WEBVTT - Could we reveal quantum gravity in a tabletop experiment? 0:00:08.520 --> 0:00:10.039 Hey, Daniel, who do you think would win in a 0:00:10.080 --> 0:00:13.039 fight theoretical or an experimental physicist? 0:00:13.200 --> 0:00:16.360 That depends are we're talking arm wrestling or like integration 0:00:16.520 --> 0:00:20.560 competitions into what mathematical race? 0:00:22.040 --> 0:00:24.520 Then I think I would put my money into theoretical physicist. 0:00:24.600 --> 0:00:25.920 I mean, no offense. 0:00:27.520 --> 0:00:29.639 Maybe we have to do the experiment, or maybe you. 0:00:29.600 --> 0:00:32.519 Should keep this theoretical. I don't know if you want 0:00:32.520 --> 0:00:33.240 to pick a fight. 0:00:33.240 --> 0:00:35.599 Well, maybe the two sides of the field just compliment 0:00:35.640 --> 0:00:36.480 each other beautifully. 0:00:36.640 --> 0:00:38.680 Is that all it takes us? Some compliments and you 0:00:38.720 --> 0:00:39.920 guys are back as friends. 0:00:40.000 --> 0:00:42.200 Theories are cheap, right, They don't need money for experiments, 0:00:42.240 --> 0:00:44.320 They just need compliments in theory. 0:00:44.640 --> 0:01:02.760 In my experience, hi am Warham, a cartoonist and the 0:01:02.760 --> 0:01:04.200 creator of PhD comics. 0:01:04.319 --> 0:01:07.240 Hi, I'm Daniel. I'm a particle physicist and a professor 0:01:07.280 --> 0:01:09.839 at UC Irvine, And back in the day, I did 0:01:09.880 --> 0:01:11.120 want to be a theorist. 0:01:11.440 --> 0:01:13.240 Back in the day, How old were you. 0:01:13.319 --> 0:01:15.120 When I started grad school? I wasn't sure if I 0:01:15.160 --> 0:01:18.600 wanted to do experimental or theoretical physics, So I guess 0:01:18.640 --> 0:01:20.920 I was in my early twenties, which by now is 0:01:20.959 --> 0:01:22.199 pretty far back in the day. 0:01:22.400 --> 0:01:24.679 Did you actually get a choice, like they offer you 0:01:24.720 --> 0:01:26.679 an option of which way to go, or do you 0:01:26.720 --> 0:01:28.440 have to, like, I don't know, test into it. 0:01:28.560 --> 0:01:30.759 You definitely have to kind of try out and work 0:01:30.800 --> 0:01:33.080 with the theorists if you want to be a theorist. 0:01:33.680 --> 0:01:36.039 But you have all the options when you start grad school. 0:01:36.200 --> 0:01:38.640 You could end up being an experimental particle physicist or 0:01:38.640 --> 0:01:43.000 a theoretical cosmologist or whatever. All those paths are available. 0:01:43.040 --> 0:01:45.440 You just gotta like it enough and be good at it. 0:01:46.200 --> 0:01:48.120 So what happened? Why didn't you pick the theory? 0:01:48.320 --> 0:01:51.200 I discovered I just didn't like writing down equations as 0:01:51.280 --> 0:01:53.320 much as a theorist. They would sit there and like 0:01:53.400 --> 0:01:57.040 develop several different mathematical fonts to write their equations in, 0:01:57.120 --> 0:01:59.440 and I was like, wow, I'm just not loving this 0:01:59.480 --> 0:02:00.760 as much as they're loving this. 0:02:00.960 --> 0:02:02.520 It sounds like you were against the idea of it 0:02:02.560 --> 0:02:03.040 in theory. 0:02:04.800 --> 0:02:07.240 My experience was the experiments were more fun. 0:02:07.280 --> 0:02:09.680 But anyways, Welcome to our podcast Daniel and Jorge Explain 0:02:09.760 --> 0:02:12.560 the Universe, a production of iHeartRadio. 0:02:11.880 --> 0:02:14.560 In which we try to blur the line between theory 0:02:14.600 --> 0:02:17.519 and experiment. We want to talk about all the concepts 0:02:17.560 --> 0:02:20.400 in theoretical physics that try to explain what's going on 0:02:20.520 --> 0:02:23.360 in our world, but we also try to touch back 0:02:23.400 --> 0:02:26.880 on the ground and understand what experiments are telling us 0:02:26.880 --> 0:02:30.680 about the nature of reality, what is Nature actually saying 0:02:30.720 --> 0:02:33.959 to us as she spins the story of the universe, 0:02:34.320 --> 0:02:36.440 And then we try to explain all of it to you. 0:02:36.639 --> 0:02:39.119 That's right, because it is a pretty storied universe, full 0:02:39.120 --> 0:02:42.000 of amazing little details and facts and things to discover 0:02:42.120 --> 0:02:44.520 out there that we are still puzzling over and which 0:02:44.560 --> 0:02:50.399 require all kinds of scientists to figure out, theorists and experimentalists. 