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Speaker 1: Humans have been working on the mysteries of the universe

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Speaker 1: for quite a long time now, and you know, we've

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Speaker 1: made some pretty good progress. But about one hundred years

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Speaker 1: ago we kind of got stuck. We have two big

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Speaker 1: theories of physics, general relativity that describes space and time

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Speaker 1: and how stuff moves, and quantum mechanics that describes probabilities

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Speaker 1: and how tiny particles behave when you're not looking. Bringing

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Speaker 1: these two ideas together into one harmonious explanation of everything

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Speaker 1: has been a real puzzle that it's stumped some of

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Speaker 1: the greatest thinkers in history. We have some famous ideas

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Speaker 1: that try to tackle it, string theory, loop quantum gravity.

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Speaker 1: Then there's some fringe ideas that very few people take

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Speaker 1: very seriously from Wolfram or Weinstein. But what if everybody's

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Speaker 1: taking the wrong approach. What if instead of attacking the problem,

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Speaker 1: the right strategy is to actually sidestep it and reframe

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Speaker 1: the question. That's what we'll do today when we talk

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Speaker 1: about post quantum gravity. Welcome to Daniel and Kelly's Extraordinary Universe.

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Speaker 2: Hello, I'm Kelly Wiener Smith. I'm a parasitologist, and today

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Speaker 2: we are going way out of my comfort zone. To

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Speaker 2: discuss post quantum gravity.

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Speaker 1: Hi, I'm Daniel. I'm a particle physicist, and I don't

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Speaker 1: even understand pre quantum gravity, not to mention a post

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Speaker 1: quantum gravity.

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Speaker 2: You know, Daniel. One thing I appreciate about my field

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Speaker 2: of study that I don't think I really appreciated enough

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Speaker 2: before this conversation that we had is how great it

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Speaker 2: is that I know where fish are and I can

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Speaker 2: count them.

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Speaker 1: That's because you've never met a post quantum fish yet, Kelly.

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Speaker 1: They're very unpredictable.

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Speaker 2: Uh, you know, No, I think that the fish are

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Speaker 2: going to be way more predictable. And Zachowy likes to

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Speaker 2: Joe that Kelly's job is to figure out what the

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Speaker 2: fish are up to, and I think that's way easier

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Speaker 2: than figuring out what the electrodes are up to. So

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Speaker 2: what kind of personality type do you think ends up

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Speaker 2: in both of these fields?

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Speaker 1: Hmmm, Yeah, that's a great question. You know, I think

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Speaker 1: people who like to deal with the tiniest bits of

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Speaker 1: the universe or people have to give up the concreteness

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Speaker 1: of being able to see what you're working on, you know,

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Speaker 1: being able to like look at your experiment and say, oh,

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Speaker 1: I see what it's doing, or I can take pictures

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Speaker 1: of it at least. So this is like leap into

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Speaker 1: the abstract realm where you just have this like mathematical scaffolding.

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Speaker 1: You have to just sort of trust and you've got

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Speaker 1: to be sort of into that, you know, mental puzzles

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Speaker 1: and mathematical mazes. But it's often unsatisfactory, right, Like we

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Speaker 1: build these huge machines, we collide these particles. We can't

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Speaker 1: even really look at what's happening at their core.

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Speaker 2: So I agree with you and I disagree. So like

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Speaker 2: I study fish, but what I'm really interested are the

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Speaker 2: parasites that are inside of them that I can't see

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Speaker 2: and that I have to like try to study in

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Speaker 2: indirect ways. Maybe I split the difference. Still, I guess

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Speaker 2: at the end of the day, I humanely euthanize those

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Speaker 2: fish and open them up to see the parasites, which

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Speaker 2: is much easier than seeing the particles.

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Speaker 1: I have this fantasy sometimes about solving most of science

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Speaker 1: just by having like perfect visualization. Like if you could

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Speaker 1: watch anything anywhere in the universe at any time, you

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Speaker 1: could like zoom in arbitrarily and just look at stuff,

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Speaker 1: you would understand what happens, like how do viruses kill

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Speaker 1: a bacteria? We'll just watch it, you know. But most

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Speaker 1: of our science is so limited by not having the instrumentation,

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Speaker 1: but not being able to see what we want to see,

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Speaker 1: and having to have all these indirect probes growing bacteria,

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Speaker 1: seeing if they die, speculating about the mechanisms. It'd be

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Speaker 1: amazing to be able to just like X ray the

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Speaker 1: whole universe and know what's happening, because then the explanations

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Speaker 1: I feel like they would just fall out.

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Speaker 2: Well, so to be the wet blanket we've all come

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Speaker 2: to know and love, hopefully know and love. When we

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Speaker 2: were doing the Human Genome project, we felt like as

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Speaker 2: soon as we could like get down to that level,

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Speaker 2: suddenly we'd be able to like solve all cancer. And

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Speaker 2: now we're working on the Human Connectome project, where we're

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Speaker 2: going to like know about all the connections in our brains,

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Speaker 2: like all of our neurons connect. But I don't feel

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Speaker 2: like immediately being able to see things at a higher

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Speaker 2: level gives us the answer. And I guess you're saying,

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Speaker 2: once we can see everything, I still feel like there

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Speaker 2: would be decades of us trying to be like, well,

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Speaker 2: what the heck does it all mean?

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Speaker 1: I just need one more level of zoom, bro, just

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Speaker 1: one more level of zoom, and I'll figure it all out,

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Speaker 1: all right.

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Speaker 2: Well, the biologists have been saying that for a long time,

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Speaker 2: and so far we're discovering it just makes things more complicated.

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Speaker 2: But I hope you're right.

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Speaker 3: All right.

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Speaker 1: Well, a big group of physicists have been zooming in

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Speaker 1: on the fundamental nature of the universe, trying to understand

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Speaker 1: how it all works, what is the universe really made

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Speaker 1: out of it as smallest scale, what is the bedrock

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Speaker 1: foundation of the universe itself? And they've all run into

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Speaker 1: a problem trying to understand how gravity and quantum mechanics

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Speaker 1: come together. So we've had a few episodes about various

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Speaker 1: solutions to one of the biggest puzzles in modern physics,

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Speaker 1: quantum gravity. And today we have another episode talking about

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Speaker 1: the theory of post quantum gravity, which is an intriguingly

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Speaker 1: compact name for.

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Speaker 2: A Yeah, and you know, I've really been enjoying looking

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Speaker 2: at this problem of quantum mechanics in general relativity and

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Speaker 2: how we make them work together from a couple different angles,

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Speaker 2: And you know, it's exciting to get to hear yet

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Speaker 2: another angle today.

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Speaker 1: Soon we'll be interviewing fish about it, right, to see

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Speaker 1: what is the fish perspective on quantum gravity.

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Speaker 2: I mean, I hope, but you know, as an experimentalist,

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Speaker 2: I love that at the end there are actually experiments

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Speaker 2: that can be tested to figure out if this one

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Speaker 2: is correct or not. So everyone's just going to have

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Speaker 2: to hold their breath, that's right.

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Speaker 1: And so at the beginning, as usual, we ask people

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Speaker 1: what they think about post quantum gravity or if they

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Speaker 1: even know what it is. Thank you very much to

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Speaker 1: everybody who sends in their thoughts. If you'd like to

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Speaker 1: be part of this crew, just email us to questions

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Speaker 1: at danielon Kelly dot org. You can play along at home.

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Speaker 1: It's very easy and fun. So think about it for

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Speaker 1: a minute. What do you think the theory of post

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Speaker 1: quantum gravity is. Here's what our listeners had to say.

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Speaker 4: Post quantum gravity, I would guess, is the products of

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Speaker 4: finding out theory that explains quantum gravity. So what the

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Speaker 4: benefits of such a theory would be.

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Speaker 5: The name suggests that there is two types of gravity,

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Speaker 5: one that does before quantum effects and one that does

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Speaker 5: after quantum effects, and post quantum gravity would be that

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Speaker 5: it only appears if quantum effects are done with their work.

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Speaker 2: I would guess that the gravity that happened after the

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Speaker 2: Big Bang turned everything from actual energy into matter.

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Speaker 6: When you spill your cereal in the morning for your coffee.

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Speaker 4: The reality inside a black hole where the quantum realm

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Speaker 4: is affected by gravity at its most extreme.

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Speaker 7: Gravity that happens when particles drink their milk and grow

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Speaker 7: up from being quantum particles to regular sized particles, and

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Speaker 7: then they have regular size gravity. Pop quiz Tricia, what

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Speaker 7: is post quantum gravity gravity?

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Speaker 2: After children?

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Speaker 1: I think that's postpartum.

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Speaker 8: Post quantum gravity is like post punk rock, the notion

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Speaker 8: of that the idea is not working.

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Speaker 3: We should just move on.

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Speaker 2: I thought that gravity was basically irrelevant at the quantum level,

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Speaker 2: so I can't say I have any idea.

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Speaker 1: I would say the new physics that we find after

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Speaker 1: we find quantum gravity.

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Speaker 6: Post quantum gravity is the conceptualization of what we think

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Speaker 6: of is gravity, taking into account that neither general relativity

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Speaker 6: nor quantum mechanics completely explains the phenomenon that we experience.

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Speaker 3: Maybe post quantum gravity is the state of society after

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Speaker 3: we figure out what quantum gravity is, and we're bored

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Speaker 3: because we don't have any more problems to solve.

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Speaker 1: David Letterman's theory of gravity. What you start looking for

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Speaker 1: when you give up on the idea of quantum gravity.

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Speaker 9: I believe it is a term that refers to the

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Speaker 9: facts of gravity beyond this scale of particles, maybe a

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Speaker 9: theory that tries to explain gravity in an ever smaller scale.

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Speaker 10: Post quantum gravity is a theory of gravity that finally

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Speaker 10: reveals that gravity actually does not obey quantum rules.

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Speaker 8: I think post quantum gravity might be the effects of

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Speaker 8: gravity at quantum scales, down at the particle level, where

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Speaker 8: it's got to fight it out with all the other

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Speaker 8: forces down there.

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Speaker 1: All right, Kelly, did these responses align with your thoughts

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Speaker 1: of post quantum gravity.

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Speaker 2: That we were all apparently comparably confused? Yes, I feel

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Speaker 2: like we have a particularly intelligent audience. It made me

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Speaker 2: feel better that I did not know what post quantum

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Speaker 2: gravity was when I heard these answers.

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Speaker 1: So well. It's sort of a niche idea in a

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Speaker 1: niche field of quantum gravity, but it's one that a

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Speaker 1: listener wrote into me and said, Hey, can you explain

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Speaker 1: what this is? I was trying to read an article

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Speaker 1: about it and I couldn't understand it. So as an

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Speaker 1: invitation to everybody out there, if you're reading about some

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Speaker 1: niche theory in physics and it doesn't make sense to you,

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Speaker 1: please write to us. We will break it down for you.

