WEBVTT - Daniel Whiteson, Time Traveler 0:00:00.160 --> 0:00:05.200 My welcome to Stuff to Blow Your Mind, the production 0:00:05.240 --> 0:00:14.120 of My Heart Radio. Hey you welcome to Stuff to 0:00:14.120 --> 0:00:17.000 Blow Your Mind. My name is Robert Lamb and I'm 0:00:17.040 --> 0:00:20.200 Joe McCormick. And for today's episode, we're going to be 0:00:20.280 --> 0:00:24.480 chatting with Daniel Whitson, who is a particle physicist and 0:00:24.560 --> 0:00:28.280 science communicator and one of the hosts of the podcast 0:00:28.440 --> 0:00:32.360 Daniel and Jorge Explain the Universe. This is Daniel's third 0:00:32.440 --> 0:00:35.480 time hopping on the show with us. The previous episodes 0:00:35.560 --> 0:00:40.479 were in September of twenty nineteen and April of And 0:00:40.760 --> 0:00:42.680 for this episode, we're gonna be talking about a book. 0:00:43.000 --> 0:00:46.320 Daniel and his co host and co author Jorge cham 0:00:46.320 --> 0:00:50.200 have a new book called Frequently Asked Questions about the Universe. 0:00:50.880 --> 0:00:52.760 So it was a real pleasure to have Daniel on 0:00:52.800 --> 0:00:55.200 the show for the hat trick, and I guess without 0:00:55.240 --> 0:01:01.040 any further delay, we will go right into the interview. Daniel, 0:01:01.120 --> 0:01:02.920 welcome back to the show. We're so glad you're here. 0:01:03.240 --> 0:01:05.280 Thanks very much for having me back. Always fun to 0:01:05.280 --> 0:01:08.640 talk to you guys about things that blew my mind. Awesome, 0:01:08.680 --> 0:01:12.400 So um, the podcast Daniel and Jorge Explain the Universe 0:01:12.520 --> 0:01:16.000 still going strong um. How how far are you into 0:01:16.080 --> 0:01:20.399 explaining the universe in its entirely? We have explained zero 0:01:20.400 --> 0:01:23.759 point zero zero zero zero zero zero zero zero one 0:01:23.800 --> 0:01:27.399 percent of the universe so far. Nice. I actually I 0:01:27.440 --> 0:01:29.480 was looking at your recent episodes and I saw did 0:01:29.520 --> 0:01:31.680 you recently do one that was an interview with Sean 0:01:31.760 --> 0:01:35.840 Carroll about the uh, the many worlds interpretation of quantum mechanics. 0:01:35.920 --> 0:01:38.920 I know, I know he favors that, right. Yeah, we 0:01:38.959 --> 0:01:41.640 actually have a series where I interview an expert on 0:01:41.800 --> 0:01:45.160 each of the interpretations of quantum mechanics. We did one 0:01:45.160 --> 0:01:48.640 on Copenhagen interpretation with annam Becker, We did one on 0:01:49.120 --> 0:01:53.320 the relational interpretation of quantum mechanics with Carlo Rovelli, and 0:01:53.320 --> 0:01:56.320 then we talked to Sean about many Worlds interpretation. And 0:01:56.440 --> 0:01:58.279 just a couple of weeks ago we did one about 0:01:58.280 --> 0:02:02.440 the pilot wave theory of quantum mechanics, which totally blew 0:02:02.480 --> 0:02:07.800 my mind. Really much overlooked and unnecessarily maligned interpretation of 0:02:07.840 --> 0:02:11.839 quantum mechanics, in my opinion, malign, like people are being 0:02:11.880 --> 0:02:15.760 mean to it. Well, there's this famous proof by John 0:02:15.840 --> 0:02:19.799 von Neuman like seventy years ago demonstrating that it was 0:02:19.880 --> 0:02:23.160 essentially impossible, and because von Neuman is such a giant 0:02:23.200 --> 0:02:26.160 of the field, everybody thought, well, that's that. Turns out 0:02:26.200 --> 0:02:30.200 he was wrong, though, and and it took people years 0:02:30.240 --> 0:02:32.200 to figure it out. It was Bell actually who figured 0:02:32.240 --> 0:02:35.240 out that Noeman was wrong, and that it's possible to 0:02:35.320 --> 0:02:40.440 have a theory of quantum mechanics with hidden variables that's deterministic, 0:02:40.480 --> 0:02:44.359 that's not random at all um. But still to this day, 0:02:44.400 --> 0:02:48.720 nobody really takes pilot wave theory seriously, to Bell's great restoration. 