Daniel and Kelly’s Extraordinary Universe - Is it possible to travel to other stars?

Episode Date: June 10, 2025

Daniel and Kelly explore the hurdles and potential technologies for bringing humans to alien solar systems.See omnystudio.com/listener for privacy information.

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for humanity around an alien star? Is there a physics obstacle, or is it just place? political will or lots of engineering. If it's actually impossible, that might explain another great mystery, why no aliens have visited us. Maybe we aren't alone in being marooned on our home stars. Maybe everyone is just stuck at home. Today on the pod, we'll dive into the challenges, the technologies, and the potential for taking humanity to those distant stars. Welcome to Daniel and Kelly's extraordinary accessible universe. Hello, I'm Kelly Weiner-Smith. I study parasites and space.

And if I could travel to the next nearest star, I wouldn't. What about you, Daniel? Hi, I'm Daniel. I'm a particle physicist, and I want to send somebody to the nearest star, but I definitely don't want to go myself. So why would you not want to go yourself? Can you imagine all the physics stuff you could ponder and maybe even figure out on an interstellar trip?

Oh, yeah, I'd be desperate to talk to the aliens about physics, to see what kind of weird technology they've invented. But I'm just not that much of a traveler these days. Honestly, I don't even like to get on an airplane. So a spaceship, absolutely not. The seats are going to be uncomfortable, the snacks are going to be weird. You know, I'm a homebody these days.

But you were born in one country, lived in another country, and had kids in a third country, right? That's true. You used to be an amazing traveler, but now it's home, huh? Time marches on, absolutely. Yes, I can no longer sleep anywhere, any time. My stomach is more sensitive. You know, youth is wasted on the young. Well, it sounds like you made good use of your youth traveling all those places.

How about you? Why don't you want to go to another planet? If we ended up in another planet and there was life there, that would be. incredible. I feel like you don't necessarily know what you're going to get until you get there. But the thought of leaving, like, so it's spring while we're recording, which means in the next couple weeks on the side of my barn, the light that's on outside is going to attract little tree frogs and all kinds of colorful moths. I just can't imagine leaving this planet and leaving all of that stuff behind to spend like decades traveling in a tin can. But maybe they'd have even

better moths and then I would feel regret. It'd be amazing though if we got to that planet and then we're like, this place is kind of ugly, you know, I wonder if like every planet has its own beauty or if our evolutionary history on Earth primes us to only find, you know, the Cierras and the Blue Ridge Mountains beautiful. See how I included Virginia there? I appreciate that. Yeah. No, we're being nice to each other on this episode, I guess. That's great. I also, it would really stink to know that you were never going to see any, anybody else that you had known your whole life ever again. And like, the farther away you get, the harder it is to communicate. But so let's into the meat of our discussion today, and we're talking about, is it possible for humans to

travel to other stars? Yeah, and I think this question is interesting and important because while we want to explore the universe robotically and gather information telescopically, I think also we have a primal need to explore to go places and to expand out into the universe. I think it's a question a lot of people have about whether it's possible today, whether it might be possible in a thousand years, or whether it might never be possible for humans. to cross the vast oceans of space to other stars. So I went out and I asked our listeners if they thought it was possible. If you would like to play for this part of the podcast in the future, please don't be shy.

Write to us at questions at danielandkelly.org. We would love to have your voice on the pod. So think about for a minute. Do you think it's possible for humans to travel to other stars? Here's what our audience had to say. We can imagine traveling to other solar systems, but can we deal with? with all the questions. Radiation, food, power sources, generation ships. We just don't know enough at this point. I would say no for the foreseeable future, but in multiple generations, after

multiple generations, I think that would be possible. It would take thousands of years, and the ship would have to survive. You'd go through accidents. There would be warring factions on the ship. You'd need a multi-generational ship. I'd say it's possible, but not within the next hundred years, there are a lot of hurdles to overcome. Propulsion being a major one in supplies. Maybe if you had a fusion-powered plasma or something. I bet we could get a human being there in their lifetime with acceleration that won't kill them, but I think stopping will be really hard and definitely turning around will be nearly impossible.

So they can't come back. Humans don't have enough time to live. and our bodies would be destroyed by the acceleration required to reach another solar system. With today's technology, I don't believe it is strictly possible. I think it's definitely possible. There's no law that says we couldn't do it, but there's still some major challenges ahead of us. Anything is possible, but humans traveling to another soul system may be a single. stretch. No. It is impossible. I don't think a single person could make it to another solar system,

but maybe multiple generations. Even if you could travel at the speed of light, it would still take years to get to the nearest star, which we don't even know if there's a solar system around, and you'd have to confirm that first. Even if we could, technology got better, we'd still probably need a generational starship. Unless we destroy ourselves with war. or refusal to confront climate change, we will eventually reach other solar systems. Maybe if money isn't a problem in the future, if we tackle certain things, maybe we could, but I think it's for now the ultimate pipe dream.

