Past, Present, Future: Time Travel with Brian Greene

What type of time travel is in “A Christmas Carol”? Neil deGrasse Tyson and comedian Chuck Nice answer fan questions on time travel, paradoxes, and wormholes with theoretical physicist, Brian Greene. Did Ebenezer Scrooge get pulled through a wormhole? (Originally Aired December 20, 2022)

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:01Neil deGrasse Tyson?

  Welcome 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 gonna be talking about time travel, inspired by Dickens' classic novel, A Christmas Carol. Chuck, love to have you on this.

• 0:27Chuck Nice?

  Always a pleasure to be here. Although I'm not actually, I'm not actually here. I'm coming to you from the future. You should just know that.

• 0:37Neil deGrasse Tyson?

  Thanks for that heads up. In case that matters.

• 0:40Chuck Nice?

  Let me say it to you this way. Neil, it's your kids. They're in trouble.

• 0:52Neil deGrasse Tyson?

  Thank you, Doc Brown. Realize when they, they went into the year 2015. Just thought I'd remind you of that.

• 1:00Chuck Nice?

  Wow.

• 1:02Brian Greene?

  Wow.

• 1:03Neil deGrasse Tyson?

  Yeah, yeah, yeah, we'll, we'll get back to that. So I, well, 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 Greene. Brian, dude.

• 1:18Chuck Nice?

  Our returning champion.

• 1:20Neil deGrasse Tyson?

  Returning.

• 1:21Chuck Nice?

  Our returning champion, Brian Greene. Ladies and gentlemen.

• 1:27Neil deGrasse Tyson?

  He's professor of, of physics and math. Is that right? Both of those?

• 1:32Brian Greene?

  Yeah, that's right. That's right.

• 1:33Neil deGrasse Tyson?

  Dang.

• 1:34Chuck Nice?

  Isn't that, wait, as a layperson, isn't that redundant?

• 1:39Brian Greene?

  It does sound redundant. Yeah, but I don't touch equipment. That's what it emphasizes.

• 1:43Neil deGrasse Tyson?

  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 bestselling books. And let me get the title correct of his latest book, because that, that, 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's a, that title doesn't leave anything untouched.

• 2:16Brian Greene?

  You need, you need all the good search words so that you come up on anything anyone ever puts in.

• 2:21Neil deGrasse Tyson?

  That's, that's great. And who's the publisher of that? We'll give them a shout out.

• 2:25Brian Greene?

  SEO optimized. Kenop.

• 2:27Neil deGrasse Tyson?

  Kenop, very nice, very nice. And of course, many people know you, know of you through being co-founder, uh, I think with your wife, is that correct? Of the World Science Festival? Yeah, with Tracy Day, right. Yeah, Tracy Day. I met Tracy Day before I knew you, when she was a news reporter. Was it for NBC? Is that right? Uh, ABC, yeah. ABC. Yeah, yeah. She did an interview with me. I don't think I-

• 2:49Brian Greene?

  Oh, I didn't know that. She never, she never mentioned it. Geez.

• 2:54Neil deGrasse Tyson?

  Yeah, we go way back, right before.

• 2:58Neil deGrasse Tyson?

  So let's put some context on the table. So, uh, 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, is that, yeah, yeah. Sure, of course. Yeah, yeah.

• 3:15Brian Greene?

  It was the best of times.

• 3:16Chuck Nice?

  And the worst of times. Right now, we're, we're in the second part of that right now. Yes, exactly. Yeah, we're-

• 3:26Neil deGrasse Tyson?

  So, uh, written a long time ago, 1843. And by some of our, 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's The Time Machine. So itself, quite a, quite a remarkable step to take in storytelling. So, so Brian, you know, before we had Einstein's relativity, uh, do you know, how did anybody think about time travel?

• 4:03Brian Greene?

  Yeah, it's a good question.

• 4:04Neil deGrasse Tyson?

  What was it missing? What, what, what, what, let me say that differently.

• 4:08Brian Greene?

  Yeah.

• 4:08Neil deGrasse Tyson?

  What, 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, you know, 1905 special relativity paper and the 1916 general relativity.

• 4:25Brian Greene?

  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. So 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.

• 5:05Brian Greene?

  And that Einstein shattered by showing that time elapses at different rates depending on all those qualities. How you move, the gravity you experience, that's the main new feature.

• 5:15Chuck Nice?

  Awesome.

• 5:16Neil deGrasse Tyson?

  Interesting.

• 5:16Chuck Nice?

  That is amazing. So now that you said that, because please, one of you, um,

• 5:24Chuck Nice?

  either one of you, I'm filthy with physicists right now, I can.

• 5:29Neil deGrasse Tyson?

  You just, you just.

• 5:31Chuck Nice?

  It's raining physicists. Raining physicists. I'm an embarrassment of physicists, Richard.

• 5:40Chuck Nice?

  So, this is just, 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?

• 6:24Neil deGrasse Tyson?

  Oh, good one.

• 6:25Brian Greene?

  Well, I shouldn't jump in. Maybe Neil wants to take this.

• 6:27Neil deGrasse Tyson?

  Well, no, so why don't I, I'll just start with 1905 and you take, pick us up at 1916, okay? So what we learned with 1905 special relativity is that the faster you go, all right, you will perceive events around you as, well, okay.

• 6:45Neil deGrasse Tyson?

  If you watch someone go fast, you will perceive their click ticking clock to move more slowly than yourself. And we call the special theory of relativity because it 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 then 10 years later, he throws in a whole other fact. And Brian, pick it up from there.

• 7:12Brian Greene?