0:02:48.800 --> 0:02:52.079 And I think it's because Neuman sort of threw shade 0:02:52.120 --> 0:02:54.760 on it decades ago and it never really recovered. I 0:02:54.760 --> 0:02:57.560 guess that's always dangerous when there's like a famously smart 0:02:57.600 --> 0:03:00.360 person who has an opinion absolutely and I find that 0:03:00.400 --> 0:03:04.480 physics Nobel Prize winners are especially guilty of this imagining 0:03:04.600 --> 0:03:07.480 that they are experts in every corner of everything and 0:03:07.520 --> 0:03:12.280 opining on economics or you know, social politics or whatever. 0:03:12.680 --> 0:03:14.520 If you have Nobel Prize winner in front of your name, 0:03:14.560 --> 0:03:17.639 you're an expert. Uh So, today we wanted to talk 0:03:17.680 --> 0:03:20.600 about a couple of chapters that are in a book 0:03:20.600 --> 0:03:22.920 of yours. Did that come out earlier this year? Tell 0:03:23.000 --> 0:03:25.120 us a bit about the book. Yeah, so the book 0:03:25.200 --> 0:03:28.200 is called Frequently Asked Questions about the Universe that I 0:03:28.240 --> 0:03:30.680 wrote together with my co host on the podcast and 0:03:30.760 --> 0:03:35.080 longtime collaborator Or Cham who's also famous for being the 0:03:35.120 --> 0:03:39.720 genius behind PhD comics. And the book comes from noticing 0:03:39.800 --> 0:03:43.840 that people who write into our podcast often ask similar 0:03:43.880 --> 0:03:46.400 types of questions. There are a few things that seems 0:03:46.440 --> 0:03:50.440 like everybody just wants to know about or understand or 0:03:50.520 --> 0:03:52.800 the things that people grapple with. You know, I'm a 0:03:52.800 --> 0:03:56.160 professional particle physicist in my day job, um, and so 0:03:56.200 --> 0:03:59.000 I like asking questions about, you know, the deep nature 0:03:59.120 --> 0:04:02.440 of the universe, usend, how our space and time really related. 0:04:02.760 --> 0:04:05.200 But you don't have to be a professor of physics 0:04:05.200 --> 0:04:08.560 to find these things interesting. And we feel like, in 0:04:08.560 --> 0:04:13.520 a sense, you know, curiosity is democratic. Everybody wonders about 0:04:13.520 --> 0:04:16.279 these things. So we wanted to try to attack some 0:04:16.360 --> 0:04:19.720 of these really big questions that everybody wonders about in 0:04:19.720 --> 0:04:22.279 an approachable way, in a way that doesn't require you 0:04:22.320 --> 0:04:26.680 to really have any knowledge of modern physics at all. Yeah. 0:04:26.520 --> 0:04:29.200 I've really been enjoying the chapter as I was reading. 0:04:29.440 --> 0:04:31.400 One thing I like that you do in this book 0:04:31.480 --> 0:04:34.080 is um is that you know, it's not like a 0:04:34.360 --> 0:04:37.360 continuous narrative that has to you have to have read 0:04:37.440 --> 0:04:40.320 everything that came before in order to understand. Like, the 0:04:40.400 --> 0:04:43.680 chapters can be consumed pretty much on their own, right. Yeah, 0:04:43.720 --> 0:04:46.560 we figured, you know, each chapter should be like one 0:04:46.600 --> 0:04:48.880 long bathroom break. So I mean, I'm not telling you 0:04:48.880 --> 0:04:51.120 where to read it, but if you're looking for reading 0:04:51.360 --> 0:04:54.400 while you're busy sitting down doing something else, each chapter 0:04:54.560 --> 0:04:57.719 you know, should entertain you while you're doing your business. 0:04:57.760 --> 0:04:59.840 Just you know, don't get so distracted that you forget 0:04:59.880 --> 0:05:04.359 to flush right now. Obviously this edition would lack the 0:05:04.440 --> 0:05:07.440 wonderful illustrations that are in the print and the Kendall version. 0:05:07.760 --> 0:05:10.599 But um, but you guys put together an audio version 0:05:10.600 --> 0:05:13.159 as well, right, yes we did. We got to record 0:05:13.240 --> 0:05:15.559 the audio version of the book, which is out now also, 0:05:16.000 --> 0:05:19.520 and the chapters are read by me and by Jorge alternating, 0:05:19.880 --> 0:05:22.240 which is a lot of fun just sort of hear 0:05:22.320 --> 0:05:25.440 your words come to life. But yes, the audiobook does 0:05:25.520 --> 0:05:29.000 miss some other the real genius of Jorge's drawings. Um. 0:05:29.320 --> 0:05:31.920 Jorge and I started working together on science communication more 0:05:31.960 --> 0:05:33.840 than ten years ago when I reached out to him 0:05:33.839 --> 0:05:37.480 because I thought that cartoons would be a really great 0:05:37.640 --> 0:05:41.760 medium for communicating science because they don't take themselves seriously, 0:05:42.000 --> 0:05:44.159 you know, they're a cartoon is different from like a 0:05:44.320 --> 0:05:47.400 figure into science paper, you know, which is very official 0:05:47.480 --> 0:05:50.120 and formal. A cartoon like makes fun of itself and 0:05:50.279 --> 0:05:52.839 is easy to you know, hang out with and accessible. 