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Speaker 1: The big picture task.

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Speaker 2: The ultimate quest of physics is to make sense of

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Speaker 2: the universe, to sort through our amazing and bonkers experience

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Speaker 2: and find as compact and simple an explanation as possible

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Speaker 2: for why what happens happens. We've made a lot of progress,

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Speaker 2: but we've also struggled together. Do quantum particles really collapse

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Speaker 2: together across space time? Why is gravity so difficult to

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Speaker 2: unify with quantum theory? Why does time flow forward? Sometimes?

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Speaker 2: I wonder if the reason we feel stuck is that

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Speaker 2: we started from the wrong place. What if there's a simple,

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Speaker 2: basic assumption that we're making that has led us down

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Speaker 2: the wrong path. It's happened to us before, like when

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Speaker 2: we assumed that time flows the same way across the universe,

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Speaker 2: or that we were at the center of the Solar system,

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Speaker 2: or that light needed medium to propagate. When we removed

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Speaker 2: the mistaken assumption, the veil was pulled from our eyes

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Speaker 2: in a simpler, clear if weirder explanation emerged, Could that

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Speaker 2: be the cause of our current head scratching? Is there

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Speaker 2: some requirement we've imposed on physics that's keeping us from

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Speaker 2: seeing the simple explanation staring us in the face. Today

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Speaker 2: on the podcast, will knock on the doors the most

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Speaker 2: basic intuitive foundational concepts in physics and ask do we

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Speaker 2: really need them? We'll question locality, do things have to

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Speaker 2: be in the same place in order to interact? And

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Speaker 2: its cousin causality, do causes have to come before effects?

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Speaker 2: Welcome to Daniel and Kelly's extraordinary counterintuitive Universe.

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Speaker 3: Hello. I'm Kelly Readersmith. I study parasites and and after

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Speaker 3: today's conversation, I feel really lucky that locality and causality

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Speaker 3: just make a lot more sense in biology.

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Speaker 1: Hi, I'm Daniel.

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Speaker 2: I'm a particle physicist, and I like to pretend sometimes

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Speaker 2: to be a philosopher.

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Speaker 3: Ooh yeah, I enjoy philosophy too.

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Speaker 4: Well.

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Speaker 2: One of the things I love about physics is that

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Speaker 2: it butts right up against philosophical questions. We're asking about

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Speaker 2: the deep, fundamental nature of the universe, and so the

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Speaker 2: philosophical questions are like immediate and obvious. I hear a

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Speaker 2: little bit less about like the philosophy of biology. You know,

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Speaker 2: what does it mean to be a parasite?

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Speaker 1: This sort of stuff?

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Speaker 3: Well, I mean, we have plenty of arguments about that

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Speaker 3: at the conference as we go to. But I feel

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Speaker 3: like for us philosophy, Well, what's the difference between philosophy

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Speaker 3: and ethics? Is ethics a subset of philosophy? Because we

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Speaker 3: talk a lot about ethics associated with the biology stuff

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Speaker 3: that we're working on. So I'm more used to tackling

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Speaker 3: ethical questions than like, you know, what does causality even mean?

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Speaker 2: Well, I think you can your finger on it, because

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Speaker 2: anytime you get bogged down in a conversation by defining

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Speaker 2: your terms, that's when you know you're doing philosophy.

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Speaker 3: Ah, there we go. All right, Well, we spend a

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Speaker 3: glorious amount of time talking about how to define terms today,

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Speaker 3: and we've got Sean Carroll to help us do that,

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Speaker 3: and I love all of the stories that he tells.

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Speaker 3: But before we jump into our interview with Sean, we

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Speaker 3: should hear what our audience thinks about what it means

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Speaker 3: to be local and causal.

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Speaker 2: That's right, as usual, I went out there to our

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Speaker 2: group of volunteers to see what they thought about these

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Speaker 2: fundamental concepts in physics and philosophy. So think about it

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Speaker 2: for a moment before you hear these answers. Do you

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Speaker 2: think physics has to be local and causal. Here's what

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Speaker 2: our listeners had to say.

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Speaker 5: Physics used to be local to me years ago when

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Speaker 5: I dated a woman whose father was a physicist. Splash, Yes,

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Speaker 5: chemist at Bell Labs. But it's not anymore.

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Speaker 2: Observations and experiments at much larger scale, much smaller scales

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Speaker 2: seem to show that both of those concepts break down

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Speaker 2: in the right circumstances.

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Speaker 1: Most physicists assume causality.

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Speaker 6: I don't think physics have to be local, but I

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Speaker 6: think they have to be caucal to have causality.

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Speaker 7: I think classical physics is local and causal, but I

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Speaker 7: think quantum mechanics, with its spontaneous random activity and its

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Speaker 7: ability to have spooky action at a distance, breaks that.

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Speaker 4: A little bit.

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Speaker 6: Physics has to be local in causal, at least in

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Speaker 6: our quarter of the universe. Otherwise Einstein's spooky action would

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Speaker 6: have your lamp turned on before you hit the switch,

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Speaker 6: Newton would drop his apple in reverse, and all hell

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Speaker 6: would have broken loose yesterday.

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Speaker 4: I would think physics has to be local in order

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Speaker 4: to be testimal, so that the results are always predictable.

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Speaker 4: But I'm not sure what causal means. Maybe cause and effect.

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Speaker 7: I think It's really interesting to think of physics as

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Speaker 7: not being local, because the implications would kind of crazy,

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Speaker 7: and maybe it would explain some of the stuff that

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Speaker 7: we don't really understand yet.

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Speaker 6: How about it has to be at least as local

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Speaker 6: as gravity and at least as causal as quantum decay.

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Speaker 8: What an interesting question? Does physics have to be local

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Speaker 8: and or casual? Well, if it's a dating app I

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Speaker 8: was supposed to be local, and because I had a

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Speaker 8: casual relationship with physics in high school, I really don't

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Speaker 8: get the question. But I would say, isn't physics universal

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Speaker 8: and cosmological?

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Speaker 6: Maybe things change based on locality and conditions.

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Speaker 7: I say, yes, physics has to be local and clusal.

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Speaker 2: Love these answers both insightful and hilarious, especially the person

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Speaker 2: who misinterpreted causal to be casual.

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Speaker 3: Are you sure you spelled it right in your email, Daniel.

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Speaker 1: I am not sure. Maybe that was the cause.

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Speaker 3: Of Oh yeah, See most things in my world there's

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Speaker 3: a cause and effect relationship and there's no going back

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Speaker 3: in time.

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Speaker 2: Well, you know, this really is a fundamental issue in philosophy.

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Speaker 2: We are just discussing on the discord this morning, Like

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Speaker 2: does the universe have to make sense? Does there have

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Speaker 2: to be an explanation for everything? I think as humans,

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Speaker 2: we are curious and we want to know answers to

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Speaker 2: questions about the universe because we assume that there are

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Speaker 2: answers right, that there is a thing that is happening

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Speaker 2: and we can figure it out somehow. And sometimes these

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Speaker 2: discussions of like locality and causality tell me that the

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Speaker 2: universe could be very, very different from the way that

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Speaker 2: we assume that it is, and in a way that

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Speaker 2: might never make sense to us.

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Speaker 3: And you know, the more we learn about quantum mechanics,

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Speaker 3: the more the more it breaks my brain, to be honest,

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Speaker 3: but the more yeah interesting questions, I realize that there

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Speaker 3: are left to ask like questions about locality and causality

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Speaker 3: at the quantum mechanic level. That is what I will

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Speaker 3: be thinking about tonight when I'm trying to fall asleep

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Speaker 3: and I'm not sure if it's going to keep me

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Speaker 3: up or make me fall asleep more quickly. So we'll see.

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Speaker 2: And to dig deep into these topics, we invited an

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Speaker 2: expert in physics and philosophy on the show and a

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Speaker 2: friend of the podcast, Sean Carroll, who's not afraid to

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Speaker 2: get bogged down defining terms.

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Speaker 3: Yeah, things got heated between you two at a couple points,

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Speaker 3: so let's jump in.

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Speaker 1: All right, it's my pleasure to welcome to the podcast.

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Speaker 1: Shawn Carroll Sean is a.

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Speaker 2: Theoretical physicist who's done impouring work on the foundations of

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Speaker 2: quantum mechanics, cosmology, and the arrow of time. He holds

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Speaker 2: a joint appointment between physics and philosophy at Johns Hopkins.

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Speaker 2: He's also a prolific science communicator, the host of the

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Speaker 2: mind ski podcast, which I hardly recommend for its impressively

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Speaker 2: deep dives, and author several books such as The Biggest

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Speaker 2: Ideas in the Universe. Shawn, Welcome to the podcast.

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Speaker 4: Thanks very much for having me.

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Speaker 9: This is the first time I have been on a

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Speaker 9: podcast hosted by two previous guests of my podcast, So

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Speaker 9: I like how we are closing the triangle.

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Speaker 3: This is exciting.

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Speaker 2: We are accelerating towards the podcast singularity. Well, speaking of Singularity,

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Speaker 2: today we're going to die really deep on something that

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Speaker 2: I've always wanted to understand better in physics, and I

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Speaker 2: can't imagine a better person to ask hard questions about

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Speaker 2: issues on the boundary of philosophy in physics. I mean,

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Speaker 2: I know a lot of people who are physicists to

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Speaker 2: interest in philosophy. I know a lot of philosophers who

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Speaker 2: do some physics as well, But I don't think I

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Speaker 2: know anybody else who literally has an appointment in physics

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Speaker 2: and in philosophy. So congratulations on straddling that barrier.

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Speaker 4: Thanks.

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Speaker 9: So it's not easy to make it happen, you know,

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Speaker 9: as you know academia, we love our little silos, and

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Speaker 9: I mean there's plenty of people in universities who are

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Speaker 9: cross appointed between departments, but a humanities and a science

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Speaker 9: thing getting together is really hard to pull off.

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Speaker 3: Yeah, and really awesome.

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Speaker 9: I think it's awesome. I'm having a good time, so

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Speaker 9: you know, it makes me feel special.

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Speaker 2: Well, I don't have a joint appointment philosophy. I do

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Speaker 2: have a courtesy appointment in the philosophy department, which is

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Speaker 2: just because I showed up to enough philosophy seminars to

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Speaker 2: ask awkward questions that they were like.

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Speaker 9: Who are you?

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Speaker 1: And then they're like, oh, do you want to courtesy appointment?

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Speaker 1: It gives you nothing. That's very yeah, exactly.

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Speaker 3: It's like they're encouraging you to keep coming back though.

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Speaker 2: That's nice, Yeah, exactly, yeah, yeah, although I did learn

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Speaker 2: there's a big difference between the two fields. And physics seminars,

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Speaker 2: it's totally normal to interrupt with questions. You don't understand something,

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Speaker 2: raise your hands, speak up, start a discussion. If I

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Speaker 2: give a physics seminar, I feel like it's a failure

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Speaker 2: if nobody's interrupted to discuss something. First time I asked

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Speaker 2: a question in the middle of a philosophy seminar, everybody

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Speaker 2: in the room turned to me with horror. So it's

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Speaker 2: like objecting in the middle of a wedding or something.

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Speaker 1: You know, like hold your question to the end, sir.

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Speaker 9: But then also at a philosophy colloquium, you know, often

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Speaker 9: they will let the speaker talk for an hour, take

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Speaker 9: a five minute break, and then come back for an

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Speaker 9: hour of questions, which physicists would never do. Physicists be like,

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Speaker 9: what you expected us to listen to? What was happening

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Speaker 9: into the seminar? What's going on here?

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Speaker 2: All right, Well, let's dive into the topic for today.

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Speaker 2: We're talking about locality and causality, and these of course

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Speaker 2: are intermingled, but we're going to take them one at

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Speaker 2: a time if we can. So let's start with locality, Sean,

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Speaker 2: how do you interpret the concept of locality? What does

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Speaker 2: locality mean in physics? It doesn't mean like, get your

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Speaker 2: donuts from the store around the corner.

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Speaker 9: It kind of does. I mean, it's pretty close to that.

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Speaker 9: It's the idea that the donuts aren't that far away,

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Speaker 9: the donuts that you're actually going to want. You know,

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Speaker 9: when I get donuts, I'm more likely to go to

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Speaker 9: the place a couple blocks away than to the place

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Speaker 9: three thousand miles away. And fundamentally that's because of locality.

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Speaker 4: You know.

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Speaker 9: Locality comes in different ways in different stretches of physics.

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Speaker 9: But the basic idea is that there's this thing called space,

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Speaker 9: and indeed we can promote it to space time and

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Speaker 9: we can talk about that too. But there are places

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Speaker 9: where we're located in the universe. And that kind of sounds,

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Speaker 9: you know, obvious. So yes, there are places where we're

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Speaker 9: located in the universe, but it's not so obvious. It's

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Speaker 9: a very strong claim that what the universe is made

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Speaker 9: out of is space and things inside of space. Right,

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Speaker 9: Like I have a location, you have a location. This

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Speaker 9: electron has a location, et cetera. And then furthermore that

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Speaker 9: when these things bump into each other. This sort of

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Speaker 9: the basic ideal locality is there is space and things

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Speaker 9: in it. The next level ideal locality is that when

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Speaker 9: different things interact, they do so at the same point

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Speaker 9: in space time, or at least at neighboring points. And

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Speaker 9: you might say, well, wait a minute, Like the Sun

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Speaker 9: exerts a gravitational field on the Earth, even though it's

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Speaker 9: very far away, but there is a field in between

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Speaker 9: the Sun and the Earth. And it's more like the

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Speaker 9: Sun effects it's gravitational field right at the Sun, and

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Speaker 9: that affects the gravitational field right next to that. And

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Speaker 9: you work your way up to what's going on here

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Speaker 9: on Earth.

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Speaker 2: But let me back you up because you said something

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Speaker 2: which already blew my mind, Yeah, which is that it's

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Speaker 2: a strong claim essentially to say that locations exist before

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Speaker 2: we get to locality requirements, that things have to interact

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Speaker 2: at the same location. You're saying, it's a big idea

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Speaker 2: that there is space and there are locations, right, And

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Speaker 2: if that's a strong claim, like, what's the opposite, Like,

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Speaker 2: are you suggesting it's possible to have a universe without

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Speaker 2: locations or.

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Speaker 3: A physicists just making things complicated.

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Speaker 7: No.

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Speaker 9: In fact, what I was just about to say is

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Speaker 9: the thing about that version of locality I just said

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Speaker 9: that the world is made of things located in space,

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Speaker 9: is that it's false. It is clearly not true, because

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Speaker 9: there's this thing called quantum mechanics, and quantum mechanics says

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Speaker 9: that's not what the world is made out of. In

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Speaker 9: again various different levels of precision. Even if you just

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Speaker 9: have one electron, there's a wave function for the electron,

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Speaker 9: and that wave function has some profile throughout space, so

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Speaker 9: there's no such thing as the point at which the

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Speaker 9: electron is located. But you might say, like, okay, but fine,

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Speaker 9: there is something called the value of the electrons wave

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Speaker 9: function at every point in space. It's kind of like

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Speaker 9: the value of the electric field or the gravitational field.

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Speaker 9: Is still a kind of locality, which is fine until

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Speaker 9: you have two electrons. When you have two electrons, now

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Speaker 9: there's not the wave function of electron one and the

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Speaker 9: wave function of electron two. There's the wave function quantum

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Speaker 9: mechanically for the system of both electrons at once, and

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Speaker 9: that's what the world is. And that's not things located

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Speaker 9: in space. It's something much weirder than that. And so

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Speaker 9: quantum mechanics, and I'm sure we'll get into it, but

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Speaker 9: quantum mechanics just makes the world look really not local

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Speaker 9: at all. And then the question I would argue is

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Speaker 9: why does it kind of look local?

