Past, Present, Future: Time Travel with Brian Greene

Summary

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

Podcast Information:

• Title: StarTalk Radio
• Host: Neil deGrasse Tyson
• Guest: Brian Greene, Theoretical Physicist and Mathematician
• Episode Title: Past, Present, Future: Time Travel with Brian Greene
• Release Date: December 24, 2024

---

Introduction to Time Travel

In this episode of StarTalk Radio, astrophysicist Neil deGrasse Tyson delves into the intriguing concept of time travel alongside his guest, renowned theoretical physicist Brian Greene. The conversation is set against the backdrop of Charles Dickens' classic novel A Christmas Carol, which is often cited as one of the earliest literary explorations of time travel.

Neil deGrasse Tyson [02:40]:
"Welcome to StarTalk, your place in the universe where science and pop culture collide."

Chuck Nice [03:06]:
"Always a pleasure to be here. Although I'm not actually, I'm not actually here. I'm coming to you from the future."

This humorous opening by co-host Chuck Nice sets a lighthearted tone for the deep scientific discussions that follow.

---

Einstein's Relativity and Its Implications for Time Travel

Neil and Brian begin by exploring how Einstein's theories of relativity revolutionized our understanding of time.

Brian Greene [07:04]:
"Time is much more individualistic than anybody would have ever thought. The common view of time was it's universal... Einstein shattered that by showing that time elapses at different rates depending on how you move and your gravitational experience."

They discuss how special relativity introduced the idea that faster movement results in slower ticking clocks relative to a stationary observer. General relativity further expanded this by demonstrating that gravity affects the passage of time, leading to phenomena like time dilation experienced by satellites.

Brian Greene [07:54]:
"The clock on Earth truly is ticking off time at a slower rate than the clock that's floating up there in space."

This discussion highlights real-world applications, such as the need to adjust satellite clocks to account for both their high velocity and their distance from Earth's gravitational pull, ensuring the accuracy of our communication networks.

---

Experimental Evidence of Time Dilation

Neil brings up historical experiments that validate Einstein's predictions.

Brian Greene [11:24]:
"When they tested these ideas on a Pan Am jet in the 1970s... the net change is not what you would have anticipated. But when you take all the effects into account, motion and gravity that Einstein delineated, the prediction agrees with the observations. Spot on."

He references experiments where atomic clocks on fast-moving jets showed measurable differences compared to those left on the ground, confirming the theory that both velocity and gravity influence the passage of time.

---

Philosophical Implications and Paradoxes of Time Travel

The conversation shifts to the philosophical dilemmas posed by time travel, particularly the possibility of altering past events.

Chuck Nice [15:45]:
"Suicide via time travel. So those constrictions, since I'm coming from a place that is already set..."

Brian Greene [17:42]:
"If you travel back to an earlier moment in time before your own birth, then you were always part of that moment. Therefore, the causal chain of events that resulted in your own birth remains unchanged."

Brian discusses the concept of chronology protection, a conjecture by Stephen Hawking, which suggests that the laws of physics prevent time travel from creating paradoxes, such as altering one's own existence.

Neil deGrasse Tyson [23:45]:
"But, Brian, it takes away free will."

Brian Greene [23:48]:
"This is a version of negating free will. But I am one of those people who doesn't believe in any variety of free will of the traditional sort."

This segment delves into the deterministic nature of time travel within certain theoretical frameworks, questioning the extent of human free will.

---

Time Travel in Popular Media

Neil and Brian analyze how time travel is portrayed in films, assessing their scientific accuracy.

Brian Greene [31:07]:
"Interstellar is probably the best one where you have Matthew McConaughey going near a black hole. Time slows down... those are accurate features of Einstein's general theory of relativity."

They commend Interstellar for its faithful representation of time dilation near black holes, a concept grounded in general relativity. Conversely, they critique other portrayals for lacking scientific precision.

Neil deGrasse Tyson [32:11]:
"Kip Thorne helped pioneer LIGO and received the Nobel Prize, and he was the co-recipient of it a few years ago. And Henry did the movie."

This highlights the collaboration between physicists and filmmakers to enhance the scientific authenticity of cinematic narratives.

---

Advanced Concepts: Tachyons and Wormholes

The discussion moves to more speculative aspects of time travel, including hypothetical particles and spacetime structures.

Chuck Nice [49:03]:
"Is there any idea on how to try and detect the theorized tachyon particle?"

Brian Greene [50:20]:
"A tachyon is a particle that can go faster than the speed of light. If a particle can go faster than light, there are observers who will see it reach its target before it was emitted by the source."

They explore the concept of tachyons—hypothetical particles that move faster than light—and the implications such particles would have on our understanding of causality and the directionality of time.

Brian Greene [37:00]:
"The richness of wormholes really arises when you have a time difference between the two openings. It's fun to have a tunnel through space, but it is mind-blowing to have a tunnel through time."

Wormholes, theoretical passages through spacetime, are discussed as potential mechanisms for time travel, allowing movement between different points in both space and time.

---

Black Holes and the Information Paradox

The conversation touches upon the black hole information paradox, a pivotal topic in modern physics.

Chuck Nice [45:03]:
"So when you go into a black hole, I'm a chair. I go into a black hole, I get broken down into just particles, okay? That's all because the gravity's so strong now, just the string of particles. I come out during the evaporation. Would I be able to be a chair again?"

Brian Greene [46:24]:
"Stephen Hawking told us that particles will come out and they do carry the imprint of the fact that they were once a chair. And if you measure the particles appropriately, you could reconstruct the chair when those particles come out."

They discuss how, contrary to Hawking's initial assertions that information is lost in black holes, advances in string theory and quantum mechanics suggest that information is preserved, albeit in a highly scrambled form.

---

Cosmic Queries and Audience Engagement

Throughout the episode, Neil and Brian address questions from listeners, further unpacking the complexities of time travel.

Listener Question [24:26]:
"If controlled backwards time travel was possible today, where do you think your matter or information would travel?"

