Latest / Daniel and Kelly’s Extraordinary Universe / The Many-Worlds Interpretation of Quantum Mechanics

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The Many-Worlds Interpretation of Quantum Mechanics · Daniel and Kelly’s Extraordinary Universe

0:08
Speaker 1: What if your idea of how the universe works is
0:12
Speaker 1: just wrong? I mean, you live in a world that
0:15
Speaker 1: seems to make sense of you, that seems to follow
0:18
Speaker 1: rules that you're familiar with. But what if that's just wrong?
0:22
Speaker 1: Could reality what's actually out there beyond our brains and
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Speaker 1: our senses. Could it be something so strange and bizarre
0:30
Speaker 1: that we would hardly recognize it. Could it be dramatically
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Speaker 1: different from the glimpses we get through our senses and experiments.
0:37
Speaker 1: There's a vital clue that might just point us in
0:40
Speaker 1: that direction, something that has puzzled physicists and philosophers for
0:44
Speaker 1: nearly one hundred years, and that may take another hundred
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Speaker 1: years to solve. Solving it might require us to swallow
0:52
Speaker 1: a picture of reality that is mind bending lee strange
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Speaker 1: to our little human brains. I'm Daniel, I'm a particle physicist,
1:15
Speaker 1: and I'm drawn to the possibility that the universe might
1:18
Speaker 1: be very different from the way we imagine it. What
1:22
Speaker 1: is the goal of physics, anyway, if not to reveal
1:25
Speaker 1: the true nature of reality to us. We build mathematical
1:29
Speaker 1: stories in our minds and apply them to our experiences.
1:33
Speaker 1: But why immediately it's because we want to predict what
1:36
Speaker 1: will happen when we throw a stone or jump a river.
1:40
Speaker 1: But going deeper it gives us the chance to ask
1:43
Speaker 1: questions about how the universe works. If our mathematical story
1:48
Speaker 1: describes the universe, then we can look at that math
1:51
Speaker 1: and ask why the universe seems to follow that and
1:54
Speaker 1: what it all means. And sometimes the universe out there
1:58
Speaker 1: seems to insist on a math modical story that we
2:01
Speaker 1: find very weird, shocking, almost And that is the goal
2:05
Speaker 1: of physics, not just to give us the power to
2:08
Speaker 1: throw rocks and jump rivers, but to reveal the truth.
2:11
Speaker 1: And the most exciting moments are when the truth and
2:14
Speaker 1: our intuition clash dramatically, when the universe says to us, know,
2:18
Speaker 1: your ideas about the universe are just wrong. And that's
2:22
Speaker 1: the goal of this podcast. Daniel and Jorge Explain the Universe,
2:25
Speaker 1: a production of I Heart Radio in which we tackle
2:28
Speaker 1: the biggest and hardest and nastiest and funnest questions of
2:32
Speaker 1: the universe. The ones that make your brain twist, the
2:35
Speaker 1: ones that slip away from you just as you thought
2:38
Speaker 1: you had figured them out. The ones that might elude
2:40
Speaker 1: humanity for centuries or forever. We don't shy away from
2:44
Speaker 1: any questions on the podcast, but we seek to approach
2:47
Speaker 1: them and explain our knowledge and our ignorance to you.
2:50
Speaker 1: My friend and co host Jorge is on a break,
2:53
Speaker 1: but I have a special treat for you today. We
2:55
Speaker 1: are very lucky to have as a guest one of
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Speaker 1: my favorite physicists and one of my writers about physics.
3:01
Speaker 1: Today we'll be talking to Professor Sean Carroll about some
3:04
Speaker 1: of the problems at the heart of quantum mechanics and
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Speaker 1: a potential solution. So today on the podcast, we'll be
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Speaker 1: answering the question what is the many worlds interpretation of
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Speaker 1: quantum mechanics. So it's my great pleasure to introduce Professor
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Speaker 1: Sean Carroll. He's a theoretical physicist at cal Tech, and
3:27
Speaker 1: he's known for his work on cosmology, general relativity, and
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Speaker 1: the foundations of quantum mechanics. He's also the author of
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Speaker 1: several widely acclaimed and widely read books, including Something People
3:37
Speaker 1: Hidden and The Big Picture, and is the host of
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Speaker 1: the podcast Mindscape, which might actually be nearlier than this podcast. Today,
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Speaker 1: Sean is here to talk to us about the many
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Speaker 1: worlds interpretation of quantum mechanics and the measurement problem in
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Speaker 1: quantum mechanics. Sean, Welcome to the podcast. Thanks very much
3:52
Speaker 1: for having me here. Wonderful to have you. So I
3:55
Speaker 1: want to die right in and before we talk about
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Speaker 1: what the many worlds interpretation is, I want to get
4:00
Speaker 1: review on what problem it solves, Like why do we
4:03
Speaker 1: need so many interpretations of quantum mechanics. What problem is
4:06
Speaker 1: it that they are trying to address. I think there's
4:08
Speaker 1: actually two problems. I mean, this is the right question,
4:10
Speaker 1: because are we just wasting our time or it's not. Honestly,
4:12
Speaker 1: it's not a lot of time compared to other physicists
4:14
Speaker 1: thinking about other things. The foundations of quantum mechanics is
4:17
Speaker 1: a minority pursuit. But I think there are two problems,
4:20
Speaker 1: and there's such looming large problems, and quantum mechanics is
4:24
Speaker 1: so important to modern physics that I do wish we
4:27
Speaker 1: were spending more time on them. So in Quantum Mechanics,
4:30
Speaker 1: I'll try to give my briefest version of quantum mechanics
4:32
Speaker 1: that we can we talk about objects in the universe,
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Speaker 1: whether it's an electron or whatever, in a different way
4:40
Speaker 1: than we talked about them in classical mechanics and Isaac
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Speaker 1: Newton's view the universe. And Newton's view, we would have
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Speaker 1: a position a location in space for a particle, and
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Speaker 1: we would also have a velocity. And if you knew
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Speaker 1: the positions of velocities of everything in the universe, in principle,
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Speaker 1: you could predict what would happen, and you could measure
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Speaker 1: what would happen as much as you want. In quantum
4:57
Speaker 1: mechanics we say no, no, no, that's not how we
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Speaker 1: describe reality. There's something called the wave function, which, for
5:04
Speaker 1: a single particle like an electron, is just very wave
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Speaker 1: like basically at every point in space that has a value.
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Speaker 1: But positions and velocities are not properties of the electron anymore.
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Speaker 1: There are things we can observe about it, and the
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Speaker 1: wave function tells us the probability that we'll get different
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Speaker 1: answers if we observe like the position or the momentum.
5:27
Speaker 1: The momentum is just the mass times the velocity. So
5:30
Speaker 1: this raises two big questions. One is what is the
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Speaker 1: wave function? Is it supposed to be the real world?
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Speaker 1: Is it that somehow we're not measuring the real world
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Speaker 1: exactly when we do our measurements, but it really is
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Speaker 1: described by this weird thing called the wave function that
5:46
Speaker 1: we don't have direct access to. Or is it just
5:49
Speaker 1: part of the world, And there's other extra variables in
5:52
Speaker 1: addition to the wave function, hidden variables we sometimes called them.
5:55
Speaker 1: Or does the wave function have nothing to do with
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Speaker 1: the real world, That it's just a way of printdicting
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Speaker 1: the experimental outcomes, and the real world is something much
6:03
Speaker 1: more definite than that. So all three of these options
6:05
Speaker 1: are very much on the table. This is what I
6:07
Speaker 1: call the reality problem, Like what is the real world?
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Speaker 1: Is that the wave function or something weirder, something else,
6:12
Speaker 1: I should say, not necessarily weirder. The other problem with
6:15
Speaker 1: that brief version of quantum mechanics I just gave you
6:17
Speaker 1: is that it involves the word measure or observe. Right,
6:22
Speaker 1: No other fundamental theory of physics uses those words at all.
6:26
Speaker 1: They just assume you can measure whatever you want. But
6:28
Speaker 1: in quantum mechanics it seems to be the case that
6:32
Speaker 1: you need separate rules for describing systems when you're not
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Speaker 1: measuring them and describing them when you are measuring them.
6:40
Speaker 1: And so this raises what we call the measurement problem.
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Speaker 1: Which is what's up with that? Which includes like, what
6:47
Speaker 1: do you mean measure? Does that? You know what? What
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Speaker 1: does the definition of your measurement is? Doesn't need to
6:51
Speaker 1: be a conscious creature? Could it be a robot or
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Speaker 1: a video camera? What if you just measure it badly?
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Speaker 1: Does the same thing happen? When does it happen? How
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Speaker 1: quickly does it happen? Why is there even a separate
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Speaker 1: set of rules. So a whole bunch of questions get
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Speaker 1: swept under the rug of the measurement problem in quantum mechanics.
7:06
Speaker 1: And I think both these are big, really big problems.
7:08
Speaker 1: If we want to think the quantum mechanics is the
7:10
Speaker 1: right theory of how reality works, we need to know
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Speaker 1: what reality is, and we need to know why this
7:14
Speaker 1: measurement process plays such a special role. And you make
7:17
Speaker 1: a really interesting distinction there. You say, an electron, we
7:20
Speaker 1: can observe these properties of it, or we can observe
7:22
Speaker 1: these quantities, but it no longer has these properties that
7:26
Speaker 1: its velocity's position are not like aspects of the electron.