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Speaker 1: And we're lucky enough on this episode to have as

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Speaker 1: a guest the guy who then did post quantum gravity

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Speaker 1: and does a pretty solid job of explaining it.

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Speaker 2: Totally solid. Yes, it's so nice when you can have

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Speaker 2: the experts come in to tell you what's up.

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Speaker 1: And he's got a beautiful voice as well. So here's

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Speaker 1: our interview with Jonathan Oppenheim. So it's my pleasure to

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Speaker 1: welcome to the podcast professor Jonathan Oppenheim, a physics professor

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Speaker 1: at University College London and proponent of the theory of

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Speaker 1: post quantum gravity. Jonathan, thank you very much for talking

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Speaker 1: to us, Thanks for having me. So first I'd like

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Speaker 1: to set the stage and understand what is it we

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Speaker 1: are trying to solve. Why is everybody after quantum gravity.

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Speaker 1: Why can't we just have general relativity to describe big

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Speaker 1: stuff and quantum mechanics to describe small stuff and be

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Speaker 1: happy with that. Why do we need one unified theory

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Speaker 1: of the universe.

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Speaker 3: Well, the two theories are frameworks general relativity and quantum mechanics.

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Speaker 3: They're inconsistent, so they can't really live together in a

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Speaker 3: mathematically consistent way, and so we know that either one

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Speaker 3: of them is wrong or both of them are wrong,

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Speaker 3: and so we know that they cannot be a correct

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Speaker 3: description of nature.

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Speaker 1: What if they always describe different domains, Do they need

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Speaker 1: to give us a single, unified sort of conceptual understanding

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Speaker 1: of the universe or do they actually disagree about what

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Speaker 1: happens in the universe. Do they conflict in terms of

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Speaker 1: their predictions.

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Speaker 3: Yeah, they conflict. I mean the regimes that we're used to,

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Speaker 3: it doesn't matter so much. But now that we're exploring

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Speaker 3: the quantum realm, and we're exploring the quantum realm, when

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Speaker 3: it comes to larger and larger particles than it does

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Speaker 3: start to matter. If you think of like a very

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Speaker 3: small gold atom, which is put in a superposition of

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Speaker 3: two places at once, which is something we can do

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Speaker 3: with gold atoms, then gold atoms gravitate in a very

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Speaker 3: small way. But we don't have a theory which would

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Speaker 3: describe that, and maybe that's not a realm that we

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Speaker 3: care that much about. But we know from the history

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Speaker 3: of physics that when we have these inconsistencies and contradictions

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Speaker 3: in our theory, that once we start to try and

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Speaker 3: fix them, everything just starts to unravel. I mean, if

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Speaker 3: you look at the history of quantum mechanics, there was

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Speaker 3: only one or two little places where things were seemed strange.

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Speaker 3: And once we started really looking at how we could

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Speaker 3: reconcile and explain certain phenomena back in the beginning of

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Speaker 3: the nineteen hundreds, we realized that all of our laws

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Speaker 3: of physics were wrong and there was the whole quantum revolution.

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Speaker 3: So we know from history that when there are these contradictions,

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Speaker 3: they usually spell the beginning of a new era of physics.

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Speaker 2: Could you for the biologists dig in a little bit

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Speaker 2: more to the gold superposition atoms.

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Speaker 3: So quantum mechanics is strange. And in the world that

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Speaker 3: we inhibit, we think of things as being in definite places.

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Speaker 3: So a coffee mug is in one place or it's

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Speaker 3: in another place. But as you go to smaller and

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Speaker 3: smaller particles, in smaller and smaller systems, then things behave

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Speaker 3: very differently. So gold atom, which is not like a

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Speaker 3: coffee cup. We'd say that it's in a superposition of

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Speaker 3: being in many places at the same time. So in

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Speaker 3: some sense, it doesn't have a definite position, it doesn't

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Speaker 3: have a definite velocity. As we get smaller and smaller,

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Speaker 3: things just behave very weirdly and that's what quantum mechanics is.

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Speaker 2: And so quantum mechanics can explain the weirdness of what

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Speaker 2: the gold atom is doing, but general relativity cannot, is

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Speaker 2: that right, That's right, that's right.

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Speaker 3: I mean general relativity is our theory of gravity, and

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Speaker 3: it describes planets and the Solar system and the way

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Speaker 3: the universe evolves and the Big Bang, and so we

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Speaker 3: used to it explained those very big things, but well

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Speaker 3: contradicts what's happening on the small scale.

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Speaker 1: So we have this theory of the very large gener relativity,

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Speaker 1: which requires us to know where things are and when

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Speaker 1: they're there in order to describe how space is bending

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Speaker 1: and how things gravitate. But then we have quantum mechanics,

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Speaker 1: which says things can have an uncertainty in their location,

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Speaker 1: and these two things are fundamentally in contradiction.

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Speaker 3: That's a good way of putting in it, and maybe

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Speaker 3: just to say that you mentioned space time, and the

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Speaker 3: thing to say maybe, is that our theory of gravity.

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Speaker 3: What Einstein one of the massive contributions to science is

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Speaker 3: he taught us that gravity is really space time bending.

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Speaker 3: So we know that large objects bend space time, that's

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Speaker 3: what gravity is. And very small objects, because they don't

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Speaker 3: have a definite position, as you said, we don't know

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Speaker 3: how space time should bend because space time doesn't even

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Speaker 3: know where they are. So that's why you get these contradictions.

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Speaker 1: And you have to cook up this example of a

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Speaker 1: gold atom because typically general relativity applies to the very

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Speaker 1: large things planets and galaxies, and quant mechanics are the

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Speaker 1: very small. So you need something sort of on the

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Speaker 1: edge there where it's large enough where we can measure

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Speaker 1: its gravity, but it's small enough the quantum mechanical effects

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Speaker 1: are still relevant. Is that why you come up with

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Speaker 1: this idea of a gold at them? And why gold

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Speaker 1: in particular would it work also for a lead atom.

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Speaker 3: I mean, I love gold because it's it's actually because

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Speaker 3: gold is very dense, and so it turns out that

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Speaker 3: you run into trouble actually with very dense objects rather

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Speaker 3: than very heavy objects. That's one of the things we've

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Speaker 3: found out. But you know, it's true that we generally

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Speaker 3: think of the gravitational field as being caused by the

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Speaker 3: Moon and the Earth and things like that. But one

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Speaker 3: kilogram mass will bend. We can feel that gravitation pool

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Speaker 3: of a one kilogram mass. And now we're doing experiments

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Speaker 3: where we can feel the pull of gold, which is

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Speaker 3: a millimeter sphere. We can feel the gravitational pull of

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Speaker 3: that kind of an object. But when they get much

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Speaker 3: smaller than that, then we don't know what happens. And

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Speaker 3: we haven't even been able to really perform the experiments

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Speaker 3: that will tell us what happens at that scale.

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Speaker 1: Right, So we need some sort of unified picture of

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Speaker 1: gravivity and quantum mechanics. Why don't we just do that?

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Speaker 1: I mean, we did it for electromagnetism. We had classical

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Speaker 1: theory of electromagnetism, and then folks quantized it and gave

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Speaker 1: us a theory of quantum electrodynamics. Why can't we also

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Speaker 1: just do that for gravity? Take space time, consider that

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Speaker 1: a field, quantize it. Bob's your uncle.

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Speaker 3: Yeah, I mean that's what everyone has thought, and that's

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Speaker 3: what we've been thinking for the last one hundred years,

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Speaker 3: which is probably the amount of time that we've been

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Speaker 3: failing to quantize gravity. So yeah, you have this contradiction.

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Speaker 3: You have gravity which is not quantum. You want to

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Speaker 3: make it quantum to fit in with everything else. That

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Speaker 3: would be I think the thing that everyone thought that's

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Speaker 3: what we should be doing. But we're finding that really troublesome,

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Speaker 3: and there's reasons for that. And one of the I

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Speaker 3: think big reasons is that gravity is really different from

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Speaker 3: the other forces. So we're used to electromagnetism, which is

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Speaker 3: what keeps my two fingers from being able to push

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Speaker 3: through each other. They're repulsed by the electromagnetic force. Gravity

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Speaker 3: just behaves very differently. And what Einstein taught us about

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Speaker 3: gravity is that, unlike the other forces, gravity is spacetime bending.

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Speaker 3: So the reason that the Earth goes around the Sun

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Speaker 3: is that the Sun causes space time to bend in

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Speaker 3: just such a way that the Earth instead of rolling

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Speaker 3: past it orbits around it. So it's slightly weirder than

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Speaker 3: that actually. I mean, you often in science centers and stuff,

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Speaker 3: the vehicle demonstration where you see a big planet like

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Speaker 3: the Sun sitting in the middle of a cone and

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Speaker 3: then the Earth goes around the Sun as if it's

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Speaker 3: caught in this vortex or funnel. It's the kind of

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Speaker 3: demonstration we often see to kind of give us a

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Speaker 3: sense of why gravity as space time bending is causing

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Speaker 3: the Earth to orbit the Sun.

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Speaker 1: Since you brought that up, do you find that to

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Speaker 1: be an accurate description, like a useful mental model of

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Speaker 1: how gravity works? Because I find it to be very confusing.

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Speaker 1: You have like a two D situation, and then you're

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Speaker 1: adding curvature in a third dimension, where general relativity has

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Speaker 1: the curvature to be intrinsic in the three D space.

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Speaker 1: Do you find those demonstrations to be misleading or do

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Speaker 1: you find them to be a useful description of what's

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Speaker 1: happening in general relativity?

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Speaker 3: All our descriptions are misleading. So what we do is

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Speaker 3: we tell ourselves lies and each other lies, and they're

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Speaker 3: useful for a bit, and then we replace it with

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Speaker 3: a better lie. And I feel like the funnel is

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Speaker 3: a reasonable lie because it gives you a partial sense

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Speaker 3: of what's happening. And like when I say a gold

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Speaker 3: at them can be in a superposition of two places

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Speaker 3: at once, that's also a bit of a lie. Because

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Speaker 3: you could just as well say it's in a superposition

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Speaker 3: of being in neither place at once. This is a

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Speaker 3: mental picture that we're using and it has some truth

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Speaker 3: to it. Also, the are parts where it fails and

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Speaker 3: the funnel that you see at many sigence centers, which

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Speaker 3: is meant to explain why space time bending causes the

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Speaker 3: Earth to go around the Sun. Well, a better lie

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Speaker 3: is that what's really happening is that time is slowing

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Speaker 3: down you get closer to the Sun, and the reason

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Speaker 3: that the Earth is going around the Sun is because

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Speaker 3: time slows down closer to the Sun, and the Earth

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Speaker 3: wants to travel in a path which causes clocks to

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Speaker 3: tick the least number of times. And so it's actually

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Speaker 3: time that's bending and traveling at a different rate at

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Speaker 3: different places in space, and that's why the Earth goes

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Speaker 3: around the Sun.