Brian Greene [37:24]:
"If you can time travel, you also need to space travel. It's really space-time travel. You have to pick the location in time and the location in space."

This emphasizes the intertwined nature of space and time, necessitating precise navigation through both dimensions to achieve controlled time travel.

Listener Question [53:03]:
"If we developed a way to time travel backwards, could we travel to a time before the Big Bang, or would we just break science altogether?"

Brian Greene [53:24]:
"The Big Bang could be the origin of time itself. The notion of 'before' makes sense only when time exists."

They explore the speculative idea of traveling to the very beginning of the universe, concluding that time as we understand it may have originated with the Big Bang, rendering the concept of "before" meaningless within our current framework.

---

Conclusion and Reflections

As the episode wraps up, Neil and Brian reflect on the profound implications of time travel theories, intertwining scientific principles with philosophical musings.

Neil deGrasse Tyson [57:36]:
"It's been a delight to have you back on StarTalk."

Brian Greene [57:08]:
"Always a pleasure."

Their engaging dialogue not only elucidates complex scientific ideas but also inspires listeners to ponder the nature of time, existence, and the universe.

Neil deGrasse Tyson [57:37]:
"Cosmic queries. I am Neil Degrasse Tyson, your personal astrophysicist. As always, I bid you to keep looking up."

The episode concludes with an invitation to continue exploring the cosmos, encouraging curiosity and scientific inquiry.

---

Notable Quotes with Timestamps

• Brian Greene [07:04]:
  "Time is much more individualistic than anybody would have ever thought. The common view of time was it's universal... Einstein shattered that by showing that time elapses at different rates depending on how you move and your gravitational experience."

• Brian Greene [11:24]:
  "When they tested these ideas on a Pan Am jet in the 1970s... the net change is not what you would have anticipated. But when you take all the effects into account, motion and gravity that Einstein delineated, the prediction agrees with the observations. Spot on."

• Brian Greene [17:42]:
  "If you travel back to an earlier moment in time before your own birth, then you were always part of that moment. Therefore, the causal chain of events that resulted in your own birth remains unchanged."

• Brian Greene [31:07]:
  "Interstellar is probably the best one where you have Matthew McConaughey going near a black hole. Time slows down... those are accurate features of Einstein's general theory of relativity."

• Brian Greene [50:20]:
  "A tachyon is a particle that can go faster than the speed of light. If a particle can go faster than light, there are observers who will see it reach its target before it was emitted by the source."

• Brian Greene [53:24]:
  "The Big Bang could be the origin of time itself. The notion of 'before' makes sense only when time exists."

---

Final Thoughts

This episode of StarTalk Radio masterfully blends scientific rigor with accessible explanations, making complex theories about time travel and relativity comprehensible and engaging for a broad audience. Brian Greene's expertise, coupled with Neil's charismatic hosting, offers listeners a comprehensive exploration of one of science's most captivating topics.

Whether you're a seasoned science enthusiast or a curious newcomer, "Past, Present, Future: Time Travel with Brian Greene" provides valuable insights into the fabric of our universe and the tantalizing possibilities that lie within the enigmatic concept of time travel.

---

Keep looking up and stay curious!

Transcript

[Neil DeGrasse Tyson]

Now. AT T Mobile get four 5G phones on us and four lines for $25 a line per month when you switch with eligible trade ins. All on America's largest 5G network. Minimum of 4 lines for 25 per line per month with auto pay discount using debit or bank account, $5 more per line without autopay plus taxes and fees and $10 device connection charge phones via 24 monthly bill credits for well qualified customers. Contact us before canceling entire account to continue bill credits or credit stop and balance on a required finance agreement too. Bill credits end if you pay off devices early.

[Chuck Nice]

CT mobile.com oh it feels great to make progress on a Language Learning new words and phrases Are you feeling ready to make conversation this holiday season? Share a new language with your loved ones. A lifetime membership to Rosetta Stone makes a meaningful present for friends and family. My brother vowed to move to Spain recently and he's always walking around now saying things like in Espana, la comida es deliciosa. And soon the two of us will have a secret language that everybody who speaks Spanish knows. Start learning today with Rosetta Stone's Lifetime Membership Holiday Special. Visit rosettastone.com startalk for unlimited access to 25 language courses for the rest of your life. Available for a short time@Rosetta Stone.com startalk with incredible new features and upgrades, there's never been a better time to armor up Critically acclaimed action RPG Diablo 4. Get ready to continue the epic story in your battle against evil in the new expansion Vessel of Hatred. As darkness spreads through the lands of Sanctuary, it's up to you to fight back the encroaching corruption. Reap the benefits of massive updates to character progression loot, systems difficulties and tons of added activities. Face off against new and iconic bosses teeming with the spoils of Sanctuary. Plus harness formidable powers of the jungle with the all new Spirit Born class now yours to customize and progress alongside five other iconic classes. Embark on the epic journey solo or with friends. You'll quickly understand why Diablo 4 has been called one of the best action RPGs of the last decade by PC Gamer. Forge your own path through hell torn lands of Sanctuary. Save 35% off Diablo 4 and Vessel of Hatred in the expansion bundle today. Rated M for mature.

[Neil DeGrasse Tyson]

Welcome to StarTalk, your place in the universe where science and pop culture collide. StarTalk begins right now. This is StarTalking Cosmic Queries Edition and today we're going to be talking about time travel inspired by Dickens classic novel A Christmas Carol, Chuck Love to have you on this.

[Chuck Nice]

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

[Neil DeGrasse Tyson]

Thanks for that. Heads up.

[Chuck Nice]

In case that matters, let me say it to you this way, Neil.

[Neil DeGrasse Tyson]

It's your kids, they're in troubles. Thank you, Doc Brown realize when they went into the year 2015. Just thought I'd remind you of that.

[Chuck Nice]

Wow. Wow.

[Neil DeGrasse Tyson]

Yeah, yeah, yeah, we'll get back to that. So while I know a little bit about time travel, I don't know nearly as much as our guest, which is why we brought him on. A good friend and colleague from up the street at Columbia University, Brian Greene. Brian, dude.