7:30
Speaker 1: You've like separated the electron from these things we can
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Speaker 1: learn about it. Yeah, and actually in doing that, I've
7:35
Speaker 1: already cheated I've already sort of slipped into my favorite
7:40
Speaker 1: way of thinking about quantum mechanics because there are people
7:43
Speaker 1: who would say that the wave function is just a
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Speaker 1: way to predict the outcomes of what you measure, and
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Speaker 1: there really is something called the position, something called the velocity.
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Speaker 1: We just don't know how to predict what it's going
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Speaker 1: to be until we measure them. Whereas someone like myself
7:58
Speaker 1: is a way function realist, my point of view is, look,
8:01
Speaker 1: every version of quantum mechanics uses something like the wave function, Okay,
8:07
Speaker 1: either way function or something completely equivalent to it. So
8:10
Speaker 1: the simplest, most minimal version of quantum mechanics would only
8:14
Speaker 1: use the wave function, right, Like, not as a route
8:17
Speaker 1: to get to somewhere else or as part of the story,
8:19
Speaker 1: but the whole story. Like why not imagine that the
8:21
Speaker 1: wave function is actually what the world is. And when
8:25
Speaker 1: that's true in that perspective, it becomes the case that
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Speaker 1: things like positions and velocities are not features of the
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Speaker 1: wave function, there possible experimental outcomes. And that's the biggest
8:37
Speaker 1: conceptual hurdle here, because we all look at things and
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Speaker 1: we think they have positions, and we think they have velocities,
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Speaker 1: and if you're this wave function realist kind of version
8:45
Speaker 1: of quantum mechanics, no longer is that true. So let's
8:48
Speaker 1: move from positions velocities to something that's more binary, because
8:51
Speaker 1: I think it's easier to think about Let's talk about
8:54
Speaker 1: the electron and it's spin like, maybe it's spin up
8:56
Speaker 1: or maybe it's spin down. So then what's the problem
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Speaker 1: with the orthodox the Copenhagen approach to quantum mechanics where
9:03
Speaker 1: you say, I have a way function that describes the
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Speaker 1: probabilities of this electron being spin up or spin down,
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Speaker 1: and then when I make a measurement, when I poke
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Speaker 1: it with my finger, the universe rolls the diet and says, okay,
9:13
Speaker 1: well you had a sixty chance of being spin up,
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Speaker 1: so you've got to spin up or nope, you've got
9:17
Speaker 1: to spin down this time. What's the problem with taking
9:19
Speaker 1: that approach? Well, you already said when I measure the
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Speaker 1: spin or when I poke it, I want something more
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Speaker 1: definite than that. If what I'm talking about here is
9:27
Speaker 1: a really fundamental theory of physics, I should not be
9:31
Speaker 1: able to rely on weasel words about poking and measuring,
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Speaker 1: or at least I should give a super duper rigorous
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Speaker 1: definition of what exactly that is and the originators of
9:43
Speaker 1: quantum mechanics and it's conventional textbook form. The Copenhagen interpretation
9:48
Speaker 1: resolutely refused to do this. The strategy they adopted was
9:52
Speaker 1: to say that observers like you and me just aren't
9:56
Speaker 1: subject to the rules of quantum mechanics. We are classical.
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Speaker 1: We are as if quantum mechanics never happened. Okay, you
10:02
Speaker 1: and I, and this is perfectly compatible with our everyday
10:05
Speaker 1: experience of the world. But then they say, but individual
10:08
Speaker 1: particles or atoms obey the rules of quantum mechanics. And
10:11
Speaker 1: you come along and say, well, but I'm made of atoms.
10:15
Speaker 1: How could it be with my atoms obey quantum mechanics
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Speaker 1: and I obey classical mechanics. And so the real problem
10:20
Speaker 1: with this sort of conventional textbook version of quantum mechanics
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Speaker 1: is that it's just not a deaf and physical theory.
10:27
Speaker 1: Is it's not even something you can compare to other things.
10:30
Speaker 1: It assumes that we're in a regime where this division
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Speaker 1: between quantum and classical is good enough to get us by.
10:37
Speaker 1: And I think that when it comes to fundamental physics,
10:39
Speaker 1: we need to do better than that. And for example,
10:41
Speaker 1: if I'm poking something with my finger, you could say, well,
10:43
Speaker 1: I'm classical, so it should collapse the wave function. But
10:46
Speaker 1: then you can imagine the very very tip of my
10:49
Speaker 1: finger is just a quantum particle, and that shouldn't collapse
10:52
Speaker 1: the wave function. And so at what point is that
10:54
Speaker 1: wave function collapsing happening? Is it two layers of quantum particles?
10:58
Speaker 1: Is it ten? Isn't it gets to my neuron? As
11:00
Speaker 1: you said, there's no good answer to that. Yeah, exactly right.
11:02
Speaker 1: And you know what if you missed the particle, or
11:04
Speaker 1: what if you like graves it. You know, all of
11:06
Speaker 1: these questions you can ask are just you're told you're
11:09
Speaker 1: not allowed to ask them in the conventional way of
11:11
Speaker 1: thinking about quantum mechanics. Well, I mean I am a
11:13
Speaker 1: professional particle physicist. I think I know how to poke
11:15
Speaker 1: a particle when I want to, but I won't take
11:18
Speaker 1: on ridge at your example. Um, So then what is
11:21
Speaker 1: the solution offered by the many world interpretation? How does
11:24
Speaker 1: that solve this problem? So many worlds came about from
11:27
Speaker 1: a graduate student, Hugh Everett. So this is always what
11:30
Speaker 1: you should aspire to do as a graduate student overthrow
11:33
Speaker 1: the fundamental nature of reality. And interestingly, Everett was working
11:38
Speaker 1: with John Wheeler, who was a very famous physicist who
11:41
Speaker 1: was an acolyte of Neil's Bore, the grandfather of the
11:44
Speaker 1: Copenhagen interpretation, and Wheeler gave him the following thesis problem
11:49
Speaker 1: quantized gravity. This turns out to be very hard quantizing gravity.
11:54
Speaker 1: We use the word quantized as a verb to turn
11:56
Speaker 1: an existing classical theory into a quantum theory. And what
12:00
Speaker 1: happens with gravity gravity we understand classically pretty well in
12:04
Speaker 1: a theory called general relativity given to us by Einstein.
12:07
Speaker 1: And the point is that the reason why it becomes
12:09
Speaker 1: a problem for quantum mechanics is both technical, like when
12:13
Speaker 1: you try to quantize gravity, run into infinities and other
12:16
Speaker 1: things you don't like, But there's also conceptual problems. Everett said, Look,
12:20
Speaker 1: in the Copenhagen view of quantum mechanics, it's crucial that
12:25
Speaker 1: I have the quantum system I'm looking at and the
12:28
Speaker 1: outside observer poking it. But if I'm quantizing the universe,
12:34
Speaker 1: then I don't have an outside observer. That I have
12:37
Speaker 1: the whole universe, and I should include all the possible
12:40
Speaker 1: observers in there. So he started thinking about that. He said,
12:42
Speaker 1: what happens if we just include the quantum state of
12:46
Speaker 1: observers as well. I don't know why it took twenty
12:49
Speaker 1: years for people to guess this, but he has a
12:50
Speaker 1: very natural place to go. What happens if you let
12:53
Speaker 1: the observer be part of the way function rather than
12:56
Speaker 1: treating them differently. So consider that electron that we have
13:00
Speaker 1: at where you could get either spin up or spin down, right,
13:02
Speaker 1: and consider the following possibility. Since you're a particle physicist,
13:06
Speaker 1: we're going to assume that you're pretty good and measuring
13:10
Speaker 1: the spin of the electron. And what that means is
13:12
Speaker 1: if the electron was absolutely spin up, we're going to
13:16
Speaker 1: grant you that you would always measure it to be
13:19
Speaker 1: spin up. So that means that you, as a physical system,
13:21
Speaker 1: would evolve into a state where your brain says, I
13:25
Speaker 1: measured its spin up, and likewise for spin down, if
13:28
Speaker 1: the electron was a spin down, we're going to grant
13:30
Speaker 1: you that you would say, yep, I measured that to
13:33
Speaker 1: be spin down. Now you're saying that I can do
13:35
Speaker 1: something which is unfamiliar to me, which is I can
13:38
Speaker 1: be in a quantum state, I can have a superposition
13:41
Speaker 1: of having measured one thing and the other thing. Well,
13:44
Speaker 1: I haven't said that yet. I'm about to say that,
13:47
Speaker 1: but it was so far saying is if the electron
13:49
Speaker 1: was a percent spin up and you measured its spin,
13:52
Speaker 1: you would find it to be spin up. You're not
13:54
Speaker 1: in a superposition of anything, right, And likewise, if it's
13:56
Speaker 1: a hundred percent spin down, you would be spin down.
13:58
Speaker 1: Let's assume that. Let's assume we want to be getting
14:01
Speaker 1: the right answer when the answer is definite and known already. Okay,
14:06
Speaker 1: I mean that's the least that we can ask. I'm
14:08
Speaker 1: a reliable measuring device so far, yeah, exactly. But then
14:11
Speaker 1: if you are a quantum system, that's all you need
14:15
Speaker 1: to know because quantum mechanics, in the technical jargon, it's linear.
