Travels in Time

Is time travel possible? Or has it already happened? Get ready to spend some time pondering time, because in this episode of StarTalk, host Neil deGrasse Tyson is joined by co-host J. Richard Gott, author of Time Travel in Einstein's Universe and an expert in exotic astrophysics. Join us as we explore time travel, black holes, and worm holes, and analyze some Hollywood time travel films in terms of their scientific accuracy. Rich enlightens us on the fact that time travel is mathematically possible, citing Einstein’s Theory of Special Relativity to explain the phenomenon. In fact, there’s some time travel going on in the recording studio when Rich explains that Neil is seeing him as he was 3 billionths of a second before…maybe solving the secret to looking younger? Next up on the space-time continuum is the question of whether gravity effects time travel. Rich tells us that on a planet like Mercury, whose gravitational pull from the Sun is so strong, time ticks slower. You’ll hear some discussion about the accuracy of the movie The Time Machine, and how building a time machine like they did, as long as it uses the mass of Jupiter, just may give us enough energy to travel forward in time to find out the results of the next election! Another way to travel through time may be to orbit a black hole, but Rich warns us against getting too close to the point of no return unless we want to be spaghettified. You’ll also hear a little about Hollywood’s take on time travel depicted in Back to the Future, as well as in Planet of the Apes, for which Rich suggests that the first clue that time travel had occurred should have been that the apes were speaking English. The episode gets even more interesting when Neil and Rich take phone calls from fans. They answer questions about the expansion of space, seeing back in time through modern telescopes, and how the fabric of space stretches faster than the speed of light. Investigate wormholes, curved spacetime, general relativity, and all that cosmic craziness. Even crazier, though, may be when Neil recalls a strange memory of being confused for Michael Jackson when he was a kid, sparking his curiosity about whether MJ could’ve traveled back in time to get his old nose back? Rich says that even though we can travel into the past, we can’t age in reverse. Ponder the Grandmother Paradox, and whether time travellers have the ability to change the past. Finally, a fan asks how the past, present, and future coincide with the continuum of time. Rich has plenty of answers, but we’ll leave you with his reference to Einstein: “time plays out like a movie.” So, to make the most of your movie, we encourage you to sit back, relax, and enjoy the show. NOTE: All-Access subscribers can listen to this entire episode commercial-free here: https://www.startalkradio.net/all-access/travels-in-time/?_sf_s=travels+in+time

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

• 0:06Speaker 00

  7.3, 7.6, but I...

• 0:20Speaker 01

  Our universe is filled with secrets and mysteries, leaving us with many questions to be answered.

• 0:33Speaker 01

  Now, more than ever, we find ourselves searching for those answers as the very fabric of space, science, and society are converging.

• 0:44Speaker 01

  Here, for the first time, these worlds collide. As we give you the knowledge that breaks the barrier between what is science and what is merely pop culture. This, this, this is StarTalk. Now, here's your hosts, astrophysicist Dr. Neil deGrasse Tyson and comedian Lynn Koplitz. StarTalk.

• 1:12Neil deGrasse Tyson?

  Welcome back to StarTalk. I'm your host, astrophysicist Neil deGrasse Tyson. And my co-host, Lynn Koplitz, professional comedian, is actually on the road right now. She's performing at the Pittsburgh Improv. You might catch her tonight if you happen to be in the area. And she's also an actress and appears in Xerox on the Independent Film Channel on Sunday nights. Check it out. You can go to Lynn Koplitz's homepage at Lynn-Koplitz.com. That's Lynn with an E. Again, I'm your host, Neil deGrasse Tyson. Welcome back to StarTalk. In lieu of my charming co-host Lynn Koplitz, I snatched from the halls of Princeton University

• 1:57Neil deGrasse Tyson?

  an old-time friend and colleague of mine, Professor J. Richard Gott.

• 2:02Neil deGrasse Tyson?

  Richard Gott, welcome to StarTalk Radio.

• 2:06Speaker 03

  Hi Neil, glad to be here.

• 2:08Neil deGrasse Tyson?

  Richard Gott is one of the world's experts on all kinds of exotic astrophysics. And today's subject is going to be time travel. Is time travel possible? And what sorts of frontier astrophysical phenomena? Black holes, warped space, wormholes? What sorts of astrophysical phenomena enable it or prevent it? That's today's show on StarTalk. Now, as you'd... And Rich, we... Every time when we begin StarTalk, we find out what our good friend, Bill Nye, has to say about black holes and Big Bang and the universe. So, you're gonna hang for the whole hour?

• 2:47Speaker 03

  Sure.

• 2:48Neil deGrasse Tyson?

  And let's see what Bill Nye says to start us off. Check him out.

• 2:54Speaker 04

  Hey, hey, Bill Nye the Science Guy here. As the singer Hank Williams pointed out, we'll never get out of this world alive. For us, the conveyor belt of time goes one way, and we're on it. No way to live forever. And getting out of here? Forget it! Gravity's got us stuck! But since this is StarTalk Radio, we can tell you there is a way out. Find a black hole and fall in. That's a star. Or was a star, so massive that nothing can get out, not even pure energy. Light is sucked in along with everything else. In there, you could end up in another part of the universe, or another universe, at another time. You're still dead, though. This black hole business is not intuitive. And we may have just discovered an intermediate version, and special clouds of interstellar gas called blobs, detectable with X-rays.

• 3:37Speaker 04

  Fundamentally, from our universe, is there a way to see to some other side? There has to be a lot more to the story of stars than we know right now. But StarTalk listeners, you and I, can't help but struggle and strive to understand the world that keeps us alive. Listen on.

• 3:54Neil deGrasse Tyson?

  That was Bill Nye, frenetic as he is in every minute that he gives us. You're listening to StarTalk Radio. You can call us at 1-877-5-Star-Talk if you have a question on the frontier of astrophysics, be it about the Big Bang, the black holes that we've been discovering, wormholes, and in particular today's subject, time travel. So Rich, my guest host for today, Rich, is time travel possible?

• 4:21Speaker 03

  Well, yes. Einstein showed that time travel to the future was possible in his 1905 theory of special relativity.

• 4:30Neil deGrasse Tyson?

  Mm-hmm. And you say that as though you've done it. Yeah, it's possible. Like, yeah, it's possible to go to Baltimore, you know?

• 4:37Speaker 03

  Well, we do have time travelers with us even today. The greatest time traveler up to date is Sergey Krkalev. He's a Russian astronaut. He spent time up on the Mir space station and in a few shuttle flights.

• 4:50Neil deGrasse Tyson?

  Now, the Mir space station preceded the current International Space Station.

• 4:54Speaker 03

  That's right.

• 4:54Neil deGrasse Tyson?

  In case people forgot that, just to include them in, yeah.

• 4:57Speaker 03

  And so the mechanism that he's using there is that Einstein showed that in special relativity that moving clocks tick slowly.

• 5:04Neil deGrasse Tyson?

  Well, how long was he on Mir?

• 5:07Speaker 03

  Well, the total time he spent in orbit was about 803 days.

• 5:11Neil deGrasse Tyson?

  So years, he spent years.

• 5:12Speaker 03

  Yeah, yeah.

• 5:13Neil deGrasse Tyson?

  Yeah, and so the space station, the Mir, in orbit is going like 18,000 miles an hour. So is that fast enough to have measurable effects?