0:02:49.480 --> 0:02:51.720 And in the history of physics we have made progress 0:02:51.760 --> 0:02:54.880 in lots of different ways. Sometimes the theorists have come 0:02:54.960 --> 0:02:58.080 up with a clever idea, a suspicion about how the 0:02:58.240 --> 0:03:01.919 universe might work, with lots of cool directions for experimentalists. 0:03:02.000 --> 0:03:04.680 Go out and check this thing, Measure how light bends 0:03:04.720 --> 0:03:07.440 around the sun, see if you can find the Higgs boson. 0:03:07.760 --> 0:03:11.120 Those can be wonderful directions to help unravel the mysteries 0:03:11.160 --> 0:03:14.960 of the universe. But sometimes the experimentalists lead the way, 0:03:15.040 --> 0:03:18.040 turning on particle smashers and discovering gobs and gobs of 0:03:18.080 --> 0:03:20.080 new particles that nobody expected. 0:03:20.880 --> 0:03:23.000 I guess my question, Daniel is why can't you be both? 0:03:23.160 --> 0:03:25.679 Why can't you be a theoretical and an experimental physicist. 0:03:26.200 --> 0:03:29.680 I'm doing my best. Actually i'm doing my best. But 0:03:29.840 --> 0:03:32.639 the reality of academia these days is to get one 0:03:32.639 --> 0:03:35.160 of these jobs, you have to be the world's expert 0:03:35.240 --> 0:03:38.600 in some subfield. And that makes it really hard to 0:03:38.760 --> 0:03:41.600 sort of live between two fields because you have to 0:03:41.640 --> 0:03:44.640 be like the top person in that field that year. 0:03:44.840 --> 0:03:46.600 And so if the theorists aren't sure, if you're a 0:03:46.600 --> 0:03:49.240 theorist and the experimentalist aren't sure if you're an experimentalist, 0:03:49.520 --> 0:03:52.080 nobody's going to give you that job. So you've got 0:03:52.120 --> 0:03:53.960 to sort of get the job in one category and 0:03:54.000 --> 0:03:56.360 then inch your way over to the other one if 0:03:56.360 --> 0:04:00.720 you're interested. That's kind of cliquish, it's definitely very cliquiche. Absolutely, 0:04:01.040 --> 0:04:04.120 these fields form and then they protect themselves and it 0:04:04.120 --> 0:04:07.400 can be hard for new kinds of subfields to emerge, 0:04:07.440 --> 0:04:10.480 Like right now we have the emergence of physicists who 0:04:10.480 --> 0:04:12.920 are experts in machine learning, and people aren't sure is 0:04:12.920 --> 0:04:16.200 that theoretical is it experimental? Because you're running a bunch 0:04:16.240 --> 0:04:19.719 of calculations nobody's really sure. Everybody knows that it's valuable, 0:04:19.880 --> 0:04:21.800 but we aren't quite sure where to put them. 0:04:21.880 --> 0:04:25.400 That's because they're robots? Are they in disguise? 0:04:27.640 --> 0:04:29.680 We're all just biological robots? 0:04:29.720 --> 0:04:29.920 Man? 0:04:30.040 --> 0:04:30.640 Oh, there you go. 0:04:31.760 --> 0:04:33.600 Aren't you the expert in squishy robots? 0:04:33.760 --> 0:04:36.080 I am, Yeah, Well I used to be, at least 0:04:36.160 --> 0:04:38.480 a lifetime ago. We're a couple of lifetimes ago. 0:04:38.600 --> 0:04:40.760 Now back in the day. Is there such a thing 0:04:40.800 --> 0:04:42.320 as a theoretical roboticist? 0:04:42.560 --> 0:04:45.599 Uh? Yeah, there's a lot of theory in robotics as well. 0:04:45.800 --> 0:04:47.559 But no, as we I guess we're not as clique 0:04:47.560 --> 0:04:50.080 as you're just a roboticist. If you're into robots, you're 0:04:50.120 --> 0:04:51.560 just a roboticist. 