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Speaker 1: We all know that quantum mechanics is plenty confusing already,

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Speaker 1: particles can weirdly maintain multiple possible states at the same time,

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Speaker 1: there's instantaneous collapse of entangled particle states across various space

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Speaker 1: time distances. And on top of all of that, there

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Speaker 1: are these articles you see in popular science venues claiming

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Speaker 1: that quantum mechanics proves you can change the past. Well,

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Speaker 1: spoiler alert, these are wrong. They're wrong when they were written,

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Speaker 1: which means that they can't use quantum mechanics to go

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Speaker 1: back in time to fix them later. But there are

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Speaker 1: some fascinating issues about how time works in quantum mechanics,

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Speaker 1: and there are some amazing and crazy ideas for retro

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Speaker 1: causality where the past can depend on the future. These

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Speaker 1: ideas might actually help unravel some confusion about instantaneous collapse

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Speaker 1: and and also avoid creating signaling paradoxes. So we can't

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Speaker 1: send you back in time to kill your grandfather. But

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Speaker 1: today we are going to dig into all of that

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Speaker 1: and hope that you understand time in quantum mechanics better

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Speaker 1: after you hear this episode than you did before. Welcome

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Speaker 1: to Daniel and Kelly's extraordinarily timely universe.

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Speaker 2: Hello. I'm Kelly Waitersmith. I study parasites and space, and

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Speaker 2: after looking at Daniel's outline for today, I wonder why

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Speaker 2: physicists are always trying to confuse us. Hi.

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Speaker 1: I'm Daniel. I'm a particle physicist, and I'd like to

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Speaker 1: go back in time and tell young Daniel that instead

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Speaker 1: he should work on quantum foundations. But I can't because

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Speaker 1: time flows forwards.

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Speaker 2: All right, Well, so Daniel, here's my question for you.

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Speaker 2: So if you could go back in time and meddle

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Speaker 2: with one historical moment, but not Hitler because that's too obvious,

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Speaker 2: what would you meddle with.

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Speaker 1: I would love to somehow preserve more knowledge from ancient societies.

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Speaker 1: So I would love to, for example, keep the Spaniards

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Speaker 1: from burning all of the Mayan books, or stop the

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Speaker 1: library of Alexandria from burning down, or just like you know,

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Speaker 1: go and scan a bunch of scrolls and a bunch

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Speaker 1: of libraries in the ancient Greek world, because we have

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Speaker 1: like a tiny fraction of ancient writing. And not that

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Speaker 1: I think that they like sold quantum gravity back then

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Speaker 1: and we've lost it or anything, But it's just an

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Speaker 1: amazing window into what people were thinking about, and the

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Speaker 1: tiny fraction that we do have is fascinating and insightful.

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Speaker 1: So I'm always frustrated by lost knowledge.

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Speaker 2: That is a great answer.

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Speaker 1: About you, Kelly. What would you go back and change?

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Speaker 1: Would you go back and become a particle physicist?

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Speaker 2: No, definitely, not that anything, but that No, Yeah, I

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Speaker 2: don't know. I kind of want to take your answer

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Speaker 2: because I was thinking that maybe maybe I would go

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Speaker 2: back and like thwart Lenin's revolution, because then you could

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Speaker 2: avoid the Hulladamoor and the Stalins, Gulogs and you know,

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Speaker 2: all of all of that other you know, Lisenko and

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Speaker 2: how he you know, slowed Soviet science for so long,

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Speaker 2: all of that stuff. But I don't know your answer.

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Speaker 2: Although your answer was not just like one moment in

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Speaker 2: history and everything that followed from it. You were you

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Speaker 2: were like hopping through multiple moments in history. That's kind

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Speaker 2: of cheating, But that would be a good use for

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Speaker 2: a time machine, Yeah, I.

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Speaker 1: Think it would. Or you could go back and like

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Speaker 1: redesign the butterfly ballot in Florida in the two thousands

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Speaker 1: and really changed the course of history.

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Speaker 2: Whoa, whoa, All right, but today you're gonna let us

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Speaker 2: know what our options are here.

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Speaker 1: That's right. Today we are digging into a fascinating question

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Speaker 1: in quantum foundations, what quantum mechanics really says about the

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Speaker 1: nature of reality and what it says about the nature

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Speaker 1: of time. We had an episode recently about quantum entanglement

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Speaker 1: and instantaneous collapse across space, which never really sits well

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Speaker 1: with people because of special relativity, and today we're not

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Speaker 1: going to avoid that issue. We're going to dig deep

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Speaker 1: into what it means and whether all the popular science

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Speaker 1: that says quantum mechanics has proved you could change the

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Speaker 1: past is right or wrong.

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Speaker 2: Wait a minute, headlines and popside articles could be wrong.

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Speaker 2: I don't that doesn't seem like a good premise for

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Speaker 2: a podcast. Impossible.

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Speaker 1: I know it's crazy, but there is one famous experiment

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Speaker 1: in quantum mechanics which is consistently always completely misrepresented, not misinterpreted,

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Speaker 1: but the actual results of the experiment are misrepresented in

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Speaker 1: dramatic fashion, which leads people astray. So there's really interesting

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Speaker 1: questions about time and order of events in quantum mechanics,

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Speaker 1: but it's often cartoonized and misrepresented. So we're going to

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Speaker 1: clear all that up today and hopefully it penetrates back

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Speaker 1: in time to clarify things for you before you even

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Speaker 1: heard this episode. Amazing, But first, I was wondering what

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Speaker 1: people thought about the question of quantum mechanics in time.

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Speaker 1: So I went out there and I asked our amazing,

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Speaker 1: good looking, intelligent, cat loving dog parenting listeners whether quantum

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Speaker 1: mechanics can change the past. Here's what folks had to say.

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Speaker 3: I love you, professor, but good heavens, not even a

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Speaker 3: little googling. I assume you mean more than make them

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Speaker 3: more precise. So perhaps it's a Heisenberg thing.

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Speaker 4: So weird.

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Speaker 1: I have absolutely no idea.

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Speaker 4: I know, I love this fantastic question.

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Speaker 5: It can be overwhelming food for thought. I cannot wait

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Speaker 5: for the podcast.

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Speaker 1: No, I don't think so.

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Speaker 4: I think quantum measurements can change the state of the

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Speaker 4: quantum object when measured, but not the past.

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Speaker 1: Well it depends. Oh sorry, this wasn't a biology question.

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Speaker 5: Right, Making quantum measurements is there's a possibility that it

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Speaker 5: can affect the past.

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Speaker 4: I don't currently know how to tell the difference between

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Speaker 4: spo action at a distance and an action that propagates

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Speaker 4: into the past, but maybe when future me listens to

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Speaker 4: the podcast, I can find a way to send the

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Speaker 4: answer back to present me using quantum entanglement.

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Speaker 6: Changing the past sounds like science fiction to me, but

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Speaker 6: on the smallest quantum scale things are weird.

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Speaker 1: Maybe it's possible.

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Speaker 2: I don't know.

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Speaker 1: No, that would be insane, but I know how this

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Speaker 1: show goes, so maybe.

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Speaker 7: I think even if entanglement somehow violates causality and allows

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Speaker 7: things to happen simultaneously, I don't know how that would

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Speaker 7: allow it to be going backwards in time. So no,

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Speaker 7: I don't think it can change the past. I'm gonna

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Speaker 7: say no, but perhaps they can change our interpretation of

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Speaker 7: the past.

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Speaker 6: Before meation goes from one particle to the past and

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Speaker 6: the retransmitted to the other particles, so they are correlated

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Speaker 6: in real time. If the past changes with a quantum measurement,

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Speaker 6: then all recordings and on memory will change as well.

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Speaker 6: So there's no way to know for sure.

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Speaker 5: The unpublished sci fi writer and meshauts yes enthusiastically, but

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Speaker 5: the logical curious child to me says no, because that

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Speaker 5: would then require any causality after the event, but before

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Speaker 5: the observation the change as well well.

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Speaker 2: Our extraordinaries are clever in the past, present, and future.

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Speaker 2: And these were great answers and yes.

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Speaker 1: Not even a little bit of googling, and thank you

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Speaker 1: for trying. My goal here is really just to reveal

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Speaker 1: what people already know, not what they could look up

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Speaker 1: in a few minutes online, to get a sense for

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Speaker 1: where people's brains are at, because that's where we want

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Speaker 1: to meet you. We want to clarify all this stuff,

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Speaker 1: and so we want to know what you already know.

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Speaker 2: And you know, these are supposed to be our person

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Speaker 2: on the street question, so it's like we just stopped

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Speaker 2: you on the street, but actually we stopped you at

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Speaker 2: your computer. But Daniel, I think the place we should

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Speaker 2: start probably is what causality means. And we had a

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Speaker 2: really great entire conversation with Sean Carroll about this. But

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Speaker 2: in case folks don't have a whole hour to catch

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Speaker 2: up on that conversation, how about you give us the

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Speaker 2: short version.

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Speaker 1: Yeah, causality really is at the heart of this and

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Speaker 1: how we think about physics as a way to explain

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Speaker 1: the universe, and we impose causality when we say that

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Speaker 1: the past determines the future, which sounds like pretty obvious, right,

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Speaker 1: because the choices you make now determine the future. Causes

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Speaker 1: happen before effects. I shoot an arrow, and an arrow

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Speaker 1: has to fly and hit the target after I shoot

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Speaker 1: the arrow. There's like a natural order to events, and

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Speaker 1: that seems really crisp and clear immediately, But there's a

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Speaker 1: whole philosophical swamp here of thinking about, like, well, which

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Speaker 1: causes exactly influence those effects, Like if Kelly shouts at

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Speaker 1: me just before I release the arrow and it changes

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Speaker 1: the direction of the arrow, then Kelly is also a

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Speaker 1: cause of the cause, And what she had for breakfast

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Speaker 1: this morning is a cause for whether or not She's

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Speaker 1: going to shout at me while I'm shooting the arrow,

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Speaker 1: and you know how well she slept last night or whatever.

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Speaker 1: You could go back in time and basically then the

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Speaker 1: whole universe is a cause. So you could think about

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Speaker 1: light cones, you can think about dominant causes. It turns

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Speaker 1: out to be a big philosophical mess. And yes, please

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Speaker 1: do dig into our episode with Sean Carroll about causality

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Speaker 1: and locality.

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Speaker 2: So the philosophical swamp is identifying what counts as a cause.

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Speaker 2: Is that what you were saying, So like we're not

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Speaker 2: talking about temporal ordering yet, we're just talking about how

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Speaker 2: many things get to be included as a cause when

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Speaker 2: you were trying to understand something.

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Speaker 1: Yeah, it gets a little bit messy because there are

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Speaker 1: like things in the past which don't affect your decision,

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Speaker 1: but they did happen in the past, and so like

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Speaker 1: now you're deciding which things are important, which things are

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Speaker 1: central to that decision having been made. So it's not

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Speaker 1: just as crisp and clean as saying like the whole

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Speaker 1: past the terms the future. It's like some elements of

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Speaker 1: the past. And then of course there's the fact that

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Speaker 1: information doesn't propagate faster than light, So some things in

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Speaker 1: the past can't affect your decisions, like things that are

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Speaker 1: happening and Andromeda right now can't affect the decisions I'm making,

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Speaker 1: And they can't affect the decisions I'm making tomorrow or

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Speaker 1: the next day. They can't affect decisions I'm making for

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Speaker 1: millions of years, because that information will take millions of

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Speaker 1: years to get here. So that's sort of the beginning

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Speaker 1: of the structure of the question of causality and locality.