• 5:20Speaker 03

  It slows down your clock just a little. So in fact, he came back after all of his voyages. He was 1/48 of a second younger than he would have been if he'd stayed home.

• 5:34Neil deGrasse Tyson?

  So he traveled to the future 1/48 of a second.

• 5:37Speaker 03

  1/48 of a second. So when he got back, see, he found the Earth to be 1/48 of a second toward the future. Now, that didn't seem like much.

• 5:48Neil deGrasse Tyson?

  No, not only does it not seem like much, it isn't much, Rich.

• 5:52Speaker 03

  Well, um, someone asked me once, "Well, why is it so easy to travel in space, but so hard to travel in time?" And the answer is, we haven't been very far in space either. Um, the furthest we've been in space is to the moon, which Einstein would tell us is 1.3 light seconds away.

• 6:09Neil deGrasse Tyson?

  You would need Ein- you can just calculate that. You would need Einstein to tell you.

• 6:12Speaker 03

  Well, he would tell you that if you're gonna compare distances in space with distances in time, you should use the speed of light.

• 6:19Neil deGrasse Tyson?

  Ah, okay.

• 6:20Speaker 03

  So we say Alpha Centauri is four light years away.

• 6:23Neil deGrasse Tyson?

  The nearest star to the sun, star system.

• 6:26Speaker 03

  So it takes light four years to get here. So we've been 1.3 seconds in light seconds in space and forty eight, 1/48 of a second in time. So, uh, Mr. Krickeloff is like Lindberg. He's like traveled a distance in time equal to going across the Atlantic, basically.

• 6:44Neil deGrasse Tyson?

  Oh, I see. Okay. So he's, that's just the beginning, then, just as Lindberg.

• 6:48Speaker 03

  That's the first step.

• 6:49Neil deGrasse Tyson?

  By the way, Rich, you have a book called Time Travel in Einstein's Universe.

• 6:52Speaker 03

  Yes.

• 6:53Neil deGrasse Tyson?

  Came out a couple of years ago, but it's available in paperback, published by Houghton Mifflin.

• 6:56Speaker 03

  Right.

• 6:57Neil deGrasse Tyson?

  And we'd like to actually, uh, give away some free copies of that book to callers whose questions may have earned the right to receive a free book. We'll be listening for your calls later in the show. So, we got Kirk- Krickeloff, Krickeloff?

• 7:11Speaker 03

  Krickeloff.

• 7:12Neil deGrasse Tyson?

  Krickeloff.

• 7:13Speaker 03

  Sergey Krickeloff. Now, the previous guy who had the record was Sergey, Sergey Avdeev. So, he was a 50th of a second. So apparently, it's good to be named Sergey.

• 7:23Neil deGrasse Tyson?

  So the Russian, so we don't have any Americans are in the running for this, I guess.

• 7:27Speaker 03

  Not, not in the running currently.

• 7:29Neil deGrasse Tyson?

  So we're nicer to our people by not leaving them in orbit for years and forgetting about them.

• 7:33Speaker 03

  I think that's right.

• 7:34Neil deGrasse Tyson?

  So, do you have like a, a, a sort of layman's account for how you can actually uh, travel into the future using Einstein's rules? What is it about the physics of relativity that enables this?

• 7:45Speaker 03

  Well, we all know that Einstein was a genius. So, well, what did he do that really made him a genius? Well, special relativity, he built off of two postulates. One was that motion is relative. And you've seen this when you're on an airplane going at 500 miles an hour and the windows are down, you're watching some bad movie, and, and it seems just like you're on the ground.

• 8:06Neil deGrasse Tyson?

  Yeah.

• 8:07Speaker 03

  In fact.

• 8:07Neil deGrasse Tyson?

  If there's no turbulence, if there's no turbulence.

• 8:09Speaker 03

  If there's no turbulence and you're not turning. Uh, in fact, sometimes you actually are on the ground. So.

• 8:15Neil deGrasse Tyson?

  Most, most times lately.

• 8:17Speaker 03

  Well, that, that was it, that it's motion is relative. You can't tell whether you're moving or not. He thought that, that applied to Newton's laws of physics and he thought that should apply to all the laws of physics, including electrodynamics. The second.

• 8:30Neil deGrasse Tyson?

  Electrodynamics is study of, uh, uh, properties of light, basically.

• 8:33Speaker 03

  Yeah.

• 8:34Neil deGrasse Tyson?

  Light and particles interacting with them, yeah?

• 8:36Speaker 03

  Yes, and so the second postulate was that the speed of light should always be observed by you. A light beam should pass you at always the same speed, three hundred thousand kilometers a second. And that was because Maxwell showed that the speed of light was, could be calculated from a ratio of magnetic to electric forces. And when he got that velocity, he said, eureka, this is the velocity astronomers have already measured to be the velocity of light. So I know that electromagnetic waves, which I've just calculated.

• 9:06Neil deGrasse Tyson?

  Light.

• 9:07Speaker 03

  Is light.

• 9:08Neil deGrasse Tyson?

  Okay.

• 9:08Speaker 03

  So, uh, Einstein thought that we should see that always at that speed. If we didn't, we would know we were moving. If we saw it going by two hundred thousand kilometers a second, we say, hey, I'm going by, I must be moving at a hundred thousand. So, and that wasn't allowed by the first postulate. So with those two postulates, he proved a lot of theorems that you could then check. He, he did thought experiments. So one of the experiments that he thought of was.

• 9:34Neil deGrasse Tyson?

  So these, as a theorist without a budget to create a lab, he thinks up experiments in his head.

• 9:37Speaker 03

  He thought up the experiment. Nobody really done anything like this before. He thought up an experiment. And one of the experiments was, he said, suppose I have a light clock and I just have a mirror at the bottom and a mirror at the top. I'll bounce a light beam back and forth.

• 9:51Neil deGrasse Tyson?

  This is a mirror on the floor and a mirror on the ceiling.

• 9:54Speaker 03

  That's right.

• 9:54Neil deGrasse Tyson?

  Start a beam of light bouncing between the two and there it goes.

• 9:57Speaker 03

  And it'll take, uh, uh, a number of nanoseconds to go from the top to the bottom.

• 10:02Neil deGrasse Tyson?

  Billionths of a second.

• 10:03Speaker 03

  Yes, if it's three feet tall, your, your clock will take three nanoseconds to, to, to beat.

• 10:09Neil deGrasse Tyson?

  A, a foot per nanosecond.

• 10:11Speaker 03

  That's the velocity of light. It's very easy to remember in those.

• 10:14Neil deGrasse Tyson?

  Yeah, yeah. So you're sitting now for me, three billionths of a light second away, three billion, three billionths of a second.

• 10:22Speaker 03

  Yes.

• 10:22Neil deGrasse Tyson?

  Away from me.

• 10:23Speaker 03

  In fact, you're seeing me not as I am now, but as I was three billionths of a second ago.

• 10:28Neil deGrasse Tyson?

  And you look marvelous.

• 10:29Speaker 03

  In the past. I'm a bit younger than I really am. I look a bit younger. The further you move away, the younger you look.

• 10:40Speaker 03

  So this is something to remember. Anyway, the, um, you have the light clock and then-

• 10:45Neil deGrasse Tyson?

  So I'm in this closed room and I see the light beam just going back and forth and I can count, I can keep time this way.