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Speaker 4: Right?

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Speaker 9: You know, why do we get along so well thinking

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Speaker 9: that the world looks local if quantum mechanics is trying

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Speaker 9: to tell us something different?

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Speaker 3: Does this question about what local means? You know, does

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Speaker 3: it not make sense at the quantum level, but when

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Speaker 3: you go past the quantum level, it totally makes sense.

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Speaker 1: Yes.

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Speaker 9: But I will just like be I got to be

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Speaker 9: a stickler in this whole conversation, right because where this

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Speaker 9: is one of those things where we think we know

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Speaker 9: what the words mean, and we're digging deeply into what

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Speaker 9: they mean, and so we can't be beholden to our

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Speaker 9: folk wisdom about what these words mean. So when you say,

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Speaker 9: like the quantum level and not the quantum level, I

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Speaker 9: just want to remind everyone everything is the quantum level.

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Speaker 9: There's no non quantum level. The world is quantum. It's

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Speaker 9: not like you get quantum when you look at things

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Speaker 9: that are very small it's the eppisode way around. You

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Speaker 9: get classical when you look at things that are very big,

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Speaker 9: So classical mechanics, allah, Isaac Newton, et cetera becomes a

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Speaker 9: good approximation when things are big and ponderous and macroscopic

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Speaker 9: and so forth. And that's the world in which, yes,

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Speaker 9: you're completely right. The world looks in the classical limit

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Speaker 9: as if it's made of objects with locations bumping into

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Speaker 9: other objects when they are at the same location.

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Speaker 2: Okay, and before we get quantum, like Isaac Newton's theory

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Speaker 2: of gravity's already non local right, like.

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Speaker 9: We already got quantum, Daniel, it is too late. You've

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Speaker 9: been quantum this whole time. Why are you listening?

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Speaker 2: Yes, and I acknowledge that Newton is made of quantum objects. Yeah,

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Speaker 2: in that sense, he is quantum, but he didn't know

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Speaker 2: about quantum. And when he was developing his theory of gravity,

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Speaker 2: he invented a concept which had instantaneous communication across space

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Speaker 2: and time. And so if people think about non localities,

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Speaker 2: this new fangled thing that emerge from quantum mechanics, isn't

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Speaker 2: it sort of the old fangle thing that we sort

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Speaker 2: of accepted for a while and then gave up.

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Speaker 9: And now we're returning to we had to be very

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Speaker 9: very careful. Once again, Sorry for being so careful, but

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Speaker 9: please be careful. There are once again two different levels

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Speaker 9: of locality that we have to distinguish between. One is

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Speaker 9: just what I said, like, the equations of physics are local.

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Speaker 9: Means that you write down the things the world is

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Speaker 9: made of, and the whole equation is a function of position. Right,

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Speaker 9: so in Newtonian gravity, you're right, that's not quite true.

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Speaker 9: Like when Isaac Newton literally wrote down the inverse square

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Speaker 9: law for gravity, the rule that says that the gravitational

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Speaker 9: force is weaker when you're further away, stronger when you're

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Speaker 9: close by a factor of one over the distance squared.

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Speaker 9: And he worried that this seemed non local, that here's

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Speaker 9: the Earth. The Earth is being pulled on by the

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Speaker 9: Sun and the Moon and everything in the universe instantaneously.

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Speaker 9: Because he's Isac Newton, he doesn't know about relativity. It

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Speaker 9: doesn't know about special relativity anyway. So there is absolute space,

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Speaker 9: absolute time. And Newton in the principia said like, yeah,

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Speaker 9: this bothers me, the fact that somehow the information about

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Speaker 9: the gravitational field of everything in the universe is conveyed

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Speaker 9: to the Earth immediately, and he literally said, I'm going

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Speaker 9: to leave this for future generations to sort out. I

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Speaker 9: don't like it. It bugs me. I'm not smart enough.

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Speaker 9: I'mder Isaac Newton. You know, eventually we will figure it out. Now,

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Speaker 9: there's a sort of little known thing that happened, but

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Speaker 9: I think is really crucial. In a circa eighteen hundred,

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Speaker 9: Pierre Simone Laplace solved that problem. And no one ever

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Speaker 9: gives them credit for this, but Laplace pointed out that

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Speaker 9: you can write down Newtonian gravity as a field theory.

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Speaker 9: He invented the idea of the gravitational potential. And by

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Speaker 9: a field theory, we just mean rather than just saying

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Speaker 9: there's a force acting on the Earth, and to know

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Speaker 9: what the force is, you have to know what everything

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Speaker 9: in the universe is doing. He says, there is something

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Speaker 9: called the gravitational potential field, and it's a field, so

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Speaker 9: it has a value at every point in space, and

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Speaker 9: the equation that that field obeys is entirely local. What

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Speaker 9: is happening through the field right here at some point

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Speaker 9: only depends on what's happening at right next door points.

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Speaker 9: And it's exactly like we said at the beginning, that

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Speaker 9: the Sun creates a dimple in the gravitational field at

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Speaker 9: where thus is, and that pulls on the field next

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Speaker 9: to it, and that pulls on the field next to it,

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Speaker 9: and it works its way out to the Earth. So,

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Speaker 9: in that sense, because there is a local field theory

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Speaker 9: description of Newtonian gravity, Lutonian gravity is entirely local.

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Speaker 1: But is it still instantaneous?

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Speaker 9: Well, you've skipped ahead to the world of relativity, where

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Speaker 9: there's a speed limit. So when relativity comes along, which

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Speaker 9: is long after either Newton or laplace, now we have

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Speaker 9: an idea that even if the laws of physics are

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Speaker 9: local in space time, which they are, like Einstein's equations

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Speaker 9: and Maxwell's equations and whatever, still, if something happens at

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Speaker 9: a particular point in space time, the effects of that

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Speaker 9: thing only ripple out slower than or at the speed

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Speaker 9: of light. Right, You do not affect things instantaneously far away.

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Speaker 9: And what that means is that we can have a better, stronger,

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Speaker 9: more satisfying version of locality, which is that not only

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Speaker 9: do the equations simply exist at every point in space

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Speaker 9: or every point in space time, but that you're not

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Speaker 9: being affected by things infinitely far away instantaneously. So we

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Speaker 9: have ex post facto retrofitted our notion of locality to

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Speaker 9: demand not instantaneous communication between two different points of space.

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Speaker 9: For Newton, that would have been fine. For in the

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Speaker 9: post Einstein world, that's no longer okay, because the speed

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Speaker 9: of light is a fundamental limit.

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Speaker 3: Okay, So the biologist who lives in the world where

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Speaker 3: locality works, fine, thank you very much. Is so if

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Speaker 3: I can sort of summarize what's happening. So locality is

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Speaker 3: a problem because at the quantum level it doesn't really

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Speaker 3: work and you scale up. But it sounds like you

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Speaker 3: were just saying that locality is fine when you're talking

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Speaker 3: about things like the Sun impacting the Earth because you've

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Speaker 3: got a field and you're connected every point along the way,

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Speaker 3: And so is our conversation then mostly about the quantum level.

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Speaker 9: Now am I following, Well, there's lots of conversations. Okay,

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Speaker 9: you know, there's definitely that. The distinction we were just

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Speaker 9: drawing is between pre relativity classical physics and post relativity

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Speaker 9: classical physics, where we have glommed onto the notion of locality.

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Speaker 9: The idea that signals don't travel faster than light, so

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Speaker 9: there cannot be any instantaneous communication across distances. You know,

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Speaker 9: in Isaac Newton's world, I could send a signal to

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Speaker 9: the Andromeda Galaxy instantaneously by taking a planet and shaking

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Speaker 9: it a little bit right. Its gravitational force in principle,

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Speaker 9: although not in practice, would change instantaneously in the Andromeda Galaxy.

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Speaker 9: And so is it local, You know, kind of it's

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Speaker 9: fading away, but still strictly speaking, it's hard to wrap

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Speaker 9: your brain around. But now, in Einstein's universe, if you

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Speaker 9: shake a planet, it's going to take you a million

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Speaker 9: light years for that signal to get to the Andromeda Galaxy.

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Speaker 9: But still that's all within the limit that we called

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Speaker 9: the classical limit of quantum mechanics. And so one place

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Speaker 9: where our notions of locality are challenged and need to

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Speaker 9: be updated is in the quantum realm. But by the way,

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Speaker 9: another place that their challenge need to be updated is

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Speaker 9: in things like biology. Oh no, yes, because you know

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Speaker 9: in biology what happens is most of the biological organisms

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Speaker 9: that you and I know and love are moving very

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Speaker 9: slow compared to the speed of light. Yeap so even

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Speaker 9: though they're big and they're in the classical world and

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Speaker 9: they bump into each other, when things do happen, they

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Speaker 9: happen essentially instantaneously, right. The signals can get to, you know,

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Speaker 9: across the organism very very fast, and so it can

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Speaker 9: often be useful at that higher emergent level of biology

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Speaker 9: to have both local things like here's a little cell

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Speaker 9: or there's a little bacterium or whatever, but also global things, right,

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Speaker 9: global variables. What is the pH of your solution that

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Speaker 9: you're in, or you know, what is the gradient of nutrients?

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Speaker 9: Or is the human being happy or sad? Like, that's

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Speaker 9: not a local thing. Where's the happiness, where's the sadness?

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Speaker 4: Right?

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Speaker 9: So, both at the microscopic quantum level and at the

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Speaker 9: emergent non physics y higher levels, the strict notions of

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Speaker 9: locality that we have in classical relativistic physics become a

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Speaker 9: little shaky.

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Speaker 2: All right, So let's trace it historically. We start with

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Speaker 2: these concepts where you can have instantaneous interaction across space

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Speaker 2: and time. Then we get special relativity, which gives us

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Speaker 2: a cool concept of locality. You can only be influenced

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Speaker 2: by things in your past like cone and influence things

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Speaker 2: in your future like cone, because signals take time to propagate,

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Speaker 2: which is still a cool concept of locality. It means that,

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Speaker 2: like very much, something interacts with something else, which interacts

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Speaker 2: with something else, so you have to have like a

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Speaker 2: chain of interactions or you know, wiggle or propagation of

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Speaker 2: this interaction. But what about general relativity? Is general relativity

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Speaker 2: local in the same way that special relativity is.

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Speaker 9: General relativity is almost as local as special relativity is,

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Speaker 9: so special relativity came along in nineteen oh five. This

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Speaker 9: is Einstein's idea that you can reconcile the non existence

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Speaker 9: of any preferred frame of reference in the universe. There's

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Speaker 9: no ether or anything like that, with the fact that

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Speaker 9: everyone thinks that the speed of light is the same.

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Speaker 9: All you have to do, says Einstein, is give up

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Speaker 9: your conventional notions of what space is and what time

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00:23:32,480 --> 00:23:36,000
Speaker 9: is and marry them together. Yeah, that's why he's Einstein,

438
00:23:36,119 --> 00:23:39,720
Speaker 9: you know, marry them together to make space time. And

439
00:23:39,760 --> 00:23:43,359
Speaker 9: then ten years later in general relativity he says, you

440
00:23:43,400 --> 00:23:46,720
Speaker 9: know what I forgot to tell you, but space time.

441
00:23:46,880 --> 00:23:50,080
Speaker 9: This arena in which the game of physics is played

442
00:23:50,760 --> 00:23:53,879
Speaker 9: has a life of its own, it's dynamical, it has

443
00:23:53,920 --> 00:23:57,720
Speaker 9: a geometry, it can be curved, it can respond to

444
00:23:58,160 --> 00:24:01,080
Speaker 9: matter and energy and their motion in the universe. They're

445
00:24:01,119 --> 00:24:03,680
Speaker 9: a player, They're not just the arena in which the

446
00:24:03,720 --> 00:24:09,320
Speaker 9: game is played. So that makes things much trickier when

447
00:24:09,320 --> 00:24:12,439
Speaker 9: it comes to saying, you know, what depends on what,

448
00:24:13,440 --> 00:24:17,119
Speaker 9: like the future of the universe, the future of the

449
00:24:17,119 --> 00:24:20,440
Speaker 9: curvature of the universe, et cetera. In general relativity, in principle,

450
00:24:20,520 --> 00:24:24,680
Speaker 9: this is a deterministic consequence of what's happening in the

451
00:24:24,720 --> 00:24:27,760
Speaker 9: universe right now. But there can be limitations on that.

452
00:24:27,840 --> 00:24:30,760
Speaker 9: There can be weird global structures in the universe. You know,

453
00:24:30,800 --> 00:24:34,639
Speaker 9: there can be closed time like curves where the timelike

454
00:24:34,800 --> 00:24:37,720
Speaker 9: trajectories that a person can travel on in a rocket

455
00:24:37,760 --> 00:24:43,080
Speaker 9: ship can loop back on themselves. Space can be curled

456
00:24:43,160 --> 00:24:45,800
Speaker 9: up on itself. Space can be a tourist. There can

457
00:24:45,840 --> 00:24:48,240
Speaker 9: be extra dimensions of space that are very tiny, and

458
00:24:48,280 --> 00:24:52,359
Speaker 9: things like that. So the fundamental equations of general relativity

459
00:24:52,359 --> 00:24:55,560
Speaker 9: are still one hundred percent local, but there are global

460
00:24:55,600 --> 00:24:58,600
Speaker 9: consequences of those equations that get things a little more subtle.

461
00:24:58,680 --> 00:25:00,760
Speaker 2: Well, what about concepts like work? I mean, if we're

462
00:25:00,800 --> 00:25:05,040
Speaker 2: talking about locality. General relativity gives us more flexibility and

463
00:25:05,080 --> 00:25:09,000
Speaker 2: what locality means because it changes which points of space

464
00:25:09,040 --> 00:25:11,640
Speaker 2: are near each other. Right, So in principle, you open

465
00:25:11,640 --> 00:25:15,680
Speaker 2: a wormhole between our galaxy and Drameda. Now some part

466
00:25:15,680 --> 00:25:18,840
Speaker 2: of Andrameda is local. Is that concept of locality so

467
00:25:19,000 --> 00:25:20,520
Speaker 2: to preserved through the wormhole.

468
00:25:21,040 --> 00:25:25,000
Speaker 9: There is absolutely a concept of locality that is preserved

469
00:25:25,040 --> 00:25:27,600
Speaker 9: even in the presence of wormholes. Like if you say,

470
00:25:28,040 --> 00:25:31,119
Speaker 9: if I take a limit where I look at creatures

471
00:25:31,400 --> 00:25:35,480
Speaker 9: or points of light or particles or whatever that are

472
00:25:35,520 --> 00:25:39,280
Speaker 9: small compared to the curvature and the topology of space time,

473
00:25:39,720 --> 00:25:42,439
Speaker 9: everything is local. All of the equations are local, all

474
00:25:42,480 --> 00:25:45,840
Speaker 9: the dynamics are local. But you're right, you can imagine.

475
00:25:45,880 --> 00:25:48,880
Speaker 9: General relativity gives you this freedom to imagine. Okay, I'm

476
00:25:48,920 --> 00:25:52,040
Speaker 9: going to make a shortcut in space time. I'm going

477
00:25:52,080 --> 00:25:55,840
Speaker 9: to construct a wormhole that attaches two different parts of

478
00:25:55,880 --> 00:25:58,480
Speaker 9: space that I thought were far away. But if I

479
00:25:58,520 --> 00:26:00,760
Speaker 9: go from point A to point B three the wormhole,

480
00:26:01,040 --> 00:26:03,439
Speaker 9: I'm going to say, it doesn't take that long at all.

