• 8:08
  Speaker 4: completely arbitrary.
• 8:09
  Speaker 2: So we have this situation where the core math is
• 8:12
  Speaker 2: beautiful and continuous and predictive, but our interpretation of it
• 8:16
  Speaker 2: requires this ugly, unexplained, arbitrary snap. Every time we decide
• 8:21
  Speaker 2: to measure something exactly.
• 8:22
  Speaker 3: It's a patch.
• 8:23
  Speaker 4: It's a software fix for a bug in our understanding
• 8:26
  Speaker 4: of reality, and a lot of physicists, even back then,
• 8:29
  Speaker 4: we're deeply unhappy with it.
• 8:31
  Speaker 2: So enter Hugh Everett the Third. It's nineteen fifty seven.
• 8:34
  Speaker 2: He's a graduate student at Princeton studying under the famous
• 8:37
  Speaker 2: John Wheeler. He looks at this whole mass.
• 8:39
  Speaker 3: This whole Frankenstein's monster of a theory.
• 8:42
  Speaker 2: And he proposes something just breathtakingly radical.
• 8:45
  Speaker 4: He asks a question that is almost childlike in its simplicity,
• 8:48
  Speaker 4: it's so powerful. He just asks, what if the Schrodinger
• 8:51
  Speaker 4: equation is just true all the time, all the time,
• 8:54
  Speaker 4: for everything, no exceptions.
• 8:56
  Speaker 2: Meaning, what if the wave function never collapses?
• 9:01
  Speaker 4: Right? What if we just take that second role, the
• 9:03
  Speaker 4: collapse postulate, and we just delete it, We throw it
• 9:06
  Speaker 4: in the trash. What if we assume that the wave
• 9:08
  Speaker 4: evolves smoothly forever, even during a measurement.
• 9:12
  Speaker 2: Okay, but that's the whole problem, right, Yeah, if the
• 9:15
  Speaker 2: waves never collapses, then the superposition never goes away.
• 9:18
  Speaker 4: It doesn't.
• 9:19
  Speaker 2: The electron is still in multiple places at once, but
• 9:22
  Speaker 2: I see it in one place. My detector gives me
• 9:25
  Speaker 2: one click. How did Everett reconcile the math? Says it's
• 9:29
  Speaker 2: everywhere with my eyes say it's here.
• 9:31
  Speaker 3: This is his genius.
• 9:32
  Speaker 4: This is where we have to talk about entanglement, right,
• 9:35
  Speaker 4: This is the engine that drives the whole many world's idea.
• 9:38
  Speaker 2: Usually when we hear entanglement, we think of that spooky
• 9:41
  Speaker 2: action at a distance between two particles on our part.
• 9:44
  Speaker 3: Einstein's famous phrase.
• 9:45
  Speaker 2: Yeah, but ever it applied it in a new way, right.
• 9:47
  Speaker 2: He applied it to the observer.
• 9:49
  Speaker 3: He did.
• 9:49
  Speaker 4: He took the Copenhagen interpretation's biggest problem, the special status
• 9:54
  Speaker 4: of the observer, and turn it into his solution. He
• 9:57
  Speaker 4: just treated the observer, the scientist, the camera as just
• 10:01
  Speaker 4: another quantum system.
• 10:02
  Speaker 2: Which I mean that just makes sense. We are made
• 10:04
  Speaker 2: of atoms. Of course, atoms obey the laws of quantum mechanics.
• 10:09
  Speaker 2: Therefore we obey the laws of quantum mechanics exactly.
• 10:13
  Speaker 3: There is no cut, there is no classical world.
• 10:15
  Speaker 4: It's all just quantum mechanics from the bottom all the
• 10:18
  Speaker 4: way to the top.
• 10:18
  Speaker 2: So let's trace the chain of events in Everetts view.
• 10:21
  Speaker 2: Walk me through a measurement.
• 10:22
  Speaker 4: Okay, you have an electron in a superposition. Let's say
• 10:26
  Speaker 4: it's fifty percent chance of being at point A and
• 10:28
  Speaker 4: fifty percent chance.
• 10:29
  Speaker 3: Of being at point B.
• 10:30
  Speaker 2: A simple two state system.
• 10:31
  Speaker 3: Simple two state system.
• 10:33
  Speaker 4: Yeah, Now, you the observer, want to find out where
• 10:36
  Speaker 4: it is. So you set up a detector. Maybe it's
• 10:39
  Speaker 4: a device that flashes a red light if the electron
• 10:41
  Speaker 4: is at A and a green light if it's at bay.
• 10:45
  Speaker 4: The electron interacts with your detector at that moment they
• 10:48
  Speaker 4: become entangled. Now the detector is also in a superposition.
• 10:51
  Speaker 2: So it's not just the electron anymore. The detector is
• 10:54
  Speaker 2: now in a state of gonna flash red, gonna flash green.
• 10:58
  Speaker 4: Precisely, position has grown to include the detector. Now the
• 11:02
  Speaker 4: light from that flash travels to your eye.
• 11:05
  Speaker 2: The photon hits my retina.
• 11:06
  Speaker 4: Now your eye is entangled with the detector, which is
• 11:09
  Speaker 4: entangled with the electron. Your retina is in a superposition
• 11:13
  Speaker 4: of registered a red flash and registered a green flash, and.
• 11:16
  Speaker 2: That signal goes to my brain.
• 11:18
  Speaker 4: And now you are entangled your brain state. The specific
• 11:22
  Speaker 4: pattern of neurons firing enters a superposition. There is a
• 11:26
  Speaker 4: state of you that saw the red light and thinks ah,
• 11:29
  Speaker 4: the electron was at A in a state of U
• 11:31
  Speaker 4: that saw the green light and thinks ah, it was
• 11:33
  Speaker 4: at B.
• 11:34
  Speaker 2: So the superposition didn't disappear, It didn't collapse, It just grew,
• 11:38
  Speaker 2: It spread, it.
• 11:39
  Speaker 4: Spread, it engulfed you. The wave function of the entire
• 11:42
  Speaker 4: universe now contains two distinct components, two branches, and.