0:04:51.040 --> 0:04:53.880 New York because you just build your own friends. You're like, hey, look, 0:04:53.880 --> 0:04:56.159 I don't need people's friends. I can build my own. 0:04:56.600 --> 0:04:58.320 Yeah. But as you said, I guess you need both 0:04:58.440 --> 0:05:01.839 kinds of endeavors or search. You need experimental research and 0:05:01.839 --> 0:05:03.960 you need theoretical research in order to figure out how 0:05:03.960 --> 0:05:06.240 things work in the universe. Because I guess you need 0:05:06.279 --> 0:05:07.840 to come up with a theory so that you can 0:05:08.000 --> 0:05:10.360 prove it with an experiment, and you need an experiment 0:05:10.360 --> 0:05:12.640 to prove the theories. Otherwise there's no science. 0:05:13.040 --> 0:05:15.400 That's sort of a theoretical way of thinking about it, 0:05:15.560 --> 0:05:17.760 that we come up with the theories and improve them 0:05:17.760 --> 0:05:22.280 with experiment. Remember that sometimes experiments don't just prove theories. 0:05:22.320 --> 0:05:25.040 They blow up theories and tell us that the universe 0:05:25.160 --> 0:05:27.800 is different from the way we understand it and operates 0:05:27.800 --> 0:05:30.560 in some other way we don't yet understand. Like the 0:05:30.560 --> 0:05:33.520 photoelectric effect was a demonstration that boy, we really don't 0:05:33.600 --> 0:05:35.640 understand at light and how it works, and it took 0:05:35.720 --> 0:05:38.520 a few years before the theorist came up with any sort 0:05:38.560 --> 0:05:39.640 of explanation for it. 0:05:39.800 --> 0:05:43.520 Yeah, but I guess experimenter's lunches kind of experiment blindly, right. 0:05:43.560 --> 0:05:46.040 You usually have some sort of theory at hand when 0:05:46.080 --> 0:05:49.320 you design your experiments, when you go out there and 0:05:49.440 --> 0:05:50.120 turn stuff on. 0:05:50.279 --> 0:05:52.120 It's a bit of a raging debate right now in 0:05:52.200 --> 0:05:55.680 experimental physics whether we should be focused on searching for 0:05:55.720 --> 0:05:59.160 the ideas that theoretical physicists are suggesting, or whether we 0:05:59.200 --> 0:06:03.000 should be developing strategies that are more just exploratory that 0:06:03.160 --> 0:06:06.400 leave us open to surprises. Like when you turn on 0:06:06.440 --> 0:06:09.279 the Hubble Space telescope and look out into space. Sure, 0:06:09.400 --> 0:06:11.440 you want to see the things that you had in 0:06:11.480 --> 0:06:13.640 mind to look at, but you're also open to like 0:06:13.880 --> 0:06:17.039 seeing aliens waving at you, or seeing new kinds of 0:06:17.080 --> 0:06:19.279 stuff you didn't even expect to see. 0:06:19.440 --> 0:06:21.039 But I guess also at the same time, we're getting 0:06:21.040 --> 0:06:24.200 to a spot where you know, things are so complex 0:06:24.320 --> 0:06:27.000 and so subtle and so hidden that you kind of 0:06:27.120 --> 0:06:29.680 need to know what you're looking for in a way, right, 0:06:29.680 --> 0:06:31.679 it's kind of hard to just like look for everything. 0:06:31.760 --> 0:06:34.039 It is really hard to look for everything you really 0:06:34.040 --> 0:06:36.560 put your finger on it, especially when your data is 0:06:36.680 --> 0:06:39.440 very statistical. If you do like a single experiment and 0:06:39.480 --> 0:06:42.280 you get some weird result, you might be able to say, hey, look, 0:06:42.320 --> 0:06:45.400 there's definitely something new here. But if the data are subtle, 0:06:45.440 --> 0:06:48.599 if the new things appear as like trends in your data, 0:06:49.000 --> 0:06:50.680 then you're right, it can be hard to know how 0:06:50.720 --> 0:06:52.640 to find them. So then you have to play some 0:06:52.680 --> 0:06:55.599 clever statistical arguments and say, well, you're the kinds of 0:06:55.680 --> 0:06:57.599 things that we could see, and here are the ways 0:06:57.600 --> 0:06:59.480 that we could search for them. So you have to 0:06:59.520 --> 0:07:01.599 do a little bit more work to define the kinds 0:07:01.600 --> 0:07:04.120 of things you might be able to see. Even if 0:07:04.120 --> 0:07:06.960 you aren't sure which specifically might pop up in your data. 0:07:07.120 --> 0:07:10.240 Well, sometimes there are cases where both the theories and 0:07:10.400 --> 0:07:13.400 the experimental lists are stumped. And that is the case 0:07:13.400 --> 0:07:16.560 where non physics. There's kind of a big hole in 0:07:16.560 --> 0:07:19.080 physics in terms of our knowledge of how things work 0:07:19.080 --> 0:07:19.680 in the universe. 