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Speaker 2: Why does Earth want that?

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Speaker 1: Earth is laid on its deadlines, Kelly, that's why I

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Speaker 1: hear you Earth, I hear you.

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Speaker 3: Yeah, Earth is traveling the shortest distance between two points.

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Speaker 3: And why it wants to do that, I don't know.

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Speaker 3: And there's lots of principles in physics where things want

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Speaker 3: to do. The principle least action, which is how we

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Speaker 3: derive most of our physical laws and was famous The

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Speaker 3: movie Arrival is about that principle, and we don't really

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Speaker 3: know why that is, but that just seems to be

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Speaker 3: the way it is.

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Speaker 1: I think it's at the level philosophically of if you

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Speaker 1: may this assumption, you get a model which works and

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Speaker 1: describes the universe very well, and then you can come

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Speaker 1: back to, well, what does that mean about the universe. Well,

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Speaker 1: we're still shrugging our shoulders over that one.

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Speaker 3: I think the thing you learn in physics when you're

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Speaker 3: in grade school, which is that you know when you

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Speaker 3: are in outer space and there's no force acting on you,

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Speaker 3: you move in a straight line. That's an example of that.

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Speaker 3: Why do we move in a straight line when there's

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Speaker 3: no force applied to it? Well, we're just taking the shortest.

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Speaker 1: Distance in some set.

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Speaker 3: We're taking the shortest path, the easiest path in some sense,

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Speaker 3: And that's an particular example of that. But things could

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Speaker 3: be different, but they're not.

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Speaker 1: It's also interesting to me how some things, when you

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Speaker 1: say then, people will just accept them because they sound intuitive,

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Speaker 1: and other things demand an explanation. Why do clocks tick

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Speaker 1: slower when you see them moving at high speed. That

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Speaker 1: needs an explanation. Why would clocks always tick at the

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Speaker 1: same speed. Why doesn't that need an explanation. So there's

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Speaker 1: an imbalance there because some things confront or intuition and

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Speaker 1: some things support it.

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Speaker 3: I mean, maybe this is not for this podcast, but

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Speaker 3: I could give you a good reason why we think

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Speaker 3: that the Earth is traveling the shortest distance in curved space.

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Speaker 3: Please do if you're willing to accept Newton's law, which

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Speaker 3: is that in empty space, my velocity should stay the

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Speaker 3: same if no force is acting upon me, and if

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Speaker 3: I should just move in a straight line in empty space.

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Speaker 3: If you accept that, then you can now ask the question, well,

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Speaker 3: if there's no push or pull on me, and I'm

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Speaker 3: in a curved space, how should I move? I think

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Speaker 3: it's free one answer, which is you should take the

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Speaker 3: shortest path, because that's what the satellite is doing an

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Speaker 3: empty space, when it's going around the Earth or when

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Speaker 3: it's going in a straight line, it's just taking the

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Speaker 3: shortest path.

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Speaker 1: I see. So you're making the argument that in flat space,

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Speaker 1: a straight line is the shortest path, and so if

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Speaker 1: you generalize to any space, including curved space, you should

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Speaker 1: always still take the shortest path, which in this case

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Speaker 1: is not a straight line. That's right, fascinating. So I

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Speaker 1: think we interrupted you as you were explaining why gravity

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Speaker 1: is weird and different and why it's harder to quantize

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Speaker 1: than electromagnetism. And you were explaining how gravity is actually

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Speaker 1: the curvature of space. Why does that make it harder

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Speaker 1: to quantize it. Why can't we take the same tricks

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Speaker 1: we applied to the electromagnetic field and apply them to

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Speaker 1: the metric of space time or the curvature of space time.

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Speaker 11: I mean, there's a bunch of technical reasons why we

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Speaker 11: run into trouble, but I think great conceptually is this

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Speaker 11: idea that gravity is about time flowing at a.

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Speaker 3: Different rate in different points in space, and it's about

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Speaker 3: the flow of time how fast it flows. And if

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Speaker 3: you think about it, it's almost like what are we

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Speaker 3: doing as physicists.

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Speaker 11: What we're doing, is physicists, is we're describing how things.

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Speaker 3: Change relative to our clock. So we have a clock

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Speaker 3: and it's telling the time, and we say, okay, at

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Speaker 3: nine am, the particles were in this configuration, and then

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Speaker 3: I predicted some later time they're going to be in

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Speaker 3: some other configuration. And so we're predicting the future based

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Speaker 3: on the past, and in order to do that, we

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Speaker 3: need to know how our clocks and our rods are

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Speaker 3: all behaving. And if we're going to quantize that, if

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Speaker 3: we're going to make our clocks and our rods and

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Speaker 3: the speeds at which the clock ticks and the length

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Speaker 3: of our rods, if that is going to be quantum,

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Speaker 3: then personally, I don't know how to describe physics anymore,

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Speaker 3: because you cut my legs out from underneath me, and

440
00:22:27,800 --> 00:22:30,800
Speaker 3: I no longer am able to really, I think, talk

441
00:22:30,960 --> 00:22:36,119
Speaker 3: about how things evolved relative to some time. If the

442
00:22:36,359 --> 00:22:39,400
Speaker 3: rate at which time is flowing, if that itself becomes

443
00:22:39,960 --> 00:22:42,159
Speaker 3: the thing I have to quantize and doesn't have a

444
00:22:42,200 --> 00:22:44,879
Speaker 3: definite value, because I guess remember that the thing that

445
00:22:45,320 --> 00:22:50,080
Speaker 3: distinguished classical things and quantum things was that classical things

446
00:22:50,119 --> 00:22:53,320
Speaker 3: have definite values. The coffee mug is at a definite position,

447
00:22:54,119 --> 00:22:57,840
Speaker 3: and quantum things don't. The gold atom that its quantum

448
00:22:57,880 --> 00:23:01,000
Speaker 3: can be as if it's in any places at once.

449
00:23:01,160 --> 00:23:03,600
Speaker 3: And so if your time can be running at many

450
00:23:03,640 --> 00:23:08,080
Speaker 3: different speeds, how do I tell you what is happening

451
00:23:08,119 --> 00:23:10,800
Speaker 3: relative to the time If I can't even really tell

452
00:23:10,840 --> 00:23:12,639
Speaker 3: you what time my clock is running.

453
00:23:12,680 --> 00:23:15,280
Speaker 1: Now, I see. So for an electron, if we just

454
00:23:15,320 --> 00:23:18,399
Speaker 1: ignore gravity and we think about it quantum mechanically, we

455
00:23:18,440 --> 00:23:21,240
Speaker 1: can handle its uncertainty and we can propagate that forward

456
00:23:21,280 --> 00:23:23,520
Speaker 1: in time. We have the Shorteninger equation, and we can

457
00:23:23,600 --> 00:23:26,800
Speaker 1: even allow for that uncertainty to create new uncertainties and

458
00:23:26,800 --> 00:23:29,480
Speaker 1: more uncertainties, because we always agree on a clock and

459
00:23:29,520 --> 00:23:31,800
Speaker 1: we can say what time is, and we can propagate

460
00:23:31,800 --> 00:23:34,879
Speaker 1: things forward and calculate how this uncertainty envelope is going

461
00:23:34,960 --> 00:23:37,879
Speaker 1: to change. But if I'm now talking about something that

462
00:23:37,920 --> 00:23:42,679
Speaker 1: has gravity, the additional complexity that time itself is changing

463
00:23:42,720 --> 00:23:45,080
Speaker 1: in a way that depends on what's happening. And so

464
00:23:45,640 --> 00:23:48,760
Speaker 1: time is bent by the object, and its motion then

465
00:23:48,840 --> 00:23:51,240
Speaker 1: depends on time. And so this this back and forth

466
00:23:51,280 --> 00:23:54,800
Speaker 1: interaction between the motion and time itself. Is that the complexity.

467
00:23:55,040 --> 00:23:56,760
Speaker 3: That's definitely part of it. Yeah, that's a big part

468
00:23:56,800 --> 00:23:58,920
Speaker 3: of it. It's not even clear what I mean by

469
00:23:58,960 --> 00:24:01,199
Speaker 3: the past and the future. It's not even definite that

470
00:24:01,280 --> 00:24:04,280
Speaker 3: something is to the past or to the future of

471
00:24:04,720 --> 00:24:07,000
Speaker 3: some other event. If I have two events that are happening,

472
00:24:07,920 --> 00:24:10,960
Speaker 3: you know, the mere act of setting initial conditions, which

473
00:24:11,000 --> 00:24:12,520
Speaker 3: is what we do in physics, we say, here are

474
00:24:12,520 --> 00:24:14,800
Speaker 3: the initial conditions, this is what's going to happen in

475
00:24:14,800 --> 00:24:17,400
Speaker 3: the future. The mere act of doing that, I think

476
00:24:17,520 --> 00:24:23,080
Speaker 3: becomes problematic if you're trying to quantize the space time itself,

477
00:24:23,320 --> 00:24:26,959
Speaker 3: or at least the flow of time and the distances.

478
00:24:27,160 --> 00:24:29,639
Speaker 3: If those are being quantized, then I think it's very

479
00:24:29,680 --> 00:24:33,080
Speaker 3: difficult to even ask the questions that we're used to

480
00:24:33,119 --> 00:24:49,160
Speaker 3: asking as physicists.

481
00:24:51,160 --> 00:24:53,680
Speaker 1: We often hear the quantum gravity is hard because there

482
00:24:53,720 --> 00:24:57,320
Speaker 1: are infinities, and like, maybe the universe is infinite, maybe

483
00:24:57,320 --> 00:25:00,879
Speaker 1: there's an infinite number of locations between me in this microphone.

484
00:25:01,400 --> 00:25:03,680
Speaker 1: Can you help us understand where the infinities come from

485
00:25:03,720 --> 00:25:05,800
Speaker 1: and also why they're a problem. Why can't we have

486
00:25:05,920 --> 00:25:07,360
Speaker 1: infinities in our theories.

487
00:25:07,720 --> 00:25:11,320
Speaker 3: Well, so this is something which is called renormalization, and

488
00:25:11,440 --> 00:25:15,040
Speaker 3: what that really just means is that most of our

489
00:25:15,040 --> 00:25:19,280
Speaker 3: physical theories they break down at very short distances. With

490
00:25:19,400 --> 00:25:22,000
Speaker 3: most of the theories that we have at the moment,

491
00:25:22,880 --> 00:25:26,480
Speaker 3: they are valid at very short distances, but gravity is not.

