Traveling Through Space and Time, with Janna Levin
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Welcome to StarTalk All-Stars. Hi, Matt. Hey, Jana. I am your All-Stars host, Jana Levin. I am a professor of physics and astronomy at Barnard College of Columbia University and also director of sciences here at Pioneer Works, where we are. And I'm welcoming Matt, who is co-host of Probably Science, as well as a hysterical comedian. Hey.

Thank you. But his role here is just to like be my pal today. I want to know how you end up on the all-stars. Like how does this work? Yeah, I don't know how did you end up on the all-stars? Who selected you? I don't know. I think I just got selected. I think you got called up to the main team and then I'm just subbing in. I think I selected you. Well, I appreciate that. Well, welcome. Today we're going to do Cosmic Queries. It's been a long day and we're like, you've been traveling the world. I have. By the world, you mean the west to the east coast of America. Isn't that the world? Yeah.

It is as far as baseball is concerned. It's the World Series area. And if only we could, you know, control time, then it would all be so much more relaxing to have this conversation. Well, there may or may not be questions about that right now. Oh my gosh, really? I don't know. I had no idea. What do we got? Well, these questions, these are all themed. The hashtag was...

CQ interstellar travel. I think the CQ is cosmic query. So it's just the interstellar travel bit. I should have known that lingo. Right. I feel a little slow. So everything here is interstellar travel based. Manos on Twitter says, unless we come up with a way to rip space time, what brackets wormholes, et cetera, it's not going to happen. Am I right? Is Manos right? About temporal travel. Yeah.

Well, I mean... About space travel, I think, specifically. Okay, well, so here's the thing about... Well, let's do time travel first. So here's the thing about time travel is that the question... There's two points to that question. One is whether we're going to do it

Right. Through technology or whether it's possible in terms of nature. So it's possible nature could create its own time machine and that we don't have to do it technologically, but it wouldn't be us. Right. Some some other entity in the universe like you can have two cosmic strings crossing that create these weird conical folds that allow some like some critter.

not where we live, right, but sound critter to do a loop where they come back in time. So that's actually naturally possible. It doesn't require technology. So that doesn't contravene the rules of physics, the laws of physics as we know them? You know, it's really disturbing, but the laws of physics, as Einstein set them down, allow for time travel.

And there's a couple of circumstances that occur naturally, but not in our universe. So Gödel, the famous mathematician, knew that there was a rotating space-time. Like if the entire universe was spinning, which is not our universe, right? So it's not going to be us. But if the entire universe was spinning, that he could find a path, a world line, the line of some naturally occurring entity that would go into its past. And he used to walk to the Institute in Princeton with Einstein and talk about it.

And Einstein was astounded that Gödel found the solution, but didn't disbelieve it. Understood that it was allowed, and his theory was very concerned. But then didn't Gödel also starve to death because his wife went to hospital? Yeah. So, you know. So that happened, but... So on the one hand, he knew a lot more about space and time than I do, but on the other hand, I can cook. Yeah, you can feed yourself. So I'd say we're pretty much equal.

Yeah, you know, it's, you know, nature gives greatness and weakness in equal measure. And that's a lot. We're all about average. All right. We're all about average at the end of the day. So interstellar travel, is that going to be possible without something like wormholes? So, man, if we could figure out what the dark energy was and we could make it.

then you might be able to do warp drive, which would allow interstellar travel. So right now, Voyager spacecraft has just broken out of the sun's magnetic influence, right? It's barely, barely out of the solar system. It's the first interstellar probe humans have ever sent, right? And we mailed it in like the 70s. So it's taken its time to get out there. And it'll take something like 10,000 years for Voyager to hit another star system or something like that.

But if you were traveling at the speed of light, you could get there in a few years. So if we can harness things like dark energy, if we figure out what it is and we can make it at the Large Hadron Collider and we can like put it in a barrel and take it out into space, we could suck distant parts of space closer, step across it and then expand it again.

And so it wouldn't be that we'd be traveling faster than the speed of light. It's that we'd be pulling space-time in, making a small step for humankind, and then pushing it back out again. And you can do that faster than the speed of light and not violate any laws. So that's kind of what warp drive would look like. Not technologically impossible. It's just very unlikely given the way, you know, the vote goes in Congress this week. You know what I'm saying? Right. Like the way North Korea is looking.

So we're likely to blow ourselves up before... I got you. Yeah. Well, that's a dark way to end it. So Neil Cochran on Twitter asks, if we can achieve the speed to explore the universe, isn't inertial dampening going to be an issue? Inertial dampening? Um...

