StarTalk Radio - Past, Present, Future: Time Travel with Brian Greene

Episode Date: December 24, 2024

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)NOTE: StarTalk+ Patrons can listen to this entire episode commercial-free here: https://startalkmedia.com/show/past-present-future-time-travel-with-brian-greene/ Subscribe to SiriusXM Podcasts+ on Apple Podcasts to listen to new episodes ad-free and a whole week early.

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 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. Yeah. In case that matters. Let me say it to you this way. Neil, it's your kids. They're in trouble. Thank you, Doc Brown. Realize when they went into the year 2015, just thought I'd remind you of that.

Wow. Wow. Yeah, yeah, yeah. We'll get back of that. Wow. Wow. Yeah, yeah, yeah. We'll get back to that. So, while 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. How you doing? Returning champion. Our returning champion, Brian Green, ladies and gentlemen Brian he's professor of

of physics and math is that right both of those yeah that's right that's right dang isn't that

wait as a layperson isn't that redundant it 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. 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. That's great. And who's the publisher of that? We'll give them a shout out. SEO optimized. Knopf. Knopf. Knopf. Very nice. Very nice. on anything anyone ever puts in. That'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, know of you, through being co-founder, I think with your wife.

Is that correct? Of the World Science Festival? That was 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 didn't know that she never she never mentioned it yeah we go way back right before 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 wrote a bunch of stuff and i think he wrote a tale of two cities is that yeah yeah sure of course yeah yeah it was the best of the worst of times right now we're in the second so written 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 a well before like a half a century before H.G. Wells' The Time Machine. So itself quite a remarkable step to take in storytelling. So Brian, you know, before we had

Einstein's relativity, do you know, how did anybody think about time travel? I mean, Yeah, it's a good question. How did anybody think about time travel? I mean, what was it missing? What, what, what, what, let me say that differently. 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, uh, you know, 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. 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 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. That'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. It's raining physicists. I'm an embarrassment of physicists. Richard. So 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? So what we learn with 1905 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 click 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. 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 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?

It'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, MEO, every time I say middle Earth, 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. 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. So 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, on the Panmac 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 tick off time more slowly because it's in motion. because you would think that clock should tick off 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.

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 observation spot on. And so these effects are truly real. So I didn't know about that experiment. And Brian, please tell everyone. Haefel and Keating were the two scientists.

Tell everyone what Pan Am is? Yes. I'm just saying. That was the southwest of its day. Right. 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. Yeah, super cool. That's very cool. So it turns out, so I didn't know that about the Pan 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. That's true.

You know, Einstein can 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. It's totally nuts. 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. And 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? 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? 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, is 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 stay, 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?

Yeah, without bloodshed. You just have to put your parents in an argument at a time they might have made love. Right. And 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 to be further, Brian. Slow down, Chuck. Slow down. Slow down. 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?

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 moment. There aren't two versions. There aren't two versions of a given moment in time, right?

A moment in time can't change because what would be the parameter through which that change would occur it 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. Ooh. So you are simply... Wait, wait, so Brian, wait, wait, 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. And then she says, no, wait, I can fix that. He goes back in time and prevents her from crossing the street, leaves her, and 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 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. 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 a multiverse. I'm Nicholas Costella, and I'm a proud supporter of StarTalk on Patreon.

This is StarTalk with Neil deGrasse Tyson. so so brian in the case where chuck chuck's uh 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, 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. So 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 it's part of that causal fabric. And that in this approach is the immutable quality of the timeline. Wow. Cool. So it's, it's reverse destiny. Well, it's, 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, all right, 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, well, they describe it to him. So when will I die? You're always dying. When was I born? You're always being born. When, 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. 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 query. Go, go. Brian, stop tempting us. Brian, stop tempting us. Shut up, Brian. Let's act like we don't have an audience. Brian. While's act like we don't have an audience. Brian, stop being so damn interesting.

I really am. And let us get to our Cosmic Queries. Oh, man. Okay. All right. Let me get to the queries. 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. Yeah. Right. So if they have no time, why don't... Who cares whether they move forwards or backwards?

It doesn't make a difference. Right. There is no forward and backwards for a photon. Right. 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 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, 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. There's no notion of aging from the photon's perspective, whatever that actually means. Okay. You heard that. Brian cast shade on the photon. He said it has no emotions, no feelings. cast shade on the photon. He said it has no emotions, no feelings. Basically, Brian thinks photons

are stupid. Okay. That's Brian. All right, Brian. We know where you're coming from. All right. Keep going, Chuck. Here we go. I like this. It'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? Yeah, because, I mean, think about it. If we could all. If you have bad actors going through. Right. 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. 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? Because 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. 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 is 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. So 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? Alright. 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 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? And 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. Well, 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. 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. So, 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 Cabot. Dick Cabot in two different timeframes. 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 um feels like being buried alive but uh excuse me brian black don't crack so that wouldn't be necessary that's true i'm i'm 87 i'm 87 years old right yeah well wait so brian were you in the movie brian were you in the movie i missed it it's like a scene where i I'm in a television set being interviewed in the past

and another television set in the future to set up this time warpy thing. Oh, cool. Damn. Chuck, that's not his first movie. He was 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 Greene. We need expertise on this one. You were great in it. You were like total natural. I loved it.