0:07:19.840 --> 0:07:22.400 That's right, at the most fundamental level, we still don't 0:07:22.440 --> 0:07:26.040 really understand the basic rules of physics. We have two 0:07:26.600 --> 0:07:30.440 pillars of modern physics relativity that tells us about space, 0:07:30.480 --> 0:07:33.400 time and gravity, and quantum mechanics that tells us about 0:07:33.480 --> 0:07:36.240 particles and forces, and we just don't know how to 0:07:36.360 --> 0:07:38.840 bring them together. And it's important because it has to 0:07:38.920 --> 0:07:41.400 do with one of the most basic questions in physics, 0:07:41.400 --> 0:07:44.160 which is what is the universe made out of? What 0:07:44.320 --> 0:07:47.600 is the fundamental fabric of reality? After all? 0:07:47.880 --> 0:07:50.480 Yeah? And is it soft and comfortable? Is what I 0:07:50.520 --> 0:07:50.920 want to know. 0:07:51.200 --> 0:07:52.960 It seems to have a little bit of spandex in it. 0:07:53.000 --> 0:07:55.800 Here, you guys, long is a stretchy that can accommodate 0:07:56.120 --> 0:07:56.840 all sizes. 0:07:56.960 --> 0:07:59.120 Because my waste is not the size it was back 0:07:59.120 --> 0:07:59.840 in the day. 0:08:00.080 --> 0:08:02.960 You want the universe to kind of expand with you, 0:08:02.960 --> 0:08:05.040 your mind and your waste. But yeah, there's kind of 0:08:05.080 --> 0:08:07.280 a big hole in our understanding of the universe, and 0:08:07.280 --> 0:08:09.960 it has to do with gravity. We're not quite sure 0:08:10.000 --> 0:08:13.240 where gravity falls, whether it falls or it fits with 0:08:13.400 --> 0:08:18.080 quantum mechanic skill theory, or whether it works the way 0:08:18.080 --> 0:08:20.440 that Einstein envisioned in special relativity. 0:08:20.520 --> 0:08:23.280 Right, that's right. Einstein's special relativity tells us about light 0:08:23.320 --> 0:08:26.400 and how it propagates. His theory of general relativity tells 0:08:26.440 --> 0:08:29.720 us about space time and how it bends. And these 0:08:29.720 --> 0:08:32.640 two theories are in conflict and tell us very different 0:08:32.679 --> 0:08:35.240 stories about the nature of the universe. But so far 0:08:35.400 --> 0:08:37.160 we haven't been able to figure out a way to 0:08:37.320 --> 0:08:41.720 test them without building a solar system sized particle collider 0:08:42.080 --> 0:08:46.360 or peering inside a black hole. So experimental physicists have 0:08:46.440 --> 0:08:50.120 not really been able to contribute to this conversation until now. 0:08:50.280 --> 0:08:52.520 So the deal in the podcast, we'll be asking the question, 0:08:57.800 --> 0:09:02.080 can we test quantum gravity in a tabletop experiment? And 0:09:02.240 --> 0:09:04.720 right here the word tabletop I think of board games. 0:09:07.240 --> 0:09:09.960 Is this what we're talking about? Like a little cardboard 0:09:10.080 --> 0:09:14.400 unfolding thing with pieces, And then you test quantum gravity exactly. 0:09:14.400 --> 0:09:16.679 You can download the schematics from the internet and print 0:09:16.679 --> 0:09:19.760 out your own Nobel Prize winning experiment. Do you go? 0:09:19.880 --> 0:09:23.920 Is it called settlers of quarks or quantum ton. 0:09:24.400 --> 0:09:26.400 I'll leave you to do the branding of it. But 0:09:26.440 --> 0:09:29.680 when we say tabletop experiment in physics, we basically mean 0:09:30.080 --> 0:09:33.640 something not like the large Hadron collider or something that 0:09:33.679 --> 0:09:37.640 doesn't require a ten billion dollar facility staffed by thousands 0:09:37.679 --> 0:09:39.720 of people. We mean the kind of thing a single 0:09:39.760 --> 0:09:42.560 physicist could do in their laboratory, in the basement of 0:09:42.600 --> 0:09:43.960 your nearby university. 