[Chuck Nice]

Our returning.

[Brian Greene]

How are you doing, champion?

[Neil DeGrasse Tyson]

Returning.

[Chuck Nice]

Yes, our returning champion, Brian Greene, ladies and gentlemen.

[Neil DeGrasse Tyson]

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

[Brian Greene]

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

[Chuck Nice]

Dang.

[Neil DeGrasse Tyson]

Dang.

[Chuck Nice]

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

[Brian Greene]

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

[Neil DeGrasse Tyson]

Oh yeah, it's like get the hell out of the lab. You're one of those theorists. Yes. Yeah. So Brian is a theoretical physicist and a longtime friend and it's just a delight with best selling books. And let me get the title correct of his latest book because this title leaves nothing out here it is until the end of Time. Mind Matter and Our Search for Meaning in an Evolving Universe came out last year. Brian, that title doesn't leave anything untouched.

[Brian Greene]

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

[Chuck Nice]

Yeah, that's great.

[Neil DeGrasse Tyson]

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

[Brian Greene]

SEO Optimized. Knopf.

[Neil DeGrasse Tyson]

Knopf. Very nice, very nice. And of course many people know you or know of you through being co founder, I think, with your wife, is that correct? Of the World Science Foundation.

[Chuck Nice]

Yeah.

[Neil DeGrasse Tyson]

With Tracy Day.

[Brian Greene]

Correct.

[Neil DeGrasse Tyson]

Yeah. Tracy Day. I met Traci Day before I knew you, when she was a news reporter. Was it for NBC, is that right?

[Brian Greene]

Abc.

[Neil DeGrasse Tyson]

Abc, yeah. Yeah. She did an interview with me.

[Brian Greene]

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

[Neil DeGrasse Tyson]

Yeah, we go way back right before. So let's put some context on the table. So Charles Dickens, which is, you know, he wrote Oliver, he wrote A Tale of Two Cities, he wrote A Christmas. I mean he wrote a bunch of stuff and I think he wrote A Tale of Two Cities. Is that. Yeah, yeah, sure, of course. Yeah, yeah.

[Chuck Nice]

It was the best of times and the worst of times. Right now we're in the second part of that right now.

[Neil DeGrasse Tyson]

Yes, exactly.

[Chuck Nice]

Yeah.

[Neil DeGrasse Tyson]

So written long time ago, 1843. And by some measures, it may be the first sort of modern literary example of time travel or the implications of time travel. And so that's well before, like a half a century before H.G. wells, the time machine, so itself, quite a. Quite a remarkable step to take in storytelling. So, Brian, before we had Einstein's relativity, do you know, how did anybody think about time travel?

[Brian Greene]

Yeah, it's a good question.

[Neil DeGrasse Tyson]

What was it missing? Let me say that differently. What do we, in our enlightened modern times, with the benefit of relativity, get to say about time travel that they couldn't say before Einstein, before the 1905 special relativity paper and the 1916 general relativity?

[Brian Greene]

Well, yeah. Well, I mean, I look at us as the unique species on the planet that's able to lift ourselves up above the timeline and think about the past and future. So that immediately gives us at least the imaginative capacity to travel to the past and the future. But of course, you're going beyond that. You want to know about real time travel. And what we can say today that they couldn't say back then is that time is much more individualistic than anybody would have ever thought. The common view of time was it's universal. It's the same for me, for you, for anyone else, regardless of what we're doing, what we're experiencing, how we're moving. And that Einstein shattered by showing that time elapses at different rates depending on all those qualities, how you move, the gravity experience, that's the main new feature.

[Chuck Nice]

Awesome.

[Neil DeGrasse Tyson]

Interesting.

[Chuck Nice]

That is amazing. So now that you said that, because, please, one of you, either one of you, I'm filthy with physicists right now.

[Neil DeGrasse Tyson]

I. Can you just. You just. It's raining physicists.

[Chuck Nice]

Raining physicists. I have an embarrassment of physicist riches. So this is just wonderful every time I think about it. But if something is, let's say, let's go as close as geosynchronous orbit, like a satellite, and it's traveling at the speed and they have to adjust the clocks because time is ticking differently for that thing up there than it is for us down here. And can you please just speak to that? And can you speak to. Is it a literal elapsed difference, like running clock. Running clock. Or is it something that causes it to seem like in a lapsed difference.

[Neil DeGrasse Tyson]

Oh, good.

[Brian Greene]

I shouldn't jump In. Maybe Neil wants to take this.

[Neil DeGrasse Tyson]

Well, no. So why don't I. I'll just start with 1905, and you take. Pick us up at 1916. Okay, so what we learned with 1905, special relativity is that the faster you go, all right, you will perceive events around you as well. Okay. If you watch someone go fast, you will perceive their ticking clock to move more slowly than yourself. And we call this special theory of relativity because it only involved motion that did not accelerate. It was a very sort of basic case of motion. And so once you got used to that, then. Then 10 years later, he throws in a whole other fact. And Brian, pick it up from there.

[Brian Greene]

Yeah. The new fact that Neil's referring to is that gravity also affects the passage of time. So, Chuck, in the example that you gave, if you have a clock on planet Earth and a clock that's hovering above the Earth some number of miles up there, those clocks will really tick off time at different rates because they're experiencing different forces of gravity. The stronger the pull of gravity, the slower the clock ticks. So the clock on Earth truly is ticking off time at a slower rate than the clock that's floating up there in space.

[Neil DeGrasse Tyson]

So what's interesting there is that the geosynchronous satellites, because, Chuck, you started this by saying they have whatever is their orbital speed, which has some significance, right? It's a few miles per second, really. And up at sort of middle. You know, Brian, every time I talk about, you know, there's leo, low Earth orbit and geo. But middle Earth orbit, you know, mio. Every time I say middle Earth, people are thinking Lord of the Rings. Yeah.

[Chuck Nice]

Oh, don't worry, Mr. Frodo.

[Neil DeGrasse Tyson]

We'Ve got this.