What is inertial dampening? Inertial dampening really only matters, well, I guess it matters anywhere because there's gravity anywhere. So it's hard to push a car, but it's easier to push me around. Right. Right. And that's because I have less inertia than the car. But in completely outer space, it's not that expensive to push anything. Right. It's really easy. Yeah.

It's really quite easy. But F equals MA, you know, Newton's laws. Oh my God, I said an equation in audio, but everyone knows F equals MA. And if they don't, they're going to learn it today. Newton's laws, the force equals the mass times the acceleration. Yes, it requires greater force, the greater something, the greater the inertial mass. That's true. But it's still pretty easy to push a car in outer space. It's a lot easier than it is to do it down here on Earth. So you just surely, whenever you're moving anything across space, you just need as much energy

deceleration as you have acceleration. Yeah. I mean, the thing, the difference is, is that if I push something in space, okay, maybe it's a little bit expensive to get it going. Yeah. But once it's going, like I can pretty much just kick back and it's going to go forever. Right. That's going to be very little that slows it down or stops it. Unlike on earth where there's all kinds of forces, friction and gravity that slows things down. And you know, if I get something launched and

interstellar space it'll keep going for a long long time i see you reading i am looking pensive what do you i am reading so um so marco pedroso do i get to ask you some of these questions yes throw these questions at me professor of faster physics and see how the idiot comedian pairs um come on you have a degree in mass i i do but i i scrape my way through it

And that was well over a decade ago. And I have done nothing with it since, apart from create a podcast that tenuously has science in the title. It's only probably science. That's why we put it in there. It's not definitively science. It was named very carefully. The only thing we do carefully on that show. I've been on that show. You have. We carefully book people who know a lot more than we do. That's the other thing we do carefully. Marco Pedroso on Twitter says...

Guys, honest question. I like that little preface. As opposed to the rest aren't? Like, is he implicating the others? Yeah, or implicating the- His other friends who like 30 seconds earlier pressed send? Every other question you're getting is bullshit, but this one- This one's honest. Well, my answer may or may not be, depending. Yeah, so honest question. Ships navigating deep space should be shrouded in total darkness, right? Yes.

Well, I mean, except for the light from the galaxy. That's not total darkness. What's deep space? Galactic space? I don't know. What counts as deep space? It would be pretty dark. Okay, away from the sun, it gets dark. I mean, it's pretty dark at Pluto. It gets dark the further away you are from any individual stars. I mean, I guess you could go intergalactic.

And that's pretty dark. Yeah. But then you would still have... Wait, we're only on the first clause. He's got a comma here. There's more coming. Wait, he would have what? No, that's the full question. So, yeah. So, hang on. Just thinking of... I don't know what you count as deep space. Do you mean, is deep space the space between individual stars? Or is it the space between individual galaxies? Or... Can we write him back and ask him what he means? I don't know. I mean, it's an honest but ambiguous question. If it's interstellar, it's dim. If it's intergalactic, it's really dim.

Pretty dark. But you are still getting the light from millions of galaxies. Sure. But each of those is just a pinpoint, like a single star, right? Yeah, like if you were a planet, a rogue planet that had somehow been ejected from the galaxy. Which I'm sure happens, right? Okay, planets it's hard to eject, but maybe stars and entire system. So like let's say when we collide with Andromeda, we're somehow...

the two galaxies, the Milky Way galaxy and Andromeda galaxy collide and our solar system gets knocked about. Right. And it gets thrown, possibly unlikely, but possibly completely out of the galaxy. So the whole solar system's floating.

Intergalactic. And takes us with it because we're just still traveling in its wake and its gravitational field. Yeah. It's actually, it's probably kind of normal for us because the sun's still illuminating us. Right. So we have to lose the sun. Okay. Okay. So let's say we make up some scenario where some evil genius ejects the planet and sends it intergalactic. It's going to be dark. Yeah. Yeah.

If you could survive that utter darkness, you would see galaxies and you would see other things in the universe, but you would not have the warmth to survive, probably. Or the light to see anything in your locality. You couldn't see it. Right, exactly. There wouldn't be enough light coming from these galaxies or stars to then bounce off things and illuminate. Right, but imagine the decrease in light pollution. I was just thinking that. That would probably be a great way to go stargazing. Great view.

Great stargazing. Seriously good astronomy. Because right now we're in New York City, which has to be one of the worst places in the world for light pollution, right? It is terrible, but we are in Red Hook. And so we're in, as far as New York City goes, we're probably ideally located because we're right on the water. We're looking at Manhattan, which is way, way over there. And there's not a lot of street lights or a lot of light pollution around here. We actually have pretty good viewing. That's pretty good. I live in Los Angeles most of the year, and that's...

