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What is post-quantum gravity? · Daniel and Kelly’s Extraordinary Universe

• 0:05
  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
• 0:11
  Speaker 1: made some pretty good progress. But about one hundred years
• 0:14
  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
• 0:27
  Speaker 1: and how tiny particles behave when you're not looking. Bringing
• 0:30
  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
• 0:37
  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.
• 1:22
  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
• 1:36
  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
• 1:43
  Speaker 2: before this conversation that we had is how great it
• 1:46
  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
• 2:01
  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.
• 2:31
  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
• 2:40
  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
• 2:54
  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.
• 3:05
  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
• 3:38
  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
• 3:45
  Speaker 2: to know and love, hopefully know and love. When we
• 3:49
  Speaker 2: were doing the Human Genome project, we felt like as
• 3:51
  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
• 3:55
  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,
• 4:01
  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,
• 4:20
  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
• 5:37
  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
• 5:50
  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
• 6:33
  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
• 9:07
  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
• 9:10
  Speaker 1: about it and I couldn't understand it. So as an
• 9:13
  Speaker 1: invitation to everybody out there, if you're reading about some
• 9:16
  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
• 9:34
  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,
• 10:48
  Speaker 3: it doesn't matter so much. But now that we're exploring
• 10:51
  Speaker 3: the quantum realm, and we're exploring the quantum realm, when
• 10:54
  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
• 11:04
  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
• 11:15
  Speaker 3: care that much about. But we know from the history
• 11:17
  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
• 11:25
  Speaker 3: fix them, everything just starts to unravel. I mean, if
• 11:27
  Speaker 3: you look at the history of quantum mechanics, there was
• 11:30
  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
• 11:40
  Speaker 3: the nineteen hundreds, we realized that all of our laws
• 11:43
  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
• 12:02
  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
• 12:08
  Speaker 3: in another place. But as you go to smaller and
• 12:11
  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
• 12:21
  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
• 12:45
  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
• 13:35
  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
• 13:45
  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
• 13:57
  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
• 14:05
  Speaker 1: are still relevant. Is that why you come up with
• 14:07
  Speaker 1: this idea of a gold at them? And why gold
• 14:09
  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
• 14:16
  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
• 14:22
  Speaker 3: than very heavy objects. That's one of the things we've
• 14:24
  Speaker 3: found out. But you know, it's true that we generally
• 14:27
  Speaker 3: think of the gravitational field as being caused by the
• 14:31
  Speaker 3: Moon and the Earth and things like that. But one
• 14:33
  Speaker 3: kilogram mass will bend. We can feel that gravitation pool
• 14:36
  Speaker 3: of a one kilogram mass. And now we're doing experiments
• 14:41
  Speaker 3: where we can feel the pull of gold, which is
• 14:44
  Speaker 3: a millimeter sphere. We can feel the gravitational pull of
• 14:47
  Speaker 3: that kind of an object. But when they get much
• 14:50
  Speaker 3: smaller than that, then we don't know what happens. And
• 14:52
  Speaker 3: we haven't even been able to really perform the experiments
• 14:54
  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
• 14:59
  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
• 15:06
  Speaker 1: theory of electromagnetism, and then folks quantized it and gave
• 15:10
  Speaker 1: us a theory of quantum electrodynamics. Why can't we also
• 15:13
  Speaker 1: just do that for gravity? Take space time, consider that
• 15:16
  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,
• 15:24
  Speaker 3: which is probably the amount of time that we've been
• 15:26
  Speaker 3: failing to quantize gravity. So yeah, you have this contradiction.
• 15:30
  Speaker 3: You have gravity which is not quantum. You want to
• 15:33
  Speaker 3: make it quantum to fit in with everything else. That
• 15:35
  Speaker 3: would be I think the thing that everyone thought that's
• 15:38
  Speaker 3: what we should be doing. But we're finding that really troublesome,
• 15:41
  Speaker 3: and there's reasons for that. And one of the I
• 15:43
  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
• 15:49
  Speaker 3: what keeps my two fingers from being able to push
• 15:52
  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
• 16:00
  Speaker 3: gravity is that, unlike the other forces, gravity is spacetime bending.
• 16:04
  Speaker 3: So the reason that the Earth goes around the Sun
• 16:08
  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
• 16:14
  Speaker 3: past it orbits around it. So it's slightly weirder than
• 16:19
  Speaker 3: that actually. I mean, you often in science centers and stuff,
• 16:22
  Speaker 3: the vehicle demonstration where you see a big planet like
• 16:25
  Speaker 3: the Sun sitting in the middle of a cone and
• 16:28
  Speaker 3: then the Earth goes around the Sun as if it's
• 16:32
  Speaker 3: caught in this vortex or funnel. It's the kind of
• 16:35
  Speaker 3: demonstration we often see to kind of give us a
• 16:37
  Speaker 3: sense of why gravity as space time bending is causing
• 16:41
  Speaker 3: the Earth to orbit the Sun.
• 16:43
  Speaker 1: Since you brought that up, do you find that to
• 16:44
  Speaker 1: be an accurate description, like a useful mental model of
• 16:47
  Speaker 1: how gravity works? Because I find it to be very confusing.
• 16:51
  Speaker 1: You have like a two D situation, and then you're
• 16:53
  Speaker 1: adding curvature in a third dimension, where general relativity has
• 16:57
  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
• 17:03
  Speaker 1: you find them to be a useful description of what's
• 17:05
  Speaker 1: happening in general relativity?
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  Speaker 3: All our descriptions are misleading. So what we do is
• 17:11
  Speaker 3: we tell ourselves lies and each other lies, and they're
• 17:14
  Speaker 3: useful for a bit, and then we replace it with
• 17:16
  Speaker 3: a better lie. And I feel like the funnel is
• 17:20
  Speaker 3: a reasonable lie because it gives you a partial sense
• 17:24
  Speaker 3: of what's happening. And like when I say a gold
• 17:27
  Speaker 3: at them can be in a superposition of two places
• 17:29
  Speaker 3: at once, that's also a bit of a lie. Because
• 17:32
  Speaker 3: you could just as well say it's in a superposition
• 17:34
  Speaker 3: of being in neither place at once. This is a
• 17:37
  Speaker 3: mental picture that we're using and it has some truth
• 17:40
  Speaker 3: to it. Also, the are parts where it fails and
• 17:44
  Speaker 3: the funnel that you see at many sigence centers, which
• 17:47
  Speaker 3: is meant to explain why space time bending causes the
• 17:51
  Speaker 3: Earth to go around the Sun. Well, a better lie
• 17:55
  Speaker 3: is that what's really happening is that time is slowing
• 17:59
  Speaker 3: down you get closer to the Sun, and the reason
• 18:02
  Speaker 3: that the Earth is going around the Sun is because
• 18:05
  Speaker 3: time slows down closer to the Sun, and the Earth
• 18:08
  Speaker 3: wants to travel in a path which causes clocks to
• 18:13
  Speaker 3: tick the least number of times. And so it's actually
• 18:15
  Speaker 3: time that's bending and traveling at a different rate at
• 18:19
  Speaker 3: different places in space, and that's why the Earth goes
• 18:23
  Speaker 3: around the Sun.
