WEBVTT - From the Vault: Daniel Whiteson, Time Traveler 0:00:05.760 --> 0:00:07.680 Hey, you, welcome to Stuff to Blow your Mind. My 0:00:07.760 --> 0:00:11.200 name is Robert Lamb and I'm Joe McCormick. And, uh, 0:00:11.240 --> 0:00:13.000 this week, Rob and I are out on breaks, so 0:00:13.039 --> 0:00:16.080 we've got some great Vault episodes for you. This one 0:00:16.160 --> 0:00:20.840 originally aired December ninth, and it's our interview with Daniel 0:00:20.880 --> 0:00:25.079 Whitson from the podcast Daniel and Jorge Explain the Universe. 0:00:25.320 --> 0:00:27.560 I think we talk all kinds of time travel and 0:00:27.600 --> 0:00:33.040 stuff like that. Welcome to Stuff to Blow your Mind, 0:00:33.320 --> 0:00:42.520 production of My Heart Radio. Hey, you, welcome to Stuff 0:00:42.560 --> 0:00:45.280 to Blow your Mind. My name is Robert Lamb and 0:00:45.320 --> 0:00:48.600 I'm Joe McCormick. And for today's episode, we're going to 0:00:48.640 --> 0:00:52.800 be chatting with Daniel Whitson, who is a particle physicist 0:00:52.880 --> 0:00:56.120 and science communicator and one of the hosts of the 0:00:56.160 --> 0:01:00.480 podcast Daniel and Jorge Explain the Universe. This is Enniel's 0:01:00.600 --> 0:01:03.360 third time hopping on the show with us. The previous 0:01:03.440 --> 0:01:07.640 episodes were in September of twenty nineteen and April of 0:01:08.880 --> 0:01:11.200 And for this episode, we're gonna be talking about a book. 0:01:11.520 --> 0:01:14.600 Daniel and his co host and co author Jorge him 0:01:14.840 --> 0:01:18.720 have a new book called Frequently Asked Questions About the universe. 0:01:19.400 --> 0:01:21.280 So it was a real pleasure to have Daniel on 0:01:21.319 --> 0:01:23.720 the show for the hat trick, and I guess without 0:01:23.760 --> 0:01:29.560 any further delay, we will go right into the interview. Daniel, 0:01:29.640 --> 0:01:31.440 welcome back to the show. We're so glad you're here. 0:01:31.760 --> 0:01:33.720 Thanks very much for having me back. I always fun 0:01:33.720 --> 0:01:37.320 to talk to you guys about things that blew my mind. Awesome. So, 0:01:37.480 --> 0:01:41.240 um the podcast Daniel and Jorge explain the Universe still 0:01:41.280 --> 0:01:45.119 going strong? Um, how how far are you into explaining 0:01:45.280 --> 0:01:49.160 the universe in its entirely. We have explained zero point 0:01:49.320 --> 0:01:52.600 zero zero zero zero zer zero zero zero one percent 0:01:52.720 --> 0:01:55.920 of the universe so far. Nice. I uh, actually I 0:01:55.960 --> 0:01:58.000 was looking at your recent episodes and I saw did 0:01:58.040 --> 0:02:00.240 you recently do one that was an interview with Shawn 0:02:00.320 --> 0:02:04.400 Carroll about the uh, the many world's interpretation of quantum mechanics. 0:02:04.440 --> 0:02:07.440 I know, I know he favors that, right. Yeah. We 0:02:07.480 --> 0:02:10.160 actually have a series where I interview an expert on 0:02:10.280 --> 0:02:13.680 each of the interpretations of quantum mechanics. We did one 0:02:13.680 --> 0:02:17.120 on Copenhagen interpretation with Anna Becker, we did one on 0:02:17.600 --> 0:02:21.800 the relational interpretation of quantum mechanics with Carlo Rovelli, and 0:02:21.840 --> 0:02:24.840 then we talked to Sean about many world's interpretation, and 0:02:24.960 --> 0:02:26.799 just a couple of weeks ago we did one about 0:02:26.800 --> 0:02:30.920 the pilot wave theory of quantum mechanics, which totally blew 0:02:31.000 --> 0:02:36.280 my mind. Really much overlooked and unnecessarily maligned interpretation of 0:02:36.360 --> 0:02:40.359 quantum mechanics, in my opinion, malign, like people are being 0:02:40.400 --> 0:02:44.280 mean to it. Well, there's this famous proof by John 0:02:44.360 --> 0:02:48.320 von Neumann like seventy years ago demonstrating that it was 0:02:48.400 --> 0:02:51.680 essentially impossible, and because von Neuman is such a giant 0:02:51.720 --> 0:02:54.679 of the field, everybody thought, well, that's that. Turns out 0:02:54.720 --> 0:02:58.800 he was wrong, though, and it took people years to 0:02:58.840 --> 0:03:00.919 figure it out. It was Belle actually who figured out 0:03:00.919 --> 0:03:03.960 that Nouman was wrong, and that it's possible to have 0:03:04.240 --> 0:03:08.960 a theory of quantum mechanics with hidden variables that's deterministic, 0:03:09.000 --> 0:03:12.840 that's not random at all um. But still to this day, 0:03:12.919 --> 0:03:17.239 nobody really takes pilot wave theory seriously, to Bell's great restoration, 0:03:17.320 --> 0:03:20.560 and I think it's because Neuman sort of through shade 0:03:20.600 --> 0:03:23.239 on it decades ago and it never really recovered. I 0:03:23.280 --> 0:03:26.079 guess that's always dangerous when there's like a famously smart 0:03:26.120 --> 0:03:28.840 person who has an opinion, absolutely, And I find that 0:03:28.919 --> 0:03:33.000 physics Nobel Prize winners are especially guilty of this imagining 0:03:33.120 --> 0:03:36.000 that they are experts in every corner of everything and 0:03:36.040 --> 0:03:40.760 opining on economics or you know, social politics or whatever. 0:03:41.160 --> 0:03:43.040 If you have Nobel Prize winner in front of your name, 0:03:43.080 --> 0:03:46.160 you're an expert. Uh So, today we wanted to talk 0:03:46.200 --> 0:03:49.119 about a couple of chapters that are in a book 0:03:49.120 --> 0:03:51.480 of yours. Did that come out earlier this year? Tell 0:03:51.520 --> 0:03:53.640 us a bit about the book. Yeah. So the book 0:03:53.720 --> 0:03:56.720 is called Frequently Asked Questions about the Universe that I 0:03:56.760 --> 0:03:59.200 wrote together with my co host on the podcast and 0:03:59.240 --> 0:04:03.040 longtime collibrator Orge h. Cham who's also famous for being 0:04:03.080 --> 0:04:07.600 the genius behind PhD Comics. And the book comes from 0:04:07.640 --> 0:04:11.880 noticing that people who write into our podcast often ask 0:04:12.000 --> 0:04:14.520 similar types of questions. There are a few things that 0:04:14.600 --> 0:04:18.400 seems like everybody just wants to know about or understand, 0:04:18.880 --> 0:04:21.200 or the things that people grapple with. You know, I'm 0:04:21.200 --> 0:04:24.520 a professional particle physicist in my day job, um, and 0:04:24.560 --> 0:04:27.040 so I like asking questions about, you know, the deep 0:04:27.160 --> 0:04:30.120 nature of the universe and how our space and time 0:04:30.160 --> 0:04:33.040 really related. But you don't have to be a professor 0:04:33.120 --> 0:04:36.680 of physics to find these things interesting. And we feel like, 0:04:36.960 --> 0:04:41.680 in a sense, you know, curiosity is democratic. Everybody wonders 0:04:41.720 --> 0:04:44.640 about these things, so we wanted to try to attack 0:04:44.720 --> 0:04:47.679 some of these really big questions that everybody wonders about 0:04:48.200 --> 0:04:50.720 in an approachable way, in the way that doesn't require 0:04:50.720 --> 0:04:55.200 you to really have any knowledge of modern physics at all. Yeah. 0:04:55.040 --> 0:04:57.720 I've really been enjoying the chapter as I was reading. 0:04:57.960 --> 0:05:00.800 One thing I like that you do in this book, um, 0:05:01.200 --> 0:05:04.120 is that you know, it's not like a continuous narrative 0:05:04.240 --> 0:05:06.640 that has to you have to have read everything that 0:05:06.720 --> 0:05:09.520 came before in order to understand. Like, the chapters can 0:05:09.520 --> 0:05:12.799 be consumed pretty much on their own, right. Yeah, we figured, 0:05:12.839 --> 0:05:16.159 you know, each chapter should be like one long bathroom break. 