14:19
Speaker 1: So what that means is if the electron is definitely
14:22
Speaker 1: spin up, you always get spin up. If it's definitely
14:24
Speaker 1: spin down, you always get spin down. Then when it's
14:27
Speaker 1: in a combination, when it's in a superposition of both,
14:31
Speaker 1: we know what you're going to evolve into the wave
14:34
Speaker 1: function of you, plus the electron will evolve into part
14:38
Speaker 1: of it where the electrons spin up and you measured
14:41
Speaker 1: its spin up, and a part where the electron is
14:44
Speaker 1: spin down and you measured it'spin down. So this is
14:47
Speaker 1: taking advantage of the quantum mechanical feature of entanglement that
14:50
Speaker 1: you don't separately say, well, here's the wave function for you,
14:54
Speaker 1: here's the way function for the electron, etcetera. There's only
14:56
Speaker 1: one way function for everything. And in fact, ever it
14:59
Speaker 1: referred to his own theory, not as many worlds, but
15:02
Speaker 1: as the theory of the universal wave function. And so
15:06
Speaker 1: everyone agrees with this. By the way, everyone agrees that
15:09
Speaker 1: if you treat you as a quantum system and you
15:11
Speaker 1: measure that spin, you evolve into an entangled superposition, part
15:15
Speaker 1: of which says the electron has spin up and that's
15:17
Speaker 1: what you saw likewise for spin down. Everett's only move
15:21
Speaker 1: is to say, and that's okay, there's nothing wrong with that.
15:26
Speaker 1: So the immediate visceral responses that can't be right, because
15:30
Speaker 1: I've measured electrons before and I've never felt like I
15:33
Speaker 1: was in a superposition. And Everett says, that's fine, because
15:36
Speaker 1: you've misidentified yourself in the wave function. You think that
15:41
Speaker 1: you're this combination of having measured spin up and having
15:44
Speaker 1: measured spin down. But that's not right because you're entangled
15:47
Speaker 1: with the electron. There's two parts of the wave function,
15:50
Speaker 1: one of which is very consistent. The electron has spin
15:53
Speaker 1: up and you measured to be spin up. The other
15:54
Speaker 1: part is also very consistent, the electron has been down
15:56
Speaker 1: and you measure to be spin down. And Everett points
15:59
Speaker 1: out that in the future evolution of the wave function,
16:03
Speaker 1: these two parts of the wave function will never interfere
16:06
Speaker 1: or interact with each other. Ever, again, they have no
16:09
Speaker 1: influence on what each other are doing. If I poke,
16:12
Speaker 1: as you say, if I change or alter what's going
16:15
Speaker 1: on in part of the wave function where the spin
16:18
Speaker 1: was up, let's say, the part where spin is down
16:20
Speaker 1: doesn't know it is not influenced by that. So it
16:23
Speaker 1: is as if these two parts of the wave function
16:26
Speaker 1: are now describing separate worlds. And the crucial thing to
16:29
Speaker 1: keep in mind, whether or not do you like many worlds,
16:31
Speaker 1: is that ever didn't put in a bunch of worlds.
16:34
Speaker 1: All he said was that we're going to take way
16:37
Speaker 1: functions seriously and include observers in them. The world's come
16:40
Speaker 1: along for free. Once you believe that the electron can
16:43
Speaker 1: be in a superposition of spin up and spin down.
16:46
Speaker 1: You've got to be able to believe that observers can
16:48
Speaker 1: be in the superposition of I measured spin up and
16:51
Speaker 1: I measured spin down. I see. So he avoids this
16:53
Speaker 1: distinction between quantum observers which don't collapse the wave function,
16:58
Speaker 1: and classical observers, which do collapse it by saying everything
17:01
Speaker 1: is quantum, nothing ever collapses the way function, that the
17:04
Speaker 1: wave function is the universe that just keeps going, but
17:07
Speaker 1: you experience one outcome rather than the other because you
17:10
Speaker 1: are no longer every part of the way function. You
17:13
Speaker 1: are part of the way function that experienced spin up,
17:16
Speaker 1: or you are part of the way function that experience
17:18
Speaker 1: spin down. Is that a fair summary? That is precisely right. Actually,
17:21
Speaker 1: that's an excellent summary. So I think that if we're
17:24
Speaker 1: being fair, we should all agree on the pros and cons.
17:30
Speaker 1: At this point of view. The pro is it's just
17:33
Speaker 1: quantum mechanics already taken at face value. There's a wave
17:36
Speaker 1: function for everything. Like you said, everything is quantum, and
17:39
Speaker 1: it obeys one single equation, the Schrodinger equation or some
17:43
Speaker 1: equivalent version thereof. You don't need new variables, you don't
17:47
Speaker 1: need new dynamical laws. You don't need any extra stuff.
17:50
Speaker 1: So at the level of writing down the theory, it's
17:54
Speaker 1: the simplest possible version of quantum mechanics, but of course
17:57
Speaker 1: the cons are at the level of coming to terms
18:01
Speaker 1: with it, it's the biggest possible imaginative leap, right because
18:05
Speaker 1: we're saying that every single time we measure the spin
18:08
Speaker 1: of an electron, a new world is created. And you know,
18:11
Speaker 1: you gotta give the skeptics a fair nod to say, yeah, like,
18:15
Speaker 1: that's a lot to buy, and we proponents of it
18:18
Speaker 1: will say, but you already bought it when you bought
18:20
Speaker 1: quantum mechanics, like you Nigally, We're already there. We're just
18:24
Speaker 1: putting your face in it. Well, what do you think
18:26
Speaker 1: that moment was like for Everett when he you know,
18:28
Speaker 1: followed this line of thinking and then had this perhaps
18:32
Speaker 1: moment of understanding where he realizes, hold on a second,
18:35
Speaker 1: maybe the universe has all these different layers and he's
18:37
Speaker 1: much vaster and much more complex than we imagine. What
18:40
Speaker 1: was that moment? Like, did he ever write about that,
18:42
Speaker 1: you know, moment of epiphany or or realization or understanding?
18:45
Speaker 1: As far as I know, he didn't directly write about that, no,
18:49
Speaker 1: but he wrote a lot. In fact, you know, Wheeler,
18:52
Speaker 1: who is his advisor, was trying to pretend Wheeler himself
18:56
Speaker 1: was in a superposition of effort has done something radical
18:59
Speaker 1: and interesting, and Everett is just going along with the
19:02
Speaker 1: conventional Copenhagen interpretation. Because Wheeler didn't want to annoy his
19:07
Speaker 1: own mentor Neil's bore, so he had Everett both visit
19:10
Speaker 1: Copenhagen literally, and you know bores people visit Princeton where
19:14
Speaker 1: they were living themselves. And letters went back and forth.
19:17
Speaker 1: So there is a lot of writing about this. And
19:20
Speaker 1: the one thing I will say is that you know, again,
19:22
Speaker 1: as as working physicists, both of us, some physicists are
19:26
Speaker 1: just lucky. Sometimes right you're in the right place at
19:28
Speaker 1: the right time, you get either the right experimental data,
19:31
Speaker 1: you get the right idea, and and good for you,
19:33
Speaker 1: and you get credit for that, but we do inevitably
19:38
Speaker 1: separate that out from how good you are, how smart
19:42
Speaker 1: you are, how brilliant you are. Right like, I mean,
19:43
Speaker 1: there's brilliant people who just never were in the right
19:46
Speaker 1: place at the right time, and there's some people got lucky.
19:48
Speaker 1: And if you didn't know any better, which I didn't
19:50
Speaker 1: when I first started thinking about this. H Everett would
19:53
Speaker 1: be the classic example of someone who got lucky, right,
19:56
Speaker 1: someone who's just in the right place at the right time.
19:58
Speaker 1: You only at one idea. He left physics to graduate
20:00
Speaker 1: school and moved on to other things. But you read
20:03
Speaker 1: what he wrote about this stuff and you realize, oh, nope,
20:06
Speaker 1: Actually he was brilliant. He completely understood what he was doing.
20:10
Speaker 1: And this is what I mean by being brilliant, because
20:12
Speaker 1: very often, like someone will have an idea in theoretical physics,
20:15
Speaker 1: and someone else, super duper smart, will understand and appreciate
20:20
Speaker 1: the implications of that idea and spell it all out.
20:23
Speaker 1: And Ever did both. He really appreciated exactly what he
20:26
Speaker 1: was saying, and in these letters going back and forth
20:29
Speaker 1: to the giants of quantum mechanics back in Europe, Everett
20:32
Speaker 1: more than held his own and in fact, he kind
20:35
Speaker 1: of ran rings around him. So I don't know how
20:38
Speaker 1: he felt when he first came up with the idea,
20:41
Speaker 1: but I do give him credit for really thinking through
20:44
Speaker 1: the implications of that idea. Wonderful. Well, I want to
20:47
Speaker 1: talk more about the implications of many worlds and what
20:49
Speaker 1: it means. But first, let's take a quick brick. All right,
21:04
Speaker 1: we're back, and we're talking about the mind blowing idea
21:08
Speaker 1: that maybe the universe is more than just what we see,
21:10
Speaker 1: that there are many universes out there, part of this
21:13
Speaker 1: quantum multiverse, where the universe is constantly splitting based on
21:17
Speaker 1: the various possibilities of what could happen every time quantum
21:20
Speaker 1: particles interact. And I think that the question that probably
21:23
Speaker 1: most listeners have, it's probably a question you hear a lot,
21:26
Speaker 1: is are these other worlds real? In what sense? Are
21:30
Speaker 1: they real? Like? Is this a calculational tool to help
21:33
Speaker 1: us understand our experience or are those worlds like in
21:36
Speaker 1: some sense really out there? Yeah? I think they're real.
