WEBVTT - What is post-quantum gravity? 0:00:05.880 --> 0:00:08.760 Humans have been working on the mysteries of the universe 0:00:08.840 --> 0:00:11.280 for quite a long time now, and you know, we've 0:00:11.320 --> 0:00:14.760 made some pretty good progress. But about one hundred years 0:00:14.760 --> 0:00:18.239 ago we kind of got stuck. We have two big 0:00:18.280 --> 0:00:22.400 theories of physics, general relativity that describes space and time 0:00:22.480 --> 0:00:26.960 and how stuff moves, and quantum mechanics that describes probabilities 0:00:27.000 --> 0:00:30.440 and how tiny particles behave when you're not looking. Bringing 0:00:30.480 --> 0:00:35.120 these two ideas together into one harmonious explanation of everything 0:00:35.159 --> 0:00:37.280 has been a real puzzle that it's stumped some of 0:00:37.280 --> 0:00:41.199 the greatest thinkers in history. We have some famous ideas 0:00:41.200 --> 0:00:44.120 that try to tackle it, string theory, loop quantum gravity. 0:00:44.640 --> 0:00:47.400 Then there's some fringe ideas that very few people take 0:00:47.520 --> 0:00:51.839 very seriously from Wolfram or Weinstein. But what if everybody's 0:00:51.960 --> 0:00:55.840 taking the wrong approach. What if instead of attacking the problem, 0:00:56.040 --> 0:00:59.640 the right strategy is to actually sidestep it and reframe 0:00:59.800 --> 0:01:02.639 the question. That's what we'll do today when we talk 0:01:02.680 --> 0:01:08.720 about post quantum gravity. Welcome to Daniel and Kelly's Extraordinary Universe. 0:01:22.480 --> 0:01:25.720 Hello, I'm Kelly Wiener Smith. I'm a parasitologist, and today 0:01:25.760 --> 0:01:28.160 we are going way out of my comfort zone. To 0:01:28.200 --> 0:01:30.240 discuss post quantum gravity. 0:01:31.200 --> 0:01:33.680 Hi, I'm Daniel. I'm a particle physicist, and I don't 0:01:33.720 --> 0:01:36.840 even understand pre quantum gravity, not to mention a post 0:01:36.920 --> 0:01:37.759 quantum gravity. 0:01:38.040 --> 0:01:40.800 You know, Daniel. One thing I appreciate about my field 0:01:40.840 --> 0:01:43.520 of study that I don't think I really appreciated enough 0:01:43.760 --> 0:01:46.520 before this conversation that we had is how great it 0:01:46.600 --> 0:01:48.200 is that I know where fish are and I can 0:01:48.200 --> 0:01:48.680 count them. 0:01:51.320 --> 0:01:54.320 That's because you've never met a post quantum fish yet, Kelly. 0:01:54.360 --> 0:01:55.480 They're very unpredictable. 0:01:55.960 --> 0:01:57.840 Uh, you know, No, I think that the fish are 0:01:57.840 --> 0:01:59.760 going to be way more predictable. And Zachowy likes to 0:01:59.800 --> 0:02:01.560 Joe that Kelly's job is to figure out what the 0:02:01.560 --> 0:02:03.720 fish are up to, and I think that's way easier 0:02:03.760 --> 0:02:06.360 than figuring out what the electrodes are up to. So 0:02:06.400 --> 0:02:08.280 what kind of personality type do you think ends up 0:02:08.280 --> 0:02:09.320 in both of these fields? 0:02:09.800 --> 0:02:12.639 Hmmm, Yeah, that's a great question. You know, I think 0:02:12.720 --> 0:02:15.400 people who like to deal with the tiniest bits of 0:02:15.400 --> 0:02:17.960 the universe or people have to give up the concreteness 0:02:18.040 --> 0:02:20.880 of being able to see what you're working on, you know, 0:02:20.960 --> 0:02:23.240 being able to like look at your experiment and say, oh, 0:02:23.280 --> 0:02:25.079 I see what it's doing, or I can take pictures 0:02:25.120 --> 0:02:27.839 of it at least. So this is like leap into 0:02:27.880 --> 0:02:31.519 the abstract realm where you just have this like mathematical scaffolding. 