0:05:16.240 --> 0:05:18.120 So I mean, I'm not telling you where to read it, 0:05:18.160 --> 0:05:21.400 but looking you're reading while you're busy sitting down doing 0:05:21.440 --> 0:05:24.919 something else. Each chapter, you know, should entertain you while 0:05:24.960 --> 0:05:27.359 you're doing your business, just you know, don't get so 0:05:27.400 --> 0:05:30.800 distracted that you forget to flush right now. Obviously this 0:05:31.200 --> 0:05:34.640 edition would lack the wonderful illustrations that are in the 0:05:34.960 --> 0:05:37.680 print and the Kindle version, but um, but you guys 0:05:37.800 --> 0:05:40.760 put together an audio version as well. Right, yes we did. 0:05:40.800 --> 0:05:43.040 We got to record the audio version of the book, 0:05:43.080 --> 0:05:45.880 which is out now also, and the chapters are read 0:05:45.920 --> 0:05:49.040 by me and by Jorge alternating, which is a lot 0:05:49.040 --> 0:05:52.159 of fun just sort of hear your words come to life. 0:05:52.680 --> 0:05:55.080 But yes, the audio book does miss some other the 0:05:55.120 --> 0:05:58.599 real genius of Jorge's drawings, Um. Jorge and I started 0:05:58.600 --> 0:06:01.080 working together on science communication and more than ten years 0:06:01.080 --> 0:06:03.159 ago when I reached out to him because I thought 0:06:03.480 --> 0:06:08.120 that cartoons would be a really great medium for communicating science, 0:06:08.480 --> 0:06:11.600 because they don't take themselves seriously, you know, They're a cartoon. 0:06:11.720 --> 0:06:14.960 Is different from like a figure into science paper, you know, 0:06:15.000 --> 0:06:17.680 which is very official and formal. A cartoon like makes 0:06:17.680 --> 0:06:20.000 fun of itself and is easy to you know, hang 0:06:20.040 --> 0:06:23.080 out with and accessible. And Jorge was great at that 0:06:23.160 --> 0:06:25.760 kind of stuff. So he and I started working together 0:06:26.000 --> 0:06:30.119 on explaining science using cartoons a long time ago. Um, 0:06:30.200 --> 0:06:33.000 And one thing I really value about his cartoons is 0:06:33.440 --> 0:06:36.640 not just that they are good visual explainers. He has 0:06:36.680 --> 0:06:40.600 a real visual skill for explaining something simply on the page, 0:06:40.839 --> 0:06:42.840 but also that there's sort of a second voice there 0:06:42.920 --> 0:06:44.880 you can hear, like in the text, is the voice 0:06:44.880 --> 0:06:47.360 of me as a physicist, and then in the cartoons 0:06:47.400 --> 0:06:49.440 you can hear sort of his response to some of 0:06:49.440 --> 0:06:52.360 the crazy ideas um and that sort of mirrors the 0:06:52.400 --> 0:06:55.240 way the podcast works. On the podcast, I'm talking about 0:06:55.240 --> 0:06:57.600 physics and Jorges you know something like that doesn't make 0:06:57.640 --> 0:06:59.720 any sense, or how could that possibly be? Or what 0:07:00.120 --> 0:07:02.400 you got to explain that again, So it sort of 0:07:02.400 --> 0:07:05.320 tries to capture those two voices. Yeah, I really liked 0:07:05.360 --> 0:07:08.080 that the illustrations almost seem kind of riffing on the 0:07:08.080 --> 0:07:11.880 written contents of the book. Well, so the parts of 0:07:11.880 --> 0:07:13.600 the book that we wanted to focus on today, I 0:07:13.600 --> 0:07:17.760 think we're mostly centered around the idea of time and 0:07:17.800 --> 0:07:20.320 so maybe maybe a good place to start is you 0:07:20.360 --> 0:07:22.040 have a chapter in the book where you talk about 0:07:22.120 --> 0:07:26.600 time travel and you make some arguments about which types 0:07:26.640 --> 0:07:30.000 of time travel are plausible from a physics perspective and 0:07:30.040 --> 0:07:32.400 which are not. So maybe that would be a good 0:07:32.400 --> 0:07:34.240 place to start. Give us the way of the land, 0:07:34.280 --> 0:07:37.120 like what types of time travel are the least consistent 0:07:37.200 --> 0:07:39.720 with the known laws of physics and which are the 0:07:39.720 --> 0:07:44.560 most consistent. Yeah. Sure, so for those of your listeners 0:07:44.600 --> 0:07:47.760 who are busy building their time travel devices, of the 0:07:48.280 --> 0:07:51.520 usable advice well, you know, the kind of time travel 0:07:51.640 --> 0:07:54.640 that's most inconsistent with the law of physics is the 0:07:54.720 --> 0:07:56.960 kind that most people want to do, you know. It 0:07:57.240 --> 0:08:00.000 is I want to go back in time and change 0:08:00.000 --> 0:08:03.040 age something. I want to not spill my coffee on 0:08:03.080 --> 0:08:05.720 my lap, or I wanted to go, you know, not 0:08:06.480 --> 0:08:08.120 make a mistake, or I want to go ask that 