WEBVTT - We Get Back In Time 0:00:00.160 --> 0:00:07.240 Brought to you by Toyota. Let's go places. Welcome to 0:00:07.400 --> 0:00:15.320 Forward Thinking. Hey everyone, and welcome to Forward Thinking, the 0:00:15.400 --> 0:00:18.000 podcast that looks the future and says, tell me, dr 0:00:18.120 --> 0:00:20.680 where are we going this time? I'm Jonathan Strickland, I'm 0:00:20.760 --> 0:00:23.880 Lauren Vocabon, and I'm Joe McCormick. Joe, I I've got 0:00:23.880 --> 0:00:27.400 a question for you. Okay, I've really been thinking about 0:00:27.400 --> 0:00:30.159 this for a while. What if I had if I 0:00:30.160 --> 0:00:33.559 had to nail you down and say, what movie that 0:00:33.680 --> 0:00:36.480 involves time travel do you feel really gets it right? 0:00:36.520 --> 0:00:40.080 What's the best depiction of time travel in the movies? 0:00:40.240 --> 0:00:42.040 What would you say? Well, I don't know if it's 0:00:42.080 --> 0:00:45.520 possible to get time travel right in the movies, because 0:00:45.560 --> 0:00:48.800 I'm not sure if it's possible to actually do it period. 0:00:49.200 --> 0:00:52.880 The movie that that convinces me the most is a little, 0:00:52.960 --> 0:00:55.680 uh indie sci fi movie called Primer. Have you all 0:00:55.720 --> 0:00:59.520 seen this now? So? Yeah, so in your version, time 0:00:59.520 --> 0:01:02.560 travel and evolves getting a storage unit. Yeah, so you 0:01:02.640 --> 0:01:05.160 run a U haul and you put a little box 0:01:05.200 --> 0:01:08.080 in it and you go sleep in the box, and um, 0:01:08.680 --> 0:01:12.480 and then everything gets really confusing. But there there there 0:01:12.480 --> 0:01:14.679 are a couple of things I really like about the 0:01:14.680 --> 0:01:18.800 way it depicted time travel. Number one is that it 0:01:18.840 --> 0:01:22.840 depicts it the technology, the technology aspect of machine they 0:01:22.880 --> 0:01:26.280 build feels like a real project. You know, you see 0:01:26.319 --> 0:01:29.600 the scenes where they're designing it, and it doesn't sound 0:01:29.680 --> 0:01:33.199 like a bunch of sci fi magic talk. You feel 0:01:33.200 --> 0:01:36.720 like these are these guys are they're real engineers who 0:01:36.720 --> 0:01:39.680 were talking about the kinds of problems engineers encounter when 0:01:39.680 --> 0:01:42.360 they're building a machine. Number two, The thing I like 0:01:42.480 --> 0:01:45.279 about it is that it's the thing I said before, 0:01:45.440 --> 0:01:49.920 It gets really confusing when once they keep using the machine, 0:01:50.200 --> 0:01:52.360 it's hard to keep track of what's going on. It's 0:01:52.360 --> 0:01:54.880 a movie that challenges you too, because if you if 0:01:54.920 --> 0:01:57.520 you aren't willing to really pay attention and listen and 0:01:57.560 --> 0:02:00.520 try and be engaged, you could be lost before there's 0:02:00.680 --> 0:02:02.720 any time travel at all. In fact, you can be 0:02:02.760 --> 0:02:08.520 lost before you realize what the heck they're trying to build. Yeah, definitely, Yeah, 0:02:08.600 --> 0:02:10.280 all right, Lauren, what about you do you have? Do 0:02:10.280 --> 0:02:12.480 you have a time travel like a favorite kind of 0:02:12.520 --> 0:02:15.840 depiction of time travel? Well, I think that probably the 0:02:16.320 --> 0:02:20.720 closest to what will eventually be reality is is what 0:02:20.760 --> 0:02:23.960 the Simpsons portrayed in the Treehouse of Horrors episode Time 0:02:24.000 --> 0:02:27.800 and Punishments, which of course was was kind of based 0:02:27.880 --> 0:02:30.640 on the old Bradberry story. A sound of Thunder is 0:02:30.639 --> 0:02:32.720 that the one where Homer goes back in time accidentally 0:02:32.720 --> 0:02:35.840 steps on a on a butterfly and changes everything all right, right, 0:02:35.919 --> 0:02:38.800 comes back to the to the present and uh and 0:02:38.800 --> 0:02:42.000 and winds up in what seems to be this amazing 0:02:42.080 --> 0:02:47.519 perfect dimension, right and well, but but but he goes 0:02:47.600 --> 0:02:49.720 he goes like they're all like rich and fabulous, and 0:02:49.840 --> 