[Chuck Nice]

We'll get that traffic satellite fixed, sir. We'll do it.

[Neil DeGrasse Tyson]

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

[Brian Greene]

There is, and I don't know the exact numbers in that particular case, but I do know that when they tested these ideas on a Pan Am jet in the 1970s, where they had an atomic clock that they left on the tarmac, and the other they strapped into the passenger seat, maybe it's first class, I don't know. In that particular case, it is the speeding up of time from the gravitational difference, that wins out. So whenever I teach this, I'm always careful with my language because you would think that clock should tick up time more slowly because it's in motion. But when you take the competition into effect, in the end, the net change is not what you would have anticipated. But none of that really matters. What really matters is when you take all the effects into account. Motion and gravity that Einstein delineated, the prediction agrees with the observations. Spot on. And so these effects are truly real.

[Neil DeGrasse Tyson]

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

[Brian Greene]

Hafel and Keating were the two.

[Neil DeGrasse Tyson]

Tell everyone what Pan Am is.

[Chuck Nice]

Yes.

[Brian Greene]

I'm just saying that was the southwest of its day.

[Neil DeGrasse Tyson]

Right? So Pan Am actually was also the space shuttle in 2001 A Space Odyssey. It was a Pan Am space shuttle. Yeah, yeah. Going up to the space station where There was an AT&T phone and. And a Howard Johnson's hotel. Super cool.

[Chuck Nice]

That's very cool.

[Neil DeGrasse Tyson]

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

[Brian Greene]

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

[Neil DeGrasse Tyson]

And one last point about Ebenezer Scrooge. He was observing different Christmases. But the big issue for any storyteller is whether you can interact either with the past or with the present that you then have access to. And so what does Einstein say about that?

[Brian Greene]

Well, he doesn't say a whole lot about travel to the past because Everything that he was doing, if you actually follow it through, you can move in such a way or experience a gravitational field that allows you to go into the future. And if you're in the future, he said, sure, you interact with anything that you want to interact with. It's just that you are at a later time than your own clock would suggest. But when it comes to the past, I haven't read anything that he wrote. But it's a real conundrum that, you know, back to the future popularized, right? If you go to the past, can you change things in such a way that might prevent your own birth? And then you're in a logical paradox. How could that ever happen? And people have struggled with that. And we can talk about the solutions if it's relevant to the topic here. But yeah, and one of those solutions, you can interact, but you're constrained. You don't have the freedom that you would have thought you would have had to mess things up.

[Chuck Nice]

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

[Neil DeGrasse Tyson]

Oh, okay.

[Chuck Nice]

Suicide via time travel. So those constrictions, since I'm coming from a place that is already set, okay. I go back at my origin, before my origin. I'm sorry. And then I. Let's just say, you know, I caused my parents to hate each other. I'm doing the reverse of the movie, right? And they end up hating each other.

[Neil DeGrasse Tyson]

Cause you can do this without. Without bloodshed, right?

[Chuck Nice]

Yeah, without bloodshed.

[Neil DeGrasse Tyson]

You just have to get your. Put your parents in an argument at a time they might have made love, right? And then you're not conceived in that moment.

[Chuck Nice]

Exactly. Or I just do something really nasty, but not, you know, like hit on my mom. And she's just like, you are adorable. And I'm like, yeah, don't ever talk to that guy. And.

[Neil DeGrasse Tyson]

Oh, so.

[Brian Greene]

But before this gets too far out of hand.

[Chuck Nice]

I wasn't going any further. Brock.

[Neil DeGrasse Tyson]

Slow down, Chuck. Slow down, slow down.

[Chuck Nice]

Let me just ask this. So would then kind of like the box of time force those events to happen anyway in a different circumstance, even though I just screwed up the circumstance under which I came into being?

[Brian Greene]

No, I wouldn't frame it that way.

[Chuck Nice]

Okay.

[Brian Greene]

The answer that I would give on that is that if you travel back to an earlier moment in time before your own birth, then you were always at that. There aren't two versions. There aren't two versions of a given moment in time. Right. A moment in time. A moment in time can't change because what would be the parameter through which that change would occur? It'd have to be time. But we're talking about individual moments. So if you go to the past, you were always part of that moment, and therefore you were always part of the causal chain of events that resulted in your own birth. Ooh, so you.

[Neil DeGrasse Tyson]

Okay, so wait, wait. So Bryan Bryant in the Time Machine, the character has a love interest and he. And then she crosses the street and is hit by a. By a, you know, by a horse and buggy. Back then, I guess that was a deadly thing. And then she says, no, wait, I can fix that. He goes back in time and like, prevents her from crossing the street, leaves her. Then she gets mugged and dies from a mugging. And then he prevents the mugging and a third time, and then she dies some other way. And he figures out that her death was something inherent in the timeline and there's nothing he can do to change it.

[Chuck Nice]

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

[Brian Greene]

That's correct. All events just are. They're unchangeable, they're immutable. So the events can't change. And you simply, if you go back to the past, are fulfilling the precise set of events that allowed you to exist and to go on that journey in the first place. Now, look, I'm not saying this is the only resolution to these ideas because, Neil, the example that you gave brings to mind another proposal which is this idea of a multiverse.

[Yara Shahidi]

Running a small business takes endurance, determination, and the right support to reach your goals. And MasterCard is here to help fuel that journey in a fast paced digital world with innovative tools and resources. We're here to guide businesses every step of the way digitally. Because when small business wins, everyone wins. Let's power up our communities one small business and one step at a time, keeping the community running strong. Priceless.

[Neil DeGrasse Tyson]

After investing billions to light up our network, T Mobile is America's largest 5G network. Plus right now you can switch keep your phone and we'll pay it off up to $800. See how you can save on every plan vs Verizon and at&t@t mobile.com KeepAndSwitch up to four lines via virtual prepaid card. Allow 15 days qualifying unlock device credit service ported 90 plus days with device ineligible carrier and timely Redemption required card has no cash access and expires in six months.

