Past, Present, Future: Time Travel with Brian Greene

0:01Welcome to StarTalk, your place in the universe where science and pop culture collide. StarTalk begins right now. This is StarTalk, Cosmic Queries Edition. And today we're going to be talking about time travel, inspired by Dickens' classic novel, A Christmas Carol. Chuck, love to have you on this. Always a pleasure to be here. Although I'm not actually here. I'm coming to you from the future. You should just know that. Thanks for that heads up. in case that matters. Let me say it to you this way. Neil, it's your kids.

0:48They're in trouble. Thank you, Doc Brown.

0:56Realize when they went into the year 2015, just thought I'd remind you of that. Wow. Wow. Yeah, yeah, yeah. We'll get back to that. So I know a little bit about time travel. I don't know nearly as much as our guest, which is why we brought him on a good friend and colleague from up the street at Columbia University, Brian Green. Brian, dude. Our returning champion. Returning champion. Yes. Our returning champion, Brian Green, ladies and gentlemen. He's professor of physics and math. Is that right? Both of those? Yeah, that's right. That's right. Dang. Isn't that, wait. as a lay person. Isn't that redundant?

1:38It does sound redundant. Yeah, but I don't touch equipment. That's what it emphasizes. Oh, yeah. It's like, get the hell out of the lab. You're one of those theorists. Yes. Yeah, so Brian is a theoretical physicist and a longtime friend. And it's just a delight with best-selling books. And let me get the title correct of his latest book because this title leaves nothing out. Here it is. Until the end of time. Mind, Matter, and Our Search for Meaning in an Evolving Universe came out last year. Brian, that title doesn't leave anything untouched. You need all the good search words so that you can come up on anything anyone ever puts in.

2:22That's great. And who's the publisher of that? We'll give them a shout out. SEO Optimized. Knopf. Knopf. Very nice. Very nice. And of course, many people know you or know of you through being co-founder, I think, with your wife. Is that correct? Of the World Science Festival? Yeah, with Tracy Day. Yeah, Tracy Day. I met Tracy Day before I knew you when she was a news reporter. Was it for NBC? Is that right? ABC. ABC. Yeah, yeah. She did an interview with me. I don't think I... Oh, I didn't know this. She never mentioned it. Yeah, we go way back. Right before. So let's put some context on the table.

3:00So Charles Dickens, which is, you know, he wrote Oliver. He wrote A Tale of Two Cities. He wrote A Christmas. I mean, he wrote a bunch of stuff. And I think he wrote A Tale of Two Cities. Yeah, yeah. Sure, yeah, of course. Yeah, yeah. It was the best of times. And the worst of times. Right now, we're in the second part of that right now. Yes, exactly. Yeah, we're... So, written a long time ago, 1843. And by some measures, it may be the first sort of modern literary example of time travel or the implications of time travel. And so that's well before, like a half a century before H.G. Wells' The Time Machine.

3:47So itself quite a remarkable step to take in storytelling. So, Brian, before we had Einstein's relativity, how did anybody think about time travel? Yeah, it's a good question. What was it missing? Let me say that differently. Yeah. What do we in our enlightened modern times with the benefit of relativity get to say about time travel that they couldn't say before Einstein, before the 1905 special relativity paper and the 1916 general relativity? Well, yeah. Well, I mean, you know, I look at us as the unique species on the planet that's able to lift ourselves up above the timeline and think about the past and future.

4:36So that immediately gives us at least the imaginative capacity to travel to the past and the future. But of course, you're going beyond that. You want to know about real time travel. And what we can say today that they couldn't say back then is that time is much more individualistic than anybody would have ever thought. The common view of time was it's universal. It's the same for me, for you, for anyone else, regardless of what we're doing, what we're experiencing, how we're moving. and that Einstein shattered by showing that time elapses at different rates depending on all those qualities, how you move, the gravity you experience.