0:02:52.240 he's like, Marge, give me a donut, and she's like, 0:02:52.240 --> 0:02:54.920 what's a donut? And then and then he leaves and 0:02:54.960 --> 0:03:00.359 then starts again. My favorite part of that one is 0:03:00.400 --> 0:03:02.320 where he goes back in time and just starts killing 0:03:02.360 --> 0:03:06.000 things randomly. But that also does try to sort of 0:03:06.040 --> 0:03:09.360 address a real issue that's possible with time travel. We're 0:03:09.480 --> 0:03:12.160 time travel a reality, and you were to go way 0:03:12.200 --> 0:03:14.800 back and change some small thing you have no idea 0:03:14.919 --> 0:03:18.720 how much of an effect that might have in the future. True, Now, 0:03:18.880 --> 0:03:21.400 before we get off too far down that road, because 0:03:21.400 --> 0:03:24.040 we are going to address that, it's my turn, Yeah, 0:03:24.080 --> 0:03:28.560 what's your job? Spill and Ted's excellent adventure because Sand 0:03:28.680 --> 0:03:32.480 Demons High School football rules. Yeah. I actually was just 0:03:32.560 --> 0:03:35.280 reading about how scientists at M I. T Are working 0:03:35.280 --> 0:03:39.240 on a time traveling telephone booth. Yeah, yeah, yeah, it's uh, 0:03:38.840 --> 0:03:43.760 it's it's phenomenal, powered by George Carlin magic. I was 0:03:43.800 --> 0:03:45.800 about to say, I mean, this has to involve George 0:03:45.800 --> 0:03:49.440 Carlin very short supply Now, well, I mean just because 0:03:49.440 --> 0:03:51.440 he's passed away in this current time. If it's a 0:03:51.440 --> 0:03:55.280 time machine, then that should that should be no problem, right, Okay, okay, 0:03:55.400 --> 0:03:58.480 so time travel is really fun for the movies, especially 0:03:58.520 --> 0:04:01.760 because it creates all these weird questions. We have no 0:04:01.880 --> 0:04:05.280 experience with which to compare it to, so you can 0:04:05.360 --> 0:04:08.120 you can almost try anything. Yes, And we of course 0:04:08.160 --> 0:04:11.800 have talked about time in a previous episode. We were 0:04:11.840 --> 0:04:15.760 really talking about time and general relativity and special relativity, 0:04:15.880 --> 0:04:18.280 and so we're not going to go too far into that, 0:04:18.279 --> 0:04:20.240 but we will give a little bit of a refresher 0:04:20.360 --> 0:04:22.160 so that we can have at least a kind of 0:04:22.279 --> 0:04:25.000 an agreed upon foundation upon which we will build our 0:04:25.360 --> 0:04:27.880 contraption of time machine that will fall a part of 0:04:27.920 --> 0:04:30.720 the moment's notice. Yeah, okay, so let's talk about time 0:04:30.760 --> 0:04:36.039 travel today. Is it possible? Uh? If it is possible. 0:04:36.080 --> 0:04:40.200 How might it work? And if it does actually work, 0:04:40.279 --> 0:04:42.120 what would it mean. Well, it depends on how you 0:04:42.160 --> 0:04:44.480 define it, because if you just define it as traveling 0:04:44.480 --> 0:04:52.040 through time, technically we're all doing that, not where Yeah 0:04:52.320 --> 0:04:55.000 that's a great term. Yeah, well it's true. You just 0:04:55.200 --> 0:04:58.159 you know, you just have to be and you and 0:04:58.240 --> 0:05:00.520 you and you would go through time. All right. So, 0:05:00.560 --> 0:05:03.799 first of all, defining time we've mentioned in our previous episode, 0:05:03.800 --> 0:05:05.800 it's really hard to do that without it becoming this 0:05:05.920 --> 0:05:11.320 kind of crazy recursive, uh you know, tautological definition. So 0:05:12.000 --> 0:05:14.960 Einstein said time is what you read on a clock. Yeah. 0:05:15.120 --> 0:05:18.279 One one definition you could apply is say it's the 0:05:18.400 --> 0:05:21.560 rate of change in the universe. And this is not 0:05:21.720 --> 0:05:25.920 something that is as globally consistent or universally consistent. In 0:05:25.920 --> 0:05:29.320 other words, it can you can actually have very localized 0:05:29.920 --> 0:05:32.240 rates of change that are much different than on the 0:05:32.320 --> 0:05:36.680 universal scale. As it turns out, time is in fact relative. 