0:05:53.320 --> 0:05:55.800 And Jorge was great at that kind of stuff. So 0:05:55.839 --> 0:05:59.640 he and I started working together on explaining science using 0:05:59.640 --> 0:06:02.160 cartoon is a long time ago. Um, And one thing 0:06:02.200 --> 0:06:05.960 I really value about his cartoons is not just that 0:06:06.040 --> 0:06:09.040 they are good visual explainers. He has a real visual 0:06:09.160 --> 0:06:12.680 skill for explaining something simply on the page, but also 0:06:12.760 --> 0:06:14.800 that there's sort of a second voice there you can hear, 0:06:14.839 --> 0:06:16.800 like in the text is the voice of me as 0:06:16.839 --> 0:06:19.480 a physicist, and then the cartoons you can hear sort 0:06:19.480 --> 0:06:22.640 of his response to some of the crazy ideas UM. 0:06:22.760 --> 0:06:24.960 And that sort of mirrors the way the podcast works. 0:06:24.960 --> 0:06:28.039 On the podcast, I'm talking about physics and Jorges you 0:06:28.040 --> 0:06:29.920 know something like that doesn't make any sense, or how 0:06:29.920 --> 0:06:32.279 could that possibly be? Or what you got to explain 0:06:32.320 --> 0:06:34.640 that again? Because so it sort of tries to capture 0:06:34.680 --> 0:06:37.440 those two voices. Yeah, I really liked that the the 0:06:37.480 --> 0:06:40.400 illustrations almost seem kind of riffing on the written contents 0:06:40.440 --> 0:06:43.680 of the book. Well, so, the parts of the book 0:06:43.680 --> 0:06:45.440 that we wanted to focus on today, I think we're 0:06:45.480 --> 0:06:49.680 mostly centered around the idea of time and so maybe 0:06:49.720 --> 0:06:52.000 maybe a good place to start is you have a 0:06:52.080 --> 0:06:54.400 chapter in the book where you talk about time travel 0:06:54.680 --> 0:06:58.520 and you make some arguments about which types of time 0:06:58.520 --> 0:07:02.120 travel are plausible from a physics perspective and which are not. 0:07:02.960 --> 0:07:04.440 So maybe that would be a good place to start 0:07:04.440 --> 0:07:06.080 to give us the way of the land, like what 0:07:06.120 --> 0:07:08.880 types of time travel are the least consistent with the 0:07:08.920 --> 0:07:14.280 known laws of physics and which are the most consistent. Yeah. Sure, 0:07:14.680 --> 0:07:17.200 So for those of your listeners who are busy building 0:07:17.240 --> 0:07:21.640 their time travel devices, of this useful advice. Well, you know, 0:07:21.680 --> 0:07:25.080 the kind of time travel that's most inconsistent with the 0:07:25.120 --> 0:07:27.200 law of physics is the kind that most people want 0:07:27.280 --> 0:07:29.800 to do, you know, which is I want to go 0:07:29.880 --> 0:07:33.360 back in time and change something. I want to not 0:07:33.600 --> 0:07:35.760 spill my coffee on my lap, or I wanted to 0:07:35.800 --> 0:07:38.760 go you know, not um make a mistake, or I 0:07:38.760 --> 0:07:40.920 want to go ask that person out in high school, 0:07:41.000 --> 0:07:43.160 which I was too timid to do. And now I 0:07:43.200 --> 0:07:45.640 realized I should have that kind of thing. It's not 0:07:45.720 --> 0:07:48.080 just that it's ruled out by the laws of physics. 0:07:48.320 --> 0:07:51.840 In my view, it's not even sort of internally self 0:07:51.880 --> 0:07:55.440 consistent what it means um, you know, and and a 0:07:55.480 --> 0:07:58.240 lot of people think about time travel is like I 0:07:58.280 --> 0:08:01.320 want to go back in time, as if time was 0:08:01.400 --> 0:08:04.680 a place, like if it's a somewhere you can go. 0:08:05.520 --> 0:08:08.240 It's just sort of like along a different direction or something. 