0:08:08.160 --> 0:08:10.560 person out in high school, which I was too timid 0:08:10.640 --> 0:08:12.960 to do, and now I realized I should have that 0:08:13.040 --> 0:08:15.360 kind of thing. It's not just that it's ruled out 0:08:15.400 --> 0:08:17.880 by the laws of physics. In my view, it's not 0:08:18.120 --> 0:08:23.200 even sort of internally self consistent. What it means um, 0:08:23.240 --> 0:08:25.040 you know, and and a lot of people think about 0:08:25.040 --> 0:08:28.400 time travel is like I want to go back in time, 0:08:28.560 --> 0:08:31.280 as if time was a place, like if it's a 0:08:31.520 --> 0:08:35.040 somewhere you can go. It's just sort of like along 0:08:35.080 --> 0:08:38.199 a different direction or something. And it's tempting to think 0:08:38.200 --> 0:08:40.600 about it that way because we we hear a lot 0:08:40.640 --> 0:08:43.760 about modern physics telling us that space and time are 0:08:43.800 --> 0:08:47.200 related and time is like a fourth dimension of space, 0:08:47.200 --> 0:08:49.920 and so it makes you want to think about time 0:08:49.960 --> 0:08:52.559 as a direction in which you can move and maybe 0:08:52.559 --> 0:08:55.600 you could just rewind it somehow, right, But the problem 0:08:55.679 --> 0:08:58.280 is that time. You know, first of all, we don't 0:08:58.360 --> 0:09:00.680 understand time like at all. You can dig into that 0:09:00.720 --> 0:09:03.000 in a minute if you like. Um, but the problem 0:09:03.080 --> 0:09:06.080 is that time sort of reflects how the universe changes, 0:09:06.720 --> 0:09:09.040 and so you know, I think about time is like 0:09:09.320 --> 0:09:13.400 you have a timeline. That timeline is the universe changing, 0:09:13.840 --> 0:09:15.880 Like you have the universe at one moment, and you 0:09:15.880 --> 0:09:19.439 have the universe at another moment. The next moment comes 0:09:19.520 --> 0:09:23.640 later in time, and things can't change without time. Time 0:09:23.800 --> 0:09:28.160 is that change. So the self consistency problem is that 0:09:28.360 --> 0:09:32.959 going back in time to change it changes the timeline itself. 0:09:33.160 --> 0:09:36.960 So like, how does the timeline change? If the timeline 0:09:37.280 --> 0:09:40.000 is the change, how does the timeline itself change? It 0:09:40.000 --> 0:09:43.840 would need like its own time, Like the timeline is 0:09:43.840 --> 0:09:46.960 now moving through time because it was a time before 0:09:46.960 --> 0:09:49.760 you changed in a time after you changed it, So 0:09:49.800 --> 0:09:52.559 it needs like a second dimension of time. I mean, 0:09:52.600 --> 0:09:55.320 it just sort of all becomes very complicated and falls 0:09:55.360 --> 0:09:57.720 apart as soon as you start thinking about it carefully. 0:09:58.080 --> 0:10:01.360 So going back and changing something in the past really 0:10:01.400 --> 0:10:04.360 just makes no sense from a physics point of view. Yeah, 0:10:04.679 --> 0:10:08.520 I love this because I have long kind of been 0:10:08.559 --> 0:10:11.160 skeptical about the idea of time travel into the past. 0:10:11.240 --> 0:10:13.920 And one of the reasons I had doubts about this 0:10:14.360 --> 0:10:17.160 is that wouldn't we expect to have already encountered lots 0:10:17.160 --> 0:10:20.360 of time travelers at some point in history, And there's 0:10:20.520 --> 0:10:23.160 no unambiguous evidence of that. I mean, obviously some people, 0:10:23.240 --> 0:10:26.520 you know, they're weird little things people think or time travel, 0:10:26.559 --> 0:10:29.360 but nothing that looks really clear. So it kind of 0:10:29.400 --> 0:10:32.560 makes me think that if if time travel into the 0:10:32.600 --> 0:10:36.040 past ever happens in the future, it will be of 0:10:36.080 --> 0:10:39.280 a very limited nature. Yeah, I love that as an 0:10:39.280 --> 0:10:43.440 experimental proof, you know, like, if time travel exists any 0:10:43.440 --> 0:10:46.680 time in the future, then you would expect to see it. Now. 0:10:46.720 --> 0:10:49.560 I love that. It's just such a powerful argument. It 0:10:49.559 --> 0:10:52.720 sort of reminds me of Stephen Hawking's famous invitation to 0:10:52.840 --> 0:10:56.120 time travelers, where he threw a party and then he 0:10:56.200 --> 0:10:59.600 posted the invitation later after the party. The idea of 0:10:59.640 --> 0:11:01.640 being that time travelers, you know, they should be able 0:11:01.679 --> 0:11:04.240 to get there anyway, but of course nobody should have 0:11:04.400 --> 0:11:08.520 to his party, well that we know of. He might 0:11:08.520 --> 0:11:13.280 have dispensed with them, or maybe he is a time traveler. 