• 11:46
  Speaker 2: In one component there is the U who saw a
• 11:49
  Speaker 2: and in the other a U who.
• 11:51
  Speaker 3: Saw B both physically real.
• 11:53
  Speaker 2: And because the schroding Your equation is linear, these two components,
• 11:57
  Speaker 2: these two branches, they just they continue to evolve independently
• 12:01
  Speaker 2: from that point on.
• 12:02
  Speaker 3: That's the key.
• 12:03
  Speaker 4: Once they split, for all practical purposes, they don't interact anymore.
• 12:06
  Speaker 4: This is why it's called the many world's interpretation. It's
• 12:09
  Speaker 4: not that the universe physically splits apart like a log
• 12:12
  Speaker 4: being chopped by an axe.
• 12:13
  Speaker 2: That's the sci fi image it is.
• 12:15
  Speaker 4: It's more like the universal wave function differentiates into non
• 12:18
  Speaker 4: interacting branches, each of which contains a consistent version of reality,
• 12:23
  Speaker 4: each of which feels from the inside like a complete universe.
• 12:27
  Speaker 2: This brings us inevitably to the cat. I feel like
• 12:30
  Speaker 2: we are legally required by the laws of physics podcasting
• 12:33
  Speaker 2: to discuss Schrodinger's cat.
• 12:35
  Speaker 4: We can't avoid the cat. It's the most famous thought
• 12:38
  Speaker 4: experiment in history.
• 12:39
  Speaker 2: So the classic setup a cat in a box. There's
• 12:42
  Speaker 2: a single radioactive atom, there's a Geiger counter. If the
• 12:45
  Speaker 2: atom decays, it triggers the.
• 12:47
  Speaker 4: Counter, which releases a hammer, which smashes a vial of poison,
• 12:52
  Speaker 4: and the.
• 12:52
  Speaker 2: Cat dies A bit morbid but okay, if the atom
• 12:55
  Speaker 2: doesn't decay, nothing happens.
• 12:57
  Speaker 4: Cat lives, right, and you leave it in there for
• 12:59
  Speaker 4: exactly one half life, So there's a perfect fifty to
• 13:01
  Speaker 4: fifty chance of decay.
• 13:02
  Speaker 2: And since the atom is a quantum system, it's not
• 13:05
  Speaker 2: decayed or not decayed. It's in a quantum superposition of
• 13:09
  Speaker 2: decayed ind not decayed.
• 13:10
  Speaker 4: Which means the entire system in the box becomes entangled.
• 13:14
  Speaker 4: The Geiger counter is in a superposition of triggered and
• 13:17
  Speaker 4: not triggered. The hammers fallen and not fallen, the poison
• 13:21
  Speaker 4: is released and not released.
• 13:23
  Speaker 2: And therefore the cat ends up in a superposition of
• 13:25
  Speaker 2: dead and.
• 13:25
  Speaker 4: Alive, which is the absurd conclusion Schrodinger was trying to
• 13:29
  Speaker 4: point out. He was trying to show how ridiculous the
• 13:31
  Speaker 4: Copenhagen view was when you scaled it up right.
• 13:34
  Speaker 2: He was saying, look, if you take your math seriously,
• 13:36
  Speaker 2: you get this nonsense. A cat that's both dead and alive.
• 13:40
  Speaker 4: And in the Copenhagen view, the absurdity is resolved.
• 13:42
  Speaker 3: When you open the box.
• 13:44
  Speaker 4: The active observation forces the universe to choose snap, the
• 13:48
  Speaker 4: wave function collapses, the cat is definitively dead. Or are
• 13:52
  Speaker 4: definitively alive.
• 13:54
  Speaker 2: But in everts view.
• 13:55
  Speaker 4: In everts view, the universe makes no choice. The math
• 13:58
  Speaker 4: is never absurd. The math is the reality. The atom
• 14:01
  Speaker 4: is both. The poison is released and not released.
• 14:04
  Speaker 3: The cat is dead and alive.
• 14:06
  Speaker 2: And when I open the lid.
• 14:07
  Speaker 4: You become part of the system. You become entangled with
• 14:10
  Speaker 4: the contents of the box. The superposition just expands one
• 14:14
  Speaker 4: final time to include you.
• 14:16
  Speaker 2: So the universe now contains a branch.
• 14:18
  Speaker 4: A branch with a dead cat and a very sad
• 14:21
  Speaker 4: observer who is now looking at.
• 14:23
  Speaker 2: A dead cat in another branch, and.
• 14:25
  Speaker 4: A completely separate branch with a live cat and a
• 14:28
  Speaker 4: very happy, relieved observer.
• 14:30
  Speaker 2: And the crucial point here, the thing that's so hard
• 14:32
  Speaker 2: to grasp, is that both of those observers are me.
• 14:36
  Speaker 4: Yes, they're both you. They both share your exact memories
• 14:39
  Speaker 4: up to the very moment of the split. They both
• 14:41
  Speaker 4: feel like the one and only real you.
• 14:44
  Speaker 2: The you looking at the live cat feels like the
• 14:46
  Speaker 2: only you. He has no idea his counterpart.
• 14:49
  Speaker 4: Exists, and the you looking at the dead cat feels
• 14:52
  Speaker 4: like the only you. He is now living in a
• 14:54
  Speaker 4: world where the cat died and he has no access
• 14:57
  Speaker 4: to the reality where it lived.
• 14:58
  Speaker 2: This solves the measurement problem because there is no measurement problem.
• 15:01
  Speaker 4: There's no collapse. The math just keeps running.
• 15:03
  Speaker 3: It's seamless.
• 15:04
  Speaker 2: It creates a perfectly seamless deterministic physics. The entire wave
• 15:08
  Speaker 2: function of the universe evolves according to one single rule.
• 15:12
  Speaker 4: But the cost, the cost is that we have to
• 15:15
  Speaker 4: accept that all these other outcomes, all these other branches,
• 15:19
  Speaker 4: actually exist. They are not possibilities, they are realities, which.