0:08:08.640 --> 0:08:10.880 And it's tempting to think about it that way because 0:08:10.960 --> 0:08:13.840 we we hear a lot about modern physics telling us 0:08:13.880 --> 0:08:16.920 that space and time are related and time is like 0:08:16.960 --> 0:08:19.440 a fourth dimension of space, and so it makes you 0:08:20.080 --> 0:08:22.600 want to think about time as a direction in which 0:08:22.640 --> 0:08:25.960 you can move and maybe you could just rewind it somehow, right, 0:08:26.440 --> 0:08:29.480 But the problem is that time. You know, first of all, 0:08:29.520 --> 0:08:31.640 we don't understand time like at all, and we can 0:08:31.680 --> 0:08:33.840 dig into that in a minute if you like. Um. 0:08:33.880 --> 0:08:36.200 But the problem is that time sort of reflects how 0:08:36.240 --> 0:08:39.800 the universe changes. And so you know, I think about 0:08:39.840 --> 0:08:43.640 time is like you have a timeline. That timeline is 0:08:43.760 --> 0:08:47.080 the universe changing, Like you have the universe at one moment, 0:08:47.120 --> 0:08:50.240 and you have the universe at another moment. The next 0:08:50.320 --> 0:08:54.680 moment comes later in time, and things can't change without time. 0:08:54.760 --> 0:08:59.400 Time is that change. So the self consistency problem is 0:08:59.440 --> 0:09:02.800 that going back in time to change it changes the 0:09:02.960 --> 0:09:07.839 timeline itself. So like, how does the timeline change If 0:09:07.840 --> 0:09:11.280 the timeline is the change, how does the timeline itself change? 0:09:11.400 --> 0:09:15.199 It would need like its own time, Like the timeline 0:09:15.240 --> 0:09:18.120 is now moving through time because it was a time 0:09:18.120 --> 0:09:21.079 before you changed in a time after you changed it. 0:09:21.160 --> 0:09:24.040 So it needs like a second dimension of time. I mean, 0:09:24.080 --> 0:09:26.800 it just sort of all becomes very complicated and falls 0:09:26.840 --> 0:09:29.160 apart as soon as you start thinking about it carefully. 0:09:29.559 --> 0:09:32.839 So going back and changing something in the past really 0:09:32.880 --> 0:09:35.840 just makes no sense from a physics point of view. Yeah, 0:09:36.160 --> 0:09:40.000 I love this because I have long kind of been 0:09:40.040 --> 0:09:42.640 skeptical about the idea of time travel into the past. 0:09:42.720 --> 0:09:45.400 And one of the reasons I had doubts about this 0:09:45.840 --> 0:09:48.640 is that wouldn't we expect to have already encountered lots 0:09:48.679 --> 0:09:51.839 of time travelers at some point in history? And there's 0:09:52.040 --> 0:09:54.640 no unambiguous evidence of that. I mean, obviously some people, 0:09:54.720 --> 0:09:58.000 you know, they're weird little things people think or time travel, 0:09:58.040 --> 0:10:00.840 but nothing that looks really clear. So It kind of 0:10:00.880 --> 0:10:04.080 makes me think that if if time travel into the 0:10:04.080 --> 0:10:07.560 past ever happens in the future, it will be of 0:10:07.559 --> 0:10:10.720 a very limited nature. Yeah, I love that as an 0:10:10.800 --> 0:10:14.880 experimental proof, you know, like, if time travel exists any 0:10:14.960 --> 0:10:18.160 time in the future, then you would expect to see it. Now. 0:10:18.200 --> 0:10:21.040 I love that. It's just such a powerful argument. It 0:10:21.080 --> 0:10:24.200 sort of reminds me of Stephen Hawking's famous invitation to 0:10:24.320 --> 0:10:27.600 time travelers where he threw a party and then he 0:10:27.679 --> 0:10:31.080 posted the invitation later after the party. The idea of 0:10:31.120 --> 0:10:33.120 being the time travelers, you know, they should be able 0:10:33.160 --> 0:10:35.840 to get there anyway, And of course nobody showed up 0:10:35.880 --> 0:10:40.600 to his party. Well we know he might have dispensed 0:10:40.600 --> 0:10:47.640 with them, or maybe he is a time traveler. Oh 0:10:47.679 --> 0:10:49.920 that's a good premise for a sci fi movie like 0:10:49.960 --> 0:10:53.080 The Time Traveler Hunters, trying to eliminate all evidence of 0:10:53.080 --> 0:10:57.679 the time travelers. Well that that kind of plays into um, 0:10:58.000 --> 0:10:59.839 you know, someone, what you're talking about about it being 0:11:00.080 --> 0:11:01.679 if it if it does exist in the future, then 0:11:01.679 --> 0:11:03.960 it must be limited in scope. And I guess you 0:11:03.960 --> 0:11:05.520 could look at it a couple of different We could 0:11:05.559 --> 0:11:08.240 basically just scify the hell out of it in multiple directions. 