[Yara Shahidi]

Hey, I'm Yara Shahidi and I'm the host of the Optimist Project. This is the podcast that ask what gives you hope. Each week I sit down with change makers you may or may not know from comedy, music, academia, and more to uncover what inspires them to create a better tomorrow. Join us as we find out ways that we can cultivate optimism in our own lives. You can find the Optimist Project wherever you get your podcasts. Don't forget to follow the show so you never miss an episode.

[Chuck Nice]

Nicholas I'm Nicholas Costella and I'm a.

[Brian Greene]

Proud supporter of StarTalk on Patreon.

[Chuck Nice]

This is StarTalk with Neil DeGrasse Tyson.

[Neil DeGrasse Tyson]

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

[Chuck Nice]

Yeah.

[Brian Greene]

It's a self consistent, immutable set of events. And the text, inexplicably from your perception, is what actually causes the event that you wanted it to prevent. But indeed, it's part of that causal fabric. And that, in this approach, is the immutable quality of the timeline.

[Neil DeGrasse Tyson]

Wow, cool.

[Chuck Nice]

So it's very cool. It's reverse destiny.

[Brian Greene]

Well, it's timeless destiny. So all the events of reality just exist out there. And it perhaps is human perception that orders them into this causal set. But they're just all out there, just like all the spaces out there. And this way of thinking about things, all of time may be out there too. And so there's nothing that can ever be changed.

[Neil DeGrasse Tyson]

So in Kurt Vonnegut's Slaughterhouse Five, that time travel scenario is what you're describing. Right. And because in it, his entire time he's picked up by aliens and he lives in a cage. All right? An alien zoo. But it doesn't matter to him. Cause he still has access to his entire life's timeline. And when he's describing this, or they describe it to him, he said, when will I die? You're always dying. When was I born? You're always being born. When did I go to college? You're always going to college. And so the preexisting timeline, he could just rejoin it at any point, but the destiny's already preordained.

[Brian Greene]

Yeah. And there's something comforting. There's something comforting about that. Right. Because even those of us who've lost parents or loved ones, in some sense, they still exist at the moment of time that they occupied. And that is an eternal, unchanging statement.

[Neil DeGrasse Tyson]

But, Brian, it takes away free will.

[Chuck Nice]

Ah, little fatalistic.

[Brian Greene]

So, yes, this is a version of negating free will. But I am one of those people who doesn't believe in any variety of free will of the traditional sort. So this doesn't run afoul of my intuition at all.

[Chuck Nice]

Okay, Brian, see, I know we got queries, but.

[Neil DeGrasse Tyson]

Wait, Chuck, we gotta get the question. This is a cosmic query. Go, Brian. Stop tempting us. Brian, stop tempting us. Shut up, Brian.

[Chuck Nice]

Let's act like we don't have an audience. Why don't we do that?

[Neil DeGrasse Tyson]

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

[Chuck Nice]

Okay, all right. Let me get to the queries. I'll put my stuff on the back burner. Here we go. Hey, this is Jay Salmon, who says. Or Salmon. I mean, it's Salmon, but salmon. Maybe he says, hello, Dr. Green. Because a photon of light has no mass, theoretically, can't it travel both forward and backward in time?

[Neil DeGrasse Tyson]

Well, I love that.

[Chuck Nice]

Speaking of gravity. And it's.

[Neil DeGrasse Tyson]

No, wait a minute. If it. So it moves the speed of light, right? And if time goes slower for you, the faster you go, then photons have no time at all.

[Chuck Nice]

Yeah, Right.

[Neil DeGrasse Tyson]

So if they have no time, why don't. Who cares whether they move?

[Chuck Nice]

It doesn't make sense.

[Neil DeGrasse Tyson]

Or backwards.

[Chuck Nice]

Right. There is no forward and backwards for photons.

[Brian Greene]

So I think there's two. There's two ways of answering this question. I think both are really important. One is from our perception watching the photon, and from our perception, you fire a photon and it travels through space as time elapses, it goes forward in time. But then you say, what about from the photon's Perspective, it's traveling at the speed of light. And from what Neil described earlier, when the photon looks out at the world, everything should be going infinitely slowly and therefore, in some sense, stopped. Basically, time is stopping. Now, that's poetically fine. The problem I have with taking that too seriously is, and this may seem like a footnote, but it's not. You're ascribing some kind of experience to a photon. And a photon doesn't have experience. It doesn't have consciousness. It doesn't look out at the world, even though the poetic language is useful to invoke. And so to imagine that we could travel at the speed of light and therefore there'd be no time and time would stop is a step too far. So when we look at the photon, it's like any other particle. It travels through space. It goes forward in time. If you put yourself into the shoes of the photon, whatever that means, then poetically, yes, time would stop. There's no notion of the elapsing of time. There's no notion of aging from the photon's perspective, whatever that actually means.

[Neil DeGrasse Tyson]

Okay, you heard that, Brian. Cast shade on the photon. He said it. It has no emotions, no feelings.

[Chuck Nice]

Brian thinks photons are stupid. And so.

[Neil DeGrasse Tyson]

Okay, that's Brian.

[Chuck Nice]

All right.

[Neil DeGrasse Tyson]

All right, Brian, we know where you're coming from. All right. Okay, keep going, Chuck.

[Chuck Nice]

Here we go. This is. I like this. Let's. A little more philosophical. This is Tyler J. Tyler says, how might time travel be policed or regulated if it were able to happen? Ooh, yeah. Cause, I mean, think about it. If we could all.

[Neil DeGrasse Tyson]

So if you have bad actors going through. Right, yeah.

[Brian Greene]

Well, again, you know, I'm less fearful of time travel because of the view that I hold that you couldn't actually change anything. If you could, then, yeah, we'd have to have the time police around here to avoid things happening that we didn't want to have happen.

[Neil DeGrasse Tyson]

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

[Brian Greene]

Well, he's got a few conjectures, so it depends.

[Neil DeGrasse Tyson]

You think?

[Brian Greene]

Depends exactly which one, but.

[Neil DeGrasse Tyson]

But I think, Hawkins, chronology protection.