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Speaker 2: Okay, And I always expect physics to get confusing, But

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Speaker 2: we haven't gotten the confusing part yet. Right now, ok

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Speaker 2: that all makes sense? Am I missing something?

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Speaker 1: And if you want to be like a philosopher and

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Speaker 1: go back to the ancient Greeks, then you can ask, well,

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Speaker 1: that all sounds natural, But why is it this way?

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Speaker 1: You know, why does it go in one direction? It

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Speaker 1: seems to flow from the past to the future. Why

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Speaker 1: is the past different from the future? Essentially, why does

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Speaker 1: time flow forwards?

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Speaker 5: Right?

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Speaker 1: You have this fundamental asymmetry in time, and you know,

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Speaker 1: relativity tells us time is related to space. But we

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Speaker 1: know that in space you can go like backwards and

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Speaker 1: forwards all you like, but in time it only flows forwards.

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Speaker 1: And that's fascinating and weird and not something that we understand.

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Speaker 1: I mean, there are ideas there about entropy that I

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Speaker 1: think are over sold. And it's not like nobody's worked

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Speaker 1: on this question, but it's still a fundamentally important question

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Speaker 1: about why the universe seems to flow forwards in time

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Speaker 1: and whether it actually does.

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Speaker 2: Okay, so let's just take it as a given time

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Speaker 2: flows forward, and we.

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Speaker 1: Think it's important for time to flow forward because we

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Speaker 1: want the universe to make sense, right, We want the

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Speaker 1: universe to be coherent.

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Speaker 5: At least you can.

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Speaker 1: Imagine another scenario where like you have time machines where

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Speaker 1: you can send information back in time very quickly. You

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Speaker 1: get a universe that's incoherent, right, you can get things

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Speaker 1: that are like paradoxes. If you can send signals backwards

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Speaker 1: in time, then I could like fire a death ray

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Speaker 1: into my own past and kill myself yesterday, and then like, Okay,

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Speaker 1: then I'm dead yesterday. Who's firing the death ray today? Nobody?

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Speaker 1: So I'm alive yesterday. So now I'm firing the death rays,

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Speaker 1: So now I'm dead yesterday. So you see how very

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Speaker 1: quickly sending any information backwards in time doesn't have to

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Speaker 1: be murderous, can create paradoxes. And so having a flow

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Speaker 1: of causality where things only move forwards solves that problem. Right,

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Speaker 1: So any theory you have about how time flows and

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Speaker 1: information can't have paradoxes because they contradict themselves, and the

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Speaker 1: universe is one way, so it can't be a paradox.

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Speaker 2: But so what I've learned from science fiction is that

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Speaker 2: you can go back in time. You just have to

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Speaker 2: make sure that you duck behind a wall when you

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Speaker 2: from the past is in the area, then you're okay, yeah.

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Speaker 1: That's right. And if you wear a dark cap and

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Speaker 1: look the other way, then.

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Speaker 2: The universe is saved fromsolved.

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Speaker 1: And if Daniel goes back in time and prevents the

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Speaker 1: Library of Alexandri from being burned down, then dot dot dot,

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Speaker 1: he's not born. He doesn't prevent the library from being

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Speaker 1: burned down, so it does. I don't know.

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Speaker 2: It's a big mess, all right, all right, I see

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Speaker 2: your point.

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Speaker 1: And this way of thinking about physics that time flows

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Speaker 1: from the past to the future. You have this like

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Speaker 1: chain of events, this causality and then locality that link

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Speaker 1: the past to the future. This is like treating the

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Speaker 1: universe as if it was a computer. You know, think

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Speaker 1: about how we model things in simulations. We talked about

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Speaker 1: this in our weather episode. For example, you have knowledge

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Speaker 1: of what the universe is now, and then you have

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Speaker 1: the laws of physics that determine what happens next. So

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Speaker 1: we think of the universe as sort of flowing forwards

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Speaker 1: the way a computer might simulate a universe. And this

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Speaker 1: is you know, one of these sort of dorm room

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Speaker 1: epiphanies that makes people think, oh, the universe is a

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Speaker 1: computer because we model our physics after Isaac Newton, who

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Speaker 1: sort of began this trend. You said, like, let's think

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Speaker 1: about the initial conditions and then the laws moving forward,

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Speaker 1: which happened to be differential equations, and so at any moment,

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Speaker 1: the current conditions and the laws predict the future. Think

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Speaker 1: about the universes like this is how Newton thought about it.

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Speaker 1: There's a universal clock, and there's a now, and then

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Speaker 1: the now determines the next slice of the universe, which

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Speaker 1: determines the next slice of the universe, which determines the

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Speaker 1: next slice of the universe. And if that sort of

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Speaker 1: seems like obvious to you, you know, like, Okay, that's

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Speaker 1: the way the universe does work. That the universe calculates

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Speaker 1: the future from the past using its laws. Then you're

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Speaker 1: already thinking about the universe as a computer. But it's

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Speaker 1: not necessarily the only way to think about the universe

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Speaker 1: and to think about time.

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Speaker 2: Okay, But I feel like if you think about time

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Speaker 2: as not just marching forward, but as being able to

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Speaker 2: go forward and back, then everything is absolute chaos. So

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Speaker 2: what would the alternative be.

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Speaker 1: Yeah, So there's a really important subtlety here between depending

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Speaker 1: on future events and being able to signal back in time.

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Speaker 1: Of course, being able to signal back in time or

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Speaker 1: have information flow explicitly from the future to the past

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Speaker 1: would create chaos. Just like you said, that's not allowed.

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Speaker 1: It doesn't happen full stop. But you can have the

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Speaker 1: present depend on a non controllable detail of the future.

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Speaker 1: That sounds like it's the same thing, just sort of

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Speaker 1: rewritten in physics loyally talk, but it's not. It's crucially different,

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Speaker 1: and we're going to dig into exactly what it means

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Speaker 1: right now. And so before we get into quant mechanics,

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Speaker 1: I want to bring up a classical physics example of this,

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Speaker 1: which is pretty widespread and I think not deeply enough understood.

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Speaker 1: And it's also going to set the stage for a

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Speaker 1: future episode about the principles of least action. So you know,

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Speaker 1: Newton's way to solve problems is one way to solve problems,

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Speaker 1: but sometimes it's a mess. Sometimes it's complicated. Like it

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Speaker 1: works fine for throwing a ball through the air because

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Speaker 1: it's pretty simple. But now you add like a tornado,

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Speaker 1: and you have like a squirrel dangling from a string,

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Speaker 1: and the squirrel's wearing a jet pack and whatever make

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Speaker 1: it complicated. Any student physics will know that those problems

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Speaker 1: are really hard to solve. It's just a lot of

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Speaker 1: terms in there. And Newton's method isn't flawed, it's just

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Speaker 1: not really very practical. And then it comes along another

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Speaker 1: way to solve these problems. There are different ways to

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Speaker 1: approach these problems that use different strategies and end up

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Speaker 1: with the same answer, but they're much simpler, and these

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Speaker 1: use minimal principles. So let's start with simple example, Fermat's principle.

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Speaker 1: Vermot found a way to calculate how light is going

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Speaker 1: to flow. So if you want to know, for example,

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Speaker 1: like where a beam of light is going to go

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Speaker 1: you're calculating through optics or whatever. Then one way to

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Speaker 1: do it is to think about the light moving and

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Speaker 1: it's going to hit the lens and how is that

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Speaker 1: going to change its angle? So to think about it

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Speaker 1: from a computer point of view, flowing forwards in time.

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Speaker 1: But Vermont discovered that if you know the beginning and

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Speaker 1: you know the end point, you can solve for the

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Speaker 1: path of light by finding the shortest path from A

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Speaker 1: to B. So it's called a minimal principle, right, and

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Speaker 1: that's cool and it's beautiful in some sense, but it's

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Speaker 1: also a bit of a brain bender because you have

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Speaker 1: to know B in order to predict the path. You

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Speaker 1: have to know where it ends to find the path

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Speaker 1: from A to B. So, like Newton's method says, start

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Speaker 1: from A, move the light forwards, predict that it lands

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Speaker 1: on B, Fermat's principle says, well, if you tell me

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Speaker 1: A and B, I can tell you where the light

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Speaker 1: had to have gone, cause I'm going to find the

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Speaker 1: shortest path from A to B.

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Speaker 2: Okay, but that's not going backwards in time. That's letting

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Speaker 2: an event play out and then extrapolating in between, right

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Speaker 2: sort of.

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Speaker 1: But it's saying that to know where the photon is

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Speaker 1: between A and B, you have to know what B is. Right,

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Speaker 1: So you have to have this information about the future

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Speaker 1: to calculate the now. If you like, set the moment

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Speaker 1: of nows the photon is halfway between A and B,

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Speaker 1: how does it know where it's going if you need

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Speaker 1: to know B to determine the path. So this is

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Speaker 1: an example of how those alternatives to Newton's computer can

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Speaker 1: solve the problem of the photon's path. But the path

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Speaker 1: depends on something in the future, the photon's endpoint. You

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Speaker 1: have to know where it starts and where it lands

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Speaker 1: later to know where it goes now in this alternative

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Speaker 1: to Newton's computer. Okay, so when I first read about this,

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Speaker 1: this is weird because like, yes, you know where A

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Speaker 1: and B are. But if you want to calculate what

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Speaker 1: the path of the photon is and use Fermat's principle,

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Speaker 1: then like you have to know B already has A.

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Speaker 1: Plug it into your equation, and it's like a beautiful

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Speaker 1: simple way to calculate the path. But you can't use

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Speaker 1: that to predict the path, right. You can't use that

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Speaker 1: to say, my photon started at A, tell me where

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Speaker 1: it's going to go. For Matt Fromat can't tell you

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Speaker 1: until you know B, and you don't know B until

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Speaker 1: it's already done. So even if you just knew A

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Speaker 1: and B, Like even if you just knew where the

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Speaker 1: photon left and where it landed, Firmat could tell you

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Speaker 1: what the path was without having seen it, right, just

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Speaker 1: knowing where it started and where it ended. But you

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Speaker 1: can't use Fermat's principle to derive your universe as a computer, right,

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Speaker 1: You can't use it to predict where it's going to

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Speaker 1: land before it does. Just after the fact, you could

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Speaker 1: just look back at the whole process and derive what

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Speaker 1: its path must have been.

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Speaker 2: Okay, okay, I see what you're saying now, But that

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Speaker 2: feels like you've you understand a process that happened in

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Speaker 2: the past. But that still feels different than changing the past.

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Speaker 1: Yeah, it's not change the past exactly. It's not retrocausal,

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Speaker 1: and you can't use this to send information backwards in time. Right.

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Speaker 1: The solution to the problem of where's the photon in

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Speaker 1: the middle depends on where the photon lands, But as

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Speaker 1: you say, you can't control it. It's not like I'm

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Speaker 1: changing where the photon landed to change where the photon went.

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Speaker 1: As we were saying before, the photon's path in Fermont's

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Speaker 1: minimal approach depends on the detail of the future. But

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Speaker 1: since you can't control that detail in the future that

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Speaker 1: it depends on, you can't use that to change the past.

