Pod Insights StarTalk Podcast The Physics and Fantasy of Time Travel About this episode Neil deGrasse Tyson unravels time travel with the help of theoretical physicist Michio Kaku, “Back to the Future’s” Doc Brown, aka Christopher Lloyd, “Doctor Who’s” Missy, aka Michelle Gomez, and co-host Chuck Nice. Plus, Bill Nye takes a ride in the DeLorean.
Transcript Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.
0:01 Neil deGrasse Tyson? Welcome to StarTalk. Your place in the universe where science and pop culture collide. StarTalk begins right now.
0:15 Neil deGrasse Tyson? Welcome to the Hall of the Universe of the American Museum of Natural History. I'm your host of StarTalk, Neil deGrasse Tyson, your personal astrophysicist. And tonight, we're going to be talking about the science and fantasy of time travel. And because we're featuring my interview with Christopher Lloyd, who played Doc Brown on Back to the Future. Not only that, I've got my interview with Michelle Gomez, who's a lead actress in the BBC cult series Doctor Who. So, let's do this.
0:46 Michio Kaku? Yes.
0:47 Neil deGrasse Tyson? All right.
0:52 Neil deGrasse Tyson? I love bringing in help to talk about this. Chuck, my co-host tonight, comedian co-host, thanks for joining. Of course, always a pleasure to be here, Neil. And I've also got a good friend and colleague, Professor Michio Kaku, who is a professor of theoretical physics at the City University of New York. So, Michio Kaku, thanks for being on StarTalk.
1:10 Michio Kaku? Glad to be on your show.
1:12 Neil deGrasse Tyson? Now, I've got you on here because you're an expert on so many things that transcend ordinary thought, at the frontier, the bleeding frontier of physics. You've got several books here, and one that's perhaps more relevant to us tonight, Physics of the Impossible.
1:26 Michio Kaku? Hmm.
1:27 Neil deGrasse Tyson? And I just, I just, when I saw that title, I said, "If it's impossible, how could it have physics that applies?"
1:32 Michio Kaku? Well, that's the whole point, right? That's why you want to buy the book.
1:36 Neil deGrasse Tyson? Oh, wow.
1:38 Michio Kaku? No, but you see, there really is a logic behind time travel and impossible things like jetpacks and flying cars, starships. We physicists take it seriously. We have working designs in the literature for potential time machines. Of course, none of us have ever gone into one to find out whether they work or not.
1:56 Neil deGrasse Tyson? Right, right. Well, one of the most famous time travel movies of all time
2:00 Michio Kaku? Yeah.
2:01 Neil deGrasse Tyson? is Back to the Future.
2:03 Michio Kaku? Absolutely.
2:04 Neil deGrasse Tyson? Of course, it was a trilogy, but let's focus on the first one for the moment.
2:07 Michio Kaku? Which started it all and actually is the most beloved.
2:09 Neil deGrasse Tyson? It's the most beloved, for sure. And it was 1985, I remember when it came out, and it was so well-thought-out, so well-acted and performed and conceived. And let's just refresh our memory. Let's check out a clip from Back to the Future.
2:28 Speaker 02 Eighty-eight miles per hour!
2:34 Speaker 02 Where the hell are they? The appropriate question is, when the hell are they?
2:39 Chuck Nice? Ah.
2:40 Neil deGrasse Tyson? Yeah, it's not where are you, when are you?
2:42 Chuck Nice? Yeah.
2:43 Neil deGrasse Tyson? And so in fact Einstein does come back. That was not the dog driving the car, although it looked...
2:47 Chuck Nice? It did kinda look that way.
2:48 Neil deGrasse Tyson? It did kinda look that way.
2:49 Chuck Nice? And I think Michael J. Fox was a little too muted on his response to seeing his dog disintegrate.
2:55 Neil deGrasse Tyson? Or to see a car coming at you at eighty-eight miles an hour.
2:57 Chuck Nice? At eighty-eight miles an hour and you stood there.
2:59 Neil deGrasse Tyson? And you stood there.
3:00 Chuck Nice? A lot of trust in the dog.
3:03 Neil deGrasse Tyson? So, well it's Doc Brown that I met up with for my interview for StarTalk. And he's a deep thinker and, and we just talked about that iconic role that he gave us in that film. Let's check it out.
3:18 Neil deGrasse Tyson? No one can imagine anybody else playing that role but you. So, what were you doing? What's your secret? Come on.
3:26 Speaker 04 I love the whole concept of scientists who deal with insoluble problems. I love the story of, of a noted scientist who was trying to find the solution to some problem and just didn't come. He worked on it for months and then one day he's not thinking about anything, he's getting on the bus and it comes to him, you know? And Doc Brown is trying to figure out how to induce time travel. And he falls down in the bathroom and hits his head against the sink and he suddenly envisions the flux capacitor.
4:11 Speaker 04 You know, so I, I love that kinda connection because it's kinda the way it works.
4:17 Neil deGrasse Tyson? Well, so it's interesting, that's basically the eureka effect.
4:20 Chuck Nice? Yeah.
4:21 Neil deGrasse Tyson? You get hit in the head or something happens to you abruptly and then the idea pops in. But, I'm a fan of the way Isaac Asimov interprets the eureka effect.
4:29 Chuck Nice? Which is?
4:30 Neil deGrasse Tyson? Which is?
4:32 Chuck Nice? Go ahead.
4:34 Neil deGrasse Tyson? Okay, you ready? The most exciting phrase to hear in science, the one that heralds the most discoveries,
4:40 Chuck Nice? Okay.
4:41 Neil deGrasse Tyson? is not eureka, but rather, "That's funny."
4:47 Chuck Nice? That, well, that makes sense.
4:48 Neil deGrasse Tyson? Yeah, so Michio, do you agree with this?