• 10:49Speaker 03

  That's a good clock. Now suppose there's an astronaut moving by you at four fifths the speed of light, zipping by, shoo, from left to right. Now he's got a light clock too, just like yours. And, and he sees it going up and down in the light clock from the floor to the ceiling, in his room, but his room's moving relative to yours. So when you look at his light beam, you see it go from the bottom, down here over on the left, to the upper right as he's moving from left to right. By the time he gets to the top, he's moved over to the right. So that light beam has gone on a longer diagonal path as far as you're measuring it.

• 11:27Neil deGrasse Tyson?

  It's gone on the hypotenuse of that triangle.

• 11:29Speaker 03

  The hypotenuse of the triangle.

• 11:31Neil deGrasse Tyson?

  We all remember the hypotenuse.

• 11:33Speaker 03

  We all remember that. And it's longer than one of the sides. So-

• 11:38Neil deGrasse Tyson?

  By the way, you're listening to Professor Rich Gott from Princeton University, a colleague of mine from the Department of Astrophysics there, talking, talking cosmic stuff. Go on, he's explaining how you can move into the future using relativity. So go on.

• 11:52Speaker 03

  So, so when I look at his light clock, his light clock has got to tick slow relative to mine. If he's moving at four fifths the speed of light, that diagonal path is five feet that I measure. So I must say it takes five nanoseconds for his clock to tick. So I must see him, his clock ticking at three fifths the rate of my clock. Now here's the interesting part. All of his clocks have to tick slower by that same factor.

• 12:21Neil deGrasse Tyson?

  Not just his light clock.

• 12:23Speaker 03

  Not just his light clock. His heart has to beat slower, otherwise he'd notice that relative to his heart, which is also a clock, that his light clock is ticking slow. And so because he can't know whether he's moving or not, uh, he has to, he has to age more slowly too.

• 12:40Neil deGrasse Tyson?

  So you're saying Einstein's two postulates require that he's aging less if his two postulates are correct?

• 12:46Speaker 03

  That's right.

• 12:46Neil deGrasse Tyson?

  And low and behold, experiments show that in fact, this is the case.

• 12:49Speaker 03

  They do. They took atomic clocks on airplanes, flew them east around the world where the velocity of the plane would add to the rotational velocity of the earth, and they came back slow by about 50 nanoseconds, which is what Einstein would have predicted.

• 13:05Neil deGrasse Tyson?

  And these clocks are accurate enough to measure that. When clearly you're a Russian astronaut, you could not have said, "Or let's check your heartbeat, see if it's 1/48th of a second too slow." You couldn't have done that.

• 13:14Speaker 03

  That's right.

• 13:14Neil deGrasse Tyson?

  So it's really the calculation that tells you that he went into the future by that much.

• 13:18Speaker 03

  But the implications of this are large, because if you get on a spaceship and you go at 99.995% the speed of light.

• 13:28Neil deGrasse Tyson?

  Mm-hmm.

• 13:30Speaker 03

  Out to a star 500 light-years away, and then you turn around and you come back. When you get back, the earth will be 1,000 years older, but you will have only aged 10 years.

• 13:41Neil deGrasse Tyson?

  This is how you move into the future. Now, who would want to do that, because then everyone would have just forgotten about you.

• 13:45Speaker 03

  Well, I think that'd be really interesting, you know.

• 13:49Neil deGrasse Tyson?

  So you would do this?

• 13:50Speaker 03

  Oh, sure.

• 13:52Neil deGrasse Tyson?

  Rich, we love you here in the present, Rich.

• 13:53Speaker 03

  But the year of 3,000 would be really interesting.

• 13:56Neil deGrasse Tyson?

  Okay. You would hope, if Earth is still here, if humans are still here.

• 14:03Speaker 03

  It took Marco Polo 24 years to go visit China and come back to Europe and tell people about it. And so if you went on an accelerating rocket ship, you could accelerate it, 1G acceleration, perfectly comfortable. It'd take you about 24 years going out and back on your, as far as your clock was concerned, you'd ride back, the earth would be 1,000 years in the future.

• 14:24Neil deGrasse Tyson?

  We're going to get back to this, because we know that there have been movies we've all seen that involve time travel. And I want to get from you your expert view on whether those movies got it right or wrong, or what was fun, or what irked you when you saw them. We're going to take a break from StarTalk Radio. I'm Neil deGrasse Tyson, your host. And call us and tell us what your favorite time travel movie has been, or whether you want to travel into the past or the future, if you had that choice. We're going to learn how to travel into the past as well, and learn all about what kinds of paradoxes that might create, as we've learned from films as well. We'll be back after a break. 1-877-5-StarTalk is our number.

• 15:12Speaker 01

  We want to hear from you. The phone lines are open. Call now. This is StarTalk.

• 15:23Neil deGrasse Tyson?

  That would be 1-877-5-Star-TALK. If you have a question about time travel, wondering whether it's possible, or you have a question for my host, my co-host for today's episode, and he is Professor J. Richard Gott from Princeton University. He's professor of astrophysics there. He's written a book called Time Travel in Einstein's Universe. So, we're all about that today. But first, let me ask you. You're from Princeton. I noticed you have a distinct accent from south of the Mason-Dixon line. What, where's that from?

• 15:52Speaker 03

  I'm from Louisville, Kentucky, originally.

• 15:55Neil deGrasse Tyson?

  Louisville.

• 15:56Speaker 03

  Louisville. There are many different ways to pronounce it.

• 15:59Neil deGrasse Tyson?

  Louisville, Kentucky. How many other Kentucky astrophysicists are there?

• 16:04Speaker 03

  Well, Hubble actually spent a little time in Louisville, teaching, teaching high school, early in his career.

• 16:12Neil deGrasse Tyson?

  But he surely did not speak with a Kentucky accent.

• 16:13Speaker 03

  No, he kind of had an English accent after he spent some time in England.

• 16:18Neil deGrasse Tyson?

  It sounded like he enjoyed faking that English accent.

• 16:20Speaker 03

  My accent hasn't changed that much since I've been in New Jersey.

• 16:24Neil deGrasse Tyson?

  People might remember that Princeton is where Einstein spent out his later years. He came to America and hung out at Princeton. He's still revered there, of course, right? Is that right?

• 16:34Speaker 03

  Oh, sure. At the 50th, we had the 50th anniversary of his 1905 theory of special relativity, and they, they actually put up a bust of Einstein in the, on the, off the main street there in Princeton.

• 16:49Neil deGrasse Tyson?

  Okay. Did people genuflect in front of it, or did they?

• 16:51Speaker 03

  Princeton people loved Einstein. He would eat at the restaurants, and he would, he would go into the flower shop and order like 17 roses, you know?

• 17:00Neil deGrasse Tyson?

  Just for all his girlfriends? I heard he was the ladies' man. Is that?

• 17:04Speaker 03

  Well, he did order, he did order flowers at the flower shop, and the trouble was that, that no one would cash his checks, because a check signed by Einstein was worth more than the amount. So, finally, he said, "Listen, could I clear up my checkbook? I'll give you as many autographs as you want, if you'll just cash the checks."

• 17:23Neil deGrasse Tyson?

  So, there are Einstein t-shirts? So, are there soon will be like Richard Gott t-shirts in Princeton, do you think?

• 17:29Speaker 03

  Well, I haven't seen one yet.