  Yeah, the new fact that Neil's referring to is that gravity also affects the passage of time. So 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're 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.

• 7:44Neil deGrasse Tyson?

  Well, 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? It's a few miles per second, really. 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, MEO. Every time I say Middle Earth, people are thinking Lord of the Rings.

• 8:11Chuck Nice?

  Oh, don't worry, Mr. Frodo.

• 8:16Chuck Nice?

  We'll get that traffic satellite fixer. We'll do it.

• 8:22Neil deGrasse Tyson?

  So, so Brian, so did you, have you done the math yet, since you are professor of freaking mathematics, on the geosynchronous satellites? There's, they're moving fast relative to us. So their time should, we should see their time slow down, but they're farther away from Earth's gravity. So that we should see their time speed up. So there's some context between the two.

• 8:46Brian Greene?

  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 as first class, I don't know, on the Pan Am jet. 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.

• 9:31Brian Greene?

  But 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 observations spot on. And so these effects are truly real.

• 9:45Neil deGrasse Tyson?

  So I didn't know about that experiment. And Brian, please tell everyone...

• 9:49Brian Greene?

  Hayfle and Keating were the two scientists.

• 9:53Neil deGrasse Tyson?

  Tell everyone what Pan Am is.

• 9:58Neil deGrasse Tyson?

  I'm just saying.

• 9:59Brian Greene?

  That was the southwest of its day.

• 10:05Neil deGrasse Tyson?

  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.

• 10:22Brian Greene?

  Super cool. That's very cool.

• 10:24Neil deGrasse Tyson?

  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 cellphones. They have to be pre-corrected, knowing that Einstein's ideas are correct, which is just freaking mind-blowing. That, you know, Einstein could say it in his, in his armchair, right? And it can happen in a physics paper, but to actually measure the stuff is a whole other thing.

• 10:59Brian Greene?

  It's totally insane, it'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 and 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.

• 11:35Neil deGrasse Tyson?

  And one last point about Ebenezer Scrooge. We, 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?

• 11:57Brian Greene?

  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. But 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?

• 12:38Brian Greene?

  And people have struggled with that, and we can talk about the solutions if it's relevant to the topic here, but yeah, and 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.

• 12:52Chuck Nice?

  Now, wait, is that constriction forced upon you? Like, for instance, if I were trying to get rid of my own self. So I'm gonna commit, I'm gonna commit suicide via time travel, right?

• 13:07Neil deGrasse Tyson?

  Oh, okay.

• 13:08Chuck Nice?

  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.

• 13:30Neil deGrasse Tyson?

  Because you can do this without, without bloodshed, right?

• 13:32Chuck Nice?

  Yeah, without bloodshed.

• 13:33Neil deGrasse Tyson?

  You just have to get your, put your parents in an argument at a time they might have made love.

• 13:37Chuck Nice?

  Right.

• 13:38Neil deGrasse Tyson?

  And then you're not conceived in that moment.

• 13:39Chuck Nice?

  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, um, so, but.

• 13:54Brian Greene?

  So before this gets too far out of hand.

• 13:56Chuck Nice?

  Yeah.

• 13:58Chuck Nice?

  I wasn't doing, I wasn't going any further, Brian.

• 14:02Neil deGrasse Tyson?

  Slow down, Chuck. Slow down, slow down.

• 14:05Chuck Nice?

  Let me just ask this. So are, 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?

• 14:23Brian Greene?

  No, I wouldn't frame it that way.

• 14:25Chuck Nice?

  Okay.

• 14:26Brian Greene?

  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 moment. There are two versions. There are two versions of a given moment in time, right? A moment in time, a moment in time can't change because what would be the parameter through which that change would occur? It'd 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.

• 15:01Neil deGrasse Tyson?

  Ooh.

• 15:02Brian Greene?

  So, so you are simply.

• 15:04Neil deGrasse Tyson?

  Wait, wait, so Brian, wait, wait. Brian, in the time machine. He, uh, the character has a love interest and he, and then she crosses the street and is hit by a, by a, you know, by a horse and buggy. Back then, I guess that was a deadly thing. And then she says, no, wait, I can fix that. He goes back in time and like prevents her from crossing the street, leaves her, then she gets mugged and dies from a mugging. And then she, he, 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. There was nothing he can do to change it.

• 15:39Brian Greene?

  Yeah.

• 15:39Chuck Nice?

  Brian, what you're saying is what's inherent in that timeline is you. You're not, you're not an X factor. You are part of that equation all the time.

• 15:49Brian Greene?

  That's correct. All events just are. They're unchangeable. They're immutable. So the events can't change and 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, which is this idea of a multiverse.

• 16:31Speaker 03

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

• 16:48Neil deGrasse Tyson?

  So, so, so Brian, in the case where Chuck's going back in time was part of the equation that led to his birth in the first place, even if he only just realized that, 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 gonna text him. So I text you via tachyons. You get the message before you slipped on the, on the banana peel.

• 17:33Neil deGrasse Tyson?

  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. So I ended up causing you to slip on the banana peel. Is that the same case that you just described with, with Chuck?

• 17:47Brian Greene?

  Yeah, it's a self-consistent, immutable set of events. And, 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.

• 18:09Neil deGrasse Tyson?

  Wow.

• 18:10Brian Greene?

  Cool.

• 18:10Neil deGrasse Tyson?

  So it's, it's, it's reverse destiny.

• 18:16Brian Greene?

  Well, i- i- it's timeless destiny.

• 18:19Neil deGrasse Tyson?

  Well, oh.

• 18:20Brian Greene?

  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.