4:50 Michio Kaku? Yes, I had exactly that moment. I first was working on string theory in the 1960s, and it consisted of hundreds of random formulas that you had to memorize in a book. But Einstein wanted an equation one inch long that would allow him to quote, "Read the mind of God." So I said to myself, "Hmm, that's funny. How come this theory has so many random equations? There should be this one inch equation." One day, I found it. It's called string field theory. That's my equation. I'm the co-founder of string field theory, and it summarizes a theory beyond Einstein in one inch long.
5:29 Neil deGrasse Tyson? Wait, but was that a eureka moment? Or did you just sort of?
5:32 Michio Kaku? It was a "that's funny" moment that no one was looking for it. No one was looking for this one inch equation, right? Because people were so busy memorizing and using all these little, hundreds of little formulas that they would memorize, no one was looking for that one inch equation.
5:46 Neil deGrasse Tyson? So my famous "that's funny" story is with William Herschel, who wanted to measure the temperature of the different colors of light that come through a prism.
5:56 Chuck Nice? Okay.
5:57 Neil deGrasse Tyson? But just to even think to do that is a, that's a...
6:00 Chuck Nice? Yeah, listen, that's all I can say is mushroom tea night for me.
6:07 Neil deGrasse Tyson? Wait, so he's got the spectrum laid out. He's got the spectrum laid out, and he puts a thermometer in each color. And then he says, "All right, I need a control thermometer." So he puts that thermometer beyond the red side of the spectrum, where there's no light. And then he watched the temperatures. And the one where there was no light, just beyond the red, rose to the highest. And he said, "That's funny. There's no light there."
6:31 Chuck Nice? Right.
6:32 Neil deGrasse Tyson? The man discovered infrared.
6:35 Neil deGrasse Tyson? Which is not visible. So there was light there. There was light there. You just couldn't see it. He thought he put it in a safe place. Right. That was a "that's funny" moment. And then Isaac Newton, which I think is more of a eureka moment. The man is sitting outside his backyard. Right. In Lincolnshire, alright? We filmed part of Cosmos there. Right. That was almost like a pilgrimage going up. I was feeling, I was feeling that Isaac. Well, I know that, that's your favorite, that's your favorite guy, right? That's my, that's my boy. That's your man right there. That's my man. That's your man right there. I go visit my man. Right. And his apple tree is still in the backyard, okay? So, so, so after I like, got a hold of myself, I, I, I, I bit an apple next to his tree. And then I floated up off.
7:19 Neil deGrasse Tyson? Yeah, I just floated up in that moment. That is awesome. So that is actually Isaac Newton's original apple tree. And. Right in his backyard. So the apple story is true. Well, yeah, but what happened was, he saw an apple fall. He didn't get hit in the head with it. He saw, I know, I know. He saw the apple fall. I'm not gonna, I'm gonna erase that, wait. I saw it. Erase, erase, erase. And. And he saw the moon in the sky, knowing that it orbits the earth. And so here's an apple falling straight to the earth, and something, some other orb going around the earth. Right. And in that moment, he recognized that the same force is making both things happen. So it takes Newton. Wow. To have an apple falling in a straight line, and a cosmic object going in orbit,
8:03 Neil deGrasse Tyson? to drop it into one equation. He's one, he's in your one equation club. That's right. Is that right? Okay. Michio.
8:09 Michio Kaku? No, that's the first unified field theory. Because they unified earth-like physics with heavenly physics. When the church says they were two separate things. One heavenly physics that, that God would obey, and then the venal and mortal sin that we lived in. He created the first unification in all of science.
8:31 Neil deGrasse Tyson? Interesting you said that God obeys the law of physics. That's an interesting inversion of how most people think of God. Yes, because most people think that God actually created the laws of physics. Yeah, but, but Michio has separate access to, to deities. I've learned this.
8:47 Neil deGrasse Tyson? Let's go back to my interview with, with Christopher Lloyd. It turns out though he plays a geeky scientist, he's actually got some geek in him, in like, in real life. Oh, yeah? I love finding that out in people. Let's check it out.
8:58 Speaker 04 Einstein always kind of amazes me.
9:02 Neil deGrasse Tyson? He amazes us all, by the way.
9:04 Speaker 04 Yeah, and he was just a clerk in the patent department. Nobody knew, you know, but he's going, looking at this and there's a railroad station. And he spent a lot of time doing mind, mind games, whatever they call it.
9:19 Neil deGrasse Tyson? Thought experiments, yeah, the Geddonka experiments.
9:21 Speaker 04 Having to do with if a train is moving through and somebody drops a ball and it bounces, how fast it, you know, and how it relates to the world outside.
9:30 Neil deGrasse Tyson? These are the seeds of the genesis of relativity.
9:35 Speaker 04 And there was a famous clock tower over the railroad station. So there was time, travel, all that kind of stuff going on.
9:45 Neil deGrasse Tyson? So he loved himself some Einstein.
9:47 Michio Kaku? Yeah, in fact, my favorite Einstein quote is, "If a theory cannot be explained to a child, then the theory is probably useless." Meaning that all great theories are based on pictures that even children can understand.
10:01 Neil deGrasse Tyson? The idea, and so, so in 1905, he had special theory of relativity, which involved the trains and the clocks and the timings. And then 10 years later, 11 years later, the general theory of relativity. So, Misha, relativity is a modern idea of how the world works, and it doesn't seem to emanate from anyone's sort of chair, because it's not part of anybody's common sense.
10:24 Michio Kaku? That's right, common sense says that time is like an arrow. You fire it, never comes back. One second on the earth is like one second on the moon. Einstein says, no, time is like a river. A river that meanders and speeds up and slows down. And the new wrinkle on this, that I work on, is the fact that the river of time can have whirlpools and fork into two rivers. And that, of course, is Back to the Future. Whirlpools in the river of time.
10:55 Chuck Nice? Man, that sounds like a really cool soap opera that I've...
11:01 Neil deGrasse Tyson? And now on the next episode of Whirlpools in the River of Time.