• 15:23
  Speaker 2: Brings up the immediate subjective objection, the thing everyone asks.
• 15:27
  Speaker 3: First, I know exactly what you're.
• 15:29
  Speaker 2: Going to say. I don't feel like I'm splitting. If
• 15:31
  Speaker 2: I'm constantly branching into literally billions of copies every second,
• 15:35
  Speaker 2: shouldn't I feel I don't know, stretched or diluted or something.
• 15:40
  Speaker 4: It's a very common intuition, a very natural one. But
• 15:43
  Speaker 4: the physics actually explains precisely why you don't feel it.
• 15:47
  Speaker 4: It's a phenomenon called decoherence.
• 15:49
  Speaker 2: We see this term a lot in the sources. It's
• 15:50
  Speaker 2: a more modern addition to Everett's original idea. Right, he
• 15:53
  Speaker 2: didn't have the full picture.
• 15:54
  Speaker 4: That's right, the full mathematical theory of decoherence was really
• 15:58
  Speaker 4: developed in the seventies and eighties, and it's what makes
• 16:01
  Speaker 4: MWI a truly robust modern theory. It's the wall between
• 16:05
  Speaker 4: the worlds.
• 16:06
  Speaker 2: So what is it? How does it work?
• 16:07
  Speaker 4: Think of it this way. Why do we see weird
• 16:10
  Speaker 4: quantum effects like superposition and interference in tiny isolated atoms
• 16:15
  Speaker 4: but not in big things like bowling balls, Because they're
• 16:18
  Speaker 4: it's not just the size, it's the interaction. Big things
• 16:21
  Speaker 4: are constantly interacting with the environment, right.
• 16:24
  Speaker 2: Air molecules, photons from the lights, thermal radiation. They're constantly
• 16:29
  Speaker 2: bouncing off us.
• 16:30
  Speaker 4: Billions and billions of them every nanosecond, and all those
• 16:34
  Speaker 4: tiny interactions carry away information. They scramble the phases of
• 16:38
  Speaker 4: the different parts of the wave function. This process is decoherence.
• 16:42
  Speaker 2: Okay, scramble the phases. I like the analogy of the
• 16:45
  Speaker 2: radio stations that was used in one of the papers.
• 16:48
  Speaker 2: Can we unpack that?
• 16:49
  Speaker 4: It's a perfect analogy. Imagine you have two radio stations.
• 16:52
  Speaker 4: If they are broadcasting on frequencies that are very very
• 16:54
  Speaker 4: close together, say one on one point five FM and
• 16:57
  Speaker 4: one on one point.
• 16:58
  Speaker 2: Five to one FM, You'll get in inference.
• 17:00
  Speaker 4: You'll get interference you'll hear a staticky mess. You'll hear
• 17:03
  Speaker 4: bits of both at once. That's like a quantum superposition
• 17:07
  Speaker 4: where the branches are still coherent. They can still interact
• 17:10
  Speaker 4: and interfere with each other, like.
• 17:12
  Speaker 2: In the famous double flit experiment, where an electron wave interferes.
• 17:15
  Speaker 3: With itself exactly.
• 17:17
  Speaker 4: But as the system you the cat, the detector interacts
• 17:20
  Speaker 4: with the environment, the branches get kicked by all those
• 17:23
  Speaker 4: air molecules and photons, and it causes them to drift
• 17:26
  Speaker 4: apart in frequency.
• 17:28
  Speaker 2: They become mathematically orthogonal. Orthogonal is the technical term. It's
• 17:32
  Speaker 2: a fancy word from geometry that just means they're at
• 17:34
  Speaker 2: right angles to each other in this abstract mathematical space.
• 17:37
  Speaker 3: But they have zero overlap.
• 17:38
  Speaker 2: Zero overlap. Yeah. Once decoherence happens, which for a macroscopic
• 17:43
  Speaker 2: object like a cat or a person, takes an unthinkably
• 17:46
  Speaker 2: tiny fraction of a second, the live cat branch and
• 17:49
  Speaker 2: the dead cat branch are completely out of phase with
• 17:52
  Speaker 2: each other. They can no longer affect each other in
• 17:55
  Speaker 2: any way. So if I am the live cat observer,
• 17:57
  Speaker 2: the reason I don't see that goes to the dead
• 17:59
  Speaker 2: cat is this reason I don't hear ninety eight point
• 18:02
  Speaker 2: five FM when my radio is tuned to one on
• 18:04
  Speaker 2: one point five FM.
• 18:05
  Speaker 4: It's a perfect analogy. The radio waves for that other
• 18:07
  Speaker 4: station are in the room. They're passing through your body
• 18:10
  Speaker 4: right now. They're physically present.
• 18:12
  Speaker 2: But my receiver, my reality isn't in tuned to them.
• 18:15
  Speaker 4: You are dynamically isolated from them. They can't interfere, they
• 18:18
  Speaker 4: can't make your speaker vibrate. They are, for all intents
• 18:21
  Speaker 4: and purposes, in a separate universe.
• 18:23
  Speaker 2: So the worlds aren't floating in bubbles in outer space. No,
• 18:27
  Speaker 2: they are superimposed right here, right now, in this room.
• 18:30
  Speaker 4: They occupy the same space. They just don't interact.
• 18:33
  Speaker 2: That is I mean, it's logically sound, but it is
• 18:36
  Speaker 2: emotionally terrifying.
• 18:38
  Speaker 4: It's what makes the theory so robust. Though it's not
• 18:40
  Speaker 4: an extra assumption, it's a direct consequence of the Schrewdinger
• 18:45
  Speaker 4: equation itself. It explains why the world looks classical while
• 18:49
  Speaker 4: we see one definite thing, even if the underlying reality
• 18:52
  Speaker 4: is this massive, branching quantum wave.
• 18:55
  Speaker 2: But let's play Devil's advocate for a bit, because this
• 18:58
  Speaker 2: theory is still very contras It's not like every physicist
• 19:01
  Speaker 2: is signed on. The sources identify two big persistent attacks
• 19:06
  Speaker 2: on MWI, right, the classics, the probability problem and the
• 19:10
  Speaker 2: extravagance complain, let's.