• 18:39Neil deGrasse Tyson?

  Well, so in, in, in Kurt Vonnegut's Slaughterhouse Five, uh, 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, in a cage. All right, a, a, an alien zoo, but it doesn't matter to him because he still has access to his entire life's timeline. And when he's describing this, or they describe it to him, 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 pre-existing timeline, he could just rejoin it at any point. But the, the destiny is already pre, preordained.

• 19:17Brian Greene?

  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.

• 19:31Neil deGrasse Tyson?

  But Brian, it takes away free will.

• 19:34Brian Greene?

  No.

• 19:35Chuck Nice?

  Little fatalistic.

• 19:36Brian Greene?

  So, 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.

• 19:49Chuck Nice?

  Okay, Brian, now, okay. See, I know we got queries, but I'm not.

• 19:52Neil deGrasse Tyson?

  Wait, Chuck, we gotta get to questions. This is a, this is a cosmic queries. Go, go. Brian, stop tempting us. Brian, stop tempting us. Shut up, Brian.

• 20:01Chuck Nice?

  Let's act like we don't have an audience.

• 20:07Chuck Nice?

  Why would we do that?

• 20:09Neil deGrasse Tyson?

  Brian, stop being so damn interesting. And let us get to our cosmic queries.

• 20:13Brian Greene?

  Oh, man, okay. All right, let me get to the queries. I'll, I'll, I'll put my stuff on the back burner. Here we go.

• 20:19Neil deGrasse Tyson?

  Okay.

• 20:20Brian Greene?

  Hey, this is Jay Salmon who says, or Salmon. I mean, it's, it's salmon, but salmon, maybe. Uh, he says, hello, Dr. Green. Because a photon of light has no mass theoretically, can't it travel both forward and backward in time?

• 20:37Neil deGrasse Tyson?

  Well, I love that.

• 20:38Chuck Nice?

  Speaking of gravity and it's.

• 20:40Neil deGrasse Tyson?

  No, wait a minute. If it, so it moves the speed of light.

• 20:43Brian Greene?

  Right.

• 20:43Neil deGrasse Tyson?

  And if time goes slower for you, the faster you go, then photons have no time at all.

• 20:48Brian Greene?

  Yeah.

• 20:49Neil deGrasse Tyson?

  So if they have no time, why don't, who cares whether they move forwards or backwards?

• 20:53Chuck Nice?

  Right. There is no forward and backwards for a photon.

• 20:56Brian Greene?

  So I think there's two, 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. And therefore, in some sense-

• 21:26Neil deGrasse Tyson?

  Basically stopped, basically stopped.

• 21:27Brian Greene?

  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. If 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.

• 22:12Brian Greene?

  There's no notion of aging, from the photon's perspective, whatever that actually means.

• 22:18Chuck Nice?

  Okay.

• 22:19Neil deGrasse Tyson?

  You heard that Brian cast shade on the photon. He said it has no emotions, no feelings.

• 22:24Chuck Nice?

  Basically. Brian thinks photons are stupid, and uh-

• 22:30Neil deGrasse Tyson?

  Okay, that's Brian.

• 22:32Chuck Nice?

  Alright, well that-

• 22:34Neil deGrasse Tyson?

  Alright, Brian, we know where you're coming from. Alright.

• 22:37Chuck Nice?

  Okay.

• 22:37Neil deGrasse Tyson?

  Keep going Chuck.

• 22:38Chuck Nice?

  Here we go, this is, um, 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?"

• 22:55Neil deGrasse Tyson?

  Ooh.

• 22:55Chuck Nice?

  Yeah, cuz I mean, think about it, if we could all-

• 22:57Neil deGrasse Tyson?

  So, if you have bad actors going through, right?

• 23:00Brian Greene?

  Yeah. Well, again, you know, 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.

• 23:14Neil deGrasse Tyson?

  Brian, is that the same thing as Hawking's time travel conjecture?

• 23:18Brian Greene?

  Um, well, uh, he's got a few conjectures, so it depends.

• 23:23Chuck Nice?

  You think?

• 23:24Brian Greene?

  It depends. It depends exactly which one. But, but I think Hawking has said that-

• 23:29Chuck Nice?

  Chronology protection conjecture.

• 23:32Brian Greene?

  That you can't travel to the past because it keeps history safe for historians, right? Because, 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? Um, 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,

• 24:15Brian Greene?

  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. Um, so that's another very straightforward way of explaining why we're not overrun with tourists from the future.

• 24:26Chuck Nice?

  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 Semoye, who's a- Oh, we love Kevin the Semoye. Yeah, we love Kevin the Semoye. Never sends us wine. Well, no, never sends Chuck wine. Neil doesn't need any wine.

• 24:46Chuck Nice?

  I've been to Neil's cellar. He don't need no wine. Chuck needs wine.

• 24:53Chuck Nice?

  All right. All right. Kevin the Semoye says, uh, "Dr. Green, what Hollywood film depicts time travel best in your estimation?" Um, and then he says, "Is it Frequency?"

• 25:08Brian Greene?

  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.

• 25:16Neil deGrasse Tyson?

  Well, Frequency is a film. I had never seen it or heard of it.

• 25:18Brian Greene?

  Yeah, Frequency is a film. It's a, there's a time travel element where a father and a son are able to communicate across thirty years, and the father's long since been dead. He's a firefighter. He died in a fire, so the, the son tries to tell the father, "Go left, not right," in that burning building when it happens tomorrow. And he actually, he actually saves him. So, so they do change the future in that particular case. Um, so, yeah, there's a multiverse way of thinking about it.

• 25:45Neil deGrasse Tyson?