But again, you can just go a little bit out of town. You can get into sort of Joshua Tree, that kind of area. And suddenly you're in the middle of a desert. Yeah. I mean, strictly speaking, Los Angeles is a desert. It is. I don't mean that as a jab. I just mean that literally. Like technically it's a desert. And you sometimes forget that. Yeah.

Because you're surrounded by the trappings of non-desert, like a lot of grass that has been expensively imported. Palm trees, which have been imported. With a lot of unnecessary water usage, and you forget that, and then you go out for the day. And you're looking down so much that you forget to look up at the sky. Yeah, and then you realize, why am I so exhausted today? Oh, because I haven't drunk water and I'm in a desert. Yeah.

It's a little tip for anyone who's visiting California. Drink water. Any questions about hydration? Yeah, throw them my way. Remember to tag them CQ Matt's hydration tips. Okay, so we're on to our first Instagram question, Jana. Does it come with a picture? Can I see it? It doesn't. It's just text. Rishirish on Instagram asks...

traveling at light speed is now just an engineering problem. Care to explain? So that's in quotes. The first bit of that's in quotes. I'm wondering whether that's something that Neil or someone else on the show has said at some point. Because it's,

Or this is just something that Rishi Rish has read. In quotes, it says, traveling at light speed is now just an engineering problem. I'm going to actually back him up on this. There are these nanosatellites that are like the size of a postage stamp that they want to put a couple of computer chips on and send into space. And they're going to kind of laser them like...

accelerate them with the pressure of a laser and they think that they can get them to lights very nearly light speed

Meaning a fraction, like even if it's 0.1, the speed of light, that's really fast. That's insanely fast. So light speed is 300,000 kilometers per second, so you're going 30,000 kilometers per second. That's insane. And they think they can do it with these tiny postage stamp things because they're so light. So what happens when you try to boost things to the speed of light is that it becomes energetically more and more expensive. Technically, the inertial question, we can tie it to one of the earlier questions, is

things get effectively heavier. They get more inertia. This is something I vaguely remember from, this is special relativity, right? Special relativity, just straightforward special relativity. The faster something gets, the closer to the speed of light, the heavier it becomes, the more inertia it gets. Right, the more inertia it gets, and so the harder it is to push it. And so in that sense, it is just an engineering problem. So you want to send postage stamp

sized stuff. Right. Because also in space... You can't send astronauts and fuel and rockets because that as an engineering problem is insurmountable at the moment. But in space you also have the advantages that you talked about at the beginning of the show where...

There is almost nothing slowing things down. So any acceleration is just cumulative. You can just keep accelerating things. That's right. So you can get something to some speed. Maybe it's a hundredth or a thousandth the speed of light and it'll pretty much keep going that way. I mean, it's, you know, if it's really tiny, especially because it won't be slowed down by passing by big planets or passing by new star systems. Or even particles just floating in space. Well, that could be a problem. Okay. That could be a problem. Like winds. Yeah.

Solar winds could blow you back pretty bad, you know. So that could be a concern for these projects, I think, in general. But they're talking about sending many, right? So just if they send many, it's like running a lot of horses. And you just hope a couple make it. So where is this laser that's going to be powering it going? It's like turtles. Yeah. Yeah. So, I mean, that sounds very sci-fi. I don't know that much about the technology, but I guess the lasers mounted here on Earth.

And, you know, light doesn't tire. It can make it as far as you can send it. And they'll just keep blasting this thing. So I think they recently, wasn't there from the ISS, I think one of the astronauts manually tossed out a nanosatellite. Literally. Like he was spacewalking. And he just sort of like sprinkled it. Through it. Yeah. And hoped for the best. So I think they're starting like preliminary ideas on these tiny, tiny satellites. Yeah. Yeah.

That's a, but you could never actually, it said traveling at light speed. Now at light speed itself, anything that has mass can never reach light speed, right? That's right. It can never reach light speed because it becomes infinitely heavy and infinitely hard to push just that little bit further. Right. But it can go a fraction of the speed of light. It's pretty good. So, I mean, 0.1, the speed of light, like we said, that's 30,000 kilometers per second. I mean, how long did it take you to get from California? Yeah.

Car far away is California. We would definitely, so... 3,000 miles? Yeah. And it took you six hours? I know. Okay, so you would have gotten here in a fraction of a second. And that's better. And I think we should really work towards that. That's better. Because you can even deal with economy class in that. Are you for that? I could do that, yeah. Because even like economy class in that kind of speed, you sort of eat just cramps. Yeah, two seconds. You know, the boredom of...

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I'm ready to take more of these cosmic queries. Well, let's go. Did people send us more? They certainly did. We've got a whole batch of them. I was at the beginning, I was like, are we going to have enough questions? So I was kind of spacing it out. And now I'm like, are we going to get through them? Are we going to get through them? So Beaucraft on Twitter says, assuming interstellar travel is mastered by humans, what about it excites you and what scares you?