492
00:25:27,600 --> 00:25:29,560
Speaker 3: And actually, one of the things which gives me some

493
00:25:29,720 --> 00:25:32,879
Speaker 3: faith in this post quantum theory is that it turns

494
00:25:32,920 --> 00:25:35,639
Speaker 3: out we've just recently shown that it's formally renormalizable. So

495
00:25:35,720 --> 00:25:40,199
Speaker 3: even though quantum gravity has the property that it's not

496
00:25:40,320 --> 00:25:43,960
Speaker 3: valid to very short distances, this post quantum theory does

497
00:25:43,960 --> 00:25:46,840
Speaker 3: seem to be valid at very short distances. And that's

498
00:25:46,880 --> 00:25:50,840
Speaker 3: important because this description of gravity in terms of geometry,

499
00:25:50,920 --> 00:25:53,000
Speaker 3: if you think that that really is what gravity is,

500
00:25:53,000 --> 00:25:56,040
Speaker 3: that gravity really is geometry, then you believe that this

501
00:25:56,280 --> 00:25:59,080
Speaker 3: picture that it's geometry should hold up to the very

502
00:25:59,320 --> 00:26:02,840
Speaker 3: smallest scale. And so that's one of the reasons why

503
00:26:02,880 --> 00:26:07,160
Speaker 3: we want to have our theories be predictable at short distances.

504
00:26:07,840 --> 00:26:12,920
Speaker 2: So renormalizable means it doesn't matter what scale you're looking at,

505
00:26:13,080 --> 00:26:16,119
Speaker 2: the theory still works. Would that be a fair okay?

506
00:26:16,320 --> 00:26:16,600
Speaker 6: Thanks?

507
00:26:16,960 --> 00:26:19,639
Speaker 2: The biologist needs simplification every once in a while.

508
00:26:20,600 --> 00:26:23,040
Speaker 1: And what do you mean when you say it doesn't work?

509
00:26:23,320 --> 00:26:26,359
Speaker 1: I mean, we have a theory of particles that allows

510
00:26:26,440 --> 00:26:31,040
Speaker 1: us to smash electrons together tiny distances to describe the

511
00:26:31,080 --> 00:26:33,800
Speaker 1: creation of very heavy particles. Why doesn't that work with gravity?

512
00:26:33,800 --> 00:26:36,040
Speaker 1: Why does it break down? What happens?

513
00:26:36,680 --> 00:26:39,960
Speaker 3: Well, there are two breakdowns that happen with gravity. One

514
00:26:40,119 --> 00:26:42,480
Speaker 3: is something called the black hole singularity, which is that

515
00:26:43,080 --> 00:26:46,560
Speaker 3: black holes, which are the strongest gravitational fields we know

516
00:26:47,080 --> 00:26:49,360
Speaker 3: and cause light to bends to the point where they

517
00:26:49,400 --> 00:26:52,680
Speaker 3: can't get out those You have something called the singularity,

518
00:26:52,680 --> 00:26:54,760
Speaker 3: which means we just don't know what happens, and the

519
00:26:54,760 --> 00:26:58,560
Speaker 3: center of a black hole that is maybe related but

520
00:26:58,640 --> 00:27:01,520
Speaker 3: maybe not related to another kinds of infinity that we get.

521
00:27:02,119 --> 00:27:04,800
Speaker 3: You know, the reason that gravity has this problem and

522
00:27:04,920 --> 00:27:08,560
Speaker 3: our other forces don't have this problem, it's for a

523
00:27:08,640 --> 00:27:11,000
Speaker 3: reason that I don't know that I have a good

524
00:27:11,160 --> 00:27:15,760
Speaker 3: podcasting way of saying other than that's saying that the

525
00:27:15,800 --> 00:27:19,960
Speaker 3: Newton's contents has, you know, the wrong dimension. But there

526
00:27:20,000 --> 00:27:23,360
Speaker 3: is a technical reason why the usual thing you would

527
00:27:23,400 --> 00:27:28,040
Speaker 3: do for other forces just doesn't work with gravity. The gravity,

528
00:27:28,200 --> 00:27:31,479
Speaker 3: we say, has these infinities which you cannot get rid of.

529
00:27:31,600 --> 00:27:36,560
Speaker 3: And therefore the quantum theory of gravity, at least in

530
00:27:36,600 --> 00:27:40,040
Speaker 3: the three spatial dimensions that we live in or appear

531
00:27:40,080 --> 00:27:43,600
Speaker 3: to live in in three special dimensions, it has these infinities,

532
00:27:43,640 --> 00:27:45,359
Speaker 3: and we don't know what to do. And that's actually

533
00:27:45,400 --> 00:27:49,800
Speaker 3: the reason why we have approaches like string theory and

534
00:27:49,880 --> 00:27:54,160
Speaker 3: loop one and gravity theories in which you imagine that

535
00:27:54,720 --> 00:27:59,359
Speaker 3: space time lives on a lattice or you know, instead

536
00:27:59,400 --> 00:28:02,920
Speaker 3: of having point particles, we have these extended strings. Those

537
00:28:02,960 --> 00:28:07,240
Speaker 3: are solutions to the problem that gravity has these infinities,

538
00:28:07,240 --> 00:28:10,600
Speaker 3: and that's why these other approaches have been born.

539
00:28:10,880 --> 00:28:13,159
Speaker 1: So your background is in string theory, right, You were

540
00:28:13,200 --> 00:28:15,800
Speaker 1: once a string theorist. Is that a fair description?

541
00:28:16,119 --> 00:28:18,680
Speaker 3: Well, no, I hang out with a lot of string theorists.

542
00:28:18,680 --> 00:28:22,359
Speaker 3: I have string theorist friends, and I've written some papers

543
00:28:22,440 --> 00:28:24,080
Speaker 3: that are string theory adjacent.

544
00:28:24,440 --> 00:28:26,720
Speaker 1: So you don't want to be described as a string theorist.

545
00:28:26,800 --> 00:28:28,959
Speaker 1: But you know something about string theory and you decided

546
00:28:29,000 --> 00:28:31,640
Speaker 1: to take a different path than the string theory crew.

547
00:28:32,280 --> 00:28:34,800
Speaker 1: Why did you not follow the crowd? What is it

548
00:28:34,840 --> 00:28:38,280
Speaker 1: about string theory that doesn't satisfy your desire to unify

549
00:28:38,320 --> 00:28:40,000
Speaker 1: quantum mechanics and gravity.

550
00:28:40,560 --> 00:28:42,080
Speaker 3: Well, I think one of the things is that I

551
00:28:42,120 --> 00:28:45,480
Speaker 3: think we should take this geometric picture of gravity very seriously.

552
00:28:46,440 --> 00:28:48,560
Speaker 3: I mean, maybe that's just a description which breaks down

553
00:28:48,640 --> 00:28:51,920
Speaker 3: at some scale, but let's just try and assume that

554
00:28:51,920 --> 00:28:55,400
Speaker 3: that is actually what's happening, that gravity is actually spacetime bending.

555
00:28:56,040 --> 00:28:58,400
Speaker 3: And I think if you have that as your picture

556
00:28:58,400 --> 00:29:01,680
Speaker 3: as to what is happening, then quantizing it becomes a

557
00:29:01,720 --> 00:29:05,640
Speaker 3: lot more problematic. And string theorist for example, or you know,

558
00:29:05,760 --> 00:29:09,600
Speaker 3: I would say almost everybody else the various approaches to

559
00:29:09,680 --> 00:29:13,640
Speaker 3: quantizing gravity, they somehow need to have it at some

560
00:29:13,720 --> 00:29:16,840
Speaker 3: small scale. That picture is not true and breaks down

561
00:29:17,160 --> 00:29:20,120
Speaker 3: the geometrical picture, you mean, the geometrical picture of gravity,

562
00:29:20,400 --> 00:29:24,239
Speaker 3: And so if you want to hold true to this

563
00:29:24,280 --> 00:29:27,840
Speaker 3: geometrical picture, then I don't think you can quantize it. Now,

564
00:29:27,880 --> 00:29:30,280
Speaker 3: maybe that's the wrong approach. It could be that the

565
00:29:30,320 --> 00:29:32,880
Speaker 3: geometric picture does break down and we get to the

566
00:29:32,880 --> 00:29:36,600
Speaker 3: smallest scale and space time is emergent in some way,

567
00:29:37,000 --> 00:29:39,280
Speaker 3: or you know, we're all just fuzzy dots in a

568
00:29:39,520 --> 00:29:42,640
Speaker 3: lattice of space and time. All those things are possible,

569
00:29:42,680 --> 00:29:45,640
Speaker 3: and I'm attracted to them. They sound sci fi and great,

570
00:29:47,080 --> 00:29:48,640
Speaker 3: but I don't know how to make them work. I

571
00:29:48,640 --> 00:29:51,400
Speaker 3: don't know how to think about them. And I think

572
00:29:51,440 --> 00:29:55,280
Speaker 3: it's worth trying the kind of conservative imagines geometry all

573
00:29:55,280 --> 00:29:55,960
Speaker 3: the way down.

574
00:29:55,800 --> 00:29:58,280
Speaker 2: Approaching and so of all of the things that you

575
00:29:58,280 --> 00:30:00,800
Speaker 2: could have decided, let's assume that this this is true

576
00:30:01,440 --> 00:30:04,040
Speaker 2: and then work around that. Why did you pick the

577
00:30:04,200 --> 00:30:05,160
Speaker 2: geometry thing?

578
00:30:05,800 --> 00:30:09,160
Speaker 3: Yeah, I mean, I guess there's an aesthetic element to it,

579
00:30:09,200 --> 00:30:11,880
Speaker 3: But I think also I have a feeling that I

580
00:30:11,880 --> 00:30:13,560
Speaker 3: don't think we should all be doing the same thing.

581
00:30:14,080 --> 00:30:16,640
Speaker 3: I think that's really important for science. I have a

582
00:30:16,760 --> 00:30:19,240
Speaker 3: plot and cheer my string theory friends when they make

583
00:30:19,440 --> 00:30:22,240
Speaker 3: breakthroughs and my new port of gravity friends, et cetera,

584
00:30:22,280 --> 00:30:24,280
Speaker 3: et cetera. I think we should be supporting each other.

585
00:30:24,680 --> 00:30:27,640
Speaker 3: But I also think we should be diversifying and trying

586
00:30:27,680 --> 00:30:30,360
Speaker 3: as many approaches as possible. And so I'm going to

587
00:30:30,440 --> 00:30:33,160
Speaker 3: pick this kind of from the slightly lonely route, but

588
00:30:33,280 --> 00:30:35,560
Speaker 3: I think it's important that we try these different things.

589
00:30:35,640 --> 00:30:38,120
Speaker 3: And I think it would be dangerous to put all

590
00:30:38,120 --> 00:30:40,120
Speaker 3: our eggs in one basket. We're looking for a needle

591
00:30:40,160 --> 00:30:42,640
Speaker 3: in a haystack, and we shouldn't all be looking in the.