  Wait, they called you for advice on that?

• 25:47Brian Greene?

  They called me for advice, and then they asked me if I would be in it, and I'm like, interviewing Dick Cavett. Dick Cavett in two different timeframes, and they aged me to make me look old in, 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. Um, feels like being buried alive, but, uh...

• 26:10Neil deGrasse Tyson?

  Excuse me, Brian, black don't crack, so that wouldn't be necessary. That's true. I'm, I'm eighty-seven. I'm eighty-seven years old, Brian.

• 26:20Neil deGrasse Tyson?

  Right, right, so Brian, were you in the movie? Brian, were you in the movie?

• 26:23Brian Greene?

  Yeah, there's like a scene where I'm in, in a television set being interviewed in the past, and another television set in the future to like, set up this time-warp-y thing.

• 26:31Neil deGrasse Tyson?

  Oh, cool. Damn. And that's, but, Chuck, that's not his first movie. He was in, in the Memzy movie. What's the name of that movie?

• 26:38Brian Greene?

  Uh, The Last Memzy, yeah.

• 26:39Neil deGrasse Tyson?

  Oh, The Last Memzy? The ma, he was in The Last Memzy. Nice. My boy was in The Last Memzy. Interviewed as Brian Greene. And, and, we need expertise on this one. And, and, and you were, you were great in it. You were like, total natural. I loved it.

• 26:53Brian Greene?

  Well, thank you. I appreciate that.

• 26:55Neil deGrasse Tyson?

  Okay, so, so what's the best movie? What's the best movie?

• 26:57Brian Greene?

  Yeah, I would say that, uh, Interstellar is, is probably the best one, where you have Matthew McConaughey going near a black hole, time slows down, so when he goes back to the ship, the fellow left on the ship is twenty-three years older, even though McConaughey's only like, an hour older. And then Matthew McConaughey, I can't remember his name, his character in the movie.

• 27:18Neil deGrasse Tyson?

  Just to be clear, twenty-three years older, not twenty-three years old. So the guy's gray.

• 27:23Brian Greene?

  Yeah, yeah, it's, it's sort of a very crazy scene, but it's accurate. You know, you go down to, near a black hole, and you come back, and your crew members have aged decades, and you've aged an hour. Um, but then, in a, in a very poignant moment, he, the character sees his own daughter, much older than he is. And she's sick in her bed, as you may recall, and he kind of comes into that room. So, so those are accurate features of Einstein's general theory of relativity.

• 27:50Neil deGrasse Tyson?

  And it's not just because you're buds with Kip Thorne, who was co-executive producer on that movie, who was professor of physics at Caltech. It's not just because you're friends with him.

• 27:59Brian Greene?

  That's not because I'm friends, but no doubt it was his expertise that kept the film on track, on scientific track, which is kind of a beautiful.

• 28:07Chuck Nice?

  It's a great movie.

• 28:08Neil deGrasse Tyson?

  Chuck, did you, Chuck, did you see Interstellar?

• 28:10Chuck Nice?

  Oh, God, yes. I love it. It's one of my favorite sci-fi movies.

• 28:13Neil deGrasse Tyson?

  Did you see, did you see the name of the robot?

• 28:16Chuck Nice?

  Um, it's the big square robot. I forget his name.

• 28:20Neil deGrasse Tyson?

  Yeah, the big, the big rectangular robot.

• 28:22Chuck Nice?

  Which, by the way, was the best, I think, depiction of a robot in a movie. It's just a block. Like, everybody wants robots. Everybody's like, I am a robot. No, it's just a block. They're like, here's a square that, that, that has appendages when it wants and, you know. I forget his name, though.

• 28:39Neil deGrasse Tyson?

  Do you remember the name of the robot?

• 28:41Chuck Nice?

  I do not.

• 28:43Neil deGrasse Tyson?

  Brian, do you remember the name of the robot?

• 28:45Brian Greene?

  I am embarrassed to say I can't even remember the robot, so I might as well just come.

• 28:50Neil deGrasse Tyson?

  Okay. The robot's name was Kip.

• 28:53Chuck Nice?

  Kip. Oh, there you go.

• 28:55Neil deGrasse Tyson?

  Yeah, just thought I'd tell you. By the way, Kip Thorne, of course, helped pioneer LIGO, Laser Interferential Gravitational Wave Observatory, received a Nobel Prize, was a co-recipient of it a few years ago. And he did the movie. So we got some good people out there.

• 29:14Chuck Nice?

  Nice.

• 29:15Neil deGrasse Tyson?

  Trying to raise science literacy of the world a few notches. So, all right, there it goes. So good calculations there on the time. So that's just, that's just straightforward gravitational Einsteinian physics.

• 29:28Brian Greene?

  Yeah, that's all that is.

• 29:29Chuck Nice?

  That's very cool. So, no, okay, so with respect, very quickly, okay. Do we have any?

• 29:36Neil deGrasse Tyson?

  Chuck, are you a Patreon member? It sounds like you're asking your own question.

• 29:39Chuck Nice?

  I paid this month.

• 29:46Neil deGrasse Tyson?

  You are lying. That's how, that's what liars sound like.

• 29:50Chuck Nice?

  That's exactly what liars sound like. And that is why I'm a terrible liar. Because that's exactly what I sound like.

• 29:56Neil deGrasse Tyson?

  I paid, I paid this money.

• 30:01Chuck Nice?

  Alright, so wait, real, very quickly. Do we have any information on the aging of cells?

• 30:09Chuck Nice?

  At the speed of light or in gravitational situations that we might be able to attribute to time travel?