[Brian Greene]

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

[Neil DeGrasse Tyson]

I like that one.

[Chuck Nice]

Wow, that's pretty wild, Chuck.

[Neil DeGrasse Tyson]

Keep going. What do you have?

[Chuck Nice]

All right, let's have some fun with this one. This is Kevin the sommelier, who's.

[Neil DeGrasse Tyson]

Oh, we love Kevin the sommelier.

[Chuck Nice]

We love Kevin the sommelier. Never sends us wine. Well, no, never sends Chuck wine. Neil doesn't need any wine. I've been to Neil's cellar. He don't need no wine. Chuck needs wine.

[Neil DeGrasse Tyson]

Okay, all right.

[Chuck Nice]

All right. Kevin the sommelier says, Dr. Green, what Hollywood film depicts time travel best in your estimation? And then he says, is it frequency?

[Brian Greene]

Well, yeah, I had a brief moment in frequency. I tried to convince the filmmakers on how the end should be done, but it could have been more.

[Neil DeGrasse Tyson]

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

[Brian Greene]

Yeah, it's a. There's a time travel element where a father and a son are able to communicate across 30 years. And the father's long since been dead. He's a firefighter. He died in a fire. So the. The son tries to tell the father, go left, not right in that burning building when it happens tomorrow. And he actually. Actually saves him. So. So they do change the future in that particular case. So, yeah, there's a multiverse way of thinking about it.

[Neil DeGrasse Tyson]

They called you for advice on that?

[Brian Greene]

They called me for advice, and then they asked me if I would be in it. And I'm, like, interviewed by Dick Cavett. Dick Cavett in two different time frames. And they aged me to make me look old in one. And it was the scariest thing, really. I thought it would be so simple. But I don't know if you've ever done this. And they put the plaster on your face to mold so they can get jowls and things on you. Feels like being buried alive.

[Neil DeGrasse Tyson]

But, excuse me, Brian Black don't crack, so that wouldn't be necessary.

[Chuck Nice]

That's true. I'm 87. I'm 87 years old, Bryan.

[Neil DeGrasse Tyson]

Yeah, right, right. So Brian Were you in the movie? Brian? Were you in the movie? I missed it.

[Brian Greene]

It's like a scene where I'm in a television set being interviewed in the past and another television set in the future to like set up this time warpy thing.

[Chuck Nice]

Oh, cool.

[Neil DeGrasse Tyson]

Damn, Chuck, that's not his first movie. He was in. In the Mimsy movie. What's the name of that movie?

[Brian Greene]

The Last Mimsy. Yeah.

[Chuck Nice]

Oh, the Last Mimsy.

[Neil DeGrasse Tyson]

He was in the Last Mimsy.

[Chuck Nice]

Nice.

[Neil DeGrasse Tyson]

My boy was in the Last Mimsy. Interviewed as Brian Greene. We need expertise on this one. And you were. You were great in it. You like? Total natural. I loved it.

[Brian Greene]

Well, thank you. I appreciate that.

[Neil DeGrasse Tyson]

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

[Brian Greene]

Yeah, I would say that Interstellar is. Is probably the best one where you have Matthew McConaughey going near a black hole. Time slows down. So when he goes back to the ship, the fellow left on the ship is 23 years older, even though McConaughey is only like an hour older. And then Matthew McConaughey, I can't remember his name. His character in the movie, just to be clear.

[Neil DeGrasse Tyson]

23 years older. 23 years old. So the guy's gray.

[Brian Greene]

Yeah, yeah. It's sort of a very crazy scene, but it's accurate. You know, you go down near a black hole and you come back and your crew members have aged decades and you've aged an hour. But then in a very poignant moment, the character sees his own daughter, much older than he is.

[Chuck Nice]

Right.

[Brian Greene]

She's sick in her bed, as you may recall. And he kind of comes into that room. So those are accurate features of Einstein's general theory of relativity. So it's.

[Neil DeGrasse Tyson]

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

[Brian Greene]

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

[Chuck Nice]

It's a great movie.

[Neil DeGrasse Tyson]

And Chuck, did you. Chuck, did you see Interstellar?

[Chuck Nice]

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

[Neil DeGrasse Tyson]

Is that. Did you see. Did you see the name of the robot?

[Chuck Nice]

It's the big square robot. I forget his name.

[Neil DeGrasse Tyson]

Yeah, the big. The big rectangular robot, which, by the.

[Chuck Nice]

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

[Neil DeGrasse Tyson]

Do you remember the name of the robot?

[Chuck Nice]

I do not.

[Neil DeGrasse Tyson]

Brian, do you remember the name of the robot?

[Brian Greene]

I'm embarrassed to say I can't even remember the robot. So I'm okay.

[Neil DeGrasse Tyson]

The robot's name was Kip.

[Chuck Nice]

Kip. Oh, there you go.

[Neil DeGrasse Tyson]

Yeah, just thought I'd tell you. By the way, Kip Thorne, of course, helped pioneer ligo Laser Interferometer Gravitational Wave Observatory. Received the Nobel Prize, was the co recipient of it a few years ago. And Henry did the movie. So we got some good people out there. Nice. Trying to raise science literacy of the world a few notches. So. All right, there it goes. So good calculations there on the time. So that's just straightforward gravitational Einsteinian physics.

[Brian Greene]

Yeah, that's all that is.

[Chuck Nice]

Yeah, that's very cool.

[Neil DeGrasse Tyson]

Okay. All right.

[Chuck Nice]

Okay. So with respect, very quickly. Okay, do we have any.

[Neil DeGrasse Tyson]

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

[Chuck Nice]

I paid this month.

[Neil DeGrasse Tyson]

You a liar. That's what liars sound like.

[Chuck Nice]

That's exactly what liars sound like. And that is why I'm a terrible liar, because that's exactly what I sound like. I paid this month. All right, so wait, very quickly, do we have any information on the aging of cells at the speed of light or in gravitational situations that we might be able to attribute to time travel?

[Brian Greene]

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

[Neil DeGrasse Tyson]

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

[Brian Greene]

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

[Chuck Nice]

Aha.