Yes, it is possible, but whether it is probable is a different question. And even if it is probable and even if it is undertaken, I don't think that the humans who would leave our solar system to get to the other solar system would be alive when we get to the other solar system. We should assume that the aliens at the solar systems would be hostile and therefore we should send all the people that we dislike the most. Maybe through like a cryosleep or human hibernation type thing, something like that. I believe we are marooned here.

No, we can't do that right now. Maybe someday in the future. From a physical point, yes. From engineering point, likely no. I loved the variability of answers that we got here. And I should say that as someone who interacts with the space settlement community, I have no doubt that even if somebody were to say, like, we've made a generation ship, there's a 1% chance it's going to make it.

Every seat on that ship would get filled. And so, you know, I think there's a lot of people who really love this idea. Wait, why do you think we would have no shortage of volunteers? You think there are people who, like, in reality, would actually sign up to go. They're not just, like, excited about the concept. Oh, my gosh. They send me angry emails. There have been so many people who have written me to tell me that I cannot stop them from settling space.

And I write them back and I say, I cannot stop you. That's right. And I'm not trying to. I don't think I have any power over these kinds of decisions. I just wrote a book saying it's kind of dangerous. And maybe we should think this all through before we go. But, hey, do what you want. Yep, yep.

Do your thing. I'm sure there'd be a lot of people. And, you know, it would be an absolutely incredible thing if our species did send a generation ship, for example, to another star. It would be incredible. And I love how this topic is so deeply inspired and informed by science fiction. You know, obviously there's science here. We're going to talk about all the nerdy details of propulsion mechanisms, et cetera.

But so many of the ideas here come from the creativity of science fiction authors, casting their minds forward to imagine what we might be able to build, might need to build what we might have to do to survive. Yeah. What is your favorite interstellar travel sci-fi book? Oh, wow, such a good question. One of my favorites is a book by Alistair Reynolds. I think it's called the House of Sons in which they tackle this problem by dragging stars closer to each other. So they want to have like a galactic empire across many solar systems, but they don't have faster than light travel. And they recognize that it's basically impossible to govern somebody if you, they're so far away. So they bring a bunch of suns closer to each other, make a little

solar neighborhood so you can have different solar systems, but they're not so far away. I like that idea, but it sounds kind of dangerous. Well, we're going to talk about that technology, which isn't as far-fetched as you might imagine at the end of the episode. So let's go ahead and dig right in. But first, let's talk about where would we be going? Where is our closest option here? Yeah, so the Milky Way galaxy has hundreds of billions of stars, but the whole thing is like 100,000 light years across, which is really big. And the density of stars in our neighborhood is not so high. It actually varies a lot in the center of the Milky Way. It's much, much denser. But

around where we are, we're like in the suburbs, not quite the exurbs in the very fringes of the Milky Way. But out here in the suburbs, you can expect to find a star a few light years away. And that's what we find. Alpha Centauri and Proxima Centauri are like just around four light years away. So if you wanted to get to the nearest star on easy mode, you'd be going to the closest one. It's still four light years away. And remember, the speed of light super duper fast. If you shined a laser beam at one of these stars, it'd be zipping through the cosmos at that incredible speed for four full years before it got there. And so say you get to Alpha Centauri.

Do we know that there are Earth-like planets there that we could try to explore? What would we see once we got there? Yeah, there's actually good news there. You know, until like 20-ish years ago, we had. no idea what planets were like around other stars or if there even were any. We'd only ever seen the planets in our solar system until the mid-90s when we started developing the technology to see exoplanets. Now we specifically identified 5,000 exoplanets at least. The number keeps going up and up and up and up. It's like a real pivot point in human history. And because of that,

we can make all sorts of really interesting statistical statements. Like on average, stars have a good number of planets. And specifically, Proxima Centauri and Alpha Centauri do have some planets. And so it's very unusual, actually, for stars to have no planets as far as we can tell. So if you're going to go to a nearby star, it's very likely you'll find some planets there. Whether they're Earth-like and whether they are capable of supporting life, a whole other question that we think the next generation of space telescopes will really help us crack. But for the point of view, today's conversation, let's just imagine getting to that solar system, not necessarily finding a cozy house there. Okay, so let's see, I'm 42 now. If I were going to jump

on one of these ships, and I was hoping that we would arrive by, you know, the average lifespan of a human woman. So what? That's like 86 or something like that. So we've got, let's say we've got about 40 years to get there. How fast do we need to go? Yeah, so if you traveled at the speed of light, you'd get there in four years. Of course, you can't travel at the speed of light because nothing that has mass can travel at the speed of light. So let's say you could go at 10 percent of of the speed of light, already blazingly fast, much, much faster than any human ship has achieved, crude or uncrewed, but if you did somehow manage that,

you would get to Alpha Centauri in about 40 years, right? A tenth of the speed of light, for light years, so 40 years. So that's basically what you need to achieve, but the sort of space technology we have now really just isn't capable of that. Like the fastest crude rocket, or the space shuttle, for example,

their top speeds would take them about 80,000 years to get there, not to mention the quest of like bringing in a fuel that we'll dig into in a minute. And even the uncrewed stuff like the Parker Solar Probe is the fastest thing humanity has ever built. Its top speed is about 0.064% of the speed of life. Oh no. It would take about 7,000 years to get to Alfa Centauri.