0:02:31.720 --> 0:02:33.760 You have to just sort of trust and you've got 0:02:33.800 --> 0:02:35.959 to be sort of into that, you know, mental puzzles 0:02:36.000 --> 0:02:40.600 and mathematical mazes. But it's often unsatisfactory, right, Like we 0:02:40.639 --> 0:02:43.160 build these huge machines, we collide these particles. We can't 0:02:43.200 --> 0:02:45.560 even really look at what's happening at their core. 0:02:45.960 --> 0:02:47.800 So I agree with you and I disagree. So like 0:02:47.840 --> 0:02:50.360 I study fish, but what I'm really interested are the 0:02:50.360 --> 0:02:52.960 parasites that are inside of them that I can't see 0:02:53.000 --> 0:02:54.919 and that I have to like try to study in 0:02:54.919 --> 0:02:58.079 indirect ways. Maybe I split the difference. Still, I guess 0:02:58.120 --> 0:03:00.760 at the end of the day, I humanely euthanize those 0:03:00.760 --> 0:03:02.840 fish and open them up to see the parasites, which 0:03:02.880 --> 0:03:04.800 is much easier than seeing the particles. 0:03:05.000 --> 0:03:08.240 I have this fantasy sometimes about solving most of science 0:03:08.720 --> 0:03:12.320 just by having like perfect visualization. Like if you could 0:03:12.400 --> 0:03:15.280 watch anything anywhere in the universe at any time, you 0:03:15.320 --> 0:03:19.120 could like zoom in arbitrarily and just look at stuff, 0:03:19.360 --> 0:03:21.639 you would understand what happens, like how do viruses kill 0:03:21.639 --> 0:03:24.040 a bacteria? We'll just watch it, you know. But most 0:03:24.040 --> 0:03:27.000 of our science is so limited by not having the instrumentation, 0:03:27.120 --> 0:03:29.400 but not being able to see what we want to see, 0:03:29.400 --> 0:03:32.919 and having to have all these indirect probes growing bacteria, 0:03:33.040 --> 0:03:36.320 seeing if they die, speculating about the mechanisms. It'd be 0:03:36.360 --> 0:03:38.600 amazing to be able to just like X ray the 0:03:38.640 --> 0:03:42.200 whole universe and know what's happening, because then the explanations 0:03:42.200 --> 0:03:43.520 I feel like they would just fall out. 0:03:43.800 --> 0:03:45.840 Well, so to be the wet blanket we've all come 0:03:45.880 --> 0:03:49.320 to know and love, hopefully know and love. When we 0:03:49.320 --> 0:03:51.480 were doing the Human Genome project, we felt like as 0:03:51.480 --> 0:03:53.480 soon as we could like get down to that level, 0:03:53.640 --> 0:03:55.920 suddenly we'd be able to like solve all cancer. And 0:03:55.920 --> 0:03:58.320 now we're working on the Human Connectome project, where we're 0:03:58.320 --> 0:04:01.120 going to like know about all the connections in our brains, 0:04:01.160 --> 0:04:03.600 like all of our neurons connect. But I don't feel 0:04:03.600 --> 0:04:06.360 like immediately being able to see things at a higher 0:04:06.440 --> 0:04:09.080 level gives us the answer. And I guess you're saying, 0:04:09.120 --> 0:04:12.280 once we can see everything, I still feel like there 0:04:12.280 --> 0:04:14.120 would be decades of us trying to be like, well, 0:04:14.160 --> 0:04:15.440 what the heck does it all mean? 0:04:15.720 --> 0:04:17.880 I just need one more level of zoom, bro, just 0:04:17.920 --> 0:04:20.080 one more level of zoom, and I'll figure it all out, 0:04:20.279 --> 0:04:20.560 all right. 0:04:20.560 --> 0:04:22.719 Well, the biologists have been saying that for a long time, 0:04:22.800 --> 0:04:25.960 and so far we're discovering it just makes things more complicated. 0:04:25.960 --> 0:04:26.800 But I hope you're right. 0:04:26.920 --> 0:04:27.280 All right. 0:04:27.320 --> 0:04:29.760 Well, a big group of physicists have been zooming in 0:04:29.920 --> 0:04:32.640 on the fundamental nature of the universe, trying to understand 0:04:32.680 --> 0:04:35.400 how it all works, what is the universe really made 0:04:35.400 --> 0:04:37.920 out of it as smallest scale, what is the bedrock 0:04:38.040 --> 0:04:41.440 foundation of the universe itself? And they've all run into 0:04:41.480 --> 0:04:44.880 a problem trying to understand how gravity and quantum mechanics 0:04:44.880 --> 0:04:48.040 come together. So we've had a few episodes about various 0:04:48.040 --> 0:04:51.040 solutions to one of the biggest puzzles in modern physics, 0:04:51.160 --> 0:04:54.200 quantum gravity. And today we have another episode talking about 0:04:54.200 --> 0:04:58.520 the theory of post quantum gravity, which is an intriguingly 0:04:58.600 --> 0:04:59.880 compact name for. 0:04:59.800 --> 0:05:02.640 A Yeah, and you know, I've really been enjoying looking 0:05:02.680 --> 0:05:05.800 at this problem of quantum mechanics in general relativity and 0:05:05.800 --> 0:05:08.440 how we make them work together from a couple different angles, 0:05:08.920 --> 0:05:11.159 And you know, it's exciting to get to hear yet 0:05:11.200 --> 0:05:12.200 another angle today. 