0:05:36.760 --> 0:05:40.480 It is different depending upon your perspective. And that doesn't 0:05:40.520 --> 0:05:43.720 just mean it seems relative to different people. You know, 0:05:43.839 --> 0:05:47.080 based on how long it feels like this horrible movie 0:05:47.160 --> 0:05:50.320 has been going on, or this lecture or something. It's 0:05:50.400 --> 0:05:55.880 measurably relative like it actually changes depending on how close 0:05:55.960 --> 0:06:00.680 you are to say, massive objects, or how fast you're going. Right, 0:06:01.440 --> 0:06:05.040 that's a gravitational time dilation, yeah, exactly. Yeah. And and 0:06:05.080 --> 0:06:07.640 both of these, you know, gravitational time dilation and the 0:06:08.000 --> 0:06:11.560 speed at which you're traveling. Uh, those those two effects 0:06:11.560 --> 0:06:15.000 are what make it um necessary for us to have 0:06:15.200 --> 0:06:19.479 satellites that correct their time gradually, because, as it turns out, 0:06:20.279 --> 0:06:23.000 the time on their clocks passes at a different rate 0:06:23.000 --> 0:06:25.240 than the time of our clocks down here on the 0:06:25.279 --> 0:06:28.080 ground level. Right. So, even if you were to take 0:06:28.160 --> 0:06:32.480 a fast airplane trip and you had a very precise 0:06:32.600 --> 0:06:35.359 clock on board the airplane when you landed, and you 0:06:35.440 --> 0:06:38.080 compared that against another clock they have been started the 0:06:38.120 --> 0:06:41.280 exact same time, has that same level of precision, you 0:06:41.279 --> 0:06:44.400 would see that time had passed at slightly different rates, 0:06:44.440 --> 0:06:46.520 not a lot. I mean, it's not like you land 0:06:46.560 --> 0:06:49.520 and you you land an hour earlier than what you 0:06:49.520 --> 0:06:53.000 you would have landed, uh if the hadn't gone well, 0:06:53.080 --> 0:06:55.120 not just speed, but the fact that time is actually 0:06:55.120 --> 0:06:57.880 passing in a a different rate for you. But it's it 0:06:57.960 --> 0:07:01.240 is a tiny amount, it's it's measurable with satellites. It's 0:07:01.279 --> 0:07:04.400 a little bit more important because one, the effect is 0:07:04.480 --> 0:07:08.360 larger and tow we're depending upon satellites for things like GPS, 0:07:08.440 --> 0:07:11.840 and if your time isn't exact, if you haven't corrected 0:07:11.880 --> 0:07:14.360 for this time dilation, you're not going to get an 0:07:14.360 --> 0:07:17.080 accurate reading of where you are on Earth based upon 0:07:17.080 --> 0:07:21.840 those GPS readings. So we already see dilation and effect 0:07:22.040 --> 0:07:26.600 on today. It's not like it's just hypothetical. Yeah, it's 0:07:26.680 --> 0:07:30.320 definitely just an accepted fact of science. But despite the 0:07:30.360 --> 0:07:33.680 fact that time is relative and influenced by things like 0:07:34.160 --> 0:07:37.680 nearby massive objects and the speed at which an object 0:07:37.800 --> 0:07:40.800 is traveling, one thing that we do seem to know 0:07:40.840 --> 0:07:43.080 about it is that it only seems to move in 0:07:43.120 --> 0:07:47.600 one direction, right, right. We don't seem to cause and 0:07:47.640 --> 0:07:50.840 effect goes in one way. Right. You don't have a 0:07:51.120 --> 0:07:54.360 a an effect and then later a cause happens. You 0:07:54.400 --> 0:07:57.320 have a cause and then the effect happens. Sort of 0:07:57.360 --> 0:07:59.880 the arrow of time. Yeah, and in points and only 0:08:00.000 --> 0:08:02.840 one direction as far as we can determine, at least 0:08:02.840 --> 0:08:09.560 intuitively mathematically, it's a different, different matter, I suppose. But 0:08:09.720 --> 0:08:13.480 first I would also mention that time travel inherently has 0:08:13.480 --> 0:08:17.160 a paradox attached to it, which is that you get 0:08:17.200 --> 0:08:21.520 two different, completely different experiences of time, at least based 0:08:21.560 --> 0:08:24.679 on the type of time travel where there's some time 0:08:24.760 --> 0:08:27.560 passing for the traveler as well as for the rest 0:08:27.640 --> 0:08:30.040 of the universe. So in other words, Joe, if you 0:08:30.120 --> 0:08:32.600 