0:09:44.520 --> 0:09:46.720 I see you're talking about a million dollar tabletop, not 0:09:46.760 --> 0:09:48.440 a billion dollar tabletop. 0:09:48.160 --> 0:09:51.080 Exactly, just like everybody has a million dollar table in 0:09:51.120 --> 0:09:55.520 their kitchen. No, it's really like a single physicist experiment, 0:09:55.600 --> 0:09:58.240 something you could do in a reasonable physics lab, not 0:09:58.360 --> 0:10:00.880 something people are going to be doing on their kits table. 0:10:00.840 --> 0:10:02.840 Well, as usually, we were wondering how many people out 0:10:02.840 --> 0:10:06.000 there had thought about this question or perhaps have any 0:10:06.040 --> 0:10:07.400 ideas about how to do it. 0:10:07.440 --> 0:10:09.920 So thanks very much to everybody who participates in this 0:10:10.040 --> 0:10:13.000 segment of the podcast. If you've been listening for years 0:10:13.000 --> 0:10:15.720 and would like to hear your voice speculating about the 0:10:15.720 --> 0:10:18.400 topic of the day, please write to us two questions 0:10:18.440 --> 0:10:21.600 at Danielandjorge dot com everybody's welcome. 0:10:21.840 --> 0:10:23.600 So think about it for a second. Do you think 0:10:23.840 --> 0:10:27.480 we can test quantum gravity on somebody's table? Here's what 0:10:27.520 --> 0:10:28.320 people had to say. 0:10:28.400 --> 0:10:31.000 Well, since quantum gravity is, you know, with the gravity 0:10:31.040 --> 0:10:34.120 of the really small, I don't see why the experiments 0:10:34.559 --> 0:10:36.880 with it couldn't be done on a tabletop. I just 0:10:37.040 --> 0:10:39.640 have no idea what those experiments would even begin to 0:10:39.679 --> 0:10:40.000 look like. 0:10:40.040 --> 0:10:40.240 Though. 0:10:40.559 --> 0:10:42.880 If yes, then it will come to our table soon. 0:10:43.000 --> 0:10:45.640 But till then, I don't think it is possible at all. 0:10:46.040 --> 0:10:48.840 Uh, yeah, you probably could, but probably not today. 0:10:49.480 --> 0:10:52.160 I do not feel like we could test quantum gravity 0:10:52.200 --> 0:10:55.360 in a tabletop experiment because you. 0:10:55.760 --> 0:10:59.480 Need a lot of gravity for it to work, and 0:10:59.679 --> 0:11:02.000 I don't think the Earth has that kind of gravity. 0:11:02.080 --> 0:11:05.120 All right, not a lot of optimism, I like the 0:11:05.120 --> 0:11:07.880 person who said the tabletop, I don't think so. But 0:11:08.000 --> 0:11:11.960 maybe a desktop or on the floor, or on a shelf, 0:11:12.400 --> 0:11:16.120 maybe mountain top maybe, yeah, tabletop on a mountain. There 0:11:16.160 --> 0:11:17.880 you go, lower gravity. 0:11:18.080 --> 0:11:20.320 Well, we talked recently about how to measure big g 0:11:20.520 --> 0:11:23.160 and that experiment was definitely done on a mountain side 0:11:23.400 --> 0:11:27.720 swinging pendulums next to a big mountain in Scotland. So yeah, 0:11:27.800 --> 0:11:30.600 you can do funny gravity experiments on tops of mountains. 0:11:30.760 --> 0:11:33.480 And I like the person who said probably, but not today, 0:11:33.840 --> 0:11:36.840 Like is today a bad day for that? Were they 0:11:36.880 --> 0:11:40.360 busy that day? How about next week? Next week work? 0:11:40.480 --> 0:11:42.439 Please fill out this doodle pole for when we will 0:11:42.440 --> 0:11:43.439 win a Nobel prize. 0:11:44.200 --> 0:11:48.000 There you go. Yeah, I guess people didn't feel like 0:11:48.200 --> 0:11:51.240 it could work. But let's find out, Daniel step us 0:11:51.240 --> 0:11:52.960 through this. What is quantum gravity? 