• 30:17Brian Greene?

  Yeah, well, everything that we're talking about in terms of time slowing down or speeding up, it's truly time. So any physical process, whether it's the motion of atoms or the motion of particles in a cell or motion of aspects of proteins carrying out instructions given to them by DNA, it all happens at a rate dictated by how time elapses. So when we talk about time slowing-

• 30:42Neil deGrasse Tyson?

  It's not as though-

• 30:43Brian Greene?

  It's everything slowing.

• 30:45Neil deGrasse Tyson?

  So Brian, just to be clear, because there's a point of ambiguity here, unless we say it explicitly. It's not that being in stronger gravity has some effect on your physiology that makes you age more slowly. It's an actual change in the ticking clock that's on your wall. That it's everything around you and has nothing to do with the effect of gravity, the strength of the gravity on your metabolism or anything. It has to do with the spacetime, the fabric of the spacetime that you're embedded in. Is that a fair way to say it?

• 31:22Brian Greene?

  Totally fair, and so much so that even your thoughts would slow down in a strong gravitational field. And that's why you wouldn't even notice in your local environment that anything had changed. Everything slows down, even your thoughts, so there's no obvious evidence of it. It's only when you compare time elapsing for you- With time elapsing for somebody far away in a different circumstance that you recognize, aha, things have elapsed.

• 31:48Neil deGrasse Tyson?

  Now, of course, strong gravity might still kill your ass, but that has nothing to do with the time that's ticking.

• 31:53Brian Greene?

  That's right, that's right.

• 31:54Chuck Nice?

  Amazing, amazing, amazing.

• 31:56Neil deGrasse Tyson?

  All right, Chuck, keep going.

• 31:57Chuck Nice?

  Okay, here we go. Uh, this is Logan Kent, and Logan says, hello beloved science professors, and happy holidays from Kansas Cities. Uh, I'm already giddy waiting for the episode where we get to hear about the knowledge and theories on the topic of time travel. Okay, well, we're in it.

• 32:16Neil deGrasse Tyson?

  Nice.

• 32:17Chuck Nice?

  And he says-

• 32:17Neil deGrasse Tyson?

  And he's hailing from both Kansas Cities, KC Mo and KC, uh, Kansas, I guess.

• 32:23Chuck Nice?

  Yeah, okay.

• 32:23Neil deGrasse Tyson?

  That's the plural there, right?

• 32:25Chuck Nice?

  Yeah.

• 32:25Neil deGrasse Tyson?

  Very good.

• 32:26Chuck Nice?

  Okay. He says, uh, if controlled backwards time travel was possible today, where do you think your matter or information would travel? And no, I'm not talking about what time do you want to travel, but rather if you were in a third position, third person position, observing someone travel back in time, what do you think it would look like? So the time travel itself, if you were the observer, what would you see if you were able to observe the timeline itself?

• 32:59Neil deGrasse Tyson?

  So I like that, but also just slipped in there, Brian, was some mention about information.

• 33:05Chuck Nice?

  Yes.

• 33:05Neil deGrasse Tyson?

  And, and that links to entropy, I presume. So if you can tackle both of those in the next 90 seconds before we go to break.

• 33:13Brian Greene?

  Yeah, so, so look, I think the only, the only real way to answer that question is to commit to a version of time travel to the past. And the version that I find most convincing involves wormholes. And the idea of a wormhole, I think many people know this idea. It's a tunnel from one location in space to another location, basically kind of shortcut. And if you move those openings relative to each other, you put one near a black hole, again, there'll be a time difference between the two openings. So now one opening is ahead, one opening is behind. So you go through the tunnel one direction, you go to the future. You go through the tunnel the other direction, you go to the past. So what would going to the past look like? Somebody would enter the opening of a wormhole, and they would disappear, and they'd pop out the other opening of the wormhole

• 33:58Brian Greene?

  at a different place at a different time.

• 34:01Neil deGrasse Tyson?

  So in the Marvel, in the Marvel Universe where you have Doctor Strange opening these portals, he's only moving through space. He's not actually moving through time. So that's a, that's a lost storytelling element there that they could totally do interesting things with, it seems to me.

• 34:16Brian Greene?

  Yeah, I mean, the richness of wormholes really arises when you have a time difference between the two openings. I mean, it's fun to have a tunnel through space, but it is mind blowing to have a tunnel through time.

• 34:29Neil deGrasse Tyson?

  And also, I would add that, I think I can add correctly, Brian, that as portrayed in the movie Contact, where Jodie Foster goes through this, we presume is a wormhole, to get to, to visit the aliens and then she returns, we like the idea that it's, it's like a water slide. You know, you're in this tunnel, you're in this tube and you're, you're sliding and that, but it's really just a simple hole. You step through it, right? It's not some journey. Isn't that correct? Cause you're instant, they're, they're basically in the same place.

• 35:03Brian Greene?

  Uh, they can be, but you can also have situations where the throat of the wormhole has some length to it. And then again, it would just be traveling through space. It wouldn't be some kind of, like you say, water slide or some kind of weird thing that was happening. But, uh, but you're right.

• 35:21Neil deGrasse Tyson?

  Okay. So, all right. So if you're gonna do this, at least then report on it, you, if you're in a wormhole and it's propped up nicely and it's safe for you, then you, you're just moving through space backwards in time. If the opening of that wormhole is near a black hole where time is ticking more slowly than where you came from.

• 35:43Brian Greene?

  Or if it was there for a while. Once you set the time difference between the two openings, you can then move away from the black hole.

• 35:49Neil deGrasse Tyson?

  Right.