• 19:11
  Speaker 4: Tackle extravagance first, because it's the more philosophical one. This
• 19:14
  Speaker 4: is basically the Oukham's razor argument.
• 19:16
  Speaker 2: Ouckham's razor says, the simplest explanation is usually the best one.
• 19:19
  Speaker 4: Don't multiply entities beyond necessity, And.
• 19:22
  Speaker 2: Critics look at MWI and say, hold on, you're proposing
• 19:27
  Speaker 2: an infinite or near infinite number of universes branching off
• 19:31
  Speaker 2: every microsecond, just to explain where a single electron is.
• 19:35
  Speaker 2: That is the opposite of simple, That is ontologically bloated.
• 19:38
  Speaker 4: It's often called ontological extravagance. Yeah, things wasteful. Why create
• 19:42
  Speaker 4: all these worlds?
• 19:43
  Speaker 2: But the MWI defenders and evert yourself, even back in
• 19:47
  Speaker 2: the fifties have a very clever rebuttal to.
• 19:49
  Speaker 3: This they do.
• 19:50
  Speaker 4: They argue that you have to distinguish between the complexity
• 19:53
  Speaker 4: of the theory and the complexity of the world the
• 19:56
  Speaker 4: theory describes.
• 19:57
  Speaker 2: Okay, unpack that difference for us.
• 19:58
  Speaker 4: The theory of many worlds is incre ardibly simple. You
• 20:00
  Speaker 4: could write it on a napkin. It consists of one equation,
• 20:03
  Speaker 4: the Schrodinger equation.
• 20:05
  Speaker 3: That's it.
• 20:05
  Speaker 4: No collapse postulate, no hidden variables, no measurement rules, no
• 20:09
  Speaker 4: arbitrary cut between quantum and classical. Just one beautiful linear
• 20:14
  Speaker 4: law that always applies.
• 20:15
  Speaker 2: So the input the theoretical machinery is as simple as
• 20:18
  Speaker 2: it could possibly be.
• 20:19
  Speaker 4: But that simple input, when you let it run, generates
• 20:22
  Speaker 4: a fantastically complex output the multiverse.
• 20:25
  Speaker 2: Okay, Now compare that to the Copenhagen interpretation.
• 20:28
  Speaker 4: Copenhagen gives you a simple output one world the one we.
• 20:31
  Speaker 2: See, which feels less extravagant.
• 20:33
  Speaker 4: But to get that simple output, the theory has to
• 20:36
  Speaker 4: be a mess. You need the Schrodinger equation plus the
• 20:40
  Speaker 4: collapse postulate, plus a definition of measurement, plus a rule
• 20:44
  Speaker 4: for where to place the Heisenberg cut.
• 20:46
  Speaker 2: So the argument is which is more parsimonious simple laws
• 20:50
  Speaker 2: that generate a complex universe or complex messy ad hoc
• 20:55
  Speaker 2: laws to force the universe to stay simple?
• 20:58
  Speaker 4: Exactly, and Everett argues that the entire history of science
• 21:01
  Speaker 4: has been about finding the simplest possible laws, even if
• 21:04
  Speaker 4: they reveal the universe to be larger and stranger than
• 21:07
  Speaker 4: we were comfortable with.
• 21:08
  Speaker 2: That's a great point when we realized the Earth wasn't
• 21:11
  Speaker 2: the center of the solar.
• 21:12
  Speaker 3: System, a perfect example.
• 21:13
  Speaker 2: The math game much simpler. Heliocentrism is mathematically cleaner than
• 21:17
  Speaker 2: the epicycles of geocentrism, but the universe suddenly got way
• 21:21
  Speaker 2: way bigger, and we feeled a lot less important.
• 21:23
  Speaker 4: Precisely, MWI is just the ultimate extension of that Copernican principle.
• 21:28
  Speaker 4: We aren't special. Our outcome isn't the only one. We
• 21:31
  Speaker 4: aren't the only show in town.
• 21:32
  Speaker 2: Okay, I think that's a very compelling defense for the
• 21:35
  Speaker 2: extravagance charge, But the probability problem seems stickier. This one
• 21:40
  Speaker 2: actually gave me a headache while I was reading about it.
• 21:42
  Speaker 4: It's the technical achilles heel of the theory, and it's
• 21:45
  Speaker 4: where most of the modern debate happens.
• 21:47
  Speaker 2: So here's my understanding of it. In standard quantum mechanics,
• 21:51
  Speaker 2: we use something called the Born rule to calculate probability
• 21:55
  Speaker 2: correct It says that the probability of an outcome is
• 21:58
  Speaker 2: related to the amplitude of it. It's part of the
• 22:00
  Speaker 2: wave function. If the wave amplitude is big, the probability
• 22:04
  Speaker 2: is high.
• 22:04
  Speaker 4: Probability equals amplitude squared to be precise, and it works perfectly.
• 22:09
  Speaker 4: It's one of the most well tested rules in all
• 22:11
  Speaker 4: of science.
• 22:11
  Speaker 2: Right, But in many worlds everything happens. If I flip
• 22:15
  Speaker 2: a quantum coin in one branch, it's heads, in another
• 22:17
  Speaker 2: its tails. Both outcomes exist with one hundred percent certainty
• 22:21
  Speaker 2: in the multiverse as a whole. Yes, So how can
• 22:24
  Speaker 2: I say there's a fifty percent chance of heads. The
• 22:26
  Speaker 2: concept of chance seems to lose all meaning if every
• 22:29
  Speaker 2: outcome is guaranteed to occur somewhere, or thirty percent chance
• 22:33
  Speaker 2: of rain.
• 22:33
  Speaker 3: This is a deep, deep problem.
• 22:36
  Speaker 4: If all branches are equally real, why do we experience
• 22:39
  Speaker 4: the world as if they aren't. Why do we experience
• 22:43
  Speaker 4: the statistics of the born rule?