481
00:26:03,560 --> 00:26:06,440
Speaker 9: They're actually much closer. And this was an idea that

482
00:26:06,560 --> 00:26:09,680
Speaker 9: has been around a long time. Einstein, as usual was

483
00:26:09,680 --> 00:26:11,960
Speaker 9: one of the first people to talk about wormholes. John

484
00:26:12,000 --> 00:26:14,840
Speaker 9: Wheeler gave them the name. We have enough time to

485
00:26:14,880 --> 00:26:17,560
Speaker 9: tell this one, very very amusing story here. It was

486
00:26:17,600 --> 00:26:22,560
Speaker 9: when Carl Sagan wrote this novel Contact that wormholes became

487
00:26:22,600 --> 00:26:26,720
Speaker 9: important in physics because Sagan wrote his book and he

488
00:26:26,760 --> 00:26:30,399
Speaker 9: wanted to get his hero Ellie across the galaxy very

489
00:26:30,480 --> 00:26:32,720
Speaker 9: very quickly. And Sagan, you know, was a great scientist,

490
00:26:32,720 --> 00:26:35,000
Speaker 9: but he was a planetary scientist. He was not a

491
00:26:35,040 --> 00:26:38,359
Speaker 9: fundamental theoretical physicist. He knows general relativity very well, so

492
00:26:38,400 --> 00:26:40,679
Speaker 9: he had her fall into a black hole in the

493
00:26:40,680 --> 00:26:43,239
Speaker 9: original draft of the novel. The good news is that

494
00:26:43,359 --> 00:26:46,200
Speaker 9: Carl Sagan was close friends with Kip Thorne, who does

495
00:26:46,320 --> 00:26:50,040
Speaker 9: know his general relativity very well, and had him read

496
00:26:50,080 --> 00:26:51,880
Speaker 9: the draft and Kip said, you can't have her fall

497
00:26:51,920 --> 00:26:54,399
Speaker 9: into a black hole. She'll get spaghettified and smushed, and

498
00:26:54,400 --> 00:26:56,840
Speaker 9: that's not what you want. What you want is for

499
00:26:56,880 --> 00:26:59,840
Speaker 9: her to fall into a wormhole and that will get

500
00:26:59,840 --> 00:27:02,280
Speaker 9: her across the galaxy in a very short period of time.

501
00:27:02,400 --> 00:27:05,119
Speaker 9: And so that's what happens in the novel and in

502
00:27:05,160 --> 00:27:07,960
Speaker 9: the movie. But then Kip was thinking about it, and

503
00:27:08,000 --> 00:27:09,880
Speaker 9: you know, he thought about it, and he said, look,

504
00:27:09,880 --> 00:27:14,160
Speaker 9: you know, usually we say it's bad if you can

505
00:27:14,200 --> 00:27:17,920
Speaker 9: travel faster than the speed of light, because just change

506
00:27:17,920 --> 00:27:20,480
Speaker 9: your reference frame and now it looks like you're traveling

507
00:27:20,520 --> 00:27:23,840
Speaker 9: backward in time. That's one of the reasons why conventional

508
00:27:23,840 --> 00:27:25,920
Speaker 9: physics we say you probably can't travel faster than the

509
00:27:25,920 --> 00:27:28,880
Speaker 9: speed of light. It enables time travel. And he thought

510
00:27:28,920 --> 00:27:30,880
Speaker 9: about it and his students and post talks and they

511
00:27:31,000 --> 00:27:33,600
Speaker 9: chatted about it, and they realized that's because you can

512
00:27:33,880 --> 00:27:36,280
Speaker 9: travel backward in time if you have a wormhole. And

513
00:27:36,600 --> 00:27:39,239
Speaker 9: he wrote a series of papers with his collaborators on

514
00:27:39,359 --> 00:27:43,760
Speaker 9: building time machines using wormholes. So it's a perfect example

515
00:27:44,040 --> 00:27:46,640
Speaker 9: of how locally everything looks local in a small north

516
00:27:46,640 --> 00:27:50,000
Speaker 9: region of space time everything looks local, but globally in

517
00:27:50,000 --> 00:27:53,560
Speaker 9: the presence of that wormhole, your conventional notions of locality

518
00:27:53,600 --> 00:27:53,920
Speaker 9: or all.

519
00:27:53,880 --> 00:27:55,920
Speaker 2: That stuff, Joe relativity is mine.

520
00:27:55,920 --> 00:27:56,240
Speaker 1: Benning.

521
00:27:57,119 --> 00:27:59,199
Speaker 3: Well, I hope we are acting on you at a

522
00:27:59,240 --> 00:28:01,520
Speaker 3: distance in your head, having fun at whatever location you

523
00:28:01,560 --> 00:28:03,880
Speaker 3: find yourself in, and when we get back we will

524
00:28:03,880 --> 00:28:23,800
Speaker 3: dig more into locality.

525
00:28:25,200 --> 00:28:25,520
Speaker 1: All right.

526
00:28:25,720 --> 00:28:28,359
Speaker 2: We are talking to Sean Carroll by the concepts of

527
00:28:28,440 --> 00:28:31,520
Speaker 2: locality and causality, and I think it's time to dig

528
00:28:31,600 --> 00:28:34,719
Speaker 2: into the thing we've been dancing around, which is locality

529
00:28:34,840 --> 00:28:37,800
Speaker 2: in quantum mechanics. So we've been talking about how you

530
00:28:37,840 --> 00:28:40,880
Speaker 2: have to be the same location to have interactions. But

531
00:28:41,000 --> 00:28:45,200
Speaker 2: now we have this concept in quantum mechanics of extended states.

532
00:28:45,320 --> 00:28:48,000
Speaker 2: You can have particles that interact and that are entangled

533
00:28:48,040 --> 00:28:50,680
Speaker 2: with each other, so their fates are connected, and yet

534
00:28:50,680 --> 00:28:53,600
Speaker 2: they're moving apart from each other. And we know from

535
00:28:53,600 --> 00:28:56,640
Speaker 2: Bell's experiments that if you interact with one, it can

536
00:28:56,680 --> 00:28:59,920
Speaker 2: collapse the wave function of the other. How do we

537
00:29:00,080 --> 00:29:03,520
Speaker 2: need gel locality and quantum mechanics or does quantum mechanics

538
00:29:03,600 --> 00:29:06,360
Speaker 2: require us to give up on locality? Is the universe

539
00:29:06,400 --> 00:29:07,959
Speaker 2: fundamentally non local?

540
00:29:08,080 --> 00:29:09,680
Speaker 9: So the answer you're going to get to this question

541
00:29:09,760 --> 00:29:12,160
Speaker 9: is the universe fundamentally non local because the quantum mechanics

542
00:29:12,200 --> 00:29:14,680
Speaker 9: will be completely different. If you ask a philosopher and

543
00:29:14,680 --> 00:29:18,280
Speaker 9: a physicist and they don't even they don't even disagree

544
00:29:18,320 --> 00:29:21,480
Speaker 9: with each other, they're just caring about different things.

545
00:29:21,640 --> 00:29:22,000
Speaker 4: Okay.

546
00:29:22,800 --> 00:29:27,680
Speaker 9: So, and this is the fundamental weirdness of quantum mechanics

547
00:29:27,720 --> 00:29:30,520
Speaker 9: is that when we tell you the rules for quantum mechanics,

548
00:29:31,000 --> 00:29:32,440
Speaker 9: it's kind of cludgy and awkward.

549
00:29:32,480 --> 00:29:32,640
Speaker 4: Right.

550
00:29:32,680 --> 00:29:35,920
Speaker 9: In classical mechanics, we say, here's a thing like a

551
00:29:35,960 --> 00:29:39,240
Speaker 9: planet or an electric field or something like that, and

552
00:29:39,320 --> 00:29:42,360
Speaker 9: here are the equations that govern how that thing behaves

553
00:29:42,480 --> 00:29:45,400
Speaker 9: at equals M for Newton's laws or Maxwell's equations or whatever.

554
00:29:45,480 --> 00:29:45,720
Speaker 4: That's it.

555
00:29:45,800 --> 00:29:49,000
Speaker 9: That's your theory, that's all you need. In quantum mechanics,

556
00:29:49,000 --> 00:29:51,200
Speaker 9: we say, here's a thing. We call the thing the

557
00:29:51,240 --> 00:29:54,320
Speaker 9: wave function of an electron or of a field or whatever.

558
00:29:54,360 --> 00:29:57,280
Speaker 9: Wave functions are the things. Here's an equation. Just like

559
00:29:57,320 --> 00:30:00,560
Speaker 9: classical mechanics, the equation of this case is the Shroding equation,

560
00:30:01,120 --> 00:30:03,720
Speaker 9: but you can rewrite it in different forms finement path

561
00:30:03,720 --> 00:30:06,520
Speaker 9: integrales or another way of doing it, et cetera. But

562
00:30:06,600 --> 00:30:09,320
Speaker 9: then we don't stop there. Right, Like classically we would

563
00:30:09,320 --> 00:30:11,800
Speaker 9: have stopped. In quantum mechanics, we say, oh, but there's

564
00:30:11,840 --> 00:30:15,080
Speaker 9: more rules. And the rules have to do with what

565
00:30:15,200 --> 00:30:19,440
Speaker 9: happens when you measure or observe the system. Right, you

566
00:30:19,480 --> 00:30:23,360
Speaker 9: can measure certain quantities and you can't predict the outcome

567
00:30:23,360 --> 00:30:25,640
Speaker 9: you're going to get. You can predict the probability of

568
00:30:25,680 --> 00:30:28,560
Speaker 9: getting certain outcomes, and there are rules for how that happens.

569
00:30:28,560 --> 00:30:31,520
Speaker 9: And when you make the measurement. The state the wave

570
00:30:31,520 --> 00:30:34,840
Speaker 9: function changes dramatically in something we call the collapse of

571
00:30:34,880 --> 00:30:38,680
Speaker 9: the wave function. So there's basically two sets of rules

572
00:30:38,680 --> 00:30:42,120
Speaker 9: that apply inner different circumstances. The set of rules, which,

573
00:30:42,160 --> 00:30:44,400
Speaker 9: if you want to be technical and impress your friends

574
00:30:44,400 --> 00:30:49,080
Speaker 9: at parties, are called unitary evolution. That's the part where

575
00:30:49,080 --> 00:30:51,680
Speaker 9: you're just obeying the Shortener equation and not being observed.

576
00:30:52,040 --> 00:30:55,360
Speaker 9: And then there's the measurement process, where you actually measure something.

577
00:30:55,960 --> 00:31:00,600
Speaker 9: And so the measurement process in quantum mechanics, well, let

578
00:31:00,600 --> 00:31:04,280
Speaker 9: me get it exactly right. The state of things in

579
00:31:04,360 --> 00:31:08,840
Speaker 9: quantum mechanics, the wave function is just manifestly nonlocal. It

580
00:31:08,960 --> 00:31:11,240
Speaker 9: just isn't local. It's not a thing with a value

581
00:31:11,280 --> 00:31:13,400
Speaker 9: at every point in space and time. That's not what

582
00:31:13,440 --> 00:31:15,239
Speaker 9: it is, and everyone knows that's not what it is.

583
00:31:15,920 --> 00:31:19,760
Speaker 9: But what is the importance of that. Well, on the

584
00:31:19,880 --> 00:31:25,040
Speaker 9: unitary evolution side, the laws of physics, the Shortener equation

585
00:31:25,200 --> 00:31:29,040
Speaker 9: or what have you still look perfectly local as far

586
00:31:29,080 --> 00:31:32,000
Speaker 9: as we can tell. Indeed, the whole discipline of quantum

587
00:31:32,040 --> 00:31:36,200
Speaker 9: field theory is based on the idea that the thing

588
00:31:36,280 --> 00:31:40,160
Speaker 9: that you're quantizing to make your theory are fields. That

589
00:31:40,240 --> 00:31:43,320
Speaker 9: have values in space and time and only interact with

590
00:31:43,400 --> 00:31:46,000
Speaker 9: each other at the points in space and time where

591
00:31:46,040 --> 00:31:50,840
Speaker 9: they overlap with each other. So the particle physicists, bless

592
00:31:50,880 --> 00:31:53,920
Speaker 9: their hearts, Daniel, that's what they care about. They care

593
00:31:53,960 --> 00:31:56,440
Speaker 9: about that unitary evolution part and they know that at

594
00:31:56,440 --> 00:31:57,640
Speaker 9: the end of the day they're going to measure and

595
00:31:57,640 --> 00:31:59,520
Speaker 9: the wave functions going to collapse. But who cares. They

596
00:31:59,520 --> 00:32:01,680
Speaker 9: have a lot of work to do with Fieman diagrams

597
00:32:01,720 --> 00:32:04,720
Speaker 9: and things like that, just calculating the unitary.

598
00:32:04,360 --> 00:32:06,800
Speaker 2: Evolution, and we spend a lot of money getting those

599
00:32:06,840 --> 00:32:08,960
Speaker 2: particles to the same location in space so that they

600
00:32:09,000 --> 00:32:10,000
Speaker 2: do interact with each other.

601
00:32:10,080 --> 00:32:12,400
Speaker 9: There's a picture right behind you on your zoom background

602
00:32:12,400 --> 00:32:15,200
Speaker 9: of exactly that happening. Yes, getting them to the same

603
00:32:15,200 --> 00:32:17,920
Speaker 9: point really matters. So the physicists are like, locality is

604
00:32:17,920 --> 00:32:19,760
Speaker 9: crucially important. What are you talking about? It's like the

605
00:32:19,760 --> 00:32:22,280
Speaker 9: most important thing. It's what makes quantum field theory go.

606
00:32:22,920 --> 00:32:25,120
Speaker 9: And the philosophers will say, like, who cares about all

607
00:32:25,160 --> 00:32:28,760
Speaker 9: your integrals and your Fineman diagrams and whatever. I care

608
00:32:28,800 --> 00:32:33,600
Speaker 9: about what the measurement process is about and what the

609
00:32:33,640 --> 00:32:36,000
Speaker 9: implications of that are, because that's the part of quantum

610
00:32:36,040 --> 00:32:39,200
Speaker 9: mechanics that is not very well understood and needs deeper explication.

611
00:32:39,360 --> 00:32:43,280
Speaker 9: And guess what, it's wildly non local. That is the

612
00:32:43,920 --> 00:32:49,000
Speaker 9: implication of the EPR thought experiment from Einstein, Podolski, and Rosen,

613
00:32:49,040 --> 00:32:52,080
Speaker 9: who say that particles can be entangled and the measurement

614
00:32:52,120 --> 00:32:56,400
Speaker 9: outcome you get on one particle can instantaneously affect the

615
00:32:56,440 --> 00:32:59,480
Speaker 9: allowed measurement outcomes on another particle. And then John Bell

616
00:32:59,560 --> 00:33:02,719
Speaker 9: comes along and makes that very rigorous. And again, if

617
00:33:02,720 --> 00:33:04,400
Speaker 9: I have a couple of minutes to tell an amusing

618
00:33:04,440 --> 00:33:08,760
Speaker 9: story here, it was David Boehm, who was a young

619
00:33:08,800 --> 00:33:13,160
Speaker 9: assistant professor at Princeton the nineteen fifties, who wrote a

620
00:33:13,200 --> 00:33:16,640
Speaker 9: textbook on quantum mechanics, and he says in the textbook

621
00:33:16,640 --> 00:33:19,600
Speaker 9: he quotes a theorem on John von Neumann, famous physicist

622
00:33:19,640 --> 00:33:24,240
Speaker 9: who says you can't reproduce the predictions of quantum mechanics

623
00:33:24,360 --> 00:33:27,040
Speaker 9: used in what are called hidden variables, saying that there's

624
00:33:27,040 --> 00:33:29,720
Speaker 9: a wave function but also extra variables that are being

625
00:33:29,720 --> 00:33:33,720
Speaker 9: pushed around. And Einstein reads Boehm's book and calls Boem

626
00:33:33,760 --> 00:33:36,360
Speaker 9: into his office right because he's also at the Instruhravan

627
00:33:36,400 --> 00:33:38,680
Speaker 9: study right there in Princeton and says this is wrong.