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Speaker 1: There's no retro signaling or true retrocausality here. And this

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Speaker 1: is actually the fundamental misunderstanding at the heart of the

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Speaker 1: movie Arrival, which comes from tit Channing's short story The

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Speaker 1: Story of Your Life, which is a really fun story

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Speaker 1: and really well written, and like, I'm a big fan

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Speaker 1: of his writing. But this idea that you could like

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Speaker 1: look at the process as sort of a block universe,

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Speaker 1: you know, and think about the future and the past

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Speaker 1: and use that to solve these problems doesn't mean that

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Speaker 1: the future controls the past, right, Because in these minimal

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Speaker 1: principle approaches, like Format's approach, the solution depends on the future.

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Speaker 1: But that doesn't mean that you could control the past, right.

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Speaker 1: These things are not controllable, right. You can't change be

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Speaker 1: it's like a boundary condition in your mathematical approach to

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Speaker 1: solving this problem. But you can't use it to send

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Speaker 1: messages back in time. You can't use it to like

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Speaker 1: change where the photon went right. So, like, that's a

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Speaker 1: fundamental misunderstanding about the heart of that science fiction, which

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Speaker 1: frustrates me because I feel like, well, it's a beautiful story,

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Speaker 1: but it also gives people the wrong impression that like

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Speaker 1: part of physics is retrocausal. So therefore there could be

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Speaker 1: aliens out there that like you know, exists all through

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Speaker 1: time and can change the past and all sorts of

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Speaker 1: silly stuff. And so the crucial thing to understand here

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Speaker 1: is that the universe as a computer flowing forwards in

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Speaker 1: time is not the only way to think about physics.

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Speaker 1: These other approaches to sort of block universe, minimal principle

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Speaker 1: approaches also work, and they're fascinating, but they don't send

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Speaker 1: messages back in time, they don't create paradoxes, and in

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Speaker 1: many ways they're much more powerful. So this same approach,

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Speaker 1: generalized beyond light, can be used to solve all sorts

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Speaker 1: of problems in physics. And it's the principle of least

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Speaker 1: action that like if you said some crazy system, agog

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Speaker 1: it will go and it will follow the rules of

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Speaker 1: physics by minimizing this quantity we call action. So this

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Speaker 1: principle of least action tells you what happened in a

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Speaker 1: system over time, and it lets you solve problems that

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Speaker 1: like Newton would really struggle with. And so this is

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Speaker 1: the foundation of Lagrange and mechanics and Hamiltonian mechanics, which

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Speaker 1: is also the foundation of all of quantum field theory.

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Speaker 1: So this is actually the way we solve problems in

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Speaker 1: the universe. We don't use Newton as a computer we

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Speaker 1: use these minimal principles because they're much more powerful.

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Speaker 2: So just to make sure I understand, So we were

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Speaker 2: talking about what are the alternatives to the past determining

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Speaker 2: the future, And what we've just decided is that if

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Speaker 2: you know the past and the future, you can figure

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Speaker 2: out what happened in between by knowing what happened at.

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Speaker 1: The extremes exactly. And that's much more powerful way to

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Speaker 1: solve physics problems. It is the foundation of all of

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Speaker 1: modern physics, but does not imply retrocausality. So important thing

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Speaker 1: to take away there is that you can have physics

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Speaker 1: which depends on future conditions which are not controllable, and

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Speaker 1: so you won't get paradoxes. Got it all, right, So

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Speaker 1: that's the crucial thing. And so what does that mean

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Speaker 1: about the universe? Right? Like, is the universe a computer

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Speaker 1: or does the universe like use minimal principles to figure

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Speaker 1: out after the fact what happened, right? And so the

438
00:22:43,600 --> 00:22:46,359
Speaker 1: answer is we don't really know. And things get really

439
00:22:46,400 --> 00:22:50,199
Speaker 1: sticky once you move to the quantum version of Newton's computer.

440
00:22:50,800 --> 00:22:53,240
Speaker 2: Okay, yeah, so we haven't broken my brain yet, which

441
00:22:53,320 --> 00:22:56,439
Speaker 2: must mean we're not done. So let's go ahead and

442
00:22:56,440 --> 00:22:58,440
Speaker 2: take a break, and when we get back we'll work

443
00:22:58,480 --> 00:23:20,360
Speaker 2: on break in the brain. All right, we are back,

444
00:23:20,400 --> 00:23:24,119
Speaker 2: and we're moving to the quantum version of our explanations,

445
00:23:24,240 --> 00:23:26,840
Speaker 2: So put on your quantum hats and gear up.

446
00:23:28,160 --> 00:23:31,359
Speaker 1: Yeah. So, now let's think about whether this idea of

447
00:23:31,400 --> 00:23:36,000
Speaker 1: like Newton's computer really works to describe the universe once

448
00:23:36,040 --> 00:23:38,480
Speaker 1: we start to have the rules of quantum mechanics. Because

449
00:23:38,800 --> 00:23:41,960
Speaker 1: we know that, you know Newton's classical theory of physics,

450
00:23:42,200 --> 00:23:44,959
Speaker 1: where you have all the information about the universe and

451
00:23:45,000 --> 00:23:47,760
Speaker 1: you can predict it perfectly into the future. If you

452
00:23:47,800 --> 00:23:49,800
Speaker 1: have the laws of physics, it doesn't really work because

453
00:23:49,800 --> 00:23:52,240
Speaker 1: we don't have the information. So what happens when we

454
00:23:52,240 --> 00:23:55,600
Speaker 1: try to make a quantum version of Newton's computer? And

455
00:23:55,640 --> 00:23:57,800
Speaker 1: here I have to give a shout out to Ken Wharton,

456
00:23:57,920 --> 00:24:01,400
Speaker 1: a listener who's also a professor in foundations at San

457
00:24:01,480 --> 00:24:04,240
Speaker 1: Jose State, who send me an email after the Entanglements

458
00:24:04,280 --> 00:24:07,040
Speaker 1: episode and a really fun paper to read which outlines

459
00:24:07,080 --> 00:24:09,800
Speaker 1: a lot of these fascinating ideas. Thanks very much, Ken

460
00:24:09,840 --> 00:24:10,920
Speaker 1: for sharing your insights.

461
00:24:11,000 --> 00:24:15,040
Speaker 2: Ah. Ken was my husband's Zach's physics teacher when he

462
00:24:15,119 --> 00:24:18,359
Speaker 2: was at San Jose State. Zach got like four ninths

463
00:24:18,359 --> 00:24:22,240
Speaker 2: of a physics degree before his comic took off and

464
00:24:22,280 --> 00:24:24,320
Speaker 2: he didn't have enough time to go to school anymore.

465
00:24:24,720 --> 00:24:28,040
Speaker 2: But Zach said Ken was a fantastic physics teacher.

466
00:24:28,280 --> 00:24:29,920
Speaker 1: Yeah, so we're found. I had a great chat with

467
00:24:30,000 --> 00:24:32,240
Speaker 1: him about retro causality, which inspired a lot of the

468
00:24:32,320 --> 00:24:34,920
Speaker 1: ideas in this episode. So thanks very much, Kenn.

469
00:24:35,119 --> 00:24:38,159
Speaker 2: Awesome side note, Hi, Ken, Zach really did say that

470
00:24:38,200 --> 00:24:40,280
Speaker 2: you were a great physics teacher. We talked about it

471
00:24:40,320 --> 00:24:42,520
Speaker 2: over coffee a few days ago after you had the

472
00:24:42,880 --> 00:24:44,720
Speaker 2: chat with Daniel over Blue Sky.

473
00:24:45,119 --> 00:24:47,280
Speaker 1: All right, so there's some issues here by thinking about

474
00:24:47,280 --> 00:24:49,480
Speaker 1: the universe as a computer. If it really is quantum

475
00:24:49,480 --> 00:24:53,040
Speaker 1: mechanical number one, we don't really know like what was

476
00:24:53,080 --> 00:24:55,720
Speaker 1: the initial state of the universe, Like, we don't know

477
00:24:55,800 --> 00:24:58,280
Speaker 1: the current state of the universe. We can't measure it precisely.

478
00:24:58,320 --> 00:25:01,000
Speaker 1: We can't tell the location and direction of every particle

479
00:25:01,000 --> 00:25:03,680
Speaker 1: because of the uncertainty principle. And no matter how far

480
00:25:03,720 --> 00:25:06,560
Speaker 1: back in time that's true, and the initial state of

481
00:25:06,560 --> 00:25:09,600
Speaker 1: the universe right can be undetermined in that same way.

482
00:25:09,880 --> 00:25:13,119
Speaker 1: So there's like a fundamental fuzziness which prevents the universe

483
00:25:13,359 --> 00:25:16,639
Speaker 1: from operating as a computer in that way. And the

484
00:25:16,680 --> 00:25:20,040
Speaker 1: second is that its predictions are probabilistic. Right, It does

485
00:25:20,080 --> 00:25:22,240
Speaker 1: not tell you what's going to happen. It tells you

486
00:25:22,280 --> 00:25:25,800
Speaker 1: the probabilities of various things happening. So in that sense,

487
00:25:25,840 --> 00:25:29,159
Speaker 1: the past doesn't determine the future completely, and so in

488
00:25:29,160 --> 00:25:30,840
Speaker 1: that way it gets kind of fuzzy if you want

489
00:25:30,840 --> 00:25:34,400
Speaker 1: to hang onto Newton's idea that the past predicts the future,

490
00:25:34,760 --> 00:25:36,840
Speaker 1: because we don't know the past perfectly and it doesn't

491
00:25:36,880 --> 00:25:40,080
Speaker 1: actually predict the future in quantum mechanics.

492
00:25:39,840 --> 00:25:43,240
Speaker 2: Did I follow that the initial state's not really known? Okay?

493
00:25:43,280 --> 00:25:45,960
Speaker 2: I did follow that. Why should I care?

494
00:25:46,160 --> 00:25:48,440
Speaker 1: Yeah? You might think, well, it's academic, doesn't really matter.

495
00:25:48,520 --> 00:25:51,240
Speaker 1: It predicts it in a different way. Right, We're predicting

496
00:25:51,280 --> 00:25:55,720
Speaker 1: probabilities instead of actual outcomes. Isn't that fine? Well, it

497
00:25:55,800 --> 00:25:57,760
Speaker 1: matters in situations like the one we talked about a

498
00:25:57,760 --> 00:26:00,679
Speaker 1: few episodes ago for Entangled part of and this is

499
00:26:00,720 --> 00:26:04,479
Speaker 1: like what really troubled Einstein. So the scenario there is

500
00:26:04,520 --> 00:26:07,280
Speaker 1: you have like two particles that have been created by

501
00:26:07,280 --> 00:26:10,280
Speaker 1: some process, and so they're entangled, like they can't just

502
00:26:10,320 --> 00:26:13,320
Speaker 1: have any random state. Maybe they have to conserve angularmentum,

503
00:26:13,400 --> 00:26:14,879
Speaker 1: so one has to be spin up and one has

504
00:26:14,920 --> 00:26:16,960
Speaker 1: to be spinned down. Like if you have a spin

505
00:26:17,080 --> 00:26:19,440
Speaker 1: zero state like a photon which turns into an electron

506
00:26:19,480 --> 00:26:22,120
Speaker 1: and a positron, and those are spin one half, then

507
00:26:22,119 --> 00:26:24,120
Speaker 1: one of them has to be like spin minus one half.