0:11:53.040 --> 0:11:55.800 So when we say quantum gravity, what we mean is 0:11:55.840 --> 0:12:00.360 a theory that explains both the quantum mechanical behavior super 0:12:00.440 --> 0:12:04.559 tiny particles, the way like electrons and photons do things 0:12:04.679 --> 0:12:08.439 that baseballs and basketballs and mountaintops don't do. You know, 0:12:08.480 --> 0:12:11.599 they don't move in smooth paths. They have weird quantized 0:12:11.720 --> 0:12:15.160 energy levels. They can be in a superposition of different states, 0:12:15.160 --> 0:12:18.040 like maybe they're here, maybe they're there. They can interact 0:12:18.080 --> 0:12:20.319 with each other and interfere in all sorts of complicated 0:12:20.400 --> 0:12:22.760 ways described by their wave function. And we want a 0:12:22.800 --> 0:12:26.040 theory that explains gravity as we know it. That things 0:12:26.080 --> 0:12:30.040 seem to move in these inertial pass through curved space time, 0:12:30.600 --> 0:12:33.679 and that mass in space tends to bend the path 0:12:34.040 --> 0:12:37.240 which affects the motion of other mass. So we have 0:12:37.320 --> 0:12:40.200 these two very different theories of the universe, and so 0:12:40.360 --> 0:12:43.920 far we can't bring them together. So quantum gravity would 0:12:43.960 --> 0:12:48.120 be a theory that explains both these things somehow harmoniously. 0:12:48.480 --> 0:12:50.440 But it's not a theory that we have today. 0:12:50.640 --> 0:12:52.319 Well, I guess maybe step us through a little bit 0:12:52.320 --> 0:12:54.520 of what we haven't been able to bring these two 0:12:54.559 --> 0:12:56.760 things together as far as I understand it. It's kind 0:12:56.760 --> 0:12:58.959 of due to two things, right, Like, one is that 0:12:59.240 --> 0:13:02.719 we haven't measure the gravitational force at the level of 0:13:03.240 --> 0:13:07.040 quantum particles, right, that's one thing. And also we don't 0:13:07.080 --> 0:13:11.360 know what happens to general relativity when you get down 0:13:11.400 --> 0:13:12.560 to that small level. 0:13:12.320 --> 0:13:14.679 Too exactly, I think you put your finger on it. Really, 0:13:14.720 --> 0:13:17.640 we don't know what the gravity is for little particles. 0:13:17.760 --> 0:13:20.600 The gravity for a baseball or for a moon. We 0:13:20.679 --> 0:13:22.840 think we understand and we've been able to test that. 0:13:22.960 --> 0:13:23.120 Right. 0:13:23.200 --> 0:13:26.760 We see moon's orbiting planets, we see planets orbiting suns. 0:13:26.800 --> 0:13:29.600 We see how gravity works. But that's all really really 0:13:29.640 --> 0:13:32.200 big stuff. What we don't know is what happens when 0:13:32.240 --> 0:13:36.120 you have gravity for particles, because particles are super duper tiny, 0:13:36.440 --> 0:13:39.720 which makes it really complicated. For two reasons. One is 0:13:39.720 --> 0:13:42.600 that they have almost no gravity. Remember that gravity is 0:13:42.640 --> 0:13:45.960 like the weakest force in the universe, and so the 0:13:46.000 --> 0:13:49.520 other forces overwhelm it. You try to do experiments with electrons, 0:13:49.559 --> 0:13:54.000 then their charge is much more powerful than their mass. Right, 0:13:54.000 --> 0:13:57.800 the electromagnetic force is much more powerful than the gravitational 0:13:57.840 --> 0:14:01.040 force on an electron. So it's basically possible to measure 0:14:01.200 --> 0:14:03.160 the gravitational force on an electron. 0:14:03.320 --> 0:14:04.640 Can I ask why that is? 0:14:04.679 --> 0:14:04.880 Though? 0:14:04.960 --> 0:14:09.000 Like, couldn't I shoot an electron from here to London 0:14:09.040 --> 0:14:11.600 and see if it curves with the curvature of the Earth. 0:14:11.840 --> 0:14:14.640 You could try that, absolutely, I think you probably shouldn't 0:14:14.640 --> 0:14:17.200 shoot beams across the surface of the Earth without getting 0:14:17.200 --> 0:14:20.600 signatures from everybody who might live in between. But say 0:14:20.600 --> 0:14:22.680 you did that, the electron would be effected by all 0:14:22.720 --> 0:14:26.400 sorts of charged particles between here and London, right, There'd 0:14:26.400 --> 0:14:28.640 be lots of other effects on the electron which would 0:14:28.720 --> 0:14:31.040 swamp out any gravitational effects. 