0:05:13.120 --> 0:05:15.279 Soon we'll be interviewing fish about it, right, to see 0:05:15.279 --> 0:05:17.720 what is the fish perspective on quantum gravity. 0:05:17.960 --> 0:05:20.320 I mean, I hope, but you know, as an experimentalist, 0:05:20.400 --> 0:05:22.680 I love that at the end there are actually experiments 0:05:22.680 --> 0:05:24.440 that can be tested to figure out if this one 0:05:24.560 --> 0:05:26.919 is correct or not. So everyone's just going to have 0:05:26.960 --> 0:05:29.159 to hold their breath, that's right. 0:05:29.040 --> 0:05:31.440 And so at the beginning, as usual, we ask people 0:05:31.440 --> 0:05:34.320 what they think about post quantum gravity or if they 0:05:34.360 --> 0:05:37.280 even know what it is. Thank you very much to 0:05:37.320 --> 0:05:39.640 everybody who sends in their thoughts. If you'd like to 0:05:39.680 --> 0:05:42.440 be part of this crew, just email us to questions 0:05:42.440 --> 0:05:45.039 at danielon Kelly dot org. You can play along at home. 0:05:45.080 --> 0:05:47.800 It's very easy and fun. So think about it for 0:05:47.839 --> 0:05:50.200 a minute. What do you think the theory of post 0:05:50.320 --> 0:05:54.279 quantum gravity is. Here's what our listeners had to say. 0:05:55.080 --> 0:06:00.520 Post quantum gravity, I would guess, is the products of 0:06:00.600 --> 0:06:06.520 finding out theory that explains quantum gravity. So what the 0:06:06.520 --> 0:06:08.320 benefits of such a theory would be. 0:06:08.760 --> 0:06:13.400 The name suggests that there is two types of gravity, 0:06:14.080 --> 0:06:18.880 one that does before quantum effects and one that does 0:06:19.440 --> 0:06:23.479 after quantum effects, and post quantum gravity would be that 0:06:24.800 --> 0:06:29.440 it only appears if quantum effects are done with their work. 0:06:29.960 --> 0:06:33.080 I would guess that the gravity that happened after the 0:06:33.120 --> 0:06:37.360 Big Bang turned everything from actual energy into matter. 0:06:37.520 --> 0:06:41.320 When you spill your cereal in the morning for your coffee. 0:06:41.760 --> 0:06:46.040 The reality inside a black hole where the quantum realm 0:06:46.800 --> 0:06:50.880 is affected by gravity at its most extreme. 0:06:50.880 --> 0:06:56.520 Gravity that happens when particles drink their milk and grow 0:06:56.640 --> 0:07:00.440 up from being quantum particles to regular sized particles, and 0:07:00.480 --> 0:07:03.800 then they have regular size gravity. Pop quiz Tricia, what 0:07:03.880 --> 0:07:06.400 is post quantum gravity gravity? 0:07:06.760 --> 0:07:08.680 After children? 0:07:09.640 --> 0:07:10.800 I think that's postpartum. 0:07:11.400 --> 0:07:14.960 Post quantum gravity is like post punk rock, the notion 0:07:15.040 --> 0:07:16.480 of that the idea is not working. 0:07:16.760 --> 0:07:17.720 We should just move on. 0:07:18.400 --> 0:07:23.040 I thought that gravity was basically irrelevant at the quantum level, 0:07:23.160 --> 0:07:26.360 so I can't say I have any idea. 0:07:27.000 --> 0:07:31.000 I would say the new physics that we find after 0:07:31.240 --> 0:07:32.800 we find quantum gravity. 0:07:32.960 --> 0:07:36.680 Post quantum gravity is the conceptualization of what we think 0:07:36.720 --> 0:07:41.600 of is gravity, taking into account that neither general relativity 0:07:41.960 --> 0:07:47.680 nor quantum mechanics completely explains the phenomenon that we experience. 