0:11:08.280 --> 0:11:11.520 But you know, you could say, like, well, maybe travel 0:11:11.559 --> 0:11:13.800 into the past. It has a range, and we haven't 0:11:13.800 --> 0:11:16.520 reached the point to where time machines of the future 0:11:16.520 --> 0:11:19.679 can reach us. Or it's just so tightly policed that 0:11:19.720 --> 0:11:21.560 nobody can make it back. You know, we have time 0:11:21.600 --> 0:11:24.760 cops or or something that are that are keeping people 0:11:24.880 --> 0:11:28.240 from making too much of a show of the whole thing. 0:11:28.520 --> 0:11:31.040 There's so many of those science fiction depictions of like 0:11:31.120 --> 0:11:35.240 a big time bureaucracy, you know that's managing the time flow, 0:11:35.480 --> 0:11:38.239 like you saw that in Loki and in Umbrella Academy 0:11:38.360 --> 0:11:42.040 and and It and in um that book recently, this 0:11:42.120 --> 0:11:44.640 is how you win the time war. And those can 0:11:44.679 --> 0:11:46.319 be a lot of fun, but also I feel like 0:11:46.360 --> 0:11:49.600 they're they just make no sense at all. You know, 0:11:50.000 --> 0:11:53.120 how do you have this weird administration that's separated from 0:11:53.160 --> 0:11:58.120 time and also weirdly frozen in like nineties bureaucracy. It's 0:11:58.120 --> 0:11:59.920 you know, it's fun, but not not if you think 0:12:00.000 --> 0:12:02.760 about it really at all. What also reminds me a 0:12:02.800 --> 0:12:05.160 lot of something I was actually chatting with you about 0:12:05.320 --> 0:12:07.840 a couple of weeks ago, when I interviewed you for 0:12:07.880 --> 0:12:10.520 a short freelance piece for How Stuff works dot com 0:12:10.559 --> 0:12:14.959 about UM about the zoo hypothesis. Uh you you spoke 0:12:15.000 --> 0:12:19.280 about about that for the interview, and uh you mentioned 0:12:19.280 --> 0:12:21.440 that one of the strong arguments against it is that 0:12:21.480 --> 0:12:25.200 if there is actually this, um, this conspiracy of of 0:12:25.280 --> 0:12:29.000 aliens to avoid contact with humans and and and keep 0:12:29.080 --> 0:12:31.120 us in the dark about the uh you know, the 0:12:31.160 --> 0:12:36.079 galactic civilizations just outside of our view, UM, the main 0:12:36.160 --> 0:12:40.400 argument against it is that governments as we know them, 0:12:40.440 --> 0:12:42.880 by the only model that we know the the the 0:12:42.960 --> 0:12:45.559 human model, are not really good at keeping secrets. They're 0:12:45.600 --> 0:12:48.240 not good at advanaging secrets. And it seems like you 0:12:48.240 --> 0:12:51.640 could also apply that to the idea of intelligent beings 0:12:51.679 --> 0:12:55.719 or humans and the future managing the timeline and so forth. Exactly, 0:12:55.800 --> 0:12:58.280 you know, some version of Elon Musk and the future 0:12:58.360 --> 0:13:00.640 is get his hands on it, and he's gonna launch 0:13:00.679 --> 0:13:03.720 a bunch of crazy, you know, missions, and somebody's gonna 0:13:03.720 --> 0:13:06.520 mess something up. So it's hard to imagine that people 0:13:06.520 --> 0:13:09.160 in the future having time travel and somehow keeping a 0:13:09.160 --> 0:13:13.720 secretor slipping into the past unnoticed and nobody ever, you know, 0:13:13.800 --> 0:13:16.920 breaking the protocol or something. It's it just becomes totally 0:13:16.960 --> 0:13:20.200 implausible the more you think about it. Picking up off that. 0:13:20.240 --> 0:13:22.640 I mean, this is another one of the weird things 0:13:22.679 --> 0:13:26.080 about time is it seems like time is actually one 0:13:26.160 --> 