• 17:34Neil deGrasse Tyson?

  So Ritz, tell me, the, uh, not only can you, uh, time travel by moving fast, uh, out of Einstein's theories, general theory of relativity, gravity, extreme gravity can also affect your time travel, that's, that's true as well. So any examples of that that you can cite?

• 17:50Speaker 03

  Well, if you, if you go deep in a gravitational well.

• 17:55Neil deGrasse Tyson?

  Just closer to a source of gravity.

• 17:56Speaker 03

  Closer to a source of gravity. For example, if you were on the planet Mercury, your clock would tick slightly slower there, both because Mercury is orbiting around the sun.

• 18:07Neil deGrasse Tyson?

  Really fast.

• 18:07Speaker 03

  Rapidly, more rapidly than the Earth, but also because Mercury is down, hunkered down in the gravitational well of the sun. So when, when photons come out from Mercury to us, they are, they lose energy, uh, because they got to climb out of this gravitational well, and that lengthens their wavelength, and it means that they come, the, the waves come in from wavelength to wavelength more slowly. So you see everything going in slow motion, like, so I'm talking to you very slowly from Mercury.

• 18:40Neil deGrasse Tyson?

  Had, if you were a Mercury resident and didn't vaporize in the heat of the sun, that's what you would sound like?

• 18:45Speaker 03

  A little slowly. So if we sent an astronaut to live on Mercury for 30 years and then brought him back to Earth, they'd come back about 22 seconds younger than they should have been, or they'd travel 22 seconds into the future.

• 19:00Neil deGrasse Tyson?

  So if, uh, ignoring the speed version of that, uh, time travel, just the being in a deep gravity well, he can just sit there and age more slowly?

• 19:09Speaker 03

  That's true.

• 19:09Neil deGrasse Tyson?

  So is any movie that sort of captures that aspect of it?

• 19:12Speaker 03

  Well, yes, there was a movie, um, uh, The Time Machine in 2001.

• 19:18Neil deGrasse Tyson?

  H. D. Wells, The Time Machine.

• 19:19Speaker 03

  It's, it's the same story of H. D. Wells, basic, with a few additions, but basically the same story. And he sits in this sort of spherical time machine, and, um, he just notices that the, uh, the, uh, world outside is going by rather quickly, and namely, he's going slowly through time, he's aging more slowly. Um, now you could really build one like this, about that size, it's depicted in the movie, if you put yourself inside a shell of mass that was about the mass of Jupiter. Now, the, you would age four times more slowly than the people outside, so it's a good way to hurry to the next election, you know? You could get there quicker.

• 19:59Neil deGrasse Tyson?

  You could get there one year instead of four years.

• 20:02Speaker 03

  Yeah, it'd only take you one year to get there.

• 20:04Neil deGrasse Tyson?

  So that's just Jupiter, but you imagine a black hole would accelerate this, right?

• 20:07Speaker 03

  Well, yes, the, if you wanted to go millions of years into the future, as the movie portrays him going 800,000 years into the future, for that one, you need about 100 million solar masses, and it's gonna be about the si- the shell of mass is gonna be about the size of the Earth's orbit, and you're gonna sit in there and age more slowly, and, uh, the other way to do is-

• 20:28Neil deGrasse Tyson?

  And by the way, it's one thing to say you age more slowly, but what's actually going on is, you think you're do- you're being normal, and everything around you is moving faster.

• 20:36Speaker 03

  That's right, and, and one of the-

• 20:37Neil deGrasse Tyson?

  We got a phone call. I think we have Michael on the line from Los Angeles.

• 20:40Speaker 05

  Yeah.

• 20:41Neil deGrasse Tyson?

  Is that you? How you doing? Welcome to StarTalk.

• 20:44Speaker 05

  Pretty good.

• 20:45Neil deGrasse Tyson?

  Yeah.

• 20:46Speaker 05

  You guys, you sort of answered my question, I guess. What I, as you approach heavier gravitational sources, I, my question is, is the physical ramifications that, you know, if you were on Mercury, your body's gonna have to expend more energy, and, i.e. that, you know, parts of your body's gonna break down faster, muscles and all that, so I would think physically, well, obviously, on Mercury, or, but I'm even, I guess I'm saying, how do you counteract that physical force that would be detrimental to your body?

• 21:16Neil deGrasse Tyson?

  Plus, there's a little bit about the fact that the sun would vaporize you on the surface of it.

• 21:19Speaker 05

  Right, right. But I guess it's, it's, is it the gravity that actually, it does this, or is there, uh, you know, like Einstein said, the, the fabric of that universe and stuff is being stretched?

• 21:30Neil deGrasse Tyson?

  Michael has an excellent question. So, Rich, what he, what he's asking is-

• 21:32Speaker 05

  Thank you.

• 21:33Neil deGrasse Tyson?

  Thank you, Michael. What he's asking, is it something physiological about the heavier gravity that your body's reacting to, or is there something deeper than that?

• 21:44Speaker 03

  Well, you think everything's going normally. We're gonna put you in a good space cabin, you know, to live. We're gonna protect you from the heat of the sun and that. But the thing that's gonna happen to you is that as you live out your life, if you send signals out to me, those are gonna lose energy as they come out. So their wavelength is gonna be longer. So I'm gonna look at you and see everything happening in slow motion. And you're gonna look back at the Earth and see everything happening a bit fast. So the time travel movie that we were talking about, where he was seeing thousands of years go by on the surface of the Earth,

• 22:29Speaker 03

  and he was aging only very slowly, those visible photons coming in, showing him the visible light coming in, would be shifted way into the ultraviolet and into the X-rays. So he'd be fried by looking at the vicinity of the Earth.

• 22:47Neil deGrasse Tyson?

  They missed that part of the movie.

• 22:48Speaker 03

  They missed that part, and also the part that his spaceship would have been so massive that it would have torn the Earth apart. So I do not expect to find people building time machines in their garage.

• 22:59Neil deGrasse Tyson?

  Now, have you ever consulted on a time machine movie? I mean, now that you're author of Time Travel and Einstein's Universe.

• 23:07Speaker 03

  I've consulted on other movies, and the thing is, they usually ask for your advice, and then they don't follow it.

• 23:17Speaker 03

  Because the plot needs to be spicier, so...

• 23:23Speaker 03

  That's the trouble.

• 23:25Neil deGrasse Tyson?

  So tell me about how black holes would play into this. If I orbit a black hole real close, I can be moving fast, and I'm in a deep gravitational well, so how different is the time there?

• 23:35Speaker 03

  Well, you can slow down your clock by just hanging out in the vicinity of the black hole. Don't go in. Don't go past that point of no return. But you hang out outside, and you can get quite a bit toward the future doing that. So that's a good way to go toward the future. You wanna get a big black hole, so that the tidal forces aren't too bad.

• 23:57Neil deGrasse Tyson?

  Otherwise, you'd be ripped apart.

• 23:58Speaker 03

  You don't wanna be ripped apart. But we got three billion solar mass black holes for the tidal forces at the edge. Quite fun.

• 24:06Neil deGrasse Tyson?

  We've got those in the centers of galaxies.

• 24:07Speaker 03

  Yes, yes.

• 24:08Neil deGrasse Tyson?

  So we can send you there and report back.

• 24:10Speaker 03

  We have to go there and then hang around.