11:07 Neil deGrasse Tyson? Well, up next, we're going to discuss another icon of time travel in pop culture. And that occurs in the series called Doctor Who, when StarTalk continues.
11:23 Neil deGrasse Tyson? Welcome back to StarTalk. Right here at the Rose Center for Earth and Space. And we're talking about the science and the fantasy of time travel. I got Chuck here.
11:34 Michio Kaku? Yes.
11:34 Neil deGrasse Tyson? I got Michio Kaku, an expert on so many things, including time travel. How often do you just have an expert on time travel just sitting around, right? So we reach for you. You know, so in your book, The Physics of the Impossible, you actually classify impossible things, and you classify time travel as a Class 2 impossibility. What the hell does that mean?
11:56 Michio Kaku? Class 1 impossibility are things that are actually possible, but most people think they're not. Like, for example, jetpacks, starships, teleportation. They're all physically possible. In the laboratory, we actually teleport atoms.
12:12 Neil deGrasse Tyson? So there's no law of physics preventing them?
12:14 Michio Kaku? That's right. But people think they're impossible because you see them on Star Trek.
12:18 Neil deGrasse Tyson? They've only ever seen them in a science fiction story.
12:20 Michio Kaku? That's right. But we have jetpacks. We have, uh, we can make flying cars. We can do all that stuff, except we go to Class 2 now, where we really have problems. Now we're at the cutting edge of physics where I do my research. That includes wormholes, higher dimensional travel. This includes black holes and what happens at the center of a black hole, wormholes, time travel. These are Class 2 impossibilities.
12:48 Neil deGrasse Tyson? I'm afraid to learn what Class 3 is after hearing that.
12:51 Michio Kaku? Class 3 is just darn impossible, period.
12:55 Neil deGrasse Tyson? Just plain old impossible.
12:57 Michio Kaku? Like violation of the conservation of matter and energy, getting something from nothing. Okay, that would be a Class 3 impossibility. Or my wife letting me go to a strip club. So that's Class 4.
13:08 Neil deGrasse Tyson? That's Class 4. Okay. Class 4.
13:12 Michio Kaku? Not in your lifetime.
13:14 Neil deGrasse Tyson? Well, there've been people who've tried to make time machines in the past. And most famously, we have H.G. Wells in 1895 with a story called "The Time Machine." That one took people to new places psychologically, intellectually, scientifically, and that wasn't the last time people have tried to do this. The most successful and longest-running science fiction series.
13:41 Neil deGrasse Tyson? Doctor Who. True. It's got, it's got a rabid fanbase in the UK. Frightening people.
13:48 Neil deGrasse Tyson? Just gonna say. And the, and the fanbase, we got a strong fanbase here in the United States. Yes. Including my daughter, who's a Whovian. Listen. They got words, if you are, you are a Whovian. And it, and it started in 1963. Yes. So if you, if you knew about those, you're classic Who. But then they rebooted the series in 05. If you're into that, then you're new Who. New Who? You're, you're. Who knew?
14:11 Neil deGrasse Tyson? So, that series involves time travel. Yes. Let's check out what they do.
14:19 Speaker 05 Missy, the Master, whatever you call yourself, I promise I'm much more useful to you alive.
14:31 Neil deGrasse Tyson? Evil? That is a frightening woman. So, Doctor Who follows the adventures of a humanoid alien called The Doctor. Okay? That was not The Doctor, that was The Doctor's nemesis. That nemesis is called The Master. And they occasionally will face off to one another. And all of their adventures involve moving through time in some way that is always intriguing. Always, uh, there's, they force you to think about the consequences of having done so, and what they're doing in that time and place, and what effect that'll have on everything else in the universe. And so that zap, that woman who zapped this other woman out of existence,
15:14 Neil deGrasse Tyson? the woman who plays that is called Michelle Gomez. And she is the first woman to play the role of a Time Lord. Time Lord, yeah. She's the, and so I had, she came by my office. And I said, I had to, I had to ask her about this. What are the powers that you have? Cause we all know about The Doctor's powers. They're both Time Lords, but they're nemesises, nemisai, nemesises of one another. Let's check out the actual powers of The Master.
15:42 Neil deGrasse Tyson? Could you list for me the powers of the Master?
15:46 Speaker 06 Being able to time travel.
15:48 Neil deGrasse Tyson? Time travel, but with your vortex.
15:50 Speaker 06 Yeah, at the moment with the vortex.
15:52 Neil deGrasse Tyson? With your non-Tardis vortex.
15:54 Speaker 06 Very ugly. So there's that, I would say, is basically that's...
15:59 Neil deGrasse Tyson? That's the thing.
15:59 Speaker 06 That's the thing. I mean, she has lots of... There's other gizmos and gimmicks and things that she has at her disposal to delete you. So there's...
16:10 Neil deGrasse Tyson? So not just kill me, but delete me? That sounds worse than just killing me. Because if you just kill me, my body, my dead body is here, and you have to bury me, or think about me, or carry me somewhere.
16:20 Speaker 06 But then you'll still be here in the cosmos.
16:21 Neil deGrasse Tyson? Yes, my sense of self and memory of me is there. If you delete me...
16:25 Speaker 06 Yeah, you never existed.
16:27 Neil deGrasse Tyson? That's scary.
16:30 Neil deGrasse Tyson? Well, so, so here's what's interesting. The fact that she can delete you, rather than just kill you, means she has access to your prior existence.
16:40 Chuck Nice? Right.
16:40 Neil deGrasse Tyson? Because you're deleted, it means no one even knew that you existed.
16:43 Chuck Nice? Yeah, that's some really angry girlfriend stuff right there.
16:49 Neil deGrasse Tyson? Chuck, you sound like that's happened to you.
16:51 Chuck Nice? I'm just saying.
16:54 Michio Kaku? Which girlfriend was that?