Oh, thank you. I appreciate that. Okay, so what's the best movie? What's the best movie? Yeah, I would say that Interstellar 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 23 years older, even though McConaughey is only like an hour older. And then Matthew McConaughey, I can't remember his name, his character in the movie. Just to be clear, 23 years older. Older. Not 23 years old. So the guy's gray.

Yeah. Yeah, 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. Right. She's sick in her bed, as you may recall. And he kind of comes into that room. So those are accurate features of Einstein's general theory of relativity. 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. Not because we're friends, but no doubt it was his expertise that kept the film on track, on scientific track, which is kind of a beautiful thing. It's a great movie. Chuck, did you see Interstar? Oh, God, yes. I love it. It's one of my favorite sci-fi movies. Did you see the name of the robot?

It's the big square robot. I forget his name, though. Yeah, the big rectangular robot. 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 has appendages when it wants. And, you know, I forget his name, though. Do you remember the name of the robot? I do not. Brian, do you remember the name of the robot? I'm embarrassed to Brian, do you remember the name of the robot? I'm embarrassed to say I can't even remember the robot. Okay.

The robot's name was Kip. Kip. Oh, there you go. Yeah, just thought I'd tell you. By the way, Kip Thorne, of course, helped pioneer LIGO, Laser Intrinsic Gravitational Wave Observatory, received the Nobel Prize, was the co-recipient of it a few years ago.

And he did the movie. So we got some good people out there. Nice. Trying to raise science literacy of the world a few notches. So, all right, there it goes. So good calculations there. So that's just straightforward gravitational Einsteinian physics the time so that's just that's just straightforward gravitational einsteinian physics yeah that's all that is yeah that's very cool okay so no okay so with respect very quickly you know okay do we have any wait chuck are you a patreon member it sounds

like you're asking your own question i paid this month? You a liar. That's what liars sound like. That's exactly what liars sound like. And that is why I'm a terrible liar because that's exactly what I sound like. I paid. I paid this month. I paid.

All right. So wait. Very quickly. Do we have any information on the aging of cells at the speed of light or in gravitational situations that we might be able to attribute to time travel? 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, it's everything slowing.

So Brian, just to be clear,'s 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 right it's an actual change in the ticking clock that's on your wall. 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 space-time, the fabric of the space-time that you're embedded in. Is that a fair way to say it? It's 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 elapsing for you with time elapsing for somebody far away in a different circumstance that you recognize aha, things have elapsed.

Now, of course, strong gravity might still kill your ass, but that has nothing to do with the time that's ticking. That's right. Amazing. Amazing. Amazing. All right, Chuck, keep going. Okay, here we go. This is Logan Kent, and Logan says, hello, beloved

science professors, and happy holidays from Kansas cities. 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. Nice. And he's hailing from both Kansas cities, KC Moe

and KC Kansas, I guess. That's the plural there, right? Very guess. Yeah, okay. That's the plural there, right? Yeah. Very good. Okay. He says, 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-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? So I like that, but also just slipped in there, Brian, was some mention about information. Yes. And that links to entropy, I presume. So if you can tackle both of those in the next 90 seconds before we go over this. So look, I think 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 in space, a 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 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 at a different place at a different time 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 lost storytelling element there that they could totally do interesting things with, it seems to me.

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. 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 visit the aliens, and then she returns. We like the idea that it's like a water slide. You know, you're in this tunnel, you're in this tube, and you're sliding, but it's really just a simple hole. You step through it, right? It's not some journey. Isn't that correct? Because they're basically in the same place. that correct because they're instant they're basically in the same place 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 okay so all right so if you're gonna do this at least and report on it you if you're in a wormhole and it's propped up nicely and it's safe for you, then 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. 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. Right.

Because the time difference will then persist. Wow. Jesus. That is insane. So, Chuck, in this one broadcast, you've mentioned God and Jesus together. Yeah. This must be a very significant force operating on your brain right now. Instead, I should have said, Father, time.

Ugh. all righty here we go this is jim kelly and jim kelly says hi dr tyson dr green dr comedy 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, patricidal maniacs? Just kidding. Just kidding. But how does a hypothetical paradox preclude the existence of time travel? 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 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. 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 uh 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 And I always hear physicists arguing about whether we lose or gain information every time you do something with a black hole. Yeah. I mean, I like to think of information as more concrete than that description might suggest, because information is always carried by particles. 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... Instead of Louis Hawking, you mean Stephen Hawking? That's right. That's right. They always get those confused.