0:14:31.320 --> 0:14:34.400 But I guess maybe, like from a satellite, I'm thinking, 0:14:34.520 --> 0:14:35.960 you know, I just shoot a whole bunch of them, 0:14:36.120 --> 0:14:39.400 and wouldn't the effects from other things kind of even 0:14:39.440 --> 0:14:42.000 out If you shoot a bunch of them out, Like, 0:14:42.000 --> 0:14:45.000 don't we have like quantum drives or like electron cannons. 0:14:45.320 --> 0:14:47.880 What happens if I just shoot them out there in space? 0:14:48.000 --> 0:14:49.960 Do they keep going straight or do they bend? 0:14:50.320 --> 0:14:52.840 Yeah, you could build an electron gun and put it 0:14:52.880 --> 0:14:55.840 in space and shoot them out, but still it would 0:14:55.880 --> 0:14:58.640 be dominated by the effects of other particles. Remember space 0:14:58.720 --> 0:15:02.200 is not totally empty. There's cosmic microwave background photons there, 0:15:02.280 --> 0:15:04.760 there's other charge particles from the Sun, and all of 0:15:04.760 --> 0:15:07.440 these would dominate the fate of that electron. Really, the 0:15:07.480 --> 0:15:10.840 problem is that the charge is more powerful than the mass. 0:15:11.240 --> 0:15:13.320 We talked about this once, and this is either because 0:15:13.400 --> 0:15:16.560 gravity itself is just weaker than the other forces for 0:15:16.640 --> 0:15:20.000 reasons we don't understand, or because electrons are just packed 0:15:20.000 --> 0:15:22.760 with a lot of charge compared to how much mass 0:15:22.840 --> 0:15:25.280 they have. You can think about it sort of either way. 0:15:25.320 --> 0:15:27.320 But that just means that the effect of gravity is 0:15:27.360 --> 0:15:30.040 tiny compared to the effect of electromagnetism. So to do 0:15:30.080 --> 0:15:33.360 that experiment, you'd need to isolate those particles from any 0:15:33.440 --> 0:15:35.920 sort of effect. And today we'll talk about an experiment 0:15:35.960 --> 0:15:37.480 that's going to try to do that, all. 0:15:37.480 --> 0:15:40.200 Right, So then that's where quantum gravity comes in. It's 0:15:40.360 --> 0:15:42.240 kind of a is it a theory or an idea 0:15:42.320 --> 0:15:44.680 that tries to bring these two big ideas together. 0:15:44.880 --> 0:15:47.600 It's not a theory. It's like a category of theories. 0:15:47.640 --> 0:15:50.760 It's like a dreamt of theory. What we want is 0:15:50.760 --> 0:15:52.920 a theory that bring these two things together. We don't 0:15:52.920 --> 0:15:55.480 have one. We don't know what the theory of quantum 0:15:55.560 --> 0:15:58.520 gravity is. You know, sometimes you have like ten different 0:15:58.600 --> 0:16:00.880 theories that describe the universe. The experiment has to go 0:16:00.880 --> 0:16:03.200 off and tell you which one is correct. Right now, 0:16:03.240 --> 0:16:06.200 we have zero. We have zero theories that explain quantum 0:16:06.200 --> 0:16:09.680 mechanics and gravity at the same time. So we sort 0:16:09.720 --> 0:16:12.520 of need an experimental result to be like, hey, this 0:16:12.680 --> 0:16:15.240 is the right direction though where hey, here's something to 0:16:15.280 --> 0:16:17.680 grab on to, here's a clue. But there's the second 0:16:17.720 --> 0:16:20.400 reason why these experiments are difficult that we didn't get 0:16:20.440 --> 0:16:22.520 to yet. One is just that gravity is so weak, 0:16:22.720 --> 0:16:25.880 and the other is that these particles do things that 0:16:25.920 --> 0:16:28.520 we don't know how to explain with gravity, Like particles 0:16:28.920 --> 0:16:32.000 don't have smooth paths. It's not like the electron is 0:16:32.160 --> 0:16:35.600 always somewhere, has some velocity. You know, you want to 0:16:35.640 --> 0:16:38.520 calculate the gravity of an electron, you have to know 0:16:38.680 --> 0:16:41.000 where it is, so you know how far away it is, 0:16:41.120 --> 0:16:44.200 you can calculate it's gravity. But electrons don't have specific 0:16:44.240 --> 0:16:47.760 locations that have probabilities, so we don't know. For example, 0:16:47.880 --> 0:16:50.080 if an electron, when it has probabilities to be in 0:16:50.160 --> 0:16:53.080 multiple places, does it mean it has like multiple different 0:16:53.280 --> 0:16:57.080 possible gravities. We just don't know how to do gravity 0:16:57.120 --> 0:16:59.640 for things that have uncertainties in their locations. 