• 22:44
  Speaker 2: Right? If I buy a lottery ticket, there is a
• 22:47
  Speaker 2: branch where I win. I know that for a fact
• 22:49
  Speaker 2: in mwy So why do I feel correctly that I
• 22:53
  Speaker 2: am almost certainly going to find myself in a branch
• 22:55
  Speaker 2: where I lose?
• 22:56
  Speaker 4: The modern defense, and this is largely from philosophers. A
• 22:59
  Speaker 4: physic like David Wallace and Simon Saunders relies on something
• 23:03
  Speaker 4: called self locating uncertainty and the concept of branch weight.
• 23:07
  Speaker 2: Branch weight. Okay, this sounds like we're literally weighing the universes.
• 23:11
  Speaker 4: In a sense, you are go back to the wave function.
• 23:14
  Speaker 4: It's not just a flat list of possibilities.
• 23:15
  Speaker 3: It has a shape.
• 23:17
  Speaker 4: Some parts of the wave are thicker than others, they
• 23:19
  Speaker 4: have a higher amplitude.
• 23:20
  Speaker 2: Is this the loaf of bread analogy?
• 23:21
  Speaker 4: I saw, Yes, I think that's a great one. Imagine
• 23:24
  Speaker 4: the total wave function of the universe. After you buy
• 23:26
  Speaker 4: a lottery ticket is a giant loaf of bread. After
• 23:29
  Speaker 4: the draw, you slice it. One slice, the lost the
• 23:32
  Speaker 4: lottery slice is ninety nine point nine nine nine nine
• 23:34
  Speaker 4: nine percent of.
• 23:35
  Speaker 2: The loaf, and the one the lottery slice is a microscopics.
• 23:38
  Speaker 4: Liver, a tiny crumb. Now, both slices exist, both are real.
• 23:44
  Speaker 2: But the argument is that you, the rational agent who
• 23:48
  Speaker 2: is about to experience one of these outcomes, should care
• 23:50
  Speaker 2: about the measure the weight of the brand you are
• 23:53
  Speaker 2: ending up in.
• 23:54
  Speaker 4: It's about where the bulk of U flows. The probability
• 23:58
  Speaker 4: isn't about if an outcome happened, It's about what fraction
• 24:01
  Speaker 4: of the total unice ends up in that outcome.
• 24:04
  Speaker 2: So it's not about if it happens, it's about where
• 24:06
  Speaker 2: the bulk of reality flows exactly.
• 24:09
  Speaker 4: Philosophers like David Wallace argue this using decision theory. They
• 24:12
  Speaker 4: basically prove that if you want to be a rational
• 24:14
  Speaker 4: actor in a branching universe, if you want to make
• 24:17
  Speaker 4: good bets, the only logical strategy is to bet according
• 24:21
  Speaker 4: to the weight of the branches, which just happens to
• 24:23
  Speaker 4: match the Born rule.
• 24:24
  Speaker 2: So when I say there was a one percent chance
• 24:26
  Speaker 2: of this happening, what I'm really saying is after this event,
• 24:29
  Speaker 2: one percent of the total stuff of the multiverse, including me,
• 24:33
  Speaker 2: will be in branches where this happened.
• 24:35
  Speaker 4: Yes, and therefore you should expect to find yourself to
• 24:39
  Speaker 4: wake up in the next moment in the thicker branch.
• 24:42
  Speaker 4: It's a very subtle argument, and frankly, not everyone buys it.
• 24:45
  Speaker 4: It's still hotly debated, but it's the best and most
• 24:48
  Speaker 4: rigorous answer MWI has.
• 24:50
  Speaker 2: It redefines probability from how often does this happen to
• 24:54
  Speaker 2: how much me ends up there.
• 24:55
  Speaker 3: That's a perfect summary.
• 24:56
  Speaker 2: So okay, let's briefly look at the alternatives, because is
• 25:00
  Speaker 2: not the only game in town. If we don't like
• 25:03
  Speaker 2: it and we hate the magic of Copenhagen, what's left
• 25:05
  Speaker 2: on the menu.
• 25:06
  Speaker 4: Well, the sources we looked at highlight two main contenders
• 25:09
  Speaker 4: that try to offer a different way out, pilot wave
• 25:12
  Speaker 4: theory and objective Clacks theories.
• 25:14
  Speaker 2: Let's start with pilotwave. This is also called Bohemian mechanics.
• 25:17
  Speaker 4: Right after David Bone. This theory tries to restore the
• 25:20
  Speaker 4: classical particle. It says, look, the electron is a real
• 25:23
  Speaker 4: particle and it is in one definite place at all times,
• 25:27
  Speaker 4: so no superposition of location, no superpositional location. But it
• 25:31
  Speaker 4: says the particle is guided by a pilot wave that
• 25:34
  Speaker 4: is also real and tells it where to go.
• 25:36
  Speaker 2: So the wave is real and the particle is like
• 25:38
  Speaker 2: a tiny surfer riding on it.
• 25:40
  Speaker 4: That's a great way to put it. This gets rid
• 25:42
  Speaker 4: of the multiverse and the measurement problem. There is only
• 25:46
  Speaker 4: one world and the particle is always in it. But
• 25:49
  Speaker 4: the price you.
• 25:50
  Speaker 3: Pay is steep.
• 25:51
  Speaker 2: What's the catch?
• 25:52
  Speaker 4: It requires extreme non locality. The pilot wave has to
• 25:57
  Speaker 4: instantly know the position of every other particle in the
• 25:59
  Speaker 4: U S universe to correctly guide your one particle that.
• 26:02
  Speaker 2: Sounds like it clashes pretty hard with Einstein's relativity, which
• 26:05
  Speaker 2: says nothing can travel faster than light.
• 26:07
  Speaker 3: It does.
• 26:08
  Speaker 4: It's very, very hard to reconcile with Einstein, and to
• 26:11
  Speaker 4: many physicists it feels a bit artificial, like you're forcing
• 26:14
  Speaker 4: the universe to be classical just because you want it
• 26:17
  Speaker 4: to be by inventing this extra pilot wave entity.