21:39
Speaker 1: You know, this gets into deep philosophical questions about what
21:43
Speaker 1: you mean by real right away. But you know, here's
21:46
Speaker 1: how I think about it. If we have our best
21:48
Speaker 1: explanation for what we observe, and that explanation takes the
21:51
Speaker 1: form of some physical theory, and that physical theory predicts
21:55
Speaker 1: the existence of stuff we don't observe, then we take
21:58
Speaker 1: that stuff seriously until will we have a better physical theory? Right? So,
22:02
Speaker 1: I mean, we often discover new things in the universe
22:05
Speaker 1: by doing this, whether it's you know, new planets or
22:08
Speaker 1: dark matter or whatever. And so if you take Everett's
22:12
Speaker 1: version of quantum mechanics seriously, he solves the both the
22:15
Speaker 1: reality problem and the measurement problem. The answer to the
22:18
Speaker 1: reality problem is the wave function directly describes reality. The
22:21
Speaker 1: answer to the measurement problem is when a quantum mechanical
22:24
Speaker 1: system becomes entangled with big macroscopic things, that's what counts
22:28
Speaker 1: as a measurement. And a prediction of his resolution to
22:31
Speaker 1: these two problems is that these other worlds are real.
22:34
Speaker 1: So if you want to get the benefit of his solutions,
22:39
Speaker 1: but you find the other world's distasteful, that's okay. But
22:43
Speaker 1: then you have to come up with a better theory,
22:45
Speaker 1: and you have to come up with a disappearing world's
22:48
Speaker 1: theory where you get rid of the other world. And
22:51
Speaker 1: you can do that. People have done that. I mean,
22:52
Speaker 1: I'm saying it in a sort of facetious voice, but
22:54
Speaker 1: it is in fact an ongoing research program to do
22:57
Speaker 1: exactly that. And the problem is there's a couple of problems.
23:00
Speaker 1: One is it is inevitably more complicated, right, I mean,
23:04
Speaker 1: you're adding something to a very clean, crisp formalism to
23:08
Speaker 1: get rid of parts of it that you find distasteful.
23:10
Speaker 1: And number two, it's hard to make it work. Everett
23:14
Speaker 1: is very plug and play, and especially when you go
23:17
Speaker 1: from a quantum mechanical theory of particles to a theory
23:20
Speaker 1: of fields and then to a theory of quantum gravity,
23:23
Speaker 1: which was initially his initial motivation. Every Ready in quantum
23:26
Speaker 1: mechanics is perfectly happy doing any one of those, whereas
23:29
Speaker 1: when you try to mess with it, adding more variables
23:32
Speaker 1: or adding more rules or whatever, you kind of find
23:35
Speaker 1: you have to mess with it in different ways for
23:37
Speaker 1: every version of the theory, and who knows what will
23:40
Speaker 1: end up being. So, both from a philosophical point of view,
23:44
Speaker 1: and I think from a physicist point of view, the
23:46
Speaker 1: simplicity and success of the Evert interpretation speaks to not
23:52
Speaker 1: working so hard to get rid of the other worlds.
23:54
Speaker 1: So I think that you're responding to sort of implied
23:57
Speaker 1: criticism in my question, which is about the nature of
24:00
Speaker 1: what he's real, which is totally reasonable. And you know,
24:02
Speaker 1: I think that a lot of people argue that if
24:05
Speaker 1: you can't measure it directly, if you can't interact with it,
24:08
Speaker 1: then it's not real physically, that it's can only be
24:11
Speaker 1: real philosophically. And I think that your response is probably
24:15
Speaker 1: if it's required by your theory, and your theory is
24:17
Speaker 1: the only one you have that describes what you can observe,
24:21
Speaker 1: then what's required by your theory but not observed is
24:24
Speaker 1: still real. Is that fair? Yeah? I think that's exactly right.
24:28
Speaker 1: And honestly, if all you wanted to do was to say,
24:32
Speaker 1: I believe everything that EVERT says, but I don't believe
24:34
Speaker 1: the other worlds are real, you know, knock yourself out.
24:37
Speaker 1: It's a free country, right, Like, I don't know what
24:39
Speaker 1: you get from that. I think if you face up
24:42
Speaker 1: to that perspective, you're going to have to change the physics.
24:45
Speaker 1: You're gonna have to change the equations to literally get
24:48
Speaker 1: rid of those other worlds, and you're going to get
24:49
Speaker 1: in trouble doing that. But if it's just kind of
24:52
Speaker 1: an attitude like I don't care about the other worlds
24:54
Speaker 1: because I'm not in them, then that's fine. Whatever. I
24:57
Speaker 1: do think. You know, one thing just to put on
24:59
Speaker 1: the table that maybe we'll get back to you later,
25:01
Speaker 1: is that it's not just philosophy, you know, I really
25:05
Speaker 1: do think that one of the reasons why we still
25:08
Speaker 1: struggle to understand quantum gravity, for example, as a field
25:12
Speaker 1: of theoretical physics, is exactly because we had not struggled
25:16
Speaker 1: hard enough to understand quantum mechanics. And I think that
25:19
Speaker 1: rather than sort of putting the heads in the sand
25:22
Speaker 1: and denying the existence of these other worlds, if you
25:24
Speaker 1: do take the formalism seriously, it provides new angles of
25:28
Speaker 1: fruitful approach to longstanding problems in physics. So you know,
25:33
Speaker 1: it's a reason to not give in to your first
25:37
Speaker 1: impulse to be worried about all those other worlds. Well,
25:39
Speaker 1: we were chatting with Carlo Rovelli a few weeks ago,
25:41
Speaker 1: and he said that every interpretation of quantum mechanics has
25:44
Speaker 1: a cost. And I think that a lot of people
25:46
Speaker 1: would see, you know, these infinite other universes is maybe
25:49
Speaker 1: like a cost of the many worlds interpretation. But to me,
25:52
Speaker 1: it's exactly what I got into physics to do, is
25:56
Speaker 1: to blow my mind and shake up my intuition about
25:59
Speaker 1: the universe. I don't get into physics to have the
26:01
Speaker 1: universe say, yeah, Daniel, what you thought about the universe?
26:04
Speaker 1: That's basically it I'm hoping to peel back a layer
26:07
Speaker 1: of reality and see something shocking, which at first is
26:10
Speaker 1: difficult to accept because it's different from my intuition, but
26:13
Speaker 1: that eventually, guided by mathematics, that can help some new
26:16
Speaker 1: intuition be like, Wow, the universe is different from what
26:18
Speaker 1: I imagined, and it works in this incredibly beautiful way.
26:21
Speaker 1: So to me, it's not a cost, it's a it's
26:23
Speaker 1: a feature, it's it's the goal of digging into quantum mechanics.
26:26
Speaker 1: But my question is, you just said that we've been
26:28
Speaker 1: hesitant to dig into the foundations of quantum mechanics. Why
26:31
Speaker 1: do you think that is? Why do you think that
26:33
Speaker 1: for such an important problem at the core of modern physics,
26:37
Speaker 1: that progress has been so slow. Why haven't we taken
26:40
Speaker 1: this question more seriously? I think it's a large number
26:42
Speaker 1: of reasons. And this is a very very good question
26:44
Speaker 1: also that I've talked about other people who talked about.
26:46
Speaker 1: But it's sort of a more sociology, psychology, history question, right,
26:50
Speaker 1: So it's a little on shakier ground here, So forgive me.
26:53
Speaker 1: But you know, part of it is just that we
26:55
Speaker 1: don't know how to answer it. That if there are
26:59
Speaker 1: different compete eating versions of quantum mechanics that are well
27:03
Speaker 1: defined physical theories. So you know, pilot wave theories and
27:07
Speaker 1: I guess Carlo's relational quantum mechanics, etcetera. There are various
27:10
Speaker 1: spontaneous collapse models on the market, and these are real
27:13
Speaker 1: physical theories. The problem with Copenhagen this is not a
27:16
Speaker 1: real physical theory. There's it just doesn't answer certain questions
27:19
Speaker 1: about what actually happens. But once you like put on
27:22
Speaker 1: your big boy pants and actually make a theory, then
27:24
Speaker 1: you can make predictions with that theory and you can
27:26
Speaker 1: try to experimentally test them. So I think that for
27:29
Speaker 1: a long time it was just thought to be not
27:31
Speaker 1: very fruitful to think about these ideas because we didn't
27:34
Speaker 1: know how to get any experimental data about them. And
27:37
Speaker 1: the other aspect is, you know, we had other things
27:40
Speaker 1: going on. Businesses are very good at, you know, pushing
27:44
Speaker 1: forward in directions they can make progress on. So we're
27:48
Speaker 1: talking about the thirties, forties, fifties, right, Like, there were
27:51
Speaker 1: particles in nuclei to understand, there were bombs do bill,
27:54
Speaker 1: there were superconductors to construct, and quantum field theories to invent,
27:58
Speaker 1: and normally anything you know, it goes on and on, right,
28:00
Speaker 1: So there's plenty of work to do that you could
28:03
Speaker 1: connect directly to experimental progress. So it was kind of
28:07
Speaker 1: okay to push the foundations of quantum mechanics into the background.