Interstellar travel. Well, I mean, what about it would not be exciting? You know, it's amazing. We would go to it. We have planets now. We have exoplanets by the thousands in our own galaxy. That is something strange that's happened in the last...

10 years. That's really recent, right? Really recent. The science of exoplanets has gone from we might find one to we can't stop finding these things. So now pretty much they think that, I mean, minimum one-fifth of all star systems have planets. There's 100 billion stars in the galaxy. So one-fifth is a big number, right? It's billions. So that's, again, the chance of us finding something approximating life of some way. Right. So each one of those systems could have multiple planets. Right.

So that makes more planets than there are stars. And each of those planets could also have moons because like... Yeah, and the moons are also habitable. So probably like if you were in the solar system, we talk about Mars, but like in an emergency, don't go to Mars. Go to, I don't know, Mars.

We just had something about Europa on our show, and that's the only reason I know this, but I happen to know that Europa has a lot of water and... Yeah. Well, you know I don't work on anything that happened more recently than a billion years ago, so... Right. Europa's like local politics. This is way into practicality. You're this stuff. For the Odeonee, the people, I'm pointing to a whiteboard behind us covered in symbols and squiggles. Yeah, that's the good stuff. Yeah. But so, interstellar travel would mean we could actually go visit these planets, but...

Like Interstellar, the movie, which was quite accurate in a lot of ways, because Kip Thorne, who's the great astrophysicist from Caltech, wrote the treatment for that movie and was very involved in consulting on it. And a lot of that's incredibly accurate. You would burn a lot of years traveling. It might be that your years were shortened, like you could Interstellar travel for like five years and come back to the Earth, but 100 years might have passed on Earth.

So that's a pretty big sacrifice. - That's a scary thing. - Like the guys who go up to, so I met Scott Kelly today, the astronaut. - That's right. - It was really great hanging out with him, like for the few minutes. I don't know if it was great for him. I wanted to ask him stuff and I couldn't think of anything, but he probably aged a few seconds more slowly than his twin brother.

Because of being on the International Space Station. That's right. Scott Kelly was famous, has a twin brother who was also an astronaut, right? Yes. He's also part of NASA. Yes. And part of the experiment was the brother stays on Earth and the other one goes to space. Which was a thought experiment from decades back. Insane. And...

I mean, he had many more physical effects that were much more important than the time dilation. But basically, he aged more slowly than his brother. But, you know, they're not going to notice. But if you interstellar travel, you're going to come back. And let's say Scott Kelly goes and interstellar travels and comes back. And then his brother is really, you know, maybe 70 years older. And he's a couple years older.

That is a... Okay. So that's scary. That's a great answer. That's a scary part. That is a great answer. Yeah. From Jay Z McGovern on Twitter. What is hyperspace and how does that differ from warp?

Asking for a friend, says Jay-Z. Not having it, Jay-Z. You own it. You own that question. If this is like a game show and I'm allowed to reach for a lifeline, I'm allowed to pipe this back to you because you're the popular culture guy. I hardly know what year it is. I'm surprised I know what day of the week it is. That's right. You live in a world of dimensions and figures. So as the pop guy, I think you have to tell me what hyperspace is.

I think... I'm pretty sure the only difference is the science fiction franchise that the words have come from. I don't... I'm pretty sure. I'm...

I think we're just talking Star Wars and Star Trek. Do you have stock in any of these franchises? No, I don't. And also, I'm not good on pop culture either. Well, I think hyperspace, I think what they probably mean is extra spatial dimensions. Should we just assume that that's what they mean? I don't know. Can we opt to answer whatever questions you want? I honestly think they're just two different ways that different sci-fi things have used to describe going faster than light. But you know what? Sci-fi can be very provocative in terms of generating ideas. Right. So I'm not dissing sci-fi.

But I'm going to answer the extra spatial dimension question because I can. Is that fair? Yes, go for it. So it's completely possible we live in extra spatial dimensions. I work on this as a serious science project, science thesis, that the universe has more than three spatial dimensions and that it's an illusion.