• 35:49Brian Greene?

  Because the time difference will then persist. Wow.

• 35:54Chuck Nice?

  Jesus. That is, that is insane.

• 35:59Neil deGrasse Tyson?

  Chuck, in this one broadcast, you've mentioned God and Jesus together.

• 36:02Brian Greene?

  Yeah.

• 36:03Neil deGrasse Tyson?

  This is, this is, must be a very significant force operating on your brain.

• 36:07Chuck Nice?

  Instead, I should have said, "Father, time."

• 36:38Chuck Nice?

  Alrighty, here we go. This is Jim Kelly and Jim Kelly says, "Hi, Dr. Tyson, Dr. Green, Dr. Comedy. Why do physicists assume that all time travelers are murderous, petricidal maniacs?" Just kidding. Just kidding. But how does a hypothetical paradox preclude the existence of time travel?

• 36:58Brian Greene?

  Yeah, I mean, I don't think it does. So, the hypothetical paradoxes are, you change the past in a way that, say, prevents your own existence, and we already discussed how maybe you can't do that. But the other idea that we made brief mention of is, if, instead of traveling to the past in your own universe, the laws of physics demand that you go to the past in a parallel universe, well, if you prevent your own birth in that universe, there's no paradox, where were you born in a different universe? And so that's another way in which you can have the freedom to make changes to the past, but not the past of your own world.

• 37:38Neil deGrasse Tyson?

  And that's what the Marvel Universe persistently does. That's how Spider-Man can have multiple origin stories, for example. We're actually accessing a different universe where similar things are happening, but not so different that we don't recognize the story. That's crazy. So Brian, what about information? Because information, I don't think it has mass, does it? Does it travel? Information is this intangible thing, and I always hear physicists arguing about whether we lose or gain information every time you do something with a black hole.

• 38:11Brian Greene?

  Yeah, I mean, I like to think of information as more concrete than that description might suggest, because information is always carried by particles.

• 38:22Brian Greene?

  You can have an abstract measure of information, but if you want to look at the motion of information, it's got to be the motion of stuff that carries that information. And to me, that makes it much more clear what's going on. So with black holes, the whole question was, as radiation comes out of a black hole, which Hawking, Stephen Hawking, told us will happen, are the particles-

• 38:43Neil deGrasse Tyson?

  Instead of Louis Hawking, you mean Stephen Hawking?

• 38:46Brian Greene?

  That's right, that's right. They always get those confused, different universes.

• 38:50Neil deGrasse Tyson?

  The thing of specifying, Brian, which Hawking you're referring to.

• 38:53Brian Greene?

  It was the universe where Louis Hawking actually was responsible, but let's put that to the side. Um, question is, do the particles have a relationship among each other that carries away the information of what fell in? So it's really concrete when you think about it as information carried by stuff.

• 39:12Neil deGrasse Tyson?

  Okay, because, right, because, because otherwise there's no information without stuff to carry it.

• 39:16Brian Greene?

  Right.

• 39:16Neil deGrasse Tyson?

  That's another way to say it.

• 39:17Brian Greene?

  It's hard to follow the information without that commitment.

• 39:20Chuck Nice?

  So, so, so when it comes to, like, uh, for those of us who are listening that may not be so, um, familiar with what you're talking about. So you're, when you go into a black hole, I'm a chair, I go into a black hole. I get broken down into just particles, okay? That's all, because the gravity's so strong. Now, just a string of particles, I come out during the evaporation. Would I be able to be a chair again?

• 39:47Brian Greene?

  Um, yeah.

• 39:48Chuck Nice?

  Okay?

• 39:49Brian Greene?

  That, that is the question. That is the deep question.

• 39:51Chuck Nice?

  Right.

• 39:52Brian Greene?

  And for a while Hawking said no, that your particles would come out and they'd have no memory of their earlier configuration. The information that they carried would be lost. But he was ultimately convinced by string theorists and others that that was wrong. The particles will come out and they do carry the imprint of the fact that they were once a chair. And if you measure the particles appropriately, you could reconstruct the chair when those particles come out.

• 40:20Neil deGrasse Tyson?

  Uh, but the chair itself doesn't come out. You have, you have to still make the chair. That's, that, that sounds like a cop out, actually.

• 40:26Chuck Nice?

  It's now an IKEA chair.

• 40:30Neil deGrasse Tyson?

  IKEA is in black holes. Chuck, I love that new theory. All right, Brian, I want a new research paper.

• 40:42Neil deGrasse Tyson?

  I want it on my desk in the morning, Brian. The IKEA hypothesis.

• 40:47Chuck Nice?

  Wow, that's amazing.

• 40:48Neil deGrasse Tyson?

  All right, Chuck, keep them coming.

• 40:49Chuck Nice?

  Okay, let's, let's keep it going. This is from Y. Cos. And Y. says, hello, Brian, hello, Neil. And then he puts in parentheses with a question mark. Chuck? Okay.

• 41:02Neil deGrasse Tyson?

  That ain't right.

• 41:03Chuck Nice?

  Oh, no, what's up?

• 41:04Neil deGrasse Tyson?

  We've been Lord Chuck, we've been King Chuck.

• 41:06Chuck Nice?

  Yeah.

• 41:07Neil deGrasse Tyson?

  All right, all right.

• 41:08Chuck Nice?

  Okay, he says, is it possible to calculate precisely where the universe, where in the universe Earth was in a moment to make sure that we find ourselves on Earth's surface and not in the position that Earth was in when we are time traveling? How would you compensate for the? Oh, wait, and then, wait a minute now. This is where he gets, this is when you know that Y. Cos. Was either thinking too much or maybe might have been smoking a little something. Then he goes, then he goes like this. How would you also compensate for the expansion of the universe when calculating that trajectory? So he's, he's getting it all in.