28:11
Speaker 1: That's sort of like saying, you know, my debit card
28:14
Speaker 1: still works, so I don't need to check my balance
28:16
Speaker 1: because probably everything is fine. That's right. I mean, that
28:19
Speaker 1: is part of it. But also, you know, it's very
28:23
Speaker 1: common advice when you have an enormous task in front
28:25
Speaker 1: of you to first do the parts you can do,
28:28
Speaker 1: rather than fretting about the parts you can't do. I
28:30
Speaker 1: think that what has changed recently is number one, technology
28:33
Speaker 1: has grown to the point where this idea of a
28:36
Speaker 1: division between the classical world and the quantum world as
28:39
Speaker 1: part of the fundamental description of reality has just become
28:42
Speaker 1: increasingly untenable. Right, we can make much larger quantum systems
28:46
Speaker 1: than we could back in the thirties that are in superpositions,
28:49
Speaker 1: and we need to deal with the reality of entanglement
28:52
Speaker 1: and so forth when we build quantum computers and things
28:55
Speaker 1: like that. And the other is that this, you know,
28:57
Speaker 1: enormous progress we made on under standing particle physics and
29:01
Speaker 1: field theory has slowed in the past few decades, and
29:06
Speaker 1: we're in this weird position where we built these amazingly
29:09
Speaker 1: successful theories that fit all the data, but we know
29:12
Speaker 1: they're not the final answer, right because gravity is not included,
29:14
Speaker 1: because there are these naturalness problems, etcetera, etcetera. So one
29:18
Speaker 1: strategy is just to stubbornly bull forward using the same
29:22
Speaker 1: tools we've used before. But another strategy is to take
29:24
Speaker 1: a step back and say, Okay, maybe we have to
29:27
Speaker 1: think fundamentally in a different way about these questions to
29:30
Speaker 1: make progress on them. And thinking about the foundations of
29:32
Speaker 1: quantum mechanics plays into that strategy. So I guess it's
29:35
Speaker 1: time to check the balance, huh. And we've got to
29:37
Speaker 1: figure out what's happening down there in the vision we
29:39
Speaker 1: keep getting turned down at the A T M. So yeah,
29:43
Speaker 1: So then digging deep into like what it means this
29:45
Speaker 1: many world's interpretation, The many world's interpretation says essentially that
29:50
Speaker 1: none of these universes are special or different, that these
29:52
Speaker 1: branching aspects of the way function, and it sort of
29:55
Speaker 1: avoids the collapse question that way. But I can't get
29:58
Speaker 1: around what you said earlier, which is that You're redefining
30:01
Speaker 1: what it means to be me, right, because this universe
30:04
Speaker 1: does feel special to me. I mean, I'm in this one.
30:07
Speaker 1: It's the only one that I can interact with. Is
30:09
Speaker 1: that sort of a naive objection to the many worlds
30:11
Speaker 1: interpretation to say that this one must somehow be different?
30:14
Speaker 1: Can you swap that away by just saying, well, the
30:16
Speaker 1: other Daniels also think that they're the only one who
30:18
Speaker 1: interacts with the universe. Pretty much, Yes, that is exactly
30:21
Speaker 1: how I will slipe it away. The relevant anecdote here
30:25
Speaker 1: that Everyboden's love this anecdote, so I will just share it.
30:27
Speaker 1: It is actually about Ludwig Wittgenstein, the philosopher. So one
30:31
Speaker 1: day one of his former students, Elizabeth Anscombe, was also
30:34
Speaker 1: an extremely accomplished philosopher in her own right. She comes
30:37
Speaker 1: across Wittgenstein, you know, standing in the yard at Cambridge
30:42
Speaker 1: like looking at the sun and Vichenstein was famously a
30:46
Speaker 1: little idiosyncratic. So she says, what is going on? And
30:48
Speaker 1: he says, you know, why is it? The people were
30:51
Speaker 1: reluctant to believe that the Earth rotated, rather than believing
30:55
Speaker 1: that the sun moved around the Earth. And Anscomb says, well,
30:58
Speaker 1: it just looks like the Sun moves around the earth, right,
31:02
Speaker 1: And Vickenstein says, well, what would it have looked like
31:05
Speaker 1: if the Earth rotated? So the point being that the
31:09
Speaker 1: question you should be asking is not start with an
31:12
Speaker 1: impression I see the Sun moving, and then construct a
31:17
Speaker 1: theory that fits most closely with that immediate impression. The
31:20
Speaker 1: strategy should be construct theories and then ask what observers
31:24
Speaker 1: within those theories would observe, and if it's consistent with
31:27
Speaker 1: what we observe, then it works. So the point is
31:30
Speaker 1: that in many worlds, if there's you right here and
31:33
Speaker 1: then you go and do some experiments at CERN and
31:36
Speaker 1: you observe some particles, the prediction is there are now many,
31:39
Speaker 1: many branches in which the specific pattern of particles and
31:43
Speaker 1: a collision are different in every single branch, and the
31:46
Speaker 1: version of you has now seen different things. And all
31:49
Speaker 1: of those versions of you exist, and they've seen different things,
31:53
Speaker 1: and they all think that they're special because they exist,
31:55
Speaker 1: and the other ones are kind of dubious, but there's
31:57
Speaker 1: no pointer that says this is the real, real one. Right.
32:00
Speaker 1: There are other version of the quantum mechanics that try
32:02
Speaker 1: to do that to try to say like, this is
32:04
Speaker 1: the real branch and all the other ones are fake,
32:07
Speaker 1: but it would still be true weave without that pointer
32:10
Speaker 1: that says this one is real, that all of the
32:13
Speaker 1: different versions of you on the different branches would feel
32:16
Speaker 1: equally real. So the experimental empirical prediction of this theory
32:22
Speaker 1: is exactly what we observe in the world, and I
32:25
Speaker 1: think that should be the criterion for saying whether or
32:27
Speaker 1: not it's an adequate explanation. It is interesting. It does
32:30
Speaker 1: feel somehow like a bit of sleight of hand, like
32:32
Speaker 1: you've taken the fuzziness of defining a classical object in
32:37
Speaker 1: terms of quantum mechanical particles and you sort of transformed
32:40
Speaker 1: it into like, well, I'm just going to redefine what
32:42
Speaker 1: you are. You aren't who you thought you were. You're
32:44
Speaker 1: just an element of this quantum wave function instead of
32:47
Speaker 1: being like the holistic version of you. It feels to
32:50
Speaker 1: me like, you know, when you make this step you
32:53
Speaker 1: have this interpretation of quantum mechanics, you need to say, one,
32:55
Speaker 1: what the wave function is. It's real, it's the universe,
32:58
Speaker 1: But also something about like the correspondence between the quantum
33:01
Speaker 1: state of the universe and your experience of it as
33:04
Speaker 1: an observer. Yeah, no, that's under percent true. And so again,
33:08
Speaker 1: if we're honest about the pros and cons, the physics
33:11
Speaker 1: of ever writing quantum mechanics is as simple as it
33:14
Speaker 1: can be. But the philosophy requires some new moves. And
33:20
Speaker 1: I'm a you know, on board with people who say
33:24
Speaker 1: I just can't accept those moves, like it's too much,
33:27
Speaker 1: or at least say it this way. You know, if
33:29
Speaker 1: we think we don't agree on what the correct version
33:33
Speaker 1: of quantum mechanics is, and each of us has our
33:35
Speaker 1: credence for saying, well, it's probably this, but unlikely that
33:38
Speaker 1: it's perfectly fair for one of the ingredients that goes
33:42
Speaker 1: into your choice of your personal credence to say, this
33:46
Speaker 1: redefinition of who I am is just so dramatic that
33:49
Speaker 1: I'm going to be skeptical of it. Maybe it's true,
33:51
Speaker 1: but I'm gonna be a little dubious until I'm forced
33:55
Speaker 1: into it. I think that's okay, And so I think this,
33:58
Speaker 1: I'm completely acknowledged jing that the philosophical leaps that required
34:03
Speaker 1: by many worlds are substantial, and in other versions they're
34:07
Speaker 1: just not there. They don't bother me as much like
34:09
Speaker 1: you know, I think that in physics we very often
34:12
Speaker 1: come across better understandings of the world, including of ourselves.
34:16
Speaker 1: Right like we might have thought back in the day
34:17
Speaker 1: that we were a spirit animating a fleshy machine that
34:20
Speaker 1: housed us, and now we think otherwise. I think that's okay,
34:23
Speaker 1: as long as again, as long as the model that
34:27
Speaker 1: we're building, once we understand it turns out to be
34:30
Speaker 1: completely compatible with the world we observe, then I'm on board.
34:33
Speaker 1: But then let's talk about how to use many worlds interpretation,
34:37
Speaker 1: you know, as a functioning theory of quantum mechanics, because
34:39
Speaker 1: something that is a bit slippery for me is that
34:42
Speaker 1: in the Copenhagen interpretation, I know what a probability means.