Because every day we do this experiment where we live in three dimensions. Every day around the world, billions of people confirm this experiment. There are three spatial dimensions. But it actually could be an illusion. And so one of the interesting possibilities is that something like dark energy is actually a quantum energy trapped in extra spatial dimensions that are just very, very small. Isn't extra dimensions also one of the reasons, one of the things that's posited as the reason why

Gravity is so much weaker than all the other forces. Yeah, so it's a very clever suggestion by a bunch of theoretical physicists like Nima or Connie Hamid Lisa Randall Demopolis Diwali a lot of interesting people worked on this that It's like you've diluted gravity over this huge volume, even though the extra dimensions are small. They're kind of everywhere and

Right? Because gravity is... And so it makes it very diluting. Like, if you ask which direction is up right here, there's up. But if I go a little bit over, where's up? It's right here. So the dimension up exists everywhere. Okay. Do you know what I'm saying? So those extra dimensions similarly would exist everywhere. If I was right here, I'd be like, where's the extra dimension? Well, I can't really point to it, but it's at every single point in space, there's that extra dimension. This could theoretically explain why...

tiny magnet can override the gravitational pull of the entire earth? Yeah, because the argument would be that the electromagnetic force, which is responsible for the magnetic pull, is bound to three dimensions. Maybe it's glued to a sticky brain, which is a membrane, which lives in three dimensions. But

Gravity has to live in space-time. However big space-time is, that's where gravity has to live. Which is why it's-- It is equivalent to space-time. So gravity doesn't have the option to confine itself to a smaller space, so it kind of blobs out. And as a result, it actually makes it-- it dilutes the strength of gravity. Yeah.

How widespread is that theory? Why would you know this weird? I don't know. I don't know where that little thing is. Let me come up with this kooky knowledge. But I do know that gravity is incredibly weak compared to electromagnetic force. It is incredibly weak. It's something like a trillionth of a trillionth of a trillionth.

the strength between two electrons gravitationally versus electromagnetically. Well, that conveniently leads us straight into the next question from adved on Instagram. What kind of gravitational force exists in space such that smaller bodies revolve around the bigger bodies and not fall upon them?

Well, I can put an apple into orbit just by throwing it fast enough. So the International Space Station is traveling at 17,000, over 17,000 miles an hour, I think it is. Maybe Lindsay over there has got a little fancy machine where she can, I don't know, connect to space and find out.

But I think it's over 17,000 miles per hour. And if it was going slower, it would drop to earth like a stone. Right. Okay. So it's not about sizes of bodies. It's about how much you fling them. You know, so if I take an apple and I drop it, it goes straight to the earth. Bam.

bang, splat. If I throw it, it travels a little bit further on an arc. If I throw it at 17,000 miles per hour, I could put that thing in orbit. Right. Right. At happy Apple. Doesn't matter how big it is. How is that experiment going? How far have you got it so far? Um, I've gotten like not to the wall. Okay. You see the Apple splats. It's a big room. Progressively. It's a big room. And that's impressive. And I really, these are our beautiful new science studios, which we're proud to have renovated with support from the Simons foundation. Yeah.

Good plug. Good getting in the plug in the middle of that. I didn't even think I was going to get a plug in. It occurred to me. I should have thought of that sooner. Okay, Holly Ann Lang on Instagram. When you're traveling in space, do you feel the effects of time dilation as you move closer to strong gravitational forecast? Love this stuff. Does the G-force affect your experience of time? Okay, so here's something that took me a very long time to struggle with studying relativity. Your experience of time is insane.

unchanged. You don't notice that your time is dilated relative to somebody else's time. And that's really interesting. So literally if I am falling across the event horizon of a black hole, like what could be worse than that? Right. I, which is about as strong gravitational pull as you can find. Yeah. The strongest, right. Except for the interior, which is your worst fate, which you're, you know, impending fate. Um,

You would absolutely nothing you could do would let you know that your clocks were acting in any way funny at all. You would feel totally normal. And this is something that people really struggle with. They think if your time is dilating that you experience it in a strange way, but you don't.

you think it's absolutely normal as you're crossing the event horizon, you're looking at your clock, you're looking at your watch, whatever, you know, you're juggling. I don't know what you're doing. There's no gravity. Forget it. You're not juggling. Well, you could juggle quite impressively because you're just like placing each ball in the air and then picking it out in a triangle. Right, exactly, because you're free-falling. You're free-falling like the astronauts in the space station. But it's only when you compare to somebody far away that you realize like,

oh wow something weird's going on they're going really slowly so when we're comparing our kelly twins our twin astronauts yeah each kelly you have to compare them each kelly thinks that the time is perfectly normal it's only when they re-meet up and realize that one of them is decades older than the other yeah it's nobody gains extra time it's not a way to get more squeeze more life out right okay you it doesn't work like that your experience is totally normal

No matter what. And then you've got about a millisecond before you're crushed to death in the center of the black hole. So that's not good. Yeah. Is it possible to skirt the edge of a black hole, to skirt the event horizon without falling in? It's pretty hard. It's pretty hard. It's exactly the same argument as we made earlier about inertial mass, is that it requires a tremendous amount of fuel.

right to boost you out of it right so like it's still just like you're still your apple going around the earth problem where it's just needs to be going quicker it's just an engineering problem is one of our earlier questions read so like to get off the earth

I can't remember the number. I feel like it's 200 and I don't know it's maybe 20 kilometers a second. I don't know the escape velocity from the earth something. I don't remember the exact number. But if you're getting closer and closer to the black hole, your escape velocity, how fast you need to be moving so you have to accelerate until you reach the speed becomes closer and closer to the speed of light.