0:13:31.200 of the arguments against the idea of a coherent galactic civilization, 0:13:31.720 --> 0:13:34.240 if this makes any sense, because like you think, a 0:13:34.280 --> 0:13:37.360 civilization in order to organize itself, has to have some 0:13:38.120 --> 0:13:42.560 pretty close to synchronous uh, you know, thing going on, 0:13:42.679 --> 0:13:45.960 Like things have to be happening pretty close to around 0:13:45.960 --> 0:13:47.880 the same time for them. But does it even make 0:13:47.960 --> 0:13:51.360 sense for I don't know, one planet in a galactic 0:13:51.400 --> 0:13:54.440 civilization to be part of a civilization with one on 0:13:54.440 --> 0:13:56.360 the other side of the galaxy. I mean, is there 0:13:56.400 --> 0:14:00.080 you know, can they say, uh, is there such a thing? 0:14:00.160 --> 0:14:03.400 Is what's happening right now on a planet on the 0:14:03.440 --> 0:14:06.120 other side of the galaxy? Yeah. You make a great point, 0:14:06.160 --> 0:14:09.960 because there's a speed limit to information moving through the universe, 0:14:10.400 --> 0:14:12.720 which puts an effective limit on like how well you 0:14:12.760 --> 0:14:16.280 can coordinate and organize things. Actually, think about this in 0:14:16.360 --> 0:14:19.600 cosmology all the time, because there's a like the largest 0:14:19.840 --> 0:14:22.920 thing that can exist in the universe just from very 0:14:22.920 --> 0:14:25.400 simple arguments like the age of the universe and the 0:14:25.440 --> 0:14:28.480 speed of light. You can't have an object that's like 0:14:29.200 --> 0:14:33.600 ten thousand billion light years wide, that's like coordinated, that 0:14:33.680 --> 0:14:36.920 has like a structure that's like gravitationally bound on itself, 0:14:37.360 --> 0:14:39.560 because there hasn't been time for like a photon to 0:14:39.680 --> 0:14:43.920 even cross over the entire size of that object. So 0:14:43.960 --> 0:14:46.400 there's like a limit to how big the universe can 0:14:46.440 --> 0:14:49.560 even build like a thing, not to mention like the 0:14:49.600 --> 0:14:53.800 close coordination required to like organize a galactic empire. And 0:14:53.840 --> 0:14:57.640 so yeah, absolutely, um, I think that the sheer size 0:14:57.680 --> 0:15:01.200 of space definitely limits our ability to exploit it unless 0:15:01.240 --> 0:15:06.080 it breaks down into you know, lots of different unorganized entities, 0:15:06.120 --> 0:15:08.480 Like maybe we send humans in an arc off to 0:15:08.680 --> 0:15:11.480 another star and they start their own human colony and 0:15:11.640 --> 0:15:13.640 we're not in touch and a part of them, you know, 0:15:13.720 --> 0:15:16.400 political nation state. But at least we're humans here and 0:15:16.400 --> 0:15:19.120 there are humans there. Yeah, I think that's a great 0:15:19.160 --> 0:15:28.120 way to conceptualize it. So, I guess coming back to 0:15:28.200 --> 0:15:31.080 time travel for a minute, I wanted to talk about 0:15:31.080 --> 0:15:35.040 some of the specifics you offer about physically plausible ways 0:15:35.320 --> 0:15:39.280 of traveling into the past. Uh. So you mentioned a 0:15:39.320 --> 0:15:41.560 couple of things. You mentioned the idea of wormholes, and 0:15:41.560 --> 0:15:44.000 then you also mentioned one that might be less familiar 0:15:44.000 --> 0:15:47.960 to people. The idea of an infinitely long cylinder of 0:15:48.040 --> 0:15:52.960 spinning dust, which could potentially, at least maybe depending on 0:15:53.920 --> 0:15:56.840 something about whether something about relativity is true or not, 0:15:57.440 --> 0:16:00.280 could potentially allow time travel into the past through something 0:16:00.320 --> 0:16:03.880 called time loops. Could could you explain how this would work? Like, 0:16:03.960 --> 0:16:08.040 what would this experience be like for the time traveler? Yeah, well, 0:16:08.080 --> 0:16:10.680 the short answer is, we just don't know. Uh, this 0:16:10.760 --> 0:16:14.600 is a realm where we are like on the cutting edge. Theoretically, 0:16:15.120 --> 0:16:18.240 people are looking at the rules of how space and 