16:55 Neil deGrasse Tyson? I know.
16:55 Chuck Nice? I'm gonna say, there's a lot of places on Facebook, "I don't exist."
17:01 Neil deGrasse Tyson? So, so, so the doctor uses a TARDIS, which is a call box. We might see an image of that later. Missy, which is her name, the Master, she uses a vortex manipulator to travel through time. That's basically her time machine. But the TARDIS, though, is, you know, that's an acronym, TARDIS.
17:23 Chuck Nice? I didn't know that.
17:23 Neil deGrasse Tyson? Time and Relative Dimension in Space. That's why you'll always see it capitalized.
17:27 Chuck Nice? Right.
17:27 Neil deGrasse Tyson? And it looks for all the world like a...
17:30 Chuck Nice? Phone booth.
17:30 Neil deGrasse Tyson? Yeah, a phone booth, a police call. But you walk in, and it's bigger on the inside.
17:35 Chuck Nice? Right.
17:35 Neil deGrasse Tyson? That is an entire place on the inside. Michio, that clearly involves another dimension.
17:40 Michio Kaku? That's right. These are called multiply-connected spaces. If you get scissors and paper and Scotch tape and start to cut, and then reform in different ways, you can get wormholes, you can get TARDISs, multiply-connected spaces which wrap around themselves. We physicists have catalogued these things. And so if you have enough energy, perhaps you can warp time and space into a pretzel.
18:05 Neil deGrasse Tyson? So this is a kind of a, this is a kind of a time machine.
18:07 Michio Kaku? That's right.
18:08 Neil deGrasse Tyson? So why is a pretzel a time machine?
18:10 Michio Kaku? Uh, because anything that allows you to go backwards on that sheet of paper in time is a time machine.
18:15 Neil deGrasse Tyson? So a pretzel, you, you, you think you're going someplace and by the time you turn around, you're back where you started.
18:20 Michio Kaku? That's right. In other words, whirlpools in the river of time. And you can see this by taking a sheet of paper and folding it on itself. And then punching through it, and that's called a wormhole.
18:32 Neil deGrasse Tyson? Connecting this part of the space-time continuum to this part, cheating really by not having to go through the full length.
18:37 Michio Kaku? Now, to actually build something like this, you need a machine bigger than the super collider in Geneva, Switzerland, the Large Hadron Collider.
18:45 Neil deGrasse Tyson? Oh, how much bigger?
18:46 Michio Kaku? Uh, a quadrillion times bigger than the Large Hadron Collider.
18:49 Neil deGrasse Tyson? Yeah, so that's not just bigger, Michio.
18:51 Michio Kaku? It's a lot bigger than the Large Hadron, a lot bigger than the city of Geneva. But if you had that energy, then yeah, you too might be able to rip a hole in the fabric of space and time. In fact, we think that energy is perhaps the energy of the Big Bang.
19:05 Neil deGrasse Tyson? Ah.
19:05 Michio Kaku? The Big Bang, the creation of the universe itself might have come out through a wormhole.
19:09 Neil deGrasse Tyson? So let the record show that Michio wants to tear a new one in the universe.
19:15 Neil deGrasse Tyson? When we come back, more on the physics and fantasy of time travel on StarTalk.
19:26 Neil deGrasse Tyson? We're back on StarTalk. And tonight, we're talking about time travel, the physics of it, the fantasy of it, the pop culture of it. Featuring my interview with Christopher Lloyd, who played Doc Brown on Back to the Future. Check it out.
19:44 Speaker 04 My fiancee been trying to get me to a little more technology adept. So she's, I carry all these books with me on a plane, and she says it drives her crazy. So she's trying to get me to all the Kindle.
20:00 Neil deGrasse Tyson? From the Kindle, yeah.
20:01 Speaker 04 She said, "Now, choose a book, and we're gonna be taking some long trips." And all I could think of was Steve Hawking's, um, A Short History of Time.
20:11 Neil deGrasse Tyson? Uh, Brief History of Time, yeah.
20:12 Speaker 04 Brief History of Time. And she says, "Why did you choose something that's fun?" And I said, "This is fun." And it is. I, I, I don't, don't, don't ask me to explicate about black holes or anything.
20:29 Speaker 04 But I love reading it. It's just so, so awesome.
20:34 Neil deGrasse Tyson? So, Michio, a brief history of time. Stephen Hawking, that single-handedly put physics back on the map in pop culture. Ever since Einstein was a pop, pop culture icon. He proposed the chronology protection conjecture. Did I even say that right?
20:50 Michio Kaku? That's right. However, uh, we think he's wrong on this one.
20:54 Neil deGrasse Tyson? Well, wait, wait, wait. So first of all, so he's saying that backwards time travel is impossible. A, B, you're saying we think he's wrong. Does that mean you think he's wrong? Who's the, what, when the word "we" comes out of Michio's mouth, who is the "we"?
21:08 Chuck Nice? I don't know, but I just think this is awesome. Like, just sitting here and listening to astrophysicists, uh, talk about the fact that Stephen Hawking is wrong.
21:21 Michio Kaku? He himself has stated that he is unable to prove that hypothesis, which would disprove time travel.
21:29 Neil deGrasse Tyson? But that's why it's a conjecture.
21:30 Michio Kaku? It's a conjecture, but it's never been proven. And that's why he cannot say that time travel is impossible. He tried to codify the impossibility of time travel. He failed. Meaning that it could be possible.
21:44 Chuck Nice? Damn, that was a Stephen Hawking smackdown.
21:48 Neil deGrasse Tyson? But there's some other, wait, there's some other arguments here. There's sort of the practical argument. So, for example, if in the future, a time machine is invented, where you can go backwards in time, presumably they would have already done so. Where are they today?
22:06 Michio Kaku? Well, we're gonna have invisibility in the next few decades, so maybe they're invisible.
22:11 Neil deGrasse Tyson? See, you make, you pulling stuff out of your, no, wait, Michio.