0:17:00.120 --> 0:17:02.160 You mean, we don't know how to do that if 0:17:02.200 --> 0:17:06.960 gravity was not a quantum force, right, Like you're sort 0:17:06.960 --> 0:17:10.280 of assuming that. I guess you want gravity to be 0:17:10.560 --> 0:17:12.679 like a quantum force like the other forces that we 0:17:12.720 --> 0:17:14.120 know about, right. 0:17:13.960 --> 0:17:15.719 Yeah, that's sort of one of the basic questions when 0:17:15.760 --> 0:17:17.679 you want to build the theory of quantum gravity, like 0:17:17.800 --> 0:17:21.000 is it a quantum force? If so, then two electrons 0:17:21.040 --> 0:17:25.480 interacting gravitationally wouldn't like collapse each other's wave functions. Some 0:17:25.600 --> 0:17:27.720 bits of one wave function would interact with some bits 0:17:27.760 --> 0:17:29.480 of another wave function, and they could do all sorts 0:17:29.520 --> 0:17:32.399 of weird quantum interactions. But if gravity is actually a 0:17:32.440 --> 0:17:35.480 classical force and not a quantum force, then it would 0:17:35.520 --> 0:17:38.360 collapse the wave function sort of like when you use 0:17:38.400 --> 0:17:41.240 a detector in a double slit experiment, it forces the 0:17:41.280 --> 0:17:44.040 particle to pick one of the options instead of the 0:17:44.080 --> 0:17:47.200 other one. So we just don't know, like is gravity classical, 0:17:47.359 --> 0:17:49.920 is it quantum mechanical. We just don't even know where 0:17:49.960 --> 0:17:50.359 to begin. 0:17:50.760 --> 0:17:53.119 And when you say classical, you mean like basically not 0:17:53.359 --> 0:17:56.920 quantum mechanical, like not fuzzy, not uncertain. 0:17:57.040 --> 0:18:00.600 Yeah, exactly, we mean not quantum is sort of an 0:18:00.640 --> 0:18:03.919 overused word. Some people say classical to me, not relativistic, 0:18:04.000 --> 0:18:07.960 like Newtonian, but today we mean not quantum mechanical. So 0:18:08.160 --> 0:18:11.000 we don't know if gravity, like really is just classical 0:18:11.040 --> 0:18:14.440 the way Einstein described it, thinking about space as smooth 0:18:14.560 --> 0:18:17.199 and continuous and everything having passed, or if it is 0:18:17.240 --> 0:18:19.359 a quantum effect, in which case it could either be 0:18:19.400 --> 0:18:23.760 a force like you suggested, mediated by weird gravitons, or 0:18:23.800 --> 0:18:27.840 maybe like space itself is quantum mechanical and uncertain. If 0:18:27.920 --> 0:18:31.479 gravity is the curvature of space, maybe space itself can 0:18:31.520 --> 0:18:34.040 be like maybe bent here and maybe bent there in 0:18:34.040 --> 0:18:38.440 some weird quantum mechanical way. There's so many possible directions 0:18:38.440 --> 0:18:41.639 for quantum gravity, nobody really knows which one is going 0:18:41.680 --> 0:18:45.000 to build a viable theory that even can do calculations. 0:18:45.160 --> 0:18:47.720 All right, well, let's get a little bit deeper into 0:18:47.760 --> 0:18:50.720 quantum gravity and whether or not we can test it 0:18:51.000 --> 0:18:54.280 and test it for under a billion dollars, because I 0:18:54.280 --> 0:18:57.000 guess the cheaper the better. We'll dig into that, but first, 0:18:57.040 --> 0:19:11.480 let's take a quick break. All right, we're talking about 0:19:11.560 --> 0:19:14.840 quantum gravity and whether or not that is a thing 0:19:15.000 --> 0:19:18.119 at all, whether it will bring together quantum mechanics in 0:19:18.200 --> 0:19:21.360