• 18:23
  Speaker 2: Why does Earth want that?
• 18:27
  Speaker 1: Earth is laid on its deadlines, Kelly, that's why I
• 18:30
  Speaker 1: hear you Earth, I hear you.
• 18:34
  Speaker 3: Yeah, Earth is traveling the shortest distance between two points.
• 18:37
  Speaker 3: And why it wants to do that, I don't know.
• 18:39
  Speaker 3: And there's lots of principles in physics where things want
• 18:43
  Speaker 3: to do. The principle least action, which is how we
• 18:46
  Speaker 3: derive most of our physical laws and was famous The
• 18:50
  Speaker 3: movie Arrival is about that principle, and we don't really
• 18:54
  Speaker 3: know why that is, but that just seems to be
• 18:56
  Speaker 3: the way it is.
• 18:56
  Speaker 1: I think it's at the level philosophically of if you
• 18:59
  Speaker 1: may this assumption, you get a model which works and
• 19:02
  Speaker 1: describes the universe very well, and then you can come
• 19:05
  Speaker 1: back to, well, what does that mean about the universe. Well,
• 19:07
  Speaker 1: we're still shrugging our shoulders over that one.
• 19:09
  Speaker 3: I think the thing you learn in physics when you're
• 19:11
  Speaker 3: in grade school, which is that you know when you
• 19:15
  Speaker 3: are in outer space and there's no force acting on you,
• 19:18
  Speaker 3: you move in a straight line. That's an example of that.
• 19:21
  Speaker 3: Why do we move in a straight line when there's
• 19:23
  Speaker 3: no force applied to it? Well, we're just taking the shortest.
• 19:27
  Speaker 1: Distance in some set.
• 19:28
  Speaker 3: We're taking the shortest path, the easiest path in some sense,
• 19:32
  Speaker 3: And that's an particular example of that. But things could
• 19:35
  Speaker 3: be different, but they're not.
• 19:37
  Speaker 1: It's also interesting to me how some things, when you
• 19:40
  Speaker 1: say then, people will just accept them because they sound intuitive,
• 19:43
  Speaker 1: and other things demand an explanation. Why do clocks tick
• 19:46
  Speaker 1: slower when you see them moving at high speed. That
• 19:49
  Speaker 1: needs an explanation. Why would clocks always tick at the
• 19:52
  Speaker 1: same speed. Why doesn't that need an explanation. So there's
• 19:54
  Speaker 1: an imbalance there because some things confront or intuition and
• 19:57
  Speaker 1: some things support it.
• 19:59
  Speaker 3: I mean, maybe this is not for this podcast, but
• 20:01
  Speaker 3: I could give you a good reason why we think
• 20:03
  Speaker 3: that the Earth is traveling the shortest distance in curved space.
• 20:07
  Speaker 3: Please do if you're willing to accept Newton's law, which
• 20:10
  Speaker 3: is that in empty space, my velocity should stay the
• 20:13
  Speaker 3: same if no force is acting upon me, and if
• 20:16
  Speaker 3: I should just move in a straight line in empty space.
• 20:20
  Speaker 3: If you accept that, then you can now ask the question, well,
• 20:24
  Speaker 3: if there's no push or pull on me, and I'm
• 20:26
  Speaker 3: in a curved space, how should I move? I think
• 20:29
  Speaker 3: it's free one answer, which is you should take the
• 20:31
  Speaker 3: shortest path, because that's what the satellite is doing an
• 20:35
  Speaker 3: empty space, when it's going around the Earth or when
• 20:38
  Speaker 3: it's going in a straight line, it's just taking the
• 20:40
  Speaker 3: shortest path.
• 20:41
  Speaker 1: I see. So you're making the argument that in flat space,
• 20:44
  Speaker 1: a straight line is the shortest path, and so if
• 20:46
  Speaker 1: you generalize to any space, including curved space, you should
• 20:49
  Speaker 1: always still take the shortest path, which in this case
• 20:52
  Speaker 1: is not a straight line. That's right, fascinating. So I
• 20:55
  Speaker 1: think we interrupted you as you were explaining why gravity
• 20:59
  Speaker 1: is weird and different and why it's harder to quantize
• 21:01
  Speaker 1: than electromagnetism. And you were explaining how gravity is actually
• 21:05
  Speaker 1: the curvature of space. Why does that make it harder
• 21:08
  Speaker 1: to quantize it. Why can't we take the same tricks
• 21:10
  Speaker 1: we applied to the electromagnetic field and apply them to
• 21:14
  Speaker 1: the metric of space time or the curvature of space time.
• 21:17
  Speaker 11: I mean, there's a bunch of technical reasons why we
• 21:20
  Speaker 11: run into trouble, but I think great conceptually is this
• 21:24
  Speaker 11: idea that gravity is about time flowing at a.
• 21:29
  Speaker 3: Different rate in different points in space, and it's about
• 21:33
  Speaker 3: the flow of time how fast it flows. And if
• 21:37
  Speaker 3: you think about it, it's almost like what are we
• 21:39
  Speaker 3: doing as physicists.
• 21:40
  Speaker 11: What we're doing, is physicists, is we're describing how things.
• 21:43
  Speaker 3: Change relative to our clock. So we have a clock
• 21:47
  Speaker 3: and it's telling the time, and we say, okay, at
• 21:50
  Speaker 3: nine am, the particles were in this configuration, and then
• 21:54
  Speaker 3: I predicted some later time they're going to be in
• 21:56
  Speaker 3: some other configuration. And so we're predicting the future based
• 22:01
  Speaker 3: on the past, and in order to do that, we
• 22:03
  Speaker 3: need to know how our clocks and our rods are
• 22:06
  Speaker 3: all behaving. And if we're going to quantize that, if
• 22:11
  Speaker 3: we're going to make our clocks and our rods and
• 22:13
  Speaker 3: the speeds at which the clock ticks and the length
• 22:16
  Speaker 3: of our rods, if that is going to be quantum,
• 22:20
  Speaker 3: then personally, I don't know how to describe physics anymore,
• 22:24
  Speaker 3: because you cut my legs out from underneath me, and
• 22:27
  Speaker 3: I no longer am able to really, I think, talk
• 22:30
  Speaker 3: about how things evolved relative to some time. If the
• 22:36
  Speaker 3: rate at which time is flowing, if that itself becomes
• 22:39
  Speaker 3: the thing I have to quantize and doesn't have a
• 22:42
  Speaker 3: definite value, because I guess remember that the thing that
• 22:45
  Speaker 3: distinguished classical things and quantum things was that classical things
• 22:50
  Speaker 3: have definite values. The coffee mug is at a definite position,
• 22:54
  Speaker 3: and quantum things don't. The gold atom that its quantum
• 22:57
  Speaker 3: can be as if it's in any places at once.