628
00:33:38,920 --> 00:33:42,320
Speaker 9: Like I speak German. Von Neuman's book was only in German,

629
00:33:42,400 --> 00:33:44,200
Speaker 9: and so like then, the Americans knew what was in

630
00:33:44,240 --> 00:33:46,120
Speaker 9: his book. They just quoted it because they thought he

631
00:33:46,200 --> 00:33:49,360
Speaker 9: was a genius. And Einstein said this, he made a mistake.

632
00:33:49,440 --> 00:33:52,320
Speaker 9: He doesn't cover a lot of the important possibilities that

633
00:33:52,360 --> 00:33:55,320
Speaker 9: you might want to consider here. So you have not shown,

634
00:33:55,600 --> 00:33:57,320
Speaker 9: or you're not even given a good argument that you

635
00:33:57,360 --> 00:34:00,920
Speaker 9: can't have hidden variable theories. So he was inspired by

636
00:34:00,920 --> 00:34:03,640
Speaker 9: this and he goes off and invents a hidden variable theory.

637
00:34:04,360 --> 00:34:06,600
Speaker 9: The only way to make it work, though, is to

638
00:34:06,600 --> 00:34:10,200
Speaker 9: make it non local, so the dynamics of that theory

639
00:34:10,280 --> 00:34:14,000
Speaker 9: are explicitly non local. And then everyone ignores him, because

640
00:34:14,000 --> 00:34:16,480
Speaker 9: everyone ignores the foundations of quantum mechanics. By this point

641
00:34:16,520 --> 00:34:18,799
Speaker 9: in time, it's the fifties, and then in the sixties

642
00:34:18,840 --> 00:34:23,720
Speaker 9: it's discovered by John Bell, who reads Boehm's papers, notices

643
00:34:23,760 --> 00:34:27,239
Speaker 9: that it's non local and wonders to himself, like, is

644
00:34:27,280 --> 00:34:30,880
Speaker 9: there a way of doing this hidden variable thing without

645
00:34:30,960 --> 00:34:34,000
Speaker 9: being non local? And he basically proves that the answer

646
00:34:34,080 --> 00:34:37,920
Speaker 9: is no. You cannot reproduce the predictions of quantum mechanics

647
00:34:37,960 --> 00:34:42,360
Speaker 9: without being non local, and he hid that from his friends.

648
00:34:42,360 --> 00:34:44,160
Speaker 9: He was working at CERN at the time. He didn't

649
00:34:44,200 --> 00:34:47,600
Speaker 9: tell anybody because it was considered disreputable. But a couple

650
00:34:47,640 --> 00:34:49,880
Speaker 9: of years ago people won the Nobel Prize for testing

651
00:34:49,880 --> 00:34:51,600
Speaker 9: his predictions and showing that they're correct.

652
00:34:51,880 --> 00:34:55,759
Speaker 3: So it's non local. But we discussed earlier that these

653
00:34:55,800 --> 00:34:58,239
Speaker 3: interactions can't happen faster than the speed of light, which

654
00:34:58,320 --> 00:35:00,719
Speaker 3: Daniel has also we've talked about on the show right,

655
00:35:00,760 --> 00:35:03,279
Speaker 3: that they can't communicate or whatever it is that they're

656
00:35:03,320 --> 00:35:05,880
Speaker 3: doing faster than the speed of light. So what is

657
00:35:05,920 --> 00:35:08,840
Speaker 3: happening then if it's not local and it's not traveling

658
00:35:08,840 --> 00:35:09,719
Speaker 3: faster than the speed of.

659
00:35:09,760 --> 00:35:11,880
Speaker 9: Light, Daniel, do you know what's happening? Does anyone know

660
00:35:11,920 --> 00:35:14,319
Speaker 9: what's happening? Like, if you know what's happening, you win,

661
00:35:14,560 --> 00:35:17,160
Speaker 9: You save quantum mechanics. Quantachanics has been around for one

662
00:35:17,239 --> 00:35:21,880
Speaker 9: hundred years. Just a couple of weeks ago, Nature the

663
00:35:21,960 --> 00:35:24,759
Speaker 9: Journal came out with a poll that they did of

664
00:35:24,800 --> 00:35:28,160
Speaker 9: working physicists who care about quantum mechanics, showing there's no

665
00:35:28,200 --> 00:35:31,960
Speaker 9: consensus whatsoever about what's going on at the deep levels. So, Kelly,

666
00:35:32,000 --> 00:35:34,640
Speaker 9: you're just you're right, Like, I mean, you didn't. You're

667
00:35:34,680 --> 00:35:37,600
Speaker 9: not right because you asked a question. But you're you know,

668
00:35:37,840 --> 00:35:41,719
Speaker 9: charmingly adorable about thinking that we have the answer to

669
00:35:41,760 --> 00:35:44,880
Speaker 9: that question, because this is exactly what we don't have

670
00:35:45,000 --> 00:35:48,600
Speaker 9: the answer to. And and most working physicists, by the way,

671
00:35:49,600 --> 00:35:53,040
Speaker 9: just you know, use the strategy called denial to deal

672
00:35:53,080 --> 00:35:56,160
Speaker 9: with this, like they just say, just it works, okay, Like,

673
00:35:56,239 --> 00:35:58,759
Speaker 9: don't bug me about this. I can make the predictions,

674
00:35:58,800 --> 00:36:02,279
Speaker 9: the predictions come true. But above Einstein, that's why he

675
00:36:02,320 --> 00:36:04,920
Speaker 9: wrote the CPR paper back in nineteen thirty five. You know,

676
00:36:04,960 --> 00:36:08,560
Speaker 9: he basically it's very unfair to Einstein when I'm about

677
00:36:08,560 --> 00:36:10,760
Speaker 9: to say because he had a much more sophisticated argument.

678
00:36:10,800 --> 00:36:12,640
Speaker 9: But basically he points this out. He says, I can

679
00:36:12,760 --> 00:36:16,520
Speaker 9: get two particles. They're entangled. They're very very far away.

680
00:36:16,680 --> 00:36:21,080
Speaker 9: I observe one, and apparently the formalism is telling me

681
00:36:21,239 --> 00:36:24,200
Speaker 9: that instantly changes the state of the other particle very

682
00:36:24,200 --> 00:36:27,960
Speaker 9: far away. I'm Einstein, I know that's not possible. You

683
00:36:28,000 --> 00:36:31,520
Speaker 9: can't change things instantaneously very far away. What's up with that?

684
00:36:32,640 --> 00:36:35,239
Speaker 9: And we still know what's up with that?

685
00:36:35,600 --> 00:36:38,960
Speaker 2: And I read that bell, you know, he interpreted his

686
00:36:39,080 --> 00:36:42,239
Speaker 2: thought experiments in the later actual experiments to mean, as

687
00:36:42,280 --> 00:36:45,480
Speaker 2: you say that there is no local hidden variable, right,

688
00:36:45,719 --> 00:36:49,880
Speaker 2: the particles are not carrying with them some details created

689
00:36:50,000 --> 00:36:52,960
Speaker 2: at the moment of their entanglement which actually determined the outcome,

690
00:36:53,520 --> 00:36:55,319
Speaker 2: But that there is this loophole that you could have

691
00:36:55,440 --> 00:36:59,520
Speaker 2: global hidden variables. So I think it's widely misunderstood that

692
00:36:59,520 --> 00:37:01,920
Speaker 2: Bell's mus tell you there's no hidden variables. It just

693
00:37:01,960 --> 00:37:04,359
Speaker 2: tells you there's no local hidden variables. And as you say,

694
00:37:04,680 --> 00:37:07,920
Speaker 2: a global theory is possible. But tell me about the

695
00:37:08,000 --> 00:37:10,920
Speaker 2: different interpretations of quantum mechanics. I mean, Kelly asks like

696
00:37:11,080 --> 00:37:14,040
Speaker 2: the question what is real? What is happening? And now

697
00:37:14,120 --> 00:37:16,840
Speaker 2: essentially we feel like there is no local realism. But

698
00:37:17,239 --> 00:37:20,520
Speaker 2: do the different interpretations of quantum mechanics tell different stories

699
00:37:20,520 --> 00:37:22,799
Speaker 2: about how to accommodate this? And I know we have

700
00:37:22,880 --> 00:37:25,680
Speaker 2: like Boemian mechanics where you have a pilot wave which

701
00:37:25,719 --> 00:37:28,680
Speaker 2: is explicitly non locally. You have a global theory, as

702
00:37:28,719 --> 00:37:32,400
Speaker 2: you say, But with the Copenhagen interpretation, is that a

703
00:37:32,520 --> 00:37:36,480
Speaker 2: non local theory or does Copenhagen essentially shrug away this

704
00:37:36,560 --> 00:37:38,560
Speaker 2: question the way it does most of the important.

705
00:37:38,160 --> 00:37:40,520
Speaker 9: Issues yeah, I mean, look, it really is fascinating. I

706
00:37:40,520 --> 00:37:43,760
Speaker 9: do encourage anyone with a little bit of quantum mechanics

707
00:37:43,800 --> 00:37:49,320
Speaker 9: knowledge to actually read the original EPR paper Einstein, Podolski,

708
00:37:49,360 --> 00:37:52,520
Speaker 9: and Rosen, because you know, Daniel can back me up

709
00:37:52,520 --> 00:37:54,840
Speaker 9: on this. But we physicist don't read the original papers

710
00:37:54,880 --> 00:37:56,680
Speaker 9: like that's you know, we have a textbook and that

711
00:37:56,719 --> 00:37:59,200
Speaker 9: tells us what's going on. But the original papers are

712
00:37:59,239 --> 00:38:02,319
Speaker 9: fascinating because they don't know the answer, right, They're struggling

713
00:38:02,400 --> 00:38:06,160
Speaker 9: with figuring out what's going on. So Einstein and Podolski

714
00:38:06,200 --> 00:38:08,920
Speaker 9: and Rosen, but I get the impression that the ideas

715
00:38:08,960 --> 00:38:12,920
Speaker 9: were mostly from Einstein. They didn't just say, look, there

716
00:38:13,000 --> 00:38:15,520
Speaker 9: is this spooky action at a distance that bugs us.

717
00:38:15,920 --> 00:38:19,120
Speaker 9: They were much more careful than that. They tried their

718
00:38:19,200 --> 00:38:22,600
Speaker 9: best to construct an argument that says there should be

719
00:38:22,680 --> 00:38:27,080
Speaker 9: what they called local elements of reality. And I think

720
00:38:27,080 --> 00:38:29,600
Speaker 9: that it was Bell who later called these things be

721
00:38:29,800 --> 00:38:32,799
Speaker 9: a bles be ables in the sense of things that be,

722
00:38:33,200 --> 00:38:34,800
Speaker 9: things that are things that exist.

723
00:38:34,960 --> 00:38:38,120
Speaker 2: Right, philosophers, and they're creating phrases for.

724
00:38:38,320 --> 00:38:42,400
Speaker 9: John Bell is a car carrying physicist. I gotta say.

725
00:38:42,560 --> 00:38:45,240
Speaker 1: He's doing philosophy. Though when he invents a new meaning

726
00:38:45,239 --> 00:38:45,799
Speaker 1: for the word b.

727
00:38:46,040 --> 00:38:50,000
Speaker 9: He's doing philosophy, so he invents this word beable, and

728
00:38:50,520 --> 00:38:53,640
Speaker 9: it's exactly what Einstein wanted to be the case. Einstein

729
00:38:53,719 --> 00:38:56,439
Speaker 9: wanted it to be the fact that at every point

730
00:38:56,480 --> 00:38:59,320
Speaker 9: in space time there's a fact of the matter about

731
00:38:59,400 --> 00:39:02,520
Speaker 9: what is physical going on in the universe. And as

732
00:39:02,520 --> 00:39:07,799
Speaker 9: we said, quantum mechanics doesn't say that. Bell wanted, you know,

733
00:39:07,960 --> 00:39:12,960
Speaker 9: to really understand this locality issue. And so the way

734
00:39:13,000 --> 00:39:17,719
Speaker 9: that Boem solves the problem is there are local beables,

735
00:39:17,760 --> 00:39:20,920
Speaker 9: like in Boehm's theory, there are particles. When you see

736
00:39:21,000 --> 00:39:24,120
Speaker 9: at the LHC the track of a particle, what Boem

737
00:39:24,120 --> 00:39:26,440
Speaker 9: would say is you're not seeing the wave function. You're

738
00:39:26,480 --> 00:39:29,320
Speaker 9: seeing the particle. The particles there in addition to the

739
00:39:29,360 --> 00:39:33,279
Speaker 9: wave function. But the equation that the particle follows is

740
00:39:33,360 --> 00:39:35,640
Speaker 9: non local. It depends on what all the other particles

741
00:39:35,680 --> 00:39:38,200
Speaker 9: everywhere else are doing, which is which is really weird.

742
00:39:39,000 --> 00:39:42,000
Speaker 9: And so Bell asked this question, can you come up

743
00:39:42,040 --> 00:39:45,640
Speaker 9: with any theory where everything is one hundred percent local

744
00:39:45,800 --> 00:39:47,799
Speaker 9: and none of the dynamics are local, none of the

745
00:39:47,840 --> 00:39:50,960
Speaker 9: things are non local or non local issues. I hope

746
00:39:50,960 --> 00:39:54,200
Speaker 9: I said that correctly both times and he proves the

747
00:39:54,200 --> 00:39:57,360
Speaker 9: answer is no. So the different strategies for solving this

748
00:39:57,600 --> 00:40:01,520
Speaker 9: just take very different points of view. In Bomi mechanics,

749
00:40:01,600 --> 00:40:05,680
Speaker 9: despite the bullet there's a non local evolution rule. There

750
00:40:05,719 --> 00:40:10,320
Speaker 9: are what are called objective collapse models of quantum mechanics,

751
00:40:10,360 --> 00:40:13,600
Speaker 9: where the wave function just suddenly changes all over space,

752
00:40:13,640 --> 00:40:17,120
Speaker 9: all at once. That's very non local in its own

753
00:40:18,520 --> 00:40:21,680
Speaker 9: in everready, in quantum mechanics, in the many worlds theory,

754
00:40:22,160 --> 00:40:26,600
Speaker 9: you kind of sidestep the question. One of the axioms,

755
00:40:26,600 --> 00:40:30,200
Speaker 9: one of the assumptions of premises of Bell's theorem is

756
00:40:30,239 --> 00:40:33,839
Speaker 9: that measurements have definite outcomes. When you measure the spin

757
00:40:33,880 --> 00:40:35,600
Speaker 9: of a particle, it will either be spin up or

758
00:40:35,640 --> 00:40:38,759
Speaker 9: spin down, And whatever it says is well, it's spinned

759
00:40:38,800 --> 00:40:41,120
Speaker 9: up in one universe and spin down in another universe.