34:46
Speaker 1: I'm gonna do an experiment and I'm either going to
34:48
Speaker 1: get spin up or spin down. I can look at
34:50
Speaker 1: the way of function. I can say, well projected, you know,
34:52
Speaker 1: against both of these possible outcomes, and those give me
34:55
Speaker 1: the probabilities. They just use the Born rule, and whether
34:57
Speaker 1: or not I'm a quantum or classical observer sort of
35:00
Speaker 1: separate from that. But in many worlds, everything happens, and
35:03
Speaker 1: so I can no longer say like, well, the probability
35:05
Speaker 1: of this happening is six because they both happen in
35:09
Speaker 1: some universe. How do you define probability if everything that
35:13
Speaker 1: can happen is going to happen. Yeah, I think this
35:15
Speaker 1: is the right question to ask. Like I said, there
35:17
Speaker 1: are objections to many worlds that are not very good,
35:20
Speaker 1: objections that are just misunderstandings. But there are also puzzles
35:25
Speaker 1: or problems or things we got to address that arise
35:30
Speaker 1: only in many worlds that weren't there before. So you know,
35:33
Speaker 1: I gave Everett credit for solving the reality problem and
35:36
Speaker 1: the measurement problem, an equal number of new problems arise.
35:40
Speaker 1: And the reason why I think that's okay is because
35:41
Speaker 1: I can see the solutions for these problems pretty clearly,
35:45
Speaker 1: even if we don't have them completely spelled out. One
35:48
Speaker 1: problem is just and maybe we'll get to this if
35:50
Speaker 1: you want to, but it's the structure problem, which is
35:53
Speaker 1: why does the world look so classical to us if
35:55
Speaker 1: it's really this big quantum wave function. There's a lot
35:58
Speaker 1: of details in that. And the other one, like you said,
36:00
Speaker 1: is the probability problem. The benefit of effort is that
36:03
Speaker 1: the underlying equations are lean and mean and austere, so
36:06
Speaker 1: there's no room to say, okay, the wave function evolves,
36:10
Speaker 1: and there's a rule that says the probability forgetting a
36:13
Speaker 1: measurement is is given by the wave function square. Like,
36:15
Speaker 1: there's no room to add extra rules like that. So
36:18
Speaker 1: what you need to do is derive these rules, and
36:22
Speaker 1: there are different strategies for doing it. And to be clear,
36:25
Speaker 1: the fact that the probability is given by the wave
36:27
Speaker 1: functions squared rather than just by the wave function or
36:31
Speaker 1: by the wave function cube to the logarithm or whatever,
36:33
Speaker 1: that's not the problem. Of course. It's going to be
36:36
Speaker 1: given by the wave function squared because the set of
36:39
Speaker 1: numbers which are given by the wave functions squared are
36:42
Speaker 1: the unique set of numbers that are all non negative
36:45
Speaker 1: and they add up to one and they're conserved over time.
36:48
Speaker 1: That's what you want out of a probability. And it's
36:50
Speaker 1: just Pythagoras theorem, right. The hypot new squared is the
36:53
Speaker 1: other two sides squared. That's why you take all of
36:56
Speaker 1: the different amplitudes in the wave functions squared to add
36:58
Speaker 1: them up and get one. So that not the tricky part.
37:00
Speaker 1: The tricky part is, like you said, why are their
37:03
Speaker 1: probabilities at all? Because it's a completely deterministic theory and
37:07
Speaker 1: different people have their angles on that. I think that
37:09
Speaker 1: I've solved it along with my collaborator Chip Saban's, because well,
37:14
Speaker 1: we borrowed an idea from someone else. I should give
37:15
Speaker 1: credit to who's leve Widmin. But here's the idea. When
37:18
Speaker 1: you measure that spin. So there's some amplitude saying this
37:22
Speaker 1: spin is up. There's some amplitude saying the spin is down.
37:25
Speaker 1: And you measure, and like you say, with probability one,
37:28
Speaker 1: there is now a version of you on the branch
37:30
Speaker 1: where the spin was up, in a version of you
37:32
Speaker 1: that is on the branch where the spin is down.
37:34
Speaker 1: But if you be a good physicist and you do
37:37
Speaker 1: all the details carefully, you can say, well, you know,
37:41
Speaker 1: EVERT purportedly explains to me when these measurement occurs, it's
37:44
Speaker 1: just an entanglement process. I can calculate when it happens.
37:49
Speaker 1: And the answer is, it happens incredibly quickly. The time
37:52
Speaker 1: scale for the branching to happen is shorter than the
37:55
Speaker 1: lifetime the Higgs boson for those particle physicists out there
37:58
Speaker 1: right like less intended mine is on the seconds, and
38:01
Speaker 1: so your brain doesn't work that fast. You can't actually
38:05
Speaker 1: know which branch you're on as quickly as the branching happens.
38:09
Speaker 1: So what that means is, inevitably, when the wave function
38:12
Speaker 1: does branch, there's a period of time when there are
38:15
Speaker 1: two copies of you who those the world is not identical,
38:18
Speaker 1: but those copies of you are identical, okay, And neither
38:22
Speaker 1: one of those knows what's branches on. So even if
38:25
Speaker 1: it knows the entire way function of the universe, there's
38:28
Speaker 1: something about itself that neither one of those copies of
38:32
Speaker 1: you knows, namely which branch it is on. This is
38:35
Speaker 1: called self locating uncertainty or indexical uncertainty. And in those cases,
38:40
Speaker 1: you know there's some fact about the world, but you
38:41
Speaker 1: don't know it. What do you do as a good
38:43
Speaker 1: Baysian reason or as as a good modern rational person.
38:46
Speaker 1: You assigned credences. You assigned non negative numbers that add
38:50
Speaker 1: up to one, right, that act like probabilities. So it's
38:53
Speaker 1: a subjective probability. I don't know which branch I'm on,
38:57
Speaker 1: but I'm going to sign a credence, and you might say, well,
38:59
Speaker 1: what do I care about the wave function. I'm just
39:01
Speaker 1: going to assign credences that are fifty fifty, right, because
39:03
Speaker 1: there's two options, I'm gonna sign equal credence. Turns out
39:05
Speaker 1: that doesn't work. It's inconsistent because you can then branch
39:09
Speaker 1: the wave function again. Depending on whether this spin was upper,
39:12
Speaker 1: spin was down, you have a different number of branches,
39:14
Speaker 1: and now you have to assign like one third, one third,
39:16
Speaker 1: one third. So the first guy's probability changes even though
39:20
Speaker 1: nothing happened in his world. Is that it's kind of
39:22
Speaker 1: inconsistent over time. Assigning the Born rule probabilities, giving the
39:27
Speaker 1: probability the credence the subjective probability assigned by the wave
39:31
Speaker 1: functions squared is the uniquely consistent thing you can do
39:35
Speaker 1: in this situation. So number one, there are inevitably uncertainties,
39:40
Speaker 1: and number two, the uniquely rational way to assigned credences
39:44
Speaker 1: to them is the Born rule. So then the uncertainties
39:46
Speaker 1: reflect more like our ignorance rather than some fundamental property
39:51
Speaker 1: of the universe. That's right, and people like me would
39:54
Speaker 1: go so far as to say, that's always what you
39:56
Speaker 1: mean by probability. I mean, there's something that happens in
39:59
Speaker 1: the world but we don't know, so we assign different
40:01
Speaker 1: credences to it. It's a subjectivist Bayesian version of probability.
40:05
Speaker 1: I see. So then does Many Worlds interpretation require basian
40:08
Speaker 1: probability and rule out frequentist probability. It certainly comports way
40:12
Speaker 1: more comfortably with Bayesian notions of probability. So you don't
40:16
Speaker 1: need to be as extremist as I am and think
40:18
Speaker 1: that all probabilities are fundamentally subjective to be an ever
40:21
Speaker 1: ready in But it doesn't hurt. It helps you sleep
40:24
Speaker 1: better at night. Let's put it that way. Let's take
40:26
Speaker 1: a quick break. And so then are these sort of
40:41
Speaker 1: like philosophical explorations the only way that we can make
40:45
Speaker 1: progress on these questions of the quantum foundations? I mean,
40:48
Speaker 1: if we have two interpretations of quantum mechanics, Many worlds
40:52
Speaker 1: and relational for example, and they both are you know,
40:55
Speaker 1: actual physical theories unlike Copenhagen, and they both describe everything
40:59
Speaker 1: that we he as observers, then are we just forced
41:02
Speaker 1: to make philosophical choices between them? You know, as as individuals.
41:06
Speaker 1: Are there no experiments we can do to help us
41:08
Speaker 1: resolve this question. Well, I think there's two answers to that.
41:11
Speaker 1: One one is that sometimes there are experiments that help
41:15
Speaker 1: us distinguish between them. You know, Roger penn Rose has
41:17
Speaker 1: been pushing an idea where there are objective collapses of
41:21
Speaker 1: the wave function, where the wave function violates the Shorteninger
41:23
Speaker 1: equation and really does collapse. Other people. There's a famous
41:26
Speaker 1: theory called the GRW theory Gerardi, Ramini and Webber who
41:30
Speaker 1: have a similar theory with different equations attached to it.
41:33
Speaker 1: And these are experimentally testable, and and tests are going on, right,
41:36
Speaker 1: so we're actually doing them. Theories like hidden variable theories
41:41
Speaker 1: Bomian mechanics, I think they should be experimentally distinguishable from
41:46
Speaker 1: ever ready in quantum mechanics. But the proponents of those
41:48
Speaker 1: theories say they're not. So I'm a little suspicious about that,
41:51
Speaker 1: But I haven't actually, I can't put a good proposal
41:53
Speaker 1: on the table for how to experimentally distinguish them. But
41:56
Speaker 1: I still don't quite believe the standard lure in that case.