508
00:26:24,119 --> 00:26:26,359
Speaker 1: One of them has to be spin plus one half.

509
00:26:26,720 --> 00:26:29,680
Speaker 1: And so that means that either your electron is spin

510
00:26:29,800 --> 00:26:32,720
Speaker 1: up and your positron to spin down, or you're electronic

511
00:26:32,760 --> 00:26:34,800
Speaker 1: spin down and your positron to spin up. They can't

512
00:26:34,800 --> 00:26:37,240
Speaker 1: be both spin up, and it can't be both spin down.

513
00:26:37,640 --> 00:26:40,160
Speaker 1: So in that sense they're entangled. The conditions you've used

514
00:26:40,160 --> 00:26:43,080
Speaker 1: to create these two particles means that there's a connection

515
00:26:43,200 --> 00:26:47,080
Speaker 1: between the states of the particles with me. So far, yep, okay,

516
00:26:47,560 --> 00:26:50,119
Speaker 1: And that doesn't seem too weird. It's like if I

517
00:26:50,160 --> 00:26:52,520
Speaker 1: have a bag and I know there's a red ball

518
00:26:52,560 --> 00:26:54,440
Speaker 1: and a blue ball, and Kelly and I each draw

519
00:26:54,560 --> 00:26:56,639
Speaker 1: a ball out of the bag, Then by looking at

520
00:26:56,680 --> 00:26:58,840
Speaker 1: my ball, I can tell oh, I have a blue ball,

521
00:26:58,880 --> 00:27:00,439
Speaker 1: Kelly must have a red one, or if I have

522
00:27:00,440 --> 00:27:02,520
Speaker 1: a red ball, Kelly must have a blue one. That's

523
00:27:02,560 --> 00:27:05,639
Speaker 1: not so weird. The weird thing is quantum mechanics says

524
00:27:06,280 --> 00:27:09,320
Speaker 1: that it's not just that I don't know which spin

525
00:27:09,400 --> 00:27:12,399
Speaker 1: my particle is, that it's not determined, right, that my

526
00:27:12,480 --> 00:27:16,479
Speaker 1: particle is in this uncollapsed state of a mixture of

527
00:27:16,760 --> 00:27:19,560
Speaker 1: spin up and spin down, and so is Kelly's. But

528
00:27:19,640 --> 00:27:23,600
Speaker 1: that weirdly, when I measure my electron and I say, oh,

529
00:27:23,680 --> 00:27:27,760
Speaker 1: mine spin up, that that changes Kelly's electron to now

530
00:27:27,840 --> 00:27:30,640
Speaker 1: be spin down because mine is spin up. So when

531
00:27:30,640 --> 00:27:33,160
Speaker 1: I make my measurement, it goes from undetermined to spin up.

532
00:27:33,480 --> 00:27:36,960
Speaker 1: Kelly's goes from undetermined to spin down. And the weird

533
00:27:36,960 --> 00:27:39,000
Speaker 1: thing is this is true even if our electrons are

534
00:27:39,040 --> 00:27:42,240
Speaker 1: really far apart, like we can move them light years apart.

535
00:27:42,440 --> 00:27:46,359
Speaker 1: And the rule here tells us that this happens instantaneously

536
00:27:46,400 --> 00:27:49,000
Speaker 1: across space and time. Right as soon as I make

537
00:27:49,040 --> 00:27:53,280
Speaker 1: my measurement, Kelly's changes from undetermined to spin down.

538
00:27:54,080 --> 00:27:57,480
Speaker 2: I remember this from our Entanglements episode.

539
00:27:57,200 --> 00:27:59,040
Speaker 1: Exactly, And you might ask, well, how do you know,

540
00:27:59,080 --> 00:28:01,000
Speaker 1: how can you prove it? How do you know that

541
00:28:01,040 --> 00:28:03,040
Speaker 1: it wasn't really just spin up and spin down the

542
00:28:03,080 --> 00:28:06,080
Speaker 1: whole time? And then the Tanglement episode goes through Bell's

543
00:28:06,160 --> 00:28:08,760
Speaker 1: experiment which proved this. It proves that there is no

544
00:28:09,000 --> 00:28:12,879
Speaker 1: local hidden information. That there's not like some feature of

545
00:28:13,000 --> 00:28:15,359
Speaker 1: mind which meant it had to be spin up the

546
00:28:15,359 --> 00:28:18,119
Speaker 1: whole time, and some feature of Kelly's which meant it

547
00:28:18,200 --> 00:28:20,359
Speaker 1: had to be spinned down the whole time. That it

548
00:28:20,480 --> 00:28:23,920
Speaker 1: really is undetermined that the universe chooses at the moment

549
00:28:24,080 --> 00:28:27,080
Speaker 1: one of us makes a measurement. So this is Bell's experiment,

550
00:28:27,160 --> 00:28:28,560
Speaker 1: and go back and dig into that if you want

551
00:28:28,600 --> 00:28:31,040
Speaker 1: more details about it. And a couple crucial things to

552
00:28:31,119 --> 00:28:33,880
Speaker 1: understand about Bell's experiment is number one, it only proves

553
00:28:33,880 --> 00:28:37,280
Speaker 1: that there's no local hidden information. So like you can

554
00:28:37,440 --> 00:28:41,160
Speaker 1: have some global pattern. There are theories like Boemian mechanics

555
00:28:41,240 --> 00:28:44,000
Speaker 1: or whatever. Let's say that it's controlled over space and time,

556
00:28:44,040 --> 00:28:47,280
Speaker 1: there's a global information. Those are more fringe theories. People

557
00:28:47,320 --> 00:28:49,920
Speaker 1: believe the universe is local, So there's that as a

558
00:28:49,960 --> 00:28:53,400
Speaker 1: caveat to Bell's experiment. Another important caveat to Bell's experiment,

559
00:28:53,480 --> 00:28:55,600
Speaker 1: and this is what ken Wharton's paper is all about,

560
00:28:56,000 --> 00:28:59,239
Speaker 1: is that Bell also assumes that there's no dependence on

561
00:28:59,280 --> 00:29:03,120
Speaker 1: the future, right, that the universe operates like a quantum computer,

562
00:29:03,720 --> 00:29:07,240
Speaker 1: that nothing that happens in the future can determine anything

563
00:29:07,240 --> 00:29:09,640
Speaker 1: that's happening in the present or in the past. And

564
00:29:09,680 --> 00:29:12,000
Speaker 1: you might think, yeah, I'm cool with that, no big deal.

565
00:29:12,040 --> 00:29:14,600
Speaker 1: Of course, things can't go into the past because that

566
00:29:14,600 --> 00:29:16,480
Speaker 1: would create paradoxes and we don't want that.

567
00:29:16,760 --> 00:29:20,040
Speaker 2: Mm hm. So you know, we put the a ball

568
00:29:20,160 --> 00:29:23,560
Speaker 2: goes in each of our bags, that's the past. At

569
00:29:23,600 --> 00:29:26,160
Speaker 2: some point one of us opens the bag that's the present,

570
00:29:26,600 --> 00:29:29,400
Speaker 2: and we would be talking about at some point in

571
00:29:29,440 --> 00:29:33,440
Speaker 2: the future, something happens that impacts both of those moments

572
00:29:33,840 --> 00:29:35,680
Speaker 2: that I just talked about. That's what we're talking about

573
00:29:35,680 --> 00:29:36,680
Speaker 2: in the future, right.

574
00:29:36,920 --> 00:29:39,600
Speaker 1: Yeah, Or to spoil it a little bit, Belle is saying,

575
00:29:39,960 --> 00:29:42,719
Speaker 1: it's not that when I make my measurement of my

576
00:29:43,120 --> 00:29:45,520
Speaker 1: ball and I juster, mind is spin up that it

577
00:29:45,560 --> 00:29:48,480
Speaker 1: goes back and changes the past. To mine was always spin.

578
00:29:48,360 --> 00:29:50,960
Speaker 2: Up, goes back and changes the past, so yours was

579
00:29:51,000 --> 00:29:53,520
Speaker 2: always spin up. Okay, yeah, this is what physics does

580
00:29:53,520 --> 00:29:55,000
Speaker 2: to me. I'm like, let's go ahead and make sure

581
00:29:55,000 --> 00:29:57,840
Speaker 2: I understand the definitions of past, present, and future.

582
00:29:58,200 --> 00:29:58,760
Speaker 6: Yeah.

583
00:29:59,160 --> 00:30:01,200
Speaker 1: And so you might be like, all right, that's cool.

584
00:30:01,320 --> 00:30:04,240
Speaker 1: Of course the future doesn't affect the past, and you

585
00:30:04,280 --> 00:30:06,160
Speaker 1: want to hang on to that. But even if you do.

586
00:30:06,200 --> 00:30:09,280
Speaker 1: There's already a problem with this experiment, which is this

587
00:30:09,440 --> 00:30:12,880
Speaker 1: instantaneous component, Right like in the version of the story

588
00:30:12,920 --> 00:30:15,760
Speaker 1: I just told you that seems weird, right I have

589
00:30:15,840 --> 00:30:19,600
Speaker 1: instantaneous collapse over time. I make my measurement and Kelly's

590
00:30:19,680 --> 00:30:24,840
Speaker 1: changes instantaneously. And that's a problem because in relativity, this

591
00:30:25,040 --> 00:30:28,240
Speaker 1: concept of simultaneity, like things happening at the same time,

592
00:30:28,760 --> 00:30:33,040
Speaker 1: is not universal. It's frame dependent, right Like, if you

593
00:30:33,120 --> 00:30:35,840
Speaker 1: have two events A and then B, I might think

594
00:30:35,880 --> 00:30:38,280
Speaker 1: that happened at the same time. But Kelly, moving at

595
00:30:38,280 --> 00:30:40,560
Speaker 1: a train at half the speed of light going past

596
00:30:40,600 --> 00:30:43,760
Speaker 1: me can see A happening before B, and Zach moving

597
00:30:43,760 --> 00:30:46,080
Speaker 1: in a train the other direction can see B happening

598
00:30:46,120 --> 00:30:50,480
Speaker 1: before A, and so relativity can scramble the orders of things.

599
00:30:50,920 --> 00:30:54,560
Speaker 1: And so already, if you're going to insist on instantaneous

600
00:30:54,600 --> 00:30:59,080
Speaker 1: collapse across time, then you're already allowing for a form

601
00:30:59,240 --> 00:31:03,840
Speaker 1: of retrocl causality. Because if I make my measurement and

602
00:31:03,880 --> 00:31:07,280
Speaker 1: then Kelly's is instantly collapsed, somebody moving a train past

603
00:31:07,360 --> 00:31:11,320
Speaker 1: us could see Kelly's being collapsed before I make my measurement.

604
00:31:11,640 --> 00:31:17,560
Speaker 2: No more trains, everyone has to stand still. This is

605
00:31:17,600 --> 00:31:18,280
Speaker 2: too confusing.