0:11:16.000 --> 0:11:18.319 Oh that's a good premise. For a sci fi movie 0:11:18.360 --> 0:11:21.480 like the Time Traveler Hunters trying to eliminate all evidence 0:11:21.520 --> 0:11:24.840 of the time travelers. Well that that kind of plays 0:11:24.880 --> 0:11:27.840 into um you know, some of what you're talking about 0:11:27.840 --> 0:11:29.640 about it being if it if it does exist in 0:11:29.679 --> 0:11:32.040 the future, then it must be limited in scope. And 0:11:32.160 --> 0:11:33.400 I guess you could look at it a couple of 0:11:33.400 --> 0:11:35.400 different We could basically just sci fi the hell out 0:11:35.400 --> 0:11:38.560 of it in multiple directions. But you know, you could say, like, well, 0:11:38.559 --> 0:11:41.880 maybe travel into the past. It has a range and 0:11:41.920 --> 0:11:44.599 we haven't reached the point to where time machines of 0:11:44.640 --> 0:11:47.480 the future can reach us, or it's just so tightly 0:11:47.480 --> 0:11:49.599 policed that nobody can make it back. You know, we 0:11:49.679 --> 0:11:52.600 have time cops or or something that are that are 0:11:52.679 --> 0:11:56.240 keeping people from making too much of a show of 0:11:56.240 --> 0:11:58.760 the whole thing. There's so many of those science fiction 0:11:58.760 --> 0:12:02.400 depictions of like a b time bureaucracy, you know that's 0:12:02.440 --> 0:12:05.120 managing the time flow, like you saw that in Loki 0:12:05.360 --> 0:12:09.280 and in Umbrella Academy, and and and in um that 0:12:09.280 --> 0:12:11.839 book recently, this is how you win the time war. 0:12:12.440 --> 0:12:14.200 And those can be a lot of fun, but also 0:12:14.280 --> 0:12:17.640 I feel like they're they just make no sense at all. 0:12:17.760 --> 0:12:20.600 You know, how do you have this weird administration that's 0:12:20.840 --> 0:12:25.760 separated from time and also weirdly frozen in like nineties bureaucracy. 0:12:26.400 --> 0:12:28.280 It's you know, it's fun, but not not if you 0:12:28.280 --> 0:12:31.120 think about it really at all. What also reminds me 0:12:31.200 --> 0:12:33.360 a lot of something I was actually chatting with you 0:12:33.400 --> 0:12:36.160 about a couple of weeks ago when I interviewed you 0:12:36.280 --> 0:12:38.800 for a short freelance piece for how stuff works dot 0:12:38.840 --> 0:12:43.480 com about UM about the zoo hypothesis. You you spoke 0:12:43.520 --> 0:12:47.800 about about that for the interview, and uh, you mentioned 0:12:47.800 --> 0:12:49.960 that one of the strong arguments against it is that 0:12:50.000 --> 0:12:53.719 if there is actually this, um, this conspiracy of of 0:12:53.800 --> 0:12:57.520 aliens to avoid contact with humans and and and keep 0:12:57.559 --> 0:12:59.640 us in the dark about the uh you know, the 0:12:59.640 --> 0:13:04.440 Galack tick, civilizations just outside of our view. UM, the 0:13:04.440 --> 0:13:08.440 main argument against it is that that governments as we 0:13:08.520 --> 0:13:11.160 know them, by the only model that we know the 0:13:10.960 --> 0:13:13.880 the the human model, are not really good at keeping secrets. 