• 23:01
  Speaker 3: And so if your time can be running at many
• 23:03
  Speaker 3: different speeds, how do I tell you what is happening
• 23:08
  Speaker 3: relative to the time If I can't even really tell
• 23:10
  Speaker 3: you what time my clock is running.
• 23:12
  Speaker 1: Now, I see. So for an electron, if we just
• 23:15
  Speaker 1: ignore gravity and we think about it quantum mechanically, we
• 23:18
  Speaker 1: can handle its uncertainty and we can propagate that forward
• 23:21
  Speaker 1: in time. We have the Shorteninger equation, and we can
• 23:23
  Speaker 1: even allow for that uncertainty to create new uncertainties and
• 23:26
  Speaker 1: more uncertainties, because we always agree on a clock and
• 23:29
  Speaker 1: we can say what time is, and we can propagate
• 23:31
  Speaker 1: things forward and calculate how this uncertainty envelope is going
• 23:34
  Speaker 1: to change. But if I'm now talking about something that
• 23:37
  Speaker 1: has gravity, the additional complexity that time itself is changing
• 23:42
  Speaker 1: in a way that depends on what's happening. And so
• 23:45
  Speaker 1: time is bent by the object, and its motion then
• 23:48
  Speaker 1: depends on time. And so this this back and forth
• 23:51
  Speaker 1: interaction between the motion and time itself. Is that the complexity.
• 23:55
  Speaker 3: That's definitely part of it. Yeah, that's a big part
• 23:56
  Speaker 3: of it. It's not even clear what I mean by
• 23:58
  Speaker 3: the past and the future. It's not even definite that
• 24:01
  Speaker 3: something is to the past or to the future of
• 24:04
  Speaker 3: some other event. If I have two events that are happening,
• 24:07
  Speaker 3: you know, the mere act of setting initial conditions, which
• 24:11
  Speaker 3: is what we do in physics, we say, here are
• 24:12
  Speaker 3: the initial conditions, this is what's going to happen in
• 24:14
  Speaker 3: the future. The mere act of doing that, I think
• 24:17
  Speaker 3: becomes problematic if you're trying to quantize the space time itself,
• 24:23
  Speaker 3: or at least the flow of time and the distances.
• 24:27
  Speaker 3: If those are being quantized, then I think it's very
• 24:29
  Speaker 3: difficult to even ask the questions that we're used to
• 24:33
  Speaker 3: asking as physicists.
• 24:51
  Speaker 1: We often hear the quantum gravity is hard because there
• 24:53
  Speaker 1: are infinities, and like, maybe the universe is infinite, maybe
• 24:57
  Speaker 1: there's an infinite number of locations between me in this microphone.
• 25:01
  Speaker 1: Can you help us understand where the infinities come from
• 25:03
  Speaker 1: and also why they're a problem. Why can't we have
• 25:05
  Speaker 1: infinities in our theories.
• 25:07
  Speaker 3: Well, so this is something which is called renormalization, and
• 25:11
  Speaker 3: what that really just means is that most of our
• 25:15
  Speaker 3: physical theories they break down at very short distances. With
• 25:19
  Speaker 3: most of the theories that we have at the moment,
• 25:22
  Speaker 3: they are valid at very short distances, but gravity is not.
• 25:27
  Speaker 3: And actually, one of the things which gives me some
• 25:29
  Speaker 3: faith in this post quantum theory is that it turns
• 25:32
  Speaker 3: out we've just recently shown that it's formally renormalizable. So
• 25:35
  Speaker 3: even though quantum gravity has the property that it's not
• 25:40
  Speaker 3: valid to very short distances, this post quantum theory does
• 25:43
  Speaker 3: seem to be valid at very short distances. And that's
• 25:46
  Speaker 3: important because this description of gravity in terms of geometry,
• 25:50
  Speaker 3: if you think that that really is what gravity is,
• 25:53
  Speaker 3: that gravity really is geometry, then you believe that this
• 25:56
  Speaker 3: picture that it's geometry should hold up to the very
• 25:59
  Speaker 3: smallest scale. And so that's one of the reasons why
• 26:02
  Speaker 3: we want to have our theories be predictable at short distances.
• 26:07
  Speaker 2: So renormalizable means it doesn't matter what scale you're looking at,
• 26:13
  Speaker 2: the theory still works. Would that be a fair okay?
• 26:16
  Speaker 6: Thanks?
• 26:16
  Speaker 2: The biologist needs simplification every once in a while.
• 26:20
  Speaker 1: And what do you mean when you say it doesn't work?
• 26:23
  Speaker 1: I mean, we have a theory of particles that allows
• 26:26
  Speaker 1: us to smash electrons together tiny distances to describe the
• 26:31
  Speaker 1: creation of very heavy particles. Why doesn't that work with gravity?
• 26:33
  Speaker 1: Why does it break down? What happens?
• 26:36
  Speaker 3: Well, there are two breakdowns that happen with gravity. One
• 26:40
  Speaker 3: is something called the black hole singularity, which is that
• 26:43
  Speaker 3: black holes, which are the strongest gravitational fields we know
• 26:47
  Speaker 3: and cause light to bends to the point where they
• 26:49
  Speaker 3: can't get out those You have something called the singularity,
• 26:52
  Speaker 3: which means we just don't know what happens, and the
• 26:54
  Speaker 3: center of a black hole that is maybe related but
• 26:58
  Speaker 3: maybe not related to another kinds of infinity that we get.
• 27:02
  Speaker 3: You know, the reason that gravity has this problem and
• 27:04
  Speaker 3: our other forces don't have this problem, it's for a
• 27:08
  Speaker 3: reason that I don't know that I have a good
• 27:11
  Speaker 3: podcasting way of saying other than that's saying that the
• 27:15
  Speaker 3: Newton's contents has, you know, the wrong dimension. But there
• 27:20
  Speaker 3: is a technical reason why the usual thing you would
• 27:23
  Speaker 3: do for other forces just doesn't work with gravity. The gravity,
• 27:28
  Speaker 3: we say, has these infinities which you cannot get rid of.