592
00:30:42,560 --> 00:30:45,280
Speaker 1: Same place, thank you. And I think that's very valuable

593
00:30:45,280 --> 00:30:48,840
Speaker 1: contribution to science more broadly, So let's talk about more

594
00:30:48,880 --> 00:30:52,400
Speaker 1: deeply your idea post quantum gravity. How is it that

595
00:30:52,440 --> 00:30:56,000
Speaker 1: we can avoid quantizing gravity, to take the geometry seriously

596
00:30:56,320 --> 00:30:59,600
Speaker 1: all the way down and still somehow handle the uncertainty

597
00:30:59,600 --> 00:31:03,760
Speaker 1: of quantum mechanics, I mean classical gravity. Geometric gravity says

598
00:31:03,800 --> 00:31:06,040
Speaker 1: we need to know where a particle is to know

599
00:31:06,080 --> 00:31:09,800
Speaker 1: how it bends space time. But quantum mechanics says, sometimes

600
00:31:09,880 --> 00:31:12,400
Speaker 1: that knowledge doesn't exist, not just that it's not known,

601
00:31:12,440 --> 00:31:14,880
Speaker 1: but the gold atom is partially here and partially there,

602
00:31:14,960 --> 00:31:17,280
Speaker 1: or has a probability to be here or there or neither.

603
00:31:17,720 --> 00:31:20,360
Speaker 1: So in your picture where you take geometry very seriously,

604
00:31:20,880 --> 00:31:24,200
Speaker 1: what happens to space in that situation? How do you

605
00:31:24,240 --> 00:31:25,400
Speaker 1: avoid quantizing it?

606
00:31:25,520 --> 00:31:28,080
Speaker 3: Yeah, you have to give up something, and the thing

607
00:31:28,200 --> 00:31:30,200
Speaker 3: you give up is predictability.

608
00:31:30,480 --> 00:31:33,400
Speaker 1: Didn't you say earlier that you needed predictability, that was

609
00:31:33,440 --> 00:31:34,640
Speaker 1: the problem you're doing to solve.

610
00:31:35,480 --> 00:31:39,800
Speaker 3: Yes, I did. Yeah, I mean it's very strange that

611
00:31:40,040 --> 00:31:42,840
Speaker 3: people are willing to accept that you can't predict exactly

612
00:31:42,880 --> 00:31:45,719
Speaker 3: where the particle is and has a certain probability of

613
00:31:45,760 --> 00:31:50,000
Speaker 3: being found in a certain place. So people accept that,

614
00:31:50,520 --> 00:31:52,200
Speaker 3: or maybe they don't accept that. You could do a

615
00:31:52,200 --> 00:31:54,680
Speaker 3: whole show on the different interpretations of quantum theory that

616
00:31:54,760 --> 00:31:57,000
Speaker 3: you haven't. Maybe you've done the show on this. But

617
00:31:57,520 --> 00:32:02,200
Speaker 3: quantum mechanics does have this lack of predictability, whereas classical

618
00:32:02,200 --> 00:32:05,760
Speaker 3: physics doesn't. Right like in classical general relativity. In Einstein's

619
00:32:05,760 --> 00:32:09,520
Speaker 3: theory of classical general relativity, there is only one space time, right,

620
00:32:09,840 --> 00:32:13,240
Speaker 3: and it is definite and space time has a definite configuration.

621
00:32:13,960 --> 00:32:18,840
Speaker 3: So if you're going to redd these two theories together

622
00:32:19,440 --> 00:32:23,840
Speaker 3: and you want to keep space time as classical in

623
00:32:23,880 --> 00:32:27,120
Speaker 3: the sense that it has definite features, the only way

624
00:32:27,160 --> 00:32:30,040
Speaker 3: to do it is and this is a difficult concept

625
00:32:30,080 --> 00:32:33,160
Speaker 3: to kind of get your head around. It has to

626
00:32:33,200 --> 00:32:36,120
Speaker 3: be both definite and unpredictable.

627
00:32:38,200 --> 00:32:43,239
Speaker 1: That was definitely unpredictable. Yeah, what do you mean by that?

628
00:32:43,280 --> 00:32:46,960
Speaker 1: What happens when you have a particle and it's potentially

629
00:32:47,000 --> 00:32:50,040
Speaker 1: in two different places? Does it bend space time in

630
00:32:50,120 --> 00:32:53,800
Speaker 1: both places? Is space time probably bent in both places?

631
00:32:54,280 --> 00:32:56,160
Speaker 1: Or is it random where it gets bent?

632
00:32:56,800 --> 00:33:00,760
Speaker 3: Well, space time has to be undergoing these random f suctuations.

633
00:33:01,200 --> 00:33:04,560
Speaker 3: Some people have said it's like it's wobbly. It's undergoing

634
00:33:04,560 --> 00:33:09,040
Speaker 3: these random fluctuations. The particle then will kind of bend

635
00:33:09,080 --> 00:33:11,800
Speaker 3: it in all the places that it's in, but you

636
00:33:11,840 --> 00:33:15,120
Speaker 3: won't be able to really tell where it's bending it.

637
00:33:15,400 --> 00:33:17,480
Speaker 3: So if I were to look at space time, I

638
00:33:17,480 --> 00:33:20,120
Speaker 3: wouldn't be able to tell where the particle is because

639
00:33:20,400 --> 00:33:24,800
Speaker 3: space time will be undergoing all these random fluctuations. It's

640
00:33:24,920 --> 00:33:28,160
Speaker 3: kind of wobbly and jumping around all over the place.

641
00:33:28,240 --> 00:33:30,560
Speaker 3: And so if I looked at the space time, even

642
00:33:30,600 --> 00:33:33,600
Speaker 3: though it was being bent by the particle, I wouldn't

643
00:33:33,600 --> 00:33:35,720
Speaker 3: be able to tell where the particle was. And that's

644
00:33:35,760 --> 00:33:39,800
Speaker 3: what you need in order to reconcile quantum theory with

645
00:33:40,600 --> 00:33:44,520
Speaker 3: general relativity, if you're going to keep general relativity as

646
00:33:44,760 --> 00:33:46,880
Speaker 3: a really theory of definite geometry.

647
00:33:47,360 --> 00:33:50,360
Speaker 1: I see, so space time is not emerging from some

648
00:33:50,560 --> 00:33:54,200
Speaker 1: deeper string theory. It fundamentally is the geometry of the universe.

649
00:33:54,240 --> 00:33:58,440
Speaker 1: But it's also fundamentally random. And when you say that,

650
00:33:58,760 --> 00:34:02,360
Speaker 1: you mean that it's and unknowable, the way it is

651
00:34:02,400 --> 00:34:05,440
Speaker 1: in quantum mechanics, where the information just does not exist

652
00:34:05,880 --> 00:34:09,000
Speaker 1: it's not determined until you measure it, or that it's

653
00:34:09,280 --> 00:34:12,320
Speaker 1: random but it's unknown. Like in classical physics. You know,

654
00:34:12,360 --> 00:34:14,359
Speaker 1: if I flip a coin and I don't look at

655
00:34:14,360 --> 00:34:17,520
Speaker 1: the answer, in principle, I could have calculated the outcome

656
00:34:17,520 --> 00:34:19,919
Speaker 1: of that coin. The information exists, and it is either

657
00:34:19,960 --> 00:34:22,000
Speaker 1: heads or tails under my hand, even if I haven't

658
00:34:22,000 --> 00:34:25,360
Speaker 1: looked at it. Is your space time random and unknowable

659
00:34:25,440 --> 00:34:26,480
Speaker 1: or random and unknown.

660
00:34:27,280 --> 00:34:30,239
Speaker 3: It's knowable in principle, I could know exactly what configuration

661
00:34:30,480 --> 00:34:34,600
Speaker 3: is at the present time, but it's unpredictable in that second.

662
00:34:34,680 --> 00:34:37,719
Speaker 3: Later I will not be able to predict oh, it

663
00:34:37,719 --> 00:34:40,560
Speaker 3: will evolve too, So it's more like a breakdown in

664
00:34:40,680 --> 00:34:43,760
Speaker 3: predictability rather than a breakdown in unknowing.

665
00:34:44,239 --> 00:34:44,439
Speaker 4: Nice.

666
00:34:44,440 --> 00:34:46,400
Speaker 3: And I think you're really right to make that distinction,

667
00:34:46,560 --> 00:34:49,880
Speaker 3: because you know, when people first learn quantum mechanics, they

668
00:34:49,960 --> 00:34:52,759
Speaker 3: might learn the Heisenberg and certainty principle where they say,

669
00:34:52,880 --> 00:34:54,840
Speaker 3: you know, you don't know the position of the particle,

670
00:34:54,880 --> 00:34:57,799
Speaker 3: you don't know where the gold ATM is. But I think,

671
00:34:57,800 --> 00:35:00,800
Speaker 3: as you're hinting, it's weirder than that the gold particle

672
00:35:00,840 --> 00:35:04,120
Speaker 3: doesn't have a position that does not exist at all.

673
00:35:04,560 --> 00:35:07,640
Speaker 3: And so that's maybe the difference between a kind of

674
00:35:07,680 --> 00:35:12,239
Speaker 3: a classical breakdown and predictability and a quantum one. A

675
00:35:12,239 --> 00:35:19,480
Speaker 3: classical object can be definite but unpredictable, whereas a quantum

676
00:35:19,520 --> 00:35:23,959
Speaker 3: system it's not knowable, but it can be predictable. Because

677
00:35:23,960 --> 00:35:26,440
Speaker 3: I know that's the strange thing about quantum mechanics. I

678
00:35:26,480 --> 00:35:30,120
Speaker 3: can actually predict with certainty how something called the wave

679
00:35:30,200 --> 00:35:34,440
Speaker 3: function will evolve, But the position of the particle doesn't

680
00:35:34,440 --> 00:35:39,520
Speaker 3: have definite value. So this notion of predictability, definiteness and

681
00:35:39,640 --> 00:35:41,640
Speaker 3: noable that they're all kind of tied in a very

682
00:35:41,680 --> 00:35:45,600
Speaker 3: strange not They're quite complicated anyway, in both classical mechanics

683
00:35:45,640 --> 00:35:48,279
Speaker 3: and quantum mechanics, and they really get jumbled around here.