So nothing we've built can go nearly fast enough to get Kelly to Alva Centauri before her 85th birthday. Oh, that's right. I wasn't going to go anyway. Although maybe if they had really great moths, I'm. I'm disappointed. But so I feel like there's a couple problems here. It's like, one, can you even get to the speed that you want?

But then, two, how do you get to that speed? Because, like, if you're sending humans, we've got these squishy bodies. And if you accelerate us too fast, we like, we break and smush. And so you need to, like, get fast, but not too fast. Yeah, so there's a lot going on here. I mean, one thing is just a limitation of relativity. Number one, you can't get faster than the speed of light. We'll talk about warp drives in a minute.

but assuming that you're going through space, through flat space, you can't travel faster than that. And that's a hard limit. And I think it's important for people to think about that as sort of setting the length scale of the universe because we talked a minute ago about like, wow, it's super duper fast. It is super duper fast, but space is vast compared to the speed of light. Like we wouldn't think space was so big if the speed of light was 10 times or 100 times what it is because then these things would be like less than a light year away. Or we would think space was much bigger if the speed of light was smaller.

So the speed of light sort of determines, like, what is far and what is close. And it just so happens that because of our galactic dynamics, things are a few light years away instead of tenths of light year away. But even if you're not going to get to the speed of light, this limit at the speed of light makes it hard to accelerate. Like, you keep pouring energy into your rocket. You're not going to increase your velocity by the same amount. As you go faster and faster, it takes more and more energy to increase your velocity. And as I think you were hinting, there are also limits on how quickly you, you can accelerate, like, biologically.

Yeah, and, you know, that's the interesting part. Well, it's true that we do want to deliver our passengers to Alpha Centauri, not as bags of dead goo, right? We want to take care of all their fragile little organs and make sure that they actually get there. And so if you want to get to some sort of reasonable speed so your trip doesn't take too long, you need to accelerate. And humans are not really built for huge acceleration.

I was reading some papers that said the humans can tolerate like 20G of acceleration. G there, of course, referring to the acceleration of gravity here on Earth as basic standard units of 1G. So 20G is pretty intense. And we can't really tolerate that for more than like a few seconds, maybe 10 seconds. Humans like the really tough ones can tolerate like 10G for a minute, 5G for a few minutes. But imagine being on this ship. You're going to be on it for years at least. You don't want more than like 1G or maybe even 2G for long periods.

You know, we got a lot of the early data on how many Gs humans can survive because there was this one scientist who kept getting in a sled and then slamming himself into a foam wall. And so anyway, human ingenuity. I love it. So just to make sure I have the physics understanding of this. So like, if you speed up and then you stay at that speed, you only experience the G as you're accelerating, right? Not just because you're going fast, but because you are going faster every second. Is that right? That's right. Okay. That's right. You can't experience. experience velocity directly, it's a relative thing. Inside your ship, you can't tell how fast your ship is going. But if your ship turns on the engines and tries to increase its speed, you can measure acceleration locally. It's not relative. So as your ship tries to increase its speed, you can feel that. And something that's sort of surprising to me is that you can actually accelerate at 1G and reach near the speed of light in a reasonable amount of time. Like if you're

on a rocket that can do 1G of acceleration, you can just do that for a year. year and you'll get up to like 99% of the speed of light relative to your departure location. And in that case, you would get there in less than 40 years, right? Because now instead of going 10% the speed of light, you're going the speed of light. Yeah, that's right. But you need a technology that can provide one G of acceleration for a whole year. As we're going to talk about, that's not so easy to do, right? That's an enormous amount of thrust or momentum you're imparting on your ship. There's always something in the way. There's always something in the way.

And the flip side of all of this is deceleration or negative acceleration, because you could get up to near the speed of light and then you could get to Alpha Centauri. But then you're going to be in that solar system for about 0.07 seconds if you're traveling at near the speed of light. But what you want to do is arrive there and stop, which means decelerating. And you don't want to decelerate from the speed of light to zero too quickly. Otherwise, you'll again go splat. Mm-hmm. Yeah, humans, it's a shame we're so squishy. And I think on the next episode, we're going to talk about making humans less squishy by, like, turning them into human sickles. And that might be a better way to accelerate human bodies. I don't know. You'll have to tell me all about it. We are going to talk about the human side of things in the next episode. I'm not sure that human sickles is on my outline, but we'll see what I come up with.

It is now, because I'm going to ask you about it. Okay, great. And so a typical strategy is to accelerate. on the first half of the trip and then basically turn your ship around and decelerate all the way back to your initial velocity now relative to your destination. And so you accelerate and you reach top speed momentarily halfway there and you turn around and you're slowing down the whole second half of the trip. And this is actually kind of cool because along the way you might want to feel some artificial gravity. As you're part going to tell us, humans don't like to float in space forever. It's not good for us. And so you want to feel 1G. And so having 1G of accelerates, and the 1G of deceleration, the whole trip is actually kind of cozy.