decide to go back into the past and you're taking 0:08:32.640 --> 0:08:35.120 some sort of trip where like let's say it's the 0:08:35.160 --> 0:08:39.480 Tardest from Doctor Who, there's actually time that passes for 0:08:39.600 --> 0:08:42.800 you as you take this journey. So you're leaving today 0:08:42.880 --> 0:08:45.680 and you're going back two years into the past, and 0:08:45.679 --> 0:08:47.640 it doesn't take that long. I mean, there's basically just 0:08:47.679 --> 0:08:50.480 a series of whoo, whoop whoops, and then any land. 0:08:50.600 --> 0:08:54.560 But it's not instantaneous, so you experience time moving forward 0:08:54.840 --> 0:08:58.079 to an independent observer who I guess has been around 0:08:58.080 --> 0:09:00.640 for a few centuries. Uh, they will remember the fact 0:09:00.640 --> 0:09:04.520 that you arrived before you departed. So that's two different 0:09:04.920 --> 0:09:08.840 experiences of time. My question would be, if there really 0:09:08.920 --> 0:09:12.960 is this kind of movie style traveling backward through time 0:09:13.880 --> 0:09:20.079 uh motion, where are you physically well? I mean, space 0:09:20.200 --> 0:09:23.600 and time are very much connected. We have this whole 0:09:23.600 --> 0:09:26.480 idea of the space time continuum. But where is not 0:09:26.559 --> 0:09:29.040 so much of an issue as when Uh, as it 0:09:29.040 --> 0:09:31.360 turns out, because at least with the Doctor's tartists, it 0:09:31.400 --> 0:09:34.760 travels both through space and time. Uh. If you're in 0:09:34.800 --> 0:09:37.199 a DeLorean going eighty eight miles per hours, somehow you're 0:09:37.240 --> 0:09:41.679 magically in the right place, uh, the right region as 0:09:41.720 --> 0:09:44.840 you as you would have been had somehow you've managed 0:09:44.880 --> 0:09:49.000 to track the Earth. Yeah. That thing is what always 0:09:49.120 --> 0:09:50.960 mixes me up about a lot of this time travels. 0:09:51.200 --> 0:09:54.079 I mean, in addition to many other things like the 0:09:54.120 --> 0:09:56.880 Dolorean doesn't just appear in the cold reaches of space 0:09:56.960 --> 0:10:00.160 and parting I fly immediately perishes like Doc Brown. That's 0:10:00.160 --> 0:10:02.600 a really smart guy. Um, but no, I mean, I mean, 0:10:02.679 --> 0:10:06.080 with that kind of essential paradox, even the very small 0:10:06.240 --> 0:10:10.040 differences and in time that happened between us and our satellites, 0:10:10.040 --> 0:10:12.800 for example, are actually really hard to wrap your mind around. 0:10:13.720 --> 0:10:19.320 So anything as large as time travel is is necessarily problematic. 0:10:19.840 --> 0:10:23.680 Whack I think is the accepted scientific term. Yeah, quite 0:10:23.880 --> 0:10:29.680 quite whack. Yes, it's wackness is uh is sufficient, It's 0:10:29.760 --> 0:10:33.000 it's a it's enough of a wackness to actually detect 0:10:33.040 --> 0:10:37.040 with the naked eye. Okay, so I've got a question, 0:10:37.360 --> 0:10:40.520 does time travel is there any way, we could just 0:10:40.679 --> 0:10:42.760 rule it out at the beginning to say no, we 0:10:42.880 --> 0:10:46.400 just know it's not possible because it violates some law 0:10:46.480 --> 0:10:50.000 that we are almost positive cannot be violated. All right, 0:10:50.040 --> 0:10:52.280 To answer that question, first of all, spoiler alert, According 0:10:52.320 --> 0:10:55.600 to at least some mathematical models, time travel is at 0:10:55.679 --> 0:10:59.960 least hypothetically possible. But there's still some things that we 0:11:00.000 --> 0:11:03.160 have to talk about. For example, the law of conservation. 