0:13:13.920 --> 0:13:16.680 They're not good at managing secrets. And it seems like 0:13:16.679 --> 0:13:19.719 you could also apply that to the idea of intelligent 0:13:19.800 --> 0:13:22.880 beings or humans and the future managing the timeline and 0:13:22.920 --> 0:13:26.319 so forth. Exactly, you know, some version of Elon Musk 0:13:26.320 --> 0:13:28.120 in the future is get to get his hands on it, 0:13:28.240 --> 0:13:29.920 and then he's going to launch a bunch of crazy, 0:13:30.400 --> 0:13:33.560 you know missions, and somebody's gonna mess something up. So 0:13:33.640 --> 0:13:35.800 it's hard to imagine that people in the future having 0:13:35.840 --> 0:13:39.000 time travel and somehow keeping it a secret or slipping 0:13:39.000 --> 0:13:42.720 into the past unnoticed and nobody ever, you know, breaking 0:13:42.720 --> 0:13:46.120 the protocol or something. It's it just becomes totally implausible 0:13:46.640 --> 0:13:48.720 the more you think about it. Picking up off that, 0:13:48.760 --> 0:13:51.160 I mean, this is another one of the weird things 0:13:51.200 --> 0:13:54.600 about time is it seems like time is actually one 0:13:54.679 --> 0:13:59.720 of the arguments against the idea of a coherent galactic civilization, 0:14:00.280 --> 0:14:02.760 if this makes any sense, because like you think, a 0:14:02.800 --> 0:14:05.760 civilization in order to organize itself has to have some 0:14:06.640 --> 0:14:11.080 pretty close to synchronous, uh, you know, thing going on, 0:14:11.200 --> 0:14:14.440 Like things have to be happening pretty close to around 0:14:14.480 --> 0:14:16.400 the same time for them. But does it even make 0:14:16.440 --> 0:14:19.840 sense for I don't know, one planet in a galactic 0:14:19.880 --> 0:14:22.920 civilization to be part of a civilization with one on 0:14:22.960 --> 0:14:24.880 the other side of the galaxy? I mean, is there 0:14:24.920 --> 0:14:28.360 you know, can they say, uh, is there such a 0:14:28.400 --> 0:14:31.800 thing as what's happening right now on a planet on 0:14:31.840 --> 0:14:34.080 the other side of the galaxy. Yeah, you make a 0:14:34.120 --> 0:14:37.560 great point, because there's a speed limit to information moving 0:14:37.600 --> 0:14:40.520 through the universe, which puts an effective limit on like 0:14:40.600 --> 0:14:44.040 how well you can coordinate and organize things. Makes you 0:14:44.080 --> 0:14:46.760 think about this in cosmology all the time, because there's 0:14:46.800 --> 0:14:50.440 a like the largest thing that can exist in the universe. 0:14:50.840 --> 0:14:53.440 Just from very simple arguments like the aged the universe 0:14:53.640 --> 0:14:56.520 and the speed of light. You can't have an object 0:14:56.600 --> 0:15:02.040 that's like ten thousand billion light years wide that's like coordinated, 0:15:02.080 --> 0:15:04.920 it has like a structure that's like gravitationally bound on 0:15:04.960 --> 0:15:07.960 itself because there hasn't been time for like a photon 0:15:08.040 --> 0:15:11.800 to even cross over the entire size of that object. 0:15:12.320 --> 0:15:14.760 So there's like a limit to how big the universe 0:15:14.800 --> 0:15:17.920 can even build like a thing, not to mention like 0:15:17.960 --> 0:15:22.320 the close coordination required like organize a galactic empire. And 0:15:22.360 --> 0:15:26.160 so yeah, absolutely, Um, I think that the sheer size 0:15:26.200 --> 0:15:29.720 of space definitely limits our ability to explore it unless 0:15:29.760 --> 0:15:34.600 it breaks down into you know, lots of different unorganized entities, 0:15:34.640 --> 0:15:37.000 like maybe we send humans in an arc off to 0:15:37.200 --> 0:15:40.000 another star and they start their own human colony and 0:15:40.160 --> 0:15:41.720 we're not in touch and we're not part of some 0:15:42.000 --> 0:15:44.240 you know, political nation state. But at least we are 0:15:44.280 --> 0:15:47.200 humans here and there are humans there. Yeah. I think 0:15:47.200 --> 0:15:56.160 that's a great way to conceptualize it. So, I guess 0:15:56.160 --> 0:15:59.040 coming back to time travel for a minute, I wanted 0:15:59.080 --> 0:16:01.400 to talk about some of the specifics you offer about 0:16:01.720 --> 0:16:07.000 physically plausible ways of traveling into the past. Uh. So 0:16:07.320 --> 0:16:09.160 you mentioned a couple of things. You mentioned the idea 0:16:09.160 --> 0:16:11.640 of wormholes, and then you also mentioned one that might 0:16:11.640 --> 0:16:15.120 be less familiar to people, the idea of an infinitely 0:16:15.240 --> 0:16:19.800 