0:07:48.240 --> 0:07:51.960 Maybe post quantum gravity is the state of society after 0:07:52.000 --> 0:07:54.760 we figure out what quantum gravity is, and we're bored 0:07:54.920 --> 0:07:56.960 because we don't have any more problems to solve. 0:07:57.480 --> 0:08:01.600 David Letterman's theory of gravity. What you start looking for 0:08:02.160 --> 0:08:06.200 when you give up on the idea of quantum gravity. 0:08:06.800 --> 0:08:09.200 I believe it is a term that refers to the 0:08:09.320 --> 0:08:13.240 facts of gravity beyond this scale of particles, maybe a 0:08:13.280 --> 0:08:17.760 theory that tries to explain gravity in an ever smaller scale. 0:08:18.040 --> 0:08:22.880 Post quantum gravity is a theory of gravity that finally 0:08:23.200 --> 0:08:29.480 reveals that gravity actually does not obey quantum rules. 0:08:29.840 --> 0:08:34.760 I think post quantum gravity might be the effects of 0:08:34.920 --> 0:08:39.760 gravity at quantum scales, down at the particle level, where 0:08:39.760 --> 0:08:41.800 it's got to fight it out with all the other 0:08:41.880 --> 0:08:42.880 forces down there. 0:08:43.400 --> 0:08:46.040 All right, Kelly, did these responses align with your thoughts 0:08:46.040 --> 0:08:47.679 of post quantum gravity. 0:08:47.520 --> 0:08:51.600 That we were all apparently comparably confused? Yes, I feel 0:08:51.600 --> 0:08:54.560 like we have a particularly intelligent audience. It made me 0:08:54.600 --> 0:08:57.360 feel better that I did not know what post quantum 0:08:57.360 --> 0:08:59.720 gravity was when I heard these answers. 0:08:59.720 --> 0:09:03.800 So well. It's sort of a niche idea in a 0:09:03.920 --> 0:09:07.120 niche field of quantum gravity, but it's one that a 0:09:07.200 --> 0:09:09.319 listener wrote into me and said, Hey, can you explain 0:09:09.400 --> 0:09:10.960 what this is? I was trying to read an article 0:09:10.960 --> 0:09:13.720 about it and I couldn't understand it. So as an 0:09:13.760 --> 0:09:16.000 invitation to everybody out there, if you're reading about some 0:09:16.080 --> 0:09:18.280 niche theory in physics and it doesn't make sense to you, 0:09:18.360 --> 0:09:20.520 please write to us. We will break it down for you. 0:09:20.760 --> 0:09:22.800 And we're lucky enough on this episode to have as 0:09:22.800 --> 0:09:25.319 a guest the guy who then did post quantum gravity 0:09:25.320 --> 0:09:27.800 and does a pretty solid job of explaining it. 0:09:27.960 --> 0:09:29.920 Totally solid. Yes, it's so nice when you can have 0:09:29.920 --> 0:09:31.559 the experts come in to tell you what's up. 0:09:31.640 --> 0:09:34.480 And he's got a beautiful voice as well. So here's 0:09:34.600 --> 0:09:42.440 our interview with Jonathan Oppenheim. So it's my pleasure to 0:09:42.520 --> 0:09:46.360 welcome to the podcast professor Jonathan Oppenheim, a physics professor 0:09:46.400 --> 0:09:49.480 at University College London and proponent of the theory of 0:09:49.600 --> 0:09:52.600 post quantum gravity. Jonathan, thank you very much for talking 0:09:52.600 --> 0:09:55.640 to us, Thanks for having me. So first I'd like 0:09:55.679 --> 0:09:58.200 to set the stage and understand what is it we 0:09:58.280 --> 0:10:02.080 are trying to solve. Why is everybody after quantum gravity. 0:10:02.280 --> 0:10:05.560 Why can't we just have general relativity to describe big 0:10:05.600 --> 0:10:08.240 stuff and quantum mechanics to describe small stuff and be 0:10:08.400 --> 0:10:11.640 happy with that. Why do we need one unified theory 0:10:11.720 --> 0:10:12.520 of the universe. 0:10:13.400 --> 0:10:16.960 Well, the two theories are frameworks general relativity and quantum mechanics. 0:10:16.960 --> 0:10:21.280 They're inconsistent, so they can't really live together in a 0:10:21.280 --> 0:10:24.400 mathematically consistent way, and so we know that either one 0:10:24.400 --> 0:10:26.320 of them is wrong or both of them are wrong, 0:10:26.720 --> 0:10:29.440 and so we know that they cannot be a correct 0:10:29.480 --> 0:10:30.560 description of nature. 