22:15 Michio Kaku? Maybe they're here, right here.
22:17 Neil deGrasse Tyson? You're telling me you're right and Hawking's wrong, because they've actually come back in time and they've made themselves invisible.
22:22 Michio Kaku? That's right.
22:22 Chuck Nice? But wouldn't you, though? If you went back in time, you wouldn't let people know that you were from the future, would you?
22:27 Neil deGrasse Tyson? Uh,
22:28 Chuck Nice? I mean, seriously, you, you would not go back, because first of all,
22:31 Neil deGrasse Tyson? Wait, wait, wait.
22:31 Chuck Nice? They would take you and put you in a room.
22:33 Neil deGrasse Tyson? No, no, no, wait, first. No, no, Chuck, as you know, there aren't many times in the history of this civilization that either you or I ever want to go back to.
22:43 Chuck Nice? Well, listen, I'm gonna tell you right now.
22:45 Neil deGrasse Tyson? Okay.
22:46 Chuck Nice? No, Michael J. Fox went back to 1955. There's no way in hell I'm going anywhere near 1950 anything.
22:51 Neil deGrasse Tyson? That's exactly right. That's what I'm saying.
22:53 Chuck Nice? As a matter of fact, I'm not going pa- past 1980. Things get a little sketchy for me, right after they invented hammer pants. So, if I go back and I don't see hammer pants, I'm like, nope, I'm going back now.
23:06 Neil deGrasse Tyson? Back right. So, so, okay, so they're, they're invisible. All right, but in any of these scenarios, does the universe split?
23:15 Michio Kaku? That's right. We get what is called a multiverse of universes, where time travel is possible without any contradictions. If you saw Back in the Future, Part Two, when they go backwards in time and Biff steals that almanac.
23:28 Neil deGrasse Tyson? Yes.
23:28 Michio Kaku? Doc Brown goes to the blackboard, draws a horizontal line, and then he forks that line where Biff steals the almanac. That is a physically correct description of all time travel paradox.
23:41 Neil deGrasse Tyson? Okay, let me correct you. It's from, no, it's not that it's correct, it's that it doesn't violate laws of physics. We don't know if it can ever happen that way.
23:48 Michio Kaku? That's right.
23:49 Neil deGrasse Tyson? Okay. Just to say it's correct.
23:51 Michio Kaku? It's consistent.
23:52 Neil deGrasse Tyson? Like, like you've confirmed this.
23:55 Michio Kaku? It's consistent with what we know about the laws of quantum mechanics and general relativity. It's consistent.
24:00 Neil deGrasse Tyson? So therefore it cannot be excluded?
24:03 Michio Kaku? That's right. And that's why Hawking has since backtracked on his chronology protection hypothesis. We have not been able to prove that time travel's impossible.
24:13 Neil deGrasse Tyson? Well, coming up, the science fiction of time travel meets the science fact on StarTalk.
24:26 Neil deGrasse Tyson? Welcome back to StarTalk, right here under the sphere of the Hayden Planetarium. And we're talking about time travel in pop culture. And one of my interviews featured Michelle Gomez from the pop series Doctor Who, the BBC cult classic. Let's check it out. Answer me this: the Time Lords always tended to have companions traveling with them. So if people can travel with them, then they too are traveling through time.
24:55 Speaker 06 Yes.
24:56 Neil deGrasse Tyson? So the act of traveling through time is not unique to Time Lords. If you can bring people along with you.
25:02 Speaker 06 Yes, and that does seem to be happening now. I mean, he's just sort of, anybody can just jump at the TARDIS with the doctor.
25:07 Neil deGrasse Tyson? Just jump it right before it takes off, and then you're there.
25:09 Speaker 06 But no, that's the fun of it. You've got to take, there's something about the shared experience that we need as humans as well. There's something about turning to that one person in the dark and sharing that experience. So I think that's where the Doctor and the Master are entirely different. The Doctor shares that with somebody and then gets to tell the story, and the story then gets to...
25:34 Neil deGrasse Tyson? Propagate.
25:35 Speaker 06 Yes, to propagate.
25:36 Neil deGrasse Tyson? Whereas?
25:37 Speaker 06 Whereas with the Master...
25:39 Neil deGrasse Tyson? No companions?
25:40 Speaker 06 There's no companion.
25:41 Neil deGrasse Tyson? So you travel through time alone?
25:43 Speaker 06 Yeah.
25:43 Neil deGrasse Tyson? Space-time, alone?
25:44 Speaker 06 Yeah.
25:46 Neil deGrasse Tyson? So, Michio, when you invent your time machine, are you taking friends with you?
25:51 Chuck Nice? No, I don't think so. I think it's a personal, it's a personal journey, and I don't want to be deleted if they make a mistake.
25:59 Neil deGrasse Tyson? So, Time Lords on the show, they're all from, I guess it's the planet Gallifrey. And they form a community of people, and they're capable of manipulating space and time. It's just a feature of their identity. And it's their most defining property of who and what they are and what they do. And what this show has done is it's split that identity into good and evil.
26:21 Chuck Nice? Right.
26:21 Neil deGrasse Tyson? Yeah, so you have the Doctor and you have the Master.
26:24 Chuck Nice? Right.
26:24 Neil deGrasse Tyson? And so that's just kind of how they...
26:25 Chuck Nice? And the Doctor gets to take friends, and the Master is always alone.
26:29 Neil deGrasse Tyson? Well, Master's just evil, so...
26:30 Chuck Nice? Well, that's, yes.
26:31 Neil deGrasse Tyson? Yeah, yeah.
26:32 Chuck Nice? Maybe that's because she doesn't have any friends. If she had some friends, she'd probably be a lot nicer.
26:37 Neil deGrasse Tyson? I hadn't thought that through.
26:38 Chuck Nice? Yeah.
26:39 Neil deGrasse Tyson? Right, if you're evil, nobody wants to travel with you.