• 26:20
  Speaker 2: Okay, so that's one and the other was objective collapse.
• 26:23
  Speaker 4: These are theories with names like GRW. They take a
• 26:26
  Speaker 4: different approach. They say, maybe the Schrodinger equation is just
• 26:29
  Speaker 4: slightly wrong.
• 26:30
  Speaker 2: WHOA, Okay, that is a bold move changing the fundamental
• 26:34
  Speaker 2: equation of quantum mechanics.
• 26:35
  Speaker 3: It is.
• 26:36
  Speaker 4: They add a tiny new mathematical term to the equation,
• 26:40
  Speaker 4: and this new term basically says, every once in a while,
• 26:43
  Speaker 4: completely on its own, a superposition just collapses randomly, no
• 26:48
  Speaker 4: observer needed.
• 26:49
  Speaker 2: So it's a built in self destruct mechanism for superpositions.
• 26:52
  Speaker 4: Yes, and it's set up so that for a single particle,
• 26:56
  Speaker 4: this spontaneous collapse would almost never happen, but for a
• 27:00
  Speaker 4: big object with trillions of particles, a collapse is guaranteed
• 27:03
  Speaker 4: to happen almost instantly.
• 27:05
  Speaker 2: So that's how it explains why we don't see cats
• 27:07
  Speaker 2: and superpositions exactly.
• 27:09
  Speaker 4: And the real beauty of this idea is that it's testable.
• 27:12
  Speaker 4: If the math is different, we should be able to
• 27:13
  Speaker 4: design an experiment to measure it. Have we we have,
• 27:17
  Speaker 4: And that's the problem for these theories. We keep doing
• 27:19
  Speaker 4: experiments making larger and larger objects into superpositions and keeping
• 27:23
  Speaker 4: them isolated, and we never see them collapse spontaneously.
• 27:27
  Speaker 2: So we're pushing the boundary.
• 27:28
  Speaker 4: We've done it with massive molecules, things with thousands of atoms.
• 27:31
  Speaker 4: They still act like perfect quantum waves. So the experimental
• 27:35
  Speaker 4: evidence is actually squeezing the objective collapse theories out of existence.
• 27:39
  Speaker 2: The more we confirm that the schroding your equation is
• 27:42
  Speaker 2: correct for bigger and bigger things, the more many worlds
• 27:47
  Speaker 2: just looks like the default truth you're left with if
• 27:50
  Speaker 2: you take the math series.
• 27:51
  Speaker 4: That's exactly the modern argument for it. It's the simplest
• 27:54
  Speaker 4: theory whose predictions haven't been falsified.
• 27:56
  Speaker 2: This leads us to the legacy of the theory. Touch
• 28:00
  Speaker 2: on the personal story of Hugh Everett again, because it's
• 28:02
  Speaker 2: honestly tragic.
• 28:03
  Speaker 3: It really is.
• 28:05
  Speaker 4: Here's a guy who, in his mid twenties as a
• 28:08
  Speaker 4: grad student, solves what is arguably the biggest conceptual problem
• 28:12
  Speaker 4: in all of physics. He writes this brilliant, revolutionary thesis,
• 28:17
  Speaker 4: and the reaction from the physics community crickets, crickets, or worse,
• 28:22
  Speaker 4: open mockery. Neils Bohr, who was basically the pope of
• 28:26
  Speaker 4: quantum physics at the time, completely dismissed it without even
• 28:29
  Speaker 4: really engaging with it.
• 28:30
  Speaker 2: His advisor, the famous John Wheeler, he tried to protect
• 28:33
  Speaker 2: him right, but he was also caught in the middle.
• 28:35
  Speaker 4: Wheeler was a diplomat. He saw the genius in Everett's idea,
• 28:39
  Speaker 4: but he also wanted to stay on good terms with
• 28:41
  Speaker 4: Bohr and the Copenhagen establishment, so he made effort rewrite
• 28:44
  Speaker 4: the thesis. They cut out all the most radical and
• 28:47
  Speaker 4: most exciting language about splitting worlds and observers, and they
• 28:51
  Speaker 4: called it the relative state formulation. They made it as
• 28:55
  Speaker 4: dry and obscure and mathematically dense as possible.
• 28:59
  Speaker 2: It buried the lead.
• 29:00
  Speaker 4: They completely buried it, and Everett, who was by all
• 29:03
  Speaker 4: accounts a proud and sensitive man, was just discouraged. He
• 29:07
  Speaker 4: basically said, fine, if you don't want my idea, and
• 29:09
  Speaker 4: he walked away from academic physics.
• 29:11
  Speaker 2: He went to work for the Pentagon doing Cold war analysis.
• 29:15
  Speaker 4: He did he became a defense analyst. He used his
• 29:17
  Speaker 4: incredible mathematical genius to model nuclear war strategies and weapons systems.
• 29:23
  Speaker 4: He spent the rest of his life in a suit
• 29:24
  Speaker 4: chainsmoking calculating megadeths, while his theory about the fundamental nature
• 29:29
  Speaker 4: of reality gathered dust on a shelf.
• 29:31
  Speaker 2: And he died young, didn't he before the theory really
• 29:33
  Speaker 2: started to get popular.
• 29:34
  Speaker 4: He died of a heart attack in nineteen eighty two
• 29:36
  Speaker 4: at age fifty one, just as the theory was starting
• 29:39
  Speaker 4: to be rediscovered and championed by people like Bryce DeWitt,
• 29:42
  Speaker 4: who coined the name Many Worlds, and a young David Deutsch.
• 29:45
  Speaker 4: Everett never saw his vindication.
• 29:47
  Speaker 2: Speaking of David Deutsch, this is a connection I hadn't
• 29:49
  Speaker 2: made before going through the sources, the really strong link
• 29:52
  Speaker 2: between Many Worlds and the field of quantum computing.
• 29:56
  Speaker 4: This is a crucial modern piece of the puzzle. David
• 29:59
  Speaker 4: Deutsch's one of the founding fathers of quantum computing, and
• 30:02
  Speaker 4: he is a die hard, unapologetic proponent of Many Worlds.