0:10:30.800 --> 0:10:34.160 What if they always describe different domains, Do they need 0:10:34.200 --> 0:10:37.920 to give us a single, unified sort of conceptual understanding 0:10:37.960 --> 0:10:40.480 of the universe or do they actually disagree about what 0:10:40.600 --> 0:10:43.400 happens in the universe. Do they conflict in terms of 0:10:43.440 --> 0:10:44.400 their predictions. 0:10:44.840 --> 0:10:47.520 Yeah, they conflict. I mean the regimes that we're used to, 0:10:48.080 --> 0:10:51.640 it doesn't matter so much. But now that we're exploring 0:10:51.679 --> 0:10:54.839 the quantum realm, and we're exploring the quantum realm, when 0:10:54.840 --> 0:10:57.360 it comes to larger and larger particles than it does 0:10:57.400 --> 0:10:59.200 start to matter. If you think of like a very 0:10:59.240 --> 0:11:04.040 small gold atom, which is put in a superposition of 0:11:04.040 --> 0:11:06.080 two places at once, which is something we can do 0:11:06.160 --> 0:11:10.480 with gold atoms, then gold atoms gravitate in a very 0:11:10.520 --> 0:11:12.400 small way. But we don't have a theory which would 0:11:12.440 --> 0:11:15.360 describe that, and maybe that's not a realm that we 0:11:15.480 --> 0:11:17.760 care that much about. But we know from the history 0:11:17.800 --> 0:11:22.360 of physics that when we have these inconsistencies and contradictions 0:11:22.400 --> 0:11:25.160 in our theory, that once we start to try and 0:11:25.200 --> 0:11:27.679 fix them, everything just starts to unravel. I mean, if 0:11:27.679 --> 0:11:29.959 you look at the history of quantum mechanics, there was 0:11:30.040 --> 0:11:33.560 only one or two little places where things were seemed strange. 0:11:33.600 --> 0:11:36.720 And once we started really looking at how we could 0:11:36.720 --> 0:11:40.200 reconcile and explain certain phenomena back in the beginning of 0:11:40.240 --> 0:11:43.320 the nineteen hundreds, we realized that all of our laws 0:11:43.320 --> 0:11:45.559 of physics were wrong and there was the whole quantum revolution. 0:11:45.760 --> 0:11:48.520 So we know from history that when there are these contradictions, 0:11:48.520 --> 0:11:51.400 they usually spell the beginning of a new era of physics. 0:11:51.640 --> 0:11:53.800 Could you for the biologists dig in a little bit 0:11:53.840 --> 0:11:58.280 more to the gold superposition atoms. 0:11:58.640 --> 0:12:02.080 So quantum mechanics is strange. And in the world that 0:12:02.120 --> 0:12:05.560 we inhibit, we think of things as being in definite places. 0:12:05.640 --> 0:12:08.560 So a coffee mug is in one place or it's 0:12:08.600 --> 0:12:11.720 in another place. But as you go to smaller and 0:12:11.760 --> 0:12:15.480 smaller particles, in smaller and smaller systems, then things behave 0:12:15.600 --> 0:12:18.160 very differently. So gold atom, which is not like a 0:12:18.160 --> 0:12:20.960 coffee cup. We'd say that it's in a superposition of 0:12:21.000 --> 0:12:23.600 being in many places at the same time. So in 0:12:23.640 --> 0:12:26.680 some sense, it doesn't have a definite position, it doesn't 0:12:26.679 --> 0:12:30.000 have a definite velocity. As we get smaller and smaller, 0:12:30.000 --> 0:12:32.720 things just behave very weirdly and that's what quantum mechanics is. 0:12:33.000 --> 0:12:37.120 And so quantum mechanics can explain the weirdness of what 0:12:37.160 --> 0:12:40.720 the gold atom is doing, but general relativity cannot, is 0:12:40.760 --> 0:12:42.560 that right, That's right, that's right. 0:12:42.720 --> 0:12:45.240 I mean general relativity is our theory of gravity, and 0:12:45.280 --> 0:12:49.080 it describes planets and the Solar system and the way 0:12:49.120 --> 0:12:52.160 the universe evolves and the Big Bang, and so we 0:12:52.280 --> 0:12:55.360 used to it explained those very big things, but well 0:12:55.440 --> 0:12:57.920 contradicts what's happening on the small scale. 