So if you had 1G of acceleration for one year, you would still have, what, two or three years just staying at that velocity before you start decelerating? So there would be a period where you would have no Gs in between? Yeah, absolutely. Depending on the length of the trip, if you wanted to go further, for example, you could accelerate at 1G for a year, get up to near the speed of light, zoom around, super duper fast, and then flip around and decelerate. So yeah, you can have a period in the middle where you're not accelerating.

or decelerating, then you've got to solve that gravity problem another way. All right, all right. And we'll talk about that in the next episode. And that's not the only thing that's going to potentially kill you along the way. You know, we think of space as empty, but it's not really. Like there's a lot of particles out there. There's huge amounts of gas, the interstellar medium. There's lots of little bits of tiny rocks, we call it, dust. It's out there. And if you're traveling at relativistic speeds relative to that dust, you can be in danger. You know, these tiny particles, a millimeter-sized particle, at like even if half of the speed of light

is an enormous amount of energy deposited on your ship. And so you've got to really worry about this. And cosmic rays and radiation. So your ship has to be pretty robust. You need shielding. You need like titanium or water or lead or something. All this is going to make your ship heavier. So we'll dig into that more in the next episode

where we talk about the fragility of the human body and how to survive this. But keep that in mind as we're talking about the propulsion designs because it's going to affect how much you've got to move on the way to another star. So to me, the dust feels like the first showstopper that we've encountered. What makes us think that when each piece of dust hitting us is like getting hit with a bomb, what makes us think that we can like, I mean, we're clearly not going to dodge the dust.

So what is the dust solution? Well, you know, I've read about some cool shields. There's like these whipple shields that basically break up the dust into smaller pieces so that none of them are likely to like really be devastating. There's some cool technology like self-healing shields. builds. So I think it's an engineering problem and one that we're likely to be able to crack. But yeah, it's definitely an important one. I love your optimism, Daniel. I always love your optimism. All right. So we're going to be talking about ways to get there. Are the ways that you're

going to talk to us about, ways that could get us there in a lifetime? Or is there anything else that might work for us here? Some of these solutions really can get you there in a lifetime. But because it's physics, time is a slippery concept. Like, are we talking? about the lifetime for the people you left behind or lifetime for people on board because as soon as you get up to really high speed relative to your departure planet those are not the same thing so for example say you accelerate at 1g and you get up to near the speed of light you could travel for just a few years your time a decade your time and like a hundred thousand years will have passed back home and if you're traveling near the speed

of light a hundred thousand years is enough time to get you across the galaxy in in only like a decade of your time. So Kelly gets on board the ship. She arrives at the other side of the Milky Way that no human has even clearly seen before because of all the gas and then dust. And she's only 50-ish. Meanwhile, back home, Zach is 100,000 years old.

Oh my gosh. And I'm a narcissist. So what I care about is how long it takes me. When we say it's going to take 80, it would take like 80 years to get there if you went 10% the speed of light. That's 80 my years. as a person on the ship, right?

So our frame of reference is always the people on the ship. Well, it was going to take 40 years to get there at 10% the speed of light in human years. But at 10% of speed of light is not that much of a relativistic time dilation effect. That really kicks up when you go half or three quarters of speed of light. So that's still going to be decades, earth time and ship time. If you do get like up above 50, 60, 70% the speed of light, then you start to benefit from these time dilation effects. So on the ship, it takes less time.

Got it. Okay. And on the next episode, we're going to talk about generation ships where if you just accept it's not going to happen in a lifetime because your technology can't get you that fast, how do you carry generations of humans to still get there? But we're going to take a break now. And when we get back from the break, Daniel's going to walk us through our rocket options for our trip to the stars. I'm Dr. Scott Barry Kaufman, host of the psychology podcast.

Here's a clip from an upcoming conversation about exploring human potential. I was going to schools to try to teach kids these skills, and I get eye rolling from teachers or I get students who would be like, it's easier to punch someone in the face. When you think about emotion regulation, like, you're not going to choose an adaptive strategy which is more effortful to use unless you think there's a good outcome as a result of it, if it's going to be beneficial to you. easy to say like like go you go blank yourself right it's easy it's easy to just drink the extra beer it's easy to ignore to suppress seeing a colleague who's bothering you and just like walk the