5:14That's the main new feature. Awesome. Interesting. That is amazing. So now that you said that, because please, one of you, either one of you, I'm filthy with physicists right now. You just, you just, it's raining physicists. I'm just great in physicists. I'm an embarrassment of physicists.

5:40So this is just wonderful every time I think about it. But if something is, let's say, let's go as close as geosynchronous orbit, like a satellite, and it's traveling at this speed, and they have to adjust the clocks because time is ticking differently for that thing up there than it is for us down here. And can you please just speak to that? And can you speak to, is it a literal elapsed difference? Like running clock, running clock, or is it something that causes it to seem like an elapsed difference? Oh, good one. Well, I shouldn't jump in. Maybe Neil wants to take this. Well, no, so why don't I, I'll just start with 1905, and you pick us up at 1916, okay?

6:33So what we learn with 1905's special relativity is that the faster you go, all right, you will perceive events around you as, well, okay. If you watch someone go fast, you will perceive their ticking clock to move more slowly than yourself. And we call this special theory of relativity because it only involved motion that did not accelerate. It was very sort of a basic case of motion. And so once you got used to that, then 10 years later, he throws in a whole other fact. And Brian, pick it up from there. Yeah, the new fact that Neil's referring to is that gravity also affects the passage of time.

7:18So Chuck, in the example that you gave, if you have a clock on planet Earth and a clock that's hovering above the Earth some number of miles up there, those clocks will really tick off time at different rates because they are experiencing different forces of gravity. The stronger the pull of gravity, the slower the clock ticks. So the clock on Earth truly is ticking off time at a slower rate than the clock that's floating up there in space. So what's interesting there is that the geosynchronous satellites, because, Chuck, you started this by saying they have whatever is their orbital speed, which has some significance, right?

7:54It's a few miles per second, really. Right. And up at sort of middle, you know, Brian, every time I talk about, you know, there's Leo, low Earth orbit, and Geo, but middle Earth orbit, you know, Mio, every time I say middle Earth, people are thinking Lord of the Rings. Oh, don't worry, Mr. Frodo. We've got this. We'll get that traffic satellite fixed, sir. We'll do it. So, Brian, have you done the math yet, since you are a professor of freaking mathematics, on the geosynchronous satellites? They're moving fast relative to us, so we should see their time slow down, but they're farther away from Earth's gravity, so we should see their time speed up.

8:43So there's some context between the two. There is. And I don't know the exact numbers in that particular case, but I do know that when they tested these ideas on a Pan Am jet in the 1970s, where they had an atomic clock that they left on the tarmac and the other they strapped into the passenger seat, maybe his first class. I don't know. In that particular case, it is the speeding up of time from the gravitational difference that wins out. So whenever I teach this, I'm always careful with my language because you would think that clock should take up time more slowly because it's in motion. But when you take the competition into effect, in the end, the net change is not what you would have anticipated.

9:31But none of that really matters. What really matters is when you take all the effects into account, motion and gravity, that Einstein delineated, the prediction agrees with the observation spot on. And so these effects are truly real. So I didn't know about that experiment. And Brian, please tell everyone. Hateful and Keating were the two scientists. Tell everyone what Pan Am is. Yes. I'm just saying. That was the southwest of its day. Yeah. Right. So Pan Am actually was also the space shuttle in 2001, A Space Odyssey. It was a Pan Am space shuttle. Yeah, yeah. Going up to the space station where there was an AT &T phone and a Howard Johnson's hotel.

10:21Yeah, super cool. That's very cool. So it turns out, so I didn't know that about the Pan Am. That's excellent. That remains true even for the geosynchronous satellites. So the fact that they're higher above Earth wins over the fact that they're moving fast. And so when the geosynchronous satellites hand us our times to our cell phones, they have to be pre-corrected knowing that Einstein's ideas are correct, which is just freaking mind-blowing. Einstein could say it in his armchair, right? And it can happen in a physics paper. but to actually measure the stuff is a whole other thing. It's totally insane.