760
00:40:41,160 --> 00:40:43,960
Speaker 9: So that's not quite what Bell had in mind. So

761
00:40:44,120 --> 00:40:47,480
Speaker 9: people have huge arguments over whether or not many worlds

762
00:40:47,560 --> 00:40:49,880
Speaker 9: is local or not. The answer, of courses depends on

763
00:40:49,920 --> 00:40:54,440
Speaker 9: your definitions, but that is one of the reasons to

764
00:40:54,600 --> 00:40:57,880
Speaker 9: preserve that kind of dynamical locality that you might like

765
00:40:58,000 --> 00:41:02,200
Speaker 9: many worlds Copenhagen. I'm just I'm going to like boycott

766
00:41:02,239 --> 00:41:05,120
Speaker 9: any question about the Copenhagen interpretation from now on, because

767
00:41:05,120 --> 00:41:08,239
Speaker 9: it's giving it too much credit. It's not well defined,

768
00:41:08,400 --> 00:41:14,480
Speaker 9: it's not a theory like many worlds. Bomian mechanics, spontaneous

769
00:41:14,520 --> 00:41:17,759
Speaker 9: collapse models, these are theories. They have equations and they

770
00:41:17,760 --> 00:41:21,640
Speaker 9: make predictions. Copenhagen just won't answer certain questions, which I

771
00:41:21,640 --> 00:41:24,560
Speaker 9: don't think we should reward it by taking it seriously.

772
00:41:24,680 --> 00:41:26,759
Speaker 3: Can we give a little more information about what the

773
00:41:26,760 --> 00:41:29,640
Speaker 3: Copenhagen stuff means for the biologists in the room.

774
00:41:29,840 --> 00:41:34,640
Speaker 9: Yes, yes, Copenhagen is a city in Denmark and I

775
00:41:34,719 --> 00:41:37,480
Speaker 9: got that sean full of people doing quantum mechanics and

776
00:41:37,600 --> 00:41:40,600
Speaker 9: it was a great time. You know, is again fascinating

777
00:41:40,600 --> 00:41:43,520
Speaker 9: to read the original papers. We're here in twenty twenty five.

778
00:41:43,600 --> 00:41:47,320
Speaker 9: It's the it's been dubbed the Year of Quantum because

779
00:41:47,320 --> 00:41:50,080
Speaker 9: the International Year of Quantum, because exactly one hundred years

780
00:41:50,120 --> 00:41:53,400
Speaker 9: ago the first papers came out by Heisenberg and Schrodinger

781
00:41:53,400 --> 00:41:57,120
Speaker 9: et cetera setting up quantum mechanics, and we still understand it.

782
00:41:57,200 --> 00:42:00,360
Speaker 9: But into like the ten years after nineteen ten twenty

783
00:42:00,360 --> 00:42:05,640
Speaker 9: five Nils Borr and Werner, Heisenberg and Wolfgang Powley, who

784
00:42:05,800 --> 00:42:07,960
Speaker 9: all were sort of either affiliated with or spent a

785
00:42:08,000 --> 00:42:13,120
Speaker 9: lot of time at Bor's Institute in Copenhagen, promulgated this

786
00:42:13,239 --> 00:42:16,319
Speaker 9: way of thinking about quantum mechanics. And it's exactly what

787
00:42:16,360 --> 00:42:18,239
Speaker 9: I already said. It said that you have a wave

788
00:42:18,280 --> 00:42:20,840
Speaker 9: function and it solves the Schortener equation when you're not

789
00:42:20,880 --> 00:42:25,320
Speaker 9: looking at it, and then when you do look at it,

790
00:42:25,320 --> 00:42:28,960
Speaker 9: it collapses and you get a probability. But the philosophical

791
00:42:29,080 --> 00:42:33,439
Speaker 9: side of the Copenhagen interpretation is the claim that there's

792
00:42:33,560 --> 00:42:37,799
Speaker 9: no such thing as what is happening when you are

793
00:42:37,880 --> 00:42:42,560
Speaker 9: not looking at the quantum system. And this is very

794
00:42:42,600 --> 00:42:46,040
Speaker 9: explicit in Heisenberg's papers from nineteen twenty five, and it's

795
00:42:46,040 --> 00:42:48,120
Speaker 9: one of the reasons why it's very hard to understand

796
00:42:48,120 --> 00:42:50,160
Speaker 9: these papers. Stephen Weinberg, who is one of the most

797
00:42:50,160 --> 00:42:53,879
Speaker 9: brilliant physicists of the twentieth century, said like he tried

798
00:42:53,960 --> 00:42:56,600
Speaker 9: very hard to read Heisenberg's papers and he has no

799
00:42:56,640 --> 00:42:59,799
Speaker 9: idea what was going on there. But the big philosophical

800
00:42:59,840 --> 00:43:04,000
Speaker 9: move was stop asking about where the electron is. There

801
00:43:04,120 --> 00:43:06,560
Speaker 9: is no such thing as where the electron is. There

802
00:43:06,600 --> 00:43:09,759
Speaker 9: is only where you will see it when you measure it.

803
00:43:10,239 --> 00:43:13,920
Speaker 9: And that's really the fundamental ethos of the Copenhagen interpretation.

804
00:43:14,360 --> 00:43:19,520
Speaker 9: And by that, by itself, that's fine, but it leaves

805
00:43:19,560 --> 00:43:22,640
Speaker 9: a whole bunch of questions unanswered. That's the bad part.

806
00:43:22,760 --> 00:43:26,319
Speaker 9: Number one, What is a measurement? What counts as doing

807
00:43:26,320 --> 00:43:29,000
Speaker 9: a measurement? Can a video camera do a measurement? Do

808
00:43:29,000 --> 00:43:31,080
Speaker 9: you have to be a conscious observer to do a measurement?

809
00:43:31,160 --> 00:43:33,040
Speaker 9: What if you don't have good eyesight, does that count

810
00:43:33,040 --> 00:43:39,120
Speaker 9: as a measurement? Number two, The Copenhagen interpretation says, the

811
00:43:39,280 --> 00:43:43,879
Speaker 9: classical world exists and is real. You and I are

812
00:43:43,920 --> 00:43:46,640
Speaker 9: not quantum things. You know, Daniel made the joke earlier

813
00:43:46,640 --> 00:43:50,120
Speaker 9: about Isaac Newton being made of quantum mechanical things. Werner

814
00:43:50,160 --> 00:43:53,000
Speaker 9: Heisenberg didn't think that. He thought that Isaac Newton was classical,

815
00:43:54,040 --> 00:43:55,440
Speaker 9: and that the things that you look at in a

816
00:43:55,480 --> 00:43:58,480
Speaker 9: microscope or quantum And there's literally an idea called the

817
00:43:58,520 --> 00:44:02,239
Speaker 9: Heisenberg cut. And this is somehow in the space of

818
00:44:02,280 --> 00:44:04,719
Speaker 9: all things happening in the world, there's one side of

819
00:44:04,719 --> 00:44:06,480
Speaker 9: the cut where there's the quantum stuff, and the other

820
00:44:06,520 --> 00:44:09,080
Speaker 9: side of the cut where it's the classical stuff. And

821
00:44:09,160 --> 00:44:11,319
Speaker 9: like who invented that where does that go? Like, no

822
00:44:11,320 --> 00:44:13,120
Speaker 9: one knows what is going on with any of this,

823
00:44:13,360 --> 00:44:16,719
Speaker 9: and so the Copenhagen interpretation is kind of just what

824
00:44:16,840 --> 00:44:20,560
Speaker 9: we teach students in our quantum mechanics courses. But it's

825
00:44:21,200 --> 00:44:25,160
Speaker 9: hilariously ill defined and as a as a starting point

826
00:44:25,440 --> 00:44:28,439
Speaker 9: as the kind of conjectural hypothesis that we throw out

827
00:44:28,440 --> 00:44:31,720
Speaker 9: to do physics. It's great, it's amazing, it makes perfect sense.

828
00:44:32,239 --> 00:44:34,680
Speaker 9: The weird thing is we've been pretending for one hundred

829
00:44:34,800 --> 00:44:37,680
Speaker 9: years that it's somehow a satisfactory final answer.

830
00:44:38,680 --> 00:44:42,120
Speaker 2: I think you've been slightly unfair to the Copenhagen interpretation, and.

831
00:44:42,080 --> 00:44:43,880
Speaker 9: I would be much more unfair if you wanted me to,

832
00:44:43,960 --> 00:44:45,200
Speaker 9: I would be much harder.

833
00:44:45,640 --> 00:44:48,680
Speaker 2: And I can't believe I'm in the situation of defending it.

834
00:44:48,800 --> 00:44:51,600
Speaker 2: I mean, I agree that there's a fundamental issue at

835
00:44:51,600 --> 00:44:53,280
Speaker 2: the heart of it, which is that they don't define

836
00:44:53,280 --> 00:44:56,279
Speaker 2: the distinction between you know, what causes a collapse and

837
00:44:56,320 --> 00:44:59,520
Speaker 2: what doesn't, what's a classical object and what's a quantum object. Absolutely,

838
00:45:00,080 --> 00:45:03,040
Speaker 2: and that's a fatal error. But you know, there's these

839
00:45:03,160 --> 00:45:05,640
Speaker 2: other issues of like are you conscious or you know,

840
00:45:06,280 --> 00:45:07,960
Speaker 2: does a human do it? Or an eyeball do it

841
00:45:08,040 --> 00:45:09,839
Speaker 2: or a video camera do it. I think those are

842
00:45:10,000 --> 00:45:11,960
Speaker 2: maybe side issues, but I agree at the core of it,

843
00:45:12,080 --> 00:45:13,920
Speaker 2: Copenhagen is ill defined.

844
00:45:13,960 --> 00:45:15,600
Speaker 3: Things are heating up, things are eating up.

845
00:45:15,640 --> 00:45:17,759
Speaker 9: So I gotta wait, I gotta I gotta bump in here.

846
00:45:17,760 --> 00:45:21,000
Speaker 9: You gotta interrupt, because I just gave a talk at

847
00:45:21,000 --> 00:45:23,239
Speaker 9: the American Association of Physics Teachers where I was talking

848
00:45:23,239 --> 00:45:25,600
Speaker 9: about the foundations of quantum mechanics, and for that talk,

849
00:45:26,120 --> 00:45:28,520
Speaker 9: I made a slide in which I appealed to a

850
00:45:28,600 --> 00:45:31,279
Speaker 9: th arty. So I'm going to quote some people who

851
00:45:31,320 --> 00:45:34,360
Speaker 9: are much smarter than me talking about the Copenhagen interpretation.

852
00:45:34,880 --> 00:45:38,480
Speaker 9: Albert Einstein says the theory is apt to beguile us

853
00:45:38,520 --> 00:45:41,480
Speaker 9: into error in our search for a uniform basis for physics,

854
00:45:41,840 --> 00:45:45,480
Speaker 9: because in my belief, it is an incomplete representation of

855
00:45:45,600 --> 00:45:49,520
Speaker 9: real things. Erwin Schrodinger says, I don't like it, and

856
00:45:49,560 --> 00:45:51,360
Speaker 9: I'm sorry I ever had anything to do with it.

857
00:45:53,000 --> 00:45:57,799
Speaker 9: Hugh Everett says this is a philosophical monstrosity. And then

858
00:45:57,920 --> 00:46:01,200
Speaker 9: Carl Popper, who invented false fae as the demarketse mean

859
00:46:01,239 --> 00:46:04,840
Speaker 9: science and nonscience, says the Copenhagen interpretation is a mistaken

860
00:46:04,960 --> 00:46:06,759
Speaker 9: and even vicious doctrine.

861
00:46:07,400 --> 00:46:09,320
Speaker 1: Okay, So if you think.

862
00:46:09,160 --> 00:46:11,560
Speaker 9: I'm being unfair, like the people who care about this

863
00:46:11,640 --> 00:46:15,040
Speaker 9: a lot, I think are are pretty hardcore that this

864
00:46:15,080 --> 00:46:15,800
Speaker 9: is not acceptable.

865
00:46:15,880 --> 00:46:17,279
Speaker 1: Well, I'm glad you didn't hold back.

866
00:46:18,360 --> 00:46:20,880
Speaker 2: My own personal anecdote about Copenhagen is I got to

867
00:46:20,880 --> 00:46:24,520
Speaker 2: spend a year at the Boer Institute, and when I

868
00:46:24,680 --> 00:46:26,799
Speaker 2: got there, they gave me an office, and I noticed

869
00:46:26,840 --> 00:46:28,840
Speaker 2: that the office next door to mine was quite different

870
00:46:29,120 --> 00:46:31,439
Speaker 2: in that it had a bathtub in it, and I thought,

871
00:46:31,480 --> 00:46:33,000
Speaker 2: why is there a bathtub.

872
00:46:32,520 --> 00:46:33,280
Speaker 1: In this office?

873
00:46:33,520 --> 00:46:35,360
Speaker 9: And then that can't be good, Nothing good can happen.

874
00:46:35,520 --> 00:46:37,279
Speaker 2: I learned the story that in the old days, when

875
00:46:37,320 --> 00:46:39,440
Speaker 2: you had an institute, you lived at the institute the

876
00:46:39,440 --> 00:46:41,319
Speaker 2: way like the president lives at the White House. And

877
00:46:41,360 --> 00:46:44,160
Speaker 2: so there was an apartment. And then later, after Boor

878
00:46:44,320 --> 00:46:46,120
Speaker 2: was no longer there, they're like, well, let's just turn

879
00:46:46,160 --> 00:46:48,680
Speaker 2: these into offices. And so somebody got the office with

880
00:46:48,719 --> 00:46:50,920
Speaker 2: the bathtub in it.

881
00:46:51,960 --> 00:46:52,400
Speaker 4: Why didn't they?

882
00:46:53,600 --> 00:46:55,640
Speaker 2: He had important thoughts in that tub. You know, you

883
00:46:55,680 --> 00:46:56,719
Speaker 2: can't just get rid of it.

884
00:46:57,600 --> 00:46:59,200
Speaker 3: Are you're gonna crawl in when you need to solve

885
00:46:59,200 --> 00:46:59,839
Speaker 3: your next big NB?

886
00:47:01,000 --> 00:47:03,120
Speaker 2: All right, So on that note, let's solve the next

887
00:47:03,120 --> 00:47:05,880
Speaker 2: big problem. I want to talk about quantum gravity and

888
00:47:05,920 --> 00:47:09,359
Speaker 2: the implications, but first let's detour two causality. We've talked

889
00:47:09,360 --> 00:47:12,239
Speaker 2: about locality a lot, but let's take a break, and

890
00:47:12,280 --> 00:47:36,120
Speaker 2: when we come back, we're going to talk about causality. Okay,

891
00:47:36,120 --> 00:47:39,960
Speaker 2: we're back, and we're talking to physicists and philosophers. Sometimes

892
00:47:39,960 --> 00:47:42,480
Speaker 2: when you measure him, he collapses into one state or

893
00:47:42,480 --> 00:47:46,200
Speaker 2: the other. About causality and locality in physics, So we

894
00:47:46,239 --> 00:47:49,560
Speaker 2: started the conversation about locality with the definition, and we've

895
00:47:49,600 --> 00:47:52,840
Speaker 2: already touched on some of the concepts of causality, But

896
00:47:52,880 --> 00:47:55,080
Speaker 2: could you give us a crisp definition for what we

897
00:47:55,160 --> 00:47:56,840
Speaker 2: mean by causality in physics.