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In our universe, momentum is conserved. So you have your ship, you want it to gain momentum in one direction. There needs to be some sort of compensating. momentum in the other direction. This is something you feel if you like fire a gun, for example. You feel that kickback, that's the conservation of momentum. The rifle

is shooting the bullet super duper fast but the bullet has a small mass and the rifle itself has a larger mass is moving backwards in the other direction with a compensating velocity. See, I always imagine it as I'm trying to get a boat to move and I'm imagining Daniel on a cruise ship taking all

of the white chocolate and throwing it in the ocean to get the cruise ship to move faster while also getting rid of the bad chocolate. Yeah, that's exactly right. You need to build momentum in the other direction. So imagine you are on a boat and you're tossing stones or equivalently white chocolate or hot garbage or whatever, useless stuff you happen to have on the boat, right? You create momentum in one direction for the stones and in recoil, you go the other way. And so all rocket drives operate under this same principle. You need two things. You need energy and then you need mass. So you use the energy to throw the mass out the back and you go the other way. That's the way

all of these rockets were to talk about work. And that's how like the Falcon 9 or Starship works when it takes off too, right? Yeah, exactly. And the issue here is that you need something to produce that energy and you need something to throw out the back. And when you run out of that, you can't go anymore. And because you have to bring that fuel with you, you need enough fuel to push that fuel. And then you need more fuel to push that fuel. And so there's this famous rocket equation, which tells you like how much fuel you need to get up to a certain velocity. It depends on the mass of the ship. And also depends on the specific impulse that your engine is able to provide, which is basically just like a number. Some of them are high, some of them are

low. But the bottom line is that the math tells us that there's an exponential need for fuel if you want to get up to higher velocity. So you want to go twice as fast? You don't need twice as much fuel. You need much, much more. You need exponentially more fuel. And as you increase that velocity, the amount of fuel grows ridiculously. So for example, even like the Saturn rockets, the ones that took us to the moon, when they launched, they were like 95% fuel. It's mostly fuel being taken off. And that fuel is mostly being used to push the rest of the fuel. That absolutely blew my mind when I first learned it, that like less than 10% of that giant, that giant tube was actually going to be going to the moon. And people are often confused about rockets versus escape

velocity. Remember, escape velocity is a calculation you do if you're like throwing something from the surface of the earth. You need a certain velocity because gravity is going to slow you down. And if you have higher than the escape velocity, you can leave the orbit. But rockets are not about escape velocity because rockets have constant thrust. Rockets can lift off at basically 0.00001 meters per second and still make it to space because they're pushing themselves constantly. They're like climbing a ladder rather than just getting a single push. So escape velocity is not relevant for rockets. What is relevant for rockets is this specific impulse and how fast you want to go.

So in this case, the rocket would be not just getting us off Earth, but it would stay with us and it would continue to propel us through space. Yeah, exactly. And the sort of bog standard rocket we have is a chemical rocket, where the thing that you're throwing at the back is also the way you're getting energy. Basically, you have like exploding stones that you're throwing out the back. The stones throw themselves out the back, right? because the fuel is the propellant and the source of energy. You light it on fire and the explosion goes out the back. You get pushed the other direction.

And so that's cool and, you know, obviously fuel we can find here on Earth. But the specific impulse of these engines is not huge, right? So it's not a great way to get going really, really fast unless you have incredible quantities of fuel. So this is the rocket equation at work here. So if you do a little calculation, like how much fuel does it take to get, off of Earth and to the moon, an enormous quantity, right, filling a huge Saturn rocket. How much fuel does it take to, like, get to Alpha Centauri? If you've got to burn that chemical engine the whole way, well, that goes exponential,

and the fuel tank is something like the size of Jupiter. What? There's a showstopper. Kelly's going to keep track of these showstoppers as we go, because she's the wet blanket, but I'm going to be optimistic because in the end, I'm really hopeful that we do get to another star. I mean, you do, or somebody, not me, but some human gets to go and set their eyes on an alien planet. I think we will eventually. All right.

You heard it there, folks. She said it, yes. I do, I do. But I'm not going to put any money on a date. All right. So when I was writing a city on Mars, the engineers would get grumpy with me for occasionally using the words fuel and propellant interchangeably, which we didn't do in the final manuscript. Nobody needs to freak out and write me now. This was in an early draft.

So, Daniel, what is the difference between fuel? and propellant. So propellant is anything you want to throw out the back of your rockets, right? And fuel is a special combination, which is both the source of energy and a propellant, but it doesn't have to be. Another example of a propellant that separates these things is a nuclear rocket. So rather than using fuel to explode and create energy and propellant simultaneously, you use something like a nuclear reactor, a fission reactor to produce the energy, which you then use to throw some inert mass out the back. You heat up a gas, for example, and it bubbles out the back of your rocket chip. The fuel there is like uranium, which is powering your nuclear reactor,

which is providing the energy to toss the propellant out the back. The propellant and the fuel are very separate in this technology. And so with a nuclear rocket, what would the propellant be? Could it be like anything that you heat up? Yeah, it could be anything you can heat up, but you've got to bring some mass to throw out the back, right? Stones, white chocolate, xenon, You want something pretty inert, so it's not reacting, but you also need to bring enough of it, and it's going to be dense enough that it's not going to be huge. And nuclear rockets are cool because it's something we've actually built. We had an episode where you and I talked about nuclear jet planes, and it's the same principle, right?