0:11:03.200 --> 0:11:05.880 Now we've heard this, right, the idea that you cannot 0:11:06.000 --> 0:11:09.560 create nor destroy matter or energy. You can convert one 0:11:09.559 --> 0:11:13.520 into the other, but you cannot actually destroy it or 0:11:13.559 --> 0:11:16.880 create it, right, can't make energy disappear or matter. You 0:11:16.960 --> 0:11:21.320 can convert energy into less useful states. Sure it's never 0:11:21.400 --> 0:11:24.480 going away, right, right. So in other words, when we 0:11:24.600 --> 0:11:28.000 are you know, we talk about how systems lose energy, 0:11:28.360 --> 0:11:31.400 they don't really lose energy so much as they produce 0:11:31.480 --> 0:11:36.040 other forms of energy that is useful. They're they're pooping 0:11:36.040 --> 0:11:40.040 out heat. Essentially, heat which is hard to harness, is 0:11:40.080 --> 0:11:44.880 just diffused. Right. So that law of conservation suggests that 0:11:44.920 --> 0:11:47.280 there's a problem with time travel. And that problem is this, 0:11:47.880 --> 0:11:51.200 if I send something into the past, whether it's a 0:11:51.240 --> 0:11:54.040 person or an object or time, shame, whatever it is, 0:11:54.520 --> 0:11:58.079 then that thing will suddenly be in a time where 0:11:58.080 --> 0:12:01.120 it wasn't before. That seems to suggest that I am 0:12:01.160 --> 0:12:05.040 actually creating more matter at that moment in time, and 0:12:05.080 --> 0:12:09.480 that spacetime region then was there before. Does that violate 0:12:09.520 --> 0:12:12.680 the law of conservation more so than even just creating 0:12:12.679 --> 0:12:16.000 a really weird time sonic boom? Yeah. The fact that 0:12:16.240 --> 0:12:18.760 the fact that something is there that wasn't there before. 0:12:18.800 --> 0:12:21.800 You had a uh, let's say, a certain amount of 0:12:21.840 --> 0:12:24.800 matter in that region, and now there's more matter, and 0:12:24.840 --> 0:12:28.360 it wasn't that you converted energy, it's that you've produced it. 0:12:29.440 --> 0:12:32.920 Mm hmm. I can tell you what the general answer 0:12:33.080 --> 0:12:35.360 to this is about how they get around it, which 0:12:35.360 --> 0:12:38.600 is the idea that in an infinite and unbounded universe, 0:12:39.160 --> 0:12:43.240 you have regions where conservation takes place. But it's not 0:12:43.280 --> 0:12:46.640 a global thing. It's not it's not applied across the 0:12:46.840 --> 0:12:50.439 entirety of the universe. It's so it's it's kind of complicated, 0:12:50.440 --> 0:12:53.400 but it's it's assuming that there is an infinite universe, 0:12:54.040 --> 0:12:59.040 and therefore you're not really creating more matter because the 0:12:59.080 --> 0:13:04.679 concept of infinite alone means infinite like like there's no 0:13:05.320 --> 0:13:09.840 more to infinite. You've got that that that alone is 0:13:10.400 --> 0:13:13.000 you know, it's meaningless. You're not creating a lack or 0:13:14.400 --> 0:13:18.040 additional but localized it would be a problem. But if 0:13:18.080 --> 0:13:19.559 you look at the universe as a whole, as a 0:13:19.600 --> 0:13:23.439 system and considered infinitely and bounded, you don't apply that 0:13:23.440 --> 0:13:26.400 that law in that instant. So that's the way of 0:13:26.400 --> 0:13:28.000 getting around it. Yeah, it's kind of a lot of 0:13:28.040 --> 0:13:30.440 the stuff we're gonna talk about today with these time 0:13:30.480 --> 0:13:33.200 travel rules and the ways to get around it. It 0:13:33.360 --> 0:13:37.360 reminds me a lot of if you've ever played a 0:13:37.360 --> 0:13:42.240 a pencil and paper role playing game and you've got 0:13:42.280 --> 0:13:46.000 one of those players who's trying to find a way 0:13:46.080 --> 0:13:49.400 around every rule in the game, and that's the way 0:13:49.440 --> 0:13:52.800 they play the game. I you guys have friends who 0:13:52.840 --> 0:13:54.920 don't play the game that way. I thought, I thought 0:13:54.960 --> 0:13:57.160 that was the entire point of the game. I've I've 0:13:57.440 --> 0:14:00.480 had games where people didn't do that and they were magical. 