684
00:35:48,680 --> 00:35:50,480
Speaker 1: So let me see if I understand the distinctions. So

685
00:35:50,560 --> 00:35:53,120
Speaker 1: in a classical theory, we have something which is definite,

686
00:35:53,200 --> 00:35:57,319
Speaker 1: spacetime has values and locations and bending, and it's also

687
00:35:57,360 --> 00:36:00,680
Speaker 1: predictable in that the past controls the future. The future

688
00:36:00,719 --> 00:36:03,240
Speaker 1: is determined by the past, whereas some kind of mechanics

689
00:36:03,239 --> 00:36:06,520
Speaker 1: we can predict the possible outcomes very precisely. The Shortener

690
00:36:06,560 --> 00:36:09,719
Speaker 1: equation tells us how to describe the possible outcomes, but

691
00:36:09,760 --> 00:36:13,239
Speaker 1: we don't know which one will actually be selected when

692
00:36:13,280 --> 00:36:15,719
Speaker 1: we interact with it. But in your theory you have

693
00:36:15,760 --> 00:36:19,040
Speaker 1: something which is definite but unpredictable. Does that mean that

694
00:36:19,080 --> 00:36:22,480
Speaker 1: the past doesn't completely control the future, that space time

695
00:36:22,520 --> 00:36:25,520
Speaker 1: in one moment is not determined by space time in

696
00:36:25,560 --> 00:36:26,040
Speaker 1: the past.

697
00:36:26,440 --> 00:36:27,000
Speaker 3: That's right.

698
00:36:27,160 --> 00:36:31,880
Speaker 2: Does that allow time travel? Let's get to the important part.

699
00:36:31,760 --> 00:36:36,520
Speaker 1: Here, Kelly working on her deadlines again, that's the question.

700
00:36:36,360 --> 00:36:39,440
Speaker 3: On everyone's mind. Can get somewhere faster? There's a lot

701
00:36:39,440 --> 00:36:40,120
Speaker 3: of time travel.

702
00:36:41,680 --> 00:36:45,120
Speaker 1: So if I imagine like an empty universe, right, no mass,

703
00:36:45,120 --> 00:36:49,960
Speaker 1: no radiation, nothing, Space is completely flat in your conception.

704
00:36:50,520 --> 00:36:53,719
Speaker 1: Is that fluctuating even though there's nothing happening to it,

705
00:36:53,800 --> 00:36:57,080
Speaker 1: Nothing is being inserted, nothing is moving. Is space time

706
00:36:57,120 --> 00:36:59,320
Speaker 1: still fluctuating just like randomly changing?

707
00:37:00,120 --> 00:37:02,040
Speaker 3: Yeah, but it's not that there's nothing there. There's always

708
00:37:02,080 --> 00:37:04,960
Speaker 3: something there. There's the vacuum. I mean, in quantum mechanics,

709
00:37:05,400 --> 00:37:09,319
Speaker 3: nothing's complicated. It might look like there's nothing there, but

710
00:37:09,440 --> 00:37:12,360
Speaker 3: at the small scale there are actually in a quantum

711
00:37:12,520 --> 00:37:16,680
Speaker 3: realm also these fluctuations, but in some weird way in

712
00:37:16,719 --> 00:37:20,000
Speaker 3: the quantum realm they look random, but they're not. They're

713
00:37:20,000 --> 00:37:23,440
Speaker 3: only random if you make a measurement. You know, if

714
00:37:23,440 --> 00:37:26,440
Speaker 3: you don't make a measurement, then they're not that random.

715
00:37:26,440 --> 00:37:30,160
Speaker 3: You can kind of predict how things evolve precisely. It's

716
00:37:30,200 --> 00:37:32,359
Speaker 3: a bit why this whole theory is very tied up

717
00:37:32,400 --> 00:37:36,000
Speaker 3: with this whole measurement problem in quantum mechanics, because in

718
00:37:36,040 --> 00:37:39,240
Speaker 3: this theory you don't need somebody called the measurement postulate.

719
00:37:39,560 --> 00:37:41,680
Speaker 3: In quantum mechanics, you know, you don't know where the

720
00:37:41,719 --> 00:37:43,960
Speaker 3: atom is. It could be anywhere, and then you make

721
00:37:44,000 --> 00:37:46,080
Speaker 3: a measurement and you find that it's here or there

722
00:37:46,080 --> 00:37:48,960
Speaker 3: with certain probabilities. That's called the measurement postulive. It says

723
00:37:49,400 --> 00:37:52,960
Speaker 3: that the particle when you measure it, will appear to

724
00:37:53,000 --> 00:37:56,440
Speaker 3: be somewhere with some probability. And in this theory you

725
00:37:56,480 --> 00:37:59,600
Speaker 3: don't need the measurement posture. That's where unpredictability comes in

726
00:37:59,600 --> 00:38:02,440
Speaker 3: in theory. That's the boning place that comes in, and

727
00:38:02,480 --> 00:38:05,759
Speaker 3: it's a very artificial way that it comes in, and

728
00:38:05,800 --> 00:38:07,840
Speaker 3: we don't really understand it. We kind of put that

729
00:38:07,920 --> 00:38:10,880
Speaker 3: on top of the rest of quantum theory. We put

730
00:38:10,920 --> 00:38:14,480
Speaker 3: in this measurement posture that which tells us that, okay,

731
00:38:14,560 --> 00:38:16,160
Speaker 3: you might not know where the particle is, but then

732
00:38:16,200 --> 00:38:18,960
Speaker 3: you measure it and you see it there with probability.

733
00:38:18,960 --> 00:38:21,399
Speaker 1: You have because when we measure things, we don't get

734
00:38:21,400 --> 00:38:23,600
Speaker 1: answers that are like it's half here and it's half there.

735
00:38:23,800 --> 00:38:25,640
Speaker 1: That's we get it's here or it's there. So we

736
00:38:25,680 --> 00:38:28,959
Speaker 1: have to somehow reconcile the spread and predictions of quantum

737
00:38:28,960 --> 00:38:31,240
Speaker 1: mechanics with the specific observations.

738
00:38:31,640 --> 00:38:34,000
Speaker 3: That's right, and that's where the unpredictability comes in in

739
00:38:34,080 --> 00:38:37,640
Speaker 3: quantum theory. But if you remove the measurement postulate and

740
00:38:37,680 --> 00:38:39,960
Speaker 3: in many interpretations of quantum theory. They don't like the

741
00:38:39,960 --> 00:38:43,080
Speaker 3: measurement postulate and they remove it. If you remove the

742
00:38:43,080 --> 00:38:46,680
Speaker 3: measurement postulate, then there is no unpredictability in quantum theory.

743
00:38:47,120 --> 00:38:50,000
Speaker 3: And that's what I've done here. So in this post

744
00:38:50,000 --> 00:38:53,839
Speaker 3: oneum gravity, there is no measurement postulate. So quantum theory

745
00:38:53,840 --> 00:38:57,319
Speaker 3: by itself would be predictable but not definite. So you know,

746
00:38:57,320 --> 00:39:01,960
Speaker 3: the particle doesn't have a definite position, but everything's predictable.

747
00:39:02,400 --> 00:39:05,560
Speaker 3: The particle is predictably in a superposition of being in

748
00:39:05,600 --> 00:39:08,239
Speaker 3: many places at the same time. Right right.

749
00:39:08,280 --> 00:39:10,680
Speaker 2: I feel like I might understand it better if there

750
00:39:10,680 --> 00:39:13,880
Speaker 2: were like an experiment that we could describe for how

751
00:39:13,920 --> 00:39:15,600
Speaker 2: we would test this or is this one of those

752
00:39:15,600 --> 00:39:17,839
Speaker 2: things that can't be I think string theory we can't

753
00:39:17,840 --> 00:39:20,200
Speaker 2: do an experiment to test either. Yeah. Where are we

754
00:39:20,200 --> 00:39:21,160
Speaker 2: here with experiments?

755
00:39:21,520 --> 00:39:24,000
Speaker 3: Yeah, So at the moment, there's a few experiments that

756
00:39:24,000 --> 00:39:26,359
Speaker 3: people are doing to test theory. So it turns out

757
00:39:26,400 --> 00:39:30,399
Speaker 3: that you can test both the theory specifically and then

758
00:39:30,560 --> 00:39:34,080
Speaker 3: just the proposition more generally about whether space time is quantum.

759
00:39:34,160 --> 00:39:37,560
Speaker 3: So maybe we must quantize gravity, and we now have

760
00:39:37,600 --> 00:39:40,480
Speaker 3: actually experiments which can test whether space time should be quantum,

761
00:39:40,680 --> 00:39:44,240
Speaker 3: and then those experiments will also test this particular theory,

762
00:39:44,239 --> 00:39:47,160
Speaker 3: because this is a particular theory in which space time

763
00:39:47,239 --> 00:39:49,880
Speaker 3: is not quantum. So there are experiments to do that.

764
00:39:50,600 --> 00:39:52,720
Speaker 1: And so these gold atoms you were talking about earlier,

765
00:39:53,320 --> 00:39:55,840
Speaker 1: do we have enough control of gold atoms so that

766
00:39:55,880 --> 00:39:59,640
Speaker 1: we could potentially measure their gravity and understand how there's

767
00:39:59,680 --> 00:40:02,719
Speaker 1: super position affects their bending of space time? Are we

768
00:40:02,760 --> 00:40:05,320
Speaker 1: still years away from being able to do that actual experiment.

769
00:40:05,560 --> 00:40:09,080
Speaker 3: We can measure the gravity produced by at least millimeter

770
00:40:09,200 --> 00:40:14,160
Speaker 3: sized gold spheres. But strange enough, you can test these

771
00:40:14,200 --> 00:40:18,200
Speaker 3: theories in different ways and which don't involve having to

772
00:40:18,239 --> 00:40:22,640
Speaker 3: measure gravity. They do require us to measure gravity very precisely,

773
00:40:22,920 --> 00:40:25,640
Speaker 3: but we do have that already. So we have these

774
00:40:25,680 --> 00:40:29,960
Speaker 3: satellites in empty space which have done very precise measurements

775
00:40:29,960 --> 00:40:33,359
Speaker 3: of gravity, and so we can use that, and then

776
00:40:33,640 --> 00:40:36,959
Speaker 3: we can also use you know, people are taking gold

777
00:40:37,000 --> 00:40:39,799
Speaker 3: atoms and they're putting them in this superposition of being

778
00:40:39,800 --> 00:40:43,080
Speaker 3: in many places at once, and we can use those experiments,

779
00:40:43,520 --> 00:40:46,560
Speaker 3: and then the combination of those two experiments can be

780
00:40:46,680 --> 00:40:51,200
Speaker 3: used to rule out a theory in which gravity is

781
00:40:51,200 --> 00:40:52,200
Speaker 3: not quanta.

782
00:40:52,520 --> 00:40:54,400
Speaker 2: So could you have an answer about whether or not

783
00:40:54,440 --> 00:40:57,120
Speaker 2: you're right, like in the next five years or something

784
00:40:57,360 --> 00:40:58,200
Speaker 2: that would be exciting.

785
00:40:58,760 --> 00:41:01,120
Speaker 3: Yeah, so I think it's possible in the next five years.