• 24:13Neil deGrasse Tyson?

  You're listening to StarTalk Radio. I'm your host, Neil deGrasse Tyson, with my guest co-host this week, Professor J. Richard Gott III, if I remember correctly.

• 24:21Speaker 03

  That's right.

• 24:22Neil deGrasse Tyson?

  Professor of Astrophysics at Princeton University. He's telling us about time travel, the astrophysics of it. You can call in if you have a question about black holes, the universe, the Big Bang, ways of traveling forward or backwards in time. We're at 1-877-5-StarTalk. You can also hear us streamed on StarTalkRadio.net, or you can tweet us. If you have a tweet, send it to StarTalkRadio. We're there. So, Rich, there's other movies where they go into the future.

• 24:52Speaker 03

  See, if you're coming from a black hole, it'd be stretched out. Tweet.

• 24:56Neil deGrasse Tyson?

  Oh, tweet.

• 24:57Speaker 03

  We get one like that, they're coming from a black hole.

• 25:00Neil deGrasse Tyson?

  Oh.

• 25:01Speaker 03

  Very slow tweet.

• 25:04Neil deGrasse Tyson?

  Anyone from the future or from the bad... yeah, tweet us and we'll know by the frequency of your sound. Tell me about, um, one of everyone's early favorite time travel movies was Planet of the Apes. Is that just a simple time travel into the future movie?

• 25:16Speaker 03

  That's the simplest kind. Uh, the astronaut, Charlton Heston, he goes in a spaceship, you know, it gets going too fast, near the speed of light. He doesn't really know that. He comes back, he lands on the Earth.

• 25:30Neil deGrasse Tyson?

  Well, he doesn't know it's Earth.

• 25:31Speaker 03

  He doesn't know it's Earth.

• 25:32Neil deGrasse Tyson?

  Yeah, yeah.

• 25:32Speaker 03

  He thinks it's just landed on another planet because there's these apes running everything, you know? And then, of course, I had to give away the end of the movie, but he...

• 25:41Neil deGrasse Tyson?

  But what would have cued him in is that the apes are speaking English.

• 25:45Speaker 03

  That would be a hint. You'd think he would pick up on that, you know? Why are the apes speaking English? How do they know English? But he, but he finally, at the end of the movie, he, he encounters the ruins of the Statue of Liberty, and then he says, "Oh, my."

• 26:00Neil deGrasse Tyson?

  So this is simple forward time travel using Einstein?

• 26:03Speaker 03

  Easy to do. We know you can do this. It's just a matter of money. You gotta increase the NASA budget by a lot.

• 26:08Neil deGrasse Tyson?

  So you get to move it really fast. We've got another call. John, is that you in LA?

• 26:14Speaker 06

  Yes, it is.

• 26:14Neil deGrasse Tyson?

  How you doing? What questions you have for us today?

• 26:16Speaker 06

  Oh, I, I have a question which has troubled me for a long time, and I'm hoping that somebody can answer it.

• 26:23Neil deGrasse Tyson?

  Bring your troubles onto us.

• 26:25Speaker 06

  I, I understand the universe to have at the time of the big bang been a very, very tiny point. And then we had the big bang, and since the big bang, according to modern science, I believe, thirteen point seven billion years have passed.

• 26:42Neil deGrasse Tyson?

  You got it.

• 26:43Speaker 06

  And with modern telescopes, uh, we are able to see probably more than ninety percent back in time to, uh, to galaxies that are on the very edge of, practically at the edge of the universe.

• 26:59Neil deGrasse Tyson?

  You got it.

• 27:00Speaker 06

  Those galaxies are perhaps thirteen billion light years away.

• 27:05Neil deGrasse Tyson?

  You, you can teach Astro 101 at the rate you're going, sir. It's not clear you will have any question at all at the end of this.

• 27:12Speaker 06

  Here is my question, sir. Starting from a point in thirteen, um, how did those galaxies get thirteen billion light years away in approximately, um, thirteen, uh, uh, thirteen billion light years away so quickly?

• 27:31Neil deGrasse Tyson?

  Okay, so Rich, so he's asking-

• 27:33Speaker 03

  This is a very good and very deep question.

• 27:36Neil deGrasse Tyson?

  And is there time travel going on with these galaxies?

• 27:38Speaker 03

  Well, what's hap-

• 27:39Neil deGrasse Tyson?

  Yeah, thank you, John, for that question. Rich, what do you have?

• 27:42Speaker 03

  Well, what's happening here is that in the very early universe, the, the, the galaxies are moving apart from each other because the space between the galaxies is stretching, like a big sheet of rubber. And so, uh, we see the galaxies with time getting further and further away from us because this sheet of rubber is stretching. So, at the beginning of the universe, the universe is stretching so fast that the light can't travel between the two points. So, it's the li- the, the space is stretching between those two galaxies faster than light could cross the distance between them. Einstein's theory says you can't go faster than the speed of light, someone can't pass

• 28:25Speaker 03

  you faster than the speed of light, but it never says that the space between you and someone else can't stretch faster than the speed of light. So, that's how they get out there for us to see.

• 28:36Neil deGrasse Tyson?

  So, you're talking about the fabric of the universe expanding at, with any arbitrary speed according to the tenets of the big bang?

• 28:41Speaker 03

  Cause little pieces of it are just stretching a little, but you put a lot of them together, and that makes the distance between the two stretch faster than the speed of light.

• 28:50Neil deGrasse Tyson?

  Well, I'm ready to start giving away your book, actually. In fact, I wanted to give it to John, but he hung up before we could get his address. In fact, if he calls back, I think we can, gives us the address, I think we'll give him a book. Because that was a good question, you admit.

• 29:00Speaker 03

  That was a great question.

• 29:01Neil deGrasse Tyson?

  John, if you're out there, let us know. We're going to take a break, and we're going to come back to StarTalk Radio. If you want to call us, if you have a question, a comment about what your favorite time travel movie is, a question for my co-host for this episode, Jay Richard Gott, professor of astrophysics, give us a call at 1-877-5-Star-Talk. We're streamed on StarTalkRadio.net, and you can tweet us at StarTalkRadio. See you in a bit.

• 29:26Speaker 01

  The future of space and the secrets of our planet revealed.

• 29:35Speaker 01

  This is StarTalk.

• 29:40Neil deGrasse Tyson?

  Welcome back to StarTalk. I'm your host, Neil deGrasse Tyson, astrophysicist, with a friend and colleague of mine, Professor Jay Richard Gott, professor of astrophysics at Princeton University, as guest co-host, substituting for Lynn Koppelitz, who's actually doing stand-up comedy tonight at the Pittsburgh Improv. Check her out if you're in town. So, Rich, I've got a couple of questions for you. Are your students getting smarter or dumber lately? We hear all these complaints about America getting dumb. When you teach astrophysics, how do you feel about them over the years?

• 30:12Speaker 03

  I think people today are just as smart as they were before. About the same is the answer.

• 30:19Neil deGrasse Tyson?

  Has there been, like, an Einstein in your class that you could identify as somebody...

• 30:24Speaker 03

  Well, they come along more rarely than that.

• 30:28Neil deGrasse Tyson?