• 27:31
  Speaker 3: And therefore the quantum theory of gravity, at least in
• 27:36
  Speaker 3: the three spatial dimensions that we live in or appear
• 27:40
  Speaker 3: to live in in three special dimensions, it has these infinities,
• 27:43
  Speaker 3: and we don't know what to do. And that's actually
• 27:45
  Speaker 3: the reason why we have approaches like string theory and
• 27:49
  Speaker 3: loop one and gravity theories in which you imagine that
• 27:54
  Speaker 3: space time lives on a lattice or you know, instead
• 27:59
  Speaker 3: of having point particles, we have these extended strings. Those
• 28:02
  Speaker 3: are solutions to the problem that gravity has these infinities,
• 28:07
  Speaker 3: and that's why these other approaches have been born.
• 28:10
  Speaker 1: So your background is in string theory, right, You were
• 28:13
  Speaker 1: once a string theorist. Is that a fair description?
• 28:16
  Speaker 3: Well, no, I hang out with a lot of string theorists.
• 28:18
  Speaker 3: I have string theorist friends, and I've written some papers
• 28:22
  Speaker 3: that are string theory adjacent.
• 28:24
  Speaker 1: So you don't want to be described as a string theorist.
• 28:26
  Speaker 1: But you know something about string theory and you decided
• 28:29
  Speaker 1: to take a different path than the string theory crew.
• 28:32
  Speaker 1: Why did you not follow the crowd? What is it
• 28:34
  Speaker 1: about string theory that doesn't satisfy your desire to unify
• 28:38
  Speaker 1: quantum mechanics and gravity.
• 28:40
  Speaker 3: Well, I think one of the things is that I
• 28:42
  Speaker 3: think we should take this geometric picture of gravity very seriously.
• 28:46
  Speaker 3: I mean, maybe that's just a description which breaks down
• 28:48
  Speaker 3: at some scale, but let's just try and assume that
• 28:51
  Speaker 3: that is actually what's happening, that gravity is actually spacetime bending.
• 28:56
  Speaker 3: And I think if you have that as your picture
• 28:58
  Speaker 3: as to what is happening, then quantizing it becomes a
• 29:01
  Speaker 3: lot more problematic. And string theorist for example, or you know,
• 29:05
  Speaker 3: I would say almost everybody else the various approaches to
• 29:09
  Speaker 3: quantizing gravity, they somehow need to have it at some
• 29:13
  Speaker 3: small scale. That picture is not true and breaks down
• 29:17
  Speaker 3: the geometrical picture, you mean, the geometrical picture of gravity,
• 29:20
  Speaker 3: And so if you want to hold true to this
• 29:24
  Speaker 3: geometrical picture, then I don't think you can quantize it. Now,
• 29:27
  Speaker 3: maybe that's the wrong approach. It could be that the
• 29:30
  Speaker 3: geometric picture does break down and we get to the
• 29:32
  Speaker 3: smallest scale and space time is emergent in some way,
• 29:37
  Speaker 3: or you know, we're all just fuzzy dots in a
• 29:39
  Speaker 3: lattice of space and time. All those things are possible,
• 29:42
  Speaker 3: and I'm attracted to them. They sound sci fi and great,
• 29:47
  Speaker 3: but I don't know how to make them work. I
• 29:48
  Speaker 3: don't know how to think about them. And I think
• 29:51
  Speaker 3: it's worth trying the kind of conservative imagines geometry all
• 29:55
  Speaker 3: the way down.
• 29:55
  Speaker 2: Approaching and so of all of the things that you
• 29:58
  Speaker 2: could have decided, let's assume that this this is true
• 30:01
  Speaker 2: and then work around that. Why did you pick the
• 30:04
  Speaker 2: geometry thing?
• 30:05
  Speaker 3: Yeah, I mean, I guess there's an aesthetic element to it,
• 30:09
  Speaker 3: But I think also I have a feeling that I
• 30:11
  Speaker 3: don't think we should all be doing the same thing.
• 30:14
  Speaker 3: I think that's really important for science. I have a
• 30:16
  Speaker 3: plot and cheer my string theory friends when they make
• 30:19
  Speaker 3: breakthroughs and my new port of gravity friends, et cetera,
• 30:22
  Speaker 3: et cetera. I think we should be supporting each other.
• 30:24
  Speaker 3: But I also think we should be diversifying and trying
• 30:27
  Speaker 3: as many approaches as possible. And so I'm going to
• 30:30
  Speaker 3: pick this kind of from the slightly lonely route, but
• 30:33
  Speaker 3: I think it's important that we try these different things.
• 30:35
  Speaker 3: And I think it would be dangerous to put all
• 30:38
  Speaker 3: our eggs in one basket. We're looking for a needle
• 30:40
  Speaker 3: in a haystack, and we shouldn't all be looking in the.
• 30:42
  Speaker 1: Same place, thank you. And I think that's very valuable
• 30:45
  Speaker 1: contribution to science more broadly, So let's talk about more
• 30:48
  Speaker 1: deeply your idea post quantum gravity. How is it that
• 30:52
  Speaker 1: we can avoid quantizing gravity, to take the geometry seriously
• 30:56
  Speaker 1: all the way down and still somehow handle the uncertainty
• 30:59
  Speaker 1: of quantum mechanics, I mean classical gravity. Geometric gravity says
• 31:03
  Speaker 1: we need to know where a particle is to know
• 31:06
  Speaker 1: how it bends space time. But quantum mechanics says, sometimes
• 31:09
  Speaker 1: that knowledge doesn't exist, not just that it's not known,
• 31:12
  Speaker 1: but the gold atom is partially here and partially there,
• 31:14
  Speaker 1: or has a probability to be here or there or neither.
• 31:17
  Speaker 1: So in your picture where you take geometry very seriously,
• 31:20
  Speaker 1: what happens to space in that situation? How do you
• 31:24
  Speaker 1: avoid quantizing it?
• 31:25
  Speaker 3: Yeah, you have to give up something, and the thing
• 31:28
  Speaker 3: you give up is predictability.
• 31:30
  Speaker 1: Didn't you say earlier that you needed predictability, that was
• 31:33
  Speaker 1: the problem you're doing to solve.
• 31:35
  Speaker 3: Yes, I did. Yeah, I mean it's very strange that
• 31:40
  Speaker 3: people are willing to accept that you can't predict exactly
• 31:42
  Speaker 3: where the particle is and has a certain probability of
• 31:45
  Speaker 3: being found in a certain place. So people accept that,
• 31:50
  Speaker 3: or maybe they don't accept that. You could do a
• 31:52
  Speaker 3: whole show on the different interpretations of quantum theory that
• 31:54
  Speaker 3: you haven't. Maybe you've done the show on this. But
• 31:57
  Speaker 3: quantum mechanics does have this lack of predictability, whereas classical
• 32:02
  Speaker 3: physics doesn't. Right like in classical general relativity. In Einstein's
• 32:05
  Speaker 3: theory of classical general relativity, there is only one space time, right,
• 32:09
  Speaker 3: and it is definite and space time has a definite configuration.