And the closer and closer the speed, the lighter the heart. The harder it is to accelerate, exactly. And the more inertial mass, you know, the more inertial resistance you're going to experience. So it requires just more fuel than exists in the entire solar system to boost a postage stamp off the event horizon, basically. So I've got a question for you.

So you're near the black hole. Yeah. You're accelerating faster and faster, closer and closer to the speed of light. Yeah. Due to special relativity, as you're doing that, you become heavier and heavier. Yeah. But that's your weight, not your mass, right? Yeah. Because the gravitational pull between you and the black hole is proportional to your combined masses. Yeah.

But your gravitational pull to each other doesn't increase as you get faster, right? It doesn't... Well, no, it's not that it increases when you get faster. It's that you would be better off just turning off your rockets and falling in. Right. And letting gravity do the work for you. And we do that in the solar system. We use slingshots. Like, we slingshot past Jupiter to send probes faster than they were going when they passed...

the planet by using gravity to our advantage. So if I were you and you really wanted to go into that black hole, just turn off your engines and fall. - That is actually a question the nerdy dolphin on Instagram actually asked, also Jack from Australia, it says underneath that, will exploiting the density of a black hole to slingshot a ship be an effective method of travel? - Oh yeah. - According to you, potentially yes. - Awesome, awesome.

Because you would steal a tiny amount of the energy of that black hole. There's so much energy. Like if I drop a rock from the Empire State Building, you know, even including air resistance, it hits the earth pretty hard. Right. Right. If I take a little piece of atmosphere like cotton candy off our neighboring star and drop it onto a black hole, it's going near the speed of light by the time it splats. Right. And it usually only splats because it's bumped into other stuff that's orbiting the black hole.

right? And so that creates this incredibly bright illumination. It's one of the brightest things in the entire galaxy that we have to see is the luminosity of stuff dropping onto black holes. So there's tremendous amounts of energy gravitationally just from falling that you can extract. But if you're using it as a method of travel. So instead of splatting, if you went on a little bit of a slingshot, just, you know, like it's what we were saying, the person was asking why

why do you go into orbit? It's just because you're not traveling straight down. You're like being tossed. So if you just travel a bit around it and not falling in, you could slingshot and get a lot of energy out of it. And you're stealing a tiny amount of the speed that that black hole is moving through space. It's not even that the black hole is moving through space. It's that the black hole literally slingshots you. It's literally it slingshots you. And so you're borrowing your gravitational energy that's released when you fall. So like I take a rock,

from rest if i let it go it's fastest right before it hits the ground but don't you lose that doesn't some of that speed go when you're then moving away from the black hole again because it's going to be pulling you back in yes so how do you you still can win okay i mean strictly speaking you know something like you know the whole pendulum experiment like if i take a pendulum uh-huh um

and I let it go, I have to trust that if I let it go right at my face, I have to trust that it can't swing faster and hit me in the face. So if I let it go right in front of my face, it will swing away and come back. I've seen people do these demos with these huge, huge bobs that would knock them out, but they don't flinch because they believe in the laws of physics. It can never come back closer.

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If a black hole carries the possibility of being a wormhole for space travel, what happens to both ends, point A and point B, when two black holes merge?

I think the person is envisioning that inside a black hole is a wormhole. So you fall through the event horizon. So the event horizon is a region beyond which not even light can escape. So we live outside event horizons by definition. We live over on this side of the one-way window, right? But if you're a fool who crosses and goes to the center of the black hole, maybe you discover in your last microsecond of living that there's a little wormhole that connects to some other part of space-time.

And people have pontificated about this, but it's very hard to justify or prove. And it's probably not the truth, but it's an interesting idea, right? So I think the person's asking, now two black holes collide, what happens to the wormholes at the center? Right, right. I don't know. There's almost... I don't know. I don't know. It's too crazy. It's too crazy a scenario. But it would be kind of interesting, like...

Maybe they make a third bridge. I mean, I don't know. You have to, in my field, you have to be able to do the math to answer the question. And we can't do it yet. We don't know yet.

how to follow the chalk. We don't know how to write down the equations and pursue them to the end. Right. Can you test to see if those equations are right? Because by definition, you can't see inside an event horizon, right? You can't see anything that's come out of it. You know, it's a really good question. If I had the equations and I could pursue them to the end and I told you this is what I think happens, you could rightfully say to me, well, you can't test experimentally. So how can you have so much faith in it? And I would say, OK, fair enough. Like there are limits, you know, but I can't even do that.