long cylinder of spinning dust, which could potentially, at least 0:16:20.040 --> 0:16:24.840 maybe depending on something about whether something about relativity is 0:16:24.840 --> 0:16:28.000 true or not, could potentially allow time travel into the 0:16:28.000 --> 0:16:31.240 past through something called time loops. Could could you explain 0:16:31.280 --> 0:16:33.720 how this would work? Like, what would this experience be 0:16:33.920 --> 0:16:37.440 like for the time traveler? Yeah, Well, the short answer is, 0:16:37.480 --> 0:16:40.240 we just don't know. Uh. This is a realm where 0:16:40.280 --> 0:16:44.240 we are like on the cutting edge. Theoretically, people are 0:16:44.400 --> 0:16:47.960 looking at the rules of how space and time bend 0:16:48.080 --> 0:16:51.160 and twist because you know, the general relativity our theory 0:16:51.280 --> 0:16:55.520 for space and time itself essentially tells us that space 0:16:55.560 --> 0:16:59.600 and time bend in response to mass, and then tell 0:16:59.720 --> 0:17:02.320 mass is how to move. So, for example, you have 0:17:02.360 --> 0:17:04.359 an empty universe and you put a star in it, 0:17:04.640 --> 0:17:07.399 it bends the space around the star, and then the 0:17:07.400 --> 0:17:09.879 bending of that space tells things how to move, and 0:17:09.920 --> 0:17:13.399 not just through space but also through time. So you 0:17:13.480 --> 0:17:16.800 go near a black hole, for example, time is slowed down. 0:17:17.240 --> 0:17:19.920 So there's definitely some deep connection there between space and time. 0:17:20.359 --> 0:17:22.920 And what people have done is trying to explore extreme 0:17:23.000 --> 0:17:25.240 scenarios of that what happens if you do this, what 0:17:25.359 --> 0:17:28.160 happens if you do that? Is this allowed? Is that allowed? 0:17:28.560 --> 0:17:31.000 And so it's sort of like exploring the universe, but 0:17:31.119 --> 0:17:34.639 just inside our own heads. We can't necessarily yet go 0:17:34.800 --> 0:17:37.440 out there and build these things in space and say, 0:17:37.560 --> 0:17:40.680 let's see what happens experimentally, But we can do similar 0:17:40.800 --> 0:17:43.400 like fought experiments, where we say, what would happen if 0:17:43.400 --> 0:17:46.639 you did this, and let's just let's assume the equations 0:17:46.640 --> 0:17:49.600 are correct and see what happens. And so there's a 0:17:49.600 --> 0:17:52.360 couple of fund scenarios there. One, as you said, is wormholes. 0:17:52.960 --> 0:17:56.280 These aren't really crazy because they are like connections between 0:17:56.600 --> 0:17:59.600 different points in space, and when you think of space, 0:17:59.640 --> 0:18:02.280 you probably think of like just sheer emptiness, you know, 0:18:02.320 --> 0:18:04.800 the back drop the stage on which the universe happens. 0:18:05.200 --> 0:18:07.440 But now we know that space is more complex. It 0:18:07.520 --> 0:18:10.359 can bend and it can twist, and that might be 0:18:10.400 --> 0:18:11.879 something that you can put in your head. You can 0:18:11.920 --> 0:18:15.399 imagine like space bending around the Sun. But because space 0:18:15.560 --> 0:18:18.400 is like a thing with an arrangement, it could also 0:18:18.440 --> 0:18:23.280 do other really weird things, like be connected non trivially. 0:18:23.680 --> 0:18:25.440 So you have like a chunk of space over here, 0:18:25.800 --> 0:18:28.480 it can be directly connected to a chunk of space 0:18:28.560 --> 0:18:31.359 over there. What does that mean. Well, you're used to 0:18:31.400 --> 0:18:34.720 the space around you being connected to the space right 0:18:34.760 --> 0:18:36.440 next to it. That's what it means to be right 0:18:36.520 --> 0:18:38.600 next to it. Right, you take a step to the left, 0:18:38.640 --> 0:18:40.760 you move to the next sort of piece of space. 