  Okay, I was just curious, you know. I mean, you know, I used to be in the professor role, but now I don't see a new crop of students every year. But if the trend line is about stable, that's a good sign, I think. So, let me ask you, we were talking about time travel, and not to be too topical, but, you know, we lost Michael Jackson recently. He and I are the same age, by the way. I don't know if you know, but he and I were once confused for each other, back when I was 13 years old.

• 30:54Speaker 03

  Well, that's interesting.

• 30:56Neil deGrasse Tyson?

  I was left less sort of beefed up back then before I started doing athletics. So I was kind of a skinny kid and we had the same sort of skin tone and had the same Afro of the day. And three girls, I was in a, in a, in a, in Gimbel's, the department store. And these three girls were like drawing straws as he would walk up to me to see if I was Michael Jackson. And one of them came up and I had to disappoint them.

• 31:20Speaker 03

  Well, you must have looked very self-confident.

• 31:22Neil deGrasse Tyson?

  Certainly before all the surgery. I'm wondering if it's one thing to send your whole body forward and back in time, but could like, Michael Jackson get his old nose back or something? Is there a way to do something like that?

• 31:34Speaker 03

  Well, no. When you, when you, the time traveler, even though you go visit the past, you're always going forward into the future.

• 31:43Neil deGrasse Tyson?

  Into your own personal future.

• 31:44Speaker 03

  Into your own personal future. So you keep aging. It's like Magellan, in general relativity, we have curved spacetime. So it's like Magellan's crew left Europe, they went west, west, west, around the world, and they came back to Europe again. So the time traveler goes forward always toward the future as far as he's concerned, but circles back through twisted curved spacetime.

• 32:09Neil deGrasse Tyson?

  Like the surface of the earth was.

• 32:10Speaker 03

  To, just like the surface of the earth was, to an event in his own past. So you can visit the past, but you'll be older when you get back there.

• 32:18Neil deGrasse Tyson?

  You plushed your nose and everything else that Michael Jackson might have wanted back.

• 32:22Speaker 03

  You don't age in reverse when you go back in time.

• 32:25Neil deGrasse Tyson?

  I got you. Now, tell me about wormholes. We've always heard about, you know, in Contact, Jodie Foster looks like she travels through a wormhole. There's the warp space in Star Trek IV. Let's go back in time. How's that possible?

• 32:38Speaker 03

  Well, in general relativity, this is Einstein's theory of curved spacetime to explain gravity. And we believe this because it's been checked by light bending around the sun. We've done experiments that have checked this.

• 32:51Neil deGrasse Tyson?

  That means we know it. We don't have to just believe it.

• 32:53Speaker 03

  That's right. It's past the experimental test. So, general relativity, you have curved spacetime. So, there was a famous poem. There was a young lady named Bright. She traveled far faster than light. She left one day in a relative way and returned home the previous night.

• 33:10Neil deGrasse Tyson?

  Okay, there she goes.

• 33:12Speaker 03

  So we always knew that if you could go faster than the speed of light somehow, you could go back in time theoretically.

• 33:19Neil deGrasse Tyson?

  Mm hmm.

• 33:19Speaker 03

  But Einstein showed that you couldn't build a rocket that would go, that would pass someone at faster than the speed of light. That's because if you were in the rocket and you shot a laser beam toward the front of the rocket, it would never catch up to the front of the rocket.

• 33:34Neil deGrasse Tyson?

  It would go backwards in your rocket, right?

• 33:35Speaker 03

  That's right. It would hit the back end of the rocket. And so you'd know you were moving, and that's not allowed by that first postulate. So you can't build a rocket that goes faster than the speed of light, but if you can take a shortcut by jumping through a wormhole and get to a distant place in space.

• 33:53Neil deGrasse Tyson?

  Effectively faster than light would have.

• 33:54Speaker 03

  Then you beat a light beam there.

• 33:56Neil deGrasse Tyson?

  Is that what they did in Star Trek IV?

• 33:58Speaker 03

  Um.

• 33:58Neil deGrasse Tyson?

  Star Trek IV, just to remind you, you know, there's a current Star Trek movie out. I saw it and I loved it. Star Trek IV was like the Save the Whales movie. Remember that one?

• 34:07Speaker 03

  Save the whales.

• 34:08Neil deGrasse Tyson?

  They go back to 1984 because there's a whale sound in their future time and the aliens picked it up and they're waiting for the whale to reply. And then no whales left because we hunted them to extinction. And so there, there's a problem.

• 34:25Speaker 03

  There's a problem. And so what they did, of course, was they went back in time. They got a whale. They brought it to the 23rd century.

• 34:34Neil deGrasse Tyson?

  Much to the whale's surprise.

• 34:36Speaker 03

  And the whale answers the extraterrestrial.

• 34:39Neil deGrasse Tyson?

  Well, you got to bring two whales.

• 34:40Speaker 03

  You know.

• 34:40Neil deGrasse Tyson?

  You need two whales.

• 34:41Speaker 03

  Well, they brought two.

• 34:42Neil deGrasse Tyson?

  Yeah.

• 34:42Speaker 03

  But anyway, it answered the extraterrestrial with a whale, you know, song. And so everything was fine. My theory of that movie was always that the trouble started when Carl Sagan sent on, on one of the Voyager spacecraft, a record with everyone on Earth saying hello, hola, and all the different languages. And they sent the whale song. And this was a big mistake because we don't know what the whales are saying.

• 35:10Neil deGrasse Tyson?

  We know what each of ourselves are saying, even in our multiple languages. But what are you worried about? It's just a whale song.

• 35:16Speaker 03

  Well, they could be saying, help, the humans are hunting us to extinction. Come and save us, which I think was the basic plot of this movie. So they, they went to the past, they got the whales, and everything was fine. This is totally self-consistent.

• 35:30Neil deGrasse Tyson?

  Okay, alright. And so that, so that movie was, was okay. That one, they, they did it right. Much, but the whale is a little surprised to be yanked out of the present.

• 35:39Speaker 03

  And you have a whale that disappears from San Francisco one day, and they say, "Where'd the whale go?" And that's about it.

• 35:46Neil deGrasse Tyson?

  And that's about it.

• 35:47Speaker 03

  That's okay.

• 35:47Neil deGrasse Tyson?

  So, could it be that there are people who have just disappeared that are sort of open cases in the, in police log? That you think someone has zapped them into the, like Jimmy Hoffa. No one can find Jimmy Hoffa. Is he in the future?

• 36:00Speaker 03

  Maybe they should be looking for him in the 23rd century. In, in New Jersey, in the 23rd century.

• 36:09Speaker 03

  Be a good place to hide.

• 36:11Neil deGrasse Tyson?

  So wormholes, do they exist? Can we make it? Well, let me, I gotta give out the phone number here, because I can't be the only one wondering how to ask questions about this. You're listening to StarTalk. I'm your host, Neil deGrasse Tyson. You've got a question for my co-host for this session, Jay Richard Gott, professor of astrophysics and expert on everything cosmic, but especially on time travel. Author of Time Travel in Einstein's Universe. Give us a call at 1-877-5-Star-TALK. So tell me, is, is...

• 36:41Neil deGrasse Tyson?

  In time travel, wormholes, can we make a wormhole?

• 36:44Speaker 03

  Well, the wormhole is made. You have to have something to prop it open, or else it'll collapse like a black hole and kill you. So, we need some negative energy density stuff, some, some stuff that weighs less than zero to hold it open.

• 37:02Neil deGrasse Tyson?