0:12:58.200 --> 0:13:00.679 So we have this theory of the very large gener relativity, 0:13:00.720 --> 0:13:03.760 which requires us to know where things are and when 0:13:03.760 --> 0:13:06.720 they're there in order to describe how space is bending 0:13:06.760 --> 0:13:09.280 and how things gravitate. But then we have quantum mechanics, 0:13:09.480 --> 0:13:12.400 which says things can have an uncertainty in their location, 0:13:12.760 --> 0:13:15.880 and these two things are fundamentally in contradiction. 0:13:16.160 --> 0:13:17.520 That's a good way of putting in it, and maybe 0:13:17.600 --> 0:13:20.080 just to say that you mentioned space time, and the 0:13:20.120 --> 0:13:22.680 thing to say maybe, is that our theory of gravity. 0:13:22.720 --> 0:13:26.600 What Einstein one of the massive contributions to science is 0:13:26.679 --> 0:13:30.040 he taught us that gravity is really space time bending. 0:13:30.200 --> 0:13:35.040 So we know that large objects bend space time, that's 0:13:35.080 --> 0:13:38.760 what gravity is. And very small objects, because they don't 0:13:38.840 --> 0:13:42.120 have a definite position, as you said, we don't know 0:13:42.160 --> 0:13:45.319 how space time should bend because space time doesn't even 0:13:45.320 --> 0:13:48.760 know where they are. So that's why you get these contradictions. 0:13:49.040 --> 0:13:50.960 And you have to cook up this example of a 0:13:51.000 --> 0:13:54.880 gold atom because typically general relativity applies to the very 0:13:54.960 --> 0:13:57.640 large things planets and galaxies, and quant mechanics are the 0:13:57.720 --> 0:14:00.120 very small. So you need something sort of on the 0:14:00.240 --> 0:14:02.959 edge there where it's large enough where we can measure 0:14:02.960 --> 0:14:05.480 its gravity, but it's small enough the quantum mechanical effects 0:14:05.480 --> 0:14:07.480 are still relevant. Is that why you come up with 0:14:07.480 --> 0:14:09.240 this idea of a gold at them? And why gold 0:14:09.240 --> 0:14:12.839 in particular would it work also for a lead atom. 0:14:13.400 --> 0:14:16.520 I mean, I love gold because it's it's actually because 0:14:16.520 --> 0:14:18.720 gold is very dense, and so it turns out that 0:14:18.760 --> 0:14:22.400 you run into trouble actually with very dense objects rather 0:14:22.440 --> 0:14:24.680 than very heavy objects. That's one of the things we've 0:14:24.680 --> 0:14:27.640 found out. But you know, it's true that we generally 0:14:27.640 --> 0:14:31.160 think of the gravitational field as being caused by the 0:14:31.200 --> 0:14:33.480 Moon and the Earth and things like that. But one 0:14:33.560 --> 0:14:36.880 kilogram mass will bend. We can feel that gravitation pool 0:14:36.920 --> 0:14:41.119 of a one kilogram mass. And now we're doing experiments 0:14:41.160 --> 0:14:44.320 where we can feel the pull of gold, which is 0:14:44.640 --> 0:14:47.720 a millimeter sphere. We can feel the gravitational pull of 0:14:47.840 --> 0:14:50.480 that kind of an object. But when they get much 0:14:50.480 --> 0:14:52.320 smaller than that, then we don't know what happens. And 0:14:52.320 --> 0:14:54.480 we haven't even been able to really perform the experiments 0:14:54.480 --> 0:14:56.480 that will tell us what happens at that scale. 0:14:56.680 --> 0:14:59.440 Right, So we need some sort of unified picture of 0:14:59.680 --> 0:15:03.080 gravivity and quantum mechanics. Why don't we just do that? 0:15:03.160 --> 0:15:06.240 I mean, we did it for electromagnetism. We had classical 0:15:06.280 --> 0:15:10.280 theory of electromagnetism, and then folks quantized it and gave 0:15:10.360 --> 0:15:13.440 us a theory of quantum electrodynamics. Why can't we also 0:15:13.600 --> 0:15:16.600 just do that for gravity? Take space time, consider that 0:15:16.640 --> 0:15:18.920 a field, quantize it. Bob's your uncle. 0:15:19.400 --> 0:15:22.680 Yeah, I mean that's what everyone has thought, and that's 0:15:22.680 --> 0:15:24.480 what we've been thinking for the last one hundred years, 0:15:24.520 --> 0:15:26.680 which is probably the amount of time that we've been 0:15:26.720 --> 0:15:30.119 failing to quantize gravity. So yeah, you have this contradiction. 