606
00:31:18,600 --> 00:31:22,320
Speaker 1: Yeah, exactly. And so the point here is that even

607
00:31:22,360 --> 00:31:25,440
Speaker 1: if you don't want to allow retrocausality, this concept of

608
00:31:25,480 --> 00:31:30,120
Speaker 1: instant collapse, when combined with special relativity, says that you're

609
00:31:30,160 --> 00:31:34,960
Speaker 1: already in dangerous territory. You're already potentially having information moving

610
00:31:35,000 --> 00:31:39,080
Speaker 1: backwards in time. Now we're saved here because all this

611
00:31:39,200 --> 00:31:41,800
Speaker 1: information is hidden. Like remember in the case of the

612
00:31:41,800 --> 00:31:45,400
Speaker 1: classical example of the minimal principles, we're not saying that

613
00:31:45,720 --> 00:31:49,720
Speaker 1: it's changing the past. By solving this over time, knowing

614
00:31:49,760 --> 00:31:52,000
Speaker 1: that the initial state and the final state and solving

615
00:31:52,000 --> 00:31:54,680
Speaker 1: for the intermediate. We're not saying that changes the past

616
00:31:54,920 --> 00:31:57,040
Speaker 1: because you can't control the final states. So there's no

617
00:31:57,040 --> 00:31:59,640
Speaker 1: way to use that to change what was happening here.

618
00:31:59,560 --> 00:32:02,360
Speaker 1: There is no signaling back in time because the thing

619
00:32:02,400 --> 00:32:04,920
Speaker 1: that might be retrocausal, the thing where the future might

620
00:32:05,000 --> 00:32:08,040
Speaker 1: affect the past, is hidden. It's the same reason why

621
00:32:08,040 --> 00:32:11,920
Speaker 1: we can't have instantaneous communication. Like when I make my

622
00:32:12,080 --> 00:32:15,840
Speaker 1: measurement of my electron, it goes from undetermined to determined,

623
00:32:16,200 --> 00:32:19,240
Speaker 1: and yours, far away in space light years away maybe

624
00:32:19,280 --> 00:32:21,880
Speaker 1: goes from undetermined to determined. But we can't use that

625
00:32:21,920 --> 00:32:25,360
Speaker 1: to send information faster than light, even if that collapse

626
00:32:25,400 --> 00:32:28,560
Speaker 1: is instantaneous. And the reason for that is that you

627
00:32:28,600 --> 00:32:31,280
Speaker 1: can't tell that I've made my measurement. Right, I make

628
00:32:31,320 --> 00:32:34,760
Speaker 1: my measurement, it goes from undetermined to determined. You're looking

629
00:32:34,800 --> 00:32:37,160
Speaker 1: at your ball. You don't know if I've made my measurement,

630
00:32:37,160 --> 00:32:38,920
Speaker 1: and there's no way for you to tell. All you

631
00:32:38,960 --> 00:32:41,320
Speaker 1: can do is measure your ball and get like, oh,

632
00:32:41,440 --> 00:32:44,240
Speaker 1: I got spinned down. I wonder if that's because Daniel

633
00:32:44,280 --> 00:32:47,240
Speaker 1: collapsed it before I measured it, or if my measurement

634
00:32:47,280 --> 00:32:49,840
Speaker 1: collapsed it. You cannot tell. So the fact that something

635
00:32:49,840 --> 00:32:52,960
Speaker 1: has happened instantaneously across space and time is hidden from you.

636
00:32:53,400 --> 00:32:55,720
Speaker 1: That's the reason why we can't use it for communicating

637
00:32:55,760 --> 00:32:58,240
Speaker 1: instantaneously across space and time. That's the thing science fiction

638
00:32:58,320 --> 00:33:01,760
Speaker 1: always gets wrong. That that's crucial, right, That's what prevents

639
00:33:01,800 --> 00:33:05,320
Speaker 1: the paradoxes and the signaling backwards in time. That's why

640
00:33:05,400 --> 00:33:09,320
Speaker 1: this isn't a fatal flaw. Why special relativity saying from

641
00:33:09,320 --> 00:33:14,080
Speaker 1: some perspectives, what's happening here is retrocausal, is things in

642
00:33:14,120 --> 00:33:17,080
Speaker 1: the future affecting things in the past. From some observer's

643
00:33:17,080 --> 00:33:21,080
Speaker 1: point of view, Kelly's status collapsed before Daniel makes his measurement.

644
00:33:21,360 --> 00:33:25,560
Speaker 1: That's not a problem because no information is going backwards

645
00:33:25,560 --> 00:33:27,800
Speaker 1: in time. It can't be used to create any paradoxes

646
00:33:27,880 --> 00:33:31,440
Speaker 1: because that information is hidden from Kelly. Even if Kelly's

647
00:33:31,440 --> 00:33:34,240
Speaker 1: present depends on Daniel's future, it's not in a way

648
00:33:34,240 --> 00:33:36,880
Speaker 1: that she can tell, and so no information can go

649
00:33:36,960 --> 00:33:40,360
Speaker 1: backwards in time. So the punchline is you can actually

650
00:33:40,400 --> 00:33:44,360
Speaker 1: have retro causality in quantum mechanics without ruining the universe.

651
00:33:44,920 --> 00:33:47,320
Speaker 2: Okay, all right, okay, so I think I get this,

652
00:33:47,480 --> 00:33:48,960
Speaker 2: So okay, So does that mean that the only way

653
00:33:49,000 --> 00:33:52,120
Speaker 2: you could get retro causality would be like, you know,

654
00:33:52,160 --> 00:33:55,880
Speaker 2: for example, we're opening our bags and we're passing each

655
00:33:55,880 --> 00:34:00,280
Speaker 2: other really quickly on like a train, and they're like momentary,

656
00:34:00,960 --> 00:34:04,480
Speaker 2: this retrocausality thing happens, but we don't realize it, but

657
00:34:04,600 --> 00:34:07,000
Speaker 2: it happened. Yeah, and that's.

658
00:34:06,800 --> 00:34:09,080
Speaker 1: It exactly, Okay, And so you could take a whole

659
00:34:09,080 --> 00:34:12,320
Speaker 1: different view of this if you allow retrocausality, because remember

660
00:34:12,640 --> 00:34:16,000
Speaker 1: Bell's experiment, which in principle proves that the collapse is

661
00:34:16,040 --> 00:34:21,600
Speaker 1: instantaneous and goes from undetermined to determined relies on an assumption.

662
00:34:21,960 --> 00:34:25,600
Speaker 1: The assumption is no dependence on the future. So Ken

663
00:34:25,640 --> 00:34:29,359
Speaker 1: Morton's amazing and hilarious and beautiful paper says, you know,

664
00:34:29,440 --> 00:34:32,840
Speaker 1: he hasn't actually ruled out retrocausal theories. Let's tell the

665
00:34:32,880 --> 00:34:36,799
Speaker 1: story another way, because he just assumed no retrocausality, right,

666
00:34:37,160 --> 00:34:40,520
Speaker 1: So what happens if you allow retrocausality.

667
00:34:39,600 --> 00:34:41,920
Speaker 2: Well, you know what I am dying to know, but

668
00:34:42,000 --> 00:34:44,200
Speaker 2: I'm going to make everybody wait. So let's find out

669
00:34:44,200 --> 00:35:08,239
Speaker 2: what happens if you add retrocausality. After the break, All right,

670
00:35:08,320 --> 00:35:10,640
Speaker 2: we're back. We're gonna talk about what happens when you

671
00:35:10,680 --> 00:35:15,440
Speaker 2: add retro causality to Bell's experiment. But first, let's remind

672
00:35:15,440 --> 00:35:18,440
Speaker 2: ourselves real quick what did Bell do in his experiment?

673
00:35:18,760 --> 00:35:20,600
Speaker 1: So Bell has this experiment where we have these two

674
00:35:20,760 --> 00:35:24,640
Speaker 1: entangled particles and they're separated across space, and you measure

675
00:35:24,680 --> 00:35:27,200
Speaker 1: one of them and collapse it, and you ask whether

676
00:35:27,280 --> 00:35:29,840
Speaker 1: the other one is collapsing right then, or whether it

677
00:35:29,920 --> 00:35:32,319
Speaker 1: was already collapsed and you just didn't know it. It's

678
00:35:32,360 --> 00:35:34,680
Speaker 1: trying to distinguish between the cases of I don't know,

679
00:35:34,719 --> 00:35:37,279
Speaker 1: but it's already determined, and I don't know because it's

680
00:35:37,320 --> 00:35:40,759
Speaker 1: not yet determined. And he did this amazing series of

681
00:35:40,880 --> 00:35:43,560
Speaker 1: experiments where you have these particles and you make different

682
00:35:43,560 --> 00:35:46,480
Speaker 1: measurements on them, and there's different random elements of those

683
00:35:46,520 --> 00:35:50,359
Speaker 1: experiments to show the quantum correlations prove that the things

684
00:35:50,400 --> 00:35:53,480
Speaker 1: are undetermined. It's a really subtle experiment, and there's no

685
00:35:53,640 --> 00:35:57,520
Speaker 1: like individual experiment that's like this. One result is a

686
00:35:57,560 --> 00:35:59,839
Speaker 1: smoking gun of quantum mechanics. It's more like a pad

687
00:36:00,040 --> 00:36:03,239
Speaker 1: learn over many iterations of the experiment proves about how

688
00:36:03,239 --> 00:36:06,560
Speaker 1: the information is stored in the system. And the takeaway

689
00:36:06,560 --> 00:36:10,160
Speaker 1: from Bell's experiment is that there are no local hidden variables.

690
00:36:10,200 --> 00:36:14,280
Speaker 1: But again, Bell assumes no dependence on the future. Right.

691
00:36:14,680 --> 00:36:17,799
Speaker 1: So the typical conclusion we draw from Bell's experiment is

692
00:36:17,880 --> 00:36:20,359
Speaker 1: what we often say about quantum mechanics, that things have

693
00:36:20,440 --> 00:36:23,640
Speaker 1: two possibilities and they can maintain those things. And when

694
00:36:23,640 --> 00:36:26,399
Speaker 1: I measure my particle, it's gone from having those two

695
00:36:26,440 --> 00:36:29,120
Speaker 1: possibilities to deciding whether it's up or down, and that

696
00:36:29,160 --> 00:36:33,000
Speaker 1: yours collapses instantaneously across space and time. Right, that's a

697
00:36:33,040 --> 00:36:36,359
Speaker 1: typical conclusion. But there's another way to view these by

698
00:36:36,400 --> 00:36:39,919
Speaker 1: allowing a little bit of retrocausality, because Bell didn't rule

699
00:36:39,920 --> 00:36:43,560
Speaker 1: that out right. Bell assumed no future dependence. But what

700
00:36:43,600 --> 00:36:45,160
Speaker 1: if we tell the different version of the story and

701
00:36:45,200 --> 00:36:48,520
Speaker 1: it goes like this, I make my measurement, and my

702
00:36:48,680 --> 00:36:52,680
Speaker 1: particle goes from undetermined too determined, and my particle goes

703
00:36:52,719 --> 00:36:56,560
Speaker 1: from undetermined to up, and that propagates backwards in time,

704
00:36:57,040 --> 00:37:00,320
Speaker 1: so that my particle was always spin up and article

705
00:37:00,560 --> 00:37:04,239
Speaker 1: was always spin down. Okay, And so that part is

706
00:37:04,280 --> 00:37:08,160
Speaker 1: going backwards in time, right, and it yields the same

707
00:37:08,160 --> 00:37:11,239
Speaker 1: results minus spin up, and yours is spin down. You