26:41 Chuck Nice? Right, exactly.
26:41 Neil deGrasse Tyson? So, so, Michio, here's an interesting question that I've got. Physics and our understanding of the universe has advanced greatly, as it always does, every year, every decade. And so...
26:55 Neil deGrasse Tyson? It's almost come to the point where we are now taking time travel a little more seriously than ever before. I mean, in H.G. Wells, it was, "Oh, that's fantasy, and it's fun, let's just watch it." Even in Back to the Future, 1985, it's fun, we can chat about it, but nobody's thinking this. We're at a time when people like you are actually thinking this.
27:15 Michio Kaku? Yeah, several things have happened. First of all, we've discovered numerous black holes in the universe. There's one in our backyard, the Milky Way galaxy, weighs two to three million times the mass of our sun.
27:26 Neil deGrasse Tyson? Right in the center of the Milky Way.
27:27 Michio Kaku? That's right. These black holes collide, giving us ripples in the fabric of space and time.
27:31 Neil deGrasse Tyson? Backyard, in this context, means thirty-five thousand light-years away, okay? That's backyard cosmically. Go.
27:38 Michio Kaku? Right, and these black holes rip the fabric of space and time. And according to the theory, if a black hole's rotating very rapidly, it doesn't collapse to a dot, it collapses to a ring. A ring of neutrons circulating very rapidly.
27:51 Neil deGrasse Tyson? A ring, you mean like a donut?
27:53 Michio Kaku? Uh, yeah, and if you fall through the ring, you're not crushed to death, you wind up in another universe. So there is a wormhole, mathematically speaking, at the center of every black hole that is rotating very rapidly. And if you go through the wormhole a second time, you wind up on a second layer of a parallel universe. Now, why don't we get...
28:14 Neil deGrasse Tyson? So the universe is all the way down, is what you're saying?
28:16 Michio Kaku? That's right, the parallel universe is all the way down. Maybe even Elvis Presley is alive in one of those universes, right?
28:23 Neil deGrasse Tyson? Thank you very much.
28:30 Neil deGrasse Tyson? Is there a universe where Elvis Presley's saying, "You know, I'm not thankful"?
28:36 Neil deGrasse Tyson? That's evil Elvis, right?
28:37 Chuck Nice? "I'm not thankful at all."
28:41 Neil deGrasse Tyson? He even had the lip doing that.
28:43 Chuck Nice? Yeah, that's right.
28:43 Neil deGrasse Tyson? Can we stop interrupting Micio here?
28:45 Michio Kaku? I'm sorry, Micio.
28:46 Neil deGrasse Tyson? Okay, go, go.
28:47 Michio Kaku? The other reason for taking this very seriously is we have the Large Hadron Collider now. This gigantic atom smasher, uh, you know, fourteen trillion electron volts worth of energy. We can create a mini, a mini black hole, a mini Big Bang at the center of the Large Hadron Collider. And so we have to take them seriously now. At what point does known physics break down?
29:10 Neil deGrasse Tyson? Well, up next, we're gonna come to one of my favorite segments of StarTalk, the Cosmic Queries, where we field all the questions you have on time travel. Taken from the internet and our fanbase.
29:22 Chuck Nice? Mm-hmm.
29:22 Neil deGrasse Tyson? When StarTalk returns.
29:30 Neil deGrasse Tyson? We're back! StarTalk. From New York City, Manhattan, in the universe. I just love saying that.
29:39 Chuck Nice? I know.
29:41 Neil deGrasse Tyson? So Chuck, this is the Cosmic Queries edition.
29:42 Chuck Nice? Yes, it is.
29:43 Neil deGrasse Tyson? You and I do this all the time.
29:45 Chuck Nice? All the time.
29:45 Neil deGrasse Tyson? And these are questions solicited from our fanbase. And these are all on time travel. I won't necessarily know all these answers, but I got Michio as backup.
29:53 Chuck Nice? Yes, you do.
29:54 Neil deGrasse Tyson? And, and we're gonna try to fit in as many as we can, so the whole segment is gonna be lightning round edition.
30:00 Chuck Nice? There we go.
30:00 Neil deGrasse Tyson? I got my bell. Okay, let's check the bell. We got it.
30:03 Chuck Nice? The bell works so good.
30:04 Neil deGrasse Tyson? Go for it.
30:04 Chuck Nice? All right. Query number one is from @Theatrical. Is time just the universe's way of organizing space so that it doesn't run into itself?
30:11 Neil deGrasse Tyson? Ooh! Michio, why don't you hit that?
30:14 Michio Kaku? Uh, yeah, time is a parameter that measures change. That's what time is. A parameter that measures change. And because it measures change, it means that things don't collide. It separates things.
30:25 Neil deGrasse Tyson? So yes, is the answer.
30:27 Chuck Nice? All right. Query number two is from @KDRGuitars. What would happen if your atoms, eh, to your atoms, if you traveled back in time? Surely the atoms within you already exist elsewhere in the past, too.
30:39 Neil deGrasse Tyson? Ooh!
30:40 Chuck Nice? Whoa.
30:40 Neil deGrasse Tyson? Yeah, Michio, why don't you take that one, too? I could answer these, but I just wanna give Michio a chance.
30:46 Michio Kaku? The river of time means you go backwards and meet yourself in the past, and so your atoms exist simultaneously because you're on another timeline. So there are no paradoxes.
30:57 Chuck Nice? No paradoxes.
30:58 Michio Kaku? So when you meet yourself as a child, that is not really you. It is genetically equivalent to you, but it is you in a different timeline. Your atoms exist simultaneously.
31:07 Neil deGrasse Tyson? So the act of going back in time split the spacetime continuum, so you are meeting you, but they're not your atoms.
31:13 Michio Kaku? My god, he's got it! He's got it!
31:15 Neil deGrasse Tyson? Good, next one.