42:00
Speaker 1: For things like Ravelli's relational quantum mechanics, I don't understand
42:03
Speaker 1: it well enough to say I suspect that it will
42:07
Speaker 1: turn out to be fundamentally equivalent to one of the
42:11
Speaker 1: other approaches. Either it's just the way function and it's
42:13
Speaker 1: Everett with the many worlds, or there's got to be
42:15
Speaker 1: some hidden variables, or is epistemic. I think it's closest
42:18
Speaker 1: to what we call an epistemic approach. Episdemic approaches. I
42:22
Speaker 1: haven't even mentioned those yet. Those are the ones where
42:24
Speaker 1: they really say, the way function is just not reality. Okay,
42:28
Speaker 1: In Penrose's approach or g r W or hidden variables,
42:31
Speaker 1: the way function is part of reality, but it's dynamics
42:34
Speaker 1: are a little bit different than an Everett, whereas in
42:36
Speaker 1: a truly epistemic approach, the wave function is just a
42:39
Speaker 1: calculational tool and reality is something very, very different. And
42:43
Speaker 1: that's fine, But then what is reality? And I'm pretty sure,
42:48
Speaker 1: at least my strong belief from the current state of
42:52
Speaker 1: the answers people give me when I ask them, there's
42:54
Speaker 1: no good theory of what reality actually is in these models,
42:58
Speaker 1: and that you know, they're like, wait till water, we'll
43:00
Speaker 1: figure that out. And in principle, that's okay. You know
43:03
Speaker 1: that we can't ask that every theory as answer every question.
43:05
Speaker 1: As soon as it's invented. But it's also perfectly fair
43:09
Speaker 1: to be skeptical of those theories while those questions still
43:12
Speaker 1: linger out there. But okay, but the other answer to
43:15
Speaker 1: your question, sorry that it takes me so long to
43:17
Speaker 1: get to it is the proof of the pudding is
43:19
Speaker 1: in the tasting. And if I can make progress on
43:23
Speaker 1: other puzzles and physics by starting from an everady in
43:28
Speaker 1: perspective and taking it seriously, whereas my friends who are
43:32
Speaker 1: being epistemic or hidden variably or whatever don't make that progress,
43:36
Speaker 1: then by the rules of physics, I win, and vice versa. Right,
43:38
Speaker 1: if the people who are fundamentally pilot wave theories or
43:42
Speaker 1: epistemic people make progress that I don't, then they win.
43:45
Speaker 1: That's perfectly fair. So in this situation where we're not
43:48
Speaker 1: sure what the right answer is, then by all means,
43:50
Speaker 1: let people do research in their favorite areas, and you know,
43:53
Speaker 1: whoever actually discover something interesting, we'll get the credit. I
43:57
Speaker 1: like this idea of measuring competing theories by how much
43:59
Speaker 1: progress can make, sort of theoretically or philosophically, to show
44:03
Speaker 1: that it's like a you know, a functioning, working, fertile
44:06
Speaker 1: intellectual playground. But you raised this interesting question of you
44:08
Speaker 1: know what reality is? And I want to come back
44:10
Speaker 1: to something you mentioned earlier, which is why the universe
44:13
Speaker 1: if it is quantum, If the universe is a wave
44:16
Speaker 1: function and there's quantum particles and everything is governed by
44:18
Speaker 1: the shooting equation, why doesn't feel that way to us?
44:21
Speaker 1: You know? Why we have this emergent experience which is
44:25
Speaker 1: so drastically not quantum. Is that something we can ever
44:28
Speaker 1: grapple with? Or is it just like other emergent phenomenon,
44:31
Speaker 1: like asking like why is there ice cream at some
44:33
Speaker 1: points in the universe and not other points? I think
44:35
Speaker 1: there's actually again two aspects of this. There's sort of
44:38
Speaker 1: the philosophical aspect and the physical aspect. The philosophical aspect
44:41
Speaker 1: I don't have much to say about, which is just
44:43
Speaker 1: in a world like ours. Why is it that the
44:49
Speaker 1: idea of a self, the idea of an agent, the
44:52
Speaker 1: idea of a conscious creature is attached to just one
44:56
Speaker 1: branch of the wave function at a time. I already
44:58
Speaker 1: mentioned the fact that you know, fundum mentally, the different
45:01
Speaker 1: branches don't interact with each other. So if you tried
45:03
Speaker 1: to say, well, I'm going to treat reality to me
45:07
Speaker 1: as two of the branches, not any of the others.
45:10
Speaker 1: We know the other's gonna take to these two well,
45:12
Speaker 1: I think that someone would say, yeah, but you have
45:15
Speaker 1: two things that have literally no impact on each other.
45:17
Speaker 1: The analogy I use in my book is, what if
45:20
Speaker 1: there were a ghost world? What if there's a world
45:23
Speaker 1: that was sort of, you know, the same shape as
45:25
Speaker 1: the Earth and the same physical location as the Earth
45:27
Speaker 1: and space, and there were people on it, and they
45:28
Speaker 1: talked to each other, but there's zero interaction through any
45:32
Speaker 1: force of nature or any other kind of influence between
45:34
Speaker 1: us and the people on ghost world. It just doesn't
45:36
Speaker 1: make sense to call them part of the same reality, right,
45:39
Speaker 1: I mean, they're two worlds for a long times and
45:42
Speaker 1: purposes certainly. So that's the sort of philosophical move I
45:44
Speaker 1: think that we don't have, at least I'm not aware
45:47
Speaker 1: of a once and for all definition of what a
45:50
Speaker 1: world is and how you should divide up reality in
45:52
Speaker 1: that way. But that's the rough idea. You know, a
45:55
Speaker 1: set of things that interact with each other is a world.
45:57
Speaker 1: But the other interesting question is, you know I started
46:01
Speaker 1: that by saying, in a world like ours, with laws
46:03
Speaker 1: of physics like ours, but Okay, what are the features
46:06
Speaker 1: of our laws of physics that allow for each individual
46:12
Speaker 1: branch of the wave function to be mostly classical? You
46:15
Speaker 1: were pretty good at predicting the positions of planets in
46:19
Speaker 1: the sky and eclipses and so forth using Newtonian mechanics
46:22
Speaker 1: long before quantum mechanics came on the scene, right, So
46:25
Speaker 1: why is classical mechanics a good limit of quantum mechanics,
46:29
Speaker 1: especially given that you're saying there's all these other worlds
46:32
Speaker 1: out there, right, And that's a trickier question, And I
46:35
Speaker 1: think that we're just beginning to make progress on it.
46:39
Speaker 1: And the answer has things to do with ideas like
46:42
Speaker 1: entanglement and decoherence and locality, But fundamentally, you know, that's
46:46
Speaker 1: still a research level problem. Well, I hope that we
46:48
Speaker 1: make some progress on it in the future. Now, I
46:50
Speaker 1: want to take a slight turn and ask you a
46:52
Speaker 1: little bit more of a personal question. You mentioned earlier
46:55
Speaker 1: that not only are you, you know, talking out there
46:57
Speaker 1: in the public about science, which are actually at practicing
46:59
Speaker 1: physics this And you know, in my experience, most people
47:02
Speaker 1: take one of two paths. They're either a practicing scientist
47:06
Speaker 1: or they are a science communicator. You know, I don't
47:08
Speaker 1: know that for example, Bill Nye or Neil de grass
47:11
Speaker 1: Tyson is still publishing papers. But you have kept your
47:14
Speaker 1: feet in both worlds. Has it been a challenge for
47:17
Speaker 1: you to remain part of the scientific community and maintain
47:21
Speaker 1: that credibility while also being a public intellectual. Yeah, there's
47:24
Speaker 1: sort of two aspects to the challenge. One is, it's
47:27
Speaker 1: a lot of work all these things to to write
47:30
Speaker 1: papers and to advise grad students while also having a
47:33
Speaker 1: podcast and writing books and so forth. But you know what,
47:36
Speaker 1: look to be honest, it's not that much work, and
47:38
Speaker 1: I can compartmentalize it pretty easily, and it's fun for me,
47:43
Speaker 1: Like I get to do these things that all are
47:44
Speaker 1: individually very fun, and my personality is such that I'd
47:48
Speaker 1: like being able to switch gears and do different things
47:51
Speaker 1: at different times. I would get frustrated if I did
47:53
Speaker 1: the same exact kind of thing every day, So this
47:56
Speaker 1: is a good way to do that. The other aspect
47:58
Speaker 1: of the challenge is, like you said the word credibility,
48:01
Speaker 1: like what about how other people think of you? And yeah,
48:04
Speaker 1: that's absolutely a challenge because of course two things Number one,
48:09
Speaker 1: public outreach and communication is itself undervalued and or devalued,
48:14
Speaker 1: depending on who you're talking to, Like, it's considered to
48:16
Speaker 1: be a waste of good brain CPU cycles that you
48:20
Speaker 1: could be using doing research, right, and doing research is
48:23
Speaker 1: what really matters. And number two, like you also imply,
48:27
Speaker 1: there's this idea that even if you think that outreach
48:30
Speaker 1: and communication are valuable, it's very difficult to imagine being
48:35
Speaker 1: productive in both spheres of both doing research and doing
48:39
Speaker 1: those kinds of things. There are famous counter examples Carl Sagan,
48:42
Speaker 1: Stephen Hawking, etcetera. Stephen Weinberg right, who recently passed away,
48:46
Speaker 1: but they're so rare that people almost don't take them seriously.