So we're not even at the point of being like, well, experiment's going to verify or falsify my theoretical work. We're not even at the stage of being able to do the theoretical work. I've got a backup question to that as well. The event horizon is the point at which even light can't escape. Like even light is going too slowly to escape.

Yes, you can actually, like if I was falling across the event horizon and I was to flick a flashlight, like just one photon, one photon at exactly the right instant, I could get it to hover right there at the event horizon. There's a funny, funny condition where the light is racing out at the speed of light.

as it should, but the space-time in some sense is like a waterfall falling in at the speed of light. - So it's like walking up some escalators that are moving at the same speed that they're coming down. - That's exactly, and there's literally, and it's a mathematical solution in Einstein's equations where a photon hovers exactly at the event horizon trying to get out. That's crazy, but if you fall past it, like if you're behind me, you're like, "Jenna, wait!" And you fall behind me. - But you'd never be able to see that photon. - You could actually see the photon, but it would look to you like it was traveling at the speed of light 'cause you're falling in at the speed of light.

So there's no way for anybody to see it hovering except another photon. Okay. Here's my back-up question, because I think that's far more interesting than what I was about to ask. Your hair is literally standing on end. I'm becoming more like Einstein by the minute. Oh, my God, you're great. Is that how it works? You're great. You're more great than the last time I saw you. I'm growing facial hair. Everything's changing. About to marry Marilyn Monroe. Oh, my God, your eyebrows. What is happening? Anyway. But...

The event horizon is the boundary for light, but everything else, like a person or a spacecraft or a satellite or even just even a molecule or an atom, the horizon for that would be much further out, right? Because that would never be able to travel that fast.

So its boundary would be... Yeah, technically, again, back to the engineering problem, you can get arbitrarily close and in principle be able to escape. In principle. It might require more energy than exists in the entire universe to accelerate you to escape. So yes, you could probably define horizons that were more sensibly defined. Like if I used all the energy of the sun...

what's the boundary beyond which Matt can escape the black hole? And it would be much further out than the event horizon. Right. But in principle, it's the event horizon for everything. Sounds good. You're buying it? I'm buying it. I could say any old crap right now. No, you're saying it with enough confidence and plausibility that I'm fully sold. I'm very confident.

Well, we've covered it. Are you sorting through the crazy stuff now? I'm sorting through this question. We've kind of covered quite a few of these just through chance, through conversation, like what boundaries set us from achieving the speed of light? Well, you already covered that. That's the amount of energy and the momentum you need to overcome. If time is relative, can we send a satellite somewhere else to look back in time upon our Earth?

Well, we do that already to some extent. It's actually both a hindrance but also really a gift that the speed of light is fixed speed because it means that the further away something is and the longer the signal is taken to get to us, the deeper into the past we're able to look.

So it actually allows us to look into the past. So if the speed of light was infinite, we would not be able to see into the past. But we can see the universe the way it was billions of years ago because the light we're collecting now from far away is that old. And so it's like an archaeological record. It's actually spectacular. I think this question is asking one level beyond that. And that's like, could we...

I believe this question is asking, and apologies if I'm misinterpreting, but if not, I'm asking my own question now. If there was some way... I think that's your prerogative as co-host. Yeah, screw you, questioners. If there is some way using some kind of warp technology, using some way of wormholes or curving the space-time itself so we can effectively travel faster than light without breaking the speed of light...

and therefore travel further away. I think the question is asking, would it then be possible for us to look back on Earth and see our own history? Oh, I see what you're saying. Oh, I see. That's good. That's a good twist. I think it would. No, we can't, because we would have to outrace. So imagine the light leaves the Earth. And here we are a few centuries later developing the technology to try to go out into space. We would have to outrace that light pulse. And we've already established we can't do that.

So, to see into the past. But there is one trick. If the universe is finite, if it's not infinite, and if the light has to wrap around the space,

then we don't even have to go anywhere. We can just sit here on Earth and look out into space at a distant galaxy, and it would be the galaxy as it was in the past because of what we just discussed, but we might be able to realize somehow, oh man, that's actually an image of the Milky Way as it was in the past because the light wrapped around this finite space over and over again, and then finally we developed the technology to intercept it.