• 30:06
  Speaker 2: Why what's the connection? I mean, it seems like two
• 30:08
  Speaker 2: separate fields.
• 30:09
  Speaker 4: Think about how a quantum computer actually works. It's not
• 30:13
  Speaker 4: just a faster classical computer. It doesn't just check one
• 30:16
  Speaker 4: answer at a time. It puts its bits, its quibits,
• 30:19
  Speaker 4: into a massive superposition to check millions or billions of
• 30:23
  Speaker 4: possible answers simultaneously.
• 30:25
  Speaker 2: Right, it's parallel processing on an exponential scale.
• 30:28
  Speaker 4: So Deutsch asks a very simple, very practical, almost engineering
• 30:32
  Speaker 4: level question, where are those calculations actually happening?
• 30:36
  Speaker 2: I see where this is going.
• 30:37
  Speaker 4: If you have a quantum computer chip with just three
• 30:39
  Speaker 4: hundred interacting quibits, it can perform more simultaneous calculations than
• 30:44
  Speaker 4: there are atoms in the entire visible universe.
• 30:47
  Speaker 2: That's a staggering number.
• 30:48
  Speaker 4: It is, so Deutsch asks, if there is only one universe,
• 30:53
  Speaker 4: how can that one little silicon chip, which is made
• 30:56
  Speaker 4: of a finite number of atoms, hold and process a
• 30:59
  Speaker 4: quantity of information larger than the universe itself.
• 31:02
  Speaker 2: It doesn't fit. The numbers don't add up.
• 31:04
  Speaker 4: Deutsch argues that the only logical explanation is that the
• 31:07
  Speaker 4: computer is outsourcing the work. It's running the calculations and
• 31:10
  Speaker 4: the other parallel branches of the multiverse, and then using
• 31:13
  Speaker 4: quantum interference to bring the results back together in our
• 31:16
  Speaker 4: branch to give us the final answer.
• 31:18
  Speaker 2: So, in his view, a working quantum computer is functional
• 31:23
  Speaker 2: experimental proof of the multiverse exactly.
• 31:26
  Speaker 4: He says, you ask where the computation happened, I can
• 31:29
  Speaker 4: tell you where the answer came from. It came from
• 31:31
  Speaker 4: those other worlds that you can't see. It's a very bold,
• 31:34
  Speaker 4: but very concrete claim.
• 31:35
  Speaker 2: That creates a tangible bridge between this high concept, almost
• 31:39
  Speaker 2: metaphysical idea and the technology we are building in labs
• 31:43
  Speaker 2: right now.
• 31:43
  Speaker 4: It takes it out of the realm of philosophy and
• 31:45
  Speaker 4: puts it into the realm of engineering.
• 31:47
  Speaker 2: I want to spend our last section talking about the implications,
• 31:50
  Speaker 2: because if we accept this, if we just provisionally accept
• 31:53
  Speaker 2: that the math is real and this is how the
• 31:55
  Speaker 2: universe works, it changes our understanding of everything time identity debt, meaning.
• 32:00
  Speaker 4: It rewrites the human condition from the ground up.
• 32:03
  Speaker 2: Let's start with the arrow of time. We all have
• 32:05
  Speaker 2: this incredibly strong perception of time as flowing forward. The
• 32:09
  Speaker 2: past is fixed and singular, the future is open and
• 32:13
  Speaker 2: full of possibilities. How does MWI explain that in a.
• 32:17
  Speaker 3: Really beautiful way.
• 32:18
  Speaker 4: Actually in MWY, the branching structure of the universe is.
• 32:21
  Speaker 3: The arrow of time.
• 32:23
  Speaker 2: Explain that.
• 32:24
  Speaker 4: Think of the multiverse as a tree. At the bottom,
• 32:26
  Speaker 4: you have a single trunk. That's the big bang, that's
• 32:28
  Speaker 4: the past. It's a state of low entropy, high order,
• 32:32
  Speaker 4: a single history. As you move up the trunk, as
• 32:35
  Speaker 4: time moves forward, it splits into branches, and those branches
• 32:39
  Speaker 4: split into smaller branches, and they split again and again
• 32:43
  Speaker 4: into a vast complex canopy.
• 32:45
  Speaker 2: So the future is the canopy.
• 32:46
  Speaker 4: The future is high entropy precisely because it is highly branched.
• 32:51
  Speaker 4: There are vastly more ways for the universe to be
• 32:54
  Speaker 4: than there were in the past. And you can't go
• 32:56
  Speaker 4: back in time for the same reason you can't climb
• 32:58
  Speaker 4: down a tree and expect all all the branches to
• 33:00
  Speaker 4: magically merge back into one single twig.
• 33:03
  Speaker 2: The structure of the multiverse itself defines the direction of time.
• 33:06
  Speaker 4: It connects thermodynamics and the second law with quantum mechanics
• 33:10
  Speaker 4: in a really profound way.
• 33:11
  Speaker 2: That's beautiful, it really is. But then we get to identity,
• 33:15
  Speaker 2: and this is the part that I think really freaks
• 33:18
  Speaker 2: people out.
• 33:18
  Speaker 3: Oh yeah.
• 33:19
  Speaker 2: The question which one is the real me?
• 33:21
  Speaker 4: And the simple and very unsettling answer is they are.
• 33:25
  Speaker 2: All you, but they're so different. In one branch, I'm
• 33:28
  Speaker 2: a millionaire because I bought the right stock. In another,
• 33:30
  Speaker 2: I'm homeless. In one, I died in a car crash
• 33:33
  Speaker 2: this morning. How can all of those be the same person?
• 33:36
  Speaker 4: Because you share a common causal past up until the
• 33:38
  Speaker 4: moment of the split, the stock purchase, getting in the car,
• 33:41
  Speaker 4: you're the exact same physical entity. You share every memory,
• 33:45
  Speaker 4: every scar, every childhood trauma, every thought.