A jet engine operates in this same principle is very similar to a rocket. You have the fuel which explodes and pushes something out the jet airplane, this whole air compression thing, so jets don't work in space. But you could also have a nuclear jet engine where you're using a nuclear reactor to heat the stuff. up and shoot it out the back. Same basic principle, and that applies to rockets in space as well. So in this case, you heat the stuff up and you shoot the gas out the back, and that could be a nuclear rocket. But we haven't actually sent a nuclear rocket to space, right? The only kind of rocket we've ever sent to space is the chemical one. That's right. We do have test nuclear engines, which people

have built and tested and shown to work. And they once did fly an airplane with a working nuclear reactor on board, scary stuff, but it wasn't actually powering the plane. Anyway, this is like a viable technology, not just science fiction. And so you said that for chemical rockets, the container that would store the fuel would need to be as big as Jupiter. How big are we working on now if it's a nuclear rocket? So it doesn't have to be nearly as big because the fuel is much more dense, right? Uranium incredibly dense compared to like diesel or even like oxygen or whatever you're using as fuel. And so that doesn't need to be nearly as big.

but you'd still have to bring the propellant, right? You still need a lot of stuff to throw out the back. So we're not talking about the mass of Jupiter, but we're still talking a very, very large ship with a huge amount of propellant. Now, there's some folks that have ideas for, like, gathering propellant along the way, that dust you talked about or the interstellar medium.

That's stuff, right? You could use that as propellant. So there are technologies like ram jets or bazaar jets that people talk about where you have like a scoop that gathers propellant up, and then your engine, whatever you're using, a nuclear reactor or something else, for example, is throwing that out the back. And so there are ways to avoid, like, having to have a cataclysmically large ship in that way, though. There are also

a lot of people who think that those ram jets and busside jets are totally impractical for other reasons. Same. Enough said. What if the thing that you're throwing out the back of your rocket is nuclear bombs? Because why not, right? Why not? This is actually not a terrible idea in some ways because it separates the ship from the propulsion in a way that we'll talk about for solar sails. If you could blow up a series of nuclear weapons between here and Alpha Centauri

and you had a ship with like a huge shield in the back that could absorb all of the radiation and the energy that was dumped out by the nuclear bombs, then you could just sort of like ride this wave of nuclear explosions all the way to Alpha Centauri. And, you know, this is the basis of Project Orion and then later Project Datalysts, the idea having like a series of small bombs providing like this smooth acceleration. Now, you know, how do you get those bombs? How do you lay a trail of bombs from here to the next star? It was more about like near Earth navigation or getting things off planet than like actually going from star to star. But we can't not talk about blowing up nuclear bombs as a way to propel a ship. It's just got to be in the conversation.

Project Daedalus had a follow-up project called Project Icarus, because Icarus was Daedlis's son. But Icarus is the one who flew too close to the sun and his wings melted, and he fell down and died. And I was remember feeling like, isn't this supposed to be inspirational? And like, but I was told I just wasn't looking at it the right way. And again, this is why I don't get invited to the space parties. Do you think it's hard to sign up test pilots for Project Icarus. It could be, but again, I bet somebody would sign up. There's a lot of people who are much braver than I am.

Looking for volunteers for Project Crash and Burn. Anybody? Anybody, nobody? Probably a lot. Anyway, that's a cool technology. And the cool thing there is if you could somehow manage it, it separates the ship from the source of energy, right? And also from the propellant.

And so the ship itself doesn't have to be very big. Of course, that's sort of assuming a solution to the core problem, which is how to get all these nuclear bombs from here to Alpha Centauri, which is basically impossible. What do you think the aliens would think as we were, like, leaving a trail of nuclear explosions on our way to their galaxy? Do you think they'd be like, wow, they've really conquered technology? Or do you think they'd be like, oh, my gosh, how do we get them to turn around? Yeah, I can't wait for them to show up. Right. I think it's cool because we can imagine how we might detect aliens doing the same, right?

Like, if aliens have come up with this idea and they're using it around their star, we might be able to detect it if they're close enough. So that's pretty cool. But, yeah, I don't think it'd be a great way to announce our presence to the universe. They would definitely have time to set up the welcome party or the go home party. But along these lines, there's lots of permutations on this kind of propulsion. Once you separate the explosions from the propellant. So, for example, there are other ways to accelerate stuff.

Like, you could have an electric field. and you have ions, an electric field will push ions, that's what it does. And so if you had like an electric field and a bunch of ionized propellant, you could shoot that out the back. Basically, we're talking about a particle accelerator. That's exactly what a particle accelerator, like the Large Hadron Collider does. It pushes particles with electric fields and makes them go really, really fast. So build a particle accelerator, point it out the back of your ship, that's going to give you some impulse. This seems like another very large design, right?

Or how big would your particle accelerator need to be? Well, not that big, actually. And it's cool because it separates the two systems. And so your power source can be anything. It could be solar power. It could be some other crazy system. It doesn't necessarily have to be super duper big. In this case, it's nice because it doesn't even have to generate a lot of heat, right?