And so that's the only circumstance I can think of when we could see the earth and the Milky Way in the past. So we could then tune into the right time. You could figure out who shot JFK. That was exactly the example I was going to use. Weird, right? Stop it. But we couldn't... So we couldn't say, okay, the light from... The sun hits JFK and the shooter...

and flies off into space, into the universe. Right, but let's say somebody broadcasts it and somebody knows and they broadcast it into space out of the Earth's atmosphere. That's flying away at the speed of light. Yes. But that's traveling at the speed of light. Yes. But let's... But the light wraps around and it comes back to us. But let's say, ignoring that, let's say at some point we then develop this sort of warp technology where we can bend space-time itself and...

So that light has traveled one light year, but then we travel. Okay. So can we travel like five light years? That's a good example. And then sort of leapfrog ahead and capture that light. If we could figure out how to manipulate the entire space time, even if it was infinite, we could probably figure out a way to bounce the light back to us.

Right. So in theory, we could do that and then have a telescope that could see into the past. Yeah, we could do that. Yeah. In theory. But I mean, it's hard to, I think we would have to collude with a civilization that was far away because you can't, even then it would be kind of impossible to imagine because I would have to send a signal to that distant civilization and say, the light pulse is coming your way. But couldn't we, couldn't we send it back to us? But that signal would have to outrace the light pulse. You see, so it's a pretty sticky territory. We might be able to pre-plan it.

Or you know what? The distant civilization takes it upon themselves without our communication to send it back to us. That's pretty much the way in which it could possibly work. Okay. Yeah. So do we have time for another cosmic query? Oh, we do. Okay. All right. Well, this one, we've touched on this a little bit. I think this is the last question in this list.

The others are bad. Well, I think there's only one or two other questions, but they're things that we've accidentally... There's no such thing as a bad question. They're things we've accidentally already covered during the course of this. But this is something we've touched on, and I'd like to know more about it because it still confuses me greatly. So how does dark energy and dark matter play into the science of interstellar travel? And I think wrapped up in that question as well is...

What is dark energy and what is dark matter? Because you mentioned this briefly. And they're two very different things, right? So dark matter is just a proxy for a form of matter that we don't know what it is. So it just generically means we don't know what that is, except that it doesn't interact with light. There are examples of dark matter. People make out that dark matter is so exotic. It's not that exotic, but...

There are neutrinos. Neutrinos are dark. They don't interact with light. So it's not that exotic. But we don't know what is responsible for so much of the universe being in dark matter. It's like a quarter of the energy content of the universe. So that's why it's a big deal. Not because it's so strange for something to not to be dark. It's really invisible. It's not even dark. Light passes right through it. Okay. Right.

And then dark energy, similarly, is just a vague name for something else that we don't know what it is, but we see its consequences. We see that the universe is accelerating faster and faster, and we have no idea what's responsible, and so we just label it with this thing. The weird thing is that, so let's say dark energy is 70-some percent of the energy content of the universe, and dark matter is a quarter, 25%.

Why are they in roughly equal proportions? That's really confusing to people. What's the relationship between them? And so I think that it's one of the great mysteries because it means that most of what exists in the universe is just like a little bit of residue. It's like a little bit of dirt left over from the big bang. So the fact that this thing dark energy that we don't really know and this thing dark matter that we have a vague idea of but can't really detect very easily. Yeah.

are in the same proportions to the energy we know and the matter we know, they think is probably not a coincidence or maybe... Well, it just seems very strange for it to be a coincidence. I know it sounds like, well, what's the big deal? But it's just that the universe is, you know, 14 billion years old. And right now the energy content in light is like incredibly low by, you know, hundreds, you know,

10 billion lower. So why is this equal proportions? You know, so, so, um, it is, it is kind of a real question, but I think that the person was asking how would it relate to interstellar travel, right? And you could definitely use dark energy for interstellar travel. It's hard to harness dark matter. I mean, it's hard to harness dark energy for sure, since we don't know what it is, but like in principle, dark energy warps space in this

spectacular way, right? It can accelerate the universe. When we were talking about warp drive, they're usually based on dark energy. Like imagine a dark energy that has the opposite effect of the one that we observed. So it pulls things closer. That would be a dark energy phenomenon. Then you do the jump across and then you push it back out again. And so dark energy is warp drive. And then you get out on the other side and then you can see who's shot JFK. Yeah. No, I don't know. But you don't have jet lag when you get back from California. That's true.

Well, I think it's time for us to wrap up this episode. We could talk for hours. We're probably going to talk for a couple more hours. Excellent. I hope so. I very much hope so. You've been listening and watching possibly StarTalk All-Stars. I'm Jen Eleven. My guest was Matt Kirshen. Thanks, Matt. Thanks for having me. We're going to warp drive you back to California in a microsecond. Thank you. Coach, though. I hope that's okay. I can deal with it for a second. And in the meantime, we'll see you next time. Salutations from the multiverse.

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