0:14:01.880 --> 0:14:05.199 But at any rate, So another thing we can talk about, 0:14:05.280 --> 0:14:09.040 does it um? Is it? Is? It? Is it violating 0:14:09.200 --> 0:14:14.240 space itself? Um? And according to some models of general relativity, 0:14:14.040 --> 0:14:17.679 time travel is possible through something called a closed timelike 0:14:17.880 --> 0:14:22.760 curve or c t C which are quote worldlines that 0:14:22.920 --> 0:14:25.080 end at the same point in space and time as 0:14:25.120 --> 0:14:27.680 they begin. And we'll talk more about what c t 0:14:27.880 --> 0:14:30.640 C s are and how you might be able to 0:14:30.720 --> 0:14:33.920 travel through time backwards through time in fact, because a 0:14:33.920 --> 0:14:36.720 lot of people say, oh, time travel would be possible 0:14:36.720 --> 0:14:39.400 if you're talking about traveling into the future, But traveling 0:14:39.400 --> 0:14:41.600 into the past is problematic. I mean, especially if you 0:14:41.600 --> 0:14:45.080 think about using special relativity. So you're just talking about 0:14:45.120 --> 0:14:49.680 moving really really fast. So to you, time passes a 0:14:49.720 --> 0:14:52.760 different rate than it does to someone on some other 0:14:53.080 --> 0:14:55.800 you know, moving body, right, if they were to look 0:14:55.840 --> 0:14:58.680 at you, you would appear to be moving in slow motion, right, 0:14:58.720 --> 0:15:02.040 and you would see every things sped up back home. So, 0:15:02.160 --> 0:15:04.880 for instance, if we're talking about here on Earth and 0:15:04.960 --> 0:15:09.320 Joe leaves to go on an interstellar super fast joy ride, 0:15:09.840 --> 0:15:12.680 he might go for a year and be traveling at 0:15:12.720 --> 0:15:15.000 almost the speed of light and return to Earth and 0:15:15.040 --> 0:15:17.280 it will be like two centuries have passed, but to 0:15:17.360 --> 0:15:20.280 him a year has passed. Now to that that you 0:15:20.320 --> 0:15:23.360 could argue is just like basically traveling into the future. Yeah, 0:15:23.360 --> 0:15:25.640 it's like it's like traveling into the future for you Joe. 0:15:25.680 --> 0:15:27.480 For for the rest of us, it's just like boy Joe, 0:15:27.600 --> 0:15:31.600 you remember him? Whatever happened to that guy? And then 0:15:31.600 --> 0:15:35.440 maybe someday or descendants would be like, oh, you're this 0:15:35.520 --> 0:15:39.600 Joe guy who never shows up the things. But at 0:15:39.600 --> 0:15:42.840 any rate, the CTCs would allow you to travel back 0:15:42.880 --> 0:15:46.080 in time, assuming that they are actually in play in 0:15:46.120 --> 0:15:48.960 this particular model general relativity, and we'll get into that 0:15:49.000 --> 0:15:51.840 in a little bit. Okay, I've got one. Okay, what 0:15:51.920 --> 0:15:55.800 about basic causality? I mean, can you you've heard of 0:15:55.800 --> 0:16:00.760 the grandfather paradix? Obviously you have, Yeah, I wrote the 0:16:00.800 --> 0:16:05.800 notes right. Well, okay, so the grandfather paradox is a 0:16:05.840 --> 0:16:08.720 fun one. Uh. This this is a very common one. 0:16:08.800 --> 0:16:10.880 That's that's mentioned. There are a lot of different variations 0:16:10.920 --> 0:16:12.960 as well. But the the easy way of saying it 0:16:13.040 --> 0:16:15.560 is imagine that time travel is in fact possible where 0:16:15.600 --> 0:16:18.520 you can go back into time and then you go 0:16:18.680 --> 0:16:20.840 back in time. You're an assassin and you have a 0:16:20.880 --> 0:16:23.200 specific target that you have to hit in the past. 0:16:23.680 --> 0:16:26.600 You travel back in time and your target is eighteen 0:16:26.680 --> 0:16:29.400 years old, and you assassinate your target. But it turns 0:16:29.400 --> 0:16:32.440 out your target was actually your grandfather, Your grandfather didn't 0:16:32.440 --> 0:16:34.840 have kids until he was, you know, in his twenties. 0:16:35.240 --> 0:16:38.680 So you have killed your grandfather before your your mother 0:16:38.760 --> 0:16:41.440 or dad were born, which means that you wouldn't have 0:16:41.480 --> 0:16:43.480 been born, which means you couldn't have gone into the past, 0:16:43.520 --> 0:16:46.320 which means you couldn't have killed your grandfather. Thus the paradox. 