898
00:47:57,040 --> 00:48:00,200
Speaker 9: I'm glad you said it in physics, because just like locality,

899
00:48:00,000 --> 00:48:03,759
Speaker 9: the causality is a notion that physicists and philosophers both

900
00:48:03,760 --> 00:48:07,919
Speaker 9: talk about, but they mean entirely different things. So let's

901
00:48:07,960 --> 00:48:11,879
Speaker 9: first just mention like what someone who is neither would mean. Right,

902
00:48:12,000 --> 00:48:15,040
Speaker 9: You know, we talk about causes and effects. If you say, like,

903
00:48:15,160 --> 00:48:18,000
Speaker 9: why were you late for work? Someone might say, well,

904
00:48:18,040 --> 00:48:21,399
Speaker 9: because the traffic was unusually bad, there was an accident, right,

905
00:48:21,600 --> 00:48:24,680
Speaker 9: and that's a meaningful statement. There's a cause there's an accident,

906
00:48:24,880 --> 00:48:27,120
Speaker 9: there is an effect that there was traffic, and that

907
00:48:27,160 --> 00:48:29,200
Speaker 9: there's another cause there was traffic, and there's another fact

908
00:48:29,320 --> 00:48:31,960
Speaker 9: you're late for work, And there's a chain of causes

909
00:48:31,960 --> 00:48:34,960
Speaker 9: and effects that ripples throughout all of our existence. And

910
00:48:35,000 --> 00:48:38,279
Speaker 9: this goes back to you know, Aristotle thinking about this

911
00:48:38,400 --> 00:48:43,200
Speaker 9: kind of thing. So it's very, very common that we

912
00:48:43,239 --> 00:48:45,680
Speaker 9: take that kind of word that we use in everyday

913
00:48:45,760 --> 00:48:49,800
Speaker 9: language and repurpose it for some rigorous idea in physics

914
00:48:49,880 --> 00:48:53,080
Speaker 9: or philosophy, et cetera. So what the physicists have done

915
00:48:53,400 --> 00:48:57,800
Speaker 9: post Einstein is to notice this feature of relativity that

916
00:48:57,840 --> 00:49:01,000
Speaker 9: we've already talked about that when you do something at

917
00:49:01,000 --> 00:49:05,440
Speaker 9: some location, some event in space and time, the implications

918
00:49:05,640 --> 00:49:09,880
Speaker 9: the effects of that doing something can only ripple forward

919
00:49:09,880 --> 00:49:11,879
Speaker 9: in time, and they can only ripple forward in time

920
00:49:12,440 --> 00:49:17,160
Speaker 9: slower than the speed of light. So, really physicists, by

921
00:49:17,200 --> 00:49:21,080
Speaker 9: the word causality, what they usually mean is that signals

922
00:49:21,120 --> 00:49:23,279
Speaker 9: travel slower than the speed of light, or at the

923
00:49:23,320 --> 00:49:26,439
Speaker 9: speed of light, certainly no faster than the speed of light.

924
00:49:26,920 --> 00:49:29,200
Speaker 2: So an alien shoots their death ray at us from

925
00:49:29,200 --> 00:49:33,120
Speaker 2: Andromeda if they can't kill us today or tomorrow or yesterday.

926
00:49:33,280 --> 00:49:35,120
Speaker 2: They can only kill us in a million years.

927
00:49:34,960 --> 00:49:37,720
Speaker 9: Or so unless they shot at a million years ago, right, yes,

928
00:49:38,680 --> 00:49:41,960
Speaker 9: which is not well defined in relativity. So yeah, I'm

929
00:49:41,960 --> 00:49:45,919
Speaker 9: not gonna I'm worried about the aliens. So, but there's

930
00:49:45,920 --> 00:49:48,880
Speaker 9: already a tension there in that physicist's way of thinking,

931
00:49:49,200 --> 00:49:54,400
Speaker 9: because there's a super important feature of the everyday notion

932
00:49:54,480 --> 00:49:57,800
Speaker 9: of causality, which is that the cause always happens before

933
00:49:58,440 --> 00:50:02,160
Speaker 9: the effect. Right, You would not convince anyone by saying

934
00:50:02,640 --> 00:50:05,960
Speaker 9: there was an accident this morning because I was going

935
00:50:05,960 --> 00:50:08,440
Speaker 9: to be late for work, Right, That's just not how

936
00:50:08,520 --> 00:50:10,960
Speaker 9: causes and effects work. You're late for work as there

937
00:50:10,960 --> 00:50:13,640
Speaker 9: was an accident, not the other way around. But the

938
00:50:13,719 --> 00:50:17,640
Speaker 9: fundamental laws of physics, whether it's relativity or quantum mechanics

939
00:50:17,719 --> 00:50:21,840
Speaker 9: or anything, are time reversal invariant. Are invariant going forward

940
00:50:21,840 --> 00:50:25,040
Speaker 9: and backward in time. So really, when the physicists talk

941
00:50:25,080 --> 00:50:29,200
Speaker 9: about causality, they kind of mean signals propagate to the

942
00:50:29,280 --> 00:50:31,840
Speaker 9: future at or slower than the speed of light. But

943
00:50:32,000 --> 00:50:35,000
Speaker 9: also they get to us from the past at or

944
00:50:35,040 --> 00:50:37,400
Speaker 9: slower than the speed of light. Both are equally good

945
00:50:37,800 --> 00:50:43,960
Speaker 9: features of what physicists call causality and philosophers want to know. Okay,

946
00:50:43,960 --> 00:50:47,120
Speaker 9: but if that's true, why in the macroscopic world of

947
00:50:47,120 --> 00:50:49,880
Speaker 9: our everyday existence do we have this strong feeling that

948
00:50:50,000 --> 00:50:53,640
Speaker 9: causes precede effects. And probably that has something to do

949
00:50:53,680 --> 00:50:55,480
Speaker 9: with entropy in the hour of time. And it's a

950
00:50:55,480 --> 00:50:57,319
Speaker 9: whole long story, all right.

951
00:50:57,400 --> 00:51:01,640
Speaker 2: So in physics we have this concept that the past

952
00:51:01,719 --> 00:51:05,360
Speaker 2: determines the future. Essentially, it's a statement on top of

953
00:51:05,400 --> 00:51:07,640
Speaker 2: the existing laws of physics, which, as you say, are

954
00:51:07,960 --> 00:51:11,840
Speaker 2: mostly invariant with respect to time, that there's a directionality

955
00:51:11,880 --> 00:51:15,520
Speaker 2: to time. Is that a compact way to understand causality

956
00:51:15,520 --> 00:51:16,040
Speaker 2: in physics?

957
00:51:16,440 --> 00:51:18,200
Speaker 9: It might be a little bit too compact, So let

958
00:51:18,200 --> 00:51:20,640
Speaker 9: me be let's just expand it out a little bit.

959
00:51:21,600 --> 00:51:24,839
Speaker 9: You know, back in circa eighteen hundred, again our hero

960
00:51:25,000 --> 00:51:29,960
Speaker 9: Piersimone Laplace points out this existing feature of classical mechanics.

961
00:51:30,000 --> 00:51:32,759
Speaker 9: You know, Newton invented classical mechanics more or less in

962
00:51:32,800 --> 00:51:35,799
Speaker 9: its modern form in the sixteen hundreds. It took a

963
00:51:35,800 --> 00:51:40,920
Speaker 9: while for Laplace to realize the implication of Newton's laws,

964
00:51:40,960 --> 00:51:44,239
Speaker 9: which is that information about what is happening in the

965
00:51:44,320 --> 00:51:48,080
Speaker 9: universe is conserved over time. So if you live in

966
00:51:48,120 --> 00:51:51,920
Speaker 9: a purely Newtonian universe, purely classical, if you know what

967
00:51:52,080 --> 00:51:55,600
Speaker 9: happens at any one moment of time, if you know

968
00:51:55,640 --> 00:51:58,600
Speaker 9: that perfectly, you know the position and the velocity of

969
00:51:58,640 --> 00:52:02,880
Speaker 9: every single atom in the universe. Okay, then according to Laplace,

970
00:52:03,160 --> 00:52:06,920
Speaker 9: the laws of physics determine everything that will happen in

971
00:52:06,960 --> 00:52:12,160
Speaker 9: the future one hundred percent reliability. And it also determines

972
00:52:12,200 --> 00:52:14,400
Speaker 9: everything that happens in the past, because there is no

973
00:52:14,600 --> 00:52:18,640
Speaker 9: distinction between past and future in Newtonian mechanics. So that's

974
00:52:18,680 --> 00:52:21,160
Speaker 9: what we mean by information being conserved from moment to

975
00:52:21,200 --> 00:52:25,120
Speaker 9: moment in time. So this is already kind of different

976
00:52:25,560 --> 00:52:29,200
Speaker 9: than our conventional notion of causality, that the knowledge of

977
00:52:29,200 --> 00:52:32,719
Speaker 9: what is happening in the present determines equally well the

978
00:52:32,760 --> 00:52:37,319
Speaker 9: future and the past. And so to recover from that

979
00:52:37,440 --> 00:52:42,920
Speaker 9: description our folk wisdom about causes preceding effects, the fundamental

980
00:52:43,000 --> 00:52:45,400
Speaker 9: laws of physics are not enough. You also need to

981
00:52:45,400 --> 00:52:48,239
Speaker 9: put in some boundary conditions. And usually what we do

982
00:52:48,360 --> 00:52:52,480
Speaker 9: is we say the early universe, the Big Bang fourteen

983
00:52:52,480 --> 00:52:55,680
Speaker 9: billion years ago, had very very special conditions. They were

984
00:52:55,760 --> 00:52:59,239
Speaker 9: low entropy, they were a very very tiny kind of

985
00:52:59,280 --> 00:53:03,600
Speaker 9: configuration in the space of all possibilities. And because we

986
00:53:03,680 --> 00:53:06,680
Speaker 9: know that, because we know the conditions that were there

987
00:53:06,719 --> 00:53:10,719
Speaker 9: in the early universe, we have a better handle on

988
00:53:10,800 --> 00:53:13,040
Speaker 9: what things were like in the past than we do

989
00:53:13,120 --> 00:53:15,319
Speaker 9: in the future. We say the past is fixed, right,

990
00:53:15,360 --> 00:53:17,880
Speaker 9: We informally think that the past is just in the books.

991
00:53:18,239 --> 00:53:21,760
Speaker 9: There's no decision I can make now that will change

992
00:53:21,760 --> 00:53:24,399
Speaker 9: the past. But we think there's a decision I can

993
00:53:24,400 --> 00:53:27,480
Speaker 9: make now that will change the future. How to reconcile

994
00:53:27,520 --> 00:53:31,560
Speaker 9: that with laplace? The answer is, you know, I don't

995
00:53:31,600 --> 00:53:35,360
Speaker 9: know the position and velocity of every atom in the universe.

996
00:53:35,400 --> 00:53:40,120
Speaker 9: I have some incomplete macroscopic information, and that might be

997
00:53:40,280 --> 00:53:42,920
Speaker 9: enough to really fix what happened in the past, like

998
00:53:42,960 --> 00:53:45,440
Speaker 9: I have a photograph or a video record of it.

999
00:53:45,440 --> 00:53:47,640
Speaker 9: It is never enough to completely fix what happens in

1000
00:53:47,680 --> 00:53:48,120
Speaker 9: the future.

1001
00:53:48,960 --> 00:53:53,000
Speaker 2: I think that's a really subtle and underappreciated point, that

1002
00:53:53,040 --> 00:53:56,560
Speaker 2: the present determines the past in the way you say

1003
00:53:56,560 --> 00:53:59,160
Speaker 2: it in some sense. Another way to think about it

1004
00:53:59,200 --> 00:54:01,800
Speaker 2: is that from the pre you can recover the past

1005
00:54:01,920 --> 00:54:06,360
Speaker 2: because there's a unique moment, like the details of the

1006
00:54:06,440 --> 00:54:09,960
Speaker 2: universe now can only have come from a certain history

1007
00:54:10,080 --> 00:54:12,440
Speaker 2: of the past, and so in that sense you can recover.

1008
00:54:12,480 --> 00:54:15,520
Speaker 2: It's not like if I change the present, the past changes,

1009
00:54:16,040 --> 00:54:19,080
Speaker 2: but there's a unique path which I can recover from

1010
00:54:19,120 --> 00:54:21,320
Speaker 2: knowing enough about the present. I don't know if you

1011
00:54:21,400 --> 00:54:23,799
Speaker 2: saw that not great show Devs where they had this

1012
00:54:23,840 --> 00:54:28,120
Speaker 2: whole concept that try to see an image of the Crucifixion,

1013
00:54:28,120 --> 00:54:30,879
Speaker 2: for example, by measuring the motion of air particles over

1014
00:54:30,960 --> 00:54:31,840
Speaker 2: Malaysia or whatever.

1015
00:54:33,120 --> 00:54:36,319
Speaker 9: Well, yes, So one of the reasons why that is

1016
00:54:36,400 --> 00:54:40,000
Speaker 9: a TV show and not reality is because again one

1017
00:54:40,080 --> 00:54:44,720
Speaker 9: cannot emphasize enough that you don't have information about position

1018
00:54:44,800 --> 00:54:47,759
Speaker 9: and velocity of every particle in the universe. If you

1019
00:54:47,800 --> 00:54:51,480
Speaker 9: wanted to just to drive it home, if you wanted

1020
00:54:51,520 --> 00:54:54,319
Speaker 9: to recover something that was happening in an event two

1021
00:54:54,360 --> 00:54:59,400
Speaker 9: thousand years ago, even in principle, you would need to

1022
00:54:59,520 --> 00:55:02,440
Speaker 9: know everything that is going on in the universe now

1023
00:55:02,800 --> 00:55:06,640
Speaker 9: to within a two thousand light year radius, because there

1024
00:55:06,640 --> 00:55:09,439
Speaker 9: were photons emitted from the Earth back two thousand years

1025
00:55:09,480 --> 00:55:11,279
Speaker 9: ago and they've been moving away from you to speed

1026
00:55:11,280 --> 00:55:13,799
Speaker 9: of light. So if you don't have those photons, you

1027
00:55:13,880 --> 00:55:17,040
Speaker 9: cannot recover what was going on back there in the past.

1028
00:55:17,239 --> 00:55:19,760
Speaker 9: But at the same time, in principle you could, except

1029
00:55:19,760 --> 00:55:21,440
Speaker 9: there's quantum mechanics that gets in the way of this,

1030
00:55:21,480 --> 00:55:24,160
Speaker 9: But classically you could absolutely do this. So it's an

1031
00:55:24,239 --> 00:55:26,720
Speaker 9: interesting back and forth about the rigidity of the laws

1032
00:55:26,719 --> 00:55:30,440
Speaker 9: of physics. What's amazing to me is that we can

1033
00:55:30,440 --> 00:55:32,520
Speaker 9: make as much progress as we can talking about both

1034
00:55:32,560 --> 00:55:35,680
Speaker 9: the past and the future given such amazingly limited information

1035
00:55:35,760 --> 00:55:36,920
Speaker 9: about the state of the universe.

1036
00:55:37,120 --> 00:55:39,160
Speaker 2: So let's dig into the quantum mechanics of it. I mean,

1037
00:55:39,320 --> 00:55:42,919
Speaker 2: you describe something like a clockwork universe where things are deterministic,

1038
00:55:43,000 --> 00:55:45,400
Speaker 2: and in principle, if you knew all the information, you

1039
00:55:45,400 --> 00:55:47,959
Speaker 2: could predict the future, et cetera. But we know that's

1040
00:55:48,040 --> 00:55:51,319
Speaker 2: not our universe. We know there is fuzziness, and we

1041
00:55:51,400 --> 00:55:53,879
Speaker 2: talk a lot in physics, especially in popular science, about

1042
00:55:53,960 --> 00:55:58,480
Speaker 2: quantum fluctuations, which are treated as if they have no cause.