• 33:49
  Speaker 2: So I'm not a single person moving through time. I'm
• 33:51
  Speaker 2: more like a colony or a process that's constantly diversifying.
• 33:55
  Speaker 4: You are a branching system. That's the best way to
• 33:57
  Speaker 4: think about it. It completely dissolves the class classical idea
• 34:00
  Speaker 4: of a singular, indivisible soul or self that persists unchanged
• 34:06
  Speaker 4: through time.
• 34:07
  Speaker 2: It reminds me of that old philosophical question about the
• 34:09
  Speaker 2: transporter and Star.
• 34:10
  Speaker 3: Trek, the teleporter problem.
• 34:12
  Speaker 2: Yet, if it scans you, destroys the original, and reassembles
• 34:15
  Speaker 2: you somewhere else, is it still you? MWA says, it
• 34:20
  Speaker 2: copies you, doesn't destroy the original, and both are you?
• 34:23
  Speaker 4: And this leads to some very heavy ethical questions that
• 34:27
  Speaker 4: philosophers are grappling with. How so, let's say I do
• 34:30
  Speaker 4: something reckless, like I decide to drive home drunk. In
• 34:34
  Speaker 4: a single universe world, I might get lucky, I might
• 34:36
  Speaker 4: get home safe, and I think few got away with it.
• 34:39
  Speaker 4: I count that as a win. But in the multiverse,
• 34:41
  Speaker 4: in the multiverse, because there's a quantum randomness in the
• 34:44
  Speaker 4: swerve of my hands or the path of another car,
• 34:47
  Speaker 4: you have guaranteed that in some branches you killed someone,
• 34:50
  Speaker 4: you have created worlds of tragedy.
• 34:52
  Speaker 2: So moral responsibility becomes about branch weight. You have a
• 34:56
  Speaker 2: responsibility to act in a way that minimizes the measure
• 35:00
  Speaker 2: the total amount of the suffering worlds exactly.
• 35:02
  Speaker 4: You want to act in a way that maximizes the
• 35:05
  Speaker 4: good branches.
• 35:06
  Speaker 3: So to speak.
• 35:06
  Speaker 4: It forces you to think about consequences not as maybe,
• 35:09
  Speaker 4: but as somewhere.
• 35:11
  Speaker 2: That is a heavy, heavy burden.
• 35:12
  Speaker 4: It is, But on the other side of that coin,
• 35:14
  Speaker 4: it also offers a strange comfort also regret. We started
• 35:19
  Speaker 4: this whole conversation talking about the road not taken. What
• 35:23
  Speaker 4: if I had done X? MWI says you did do
• 35:27
  Speaker 4: X is not here, not in this branch. Right, you
• 35:30
  Speaker 4: don't have to mourn the lost potential as something that
• 35:33
  Speaker 4: vanished from existence. That potential was realized. The girl you
• 35:36
  Speaker 4: didn't ask out in another branch. You asked her out,
• 35:39
  Speaker 4: and maybe you're happily married. The book you didn't write,
• 35:42
  Speaker 4: you wrote it and it was a bestseller.
• 35:43
  Speaker 2: It's all happening. The symphony is playing every possible note.
• 35:46
  Speaker 4: We just only get to hear one melody line at
• 35:48
  Speaker 4: a time.
• 35:49
  Speaker 2: As we wrap up, I want to leave the listener
• 35:51
  Speaker 2: with a final thought that I found in the notes,
• 35:53
  Speaker 2: which really hammered this home for me. It's about the
• 35:56
  Speaker 2: location of these other worlds.
• 35:58
  Speaker 4: This is the thing people get wrong most often thanks
• 36:00
  Speaker 4: to science fiction.
• 36:01
  Speaker 2: We watch movies and we think parallel universes are somewhere else,
• 36:05
  Speaker 2: in another dimension through glowing portal, a galaxy, far far away.
• 36:11
  Speaker 3: Right, spatially distant.
• 36:13
  Speaker 2: But if Many Worlds is correct, they aren't far away
• 36:18
  Speaker 2: at all. They are here.
• 36:20
  Speaker 4: They are less than an atom's width away from you.
• 36:22
  Speaker 4: They are right where you are.
• 36:23
  Speaker 2: So the version of me that ordered the tea instead
• 36:26
  Speaker 2: of the coffee this morning, huh, he is sitting in
• 36:28
  Speaker 2: this exact chair right now, he.
• 36:30
  Speaker 4: Is in the same position in space, occupying the same
• 36:32
  Speaker 4: volume of space. The wave function of his body completely
• 36:36
  Speaker 4: overlaps with the wave function of your body.
• 36:38
  Speaker 2: But because we have decohered.
• 36:40
  Speaker 4: Because your atoms and his atoms are massively out of
• 36:43
  Speaker 4: phase with each other, you pass right through each other
• 36:45
  Speaker 4: like ghosts. The radio waves of his reality are washing
• 36:49
  Speaker 4: over you this very second, but you can't tune in.
• 36:51
  Speaker 2: It makes the room feel very crowded.
• 36:53
  Speaker 3: All of a sudden, it is crowded.
• 36:55
  Speaker 4: We are surrounded by infinite variations of ourselves, enacting every
• 36:59
  Speaker 4: possible tree, tragedy and triumph silently, invisibly, right under our noses.
• 37:04
  Speaker 2: And every time a neuron fires in your brain to
• 37:07
  Speaker 2: make a decision, every time a cosmic raise zigs instead
• 37:10
  Speaker 2: of zags, you are adding another ghost to the room.
• 37:13
  Speaker 4: You are building the multiverse moment by moment.
• 37:16
  Speaker 2: Well, on that particularly existential note, I think I'm gonna
• 37:19
  Speaker 2: go flip a coin and consciously split the universe.
• 37:21
  Speaker 3: Into make it a good one.
• 37:23
  Speaker 2: This has been mind bending. Thanks for walking through it
• 37:25
  Speaker 2: with me.
• 37:25
  Speaker 3: Always is my pleasure.
• 37:27
  Speaker 2: Catch him, that's branch