0:16:18.320 --> 0:16:22.320 time bend and twist because you know, general relativity, our 0:16:22.360 --> 0:16:26.360 theory for space and time itself essentially tells us that 0:16:26.600 --> 0:16:30.800 space and time bend in response to mass and then 0:16:30.840 --> 0:16:33.800 tell masses how to move. So, for example, you have 0:16:33.840 --> 0:16:36.160 an empty universe and you put a star in it, 0:16:36.160 --> 0:16:38.880 it bends the space around the star, and then the 0:16:38.920 --> 0:16:41.360 bending of that space tells things how to move, and 0:16:41.400 --> 0:16:44.920 not just through space but also through time. So you 0:16:44.960 --> 0:16:48.280 go near a black hole, for example, time is slowed down. 0:16:48.720 --> 0:16:51.120 So there's definitely some deep connection there between space and 0:16:51.120 --> 0:16:53.880 time and what people have done is trying to explore 0:16:53.920 --> 0:16:56.720 extreme scenarios about what happens if you do this, what 0:16:56.840 --> 0:16:59.640 happens if you do that? Is this allowed? Is that allowed? 0:17:00.080 --> 0:17:02.480 And so it's sort of like exploring the universe, but 0:17:02.600 --> 0:17:06.200 just inside our own heads. We can't necessarily yet go 0:17:06.280 --> 0:17:08.960 out there and build these things in space and say, 0:17:09.040 --> 0:17:12.160 let's see what happens experimentally, but we can do similar 0:17:12.280 --> 0:17:14.840 like fought experiments, where we say, what would happen if 0:17:14.880 --> 0:17:18.119 you did this, and let's just let's assume the equations 0:17:18.119 --> 0:17:21.080 are correct and see what happens. And so there's a 0:17:21.080 --> 0:17:23.840 couple of fund scenarios there. One, as you said, is wormholes. 0:17:24.440 --> 0:17:27.760 These aren't really crazy because they are like connections between 0:17:28.080 --> 0:17:31.119 different points in space. And when you think of space, 0:17:31.160 --> 0:17:33.760 you probably think of like just sheer emptiness, you know, 0:17:33.800 --> 0:17:36.639 the back drop the stage on which the universe happens. 0:17:36.680 --> 0:17:38.920 But now we know that space is more complex. It 0:17:39.000 --> 0:17:41.840 can bend and it can twist, and that might be 0:17:41.880 --> 0:17:43.359 something that you can put in your head. You can 0:17:43.400 --> 0:17:46.879 imagine like space bending around the sun. But because space 0:17:47.040 --> 0:17:49.880 is like a thing with an arrangement, it could also 0:17:49.920 --> 0:17:54.760 do other really weird things, like be connected non trivially, 0:17:55.160 --> 0:17:56.920 so you have like a chunk of space over here. 0:17:57.280 --> 0:17:59.920 It can be directly connected to a chunk of space 0:18:00.080 --> 0:18:02.840 over there. What does that mean. Well, you're used to 0:18:02.880 --> 0:18:06.200 the space around you being connected to the space right 0:18:06.240 --> 0:18:07.920 next to it. That's what it means to be right 0:18:08.000 --> 0:18:10.080 next to it. Right, you take a step to the left, 0:18:10.119 --> 0:18:12.240 you move to the next sort of piece of space. 0:18:12.720 --> 0:18:15.879 Think of it sort of like pixels on a screen. Right, Well, 0:18:15.920 --> 0:18:18.879 a wormhole is a connection between two points in space 0:18:18.920 --> 0:18:22.679 that are otherwise really distant. And so you take a 0:18:22.680 --> 0:18:25.159 step from a from one pixel and now you're in 0:18:25.160 --> 0:18:27.840 a pixel on the other side of the screen. And 0:18:27.880 --> 0:18:31.359 so that seems weird and impossible, but remember space can 0:18:31.400 --> 0:18:34.800 have all sorts of strange connections, and according to the 0:18:34.840 --> 0:18:37.960 equations of general relativity, the ones that define how space 0:18:38.000 --> 0:18:42.600 is organized, that is allowed. It is possible. And so 0:18:42.760 --> 0:18:44.960 a couple of folks at cal Tech we're thinking about, well, 0:18:45.200 --> 0:18:48.160 you know what about time? Is it possible for one 0:18:48.359 --> 0:18:50.320 end of the wormhole to be in one place and 0:18:50.359 --> 0:18:54.600 the other end to be in another time? Because, as 0:18:54.600 --> 0:18:57.800 you were mentioning earlier, like the notion of simultaneity, like 0:18:58.280 --> 0:19:02.159 when is now depends really on where you are. Also, 0:19:02.840 --> 0:19:04.680 so they have this idea to take one end of 0:19:04.720 --> 0:19:07.480 the wormhole and you accelerate it near the speed of light. 