0:16:46.600 --> 0:16:49.200 Now we're not saying there's any reason this would need 0:16:49.320 --> 0:16:53.119 to arise, but if traveling into the past really were possible, 0:16:54.120 --> 0:16:57.760 why shouldn't you be able to do something like this? Well, again, 0:16:58.400 --> 0:17:01.480 if through your action you end up negating the ability 0:17:01.560 --> 0:17:05.040 to even go back in the first place, that's the paradox. Right, 0:17:05.080 --> 0:17:08.320 You've created an incoherent loop, is what it's called. It's 0:17:08.400 --> 0:17:12.720 it's something that could not continue. Uh once it happens 0:17:12.760 --> 0:17:16.280 like once it once you were successful in doing whatever 0:17:16.320 --> 0:17:17.880 the task is, and it doesn't have to be kill 0:17:17.920 --> 0:17:20.600 your grandfather. That's the paradox. But there are a lot 0:17:20.640 --> 0:17:24.520 of different variations. Generally speaking, what they're saying is, even 0:17:24.520 --> 0:17:27.760 if time travel were possible, you probably couldn't go back 0:17:27.920 --> 0:17:32.080 and change anything, because if you did, then it could 0:17:32.200 --> 0:17:35.240 negate the possibility of you traveling back in time in 0:17:35.280 --> 0:17:38.600 the first place. So There's an argument that extends from that, 0:17:38.680 --> 0:17:41.399 saying that maybe there's no way to affect the past 0:17:41.440 --> 0:17:44.080 at all. But that also seems like a problem to me, 0:17:44.200 --> 0:17:47.840 because by appearing in a place, I mean, even if 0:17:47.880 --> 0:17:50.840 you don't really do much of anything, you're still affecting it, 0:17:51.680 --> 0:17:53.760 I mean, just by your state of existence there. Sure 0:17:53.800 --> 0:17:56.639 it's accidentally stepping on a butterfly thing and creating a 0:17:56.680 --> 0:17:59.600 universe where it rainstonuts. Yeah, it seems like it wouldn't 0:17:59.640 --> 0:18:03.840 be possible to go to a place without having any effect. Well, 0:18:03.920 --> 0:18:07.199 there's some interesting ways around this one too. One is 0:18:07.200 --> 0:18:09.680 suggesting that you would be able to do whatever you wanted. 0:18:09.800 --> 0:18:12.600 It's just whatever you did happens to be whatever has 0:18:12.640 --> 0:18:15.639 happened anyway, That makes more sense to me. So in 0:18:15.680 --> 0:18:18.600 other words, it's not that you are prevented from doing things, 0:18:18.680 --> 0:18:21.400 it's that whatever you choose to do is what has 0:18:21.480 --> 0:18:24.960 happened already and what led to the state of the 0:18:25.040 --> 0:18:26.920 universe that allowed you to travel back in time in 0:18:26.960 --> 0:18:29.840 the first place. And this becomes a coherent causal loop 0:18:30.040 --> 0:18:32.760 where you end up having You can have variations on 0:18:32.800 --> 0:18:35.760 this where the only way it could have happened is 0:18:35.840 --> 0:18:39.080 if you had traveled into the past, which I love. 0:18:39.119 --> 0:18:42.040 These two the very popular in science fiction. Well, yeah, 0:18:42.040 --> 0:18:43.680 I mean one of the one of the big ones 0:18:43.800 --> 0:18:46.960 that seems to emphasize the strangeness of this kind of 0:18:47.000 --> 0:18:49.639 loop would be, what if you only know how to 0:18:49.720 --> 0:18:52.439 invent a time machine because a time traveler from the 0:18:52.480 --> 0:18:54.879 future came back and showed you how to invent a 0:18:54.920 --> 0:18:58.080 time machine. Yeah, and there's another great example. In fact, 0:18:58.119 --> 0:19:01.040 I'll go ahead and say it. Let's just say that, uh, Lauren, 0:19:01.400 --> 0:19:06.080 you have created a way of uh, you've gone into 0:19:06.080 --> 0:19:07.879 the future. You've got a time machine where you can 0:19:07.880 --> 0:19:11.640 actually move into the future, and you you steal from 0:19:11.680 --> 0:19:15.639 the future an amazing device, and this steal things all 0:19:15.640 --> 0:19:18.159 the time, mostly from the future. Yeah. So it's a 0:19:18.200 --> 0:19:21.439 really cute nutcracker that looks like a sure from an 0:19:21.440 --> 0:19:25.479 antique store. I have a have a specific example. Lauren, 0:19:27.240 --> 0:19:34.560 Lauren the chronographer kleptomaniac, steals a device it turns great 0:19:34.680 --> 0:19:38.639 jelly into strawberry jam, and she travels back in time 0:19:38.800 --> 0:19:41.520 to present day and starts to thus turn all the 0:19:41.560 --> 0:19:45.120 grape jelly into strawberry jam. And she loves that. She's 0:19:45.160 --> 