1043
00:55:58,880 --> 00:56:00,640
Speaker 2: You know, why does there a guy lexi here? Oh

1044
00:56:00,719 --> 00:56:02,680
Speaker 2: there was a fluctuation in the early universe. Why is

1045
00:56:02,719 --> 00:56:05,400
Speaker 2: there no galaxy there? Oh, there was a fluctuation. Do

1046
00:56:05,440 --> 00:56:08,600
Speaker 2: those fluctuations in quantum mechanics do they really have no cause?

1047
00:56:08,680 --> 00:56:10,520
Speaker 2: Is there nothing that determines them?

1048
00:56:10,920 --> 00:56:15,240
Speaker 9: Well, I, at the risk of yet again being careful

1049
00:56:15,239 --> 00:56:19,799
Speaker 9: and pedantic. We have to distinguish between not having a

1050
00:56:19,880 --> 00:56:24,240
Speaker 9: cause and not being determined. Those are slightly two different things.

1051
00:56:25,080 --> 00:56:27,920
Speaker 9: Of course, you're right. In the macroscopic world, when you

1052
00:56:27,960 --> 00:56:30,759
Speaker 9: make a quantum measurement, the outcomes are not determined by

1053
00:56:30,760 --> 00:56:33,480
Speaker 9: the quantum state of the universe. As far as we know,

1054
00:56:34,239 --> 00:56:37,160
Speaker 9: there is no hidden information that would determine them. In

1055
00:56:37,320 --> 00:56:41,200
Speaker 9: theories like Bomian mechanics, there literally is hidden information that

1056
00:56:41,280 --> 00:56:44,880
Speaker 9: does determine them. So Boei mechanics is one hundred percent deterministic,

1057
00:56:45,800 --> 00:56:48,880
Speaker 9: but we literally have no access to that information. So

1058
00:56:49,160 --> 00:56:51,480
Speaker 9: you know, what good is it to think that this

1059
00:56:51,640 --> 00:56:54,080
Speaker 9: is a determined event even though we don't know, we

1060
00:56:54,280 --> 00:56:58,080
Speaker 9: cannot know what is the thing determining it. But also,

1061
00:56:58,320 --> 00:57:01,280
Speaker 9: you know, we're cheating a little bit because we're taking,

1062
00:57:01,719 --> 00:57:04,920
Speaker 9: we're borrowing this notion of causality that has kind of

1063
00:57:04,960 --> 00:57:08,680
Speaker 9: been handed down since Aristotle, the idea that you know,

1064
00:57:08,880 --> 00:57:12,640
Speaker 9: for every event we can assign a cause, which was

1065
00:57:12,719 --> 00:57:16,920
Speaker 9: never really very fundamentally rigorous, like, Okay, I'm late for work,

1066
00:57:16,960 --> 00:57:18,760
Speaker 9: and I assigned the cause to that that there was

1067
00:57:18,800 --> 00:57:21,880
Speaker 9: traffic that morning, But what about assigning the cause to

1068
00:57:21,920 --> 00:57:25,000
Speaker 9: that that space time is four dimensional? Like if it weren't,

1069
00:57:25,040 --> 00:57:26,520
Speaker 9: I wouldn't have been late for work, or you know,

1070
00:57:26,560 --> 00:57:28,760
Speaker 9: I mean like there's a whole bunch of facts about

1071
00:57:28,760 --> 00:57:32,320
Speaker 9: the universe on which this result depends. And this is

1072
00:57:32,360 --> 00:57:35,360
Speaker 9: full employment for philosophers to figure out exactly what you

1073
00:57:35,480 --> 00:57:39,960
Speaker 9: mean by the cause. But in physics we've sidestepped that

1074
00:57:40,120 --> 00:57:43,920
Speaker 9: question by replacing the ideas of cause and effect with

1075
00:57:44,000 --> 00:57:49,160
Speaker 9: a much more clear, rigorous framework, which is patterns, essentially

1076
00:57:49,200 --> 00:57:52,440
Speaker 9: differential equations that tell you from one thing another thing

1077
00:57:52,480 --> 00:57:54,920
Speaker 9: is going to follow. Or maybe you have some discrete

1078
00:57:55,000 --> 00:57:57,600
Speaker 9: version of physics or whatever it is. But an if

1079
00:57:57,640 --> 00:58:00,920
Speaker 9: then statement, you know, if this happens and that happens,

1080
00:58:01,640 --> 00:58:05,000
Speaker 9: it's not quite cause and effect, even though we speak

1081
00:58:05,040 --> 00:58:07,080
Speaker 9: that way sometimes, Like the example I like to use

1082
00:58:07,160 --> 00:58:11,720
Speaker 9: is the real numbers, right, zero, one, two, three minus

1083
00:58:11,720 --> 00:58:14,560
Speaker 9: one minus two minus three. There's a pattern there. If

1084
00:58:14,600 --> 00:58:17,439
Speaker 9: I tell you the number n, you can figure out

1085
00:58:17,440 --> 00:58:19,200
Speaker 9: what the number N plus one is. If I tell

1086
00:58:19,200 --> 00:58:21,920
Speaker 9: you five, you can figure out six. But five is

1087
00:58:21,960 --> 00:58:25,160
Speaker 9: not the cause of six, right, it's just the previous

1088
00:58:25,200 --> 00:58:28,800
Speaker 9: thing in the pattern. That's how the laws of physics work.

1089
00:58:28,920 --> 00:58:33,480
Speaker 9: It's one damn thing after another, and it's okay. If

1090
00:58:33,720 --> 00:58:37,120
Speaker 9: some of those laws are stochastic, right, Like, maybe they are,

1091
00:58:37,160 --> 00:58:40,040
Speaker 9: since we don't understand the foundations of quantum mechanics perfectly well.

1092
00:58:41,080 --> 00:58:45,280
Speaker 9: Classical mechanics was deterministic, but maybe quantum mechanics just isn't

1093
00:58:45,360 --> 00:58:47,960
Speaker 9: even at the most fundamental level. Or maybe it is,

1094
00:58:48,120 --> 00:58:52,200
Speaker 9: as I said, Bomi mechanics many worlds. These are deterministic theories,

1095
00:58:52,240 --> 00:58:53,960
Speaker 9: but we don't know which one, if either one of

1096
00:58:53,960 --> 00:58:57,600
Speaker 9: those is right. So I wouldn't say that quantum events

1097
00:58:57,680 --> 00:59:02,920
Speaker 9: don't have a cause. Events follow the patterns given to

1098
00:59:03,040 --> 00:59:06,040
Speaker 9: us by the laws of physics. As to whether those

1099
00:59:06,080 --> 00:59:09,960
Speaker 9: patterns are deterministic or stochastic, we just don't know at

1100
00:59:10,000 --> 00:59:13,320
Speaker 9: the fundamental level. We do know that as observers in

1101
00:59:13,360 --> 00:59:15,280
Speaker 9: the universe, they seem stochastic to.

1102
00:59:15,320 --> 00:59:17,760
Speaker 2: Us, right, And I didn't mean to imply that like

1103
00:59:17,840 --> 00:59:21,280
Speaker 2: anything goes in quantum mechanics, you do an experiment and

1104
00:59:21,320 --> 00:59:25,800
Speaker 2: like anything can happen. Obviously, we construct conditions and quantum

1105
00:59:25,840 --> 00:59:29,320
Speaker 2: mechanics gives us predictions for probability distributions. In that sense,

1106
00:59:29,360 --> 00:59:33,360
Speaker 2: it determines those distributions, but not the individual experiment. That's

1107
00:59:33,400 --> 00:59:34,880
Speaker 2: an important distinction, thank you.

1108
00:59:34,920 --> 00:59:37,520
Speaker 9: No, it's actually super important because a lot of people,

1109
00:59:37,600 --> 00:59:41,520
Speaker 9: when you tell them that the way that fundamental physics

1110
00:59:41,600 --> 00:59:46,120
Speaker 9: works isn't exactly in line with our informal, twenty five

1111
00:59:46,200 --> 00:59:49,320
Speaker 9: hundred year old notion of cause and effect, they instantly

1112
00:59:49,400 --> 00:59:52,240
Speaker 9: leap to, oh, then anything goes. But that's really not

1113
00:59:52,320 --> 00:59:55,160
Speaker 9: what it is. There's still laws, there are still patterns.

1114
00:59:55,280 --> 00:59:57,959
Speaker 9: Maybe there's a stochastic element to them. We're just not sure.

1115
00:59:58,000 --> 01:00:00,920
Speaker 2: So then let me ask you to speculate why, Because

1116
01:00:00,920 --> 01:00:04,120
Speaker 2: we're peering into the future where maybe folks smarter than

1117
01:00:04,200 --> 01:00:06,479
Speaker 2: us are going to unravel the nature of space time

1118
01:00:06,560 --> 01:00:09,720
Speaker 2: and gives us their quantum gravity that lets us understand

1119
01:00:09,760 --> 01:00:12,760
Speaker 2: all of this stuff. Do you think that theory is

1120
01:00:12,800 --> 01:00:16,600
Speaker 2: going to be non local and causal only in the

1121
01:00:16,600 --> 01:00:19,760
Speaker 2: way the quantum mechanics is. Or do we have no

1122
01:00:19,880 --> 01:00:22,200
Speaker 2: idea about what's going to happen with quantum gravity?

1123
01:00:22,640 --> 01:00:28,600
Speaker 9: So your second guess is probably more accurate, more more fair,

1124
01:00:28,840 --> 01:00:31,080
Speaker 9: more legitimate, more honest to say we have no idea

1125
01:00:31,160 --> 01:00:34,680
Speaker 9: what's going to happen. I have my favorite ideas, and

1126
01:00:34,720 --> 01:00:40,280
Speaker 9: my favorite idea is just taking quantum mechanics really super

1127
01:00:40,400 --> 01:00:43,960
Speaker 9: duper seriously. So what do I mean by that? I

1128
01:00:44,000 --> 01:00:48,840
Speaker 9: already said that when it comes to locality, the way

1129
01:00:48,880 --> 01:00:51,600
Speaker 9: that we represent the state of a physical system in

1130
01:00:51,680 --> 01:00:55,720
Speaker 9: quantum mechanics with a wave function or whatever is non local. Right,

1131
01:00:55,880 --> 01:00:57,680
Speaker 9: you have a wave function that depends on all the

1132
01:00:57,680 --> 01:00:59,919
Speaker 9: particles in all the fields, not on just one local

1133
01:01:00,320 --> 01:01:06,080
Speaker 9: in space. You know, that's a sort of particular specific

1134
01:01:06,200 --> 01:01:10,720
Speaker 9: version of a more general abstract statement that quantum states are,

1135
01:01:11,120 --> 01:01:15,600
Speaker 9: you know, elements of some abstract mathematical space. And space,

1136
01:01:15,680 --> 01:01:18,040
Speaker 9: good old space, good old three dimensional x, y z

1137
01:01:18,240 --> 01:01:23,480
Speaker 9: space is not there in the fundamental description of quantum mechanics.

1138
01:01:23,960 --> 01:01:28,280
Speaker 9: Time is, which is weird because the Shortinger equation has

1139
01:01:28,320 --> 01:01:30,680
Speaker 9: a T in it, but the general form of the

1140
01:01:30,680 --> 01:01:33,480
Speaker 9: Shortinger equation does not have an x in it. And

1141
01:01:33,520 --> 01:01:37,000
Speaker 9: that's intension with the spirit of relativity. And that's just true,

1142
01:01:37,040 --> 01:01:38,760
Speaker 9: and okay, we're gona have to deal with that, et cetera.

1143
01:01:39,400 --> 01:01:44,800
Speaker 9: But to me, the real deep question is not how

1144
01:01:44,840 --> 01:01:49,720
Speaker 9: do we reconcile ourselves to the apparent non locality of

1145
01:01:49,800 --> 01:01:54,000
Speaker 9: quantum measurement, the apparent spooky action at a distance and

1146
01:01:54,080 --> 01:01:56,880
Speaker 9: we measure one particle and we see an instantaneous effect

1147
01:01:56,880 --> 01:01:59,560
Speaker 9: on its quantum state. That's not the question. The question

1148
01:01:59,680 --> 01:02:02,640
Speaker 9: is because remember, there's this two sides of quantum mechanics.

1149
01:02:02,720 --> 01:02:04,920
Speaker 9: What happens when you're not looking at it, where everything

1150
01:02:04,960 --> 01:02:07,040
Speaker 9: looks local, and what happens when you look at it

1151
01:02:07,080 --> 01:02:11,200
Speaker 9: when there are these non local correlations. I'm interested in

1152
01:02:11,240 --> 01:02:14,440
Speaker 9: why things ever look local at all? You know, if

1153
01:02:14,480 --> 01:02:17,280
Speaker 9: you just think that your starting point is this abstract

1154
01:02:17,360 --> 01:02:22,120
Speaker 9: quantum mechanical state vector of the universe, Hilbert space is

1155
01:02:22,160 --> 01:02:27,360
Speaker 9: the mathematical space in which these quantum states live, why

1156
01:02:27,400 --> 01:02:30,080
Speaker 9: does it look like we live in space and have

1157
01:02:30,400 --> 01:02:35,240
Speaker 9: approximately local interactions at all? And in fact, so I've

1158
01:02:35,240 --> 01:02:37,360
Speaker 9: written papers about this, like you can actually be a

1159
01:02:37,360 --> 01:02:39,760
Speaker 9: working physicist and try to ask this question. It's a

1160
01:02:39,800 --> 01:02:43,680
Speaker 9: little bit hard to make too much progress because the

1161
01:02:43,760 --> 01:02:46,240
Speaker 9: question is so grandiose. So we don't know a lot

1162
01:02:46,280 --> 01:02:49,760
Speaker 9: about it. But I think that it's a different angle

1163
01:02:50,080 --> 01:02:53,240
Speaker 9: on quantum gravity than the traditional ones. You know, Traditionally,

1164
01:02:54,040 --> 01:02:56,640
Speaker 9: in all of physics, what we do is we invent

1165
01:02:56,680 --> 01:03:00,080
Speaker 9: a classical theory of something like electromagnetism or this in

1166
01:03:00,120 --> 01:03:04,000
Speaker 9: barmonic osclator or a propagating string and ten dimensions and

1167
01:03:04,040 --> 01:03:07,560
Speaker 9: all these are classical descriptions, and then we quantize them.

1168
01:03:07,600 --> 01:03:10,120
Speaker 9: We have some rules for turning that quantum theory into

1169
01:03:10,160 --> 01:03:14,240
Speaker 9: a classical theory, and we keep bumping into problems when

1170
01:03:14,280 --> 01:03:17,160
Speaker 9: applying these rules to the questions of quantum gravity. But

1171
01:03:17,280 --> 01:03:19,680
Speaker 9: nature doesn't work that way. Nature doesn't start with the

1172
01:03:19,680 --> 01:03:22,720
Speaker 9: classical theory and quantize it. Nature is just quantum from

1173
01:03:22,760 --> 01:03:26,640
Speaker 9: the start. So maybe the obstacle defining the right theory

1174
01:03:26,640 --> 01:03:29,240
Speaker 9: of quantum gravity is that we keep wanting to start

1175
01:03:29,280 --> 01:03:31,680
Speaker 9: with a classical theory and quantizing it. Maybe you should

1176
01:03:31,680 --> 01:03:35,400
Speaker 9: start with a quantum theory of nothing at all and

1177
01:03:35,560 --> 01:03:40,600
Speaker 9: asking under what conditions might it look like the classical

1178
01:03:41,000 --> 01:03:44,440
Speaker 9: three spatial dimensional universe with certain particles and fields and

1179
01:03:44,440 --> 01:03:48,640
Speaker 9: stuff like that. So I think that thinking about locality

1180
01:03:48,760 --> 01:03:51,960
Speaker 9: in this way might very well turn out to be

1181
01:03:52,120 --> 01:03:55,040
Speaker 9: absolutely crucial to making progress in quantum gravity.