786
00:41:01,120 --> 00:41:03,720
Speaker 3: So we're still doing calculations to figure out how close

787
00:41:03,760 --> 00:41:07,279
Speaker 3: we are. I think five years is probably reasonable. And

788
00:41:07,320 --> 00:41:10,960
Speaker 3: then there's another set of experiments which my colleagues to

789
00:41:11,040 --> 00:41:13,759
Speaker 3: Goatto Bosa has proposed as well as a number of

790
00:41:13,760 --> 00:41:19,000
Speaker 3: other people, which is about producing something called entanglement using gravity,

791
00:41:19,320 --> 00:41:22,520
Speaker 3: and those are going to require probably you know, a decade,

792
00:41:23,160 --> 00:41:26,200
Speaker 3: maybe two decades, who knows. I mean, those are very

793
00:41:26,200 --> 00:41:29,840
Speaker 3: difficult experiments. They are as difficult as building a quantum computer.

794
00:41:30,560 --> 00:41:34,000
Speaker 3: And that's another test that will require a huge effort,

795
00:41:34,040 --> 00:41:37,200
Speaker 3: but which excitingly, we can use to determine the quantum

796
00:41:37,400 --> 00:41:39,600
Speaker 3: nature of space time. So there are experiments, and I

797
00:41:39,640 --> 00:41:41,200
Speaker 3: think that's what's exciting about this field.

798
00:41:41,320 --> 00:41:43,520
Speaker 1: Well, are you excited or terrified to see the outcome

799
00:41:43,520 --> 00:41:44,400
Speaker 1: of those experiments?

800
00:41:46,840 --> 00:41:49,239
Speaker 3: I mean, I think there's a sense that you're going

801
00:41:49,280 --> 00:41:50,960
Speaker 3: to spend a lot of time doing something you want

802
00:41:50,960 --> 00:41:53,600
Speaker 3: to find out as quickly as possible if it's quite wrong.

803
00:41:54,480 --> 00:41:56,560
Speaker 3: It's not really personal in the sense that okay, you're

804
00:41:56,560 --> 00:41:59,080
Speaker 3: going to spend some time working out of theory, and

805
00:41:59,160 --> 00:42:01,640
Speaker 3: so because you've been invested yourself in it, you kind

806
00:42:01,640 --> 00:42:03,239
Speaker 3: of wanted to be true. And I think that's true

807
00:42:03,280 --> 00:42:06,000
Speaker 3: of everyone, that the strength theory is one string theory

808
00:42:06,040 --> 00:42:07,959
Speaker 3: to be true. But at the end of the day,

809
00:42:08,120 --> 00:42:10,480
Speaker 3: we're making an assumption, we're seeing with the theory critics,

810
00:42:10,520 --> 00:42:12,440
Speaker 3: and then we're testing it. And I think that's a

811
00:42:12,440 --> 00:42:13,920
Speaker 3: good thing, regardless of the outcome.

812
00:42:14,400 --> 00:42:14,640
Speaker 5: Yeah.

813
00:42:14,680 --> 00:42:17,760
Speaker 2: Absolutely, even if the answer is no, that's a good answer.

814
00:42:17,800 --> 00:42:20,680
Speaker 2: Then people can stop looking down that particular path, Like

815
00:42:20,719 --> 00:42:22,799
Speaker 2: I think it's still worthwhile no matter what the answer is.

816
00:42:23,239 --> 00:42:23,799
Speaker 3: Yeah.

817
00:42:23,920 --> 00:42:42,759
Speaker 1: Yeah, But before we can do those experiments, we can

818
00:42:42,800 --> 00:42:47,160
Speaker 1: try to tackle some like stubborn conceptual problems that arise

819
00:42:47,280 --> 00:42:51,080
Speaker 1: from interactions of quant mechanics and gravity. I was reading

820
00:42:51,320 --> 00:42:55,160
Speaker 1: your comments on solutions to the black hole information paradox

821
00:42:55,760 --> 00:42:59,120
Speaker 1: and how your approach might untangle that problem. Can you

822
00:42:59,160 --> 00:43:02,520
Speaker 1: tell us briefly how post quantum gravity solves the black

823
00:43:02,560 --> 00:43:05,880
Speaker 1: hole information paradox? What happens to information as it falls

824
00:43:05,880 --> 00:43:06,640
Speaker 1: into a black hole.

825
00:43:06,880 --> 00:43:08,279
Speaker 3: I wouldn't just as far as to say that, What

826
00:43:08,280 --> 00:43:11,200
Speaker 3: I would say is that the paradox it kind of

827
00:43:11,200 --> 00:43:15,840
Speaker 3: loses its bite. And it's this weird thing that probabilities

828
00:43:16,040 --> 00:43:19,200
Speaker 3: seem to emerge in physics in a few different places.

829
00:43:19,960 --> 00:43:22,160
Speaker 3: As physicists, we don't like it because we believe everything

830
00:43:22,200 --> 00:43:25,600
Speaker 3: should be predictable. So the measurement problem that we've discussed

831
00:43:25,640 --> 00:43:28,160
Speaker 3: as an example of that. You know, physics only lets

832
00:43:28,239 --> 00:43:31,680
Speaker 3: us predict probabilities of a particle being in certain places,

833
00:43:31,719 --> 00:43:35,279
Speaker 3: and that is kind of information destruction, right, because we

834
00:43:35,360 --> 00:43:37,480
Speaker 3: start off with something which is in a definite state

835
00:43:37,560 --> 00:43:41,080
Speaker 3: and we end up with some indeterminism like where the

836
00:43:41,120 --> 00:43:44,040
Speaker 3: particle is, and that seems that's a kind of information loss.

837
00:43:44,640 --> 00:43:49,080
Speaker 3: And so probabilities come up in quantum theory in terms

838
00:43:49,120 --> 00:43:52,120
Speaker 3: of this measurement problem that we talked about. And then

839
00:43:52,160 --> 00:43:55,480
Speaker 3: the other place where these probabilities come up and information

840
00:43:55,560 --> 00:43:57,840
Speaker 3: loss comes up is in black hole. So in a

841
00:43:57,840 --> 00:44:00,440
Speaker 3: black hole, you throw something into the black hole which

842
00:44:00,480 --> 00:44:03,440
Speaker 3: is in a definite state. The black hole sacks that

843
00:44:03,480 --> 00:44:06,719
Speaker 3: information in and then it slowly evaporates away, and when

844
00:44:06,800 --> 00:44:11,120
Speaker 3: it finishes evaporating, it appears that we're left with just

845
00:44:11,560 --> 00:44:14,920
Speaker 3: a bunch of noise, a bunch of thermal radiation, and

846
00:44:15,000 --> 00:44:19,040
Speaker 3: so predictability seems to be lost. And physicists don't like that.

847
00:44:19,239 --> 00:44:22,480
Speaker 3: I'm okay with that. I think we've just lost predictability

848
00:44:22,520 --> 00:44:25,840
Speaker 3: in measurements, So why are we so unhappy about losing

849
00:44:25,880 --> 00:44:28,640
Speaker 3: predictability in black holes? And in fact, I think they're

850
00:44:29,160 --> 00:44:32,279
Speaker 3: probably related, and in this theory they are related. So

851
00:44:32,320 --> 00:44:35,960
Speaker 3: this theory allows for information loss because it has this

852
00:44:36,480 --> 00:44:39,680
Speaker 3: probabilistic nature. And so I spent a lot of my

853
00:44:39,760 --> 00:44:42,759
Speaker 3: time previously trying to construct a theory which allowed for

854
00:44:42,800 --> 00:44:46,160
Speaker 3: information loss. I wasn't able to do it within quantum theory,

855
00:44:46,160 --> 00:44:47,759
Speaker 3: and I don't think it's possible to do it within

856
00:44:47,840 --> 00:44:51,120
Speaker 3: quantum theory. But within this theory it's possible. And in fact,

857
00:44:51,200 --> 00:44:53,040
Speaker 3: it's a feature of the theory that you have this

858
00:44:53,160 --> 00:44:56,919
Speaker 3: information loss. So if you have information loss, then there

859
00:44:56,960 --> 00:45:01,440
Speaker 3: is no black hole paradox. The paradox arises in black holes.

860
00:45:01,800 --> 00:45:04,319
Speaker 3: If you insist that information is preserved, then you get

861
00:45:04,320 --> 00:45:08,640
Speaker 3: the paradoxes. So if you insist that information is not lost,

862
00:45:08,840 --> 00:45:11,920
Speaker 3: then you run into a whole bunch of paradoxes. And

863
00:45:11,960 --> 00:45:15,840
Speaker 3: so in this theory, information is lost, no paradox.

864
00:45:16,440 --> 00:45:18,440
Speaker 1: It sort of sounds like you solve the problem by

865
00:45:18,440 --> 00:45:21,560
Speaker 1: saying it isn't actually a problem like information is lost

866
00:45:21,560 --> 00:45:23,040
Speaker 1: by a black hole, but that's okay.

867
00:45:23,280 --> 00:45:25,800
Speaker 3: Well that was always the case for the information lost paradox,

868
00:45:25,800 --> 00:45:29,720
Speaker 3: So there was always this debate in the community where

869
00:45:30,239 --> 00:45:33,440
Speaker 3: usually general relative business to people who think space time

870
00:45:33,520 --> 00:45:36,960
Speaker 3: geometry is really what's going on, who study space time geometry,

871
00:45:36,960 --> 00:45:39,719
Speaker 3: they were always or generally tended to be okay with

872
00:45:39,800 --> 00:45:43,960
Speaker 3: information loss. Information is lost, get over it, not a problem.

873
00:45:44,080 --> 00:45:47,640
Speaker 3: It was the string theorist usually or the high endy

874
00:45:47,680 --> 00:45:51,839
Speaker 3: physicists who insisted that information should be preserved, and that's

875
00:45:51,840 --> 00:45:53,600
Speaker 3: when you get a paradox. So it's only in that

876
00:45:53,640 --> 00:45:56,680
Speaker 3: case that you run into various paradoxes. If you're willing

877
00:45:56,719 --> 00:46:00,319
Speaker 3: to accept information loss, then you probably don't call it

878
00:46:00,320 --> 00:46:03,480
Speaker 3: the black hole information paradox. You call it something else,

879
00:46:03,600 --> 00:46:07,040
Speaker 3: like the black hole information and annoyance or something.

880
00:46:08,520 --> 00:46:10,359
Speaker 2: That sounds like a better title anyway.

881
00:46:10,760 --> 00:46:12,560
Speaker 3: Yeah, not as catchy.