  Because gravity would hold it closed.

• 37:04Speaker 03

  That's right.

• 37:05Neil deGrasse Tyson?

  So you need something to fight gravity.

• 37:06Speaker 03

  You need something to fight gravity and hold it open. And, and in fact, the empty space between two parallel metal plates is, uh, uh, stuff like that, that way, the vacuum between those two plates is stuff that weighs less than zero. It's called the Casimir effect. And so that's what-

• 37:25Neil deGrasse Tyson?

  So we know this stuff exists.

• 37:26Speaker 03

  Yeah, that's what Kip Thorne's using to prop open the, the wormhole and allow you to go through.

• 37:31Neil deGrasse Tyson?

  Oh, so, okay, but you gotta keep it open, otherwise it collapses on you and you're dead. This is dangerous stuff.

• 37:37Speaker 03

  And, and, and one mouth of it is like here near the earth and the other one might be four light years away. So you, you jump in the wormhole, you travel about 10 feet and you pop out near Alpha Centauri.

• 37:48Neil deGrasse Tyson?

  Oh, that works.

• 37:49Speaker 03

  That works. And, and you, you, you then have beaten a light beam there. And so if you move the wormhole mouths around appropriately, you can build a time machine.

• 37:59Neil deGrasse Tyson?

  We've got a caller, another one. Uh, John, Tucson. Is that Tucson, Arizona, John?

• 38:04Speaker 06

  No, Test in California.

• 38:06Neil deGrasse Tyson?

  Oh, excuse me, Tuscan. Tuscan, California. Excuse me. So, John, you got a question for Professor God?

• 38:12Speaker 06

  Yeah, I was wondering about, uh, the theory of relativity and there not being any absolute motion. Wouldn't rotary motion be a form of absolute motion? Because if you were like blindfolded in space and had like jetpack something started spinning in a circle, wouldn't the effects, you know, your arms going out, wouldn't that prove to you that there's, uh, that you're moving without any outside influence?

• 38:34Neil deGrasse Tyson?

  Yeah, excellent question, John.

• 38:35Speaker 03

  That, that, that's true. You can tell whether or not you're rotating or not because you get dizzy, you know?

• 38:41Neil deGrasse Tyson?

  And throw up.

• 38:42Speaker 03

  And, and, and, uh, we have the thing in the inner ear to tell you that. Um, Einstein's postulate of special relativity, um, assume uniform motion in a straight direction without turning. So that's the, when, when you write the details of his postulate, it's stated that way.

• 39:00Neil deGrasse Tyson?

  So therefore if, uh, uh, circular motion, rotational motion, if you're in a spin dryer, you know, and that's, that doesn't count in this analogy?

• 39:09Speaker 03

  You could, uh, use a spin dryer, you know, if you'd hopped in one of those and you're spinning around rapidly, you'd, a little time travel to the future, not much.

• 39:17Neil deGrasse Tyson?

  Is a spin dryer the same effect as being, so in other words, if you're in a spin dryer spinning really fast, does that simulate being in a strong gravitational field?

• 39:25Speaker 03

  Yes, it does. In fact, Einstein showed that those two things are really the same. That acceleration you get on, on the, on one of those whirligigs at the amusement park where you're, where you're pressed back, uh, hard against the back. Uh, he said, I think that's all gravity is, that kind of thing. It's the same physical thing going on. That was his genius in general relativity.

• 39:50Neil deGrasse Tyson?

  Okay, so I don't know, John, if you're still on the line, but that's, so we have the correspondence between rotary motion and gravity.

• 39:56Speaker 03

  Yes.

• 39:57Neil deGrasse Tyson?

  And you can still age, age more slowly, either on a black hole or in a spin dryer. That's what you're telling me.

• 40:04Speaker 03

  Yes, and particles that go around in our accelerators, they're going around in a circle, and they age much more slowly because they're going near the speed of light.

• 40:11Neil deGrasse Tyson?

  So, Rich, we've got a tweet. Hi, thank you. Okay, thanks, John, for calling in. And if you hang on, we'll give, we'll get you a book to you, if you can give us your, your address before you hang up. If you hung up, call back in, we'll give you, we'll give you one of Rich Gott's book, Time Travel in Einstein's Universe. We have a tweet, Rich, let me read it to you. If you were to go back in time and change something involving yourself, would you remember telling you something and not remember you being back in time? So let me see what that means. We're trying to approach a paradox here.

• 40:43Speaker 03

  Yes.

• 40:44Neil deGrasse Tyson?

  Are there paradoxes? For example, Bill and Ted's Excellent Adventure, there's a movie. You gotta love Bill and Ted. I don't know who remember, I remember, I just, I just, Netflix, just last weekend, the whole family saw it. They go back in time, collect these old historic dudes, bring them into the present, but they also meet themselves at a point back in time. It sounds like that's a paradox or something. Suppose they prevented their parents from meeting, for example, then they wouldn't be born, and then they couldn't go back in time to prevent them parents from meeting. How does that work?

• 41:13Speaker 03

  Well, this is the, basically, your grandmother paradox. Suppose you kill your grandmother, she's a young girl.

• 41:20Neil deGrasse Tyson?

  Yes, if I go back in time.

• 41:21Speaker 03

  Then you won't be born. But the trouble-

• 41:24Neil deGrasse Tyson?

  Born and alive to go back and kill my grandmother to prevent me from being born.

• 41:27Speaker 03

  That's right. But if you, if you kill your grandmother, and then you're, you're not born, then your whole world line, your whole path through space and time vanishes, and you're not there to kill her grandmother, she says, all right, after all. So that didn't really solve the paradox.

• 41:44Neil deGrasse Tyson?

  I don't understand.

• 41:45Speaker 03

  It's still a paradox.

• 41:46Neil deGrasse Tyson?

  Okay, okay.

• 41:47Speaker 03

  So, there are two solutions to the paradox. One is that the world has to be self-consistent. Time travelers don't change the past at all. They were always part of it, like the person tweeting in said. If you met yourself in the past, you have a recollection of that. You've written that down in your diary. And so, you're not, the time traveler doesn't change the world at all.

• 42:12Neil deGrasse Tyson?

  So what you're telling me, Rich, is I know my past.

• 42:14Speaker 03

  That's right.

• 42:14Neil deGrasse Tyson?

  I know, therefore, that I will not build a time machine and meet myself in my current past, because that hasn't happened in my past.

• 42:22Speaker 03

  That's right.

• 42:23Neil deGrasse Tyson?

  So therefore, no one alive today, unless they can tell you they've met themselves in the past, has met themselves in the past. Or will, in the future, meet themselves in the past of the present.

• 42:34Speaker 03

  That's right.

• 42:35Neil deGrasse Tyson?

  Does that make sense?

• 42:36Speaker 03

  So, it doesn't mean time travelers can't influence.

• 42:38Neil deGrasse Tyson?

  Well, let me give out our phone number again. If you've, plus, we've got to take a quick break. If you want to, if you've got a question about time travel, any of these movies we've been talking about, relativity, give us a call on StarTalk Radio, 1-877-5-Star-Talk. Tweet us at StarTalkRadio. We'll be back in a bit. Bringing space and science down to earth. You're listening to StarTalk. Welcome back to StarTalk. I'm your host, Neil deGrasse Tyson, with a special guest host sitting in for Lynn Copeland this week. It's Professor Richard Gott III, Professor of Astrophysics at Princeton University, a colleague of mine from the years I spent on the faculty there.