708
00:37:11,239 --> 00:37:15,399
Speaker 1: don't have this moment where like your particle collapses instantaneously

709
00:37:15,440 --> 00:37:18,200
Speaker 1: across space and time. Instead, we allow the information to

710
00:37:18,200 --> 00:37:21,799
Speaker 1: propagate backwards. And Bell's experiment again does not rule that

711
00:37:21,920 --> 00:37:24,680
Speaker 1: out because he has just assumed that this can't happen,

712
00:37:25,160 --> 00:37:27,120
Speaker 1: And so when you assume it, you close yourself off

713
00:37:27,160 --> 00:37:30,720
Speaker 1: to that conclusion. While it's understandably weird to think about

714
00:37:30,719 --> 00:37:35,200
Speaker 1: particle states depending on the future, it actually solves some

715
00:37:35,280 --> 00:37:38,080
Speaker 1: of the conceptual problems that we laid out before. In

716
00:37:38,160 --> 00:37:42,120
Speaker 1: the instantaneous collapse model, we had this bizarre frame dependence

717
00:37:42,239 --> 00:37:45,000
Speaker 1: when the order of events depends on the observer and

718
00:37:45,040 --> 00:37:48,920
Speaker 1: they're like velocity relative to the experiment. Plus we had

719
00:37:48,920 --> 00:37:52,200
Speaker 1: this instantaneous collapse across space time without any sort of

720
00:37:52,200 --> 00:37:56,560
Speaker 1: physical mediator capable of doing that. So the retrocosm model

721
00:37:56,680 --> 00:38:00,680
Speaker 1: is frame invariant because it has this local, hidden invariable

722
00:38:00,880 --> 00:38:04,359
Speaker 1: that is future dependent. So my particle was always up

723
00:38:04,400 --> 00:38:07,960
Speaker 1: and Kelly's particle was always down because they both depend

724
00:38:08,080 --> 00:38:11,160
Speaker 1: on my future measurement. So no matter what your velocity

725
00:38:11,239 --> 00:38:14,359
Speaker 1: is relative the experiment, you see the same thing. That's

726
00:38:14,400 --> 00:38:18,680
Speaker 1: pretty cool, and there's no need for unexplained transit of

727
00:38:18,760 --> 00:38:24,120
Speaker 1: information instantaneously, okay, And it also does not create paradoxes.

728
00:38:24,239 --> 00:38:27,719
Speaker 1: It's in principle information flowing backwards in time, but it's

729
00:38:27,800 --> 00:38:31,080
Speaker 1: hidden information. For the same reason that you can't use

730
00:38:31,440 --> 00:38:35,440
Speaker 1: entangled particles at a distance to send information back and forth.

731
00:38:35,760 --> 00:38:38,560
Speaker 1: In the other view of this experiment, you can't use

732
00:38:38,600 --> 00:38:41,239
Speaker 1: it here to send information backwards in time because the

733
00:38:41,280 --> 00:38:44,960
Speaker 1: information that's going backwards is hidden. Like you still can't

734
00:38:45,000 --> 00:38:48,799
Speaker 1: tell whether I've made my measurement, whether it's collapsed now,

735
00:38:48,840 --> 00:38:51,840
Speaker 1: whether it's collapsed backwards in time. You just have a particle,

736
00:38:52,040 --> 00:38:53,960
Speaker 1: you make a measurement, you get spin up, or you

737
00:38:54,000 --> 00:38:57,640
Speaker 1: get spinned down, and I should say that propagated here

738
00:38:57,760 --> 00:39:00,479
Speaker 1: is really just a figuous speech. It's not physical. There's

739
00:39:00,520 --> 00:39:04,279
Speaker 1: no sense in which my particle was undetermined earlier and

740
00:39:04,320 --> 00:39:08,399
Speaker 1: then information propagated back and later my particle was spin up.

741
00:39:08,719 --> 00:39:11,360
Speaker 1: My particle was always spin up. It's just that the

742
00:39:11,440 --> 00:39:15,280
Speaker 1: hidden information there depends on a future event. You can't

743
00:39:15,280 --> 00:39:18,439
Speaker 1: tell that that information propagated backwards in time, so there's

744
00:39:18,440 --> 00:39:20,520
Speaker 1: no way to use it to create a paradox.

745
00:39:20,920 --> 00:39:23,360
Speaker 2: This might be a silly question, but so the particles

746
00:39:23,360 --> 00:39:26,360
Speaker 2: that they were looking at, they knew they weren't I

747
00:39:26,360 --> 00:39:30,080
Speaker 2: don't know, for example, traveling quickly on trains past each other. Right,

748
00:39:30,120 --> 00:39:33,160
Speaker 2: So were they able to control for the possibility of

749
00:39:33,200 --> 00:39:37,120
Speaker 2: relativistic effects because of how they were measuring the particles

750
00:39:37,280 --> 00:39:41,640
Speaker 2: and thus control for the possibility of retrocausality or am

751
00:39:41,640 --> 00:39:42,400
Speaker 2: I missing something?

752
00:39:42,680 --> 00:39:44,960
Speaker 1: You can't control for that because you can always have

753
00:39:45,040 --> 00:39:48,239
Speaker 1: an observer moving past the experiment, Like even if they

754
00:39:48,320 --> 00:39:50,880
Speaker 1: keep the particles at rest relative to the Earth, and

755
00:39:50,920 --> 00:39:53,279
Speaker 1: all their experiments are making these measurements on the Earth,

756
00:39:53,320 --> 00:39:55,839
Speaker 1: so there's no relative velocity. You could always put zac

757
00:39:55,880 --> 00:39:58,440
Speaker 1: in a rocket ship flying past the experiment with no

758
00:39:58,520 --> 00:40:00,279
Speaker 1: change to the experiment, and he would say, see it

759
00:40:00,320 --> 00:40:04,040
Speaker 1: operating in a different order because of his velocity relative

760
00:40:04,040 --> 00:40:04,759
Speaker 1: to the experiment.

761
00:40:04,880 --> 00:40:08,239
Speaker 2: But can I be like, during this experiment, sit your

762
00:40:08,239 --> 00:40:11,920
Speaker 2: butt down and do not move because we're doing science.

763
00:40:12,239 --> 00:40:15,160
Speaker 1: Like, yeah, but it doesn't matter if nobody does it.

764
00:40:15,160 --> 00:40:17,520
Speaker 1: It just matters of whether it's possible. And it's also

765
00:40:17,680 --> 00:40:21,320
Speaker 1: always some particle out there moving past to whatever speed.

766
00:40:22,719 --> 00:40:25,320
Speaker 1: But I like your sort of school teacher approach, like, hey, everybody,

767
00:40:25,360 --> 00:40:29,680
Speaker 1: don't break the universe now, please, I'm trusting you.

768
00:40:29,680 --> 00:40:32,160
Speaker 2: You can't trust you can't trust Zach to just sit still,

769
00:40:32,200 --> 00:40:32,400
Speaker 2: you know.

770
00:40:32,800 --> 00:40:36,200
Speaker 1: Yeah, So let's again delineate the two scenarios here. The

771
00:40:36,320 --> 00:40:41,040
Speaker 1: canonical version without retrocausality says that I measure my particle

772
00:40:41,200 --> 00:40:44,160
Speaker 1: and then instantaneously across space in time, yours goes from

773
00:40:44,280 --> 00:40:47,520
Speaker 1: undetermined to determined. And we already pointed out that sort

774
00:40:47,520 --> 00:40:51,800
Speaker 1: of implies retrocausality because of the frame dependence of special

775
00:40:51,800 --> 00:40:55,200
Speaker 1: relativity and the dependence on the order of events. The

776
00:40:55,280 --> 00:40:59,440
Speaker 1: retrocausal version says, when I make my measurement of my particle,

777
00:40:59,760 --> 00:41:03,840
Speaker 1: that information propagates backwards in time and makes it so

778
00:41:03,880 --> 00:41:06,359
Speaker 1: that our particles were always the way we ended up

779
00:41:06,360 --> 00:41:10,080
Speaker 1: measuring them. So it determines that hidden information. So it

780
00:41:10,320 --> 00:41:14,399
Speaker 1: is local hidden variables, right, And you might think, hold

781
00:41:14,400 --> 00:41:16,879
Speaker 1: on a second, Bell ruled out local hidden variables. Those

782
00:41:16,920 --> 00:41:20,680
Speaker 1: are not allowed. But Bell assumed no dependence on the future.

783
00:41:21,280 --> 00:41:25,320
Speaker 1: And because this is hidden information, it does not create paradoxes.

784
00:41:25,800 --> 00:41:27,960
Speaker 1: So I think it's super fascinating because it opens your

785
00:41:27,960 --> 00:41:30,680
Speaker 1: mind up to like a really weird way for the

786
00:41:30,760 --> 00:41:35,080
Speaker 1: universe to potentially operate that's actually still consistent with experiments,

787
00:41:35,440 --> 00:41:38,319
Speaker 1: and it's just like a different interpretation of the same

788
00:41:38,440 --> 00:41:41,880
Speaker 1: data that is wild. Yeah. So now I want to

789
00:41:41,880 --> 00:41:44,960
Speaker 1: take this idea and pick apart a famous experiment that's

790
00:41:45,040 --> 00:41:49,440
Speaker 1: often used to claim that quantum mechanics does send information

791
00:41:49,520 --> 00:41:51,760
Speaker 1: into the past in a way they could be used

792
00:41:51,760 --> 00:41:54,480
Speaker 1: to signal, which is very very wrong. And it's a

793
00:41:54,480 --> 00:41:57,520
Speaker 1: fascinating experiment. It teaches us a lot about quantum mechanics,

794
00:41:57,600 --> 00:42:00,600
Speaker 1: but it's often mis explained in this same context. So

795
00:42:00,600 --> 00:42:01,520
Speaker 1: I want to take it apart.

796
00:42:01,680 --> 00:42:04,799
Speaker 2: All right. What's it called, because every great experiment has

797
00:42:04,840 --> 00:42:05,160
Speaker 2: a name.

798
00:42:07,120 --> 00:42:10,279
Speaker 1: It is very cool. It's called the quantum eraser, and

799
00:42:10,440 --> 00:42:13,200
Speaker 1: it's one of these great experiments that tries to understand

800
00:42:13,320 --> 00:42:17,239
Speaker 1: how information collapses and what an observer is. And the

801
00:42:17,280 --> 00:42:20,920
Speaker 1: idea is to instead of making your measurement directly, like

802
00:42:21,000 --> 00:42:23,120
Speaker 1: Daniel is a big classical object heuse, You're going to

803
00:42:23,160 --> 00:42:26,319
Speaker 1: use some detector which collapses the wave function. What if

804
00:42:26,320 --> 00:42:31,400
Speaker 1: we could try to extract this information without collapsing these uncertainties.

805
00:42:31,640 --> 00:42:34,480
Speaker 1: What if we could try to get the information and

806
00:42:34,560 --> 00:42:37,759
Speaker 1: maintain the quantum uncertainties Because in quantum mechanics you have

807
00:42:37,800 --> 00:42:43,160
Speaker 1: this distinction between classical objects which collapse things from superpositions

808
00:42:43,400 --> 00:42:46,800
Speaker 1: into a single choice, and quantum interactions which don't collapse.