• 41:52Neil deGrasse Tyson?

  He's all in.

• 41:54Chuck Nice?

  He's all in.

• 41:55Neil deGrasse Tyson?

  So let me, let me, let me toss to Brian here after I say something. So, so in Back to the Future, they kind of, in the original, they kind of avoided that problem. Because when Marty went back in time, he went back in time in a precise whole number of years. Okay? And so Earth would be back where it is when he goes back in time in its, in its orbit, you know, enough for the plot line. But if he went back a week or even an hour, he's not in that parking lot anymore at all. So, so Brian, tell me about all this time travel mechanisms. When in fact, the universe is changing.

• 42:38Neil deGrasse Tyson?

  Why, so you can't just show up in the same place and expect to, to, to.

• 42:43Brian Greene?

  Yeah.

• 42:43Neil deGrasse Tyson?

  You know, to, to be home again?

• 42:45Brian Greene?

  Exactly right. And so when you time travel, you also need to space travel. It's really space time travel, right? You have to pick the location in time and the location in space. And typically in these films, they only talk about the time part of things. And to make it concrete, if you're using the wormhole version of time travel, the opening of the wormhole that you're going to exit from, it's at some position in space at some moment in time when you exit. And you better be certain that you have somehow maneuvered that opening to be, say, on planet Earth if when you exit, you want to still be on planet Earth. Otherwise, you could exit back in time, but near the Andromeda Galaxy or just an empty space.

• 43:27Brian Greene?

  So you do need to dial both in in order to get to where you are intending.

• 43:34Neil deGrasse Tyson?

  Okay, interesting. I bet they don't know that, but we accept it anyway, right? As a, as it's implicit, it's implicit, I guess, is what that is.

• 43:43Brian Greene?

  Mm hmm.

• 43:44Neil deGrasse Tyson?

  Yeah. All right, Chuck, keep them coming.

• 43:47Chuck Nice?

  All right, let's go. I, I want to find this young person who.

• 43:51Neil deGrasse Tyson?

  The way you, the way you, you, you described that previous question, it sounds like we should have a cosmic queries only if the people were on weed when they wrote the question.

• 44:00Chuck Nice?

  Oh, man, that's, let me tell you something, that's a great show.

• 44:06Neil deGrasse Tyson?

  Just legal weed, not illegal.

• 44:07Chuck Nice?

  It will either be our best show or our worst show.

• 44:10Neil deGrasse Tyson?

  Or the worst. No, no, it'll, while we're doing it, we'll think it's the best.

• 44:14Chuck Nice?

  Right.

• 44:15Neil deGrasse Tyson?

  And then afterwards, we'll know it's the worst.

• 44:17Chuck Nice?

  Exactly. And that is kind of how that goes. All right, this is Savage 162, and Savage 162 says, "In the frontier of science, is there any idea on how to try and detect the theorized tachyon particle?" So.

• 44:35Neil deGrasse Tyson?

  Ooh, remind us what a tachyon is, Brian.

• 44:37Brian Greene?

  Yeah, so it's what you had mentioned earlier, Neil, this idea of a particle that can go faster than the speed of light. And when a particle can go hypothetical, I should say, a particle goes faster than the speed of light, there are observers watching that particle who will see it reach its target before it was emitted by the source. Reach its target before it was emitted by the source. So, cause and effect become flipped for the observers who are witnessing the motion of a tachyon. So, the question is, how would you ever detect these things?

• 45:12Neil deGrasse Tyson?

  Yeah.

• 45:14Brian Greene?

  And it's a bit, first of all, you know, to measure something going faster than the speed of light, that's actually not that hard. You start with a particle at one location and you fire it and you simply calculate, or you measure, I should say, how long it took to get to the target. Did it beat a light beam or not? And that's it. So, that's pretty straightforward. And indeed, you may recall, I don't know, some 10 years ago, there was a claim-

• 45:41Neil deGrasse Tyson?

  That neutrino thing in Italy, right?

• 45:43Brian Greene?

  Yeah, there's, in Italy, there was a claim that these neutrinos had gone from the source to the target and beaten a beam of light. They'd gotten there earlier than a beam of light would. Now, some of us knew that couldn't be true-

• 45:56Neil deGrasse Tyson?

  So, Brian, you know the joke about that, the Euro joke, because Europeans like joking about each other? They said, "That would've been the first time Italy arrived anywhere early."

• 46:04Brian Greene?

  Yeah, that's right, right. That's exactly right. So, that was the key tip-off immediately, that nothing gets to it. But scientifically, it would've been fascinating. It would've been fascinating. It would've been an example of something going faster than the speed of light. And so, people looked really intently at the data and ultimately realized that there was some kind of loose wire or fiber optic cable or something, I can't remember the details. And when that was repaired, the particles did not go faster than the speed of light. But in principle-

• 46:33Neil deGrasse Tyson?

  But let me add, Brian, because you might not go there, and for the benefit of Chuck, that in my field, and it surely happens in your field, somebody gets a cockamamie observation, which would later be shown to be wrong, but if it's right, it's amazing, and you have these ambulance-chasing theorists coming up with an explanation of, of course, why that must be so. Did that happen in the case of the neutrinos?

• 46:56Brian Greene?

  Oh, gosh, I'm not completely sure. There must have been a few papers because nobody could have fully resisted it within the confines of the entire field. But 99% of the people who encountered that result that I spoke to were like, "It can't be true. It just can't be true." And it wasn't true.