[Brian Greene]

Things have elapsed.

[Neil DeGrasse Tyson]

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

[Brian Greene]

That's right. That's right.

[Chuck Nice]

Amazing, amazing, amazing.

[Neil DeGrasse Tyson]

All right, Chuck, keep going.

[Chuck Nice]

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

[Neil DeGrasse Tyson]

Both Kansas Cities, Casey, Moe and Casey Kansas. I guess that's the plural there. Right.

[Chuck Nice]

Very good.

[Neil DeGrasse Tyson]

Okay.

[Chuck Nice]

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

[Neil DeGrasse Tyson]

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

[Chuck Nice]

Yes.

[Neil DeGrasse Tyson]

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

[Brian Greene]

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

[Neil DeGrasse Tyson]

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

[Brian Greene]

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

[Neil DeGrasse Tyson]

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

[Brian Greene]

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

[Neil DeGrasse Tyson]

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

[Brian Greene]

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

[Chuck Nice]

Right?

[Brian Greene]

Because the time difference will then persist.

[Neil DeGrasse Tyson]

Wow.

[Chuck Nice]

Jesus. That is insane.

[Neil DeGrasse Tyson]

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

[Chuck Nice]

Yeah.

[Neil DeGrasse Tyson]

This is. This is. Must be a very significant force operating on your brain.

[Chuck Nice]

Instead, I should have said Father Time.

[Neil DeGrasse Tyson]

After investing billions to light up our network. T Mobile is America's largest 5G network. Plus right now you can switch. Keep your phone and we'll pay it off up to $800. See how you can save on every plan versus Verizon and AT&T. @t mobile.com Keep and switch up to four lines via virtual prepaid card. Allow 15 days qualifying unlock device credit service ported 90 plus days with device ineligible carrier and timely redemption. Required card has no cash access and expires in six months.

[Yara Shahidi]

Hey, I'm Yara Shahidi, and I'm the host of the Optimist Project. This is the podcast that asks what gives you hope. Each week, I sit down with change makers you may or may not know from comedy, music, academia, and more to uncover what inspires them to create a better tomorrow. Join us as we find out ways that we can cultivate optimism in our own lives. You can find the Optimist Project wherever you get your pieces. Podcasts, don't forget to follow the show so you never miss an episode.

[Chuck Nice]

Do you ever watch TV and think, wow, I'm really good at this? You're right. With rewards on sling, watching 30 minutes of TV daily gives you chances to win up to $10,000 in cash and other monthly prizes. Sign up for Sling or Stream for free with Sling Freestream to get rewarded for watching TV. Sling lets you do that. Visit sling.com to learn more and get started. No purchase necessary for it. We're prohibited by law. Visit sling.com for more details. Alrighty, here we go. This is Jim Kelly, and Jim Kelly says. Hi, Dr. Tyson, Dr. Green, Dr. Comedy. Why do physicists. Physicists assume that all time travelers are murderous, petricidal maniacs? Just. Just kidding. But how does a hypothetical paradox preclude the existence of time travel?

[Brian Greene]

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

[Neil DeGrasse Tyson]

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

[Brian Greene]

Yeah, I Mean, I like to think of information as more concrete than that description might suggest, because information is always carried by particles. You can have an abstract measure of information, but if you want to look at the motion of information, it's got to be the motion of stuff that carries that information. And to me, that makes it much more clear what's going on. So with black holes, the whole question was, as radiation comes out of a black hole, which Hawking, Stephen Hawking told us will happen, are the particles.

[Neil DeGrasse Tyson]

Instead of Louis Hawking, you mean Stephen Hawking.

[Brian Greene]

That's right. That's right. They always get those confused. Different universes.

[Neil DeGrasse Tyson]

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

[Brian Greene]

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

[Neil DeGrasse Tyson]

Okay, because. Right, because otherwise, there's no information without stuff to carry it. That's another way.

[Brian Greene]

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

[Chuck Nice]

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

[Brian Greene]

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

[Chuck Nice]

Right?

[Brian Greene]

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

[Neil DeGrasse Tyson]

But the chair itself doesn't come out. You have to still make the chair. That sounds like a cop out.

[Chuck Nice]

It's now an IKEA chair.

[Neil DeGrasse Tyson]

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

[Chuck Nice]

Black hole.

[Neil DeGrasse Tyson]

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

[Chuck Nice]

Wow, that's amazing.

[Neil DeGrasse Tyson]

All right, Chuck, keep them coming. Keep coming.

[Chuck Nice]

Okay, let's keep it going. This is from Y coss. And Y says, hello, Brian. Hello, Neil. And Then he puts in parentheses with a question mark, Chuck. Okay, that ain't right.

[Neil DeGrasse Tyson]

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

[Chuck Nice]

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

[Neil DeGrasse Tyson]

He's all in. He's all in. So let me toss to Brian here after I say something. So in Back to the Future, they kind of. In the original, they kind of avoided that problem because when Marty went back in time, he went back in time in a precise whole number of years. Okay. And so Earth would be back where it is when he goes back in time in its orbit, you know, enough for the plotline. But if he went back a week or even an hour, he's not in that parking lot anymore at all. So, Brian, tell me about all this time travel mechanisms, when in fact, the universe is changing. So you can't just show up in the same place and expect to be home again.

[Brian Greene]

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

[Neil DeGrasse Tyson]

Okay, interesting. I bet they don't know that. But we accept it anyway, right? As a.

[Brian Greene]

Sure.

[Neil DeGrasse Tyson]

It's implicit. It's implicit, I guess, is what that is. Yeah. All right, Chuck, keep them coming.

[Chuck Nice]

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

[Neil DeGrasse Tyson]

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

[Chuck Nice]

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

[Neil DeGrasse Tyson]

Just legal weed, not illegal.

[Chuck Nice]

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

[Neil DeGrasse Tyson]

Or the worst. No, no. While we're doing it, we'll think it's the best and then afterwards we'll know it's the worst.