0:19:48.879 fantastic guests And then as she's you know, she's traveled 0:19:48.920 --> 0:19:51.479 far into the future to get this amazing device uh 0:19:51.840 --> 0:19:54.960 on on her her deathbed, many many many years into 0:19:55.000 --> 0:19:58.600 the future. She then gives this this device to a 0:19:58.640 --> 0:20:03.040 little vagabond, and and the vagabond, it turns out, ends 0:20:03.080 --> 0:20:07.239 up being the quote unquote inventor of this device. In 0:20:07.280 --> 0:20:10.480 other words, this device was never actually invented. It only 0:20:10.560 --> 0:20:14.000 exists in this loop of time. It turns out that 0:20:14.040 --> 0:20:17.520 the adult version of this vagabond is the person from 0:20:17.520 --> 0:20:20.119 whom Lawrence stole the device in the first place, and 0:20:20.160 --> 0:20:24.120 this device only exists within this closed time loop. From 0:20:24.119 --> 0:20:27.000 that perspective, yeah, that doesn't seem to make any sense, 0:20:27.000 --> 0:20:29.960 because in that case, the device was never actually built. 0:20:30.560 --> 0:20:34.520 It always just exactly Yeah. See, it's another one of 0:20:34.560 --> 0:20:37.720 those kind of and if you've ever read any hind Line, 0:20:38.280 --> 0:20:41.679 hind Line has got a billion stories that involved this 0:20:41.800 --> 0:20:45.720 kind of of closed loop where nothing in the story 0:20:45.760 --> 0:20:48.760 could have happened unless this other thing that hasn't happened 0:20:48.800 --> 0:20:51.760 had already happened. It just gets more and more confusing 0:20:51.800 --> 0:20:53.840 as it goes along. I think it's much more likely 0:20:54.000 --> 0:20:57.840 that um that grape to strawberry jam converters are a 0:20:57.920 --> 0:21:02.360 universal constant, and it therefore um that they have to exist. 0:21:02.400 --> 0:21:06.000 There must exist, unmust they must at some point exist, right. UM. 0:21:06.119 --> 0:21:08.520 I also am very fond of the theory that um 0:21:08.560 --> 0:21:11.959 that the universe kind of kind of equalizes anything that 0:21:12.000 --> 0:21:13.840 you would do and or try to do in the past, 0:21:13.920 --> 0:21:16.080 Like if you went back and tried to shoot your grandfather, 0:21:16.280 --> 0:21:19.320 um that the gun would misfire, or that that's suddenly 0:21:19.320 --> 0:21:21.840 a strong breeze with the bullet off course, right, or 0:21:21.880 --> 0:21:24.199 that that a weird reflection gun in your eyes and 0:21:24.240 --> 0:21:26.880 messed up your aim. And but the the idea being 0:21:26.880 --> 0:21:29.479 that the universe itself counteracts anything you try to do 0:21:29.600 --> 0:21:32.520 and thus prevents it from happening. Uh. I can see 0:21:32.520 --> 0:21:34.920 that if you're just saying like this is another way 0:21:34.920 --> 0:21:37.919 of saying that whatever you go back and change is 0:21:37.960 --> 0:21:41.560 what already happened. If you're talking about the universe having 0:21:41.600 --> 0:21:45.200 like an active deterrent mechanism, that that seems only possible 0:21:45.240 --> 0:21:49.720 if you're positing like time travel, police ghosts. I think 0:21:49.720 --> 0:21:52.399 that Final Destination has proven beyond a doubt to us 0:21:52.440 --> 0:21:55.560 that this kind of thing happens at the Great documentary series. 0:21:56.359 --> 0:21:59.320 But also it doesn't necessarily have to be conscious. It 0:21:59.359 --> 0:22:01.919 just has to Again, it's very similar to this is 0:22:01.960 --> 0:22:05.199 what has happened, therefore, this is what will happen if 0:22:05.240 --> 0:22:08.240 you were to go into the past. But another interesting 0:22:08.280 --> 0:22:11.359 idea that tries, you know, one way you can try 0:22:11.400 --> 0:22:13.600 and get around this. It's sort of similar to what 0:22:13.640 --> 0:22:15.760 you were talking about with the Simpsons, Lauren, this idea 0:22:15.880 --> 0:22:20.080 of parallel timelines. This is what it's like in Back 0:22:20.080 --> 0:22:22.399 to the Future and a lot of stories. Yes, you 0:22:22.480 --> 0:22:25.280 go back and you change something. It doesn't have to 0:22:25.320 --> 0:22:29.240