31:16 Chuck Nice? Sweet! I'm gonna go back and meet me and slap the hell out of that guy. Uh, our next query from Raymond Pinzon in Lake Forest, California. Let's say you can ride a photon. How would your surroundings look like while you are on it? Would you be able to perceive movement and time?
31:33 Neil deGrasse Tyson? Let me take half of it, and I'll give the other half to Michio.
31:35 Chuck Nice? Okay.
31:35 Neil deGrasse Tyson? Photons... Have no concept of time. Time does not exist for photons. If you are emitted any place in the universe, you are absorbed at your destination that very same instant. Yet to the rest of us, you will see the photon move at the speed of light across the thousands and in some cases, millions and billions of light years to its destination. So when I'm at a telescope and I'm observing the center of the galaxy as I did for my PhD thesis, those photons wandering through space ended up in my detector. They hit that in their mind the instant they were emitted from the stars at the center of the galaxy. What would the universe look like if you were surfing that photon?
32:19 Michio Kaku? You're right. The universe would be stopped because time as we know it as a parameter of change does not exist on the light beam. So you're correct.
32:29 Neil deGrasse Tyson? But what would the universe look like to you? Oh, it just stopped?
32:32 Michio Kaku? It just stopped.
32:32 Neil deGrasse Tyson? Yeah, yeah, okay.
32:33 Chuck Nice? So it'd be frozen?
32:34 Michio Kaku? Frozen, right.
32:34 Chuck Nice? Wow, that's very cool.
32:37 Neil deGrasse Tyson? Yeah.
32:37 Michio Kaku? Hit the bell.
32:42 Chuck Nice? And from at NG1083, is it true we may be able to see into the future but will never be actually able to time travel? Hmm. Weird question.
32:56 Michio Kaku? Well, already our astronauts are time travelers. Our astronauts, when they come back, are like a microsecond, a microsecond younger than a twin, if they had one, on the planet Earth.
33:08 Neil deGrasse Tyson? I tweeted how much younger Scott Kelly is than any twin that he left here on Earth after he'd been in space for 10 months. He's just running the straight Einsteinian calculations. You learn that in Relativity 101.
33:19 Michio Kaku? Right. And if you travel to Jupiter, since time is distorted around Jupiter, time slows down. It means you go forwards in time going around Jupiter. So you can become a time traveler into the future. It's going into the past that's the big problem. But forward time travel takes place with all our astronauts.
33:38 Neil deGrasse Tyson? And by the way, you are not forward time traveling to Michio himself. You are just moving forward in time. I'm like, we go okay on?
33:44 Michio Kaku? That's right. You're going faster than normal in time, right?
33:46 Neil deGrasse Tyson? And this was captured in the film Interstellar. Yeah, where time became the commodity. They were not just near Jupiter. They were near a black hole. So all these phenomena that Michio is describing happened in spades at a black hole.
34:00 Chuck Nice? Gotcha.
34:01 Michio Kaku? All right. Well, there you have it. Hit the bell.
34:05 Neil deGrasse Tyson? Well, coming up, we've got Bill Nye the Science Guy. And guess what he got his hands on? The actual DeLorean from Back to the Future.
34:14 Chuck Nice? Aw, man.
34:15 Neil deGrasse Tyson? And I don't know what he's gonna do in it, and he didn't invite me on that trip.
34:18 Chuck Nice? I know.
34:19 Neil deGrasse Tyson? But when we come back, we will find out what he does with that DeLorean on StarTalk.
34:29 Neil deGrasse Tyson? We're back on StarTalk. And we're talking about the science and pop culture of time travel. And who better to ask about that than Doctor Who star, Michelle Gomez? Check it out.
34:43 Neil deGrasse Tyson? Does the prospect of time travel intrigue you?
34:46 Speaker 06 Yes, absolutely.
34:47 Neil deGrasse Tyson? How long, your whole life?
34:48 Speaker 06 From the moment, I think my first thought would have been obsessed about
34:52 Neil deGrasse Tyson? Really?
34:52 Speaker 06 being somewhere else. It was only until now that I'm starting to realize that right here, right now, in this moment, right in this moment, everything's just fine and perfect. But that's taken me a few years to get there.
35:06 Neil deGrasse Tyson? So your timeline was a work in progress?
35:10 Speaker 06 Yes.
35:11 Neil deGrasse Tyson? Until this moment?
35:12 Speaker 06 Yes, because it had to be beaten into me, into submission. My very sort of humanness has had to be beaten into me throughout the years.
35:21 Neil deGrasse Tyson? Well, it's true for anyone. We are the sum of everything we've experienced.
35:24 Speaker 06 Yes.
35:25 Neil deGrasse Tyson? Right? And but not everyone puts it together
35:28 Speaker 06 No.
35:28 Neil deGrasse Tyson? into a new entity.
35:29 Speaker 06 Consciousness.
35:30 Neil deGrasse Tyson? A new consciousness.
35:31 Speaker 06 Yeah.
35:32 Neil deGrasse Tyson? A new consciousness.
35:33 Chuck Nice? Wow, man, you guys got really philosophical.
35:35 Neil deGrasse Tyson? Well, I'm just saying, you know, if you gotta go there, you gotta go there.
35:39 Neil deGrasse Tyson? The moment requires it of you.
35:40 Chuck Nice? Yeah.
35:41 Neil deGrasse Tyson? So, Michio, what is this? If there's time travel, what does that say about free will?
35:45 Michio Kaku? It means perhaps there's no such thing as free will. Uh, we have to
35:49 Neil deGrasse Tyson? No one wants to believe that.
35:50 Michio Kaku? Right, however, you know.
35:52 Chuck Nice? I do, my life is a wreck.
35:55 Michio Kaku? I'm a quantum physicist, and in quantum mechanics, there is uncertainty. And uncertainty means that we're not robots. We're not
36:02 Neil deGrasse Tyson? By the way, this is not uncertainty the way we normally use that word.