[Chuck Nice]

I just kind of how that goes. All right, this is Savage 160 and Savage 162 says, in the Frontier of Science, is there any idea on how to try and detect the theorized tachyon particle?

[Neil DeGrasse Tyson]

So remind us what a tachyon is, Brian.

[Brian Greene]

Yeah, so it's what you had mentioned earlier, Neil, this idea of a particle that can go faster than the speed of light. And when a particle can go hypothetical, I should say particle goes faster than the speed of light. There are observers watching that particle who will see it reach its target before it was emitted by the source. Reaches target before it was emitted by the source. So cause and effect become flipped for the observers who are witnessing the motion of a tachyon. So the question is, how would you ever detect these things? Yeah, and it's, it's, it's a bit, first of all, you know, to measure something going faster than the speed of light, that's actually not that hard. You start with a particle at one location and you fire it and you simply calculate or you measure, I should say, how long it took to get to the target, did it beat a light beam or not? And that's it. So, so that's pretty straightforward. And indeed, you may recall, I don't know, some 10 years ago, there was.

[Neil DeGrasse Tyson]

A neutrino thing in Italy. Right?

[Brian Greene]

Yeah, there's. In Italy, there was a claim that these neutrinos had gone from the source to the target and beaten a beam of light. They'd gotten there earlier than a beam of light would. Now, some of us knew that could be.

[Neil DeGrasse Tyson]

So, Brian, you know the joke about that, the euro joke, because Europeans like joking about each other. They said that would have been the first time Italy arrived anywhere early.

[Brian Greene]

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

[Neil DeGrasse Tyson]

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

[Brian Greene]

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

[Chuck Nice]

Right.

[Brian Greene]

And it wasn't true.

[Neil DeGrasse Tyson]

Right. And it wasn't true. Which is.

[Chuck Nice]

Right.

[Neil DeGrasse Tyson]

Right.

[Chuck Nice]

Wow.

[Brian Greene]

All right, cool.

[Chuck Nice]

Okay. Okay, let's see. Here we go.

[Neil DeGrasse Tyson]

Time for a few more Ryan.

[Chuck Nice]

This is just Ryan. He's like Cher just Ryan. He says, hello, doctors. If we developed a way to time travel backwards, could we travel to a time before the Big Bang, or would we just break science altogether?

[Neil DeGrasse Tyson]

I love it, Brian.

[Brian Greene]

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

[Chuck Nice]

Right.

[Neil DeGrasse Tyson]

Is this the same. I think I've heard Hawking, this is Louis Hawking, told me this, that going before the Big Bang might be like asking what's north of the North Pole. The whole grid system is defined there. And you can't. Once you're as far north as you can go, you can't go farther north. Once you're as far back in time at the beginning, you can't go further back.

[Brian Greene]

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

[Neil DeGrasse Tyson]

So you just. You'd have to pass out of our sort of space time structure and enter somebody else's space time. If you want to go before what happened with the Big Bang. That'd be interesting.

[Brian Greene]

Yeah, that's right.

[Chuck Nice]

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

[Neil DeGrasse Tyson]

Different train, you're a different thing.

[Brian Greene]

You're gonna have to pay another fare.

[Neil DeGrasse Tyson]

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

[Chuck Nice]

That's what you get for jump of the turn style.

[Neil DeGrasse Tyson]

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

[Brian Greene]

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

[Neil DeGrasse Tyson]

And if we're not interacting with it.

[Chuck Nice]

Yeah.

[Brian Greene]

Therefore they're sort of more of an observer status as opposed to a participant status. It kind of resonates with that. But, you know.

[Neil DeGrasse Tyson]

But don't you need a different universe now? Because there were some different outcomes. Right. So now there's. You gotta.

[Brian Greene]

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

[Chuck Nice]

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

[Brian Greene]

That's right.

[Chuck Nice]

So the past never changed. Yeah, that.

[Brian Greene]

I'm trying to remember the story well, and if that. That seems right.

[Neil DeGrasse Tyson]

Yeah, I think. Yeah.

[Chuck Nice]

Chuck is more current on that. Never changed. He changed.

[Neil DeGrasse Tyson]

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

[Brian Greene]

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

[Chuck Nice]

Wow. Damn. That's amazing.

[Neil DeGrasse Tyson]

All right. Damn. All right. We gotta land that plane right there. Brian, it's been a delight to have you back on StarTalk.

[Brian Greene]

Always a pleasure.

[Neil DeGrasse Tyson]

It's really. You're just up the street. We should have you on even. We should be like a regular Brian feature, right? Give Brian his own minute. Minute Brian. No, we'll market it.

[Brian Greene]

Make it two minutes. Make it two minutes.

[Chuck Nice]

You make it. Whatever he was called. Brian Time.

[Neil DeGrasse Tyson]

Brian.

[Brian Greene]

Near the speed of light. Brian Thomas, near the speed of light.

[Neil DeGrasse Tyson]

Oh, yeah. You got it. There you go, guys. Great to have you, Chuck. Always good to have you there.

[Chuck Nice]

Always a pleasure.

[Neil DeGrasse Tyson]

Cosmic queries. I am Neil Degrasse Tyson, your personal astrophysicist. As always, I bid you to keep looking up.

[Yara Shahidi]

Hey, I'm Yara Shahidi and I'm the host of the Optimist Project. This is the podcast that asks what gives you hope. Each week I sit down with change makers you may or may not know from comedy, music, academia and more to uncover what inspires them to create a better tomorrow. Join us as we find out ways that we can cultivate optimism in our own lives. You can find the Optimus Project wherever you get your podcasts. Don't forget to follow the show so you never miss an episode.

[Chuck Nice]

Do you ever watch TV and think, wow, I'm really good at this? You're right. With rewards on sling, watching 30 minutes of TV daily gives you chances to win up to $10,000 in cash and other monthly prizes. Sign up for Sling or Stream for free with Sling Freestream to get rewarded for watching TV. Sling Stream let you do that? Visit sling.com to learn more and get started. No purchase necessary for it were prohibited by law. Visit sling.com for more details.