• 32:13
  Speaker 3: So if you're going to redd these two theories together
• 32:19
  Speaker 3: and you want to keep space time as classical in
• 32:23
  Speaker 3: the sense that it has definite features, the only way
• 32:27
  Speaker 3: to do it is and this is a difficult concept
• 32:30
  Speaker 3: to kind of get your head around. It has to
• 32:33
  Speaker 3: be both definite and unpredictable.
• 32:38
  Speaker 1: That was definitely unpredictable. Yeah, what do you mean by that?
• 32:43
  Speaker 1: What happens when you have a particle and it's potentially
• 32:47
  Speaker 1: in two different places? Does it bend space time in
• 32:50
  Speaker 1: both places? Is space time probably bent in both places?
• 32:54
  Speaker 1: Or is it random where it gets bent?
• 32:56
  Speaker 3: Well, space time has to be undergoing these random f suctuations.
• 33:01
  Speaker 3: Some people have said it's like it's wobbly. It's undergoing
• 33:04
  Speaker 3: these random fluctuations. The particle then will kind of bend
• 33:09
  Speaker 3: it in all the places that it's in, but you
• 33:11
  Speaker 3: won't be able to really tell where it's bending it.
• 33:15
  Speaker 3: So if I were to look at space time, I
• 33:17
  Speaker 3: wouldn't be able to tell where the particle is because
• 33:20
  Speaker 3: space time will be undergoing all these random fluctuations. It's
• 33:24
  Speaker 3: kind of wobbly and jumping around all over the place.
• 33:28
  Speaker 3: And so if I looked at the space time, even
• 33:30
  Speaker 3: though it was being bent by the particle, I wouldn't
• 33:33
  Speaker 3: be able to tell where the particle was. And that's
• 33:35
  Speaker 3: what you need in order to reconcile quantum theory with
• 33:40
  Speaker 3: general relativity, if you're going to keep general relativity as
• 33:44
  Speaker 3: a really theory of definite geometry.
• 33:47
  Speaker 1: I see, so space time is not emerging from some
• 33:50
  Speaker 1: deeper string theory. It fundamentally is the geometry of the universe.
• 33:54
  Speaker 1: But it's also fundamentally random. And when you say that,
• 33:58
  Speaker 1: you mean that it's and unknowable, the way it is
• 34:02
  Speaker 1: in quantum mechanics, where the information just does not exist
• 34:05
  Speaker 1: it's not determined until you measure it, or that it's
• 34:09
  Speaker 1: random but it's unknown. Like in classical physics. You know,
• 34:12
  Speaker 1: if I flip a coin and I don't look at
• 34:14
  Speaker 1: the answer, in principle, I could have calculated the outcome
• 34:17
  Speaker 1: of that coin. The information exists, and it is either
• 34:19
  Speaker 1: heads or tails under my hand, even if I haven't
• 34:22
  Speaker 1: looked at it. Is your space time random and unknowable
• 34:25
  Speaker 1: or random and unknown.
• 34:27
  Speaker 3: It's knowable in principle, I could know exactly what configuration
• 34:30
  Speaker 3: is at the present time, but it's unpredictable in that second.
• 34:34
  Speaker 3: Later I will not be able to predict oh, it
• 34:37
  Speaker 3: will evolve too, So it's more like a breakdown in
• 34:40
  Speaker 3: predictability rather than a breakdown in unknowing.
• 34:44
  Speaker 4: Nice.
• 34:44
  Speaker 3: And I think you're really right to make that distinction,
• 34:46
  Speaker 3: because you know, when people first learn quantum mechanics, they
• 34:49
  Speaker 3: might learn the Heisenberg and certainty principle where they say,
• 34:52
  Speaker 3: you know, you don't know the position of the particle,
• 34:54
  Speaker 3: you don't know where the gold ATM is. But I think,
• 34:57
  Speaker 3: as you're hinting, it's weirder than that the gold particle
• 35:00
  Speaker 3: doesn't have a position that does not exist at all.
• 35:04
  Speaker 3: And so that's maybe the difference between a kind of
• 35:07
  Speaker 3: a classical breakdown and predictability and a quantum one. A
• 35:12
  Speaker 3: classical object can be definite but unpredictable, whereas a quantum
• 35:19
  Speaker 3: system it's not knowable, but it can be predictable. Because
• 35:23
  Speaker 3: I know that's the strange thing about quantum mechanics. I
• 35:26
  Speaker 3: can actually predict with certainty how something called the wave
• 35:30
  Speaker 3: function will evolve, But the position of the particle doesn't
• 35:34
  Speaker 3: have definite value. So this notion of predictability, definiteness and
• 35:39
  Speaker 3: noable that they're all kind of tied in a very
• 35:41
  Speaker 3: strange not They're quite complicated anyway, in both classical mechanics
• 35:45
  Speaker 3: and quantum mechanics, and they really get jumbled around here.
• 35:48
  Speaker 1: So let me see if I understand the distinctions. So
• 35:50
  Speaker 1: in a classical theory, we have something which is definite,
• 35:53
  Speaker 1: spacetime has values and locations and bending, and it's also
• 35:57
  Speaker 1: predictable in that the past controls the future. The future
• 36:00
  Speaker 1: is determined by the past, whereas some kind of mechanics
• 36:03
  Speaker 1: we can predict the possible outcomes very precisely. The Shortener
• 36:06
  Speaker 1: equation tells us how to describe the possible outcomes, but
• 36:09
  Speaker 1: we don't know which one will actually be selected when
• 36:13
  Speaker 1: we interact with it. But in your theory you have
• 36:15
  Speaker 1: something which is definite but unpredictable. Does that mean that
• 36:19
  Speaker 1: the past doesn't completely control the future, that space time
• 36:22
  Speaker 1: in one moment is not determined by space time in
• 36:25
  Speaker 1: the past.
• 36:26
  Speaker 3: That's right.
• 36:27
  Speaker 2: Does that allow time travel? Let's get to the important part.
• 36:31
  Speaker 1: Here, Kelly working on her deadlines again, that's the question.
• 36:36
  Speaker 3: On everyone's mind. Can get somewhere faster? There's a lot
• 36:39
  Speaker 3: of time travel.
• 36:41
  Speaker 1: So if I imagine like an empty universe, right, no mass,
• 36:45
  Speaker 1: no radiation, nothing, Space is completely flat in your conception.
• 36:50
  Speaker 1: Is that fluctuating even though there's nothing happening to it,
• 36:53
  Speaker 1: Nothing is being inserted, nothing is moving. Is space time
• 36:57
  Speaker 1: still fluctuating just like randomly changing?