• 47:13Neil deGrasse Tyson?

  And it wasn't true, which is right.

• 47:14Chuck Nice?

  Wow.

• 47:15Neil deGrasse Tyson?

  All right.

• 47:16Chuck Nice?

  Cool. Okay, okay, let's see, here we go.

• 47:20Neil deGrasse Tyson?

  Time for a few more.

• 47:21Chuck Nice?

  Ryan, this is just Ryan, he's like Cher, just Ryan. He says, "Hello, doctors. If we developed a way to time travel backwards, could we travel to a time before the Big Bang? Or would we just break science altogether?"

• 47:41Neil deGrasse Tyson?

  I love it. Brian?

• 47:43Brian Greene?

  Yeah, well, the whole question of before the Big Bang is a deep one. And it could simply be that there's no notion of before when it comes to the Big Bang, because the Big Bang could be the origin of time itself. And so the notion of before makes sense when you're talking about 1800s or 2000 B.C. or whatever. But when you get to the beginning of time itself, there's simply no conception of before. And so there may not even be a realm of reality that we could delineate as before the Big Bang.

• 48:15Neil deGrasse Tyson?

  Right. Is this the same, I think I've heard Hawking, this is Louis Hawking told me this.

• 48:21Neil deGrasse Tyson?

  That going before the Big Bang might be like asking what's north of the North Pole.

• 48:26Brian Greene?

  Yeah, that's a good analogy.

• 48:28Neil deGrasse Tyson?

  The whole grid system is defined there. And you can't, once you're as far north as you can go, you can't go farther north. Once you're as far back in time at the beginning, you can't go further back.

• 48:37Brian Greene?

  Yeah, but I should say that that's only one idea. It's an interesting and provocative one. But there are other approaches which suggest that there is a realm before the Big Bang. There may be many Big Bangs, giving rise to many universes, and our bang may not have been in any sense the first. And so there may be a realm of reality.

• 48:55Neil deGrasse Tyson?

  You'd have to pass out of our sort of spacetime structure and enter somebody else's spacetime if you want to go before what happened with the Big Bang.

• 49:03Brian Greene?

  Yeah, that's right.

• 49:04Chuck Nice?

  It's like a subway line. You reach the end of one line. I mean, you can't go any further. That's it. But you can switch to another subway. And then you can go on from there. But you're in a different thing at that point. Different train.

• 49:18Neil deGrasse Tyson?

  You're a different thing.

• 49:19Brian Greene?

  You're gonna have to pay another fare.

• 49:23Neil deGrasse Tyson?

  And Chuck, the laws of physics might be a little different. So that your body would fall into a pile of goo.

• 49:29Chuck Nice?

  That's what you get for a jump of the turnstile.

• 49:32Neil deGrasse Tyson?

  Oh, oh. Brian, just close this out if you could. What kind of time travel did, in all of your variants, did Charles Dickens invoke for Christmas past and future?

• 49:45Brian Greene?

  It sort of feels like a wormhole-y version, where they're traveling through a wormhole to the past. And maybe they're just hovering at the edge of the wormhole, not actually entering that reality.

• 49:57Neil deGrasse Tyson?

  And if we're not interacting with it.

• 49:59Brian Greene?

  Yeah, therefore, they're sort of more in observer status as opposed to a participant status. It kind of resonates with that, but, you know.

• 50:06Neil deGrasse Tyson?

  But don't you need a different universe now? Because there were some different outcomes, right? So now you've got to split.

• 50:12Brian Greene?

  That's right. And so this would be an example where the wormhole stretches from one universe to another, as opposed to from one universe to the same universe.

• 50:20Chuck Nice?

  Yeah, but the different outcomes were not directly attributable to the time travel. The different outcomes were because of his change of actions in the present, which affected the future.

• 50:35Brian Greene?

  That's right.

• 50:36Chuck Nice?

  So the past never changed. Yeah.

• 50:39Brian Greene?

  I'm trying to remember the story, Brian. That seems right.

• 50:43Neil deGrasse Tyson?

  Yeah, I think Chuck is more current on that.

• 50:45Chuck Nice?

  The past never changed. He changed.

• 50:47Neil deGrasse Tyson?

  The past was, oh, my gosh, is that what the future's gonna be? No, it's not. Because I'm gonna change myself in the present.

• 50:51Chuck Nice?

  Right.

• 50:52Brian Greene?

  Got it, yeah. So, yeah, so the basic idea is imagine three or some number of tunnels emanating from this realm to these other parallel universes. Sort of like slices of bread in a grand cosmic reality, where each slice of bread is its own universe. You just have these tunnels connecting us to them, allowing you to witness what would happen in a world where things were different.

• 51:14Chuck Nice?

  Wow. Damn. That's amazing.

• 51:17Neil deGrasse Tyson?

  All right, we gotta land that plane right there. Brian, it's been a delight to have you back on StarTalk. Always a pleasure. You're just up the street. We should have you on even. We should be like a regular Brian feature, right? Give Brian his own minute. A minute for Brian. We'll market it. Make it two minutes.

• 51:40Chuck Nice?

  Make it whatever it was called, Brian Time.

• 51:42Neil deGrasse Tyson?

  Brian Time.

• 51:46Neil deGrasse Tyson?

  Near the speed of light. Brian Time. Near the speed of light. Oh, yeah. You got it. Guys, great to have you. Chuck, always good to have you there. Always a pleasure. Cosmic queries. I am Neil deGrasse Tyson, your personal astrophysicist. As always, I bid you to keep looking up.