36:06 Michio Kaku? This is the uncertainty in position and velocity of electrons. But since we are made of electrons, it means that there's uncertainty with regards to who we are. We exist in multiple states at the same time. And in some sense, you can be two places at the same time. So for us quantum physicists, having multiple universes is commonplace.
36:28 Neil deGrasse Tyson? If you're gonna give yourself multiple universes, then I do have free will. There's a universe where some happen, I say, "I don't wanna do that, I don't want that to happen." Let me go change that, and then I birth another universe where I can do something different.
36:40 Michio Kaku? That's right. If there is uncertainty, then there are parallel universes, different time streams. And that resolves all the paradoxes of time travel. And it means that free will can exist because the timeline can fork into many roads.
36:58 Chuck Nice? So each one of those timelines actually represents a different reaction to a choice, if you were this person. So I get up and I went one way to work, as opposed to another way to work. I've created a different multiverse by doing that?
37:14 Michio Kaku? That's right. We create multiple universes simply by making decisions. Whether we wake up in the morning or sleep another hour in bed. We bifurcate, we split in half. And so for us, the time travel paradoxes are very easy to resolve because we work with parallel universes. Every time we work with a transistor. The internet, the internet is based on parallel universes. And some people don't like it, but hey, get used to it.
37:40 Neil deGrasse Tyson? Still, Michio, I think you're invoking parallel universes to make everything you say work out okay.
37:48 Michio Kaku? Hey, I think he's got it.
37:51 Michio Kaku? That's exactly right. We believe in a unification of all physical law. And we think that quantum mechanics is the framework that even unites Einstein's theory to quantum mechanics. And the highest version of quantum mechanics is string theory.
38:07 Neil deGrasse Tyson? So I worry that you know you can make another universe rather than commit yourself to this one and make it better. Ooh, can I dodge that question?
38:18 Speaker 07 Alright, so before we bring this to a close, you know we can't do it without like a video visit from Ni Times in the City.
38:26 Neil deGrasse Tyson? Yes.
38:27 Speaker 07 Bill and I, the science guy, good friend and buddy. And now, you know, he's a resident of New York now. I finally got him to live here.
38:32 Neil deGrasse Tyson? You finally got him here.
38:33 Speaker 07 Now he runs around town doing cool stuff, and I'm still here stuck in this chair.
38:38 Speaker 07 So I'm told he got his hands on an actual DeLorean that was used in the movie. Let's check this out. Look out!
38:53 Speaker 07 Is time travel possible?
38:56 Speaker 07 No, probably not. Be cool though. You could go back in time and change history. Maybe you could go back to the Titanic and convince the captain to slow down. Or maybe just go back to last weekend and stop yourself from having one more tequila shot. Or maybe you could send a colleague back in time to have sex with a waitress who turns out to be your own mother, like in the Terminator series. You know, to prevent worldwide nuclear war. That'd be cool. Science fiction is replete with time travel stories. But I think for Neil and me, our favorite's gotta be the Back to the Future series.
39:41 Speaker 07 That's where Doc Brown turns a DeLorean sports car into a time machine.
39:49 Speaker 07 What if you could travel through time?
40:01 Speaker 07 Great Scott!
40:04 Speaker 07 Suppose everything, everyone you know and love, were actually part of someone else's time travel adventure? That would mean that you and I don't even exist. And as I finish this Ni in the City segment, everything would just disappear.
40:27 Speaker 07 That would be weird.
40:34 Neil deGrasse Tyson? Where did he go?
40:39 Neil deGrasse Tyson? So, let's try to land this plane. So Chuck, what are your reflections on time travel?
40:45 Chuck Nice? I think it's great to think about, and it's a lot of fun. But would I actually want to go back in time, to be honest, and change something? No, that scares the hell out of me, to be honest.
40:55 Neil deGrasse Tyson? Michio, what wisdom do you have to drop on us?
40:57 Michio Kaku? For physicists, time travel is like children's paradise, being in a toy store as a child. All the toys to play with, all the laws of physics that we might be able to break or extend. So we're talking about a whole new terrain opening up. So this is a great time to be a physicist, because we're like children in a toy store.
41:18 Neil deGrasse Tyson? So your time machine tools and methods are in like a playpen for you?
41:26 Michio Kaku? That's right. We get to test the boundaries of theoretical physics. We get to play God. We get to change the laws of physics because the old ones don't quite work right.
41:35 Chuck Nice? Well, clearly, nothing wrong could ever go wrong from that.
41:40 Neil deGrasse Tyson? What could go wrong there?
41:41 Chuck Nice? What could go wrong?
41:45 Neil deGrasse Tyson? Maybe time machines have been invented in the future. And they are sending people back, but they're not sending them back invisible. They're sending them back as those who walk among us and end up transforming our world, whose ideas are well ahead of that of anything else that's going on around them, such as Isaac Newton himself, Einstein, Jimi Hendrix, Marie Curie. Maybe these people who we write stories about, these people who were ahead of their time, Picasso, people where they did something and no one understood it.
42:27 Neil deGrasse Tyson? We had to keep moving the timeline forward until somebody said, "Hey, they were on to something." Maybe they are the time travelers in our culture, and we don't even know it. And so if you see someone making discoveries...
42:44 Neil deGrasse Tyson? Give them a hug. They're trying to help us, save us from ourselves. That's how I think about it. Just a thought from the Hall of the Universe on StarTalk. This has been StarTalk, and I've been your host, Neil deGrasse Tyson, your personal astrophysicist. Thanks to DJ Kool-Aid, Chuck Nice. As always, I bid you to keep looking up.
Previous Calling ET, with Dr. FunkySpoon – StarTalk All-Stars Next Cosmic Queries - Expanding Our Perspectives, with Bill Nye – StarTalk All-Stars Page updated September 29, 2026 · transcript and summary are machine-generated