• 37:00
  Speaker 3: Yeah, but it's not that there's nothing there. There's always
• 37:02
  Speaker 3: something there. There's the vacuum. I mean, in quantum mechanics,
• 37:05
  Speaker 3: nothing's complicated. It might look like there's nothing there, but
• 37:09
  Speaker 3: at the small scale there are actually in a quantum
• 37:12
  Speaker 3: realm also these fluctuations, but in some weird way in
• 37:16
  Speaker 3: the quantum realm they look random, but they're not. They're
• 37:20
  Speaker 3: only random if you make a measurement. You know, if
• 37:23
  Speaker 3: you don't make a measurement, then they're not that random.
• 37:26
  Speaker 3: You can kind of predict how things evolve precisely. It's
• 37:30
  Speaker 3: a bit why this whole theory is very tied up
• 37:32
  Speaker 3: with this whole measurement problem in quantum mechanics, because in
• 37:36
  Speaker 3: this theory you don't need somebody called the measurement postulate.
• 37:39
  Speaker 3: In quantum mechanics, you know, you don't know where the
• 37:41
  Speaker 3: atom is. It could be anywhere, and then you make
• 37:44
  Speaker 3: a measurement and you find that it's here or there
• 37:46
  Speaker 3: with certain probabilities. That's called the measurement postulive. It says
• 37:49
  Speaker 3: that the particle when you measure it, will appear to
• 37:53
  Speaker 3: be somewhere with some probability. And in this theory you
• 37:56
  Speaker 3: don't need the measurement posture. That's where unpredictability comes in
• 37:59
  Speaker 3: in theory. That's the boning place that comes in, and
• 38:02
  Speaker 3: it's a very artificial way that it comes in, and
• 38:05
  Speaker 3: we don't really understand it. We kind of put that
• 38:07
  Speaker 3: on top of the rest of quantum theory. We put
• 38:10
  Speaker 3: in this measurement posture that which tells us that, okay,
• 38:14
  Speaker 3: you might not know where the particle is, but then
• 38:16
  Speaker 3: you measure it and you see it there with probability.
• 38:18
  Speaker 1: You have because when we measure things, we don't get
• 38:21
  Speaker 1: answers that are like it's half here and it's half there.
• 38:23
  Speaker 1: That's we get it's here or it's there. So we
• 38:25
  Speaker 1: have to somehow reconcile the spread and predictions of quantum
• 38:28
  Speaker 1: mechanics with the specific observations.
• 38:31
  Speaker 3: That's right, and that's where the unpredictability comes in in
• 38:34
  Speaker 3: quantum theory. But if you remove the measurement postulate and
• 38:37
  Speaker 3: in many interpretations of quantum theory. They don't like the
• 38:39
  Speaker 3: measurement postulate and they remove it. If you remove the
• 38:43
  Speaker 3: measurement postulate, then there is no unpredictability in quantum theory.
• 38:47
  Speaker 3: And that's what I've done here. So in this post
• 38:50
  Speaker 3: oneum gravity, there is no measurement postulate. So quantum theory
• 38:53
  Speaker 3: by itself would be predictable but not definite. So you know,
• 38:57
  Speaker 3: the particle doesn't have a definite position, but everything's predictable.
• 39:02
  Speaker 3: The particle is predictably in a superposition of being in
• 39:05
  Speaker 3: many places at the same time. Right right.
• 39:08
  Speaker 2: I feel like I might understand it better if there
• 39:10
  Speaker 2: were like an experiment that we could describe for how
• 39:13
  Speaker 2: we would test this or is this one of those
• 39:15
  Speaker 2: things that can't be I think string theory we can't
• 39:17
  Speaker 2: do an experiment to test either. Yeah. Where are we
• 39:20
  Speaker 2: here with experiments?
• 39:21
  Speaker 3: Yeah, So at the moment, there's a few experiments that
• 39:24
  Speaker 3: people are doing to test theory. So it turns out
• 39:26
  Speaker 3: that you can test both the theory specifically and then
• 39:30
  Speaker 3: just the proposition more generally about whether space time is quantum.
• 39:34
  Speaker 3: So maybe we must quantize gravity, and we now have
• 39:37
  Speaker 3: actually experiments which can test whether space time should be quantum,
• 39:40
  Speaker 3: and then those experiments will also test this particular theory,
• 39:44
  Speaker 3: because this is a particular theory in which space time
• 39:47
  Speaker 3: is not quantum. So there are experiments to do that.
• 39:50
  Speaker 1: And so these gold atoms you were talking about earlier,
• 39:53
  Speaker 1: do we have enough control of gold atoms so that
• 39:55
  Speaker 1: we could potentially measure their gravity and understand how there's
• 39:59
  Speaker 1: super position affects their bending of space time? Are we
• 40:02
  Speaker 1: still years away from being able to do that actual experiment.
• 40:05
  Speaker 3: We can measure the gravity produced by at least millimeter
• 40:09
  Speaker 3: sized gold spheres. But strange enough, you can test these
• 40:14
  Speaker 3: theories in different ways and which don't involve having to
• 40:18
  Speaker 3: measure gravity. They do require us to measure gravity very precisely,
• 40:22
  Speaker 3: but we do have that already. So we have these
• 40:25
  Speaker 3: satellites in empty space which have done very precise measurements
• 40:29
  Speaker 3: of gravity, and so we can use that, and then
• 40:33
  Speaker 3: we can also use you know, people are taking gold
• 40:37
  Speaker 3: atoms and they're putting them in this superposition of being
• 40:39
  Speaker 3: in many places at once, and we can use those experiments,
• 40:43
  Speaker 3: and then the combination of those two experiments can be
• 40:46
  Speaker 3: used to rule out a theory in which gravity is
• 40:51
  Speaker 3: not quanta.
• 40:52
  Speaker 2: So could you have an answer about whether or not
• 40:54
  Speaker 2: you're right, like in the next five years or something
• 40:57
  Speaker 2: that would be exciting.
• 40:58
  Speaker 3: Yeah, so I think it's possible in the next five years.
• 41:01
  Speaker 3: So we're still doing calculations to figure out how close
• 41:03
  Speaker 3: we are. I think five years is probably reasonable. And
• 41:07
  Speaker 3: then there's another set of experiments which my colleagues to
• 41:11
  Speaker 3: Goatto Bosa has proposed as well as a number of
• 41:13
  Speaker 3: other people, which is about producing something called entanglement using gravity,
• 41:19
  Speaker 3: and those are going to require probably you know, a decade,
• 41:23
  Speaker 3: maybe two decades, who knows. I mean, those are very
• 41:26
  Speaker 3: difficult experiments. They are as difficult as building a quantum computer.