10:59It's totally nuts. And just one other thing, Chuck, for your question, even if you take gravity out of the story and you take two clocks and you send one into space and it turns around, it comes back, and you literally compare the clock side by side, they will show different amounts of elapsed time. So it's real. The clock on Earth in that particular case will have ticked off more time than the clock that went on that round trip journey. So in a sense, the person carrying the clock on the round-trip journey has traveled into the future. They're seeing the Earth at a later time than their own watch would suggest it should be.

11:35And one last point about Ebenezer Scrooge. He was observing different Christmases, but the big issue for any storyteller is whether you can interact either with the past or with the present. that you then have access to. And so what does Einstein say about that? Well, he doesn't say a whole lot about travel to the past because everything that he was doing, if you actually follow it through, you can move in such a way or experience a gravitational field that allows you to go into the future. And if you're in the future, he said, sure, you interact with anything that you want to interact with. It's just that you are at a later time than your own clock would suggest.

12:20But when it comes to the past, I haven't read anything that he wrote, but it's a real conundrum that, you know, back to the future popularized, right? If you go to the past, can you change things in such a way that might prevent your own birth? And then you're in a logical paradox. How could that ever happen? And people have struggled with that. And we can talk about the solutions if it's relevant to the topic here. But, yeah, in one of those solutions, you can interact, but you're constrained. You don't have the freedom that you would have thought you would have had to mess things up. Now, wait.

12:53Is that constriction forced upon you? Like, for instance, if I were trying to get rid of my own self. So I'm going to commit suicide via time travel. Right? Oh, okay. Suicide via time travel. So those constrictions, since I'm coming from a place that is already set, okay? I go back at my origin, before my origin, I'm sorry. And then I, let's just say, you know, I caused my parents to hate each other. I'm doing the reverse of the movie, right? And they end up hating each other. Because you can do this without bloodshed, right? You just have to put your parents in an argument at a time they might have made love.

13:37And then you're not conceived in that moment. Exactly. Or I just do something really nasty, but not, you know, like hit on my mom. And she's just like, you are adorable. And I'm like, yeah, don't ever talk to that guy. And so, but. So before this gets too far out of hand. I wasn't going any further, Brian. Slow down, Chuck. Slow down, slow down. Let me just ask this. So would then kind of like the box of time force those events to happen anyway in a different circumstance, even though I just screwed up the circumstance under which I came into being? No, I wouldn't frame it that way. Okay. The answer that I would give on that is that if you travel back to an earlier moment in time before your own birth, then you were always at that.

14:36There aren't two versions. There aren't two versions of a given moment in time. A moment in time can't change because what would be the parameter through which that change would occur? It would have to be time. But we're talking about individual moments. So if you go to the past, you were always part of that moment, and therefore, you were always part of the causal chain of events that resulted in your own birth. So you are simply damn. Wait, wait, so Brian, in the time machine, the character has a love interest, and then she crosses the street and is hit by a horse and buggy. Back then, I guess that was a deadly thing.

15:20And then she says, no, wait, I can fix that. He goes back in time and like prevents her from crossing the street, leaves her, and then she gets mugged and dies from a mugging. And then he prevents the mugging in a third time, and then she dies some other way. And he figures out that her death was something inherent in the timeline, and there's nothing he can do to change it. Yeah. Brian, what you're saying is what's inherent in that timeline is you. You're not an X factor. You are part of that equation all the time. That's correct. All events just are. They're unchangeable. They're immutable. So the events can't change.

15:56And you simply, if you go back to the past, are fulfilling the precise set of events that allowed you to exist and to go on that journey in the first place. Now, look, I'm not saying this is the only resolution to these ideas because, Neil, the example that you gave brings to mind another proposal, which is this idea of a multiverse.

16:30I'm Nicholas Costella, and I'm a proud supporter of StarTalk on Patreon. This is StarTalk with Neil deGrasse Tyson.