882
00:46:13,239 --> 00:46:15,520
Speaker 1: So to accept that, you have to accept something kind

883
00:46:15,520 --> 00:46:18,520
Speaker 1: of weird about the universe, that there is a randomness,

884
00:46:18,520 --> 00:46:21,560
Speaker 1: that it's not predictable, that one moment is not determined

885
00:46:21,560 --> 00:46:24,360
Speaker 1: by the previous moment, or even the probabilities aren't determined,

886
00:46:24,680 --> 00:46:28,879
Speaker 1: that there's something fundamentally random about gravity itself. You must

887
00:46:28,920 --> 00:46:31,640
Speaker 1: hear a lot of sort of philosophical objections. Even if

888
00:46:31,719 --> 00:46:35,360
Speaker 1: the mathematics of your concept work and you can make predictions,

889
00:46:35,480 --> 00:46:39,080
Speaker 1: you can describe experiments, you must hear philosophical objections to

890
00:46:39,320 --> 00:46:42,279
Speaker 1: having a universe that works this way. Do you hear

891
00:46:42,320 --> 00:46:44,040
Speaker 1: those objections? And what are your answers to them?

892
00:46:44,200 --> 00:46:46,400
Speaker 3: So I think as business we believe we can predict everything,

893
00:46:46,440 --> 00:46:49,279
Speaker 3: and so how can we not predict something? It must

894
00:46:49,280 --> 00:46:54,600
Speaker 3: be So it's interesting. People are okay with a breakdown predictability,

895
00:46:54,640 --> 00:46:57,000
Speaker 3: as we discussed when it comes to the measurement problem

896
00:46:57,000 --> 00:46:59,759
Speaker 3: in quantum accounts. But for some reason, I think it's

897
00:46:59,800 --> 00:47:02,880
Speaker 3: part sociological, they're not willing to accept it in black holes.

898
00:47:03,640 --> 00:47:05,759
Speaker 3: I think there's a good reason that you might not

899
00:47:05,840 --> 00:47:08,279
Speaker 3: accept it in black holes. And the good reason is

900
00:47:08,320 --> 00:47:11,759
Speaker 3: that you say, well, okay, but I can just imagine

901
00:47:11,760 --> 00:47:16,200
Speaker 3: that there is this randomness somewhere else which determines you know,

902
00:47:16,280 --> 00:47:19,440
Speaker 3: I can imagine that there's this hidden environment we call

903
00:47:19,480 --> 00:47:22,840
Speaker 3: it a hidden variable, you know, a hidden system, and

904
00:47:22,920 --> 00:47:25,200
Speaker 3: if I knew what that system was, then everything would

905
00:47:25,239 --> 00:47:28,680
Speaker 3: be predictable. So everything is predictable. It's just something that

906
00:47:28,719 --> 00:47:32,440
Speaker 3: I'm not looking at and that's I think the philosophical objection.

907
00:47:33,280 --> 00:47:34,960
Speaker 3: But you know, we know that doesn't work for the

908
00:47:34,960 --> 00:47:37,560
Speaker 3: measurement problem in quantum theory. So we know that there

909
00:47:37,600 --> 00:47:41,520
Speaker 3: could be no hidden variables, no hidden environment which could

910
00:47:41,560 --> 00:47:45,160
Speaker 3: explain the randomness in quantum theory, at least not a

911
00:47:45,160 --> 00:47:48,120
Speaker 3: local hidden variable theory. There's some very famous theorem called

912
00:47:48,120 --> 00:47:51,320
Speaker 3: Bell's theorem which tells us that, and in this theory,

913
00:47:51,480 --> 00:47:53,759
Speaker 3: although we haven't proven it, I think there is no

914
00:47:53,920 --> 00:47:58,600
Speaker 3: way to construct a hidden environment or a hidden system

915
00:47:58,600 --> 00:48:03,520
Speaker 3: which would everything and make everything predictable. You know. I

916
00:48:03,520 --> 00:48:05,960
Speaker 3: think the philosophical objection is you could always imagine some

917
00:48:06,080 --> 00:48:09,240
Speaker 3: other system which, if you knew about it, you would

918
00:48:09,960 --> 00:48:12,520
Speaker 3: be able to predict things. And I think that we

919
00:48:12,640 --> 00:48:16,480
Speaker 3: know from quantum theory, and I think this is also

920
00:48:16,480 --> 00:48:18,560
Speaker 3: true in this theory that that may not be true.

921
00:48:19,040 --> 00:48:22,799
Speaker 3: But yeah, I would say that philosophically, that's probably the

922
00:48:22,800 --> 00:48:25,040
Speaker 3: biggest stumbling block if you're going to have a problem

923
00:48:25,120 --> 00:48:27,120
Speaker 3: with this theory. I think that's the reason you're going

924
00:48:27,160 --> 00:48:28,040
Speaker 3: to be skeptical.

925
00:48:28,239 --> 00:48:31,880
Speaker 1: So then my last question is a philosophical one. Imagine

926
00:48:31,920 --> 00:48:35,799
Speaker 1: that aliens have arrived on Earth and they're scientific and

927
00:48:35,840 --> 00:48:37,920
Speaker 1: we get to talk to them, and you get to

928
00:48:37,920 --> 00:48:40,520
Speaker 1: meet with their physicists. What do you think the chances

929
00:48:40,560 --> 00:48:42,840
Speaker 1: are that they have a community that does string theory

930
00:48:42,880 --> 00:48:44,840
Speaker 1: and a community that does loop quantum gravity, and a

931
00:48:44,840 --> 00:48:47,680
Speaker 1: community that does post quantum gravity, or that they even

932
00:48:47,719 --> 00:48:51,160
Speaker 1: think quantum gravity is a hard problem or an interesting problem.

933
00:48:51,560 --> 00:48:54,200
Speaker 1: Do you think that we're probing something about the universe

934
00:48:54,239 --> 00:48:58,040
Speaker 1: here or exploring questions that have to do more with

935
00:48:58,080 --> 00:48:59,880
Speaker 1: the way humans organize our thoughts.

936
00:49:00,520 --> 00:49:03,240
Speaker 3: I feel like we understand so little about the universe.

937
00:49:04,600 --> 00:49:06,440
Speaker 3: I guess I like to think of ourselves as like these,

938
00:49:06,480 --> 00:49:08,839
Speaker 3: you know, these little single cell organisms that are kind

939
00:49:08,840 --> 00:49:13,120
Speaker 3: of slowly moving towards something they think is light. If

940
00:49:13,160 --> 00:49:15,680
Speaker 3: my dog met my cat, they would have some similar

941
00:49:16,480 --> 00:49:19,240
Speaker 3: theories about the world, but they would be pretty different,

942
00:49:19,280 --> 00:49:22,560
Speaker 3: I guess. I mean, maybe my dog and my cat

943
00:49:22,600 --> 00:49:25,120
Speaker 3: would have similar theories, but maybe my dog and my

944
00:49:25,239 --> 00:49:28,960
Speaker 3: amoeba would have really different conceptions of the universe. And

945
00:49:29,000 --> 00:49:31,640
Speaker 3: I imagine that's how it would be. I mean, you know,

946
00:49:31,640 --> 00:49:35,799
Speaker 3: maybe they're more advanced civilizations were just we would look

947
00:49:35,840 --> 00:49:37,320
Speaker 3: quite foolish to them.

948
00:49:38,640 --> 00:49:40,440
Speaker 1: Let's hope when they arrived that they don't treat us

949
00:49:40,480 --> 00:49:41,719
Speaker 1: like Amba and just.

950
00:49:43,280 --> 00:49:48,360
Speaker 3: Well they would say, okay, right, because you ignorro meba.

951
00:49:47,680 --> 00:49:48,840
Speaker 2: That might be the best outcome.

952
00:49:49,960 --> 00:49:54,680
Speaker 1: Yeah, all right, Well, thank you very much for your clear,

953
00:49:54,800 --> 00:49:58,040
Speaker 1: encouraging explanations of your your post quondum gravity. My actual

954
00:49:58,080 --> 00:50:00,160
Speaker 1: last question is why did you call it post to

955
00:50:00,239 --> 00:50:01,120
Speaker 1: quantum gravity.

956
00:50:01,440 --> 00:50:05,240
Speaker 3: Oh, because one of them theories is modified in this theory,

957
00:50:05,360 --> 00:50:07,239
Speaker 3: so you have to modify it in order to make

958
00:50:07,280 --> 00:50:10,839
Speaker 3: it consistent with geometry. I mean, there's a whole kind

959
00:50:10,880 --> 00:50:15,280
Speaker 3: of literature of post quantum theories modifications to quantum mechanics,

960
00:50:15,360 --> 00:50:18,440
Speaker 3: and this fits in there, but probably in the most

961
00:50:19,800 --> 00:50:23,160
Speaker 3: gentle way you can imagine. I mean people myself, you

962
00:50:23,200 --> 00:50:25,160
Speaker 3: would have spend a lot of time trying to imagine

963
00:50:25,760 --> 00:50:28,960
Speaker 3: theories that were really different, you know, that would go

964
00:50:29,080 --> 00:50:32,640
Speaker 3: well beyond quantum theory, and they're actually almost impossible to construct,

965
00:50:32,719 --> 00:50:35,960
Speaker 3: and they may not exist. So this may be the

966
00:50:36,040 --> 00:50:38,560
Speaker 3: only kind of modification to quantum theory that we can make.

967
00:50:38,640 --> 00:50:39,040
Speaker 3: I don't know.

968
00:50:40,800 --> 00:50:43,000
Speaker 1: Wonderful. Well, thanks again very much for your time and

969
00:50:43,040 --> 00:50:44,759
Speaker 1: your explanations. Really appreciate it.

970
00:50:44,880 --> 00:50:47,239
Speaker 3: Thank you, Thanks, thanks very much.

971
00:50:54,560 --> 00:50:58,040
Speaker 2: Daniel and Kelly's Extraordinary Universe is produced by iHeart Reading

972
00:50:58,320 --> 00:51:00,719
Speaker 2: We would love to hear from you.

973
00:51:00,239 --> 00:51:00,799
Speaker 3: Really would.

974
00:51:01,040 --> 00:51:03,600
Speaker 1: We want to know what questions do you have about

975
00:51:03,640 --> 00:51:05,640
Speaker 1: this extraordinary universe.

976
00:51:05,760 --> 00:51:08,719
Speaker 2: We want to know your thoughts on recent shows, suggestions

977
00:51:08,719 --> 00:51:11,759
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978
00:51:11,760 --> 00:51:12,160
Speaker 2: back to you.

979
00:51:12,400 --> 00:51:15,920
Speaker 1: We really mean it. We answer every message. Email us

980
00:51:15,960 --> 00:51:19,040
Speaker 1: at Questions at Danielankelly dot org, or.

981
00:51:19,040 --> 00:51:21,120
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982
00:51:21,239 --> 00:51:25,200
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983
00:51:25,200 --> 00:51:28,120
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984
00:51:28,280 --> 00:51:29,799
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