And so for those hand ringers aboard, you don't have to worry about like melting down or something like that. The downside to this is that the thrust is really tiny. Like you're accelerating particles, and particles don't have a lot of mass. And there's sort of a limit to how many particles you can effectively shoot out the back. And so it can provide very long-term gentle thrust, which is nice for constant acceleration. But it can't really give you a lot of specific impulse. And so it's not a great way to like get up to a high speed in a short amount of time. But it's cool for like navigating around space a little bit.

So this could be like a generation ship thing. And so can you remind me what the difference is between thrust and specific impulse? They're basically the same thrust and specific impulse times the constant of gravity. But you can think of them as interchangeable. They're just different by units, essentially. All right. We're going to take a break now. And when we come back, we're going to talk about using antimatter to get you to the stars.

I'm Dr. Scott Barry Kaufman, host of the psychology. podcast. Here's a clip from an upcoming conversation about exploring human potential. I was going to schools to try to teach kids these skills and I get eye rolling from teachers or I get students who would be like, it's easier to punch someone in the face. When you think about emotion regulation, like you're not going to choose an adapted strategy which is more effortful to use unless you think there's a good outcome as a result of it if it's going to be beneficial to you. Because it's easy to say like go you go blank yourself, right? It's easy. It's easy. It's easy to

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and those were my recent guests, comedian Phoebe Robinson and writer Aaron Foster. On this show, I'm talking to the brightest minds in entertainment, health, wellness, and pop culture. And every week, we're going places in our communities, our careers, and ourselves. It's not about being perfect. It's about going on a journey and discovering the bright side of becoming. Few people know that better than soccer legend Ashlyn Harris.

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And I see that next we're going to talk about antimatter. And I am immediately thinking that dangerous explosions are made, be not what I want happening, you know, on the ship that I'm living on. But why is Kelly being a wimp? No, Kelly is not being a wimp. We are steadily moving from reliable technologies we know can work and probably won't kill you to crazy ideas that might not ever work and are much more likely to kill everybody on board. Oh, good. All right. But antimatter is cool because it's the most efficient way to store energy. Like matter and antimatter and antimatter annihilation is perfect conversion of matter into energy. Compare that to, for example,

chemical fuel, right? Chemical fuel has a lot of energy in it, but when you burn it, you only release a little bit of that energy from the chemical bonds. But inside those protons and inside those electrons is an enormous amount of stored energy, which we call mass. If you could capture all of that or release all of that, you would need to bring less fuel, right? And so matter, antimatter annihilation is the best way to do that. You have protons and antiprotons. They turn direct. directly into photons, which you can shoot out the back of your ship. And that's awesome because the propellant there is moving at the speed of light. So, like, you can't beat that for maximum thrust and impulse.

So far, this sounds great. Now, the downsides are, of course, Kelly worries about being annihilated herself. Yeah. Because antimatter's pretty dangerous stuff. Yeah, so you need some sort of magnetic confinement for your antimatter, basically a bottle that doesn't touch it. You know, a bottle made of matter that builds a magnetic field that confines it. Sort of the way we do for plasmas in Tokomax infusion reactors.

That's not totally implausible. But then again, you basically have a bomb on board and any loss of containment and everybody's dead. Oh, yay. Before you can do anything. So that's bad. But on the flip side, antimatter is basically impossible to make in large enough quantity.

So you're pretty much safe. You know, we make antimatter. We made it for the previous collider. We used to have the Tevatron, which was a proton-antiproton machine. And we produce antimatter at the large hadron collider all the time in collisions. So it's not actually that exotic. It's made in the atmosphere, like cosmic rays hit the atmosphere, turn into antimatter briefly. But the quantities we're

talking about are like nanograms of antimatter. You know, we can like count the atoms of antimatter we make. There's no large scale production of antimatter in order to make like enough fuel to get anybody near Alpha Centauri. I read one estimate that said making 100 milligrams of antimatter would cost about $100 trillion. Oh, my gosh. All right, so I'm curious, since we're starting to get towards the really crazy ones, out of the technologies we've talked about so far,

if you had to go to Alpha Centauri, which one of these would you want to use? Probably none of these. Okay. My favorite is one we haven't talked about yet. It's a little bit ridiculous and speculative, but it's still my favorite. All right, let's keep going then.

What's next? Next, I want to talk about a nonsense idea, which is out there, people are probably imagining, might save the day. And that's propulsion list drives. People have been thinking, okay, well, conservation of momentum requires we throw something out the back. We're tossing stones or white chocolate out the back of our spaceboat to get it moving. That's frustrating that it limits us, right?

Because you've got to bring all this stuff to throw out the back. Wouldn't it be amazing if we could generate a drive which didn't need that? Yeah, that would be amazing. And also violate conservation of momentum, but that hasn't stopped people from trying. And go for it, guys. Like, it wouldn't be the first time that we discovered in situ something that revealed we didn't understand the universe. So I'm all in favor of people doing crazy experiments. That's just like a Friday in physics, right?