48:49
Speaker 1: And especially like there's the feeling that you should first
48:53
Speaker 1: become a super successful researcher and then you'd be allowed
48:58
Speaker 1: to do a little bit of writing books and things
49:00
Speaker 1: like that. I mean, Stephen Hawking invented black hole radiation
49:02
Speaker 1: long before he wrote A Brief History of Time and
49:05
Speaker 1: so forth. And Carl Sagan, on the other hand, never
49:07
Speaker 1: got elected to the National Academy of Sciences because people
49:09
Speaker 1: thought that he spent too much time doing outreach work.
49:12
Speaker 1: So that's a challenge. But you know, I'm too old
49:14
Speaker 1: to worry about that these these days. Um, it has
49:17
Speaker 1: impacted my career in very tangible, definite ways, but I'm
49:22
Speaker 1: still having fun doing what I want to do. So
49:24
Speaker 1: there you go. Well, that's great to hear my experience
49:26
Speaker 1: a little bit of that. Also, when I talked to
49:28
Speaker 1: people in my you know, card carrying particle physics world,
49:32
Speaker 1: and they asked me, oh, are you still doing research
49:34
Speaker 1: now that you're doing outreach, it's have to remind them
49:37
Speaker 1: it's possible to do more than the one thing. So
49:39
Speaker 1: it's interesting. Let me just put it this way because
49:41
Speaker 1: I like this analogy. Physicists are not so narrow minded
49:45
Speaker 1: that they won't allow their colleagues to do anything else.
49:48
Speaker 1: Like if you were a ski jumper or a professional
49:51
Speaker 1: unicyclist or whatever, or not professional but amateur unicyclist in
49:54
Speaker 1: your spare time, other physicists would think, oh, that's cookie
49:57
Speaker 1: and fun. Good for you, and you're probably also still
49:59
Speaker 1: doing research. But there's something about writing books and giving
50:03
Speaker 1: talks and making videos and so forth that is different
50:07
Speaker 1: than writing unicycle or being a ski jumper, because that's
50:11
Speaker 1: the kind of thing that people think, well, you should
50:14
Speaker 1: be using that effort doing research, right, Like your unicycling
50:17
Speaker 1: is just a different kind of thing, so we don't
50:19
Speaker 1: think that takes away from your research. An enormous number
50:22
Speaker 1: of professional physicists are rock climbers and mountain climbers, right,
50:25
Speaker 1: You probably know many yourself, for example. That's fine. But
50:29
Speaker 1: if instead of going rock climbing, you write a book,
50:32
Speaker 1: that's actually counts against you and that it's just a
50:35
Speaker 1: weird thing in my mind, but it's a very definite
50:37
Speaker 1: syndrome that is strange. And you know this is it's
50:41
Speaker 1: interesting to explore these career paths, and I wonder is
50:44
Speaker 1: this sort of the path you envisioned for yourself. Like
50:48
Speaker 1: if you could go back in time and describe your
50:50
Speaker 1: life now to fresh faced assistant professor Sean Carroll one
50:55
Speaker 1: year into your gig at University of Chicago, how do
50:57
Speaker 1: you think that Sean would re act? Well, the specific
51:01
Speaker 1: twists and turns of my career were not what I
51:04
Speaker 1: predicted or wanted, but the ending point is pretty close.
51:07
Speaker 1: You know, I always wanted to be a broader intellectual
51:11
Speaker 1: contributor than just a narrow research physicist that you know,
51:14
Speaker 1: since I was a kid, I wanted that, and that
51:16
Speaker 1: was always the plan. In some sense, and I was
51:19
Speaker 1: charmingly naive about how happy academia would be to receive
51:24
Speaker 1: such a plan. But there was always the plan, And
51:26
Speaker 1: I do believe that the right way to do that
51:28
Speaker 1: is to first become an expert at something. Right Like,
51:30
Speaker 1: you don't start out as an expert in everything you know.
51:33
Speaker 1: You better get good at your research and your PhD
51:36
Speaker 1: project and then sort of branch out after that. But
51:39
Speaker 1: you can't predict all of the ins and outs. But
51:42
Speaker 1: maybe I would have done things differently if I had
51:44
Speaker 1: crystal ball and could predict the future. A few tweaks
51:47
Speaker 1: here and there would have helped. But you know, I
51:49
Speaker 1: think I can still hold my head high about the
51:52
Speaker 1: choices that I made along the way. Well, what advice
51:54
Speaker 1: would you give to young folks now who are excited
51:57
Speaker 1: about science communication? In my experience, so I followed a
52:01
Speaker 1: fairly narrow path and got tenure as an experimental particle
52:04
Speaker 1: physicists before trying to branch out to outreach other types
52:07
Speaker 1: of things. But I see graduate students in my own
52:09
Speaker 1: group doing outreach, and I wonder, you know, if I
52:12
Speaker 1: should advise them. Look, the field is not friendly to
52:15
Speaker 1: this kind of breath at this age. Wait till you
52:18
Speaker 1: get tenure, or if I'm telling them not to be
52:20
Speaker 1: their authentic intellectual selves, and I should encourage them and
52:23
Speaker 1: the field with it to grow and accept this kind
52:26
Speaker 1: of activity. What would be your advice or what is
52:28
Speaker 1: your advice to your students when they try to emulate
52:30
Speaker 1: the full breadth of your activities. It's enormously good and
52:33
Speaker 1: important question, and the answer is not obvious. But I
52:36
Speaker 1: can always fall back on the maxim that I should
52:38
Speaker 1: tell the truth. So I can tell the truth about
52:42
Speaker 1: factual statements about the world without necessarily coloring them by
52:46
Speaker 1: normative statements. So rather than saying don't do that until
52:50
Speaker 1: you get tenure, I can say, look, it's great that
52:53
Speaker 1: you like doing a lot of different things outreach and
52:55
Speaker 1: so forth. I have one recent student of mine who
52:58
Speaker 1: is actually super duper six US full hot property in
53:00
Speaker 1: the job market, wrote a musical and performed it, you know,
53:04
Speaker 1: at cal Tech while he was in graduate school. So
53:07
Speaker 1: he did okay. But the point is that it's hard
53:10
Speaker 1: to become a professional academic, right The numbers game is bad.
53:14
Speaker 1: I'm at cal Tech, and I tell my students that
53:16
Speaker 1: if you're at a place like cal Tech or Harvard
53:18
Speaker 1: or whatever, and you get a PhD, maybe one in
53:21
Speaker 1: four of you will eventually be tenured professors in that field.
53:25
Speaker 1: I don't know the exact numbers, and it will depend
53:27
Speaker 1: a lot on sub fields and where you get your
53:29
Speaker 1: PhD and so forth, but the numbers are against you, certainly.
53:33
Speaker 1: And the thing you can say with a good amount
53:35
Speaker 1: of confidence is that all else being fixed, doing outreachy
53:40
Speaker 1: things will lower the percentage chance that you will someday
53:44
Speaker 1: become tenured academic. Now you might still decide to do it,
53:48
Speaker 1: and that's great, but I want you to go in
53:49
Speaker 1: with your eyes open, right Like I sometimes get in
53:52
Speaker 1: trouble by being a little bit too candid with my students,
53:56
Speaker 1: because a lot of my colleagues are like now they're
53:58
Speaker 1: they're young and impressionable. We have to like juice them
54:01
Speaker 1: up about physics. And I'm like, well, yeah, but a
54:03
Speaker 1: lot of them are not going to end up being physicists.
54:05
Speaker 1: And I think that I love the idea of going
54:07
Speaker 1: to grad school. I love the idea of getting a PhD.
54:09
Speaker 1: And I think it's intrinsically worthwhile thing. And I'm not
54:12
Speaker 1: going to discourage anyone from doing that because it's a
54:15
Speaker 1: matter of intellectual growth and so forth. I think the
54:18
Speaker 1: current system where you then have to do somewhere between
54:20
Speaker 1: five and ten years of post doc afterward is not ideal.
54:24
Speaker 1: But you know, if you want to get a PhD,
54:26
Speaker 1: then I think that's the best thing in the world.
54:28
Speaker 1: And then you should know what the chances are and
54:31
Speaker 1: what the different aspects are that affect your chances along
54:34
Speaker 1: the way, and then you make your own decisions. Well,
54:36
Speaker 1: I agree with that, and I hope that other folks
54:38
Speaker 1: out there can see that it's possible to be an
54:40
Speaker 1: academic and to do scientific communication and outreach, and that
54:45
Speaker 1: encourages the community to accept that into broaden our concept
54:48
Speaker 1: of what a healthy physicist is. You're allowed to also
54:50
Speaker 1: be a unicycler or to talk about science to your
54:53
Speaker 1: friends and neighbors on the internet. All right, we've taken
54:55
Speaker 1: enough of your time. Thanks very much for joining us
54:57
Speaker 1: and for telling us about your vision of quantum mechanics.
54:59
Speaker 1: And it's then do us all the crazy mind blowing
55:02
Speaker 1: ideas involved in the many world's interpretation. It's been a pleasure.
55:05
Speaker 1: It's very fortunate that we had to think about these things.
55:07
Speaker 1: So thanks for having me on. All right, Thanks very much,
55:18
Speaker 1: thanks for listening, and remember that Daniel and Jorge explained.
55:20
Speaker 1: The Universe is a production of I Heart Radio or
55:23
Speaker 1: more podcast from my heart Radio. Visit the I Heart
55:26
Speaker 1: Radio Apple Apple Podcasts, or wherever you listen to your
55:30
Speaker 1: favorite shows. Ye