809
00:42:46,840 --> 00:42:49,400
Speaker 1: Things like if I have my electron and it's spin up,

810
00:42:49,600 --> 00:42:51,920
Speaker 1: I can send another electron to interact with it, and

811
00:42:51,960 --> 00:42:56,000
Speaker 1: that whole system can remain in a superposition of possible states.

812
00:42:56,000 --> 00:42:59,680
Speaker 1: It doesn't have to collapse. Not every interaction leads to

813
00:42:59,719 --> 00:43:03,719
Speaker 1: a lapse of superposition into a single choice. Only when

814
00:43:03,719 --> 00:43:06,440
Speaker 1: you interact with the classical object things which cannot be

815
00:43:06,480 --> 00:43:09,520
Speaker 1: in a superposition like me or a detector or my

816
00:43:09,640 --> 00:43:13,719
Speaker 1: eyeball big classical things. So the idea is like, let's

817
00:43:13,719 --> 00:43:16,440
Speaker 1: do the same experiment where you have two particles and

818
00:43:16,480 --> 00:43:19,000
Speaker 1: they're entangled, so you know one of them went left

819
00:43:19,000 --> 00:43:20,120
Speaker 1: and one of them went right, or one of them

820
00:43:20,120 --> 00:43:21,640
Speaker 1: is to spin up, one of them is to spin down,

821
00:43:22,120 --> 00:43:26,320
Speaker 1: And instead of interacting them with like some big detector

822
00:43:26,360 --> 00:43:30,520
Speaker 1: which forces them to choose, instead entangle them with another particle,

823
00:43:30,680 --> 00:43:34,480
Speaker 1: another quantum object, so you have no collapse, right, And

824
00:43:34,520 --> 00:43:37,240
Speaker 1: so this is typically done in like the double slit experiment.

825
00:43:37,480 --> 00:43:39,520
Speaker 1: In the double slit experiment, you have like electron that's

826
00:43:39,520 --> 00:43:41,719
Speaker 1: shot at two slits. You don't know if it goes

827
00:43:41,760 --> 00:43:44,000
Speaker 1: through the left slit or the right slit, and so

828
00:43:44,080 --> 00:43:47,040
Speaker 1: that creates an interference pattern because the probability for it

829
00:43:47,080 --> 00:43:48,960
Speaker 1: to go through the left interferes with the probability for

830
00:43:49,000 --> 00:43:50,480
Speaker 1: it to go to the right, and you get this

831
00:43:50,600 --> 00:43:53,560
Speaker 1: interference pattern. But if you put a detector on it

832
00:43:53,640 --> 00:43:56,799
Speaker 1: to measure which way went through, the interference pattern disappears.

833
00:43:56,920 --> 00:44:00,000
Speaker 1: That's the classic double slit experiment. So in this case

834
00:44:00,239 --> 00:44:04,200
Speaker 1: we replace the detector with a quantum detector, some electron

835
00:44:04,320 --> 00:44:07,560
Speaker 1: or something which interacts with a particle in the experiment,

836
00:44:07,719 --> 00:44:10,480
Speaker 1: but not in a way that collapses its wave function. Right.

837
00:44:10,560 --> 00:44:13,239
Speaker 1: It stores the information about whether the particle went through

838
00:44:13,239 --> 00:44:15,040
Speaker 1: the left slit or the right slit, but in a

839
00:44:15,160 --> 00:44:17,680
Speaker 1: quantum object, not in a classical detector.

840
00:44:17,960 --> 00:44:19,839
Speaker 2: Is this a thought experiment or an actual thing?

841
00:44:20,000 --> 00:44:22,240
Speaker 1: This is an actual experiment you can do. Oh okay,

842
00:44:22,920 --> 00:44:26,520
Speaker 1: so now you in principle the information is encoded in

843
00:44:26,560 --> 00:44:29,200
Speaker 1: this other electron right, and it hasn't collapsed yet, so

844
00:44:29,239 --> 00:44:32,960
Speaker 1: you've preserved the uncertainty while encoding the information so you

845
00:44:32,960 --> 00:44:35,719
Speaker 1: can find it later. So the way this story is

846
00:44:35,760 --> 00:44:39,600
Speaker 1: typically mistold is the following, and again this is incorrect.

847
00:44:40,000 --> 00:44:42,320
Speaker 1: It says that the particle continues through the double slits

848
00:44:42,360 --> 00:44:44,360
Speaker 1: and you get the interference, and that you can go

849
00:44:44,560 --> 00:44:48,960
Speaker 1: later to measure the information, the quantum information you've stored

850
00:44:48,960 --> 00:44:50,920
Speaker 1: in this electron. You can then go make a measurement

851
00:44:50,960 --> 00:44:54,200
Speaker 1: and to figure out which slit it went through, after

852
00:44:54,600 --> 00:44:57,319
Speaker 1: the particle has already hit the screen, right, so you're like, oh,

853
00:44:57,320 --> 00:44:59,239
Speaker 1: it's too late for it to change. Now I'm going

854
00:44:59,320 --> 00:45:00,800
Speaker 1: to go figure out what which you want it went through.

855
00:45:01,160 --> 00:45:04,080
Speaker 1: And the incorrect version, which you see all over popular science,

856
00:45:04,120 --> 00:45:06,799
Speaker 1: is that this goes back and changes what happens on

857
00:45:06,840 --> 00:45:09,560
Speaker 1: the screen. That like, you have an interference pattern on

858
00:45:09,560 --> 00:45:11,480
Speaker 1: the screen, and then you go and you measure it,

859
00:45:11,600 --> 00:45:15,800
Speaker 1: and the interference pattern disappears to be clear. This would

860
00:45:15,840 --> 00:45:19,640
Speaker 1: be retro signaling if it actually happened, but it doesn't.

861
00:45:20,120 --> 00:45:23,120
Speaker 1: So the incorrect version of the story is that you

862
00:45:23,200 --> 00:45:27,120
Speaker 1: take an action in the future, well after the particles

863
00:45:27,120 --> 00:45:30,000
Speaker 1: have already hit the screen and made an interference pattern,

864
00:45:30,320 --> 00:45:33,759
Speaker 1: maybe a year later, you decide you want to know

865
00:45:34,000 --> 00:45:37,000
Speaker 1: whether they went left or right, so then you access

866
00:45:37,080 --> 00:45:41,040
Speaker 1: the stored information in that quantum detector. And the incorrect

867
00:45:41,080 --> 00:45:44,520
Speaker 1: story is that the collapse then happens backwards in time,

868
00:45:45,320 --> 00:45:48,120
Speaker 1: going back to change the pattern on the screen from

869
00:45:48,120 --> 00:45:52,840
Speaker 1: a year ago from interference to no interference, which is like, what,

870
00:45:53,120 --> 00:45:56,000
Speaker 1: that's crazy and spooky. That's because that's not what happens.

871
00:45:56,760 --> 00:46:00,040
Speaker 1: That's literally just not the outcome of that experiment. It

872
00:46:00,040 --> 00:46:02,480
Speaker 1: would be really fascinating if it were, but it's not.

873
00:46:03,000 --> 00:46:05,200
Speaker 1: What happens when you do this experiment is when you

874
00:46:05,280 --> 00:46:08,000
Speaker 1: use the electron to figure out which slid it went through,

875
00:46:08,120 --> 00:46:13,040
Speaker 1: the interference pattern disappears already, to be very clear. Once

876
00:46:13,120 --> 00:46:17,600
Speaker 1: you add the quantum detector, the interference pattern disappears, not

877
00:46:17,760 --> 00:46:20,440
Speaker 1: when you open that detector a year later, but as

878
00:46:20,480 --> 00:46:22,920
Speaker 1: soon as you add it to the experiment. Whether or

879
00:46:22,960 --> 00:46:25,680
Speaker 1: not you look at the information. If you have the

880
00:46:25,760 --> 00:46:30,000
Speaker 1: quantum detector, you never see the interference pattern, whether or

881
00:46:30,080 --> 00:46:33,280
Speaker 1: not you access the stored information in the quantum detector,

882
00:46:33,719 --> 00:46:36,680
Speaker 1: because you've measured that information, you've encoded it in an

883
00:46:36,680 --> 00:46:40,040
Speaker 1: electron instead of some big classical detector. But the interference

884
00:46:40,040 --> 00:46:42,799
Speaker 1: pattern does disappear on the screen. You do not see

885
00:46:42,800 --> 00:46:47,440
Speaker 1: an interference pattern because that information has already been extracted

886
00:46:47,440 --> 00:46:48,960
Speaker 1: from the experiment.

887
00:46:48,520 --> 00:46:51,279
Speaker 2: Which there was an interference pattern, and then you look

888
00:46:51,320 --> 00:46:53,040
Speaker 2: at the electrons and it goes away.

889
00:46:53,160 --> 00:46:55,200
Speaker 1: There is an interference pattern on the screen. Then you

890
00:46:55,239 --> 00:46:58,880
Speaker 1: add the electron detector and the interference pattern goes away.

891
00:46:58,920 --> 00:47:01,080
Speaker 1: You don't have to look at the electron for the

892
00:47:01,080 --> 00:47:02,520
Speaker 1: interference pattern to go away.

893
00:47:02,800 --> 00:47:05,840
Speaker 2: Oh, just having the electron detector makes it go away,

894
00:47:05,880 --> 00:47:07,960
Speaker 2: even if you don't look at the response in the

895
00:47:08,000 --> 00:47:13,560
Speaker 2: electron detector exactly. So having a classical object add an

896
00:47:13,600 --> 00:47:18,440
Speaker 2: electron detector is all it takes. Even if the electron

897
00:47:18,480 --> 00:47:22,040
Speaker 2: detector is a quantum detector.

898
00:47:21,880 --> 00:47:25,359
Speaker 1: That's right. Even if it's a quantum detector, it still

899
00:47:25,400 --> 00:47:28,920
Speaker 1: collapses the interference pattern. What's cool is that you can

900
00:47:28,960 --> 00:47:33,279
Speaker 1: then erase that information right by, like you know, scrambling

901
00:47:33,320 --> 00:47:36,640
Speaker 1: it somehow, or you can go back and you can

902
00:47:36,880 --> 00:47:40,560
Speaker 1: sort of recover the interference pattern after the fact. You

903
00:47:40,560 --> 00:47:44,239
Speaker 1: can use the information in those electrons to pull out

904
00:47:44,280 --> 00:47:46,360
Speaker 1: an interference pattern after the fact, but it's not like

905
00:47:46,360 --> 00:47:49,719
Speaker 1: an interference pattern appears on the screen. You can use

906
00:47:49,760 --> 00:47:53,960
Speaker 1: the information in your quantum detector to separate the particles

907
00:47:54,000 --> 00:47:56,160
Speaker 1: on the screen into two groups, and if you pull

908
00:47:56,200 --> 00:47:59,240
Speaker 1: those apart, each of those will have an interference pattern

909
00:47:59,320 --> 00:48:03,160
Speaker 1: in them. So there's no like retrocausality here. It's a

910
00:48:03,200 --> 00:48:07,040
Speaker 1: fascinating experiment because it shows like how quantum information propagates

911
00:48:07,080 --> 00:48:11,040
Speaker 1: through the experiment, but in the way it's typically told that, like,

912
00:48:11,360 --> 00:48:14,680
Speaker 1: by using a quantum detector instead of a classical detector,

913
00:48:15,200 --> 00:48:18,440
Speaker 1: you are sending information back in time because if you

914
00:48:18,440 --> 00:48:20,799
Speaker 1: look at the outcome of your quantum detector later, the

915
00:48:20,840 --> 00:48:25,040
Speaker 1: interference pattern disappears. That's not true. The interference pattern disappears

916
00:48:25,080 --> 00:48:27,760
Speaker 1: as soon as you inject the detector into the system.