• 41:30
  Speaker 3: And that's another test that will require a huge effort,
• 41:34
  Speaker 3: but which excitingly, we can use to determine the quantum
• 41:37
  Speaker 3: nature of space time. So there are experiments, and I
• 41:39
  Speaker 3: think that's what's exciting about this field.
• 41:41
  Speaker 1: Well, are you excited or terrified to see the outcome
• 41:43
  Speaker 1: of those experiments?
• 41:46
  Speaker 3: I mean, I think there's a sense that you're going
• 41:49
  Speaker 3: to spend a lot of time doing something you want
• 41:50
  Speaker 3: to find out as quickly as possible if it's quite wrong.
• 41:54
  Speaker 3: It's not really personal in the sense that okay, you're
• 41:56
  Speaker 3: going to spend some time working out of theory, and
• 41:59
  Speaker 3: so because you've been invested yourself in it, you kind
• 42:01
  Speaker 3: of wanted to be true. And I think that's true
• 42:03
  Speaker 3: of everyone, that the strength theory is one string theory
• 42:06
  Speaker 3: to be true. But at the end of the day,
• 42:08
  Speaker 3: we're making an assumption, we're seeing with the theory critics,
• 42:10
  Speaker 3: and then we're testing it. And I think that's a
• 42:12
  Speaker 3: good thing, regardless of the outcome.
• 42:14
  Speaker 5: Yeah.
• 42:14
  Speaker 2: Absolutely, even if the answer is no, that's a good answer.
• 42:17
  Speaker 2: Then people can stop looking down that particular path, Like
• 42:20
  Speaker 2: I think it's still worthwhile no matter what the answer is.
• 42:23
  Speaker 3: Yeah.
• 42:23
  Speaker 1: Yeah, But before we can do those experiments, we can
• 42:42
  Speaker 1: try to tackle some like stubborn conceptual problems that arise
• 42:47
  Speaker 1: from interactions of quant mechanics and gravity. I was reading
• 42:51
  Speaker 1: your comments on solutions to the black hole information paradox
• 42:55
  Speaker 1: and how your approach might untangle that problem. Can you
• 42:59
  Speaker 1: tell us briefly how post quantum gravity solves the black
• 43:02
  Speaker 1: hole information paradox? What happens to information as it falls
• 43:05
  Speaker 1: into a black hole.
• 43:06
  Speaker 3: I wouldn't just as far as to say that, What
• 43:08
  Speaker 3: I would say is that the paradox it kind of
• 43:11
  Speaker 3: loses its bite. And it's this weird thing that probabilities
• 43:16
  Speaker 3: seem to emerge in physics in a few different places.
• 43:19
  Speaker 3: As physicists, we don't like it because we believe everything
• 43:22
  Speaker 3: should be predictable. So the measurement problem that we've discussed
• 43:25
  Speaker 3: as an example of that. You know, physics only lets
• 43:28
  Speaker 3: us predict probabilities of a particle being in certain places,
• 43:31
  Speaker 3: and that is kind of information destruction, right, because we
• 43:35
  Speaker 3: start off with something which is in a definite state
• 43:37
  Speaker 3: and we end up with some indeterminism like where the
• 43:41
  Speaker 3: particle is, and that seems that's a kind of information loss.
• 43:44
  Speaker 3: And so probabilities come up in quantum theory in terms
• 43:49
  Speaker 3: of this measurement problem that we talked about. And then
• 43:52
  Speaker 3: the other place where these probabilities come up and information
• 43:55
  Speaker 3: loss comes up is in black hole. So in a
• 43:57
  Speaker 3: black hole, you throw something into the black hole which
• 44:00
  Speaker 3: is in a definite state. The black hole sacks that
• 44:03
  Speaker 3: information in and then it slowly evaporates away, and when
• 44:06
  Speaker 3: it finishes evaporating, it appears that we're left with just
• 44:11
  Speaker 3: a bunch of noise, a bunch of thermal radiation, and
• 44:15
  Speaker 3: so predictability seems to be lost. And physicists don't like that.
• 44:19
  Speaker 3: I'm okay with that. I think we've just lost predictability
• 44:22
  Speaker 3: in measurements, So why are we so unhappy about losing
• 44:25
  Speaker 3: predictability in black holes? And in fact, I think they're
• 44:29
  Speaker 3: probably related, and in this theory they are related. So
• 44:32
  Speaker 3: this theory allows for information loss because it has this
• 44:36
  Speaker 3: probabilistic nature. And so I spent a lot of my
• 44:39
  Speaker 3: time previously trying to construct a theory which allowed for
• 44:42
  Speaker 3: information loss. I wasn't able to do it within quantum theory,
• 44:46
  Speaker 3: and I don't think it's possible to do it within
• 44:47
  Speaker 3: quantum theory. But within this theory it's possible. And in fact,
• 44:51
  Speaker 3: it's a feature of the theory that you have this
• 44:53
  Speaker 3: information loss. So if you have information loss, then there
• 44:56
  Speaker 3: is no black hole paradox. The paradox arises in black holes.
• 45:01
  Speaker 3: If you insist that information is preserved, then you get
• 45:04
  Speaker 3: the paradoxes. So if you insist that information is not lost,
• 45:08
  Speaker 3: then you run into a whole bunch of paradoxes. And
• 45:11
  Speaker 3: so in this theory, information is lost, no paradox.
• 45:16
  Speaker 1: It sort of sounds like you solve the problem by
• 45:18
  Speaker 1: saying it isn't actually a problem like information is lost
• 45:21
  Speaker 1: by a black hole, but that's okay.
• 45:23
  Speaker 3: Well that was always the case for the information lost paradox,
• 45:25
  Speaker 3: So there was always this debate in the community where
• 45:30
  Speaker 3: usually general relative business to people who think space time
• 45:33
  Speaker 3: geometry is really what's going on, who study space time geometry,
• 45:36
  Speaker 3: they were always or generally tended to be okay with
• 45:39
  Speaker 3: information loss. Information is lost, get over it, not a problem.
• 45:44
  Speaker 3: It was the string theorist usually or the high endy
• 45:47
  Speaker 3: physicists who insisted that information should be preserved, and that's
• 45:51
  Speaker 3: when you get a paradox. So it's only in that
• 45:53
  Speaker 3: case that you run into various paradoxes. If you're willing
• 45:56
  Speaker 3: to accept information loss, then you probably don't call it
• 46:00
  Speaker 3: the black hole information paradox. You call it something else,
• 46:03
  Speaker 3: like the black hole information and annoyance or something.
• 46:08
  Speaker 2: That sounds like a better title anyway.
• 46:10
  Speaker 3: Yeah, not as catchy.