16:48so Brian in the case where Chuck's going back in time with part of the equation that led to his birth in the first place even if he only just realized that uh let me give what an example that I've given before, but it seems to be consistent with that. Okay. So I see, uh, so let's say we can use tachyons, which travel backwards in time, and I can send texts via tachyons. And I watch you walk down the corridor and you slip on a banana peel. And I say to myself, Brian's my friend. I don't want him to bust his ass slipping on a banana peel. I'm going to text him. So I text you via tachyons, you get the message before you slipped on the banana peel, and you look down to the message, and by looking down, you don't see the banana peel, and you slip on the banana peel.

17:40So I ended up causing you to slip on the banana peel. Is that the same case that you just described with Chuck? Yeah, it's a self-consistent, immutable set of events. And the text, inexplicably from your perception is what actually causes the event that you wanted it to prevent. But indeed, it's part of that causal fabric. And that, in this approach, is the immutable quality of the timeline. Wow. That's cool. So it's reverse destiny.

18:17Well, it's timeless destiny. So all the events of reality just exist out there. And it perhaps is human perception that orders them into this causal set. But they're just all out there, just like all the spaces out there. And this way of thinking about things, all of time may be out there too. And so there's nothing that can ever be changed. So in Kurt Vonnegut's Slaughterhouse-Five, that time travel scenario is what you're describing, right? And because in it, his entire time, he's picked up by aliens and he lives in a cage. an alien zoo, but it doesn't matter to him because he still has access to his entire life's timeline.

19:00And when he's describing this, or they describe it to him, he said, when will I die? You're always dying. When was I born? You're always being born. When did I go to college? You're always going to college. And so the preexisting timeline, he could just rejoin it at any point, but the destiny is already preordained. Yeah, and there's something comforting. There's something comforting about that, right? Because even those of us who've lost parents or loved ones, in some sense, they still exist at the moment of time that they occupied. And that is an eternal, unchanging statement. But Brian, it takes away free will.

19:33Ah! No. A little fatalistic. So, yes, this is a version of negating free will. But I am one of those people who doesn't believe in any variety of free will of the traditional sort. So this doesn't run afoul of my intuition at all. Okay, Brian. See, I know we got queries. Wait, Chuck, we got to get to questions. This is a Cosmic Queries. Go, go. Brian, stop tempting us. Brian, stop tempting us. Shut up, Brian. Let's act like we don't have an audience.

20:07Brian, stop being so damn interesting. And let us get to our Cosmic Queries. Oh, man. Okay, all right. Let me get to the queries. Chuck. I'll put my stuff on the back burner. Here we go. Okay. Hey, this is Jay Salmon who says, or Salmon. I mean, it's Salmon, but Salmon maybe. He says, hello, Dr. Green. Because a photon of light has no mass, theoretically, can't it travel both forward and backward in time? Well, I love that. Speaking of gravity and it's - No, wait a minute. So it moves the speed of light. Right. And if time goes slower for you, the faster you go, then photons have no time at all.

20:48Yeah, right. So if they have no time, who cares whether they move forwards or backwards? Right. There is no forward and backwards for a photon. So I think there's two ways of answering this question. I think both are really important. One is from our perception watching the photon. And from our perception, you fire a photon and it travels through space as time elapses. It goes forward in time. But then you say, what about from the photon's perspective? It's traveling at the speed of light. And from what Neil described earlier, when the photon looks out at the world, everything should be going infinitely slowly.

21:25And therefore, in some sense, time is stopping. Now, that's poetically fine. The problem I have with taking that too seriously is, and this may seem like a footnote, but it's not, you're ascribing some kind of experience to a photon. And a photon doesn't have experience. It doesn't have consciousness. It doesn't look out at the world. even though the poetic language is useful to invoke. And so to imagine that we could travel at the speed of light and therefore there'd be no time and time would stop is a step too far. So when we look at the photon, it's like any other particle. It travels through space, it goes forward in time.