Different universes. Think of specifying, Brian, which Hawking you're referring to. It was the universe where Louis Hawking actually was responsible, but let's put that to the side. 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. Okay, because, right, because otherwise there's no information without stuff to carry it.

Right. That's another way to say it. It's hard to follow the information without that commitment. So when it comes to, like, 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 is so strong now just a string of particles I come out during the evaporation. Would I be able to be a chair again? Yeah.

Okay. That is the question. That is the deep question. Right. 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. But the chair itself doesn't come out. You have to still make the chair.

That sounds like a cop-out. Wait, it's now an Ikea chair. Ikea is in black holes. Chuck, I love that new theory. Alright, Brian, I want a new research paper on that. I want it on my desk in the morning, Brian.

The Ikea hypothesis. Wow. That's amazing. morning, Brian. The IKEA hypothesis. Wow. That's amazing. All right, Chuck, keep them coming. Okay, 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. That ain't right. We've been Lord Chuck. We've been King Chuck. Yeah. All right. All right. Okay. That ain't right. We've been Lord Chuck. We've been King Chuck. Yeah. All right.

All right. Okay. He says, is it possible to calculate precisely 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 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 getting it all in. He's all in. He's all in. So let me toss to Brian here after I say something. So in Back to the Future, they kind of, in the original, 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 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, Brian, tell me about all this time travel mechanisms when, in fact, the universe is changing. So you can't just show up in the same place and expect to be home again? 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. 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. So you do need to dial

both in, in order to get to where you are intending. Okay, interesting. I bet they don't know that, but we accept it anyway, right, as a it's implicit, it's implicit, I guess, is what that is. Yeah.

All right, Chuck, keep them coming. All right, let's go. It's implicit, I guess, is what that is. Yeah. All right. Chuck, keep them coming. All right. Let's go. I want to find this young person. The way 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.

Oh, man. Let me tell you something. That's a great show. No. Just legal weed, not illegal weed. It will either be our best show or our worst show. Or the worst. No, no.

While we're doing it, we'll think it's the best. Right. And then afterwards, we'll know it's the worst. Exactly. That is kind of how that goes. All right. This is Savage162. And Savage162 says,

Savage162, and Savage162 says, In the frontier of science, is there any idea on how to try and detect the theorized tachyon particle? Remind us what a tachyon is, Brian. 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 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. Reaches 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? 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 neutrino thing in Italy, right? 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. So Brian, you know the joke about that, the Euro joke, because Europeans like joking about each other. They said that would have been the first time Italy arrived anywhere early. Yeah, that's right. Right. That's exactly right. So that was the key tip off immediately. But scientifically, it would have been fascinating. It would have been fascinating. It would have 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. 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... But let me add, Brian, because you might not go there 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? 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. 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.

And it wasn't true, which is right. Wow. All right. Cool. Okay, okay. Let's see. Here we go. Time for a few more.

Ryan. This is just Ryan. He's like Cher, just Ryan. He says, hello, doctors. Time for a few more. 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? I love it.

Brian. 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, you know, 1800s or 2000 BC 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. Right. Is this the same? I think I've heard Hawking, this is Louis Hawking

told me this, that going before the Big Bang might be like asking what's north of the North Pole. 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. 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. So you'd have to pass out of our sort of space-time structure and enter somebody else's space-time if you want to

go before what happened with the Big Bang. Yeah, that's right. 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. You're a different thing. You do have to pay another fare. And Chuck, the laws of physics might be a little different so that your body would fall into a pile of goo. That's what you get for a jump of the turn style.

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? 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. And therefore not interacting with it. Yeah, therefore they're sort of more of an observer status as opposed to a participant status. It kind of resonates with that, but, you know. But don't you need a different universe now? Because there were some different outcomes, right? So now you've got to split. 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. Yeah, but the different outcomes were not directly attributable to the same universe. 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. That's right.

So the past never changed. Yeah. I'm trying to remember the story, but that seems right. Yeah, I think Chuck is more current on that. The past never changed. He changed. The past was, oh my gosh,

is that what the future is going to be? No, it's not. Because I'm going to change myself in the present. Right, got it. Yeah, exactly. 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. Wow. Damn. That's amazing. All right. We got to 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 been a delight to have you back on StarTalk. Always a pleasure. You're just up the street. We should have you on. 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. Make it two minutes. You make it whatever you want. It's called Brian time. Brian time. Near the speed of light. Brian time. Near the speed of light. Oh, yeah. You got it whatever you want. It's called Brian time. Brian time. Near the speed of light. Brian time.

Near the speed of light. Oh, yeah. You got it. There you go. Guys, great to have you. Chuck, always good to have you there. Always a pleasure. Cosmic queries.

I am Neil deGrasse Tyson. You're a personal astrophysicist. As always, I bid you to keep looking up.