0:18:41.240 --> 0:18:44.399 Think of it sort of like pixels on a screen. Right, Well, 0:18:44.440 --> 0:18:47.399 a wormhole is a connection between two points in space 0:18:47.440 --> 0:18:51.199 that are otherwise really distant. And so you take a 0:18:51.200 --> 0:18:53.679 step from a from one pixel and now you're in 0:18:53.680 --> 0:18:56.359 a pixel on the other side of the screen. And 0:18:56.400 --> 0:18:59.840 so that seems weird and impossible, but remember space can 0:18:59.880 --> 0:19:03.320 have all sorts of strange connections, and according to the 0:19:03.359 --> 0:19:06.440 equations of general relativity, the ones that define how space 0:19:06.520 --> 0:19:11.120 is organized that is allowed, it is possible, and so 0:19:11.280 --> 0:19:13.520 a couple of folks at cal Tech we're thinking about, well, 0:19:13.720 --> 0:19:16.680 you know what about time? Is it possible for one 0:19:16.880 --> 0:19:18.840 end of the wormhole to be in one place and 0:19:18.880 --> 0:19:23.080 the other end to be in another time because, as 0:19:23.119 --> 0:19:26.320 you were mentioning earlier, like the notion of simultaneity, like 0:19:26.800 --> 0:19:30.679 when is now depends really on where you are. Also, 0:19:31.359 --> 0:19:33.200 so they have this idea to take one end of 0:19:33.240 --> 0:19:36.000 the wormhole and you accelerate it near the speed of light. 0:19:36.280 --> 0:19:38.880 That effectively it can be sort of back in time. 0:19:39.560 --> 0:19:42.639 And this all works theoretically, but it also sort of 0:19:42.680 --> 0:19:46.080 contradicts other things we know, like if you go through 0:19:46.080 --> 0:19:49.159 this wormhole and you come out in the past, you know, 0:19:49.400 --> 0:19:51.959 doesn't that break things like causality? And you come out 0:19:51.960 --> 0:19:55.159 in the past and kill yourself before you um do 0:19:55.320 --> 0:19:57.480 the experiment, then you don't do the experiment, you don't 0:19:57.480 --> 0:20:00.720 come out in the past. So it appears to create paradoxes, 0:20:01.160 --> 0:20:04.399 and nobody knows like how to resolve that. Does that 0:20:04.440 --> 0:20:06.639 mean that these things are impossible? Does that mean if 0:20:06.640 --> 0:20:09.119 you did that the universe would like disappear in a 0:20:09.160 --> 0:20:12.159 puff of logic? Nobody really knows what would happen, so 0:20:12.200 --> 0:20:14.320 that it's a bit of a contradiction in the theory 0:20:14.400 --> 0:20:18.399 itself that it predicts something which seems to be disallowed 0:20:18.400 --> 0:20:21.760 by other parts of the theory. And it's a similar 0:20:21.800 --> 0:20:26.240 idea for these closed timelike curves. People said, if you 0:20:26.680 --> 0:20:31.280 create these infinitely long cylinders of spinning dust, which doesn't 0:20:31.320 --> 0:20:34.800 sound easy to do, then it bends time in this 0:20:34.840 --> 0:20:38.080 way that the time then as you move forward in time, 0:20:39.080 --> 0:20:42.600 you're actually moving sort of like sideways through space time 0:20:43.000 --> 0:20:45.480 in a way that's similar to the experience of going 0:20:45.520 --> 0:20:49.560 into a black hole. Outside a black hole, time always 0:20:49.600 --> 0:20:53.440 moves forwards. Inside a black hole, space has bent so 0:20:53.520 --> 0:20:56.480 much that space only moves towards the center of the 0:20:56.520 --> 0:21:00.280 black hole. It's like one direction to space. So if 0:21:00.280 --> 0:21:03.639 you imagine space being distorted, not quite as much as 0:21:03.640 --> 0:21:05.920 a black hole, but sort of in a similar direction 0:21:05.920 --> 0:21:09.680 that it sort of bends space sideways, then you can 0:21:09.720 --> 0:21:12.240 create these paths where something can move in a loop 0:21:12.320 --> 0:21:16.560 through time. Um, but you would be trapped on that loop, 0:21:16.600 --> 0:21:18.720 so you wouldn't be able to like change anything. It's 0:21:18.720 --> 0:21:21.280 like a fixed loop, sort of like Harry Potter style 0:21:21.760 --> 0:21:23.720 loop through time where every time you go through, it's 0:21:23.760 --> 0:21:26.640 exactly the same thing happening. And these are really fun 0:21:26.680 --> 0:21:29.600 because nobody knows like if these are actually possible, and 0:21:29.720 --> 0:21:32.560 what would happen if you actually went through them. So 0:21:32.560 --> 0:21:35.920