22:03If you put yourself into the shoes of the photon, whatever that means, then poetically, yes, time would stop. There's no notion of the elapsing of time. There's no notion of aging from the photon's perspective, whatever that actually means. Okay. You heard that. Brian cashed shade on the photon. He said it has no emotions, no feelings. Brian thinks photons are stupid. And so. Okay. That's Brian. All right. All right, Brian. We know where you're coming from. All right. Okay. Keep going, Chuck. Here we go. This is, I like this. Let's a little more philosophical. This is Tyler J. Tyler says, how might time travel be policed or regulated if it were able to happen?

22:55Yeah, because, I mean, think about it. If you have bad actors going through. Yeah. Well, again, I'm less fearful of time travel because of the view that I hold that you couldn't actually change anything. If you could, then, yeah, we'd have to have the time police around here to avoid things happening that we didn't want to have happen. Brian, is that the same thing as Hawking's time travel conjecture? Well, he's got a few conjectures. So it depends. You think? It depends exactly which one. But I think Hawking has said that. Chronology protection. Chronology protection. That you can't travel to the past because it keeps history safe for historians, right?

23:36Because, you know, if you could travel, you could change things. And it's also his explanation for why we're not overrun with tourists from the future. If you could travel to the past, wouldn't everybody, you know, from the future come and visit us and be gawking at, you know, what life was like in the 21st century? But, you know, again, they could be here looking at us right now. or another explanation which is more plausible is in almost any time travel scenario that has been dreamt up, you can never travel to a moment in time prior to the construction of the first time machine. And so if the first time machine has yet to be constructed, that would also explain why no one's come back here because they can't come back here because the machine's yet to be built.

24:20So that's another very straightforward way of explaining why we're not overrun with tourists from the future. I like that one. Wow. That's pretty wild. Chuck, keep going. What do you have? All right. Let's have some fun with this one. This is Kevin the sommelier, who's... Oh, we love Kevin the sommelier. Yeah, we love Kevin the sommelier. Never sends us wine. Well, no, never sends Chuck wine. Neil doesn't need any wine. I've been to Neil's cellar. He don't need no wine. Chuck needs wine. Okay. All right. All right. Kevin, the sommelier says, Dr. Green, what Hollywood film depicts time travel best in your estimation?

25:04And then he says, is it frequency? Well, yeah, I had a brief moment in frequency. I tried to convince the filmmakers on how the end should be done, but it could have been more accurate. Frequency is a film? I had never seen it or heard of it. Yeah, frequency is a film. There's a time travel element where a father and a son are able to communicate across 30 years. And the father's long since been dead. He's a firefighter. He died in a fire. So the son tries to tell the father, go left, not right in that burning building when it happens tomorrow. And he actually saves him. So they do change the future in that particular case.

25:42So, yeah, there's a multiverse way of thinking about it. Wait, they called you for advice on that? They called me for advice, and then they asked me if I would be in it. And I'm, like, interviewed by Dick Cavett. Dick Cavett in two different time frames. And they aged me to make me look old in one. And it was the scariest thing, really. I thought it would be so simple. But I don't know if you've ever done this. And they put the plaster on your face to mold so they can get jowls and things on you. Feels like being buried alive. Excuse me, Brian. Black don't crack, so that wouldn't be necessary.

26:14That's true. I'm 87. I'm 87 years old Brian I'm sorry Yeah Wait wait So Brian Were you in the movie? Brian Yeah Were you in the movie? I missed it It's like a scene Where I'm in a television set Being interviewed in the past And another television set In the future To like set up this time warpy thing Oh cool Damn And that's That's not his first movie He was in In the Mimsy movie What's the name of that movie? The Last Mimsy Yeah Oh The Last Mimsy? He was in The Last Mimsy Nice My boy was in The Last Mimsy Interviewed as Brian Green We need expertise on this one. You were great in it. You were like total natural.