1182
01:03:55,080 --> 01:03:58,320
Speaker 2: So I think the description you're suggesting here is some

1183
01:03:58,520 --> 01:04:01,320
Speaker 2: concept in which space it'sself is not fundamental.

1184
01:04:01,360 --> 01:04:07,560
Speaker 9: It's emergent absolutely, And that's actually remarkably common among people

1185
01:04:07,600 --> 01:04:10,680
Speaker 9: who think about quantum gravity, that space itself is not fundamental.

1186
01:04:11,040 --> 01:04:13,560
Speaker 9: What's not really well understood is what that means. Like

1187
01:04:13,640 --> 01:04:15,560
Speaker 9: there's only one way to be fundamental, and there's many

1188
01:04:15,600 --> 01:04:18,800
Speaker 9: ways to not be fundamental. So okay, space is emergent,

1189
01:04:18,920 --> 01:04:21,160
Speaker 9: it's not fundamental. What does that mean?

1190
01:04:21,240 --> 01:04:21,640
Speaker 4: What is it?

1191
01:04:21,680 --> 01:04:23,720
Speaker 9: Where did it come from? And different people have different

1192
01:04:23,720 --> 01:04:24,520
Speaker 9: opinions about that.

1193
01:04:24,680 --> 01:04:28,240
Speaker 2: And it's a real struggle because we think geometrically. I mean,

1194
01:04:28,280 --> 01:04:31,200
Speaker 2: if you tell me the universe is a bunch of

1195
01:04:31,400 --> 01:04:34,880
Speaker 2: wave functions and they're entangled together, I try to think

1196
01:04:34,920 --> 01:04:38,640
Speaker 2: where are they? I imagine them in my head. Yeah,

1197
01:04:38,680 --> 01:04:40,480
Speaker 2: because I think about where do I put it? And

1198
01:04:40,560 --> 01:04:42,120
Speaker 2: in my head I have a space and that space

1199
01:04:42,200 --> 01:04:45,280
Speaker 2: is three dimensional because I have lived in three dimensional space,

1200
01:04:45,320 --> 01:04:47,600
Speaker 2: so I think in three D. So it's a real

1201
01:04:47,640 --> 01:04:49,760
Speaker 2: struggle to counter our intuitions and to try to come

1202
01:04:49,840 --> 01:04:52,320
Speaker 2: up with a conception of the universe that doesn't make

1203
01:04:52,360 --> 01:04:53,240
Speaker 2: these assumptions.

1204
01:04:53,640 --> 01:04:55,800
Speaker 9: So one question I have that is well beyond my

1205
01:04:55,920 --> 01:05:01,000
Speaker 9: pay grade, But could you model a virtual reac environment

1206
01:05:01,440 --> 01:05:04,720
Speaker 9: in which space is four dimensional? Could you retrain your

1207
01:05:04,720 --> 01:05:10,320
Speaker 9: brain to move and live and perceive things in four

1208
01:05:10,320 --> 01:05:14,400
Speaker 9: dimensional space? Or is there something in the structure of

1209
01:05:14,440 --> 01:05:17,640
Speaker 9: our brain itself that only makes sense in three dimensions?

1210
01:05:18,120 --> 01:05:20,360
Speaker 9: You know, people like a manual cont to the philosopher

1211
01:05:20,560 --> 01:05:23,080
Speaker 9: you know, had an argument that three dimensional space was

1212
01:05:23,160 --> 01:05:26,400
Speaker 9: kind of necessary. People argue about exactly how necessary he

1213
01:05:26,440 --> 01:05:29,120
Speaker 9: thought it was. But it was almost an anthropic argument,

1214
01:05:29,200 --> 01:05:31,240
Speaker 9: you know, you tried to argue that this is the

1215
01:05:31,280 --> 01:05:33,000
Speaker 9: only way to make sense of the world is if

1216
01:05:33,040 --> 01:05:36,000
Speaker 9: you have this spatial arena in which things have locations

1217
01:05:36,000 --> 01:05:38,120
Speaker 9: and things like that. It's a what is it? A

1218
01:05:38,160 --> 01:05:42,400
Speaker 9: cautionary tale because philosophers should be careful that their ideas

1219
01:05:42,400 --> 01:05:46,560
Speaker 9: won't be overthrown by later advances in physics. But I

1220
01:05:46,560 --> 01:05:49,760
Speaker 9: don't know, and I have ideas that are very vague

1221
01:05:49,760 --> 01:05:52,920
Speaker 9: in hand wavy about why space should have emerged in

1222
01:05:52,920 --> 01:05:57,280
Speaker 9: the first place. Maybe not really anthropic, but maybe something

1223
01:05:57,320 --> 01:06:00,880
Speaker 9: about locality is very helpful if you just want complexity

1224
01:06:01,120 --> 01:06:04,360
Speaker 9: at all, Like if literally every particle in the universe

1225
01:06:04,400 --> 01:06:08,280
Speaker 9: could instantaneously affect every other particle, Like how do you

1226
01:06:08,400 --> 01:06:10,640
Speaker 9: get through the day? Like how do you make a

1227
01:06:10,640 --> 01:06:12,880
Speaker 9: living in a world like that? I don't know. So

1228
01:06:13,880 --> 01:06:16,080
Speaker 9: I do think that these questions are very deep and

1229
01:06:17,080 --> 01:06:19,400
Speaker 9: certainly not understood, but maybe understandable.

1230
01:06:19,520 --> 01:06:22,240
Speaker 2: I have a suspicion about whether we could have four

1231
01:06:22,240 --> 01:06:25,280
Speaker 2: dimensional VR. I mean, I remember trying to play video

1232
01:06:25,320 --> 01:06:27,920
Speaker 2: games with my teenager, and I played a lot of

1233
01:06:27,960 --> 01:06:30,360
Speaker 2: video games as a kid, But you know, the controller

1234
01:06:30,440 --> 01:06:32,400
Speaker 2: was simple. There was ab you know, there was a

1235
01:06:32,440 --> 01:06:35,280
Speaker 2: little directional thing you could jump on whatever. So then

1236
01:06:35,320 --> 01:06:37,800
Speaker 2: I'm trying to play Halo with my teenager and there's

1237
01:06:37,840 --> 01:06:39,600
Speaker 2: like a knob for the direction of your gun, is

1238
01:06:39,680 --> 01:06:41,800
Speaker 2: a knob for the direction you move, and a knob

1239
01:06:41,840 --> 01:06:42,680
Speaker 2: for the direction.

1240
01:06:42,480 --> 01:06:43,240
Speaker 1: Your head goes.

1241
01:06:43,800 --> 01:06:46,960
Speaker 2: And this kid is moving through essentially six dimensional space,

1242
01:06:47,440 --> 01:06:49,600
Speaker 2: you know, and he's controlling it and it's totally intuitive

1243
01:06:49,600 --> 01:06:51,800
Speaker 2: to him. And I'm like, I have to put this

1244
01:06:51,840 --> 01:06:53,960
Speaker 2: finger on this joystick and that thing on that joystick,

1245
01:06:54,000 --> 01:06:57,360
Speaker 2: And of course he know, headshotted me instantaneously every single time.

1246
01:06:57,960 --> 01:07:00,280
Speaker 2: So I think the human brain is probably plastic enough

1247
01:07:00,280 --> 01:07:02,360
Speaker 2: to be able to adapt to those kind of environments,

1248
01:07:02,400 --> 01:07:04,120
Speaker 2: but not yours mine.

1249
01:07:04,200 --> 01:07:06,480
Speaker 9: That's funny. You don't look that old Daniels. It's kind

1250
01:07:06,520 --> 01:07:08,560
Speaker 9: of weird, but I guess sorry, we're learning.

1251
01:07:08,600 --> 01:07:11,960
Speaker 2: That's the zoom filter. But I want to take our

1252
01:07:11,960 --> 01:07:15,800
Speaker 2: brains out universal and think, of course, about how aliens

1253
01:07:15,840 --> 01:07:18,640
Speaker 2: experience the universe. Do you think if we get to

1254
01:07:18,680 --> 01:07:20,920
Speaker 2: talk to aliens that they will have gone through a

1255
01:07:20,920 --> 01:07:24,720
Speaker 2: similar trajectory, you know, where they imagine the universe is

1256
01:07:24,800 --> 01:07:28,880
Speaker 2: local and causal and then they discover, oh, actually, at

1257
01:07:28,920 --> 01:07:32,120
Speaker 2: its foundations, these are just intuitive assumptions we're making in

1258
01:07:32,160 --> 01:07:34,800
Speaker 2: the universe doesn't respect them. Or do you think it's

1259
01:07:34,840 --> 01:07:38,160
Speaker 2: possible that they grew up natively to imagine a non

1260
01:07:38,240 --> 01:07:39,080
Speaker 2: local universe.

1261
01:07:40,480 --> 01:07:43,280
Speaker 9: I think that it's. Uh, there's a tension there, because

1262
01:07:43,320 --> 01:07:45,360
Speaker 9: I do think that everything is possible when you ask

1263
01:07:45,400 --> 01:07:48,480
Speaker 9: these possibility questions, but some things are easier to imagine

1264
01:07:48,480 --> 01:07:53,160
Speaker 9: than others. I do think the embodiedness of we beings

1265
01:07:53,360 --> 01:07:57,240
Speaker 9: in three dimensional space is pretty natural to imagine as

1266
01:07:57,280 --> 01:08:01,360
Speaker 9: a universal feature, even of alien life, because cause, you know,

1267
01:08:01,400 --> 01:08:05,200
Speaker 9: the classical world is really helpful to you know, making

1268
01:08:05,200 --> 01:08:07,200
Speaker 9: predictions about what's going to happen, and it's a very

1269
01:08:07,200 --> 01:08:10,640
Speaker 9: good approximation to the world. So I suspect that whatever

1270
01:08:10,760 --> 01:08:14,800
Speaker 9: trajectory of scientific understanding the aliens take, it will start

1271
01:08:14,800 --> 01:08:18,080
Speaker 9: with classical three dimensional physics and and move on from there.

1272
01:08:18,600 --> 01:08:22,200
Speaker 9: But you know, if the aliens get really good at

1273
01:08:22,200 --> 01:08:27,200
Speaker 9: either VR or uploading into the matrix or whatever, maybe

1274
01:08:27,200 --> 01:08:31,439
Speaker 9: they're very used to thinking and perceiving things in different

1275
01:08:31,520 --> 01:08:34,160
Speaker 9: numbers of dimensions. Maybe that's just a switch they flip

1276
01:08:34,200 --> 01:08:36,639
Speaker 9: when they when they go in there. I remember one

1277
01:08:36,640 --> 01:08:41,599
Speaker 9: more completely amusing but irrelevant story. I was a science

1278
01:08:41,600 --> 01:08:44,000
Speaker 9: consultant for the movie Tron Legacy.

1279
01:08:44,560 --> 01:08:44,800
Speaker 3: Fun.

1280
01:08:45,360 --> 01:08:48,439
Speaker 9: And you might remember Tron, those of us who are

1281
01:08:48,479 --> 01:08:51,479
Speaker 9: old enough. I remember Tron when it came out. It was,

1282
01:08:51,520 --> 01:08:54,799
Speaker 9: you know, one of the first early eighties Disney movies

1283
01:08:54,920 --> 01:08:57,080
Speaker 9: that had a lot of computer graphics in it, right,

1284
01:08:57,280 --> 01:08:58,760
Speaker 9: Jeff Bridges was in it, and it was you know,

1285
01:08:58,760 --> 01:09:01,840
Speaker 9: it was not greats and by any stretch, but it was.

1286
01:09:01,760 --> 01:09:03,120
Speaker 1: Fun, deeply influential.

1287
01:09:03,360 --> 01:09:06,280
Speaker 9: I well, it was great ish. It is a certain

1288
01:09:06,680 --> 01:09:07,400
Speaker 9: kind of great.

1289
01:09:07,479 --> 01:09:07,759
Speaker 1: Yeah.

1290
01:09:08,120 --> 01:09:13,480
Speaker 9: But the sequel that they had later in the two thousands,

1291
01:09:13,560 --> 01:09:15,559
Speaker 9: Tron Legacy, and I think they've had another one, right,

1292
01:09:15,640 --> 01:09:18,680
Speaker 9: but or they're making it. But so Tron Legacy was

1293
01:09:18,720 --> 01:09:25,320
Speaker 9: not as successful cinematically, And part of it was when

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01:09:25,360 --> 01:09:30,080
Speaker 9: you made Tron, computer graphics were terrible, right, Like it was,

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01:09:30,120 --> 01:09:31,960
Speaker 9: it was amazing you could do it at all, and

1296
01:09:32,000 --> 01:09:33,519
Speaker 9: you were like, oh my god, this is so mind

1297
01:09:33,560 --> 01:09:35,519
Speaker 9: blowing that you were in this thing that was obviously

1298
01:09:35,560 --> 01:09:39,439
Speaker 9: a bunch of people on scooters with neon taped to them, right,

1299
01:09:39,479 --> 01:09:41,800
Speaker 9: but you know they had some computer graphics going on.

1300
01:09:42,120 --> 01:09:45,320
Speaker 9: But by you know, twenty ten or whatever, you can

1301
01:09:45,400 --> 01:09:49,120
Speaker 9: just make everything look perfectly realistic, and so they did

1302
01:09:49,600 --> 01:09:52,920
Speaker 9: so it looked perfectly realistic, like inside the video game,

1303
01:09:53,040 --> 01:09:56,040
Speaker 9: things look like the real world. But my attitude was like,

1304
01:09:56,080 --> 01:10:00,320
Speaker 9: but we live in the real world, wants to see that.

1305
01:10:00,400 --> 01:10:03,040
Speaker 9: What we want to see is something that doesn't look

1306
01:10:03,120 --> 01:10:06,200
Speaker 9: anything like the real world, because you can make any

1307
01:10:06,240 --> 01:10:09,479
Speaker 9: world you want. And they did not go down that road.

1308
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Speaker 9: But I think that that movie still remains to be

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01:10:12,320 --> 01:10:15,320
Speaker 9: seen where people like are literally living in six dimensional

1310
01:10:15,320 --> 01:10:18,719
Speaker 9: space and fighting their motorcycle battles.

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01:10:18,760 --> 01:10:21,000
Speaker 3: Accordingly, trum Legacy would have done so much better if

1312
01:10:21,040 --> 01:10:21,839
Speaker 3: they took your advice.

1313
01:10:22,840 --> 01:10:25,560
Speaker 9: So many things in life. Yeah that could be said about.

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Speaker 2: Yeah, all right, Well, thank you for joining us today

1315
01:10:28,880 --> 01:10:31,320
Speaker 2: on this element of our struggle to understand the real

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01:10:31,320 --> 01:10:33,400
Speaker 2: world and what is real about the world.

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01:10:33,520 --> 01:10:36,120
Speaker 1: I appreciate your thoughts and comments. Sean, thanks very much

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01:10:36,160 --> 01:10:36,600
Speaker 1: for having me.

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01:10:43,520 --> 01:10:47,360
Speaker 3: Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. We

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01:11:01,120 --> 01:11:02,439
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1327
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