917
00:48:27,840 --> 00:48:29,520
Speaker 1: Quantum or classical.

918
00:48:29,800 --> 00:48:32,400
Speaker 2: Come on, man, this stuff is crazy.

919
00:48:32,640 --> 00:48:36,560
Speaker 1: Yeah, yeah, exactly, And so this stuff is weird. But

920
00:48:36,600 --> 00:48:39,640
Speaker 1: there's a retrocausal interpretation of this. The idea is that

921
00:48:39,680 --> 00:48:42,280
Speaker 1: you make a choice whether to look at this quantum

922
00:48:42,360 --> 00:48:45,840
Speaker 1: detector or not whether to collapse the quantum detector's information,

923
00:48:46,560 --> 00:48:49,560
Speaker 1: and that sends information back in time to determine what

924
00:48:49,719 --> 00:48:53,600
Speaker 1: happened in this experiment. But again it's hidden information. It's

925
00:48:53,640 --> 00:48:56,880
Speaker 1: not information you can use to send messages or to

926
00:48:56,920 --> 00:48:59,959
Speaker 1: do anything else. And you know, there's another famous exam

927
00:49:00,080 --> 00:49:02,239
Speaker 1: we don't have time to go through, which is called

928
00:49:02,280 --> 00:49:06,000
Speaker 1: the delayed choice version of the double slit experiment, which

929
00:49:06,080 --> 00:49:08,440
Speaker 1: is when you move the detector really close to the screen,

930
00:49:08,920 --> 00:49:11,760
Speaker 1: and so basically you're deciding whether to detect the particle

931
00:49:12,080 --> 00:49:14,600
Speaker 1: just before it hit the screen, well after it had

932
00:49:14,640 --> 00:49:17,359
Speaker 1: to decide which slit to go through, or whether it're

933
00:49:17,360 --> 00:49:20,000
Speaker 1: making an interference pattern or not. It's a really cool experiment.

934
00:49:20,080 --> 00:49:22,520
Speaker 1: But in both cases you can interpret these experiments in

935
00:49:22,520 --> 00:49:27,439
Speaker 1: a retrocausal way by saying that information propagates backwards in time,

936
00:49:27,480 --> 00:49:30,960
Speaker 1: but again only to control a local hidden variable. So basically,

937
00:49:31,000 --> 00:49:36,359
Speaker 1: retro causality allows you to reintroduce local hidden variables in

938
00:49:36,440 --> 00:49:40,320
Speaker 1: our experiments by allowing information to propagate backwards in time,

939
00:49:40,560 --> 00:49:43,560
Speaker 1: which is kind of uncomfortable and kind of weird because

940
00:49:43,560 --> 00:49:46,600
Speaker 1: it violates our sense that like the universe should flow

941
00:49:46,680 --> 00:49:50,080
Speaker 1: from the past into the future, you know, and that

942
00:49:50,400 --> 00:49:52,720
Speaker 1: we like to think about the universe as a computer,

943
00:49:53,120 --> 00:49:55,840
Speaker 1: that it's like calculating things on the fly. But we

944
00:49:55,880 --> 00:49:57,880
Speaker 1: don't know how the universe works, and it's certainly not

945
00:49:58,000 --> 00:50:01,200
Speaker 1: restricted to doing something that makes no.

946
00:50:01,520 --> 00:50:04,080
Speaker 2: It's not. No, it's not.

947
00:50:04,760 --> 00:50:07,120
Speaker 1: And you know, another question you might ask is like, well,

948
00:50:07,480 --> 00:50:11,200
Speaker 1: could we tell the difference between the one scenario where

949
00:50:11,200 --> 00:50:13,920
Speaker 1: like things really are undetermined until you measure them and

950
00:50:13,960 --> 00:50:17,479
Speaker 1: then it collapse instantaneously across space and the retrocausal version,

951
00:50:17,480 --> 00:50:21,280
Speaker 1: where like things propagate back in time. And I asked

952
00:50:21,280 --> 00:50:24,680
Speaker 1: this question to Ken Wharton, the Zach's physics professor who's

953
00:50:24,719 --> 00:50:28,000
Speaker 1: a philosopher of quantum foundations, and he said, maybe there

954
00:50:28,040 --> 00:50:30,720
Speaker 1: will never be an experiment. And you know, one issue

955
00:50:30,800 --> 00:50:32,960
Speaker 1: is that we don't have like a full theory of

956
00:50:33,040 --> 00:50:36,839
Speaker 1: retro causality. Ken's paper, for example, just points out that

957
00:50:37,040 --> 00:50:39,359
Speaker 1: this is allowed in theories. It doesn't have a full

958
00:50:39,440 --> 00:50:42,480
Speaker 1: theory that incorporates all of this and predicts everything. And

959
00:50:42,520 --> 00:50:44,960
Speaker 1: the problem is that all the experiments that we could

960
00:50:44,960 --> 00:50:48,000
Speaker 1: set up to test this idea would also allow for

961
00:50:48,200 --> 00:50:51,359
Speaker 1: signaling backwards in time, which would create paradoxes. So those

962
00:50:51,360 --> 00:50:52,680
Speaker 1: things definitely don't.

963
00:50:52,440 --> 00:50:54,920
Speaker 2: Work, but I thought they wouldn't allow for signaling back

964
00:50:54,960 --> 00:50:55,319
Speaker 2: in time.

965
00:50:55,520 --> 00:50:58,120
Speaker 1: Yeah, exactly. So the experiments we set up trying to

966
00:50:58,160 --> 00:51:02,080
Speaker 1: test this would also test or retro signaling, which we

967
00:51:02,120 --> 00:51:04,239
Speaker 1: already know can't work. So we don't know how to

968
00:51:04,280 --> 00:51:09,000
Speaker 1: test for this. Just retrocausality without retro signaling, got it. Okay,

969
00:51:09,120 --> 00:51:11,840
Speaker 1: So yeah, we don't know how to test for retrocausality.

970
00:51:11,920 --> 00:51:14,040
Speaker 1: But the good news is that if we take a

971
00:51:14,080 --> 00:51:18,080
Speaker 1: retrocausal formulation of quantum mechanics, it might make it easier

972
00:51:18,120 --> 00:51:21,600
Speaker 1: to solve the big open problem of quantum gravity. One

973
00:51:21,640 --> 00:51:24,640
Speaker 1: of the big sticking points there is that general relativity

974
00:51:24,680 --> 00:51:27,680
Speaker 1: demands to know where things are at all times, where

975
00:51:27,719 --> 00:51:30,440
Speaker 1: are the masses in space so we can decide how

976
00:51:30,520 --> 00:51:34,120
Speaker 1: much it's curving, And traditional quant mechanics says you can't

977
00:51:34,160 --> 00:51:38,680
Speaker 1: know that some things are undetermined, But retrocausal quantum mechanics

978
00:51:38,719 --> 00:51:42,040
Speaker 1: says that there are local, hidden variables that do determine

979
00:51:42,080 --> 00:51:45,239
Speaker 1: where the particles are. So if those can be made

980
00:51:45,320 --> 00:51:49,080
Speaker 1: like covariant, then you can marry that with ordinary general

981
00:51:49,120 --> 00:51:52,920
Speaker 1: relativity space time and maybe make some progress on quantum gravity.

982
00:51:53,680 --> 00:51:55,279
Speaker 1: And so you know. The headline is that a lot

983
00:51:55,360 --> 00:51:58,880
Speaker 1: of the popular science quantum mechanical articles about how quantum

984
00:51:58,840 --> 00:52:02,520
Speaker 1: mechanics changes the past are wrong because they imply that

985
00:52:02,560 --> 00:52:05,479
Speaker 1: you could send information back in the past, which could

986
00:52:05,480 --> 00:52:09,879
Speaker 1: create paradoxes. The more interesting but nuanced bit is that

987
00:52:10,320 --> 00:52:13,319
Speaker 1: some interpretations of quantum mechanics are consistent with a form

988
00:52:13,360 --> 00:52:17,160
Speaker 1: of retrocausality which changes local hidden variables in the past

989
00:52:17,200 --> 00:52:19,600
Speaker 1: in a way that does not allow for information to

990
00:52:19,640 --> 00:52:22,920
Speaker 1: be propagated into the past to create paradoxes, but is

991
00:52:22,960 --> 00:52:26,160
Speaker 1: a fascinating insight into how the universe works, whether it

992
00:52:26,160 --> 00:52:29,279
Speaker 1: really is a computer, whether it follows these minimal principles,

993
00:52:29,440 --> 00:52:31,359
Speaker 1: you know, whether you should think of the universe as

994
00:52:31,400 --> 00:52:33,319
Speaker 1: like a block. Maybe the universe is not figuring it

995
00:52:33,360 --> 00:52:36,240
Speaker 1: out as it goes. It's just like one big physics problem,

996
00:52:36,239 --> 00:52:38,760
Speaker 1: and some physics major at the end of the universe

997
00:52:38,880 --> 00:52:41,360
Speaker 1: is like going to solve for the whole thing given

998
00:52:41,360 --> 00:52:42,719
Speaker 1: the initial and the final state.

999
00:52:42,920 --> 00:52:44,720
Speaker 2: Oh god, I hope it doesn't mix up the pluses

1000
00:52:44,719 --> 00:52:45,400
Speaker 2: and the minuses.

1001
00:52:47,120 --> 00:52:49,400
Speaker 1: Physics major is not famously good.

1002
00:52:49,520 --> 00:52:50,440
Speaker 2: Yeah exactly.

1003
00:52:54,040 --> 00:52:56,400
Speaker 1: Yeah, all right, So thank you for taking this journey

1004
00:52:56,400 --> 00:53:00,399
Speaker 1: with us. Forwards and backwards in understanding. It turns out

1005
00:53:00,400 --> 00:53:03,520
Speaker 1: the universe is far weirder than we imagine, and maybe

1006
00:53:03,600 --> 00:53:05,640
Speaker 1: far weirder and than we could ever understand.

1007
00:53:06,000 --> 00:53:09,840
Speaker 2: Well, that was trippy and fun. Thanks Daniel. Until next time, extraordinaries,

1008
00:53:09,920 --> 00:53:19,400
Speaker 2: have a good one. Daniel and Kelly's Extraordinary Universe is

1009
00:53:19,400 --> 00:53:22,440
Speaker 2: produced by iHeartRadio. We would love to hear from you,

1010
00:53:22,680 --> 00:53:23,720
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1011
00:53:23,880 --> 00:53:26,600
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1012
00:53:26,840 --> 00:53:28,480
Speaker 1: Extraordinary Universe.

1013
00:53:28,560 --> 00:53:31,520
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1014
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1015
00:53:34,560 --> 00:53:34,960
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1016
00:53:35,239 --> 00:53:38,759
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1017
00:53:38,800 --> 00:53:41,920
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1018
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1019
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1020
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1021
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1022
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