0:15:30.320 --> 0:15:33.240 You have gravity which is not quantum. You want to 0:15:33.280 --> 0:15:35.680 make it quantum to fit in with everything else. That 0:15:35.720 --> 0:15:38.000 would be I think the thing that everyone thought that's 0:15:38.000 --> 0:15:41.040 what we should be doing. But we're finding that really troublesome, 0:15:41.160 --> 0:15:43.280 and there's reasons for that. And one of the I 0:15:43.280 --> 0:15:46.040 think big reasons is that gravity is really different from 0:15:46.040 --> 0:15:49.400 the other forces. So we're used to electromagnetism, which is 0:15:49.440 --> 0:15:52.160 what keeps my two fingers from being able to push 0:15:52.200 --> 0:15:56.760 through each other. They're repulsed by the electromagnetic force. Gravity 0:15:56.800 --> 0:16:00.000 just behaves very differently. And what Einstein taught us about 0:16:00.040 --> 0:16:04.280 gravity is that, unlike the other forces, gravity is spacetime bending. 0:16:04.400 --> 0:16:07.480 So the reason that the Earth goes around the Sun 0:16:08.200 --> 0:16:12.240 is that the Sun causes space time to bend in 0:16:12.280 --> 0:16:14.720 just such a way that the Earth instead of rolling 0:16:14.760 --> 0:16:19.080 past it orbits around it. So it's slightly weirder than 0:16:19.080 --> 0:16:22.080 that actually. I mean, you often in science centers and stuff, 0:16:22.080 --> 0:16:25.240 the vehicle demonstration where you see a big planet like 0:16:25.240 --> 0:16:28.560 the Sun sitting in the middle of a cone and 0:16:28.600 --> 0:16:31.680 then the Earth goes around the Sun as if it's 0:16:32.040 --> 0:16:35.440 caught in this vortex or funnel. It's the kind of 0:16:35.480 --> 0:16:37.680 demonstration we often see to kind of give us a 0:16:37.720 --> 0:16:41.200 sense of why gravity as space time bending is causing 0:16:41.240 --> 0:16:42.680 the Earth to orbit the Sun. 0:16:43.160 --> 0:16:44.640 Since you brought that up, do you find that to 0:16:44.720 --> 0:16:47.920 be an accurate description, like a useful mental model of 0:16:47.920 --> 0:16:50.640 how gravity works? Because I find it to be very confusing. 0:16:51.240 --> 0:16:53.760 You have like a two D situation, and then you're 0:16:53.800 --> 0:16:57.680 adding curvature in a third dimension, where general relativity has 0:16:57.720 --> 0:17:00.440 the curvature to be intrinsic in the three D space. 0:17:00.800 --> 0:17:03.200 Do you find those demonstrations to be misleading or do 0:17:03.240 --> 0:17:05.560 you find them to be a useful description of what's 0:17:05.560 --> 0:17:06.800 happening in general relativity? 0:17:07.320 --> 0:17:11.439 All our descriptions are misleading. So what we do is 0:17:11.640 --> 0:17:14.680 we tell ourselves lies and each other lies, and they're 0:17:14.760 --> 0:17:16.840 useful for a bit, and then we replace it with 0:17:16.920 --> 0:17:20.639 a better lie. And I feel like the funnel is 0:17:20.680 --> 0:17:24.240 a reasonable lie because it gives you a partial sense 0:17:24.320 --> 0:17:27.200 of what's happening. And like when I say a gold 0:17:27.200 --> 0:17:29.680 at them can be in a superposition of two places 0:17:29.720 --> 0:17:32.000 at once, that's also a bit of a lie. Because 0:17:32.480 --> 0:17:34.800 you could just as well say it's in a superposition 0:17:34.840 --> 0:17:37.239 of being in neither place at once. This is a 0:17:37.280 --> 0:17:40.679 mental picture that we're using and it has some truth 0:17:40.720 --> 0:17:43.440 to it. Also, the are parts where it fails and 0:17:44.280 --> 0:17:47.760 the funnel that you see at many sigence centers, which 0:17:47.800 --> 0:17:51.760 is meant to explain why space time bending causes the 0:17:51.800 --> 0:17:54.960 Earth to go around the Sun. Well, a better lie 0:17:55.200 --> 0:17:59.240 is that what's really happening is that time