• 43:21Neil deGrasse Tyson?

  And he's not only an expert on all things cosmic, but a particular expert on time travel, having written a book called Time Travel in Einstein's Universe. And we'd like to give the book away to callers that we take online here. So, Rich, tell me something. This, if you go back in time, this paradox problem, if you can't kill your, prevent your parents from meeting, if I know, since my parents did meet and I was born, I can't go back in time and prevent that?

• 43:53Speaker 03

  No.

• 43:54Neil deGrasse Tyson?

  So therefore, if I'm gonna go back in time, it has to be in a time machine that has yet to be invented, and I go back in time of the past of that, but not in the past of my current present.

• 44:04Speaker 03

  Yeah, one of the things, one of the things we've discovered is, that's interesting, is that you can't go back before the time machine was invented. So if we invent a time machine in the year 3000 by twisting space and time up there, we can't use it to come back to the present. We could use it to go from 3002 back to 3001, but not before 3000 when the time machine was invented.

• 44:28Neil deGrasse Tyson?

  So if a time machine has not yet been invented, nobody's going back to visit Julius Caesar.

• 44:32Speaker 03

  That's right. That's why you don't see, it's sufficient to explain why you don't see time travelers at the Kennedy assassination or other historical events.

• 44:42Neil deGrasse Tyson?

  Like the Titanic.

• 44:43Speaker 03

  Well, yes. If you had a time machine, and you'd built it before the Titanic had sailed, then you could use it. And in fact, one wag suggested that the real reason the Titanic sank was the, all the time travelers stole ways on board that added to the weight of the ship. It sank because of the time travelers there to witness the sinking.

• 45:07Neil deGrasse Tyson?

  This is crazy. We've got a call, Ron from LA. Is that you on the line?

• 45:11Speaker 07

  Yes, hello.

• 45:12Neil deGrasse Tyson?

  Hi, welcome to StarTalk.

• 45:14Speaker 07

  Oh, I'm delighted. It's a wonderful conversation. My question for your guest, for both of you, is this. We're talking a lot about time and time travel. I think many of us human beings experience time as having a past where there are no possibilities, a vivid thing, which we call the present, and then a future where there are no facts. Okay? In other words, they're different, past, present, future, as I experience it. How do you reconcile that kind of time with the time that you're talking about with people aging at different rates and so forth?

• 45:54Neil deGrasse Tyson?

  That's excellent question. Certainly worthy of a book.

• 45:57Speaker 07

  I'd love to.

• 46:00Neil deGrasse Tyson?

  So Rich, yeah, so we've got this past, present, and future, which we experience differently physiologically or neurologically, yet you're talking about this whole continuum of time. How did they square with each other?

• 46:10Speaker 03

  Well Einstein showed that time, there was a minus sign associated with the dimension of time.

• 46:17Neil deGrasse Tyson?

  Because we have height, width, depth, and time, and that had a minus sign.

• 46:20Speaker 03

  That just had a minus sign.

• 46:22Neil deGrasse Tyson?

  In his equations.

• 46:22Speaker 03

  That's all it is mathematically. And yet, the human perception of it is that like time is like a movie. Now, the past we know about because light beams go forward in time. There is an arrow of time that we think is connected with somehow the initial conditions of the universe. And light beams go forward in time, not backwards. So events, something causes something over here and then the effects occur later. So that's why we know about the past and not about the future.

• 46:55Neil deGrasse Tyson?

  Whereas we can know where we came from and where we're going because you can see the block that you walked and the block you're about to walk, but we don't have that access to the time dimension of all of this.

• 47:05Speaker 03

  We don't have access to the future yet, but the future in Einstein's view, it was one four-dimensional thing. So there was one past and one future. We do have ideas now about many worlds theory of quantum mechanics, it could be many parallel worlds. But Einstein's view was that there was just one four-dimensional thing, spacetime, and it played out like a movie.

• 47:29Neil deGrasse Tyson?

  Okay, so the two very popular movies, Back to the Future and Terminator.

• 47:33Speaker 03

  Yes.

• 47:34Neil deGrasse Tyson?

  They're both going into the past in those movies. How did they square in terms of the astrophysics?

• 47:40Speaker 03

  Well, in the Back to the Future movie, at one point when he stops his parents from meeting, his hand disappears. Well, hands just don't disappear like that. And besides, his whole path through spacetime would have disappeared. So they didn't quite solve that problem, but he did get his parents successfully back together.

• 48:04Neil deGrasse Tyson?

  One of my favorite lines is there's the estate where his homes were built. Uh, I think it was at the beginning, it was Twin Pines Estate. But one of the pine trees got knocked over when the DeLorean landed in that undeveloped land, knocked over one of the pine trees, so later on it becomes Lone Pine Estate.

• 48:22Speaker 03

  So, so they're working in the many worlds picture.

• 48:25Neil deGrasse Tyson?

  There's the Lone Pine, uh, Shopping Center, that's what it was.

• 48:28Speaker 03

  Yeah, so, so they're, they're working on the idea that there are many parallel worlds, and when he goes back in time, he causes a branch off on the parallel world.

• 48:37Neil deGrasse Tyson?

  A separate universe that's otherwise identical except for these details.

• 48:41Speaker 03

  Right.

• 48:41Neil deGrasse Tyson?

  Interesting. And Terminator, so they, uh, that, that one, we love Terminator, you gotta love Terminator.

• 48:49Speaker 03

  Well, yes, the, the, the Terminator came back to try to kill, uh, Mrs. Connor, so she wouldn't give birth to John Connor, who was the leader of the human revolution to, uh, fight back against the machines.

• 49:03Neil deGrasse Tyson?

  Who turns out to be the father of the guy who was sent back to, that would, that got really creepy.

• 49:10Speaker 03

  That guy who went back turned out to be John Connor's father, that was very convenient.

• 49:16Neil deGrasse Tyson?

  Rich, we're, we're almost out of time as the license plate of, uh, the professor in, in Back to the Future, his, uh, what was, what was his name again?

• 49:27Speaker 03

  Uh, I forget his name.

• 49:28Neil deGrasse Tyson?

  How could you forget his name? How could we forget the guy's name?

• 49:33Neil deGrasse Tyson?

  Anyhow, so, let me, let me, let me tell you, this, this has been a great show, Rich. I mean, going back in time, it's nice to know that it may be possible one day, it's not possible yet, and we're not gonna be able to visit major historical events in anybody's past right now, unless, of course, the government has a secret time machine, but I've spent some time in the government, they're no nearly, they're not nearly as competent as anyone ever holds them up for being. Any, any, any final reflectins on, on this?

• 50:02Speaker 03

  Well, it's been a nice time here with you, Neil.

• 50:03Neil deGrasse Tyson?

  Well, thank you, Rich. You've been listening to StarTalk, a radio program funded by the National Science Foundation. You can track us on StarTalkRadio.net, where you can hear past shows and get the schedule for upcoming shows. Uh, it's, there you have it. Check us out next week when Link Hoplick returns. Uh, a reminder, tonight she's performing live at the Pittsburgh Improv, Pittsburgh, Pennsylvania. We'll see you next week. Thanks for coming.

• 50:38Speaker 00

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