• 46:13
  Speaker 1: So to accept that, you have to accept something kind
• 46:15
  Speaker 1: of weird about the universe, that there is a randomness,
• 46:18
  Speaker 1: that it's not predictable, that one moment is not determined
• 46:21
  Speaker 1: by the previous moment, or even the probabilities aren't determined,
• 46:24
  Speaker 1: that there's something fundamentally random about gravity itself. You must
• 46:28
  Speaker 1: hear a lot of sort of philosophical objections. Even if
• 46:31
  Speaker 1: the mathematics of your concept work and you can make predictions,
• 46:35
  Speaker 1: you can describe experiments, you must hear philosophical objections to
• 46:39
  Speaker 1: having a universe that works this way. Do you hear
• 46:42
  Speaker 1: those objections? And what are your answers to them?
• 46:44
  Speaker 3: So I think as business we believe we can predict everything,
• 46:46
  Speaker 3: and so how can we not predict something? It must
• 46:49
  Speaker 3: be So it's interesting. People are okay with a breakdown predictability,
• 46:54
  Speaker 3: as we discussed when it comes to the measurement problem
• 46:57
  Speaker 3: in quantum accounts. But for some reason, I think it's
• 46:59
  Speaker 3: part sociological, they're not willing to accept it in black holes.
• 47:03
  Speaker 3: I think there's a good reason that you might not
• 47:05
  Speaker 3: accept it in black holes. And the good reason is
• 47:08
  Speaker 3: that you say, well, okay, but I can just imagine
• 47:11
  Speaker 3: that there is this randomness somewhere else which determines you know,
• 47:16
  Speaker 3: I can imagine that there's this hidden environment we call
• 47:19
  Speaker 3: it a hidden variable, you know, a hidden system, and
• 47:22
  Speaker 3: if I knew what that system was, then everything would
• 47:25
  Speaker 3: be predictable. So everything is predictable. It's just something that
• 47:28
  Speaker 3: I'm not looking at and that's I think the philosophical objection.
• 47:33
  Speaker 3: But you know, we know that doesn't work for the
• 47:34
  Speaker 3: measurement problem in quantum theory. So we know that there
• 47:37
  Speaker 3: could be no hidden variables, no hidden environment which could
• 47:41
  Speaker 3: explain the randomness in quantum theory, at least not a
• 47:45
  Speaker 3: local hidden variable theory. There's some very famous theorem called
• 47:48
  Speaker 3: Bell's theorem which tells us that, and in this theory,
• 47:51
  Speaker 3: although we haven't proven it, I think there is no
• 47:53
  Speaker 3: way to construct a hidden environment or a hidden system
• 47:58
  Speaker 3: which would everything and make everything predictable. You know. I
• 48:03
  Speaker 3: think the philosophical objection is you could always imagine some
• 48:06
  Speaker 3: other system which, if you knew about it, you would
• 48:09
  Speaker 3: be able to predict things. And I think that we
• 48:12
  Speaker 3: know from quantum theory, and I think this is also
• 48:16
  Speaker 3: true in this theory that that may not be true.
• 48:19
  Speaker 3: But yeah, I would say that philosophically, that's probably the
• 48:22
  Speaker 3: biggest stumbling block if you're going to have a problem
• 48:25
  Speaker 3: with this theory. I think that's the reason you're going
• 48:27
  Speaker 3: to be skeptical.
• 48:28
  Speaker 1: So then my last question is a philosophical one. Imagine
• 48:31
  Speaker 1: that aliens have arrived on Earth and they're scientific and
• 48:35
  Speaker 1: we get to talk to them, and you get to
• 48:37
  Speaker 1: meet with their physicists. What do you think the chances
• 48:40
  Speaker 1: are that they have a community that does string theory
• 48:42
  Speaker 1: and a community that does loop quantum gravity, and a
• 48:44
  Speaker 1: community that does post quantum gravity, or that they even
• 48:47
  Speaker 1: think quantum gravity is a hard problem or an interesting problem.
• 48:51
  Speaker 1: Do you think that we're probing something about the universe
• 48:54
  Speaker 1: here or exploring questions that have to do more with
• 48:58
  Speaker 1: the way humans organize our thoughts.
• 49:00
  Speaker 3: I feel like we understand so little about the universe.
• 49:04
  Speaker 3: I guess I like to think of ourselves as like these,
• 49:06
  Speaker 3: you know, these little single cell organisms that are kind
• 49:08
  Speaker 3: of slowly moving towards something they think is light. If
• 49:13
  Speaker 3: my dog met my cat, they would have some similar
• 49:16
  Speaker 3: theories about the world, but they would be pretty different,
• 49:19
  Speaker 3: I guess. I mean, maybe my dog and my cat
• 49:22
  Speaker 3: would have similar theories, but maybe my dog and my
• 49:25
  Speaker 3: amoeba would have really different conceptions of the universe. And
• 49:29
  Speaker 3: I imagine that's how it would be. I mean, you know,
• 49:31
  Speaker 3: maybe they're more advanced civilizations were just we would look
• 49:35
  Speaker 3: quite foolish to them.
• 49:38
  Speaker 1: Let's hope when they arrived that they don't treat us
• 49:40
  Speaker 1: like Amba and just.
• 49:43
  Speaker 3: Well they would say, okay, right, because you ignorro meba.
• 49:47
  Speaker 2: That might be the best outcome.
• 49:49
  Speaker 1: Yeah, all right, Well, thank you very much for your clear,
• 49:54
  Speaker 1: encouraging explanations of your your post quondum gravity. My actual
• 49:58
  Speaker 1: last question is why did you call it post to
• 50:00
  Speaker 1: quantum gravity.
• 50:01
  Speaker 3: Oh, because one of them theories is modified in this theory,
• 50:05
  Speaker 3: so you have to modify it in order to make
• 50:07
  Speaker 3: it consistent with geometry. I mean, there's a whole kind
• 50:10
  Speaker 3: of literature of post quantum theories modifications to quantum mechanics,
• 50:15
  Speaker 3: and this fits in there, but probably in the most
• 50:19
  Speaker 3: gentle way you can imagine. I mean people myself, you
• 50:23
  Speaker 3: would have spend a lot of time trying to imagine
• 50:25
  Speaker 3: theories that were really different, you know, that would go
• 50:29
  Speaker 3: well beyond quantum theory, and they're actually almost impossible to construct,
• 50:32
  Speaker 3: and they may not exist. So this may be the
• 50:36
  Speaker 3: only kind of modification to quantum theory that we can make.
• 50:38
  Speaker 3: I don't know.
• 50:40
  Speaker 1: Wonderful. Well, thanks again very much for your time and
• 50:43
  Speaker 1: your explanations. Really appreciate it.
• 50:44
  Speaker 3: Thank you, Thanks, thanks very much.
• 50:54
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• 50:58
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• 51:00
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• 51:03
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