0:19:07.760 --> 0:19:10.320 That effectively it can be sort of back in time. 0:19:11.080 --> 0:19:14.119 And this all works theoretically, but it also sort of 0:19:14.160 --> 0:19:17.560 contradicts other things we know, like if you go through 0:19:17.560 --> 0:19:20.679 this wormhole and you come out in the past, you know, 0:19:20.880 --> 0:19:23.439 doesn't that break things like causality? And you come out 0:19:23.440 --> 0:19:26.639 in the past and kill yourself before you um do 0:19:26.800 --> 0:19:28.960 the experiment, Then you don't do the experiment, you don't 0:19:28.960 --> 0:19:32.200 come out in the past. So it appears to create paradoxes, 0:19:32.640 --> 0:19:35.879 and nobody knows like how to resolve that. Does that 0:19:35.920 --> 0:19:38.119 mean that these things are impossible? Does that mean if 0:19:38.160 --> 0:19:40.600 you did that, the universe would like disappear in a 0:19:40.640 --> 0:19:43.639 puff of logic. Nobody really knows what would happen. So 0:19:43.680 --> 0:19:46.480 that's a bit of a contradiction in the theory itself 0:19:46.840 --> 0:19:50.040 that it predicts something which seems to be disallowed by 0:19:50.080 --> 0:19:53.639 other parts of the theory. And it's a similar idea 0:19:53.760 --> 0:19:58.520 for these closed timelike curves. People said, if you create 0:19:58.600 --> 0:20:02.800 these infinitely long the cylinders of spinning dust, which doesn't 0:20:02.800 --> 0:20:06.320 sound easy to do, then it bends time in this 0:20:06.320 --> 0:20:09.159 way that the time then as you move forward in 0:20:09.240 --> 0:20:13.720 time you're actually moving sort of like sideways through space 0:20:13.760 --> 0:20:16.520 time in a way that's similar to the experience of 0:20:16.720 --> 0:20:20.480 going into a black hole. Outside a black hole, time 0:20:20.640 --> 0:20:24.600 always moves forwards. Inside a black hole, space has bent 0:20:24.800 --> 0:20:27.879 so much that space only moves towards the center of 0:20:27.920 --> 0:20:31.520 the black hole. It's like one direction to space. So 0:20:31.600 --> 0:20:35.040 if you imagine space being distorted, not quite as much 0:20:35.040 --> 0:20:36.880 as a black hole, but sort of in a similar 0:20:36.920 --> 0:20:40.960 direction that it sort of bends space sideways, then you 0:20:40.960 --> 0:20:43.359 can create these paths where something can move in a 0:20:43.440 --> 0:20:47.439 loop through time. Um, but you would be trapped on 0:20:47.560 --> 0:20:50.040 that loop, so you wouldn't be able to like change anything. 0:20:50.040 --> 0:20:52.320 It's like a fixed loop, sort of like Harry Potter 0:20:52.440 --> 0:20:55.000 style loop through time, where every time you go through 0:20:55.040 --> 0:20:57.880 it's exactly the same thing happening. And these are really 0:20:57.920 --> 0:21:01.000 fun because nobody knows like if these are actually possible 0:21:01.040 --> 0:21:03.840 and what would happen if you actually went through them. 0:21:03.840 --> 0:21:06.200 So so we don't really know, for instance, like what 0:21:06.480 --> 0:21:11.840 conceivable reason there would be for a civilization to conceivably 0:21:12.320 --> 0:21:15.760 construct one of these, yeah, because we don't know practically 0:21:15.800 --> 0:21:18.600 what you could achieve. And also, an infinite cylinder of 0:21:18.680 --> 0:21:21.920 spitting dust sounds like an expensive project. You know, the 0:21:21.960 --> 0:21:25.560 word infinite seems to raise some doubts. And when it 0:21:25.560 --> 0:21:29.080 comes to wormholes, people know how to calculate whether a 0:21:29.080 --> 0:21:32.560 wormhole is allowed by the theory of general relativity. Nobody 0:21:32.600 --> 0:21:35.360 knows how to build a wormhole. You know. It's sort 0:21:35.359 --> 0:21:38.520