is slowing 0:17:59.320 --> 0:18:02.719 down you get closer to the Sun, and the reason 0:18:02.760 --> 0:18:05.280 that the Earth is going around the Sun is because 0:18:05.880 --> 0:18:08.320 time slows down closer to the Sun, and the Earth 0:18:08.359 --> 0:18:13.040 wants to travel in a path which causes clocks to 0:18:13.160 --> 0:18:15.800 tick the least number of times. And so it's actually 0:18:15.840 --> 0:18:19.840 time that's bending and traveling at a different rate at 0:18:19.840 --> 0:18:22.960 different places in space, and that's why the Earth goes 0:18:23.000 --> 0:18:23.640 around the Sun. 0:18:23.960 --> 0:18:25.000 Why does Earth want that? 0:18:27.920 --> 0:18:30.760 Earth is laid on its deadlines, Kelly, that's why I 0:18:30.800 --> 0:18:31.920 hear you Earth, I hear you. 0:18:34.240 --> 0:18:37.680 Yeah, Earth is traveling the shortest distance between two points. 0:18:37.680 --> 0:18:39.720 And why it wants to do that, I don't know. 0:18:39.840 --> 0:18:43.000 And there's lots of principles in physics where things want 0:18:43.080 --> 0:18:45.840 to do. The principle least action, which is how we 0:18:46.640 --> 0:18:50.480 derive most of our physical laws and was famous The 0:18:50.480 --> 0:18:54.160 movie Arrival is about that principle, and we don't really 0:18:54.200 --> 0:18:56.040 know why that is, but that just seems to be 0:18:56.119 --> 0:18:56.679 the way it is. 0:18:56.960 --> 0:18:59.800 I think it's at the level philosophically of if you 0:18:59.840 --> 0:19:02.439 may this assumption, you get a model which works and 0:19:02.480 --> 0:19:05.000 describes the universe very well, and then you can come 0:19:05.040 --> 0:19:07.360 back to, well, what does that mean about the universe. Well, 0:19:07.400 --> 0:19:09.359 we're still shrugging our shoulders over that one. 0:19:09.520 --> 0:19:11.760 I think the thing you learn in physics when you're 0:19:11.760 --> 0:19:14.919 in grade school, which is that you know when you 0:19:15.040 --> 0:19:18.760 are in outer space and there's no force acting on you, 0:19:18.760 --> 0:19:21.800 you move in a straight line. That's an example of that. 0:19:21.840 --> 0:19:23.800 Why do we move in a straight line when there's 0:19:23.840 --> 0:19:28.160 no force applied to it? Well, we're just taking the shortest. 0:19:27.680 --> 0:19:28.560 Distance in some set. 0:19:28.600 --> 0:19:31.280 We're taking the shortest path, the easiest path in some sense, 0:19:32.080 --> 0:19:35.520 And that's an particular example of that. But things could 0:19:35.560 --> 0:19:37.520 be different, but they're not. 0:19:37.720 --> 0:19:40.560 It's also interesting to me how some things, when you 0:19:40.600 --> 0:19:43.280 say then, people will just accept them because they sound intuitive, 0:19:43.320 --> 0:19:46.760 and other things demand an explanation. Why do clocks tick 0:19:46.840 --> 0:19:49.320 slower when you see them moving at high speed. That 0:19:49.359 --> 0:19:52.080 needs an explanation. Why would clocks always tick at the 0:19:52.119 --> 0:19:54.880 same speed. Why doesn't that need an explanation. So there's 0:19:54.880 --> 0:19:57.720 an imbalance there because some things confront or intuition and 0:19:57.760 --> 0:19:58.719 some things support it. 0:19:59.080 --> 0:20:01.159 I mean, maybe this is not for this podcast, but 0:20:01.240 --> 0:20:03.120 I could give you a good reason why we think 0:20:03.200 --> 0:20:06.959 that the Earth is traveling the shortest distance in curved space. 0:20:07.200 --> 0:20:10.560 Please do if you're willing to accept Newton's law, which 0:20:10.600 --> 0:20:13.720 is that in empty space, my velocity should stay the 0:20:13.760 --> 0:20:16.639 same if no force is acting upon me, and if 0:20:16.680 --> 0:20:19.719 I should just move in a straight line in empty space. 0:20:20.320 --> 0:20:24.640 If you accept that, then you can now ask the question, well, 0:20:24.720 --> 0:20:26.880 if there's no push or pull on me, and I'm 0:20:26.880 --> 0:20:29.720