Where you discover you were wrong about something? Yeah. Those are the best moments in physics. And nobody should be limited by, like, conventional wisdom about what's possible or impossible. But we should also be clear-eyed about what we have actually proven. And there are folks out there who claim to have built one of these things. It's called an EM drive. And about 15 years ago, there was a lot of hoopla about it.

There's a lot of coverage in the new scientist about these folks who had some, like, tenuous NASA affiliation who claimed to have built the impossible space drive. And if you look for articles, you'll see them. They're in popular mechanics. They're in Wired. They're in the new scientist. But the journalists here really didn't do their work. Like, if you look carefully at the claims, you see that the thrust that these things

produce is really, really tiny. It's basically smaller than the uncertainties. You know, like they just have jitter in their measurements, they have sources of noise, and they measure a thrust, but the thrust is consistent with zero within errors. So while I'm all four crazy new technologies and exploding understanding of the universe, the EM drive is not something that's done that. These are fascinating ideas, and there's not nothing to support the concepts. One idea is that there's a vacuum in space and that vacuum has energy to it, and experiments like the Kazimir effect show us that the vacuum is real and it's there. and people wonder if we can extract energy from the vacuum and use it to propel ourselves. But, you know, the vacuum is special.

You can't, like, row against the vacuum. You can't, like, absorb momentum or get momentum. It's emotional the way space is, for example. You can't, like, put an ore down and row through space in the same way. And this is part of space. So, anyway, EM drives, not something we can rely on and not something I would bet on. But, hey, if you're out there and you're building an EM drive, I hope you make it work. Fingers crossed. All right. So I have heard proposals where, you know, the sun is shooting out photons all the time. What if you could capture those photons and let the sun sort of push you forward? How would that work?

This is my favorite idea because it separates the ship from the propulsion completely, right? So this is called a solar sail. And the idea is that you're right. The sun is putting out photons. And not just photons, but other particles. The solar wind has momentum. So why, like, generate momentum if we have an enormous momentum generating machine already? And so all you need to do to capture that momentum is build a huge sail.

So, you know, when you're on a sailboat, A sail is just a piece of cloth that gathers that momentum. Here, the best kind of sail would be something very thin, so low mass and reflective. And imagine a photon hits the sail, it bounces off. The same way a photon hits a mirror and bounces off, right? What happens? Well, the photon is now changing its momentum. It's going one way.

Now it's going the other way. Conservation momentum says that can't happen unless something is going the opposite direction, right? Something is now going the way the photon originally was to conserve momentum. And that's the sail. This is a confusing concept for people because it's hard to imagine like light pushing something because light has no mass. And you're probably thinking, well, momentum is mass times velocity and photons have no mass. So therefore they have no momentum, right? Yeah, wrong.

And the reason is that momentum is mass times velocity only for very slow stuff. As you approach the speed of light, the equation changes a little bit. So photons do in fact have momentum even though they have no mass. So yeah, their momentum can push along a sail. So imagine a tiny little ship with an enormous sail and the photons are bouncing off of it and giving it a push. Basically, you don't need an engine. So one of the things that always sounded limiting to me with this method is that it seems

like it would work great when you're close to the sun. But the farther you get from the sun, the fewer photons are going to be hitting your sail. How would this work when you get too far away from the sun? Yeah, this is tricky. As you say, you get further from the sun, the photon density drops by 1 over R squared. That's bad. is a good idea for moving around the solar system, like navigating from here to Mercury or here to Neptune, a very light ship without fuel, it would be awesome. Getting from here to another

star would be hard. And what you need to do is build an enormous laser. Like, think about a really big laser. Now make it 10 times bigger. That's too small. You need an enormous laser, right? Really huge laser or like a big laser array that I'll focus their energy on this sail. But the cool thing is, again, you can build this on Earth or you can have it in space or something. You don't need to bring it with you. And you can point it at your ship and push it to another star. So it's not a solar sale anymore. It's like a photonic sale. And I can't imagine that a laser of that size is going to cause any geopolitical conflict at all. A weapon of that magnitude. Yeah, we're building the Death Star basically, but for good reasons.

That's right. To go to space. We're just making space smores. What's the big deal? That's right. But there's a project to do this. It's called Breakthrough Star Shot. And they want to take a bunch of nanocraft, like tiny little devices.

And they want to build ground-based lasers and send those devices to Alpha Centauri. Now, you push these tiny little devices long enough. You can get them up to like 20% of the speed of light. And so you do the math. And these things, they're estimating to launch them in like 2030-ish, 20-35. They would take about 20 years. They'd be there by like 20-55. So if I'm lucky, I'd be around when the signal comes back, you know, from Alpha Centauri.

This is something that really could happen, which is why I think it's my favorite technology. The challenge, of course, is getting a big enough sale to push a ship with people on it. You know, so far we're just talking about sending tiny nano robots. But, you know, if I want to go there, you've got to bring me. me, my toilet, my bed, you know, all sorts of stuff needs to go with me. So we're not talking nanoships anymore. So we're talking enormous sales. And we really need like better technology for enormous sales that are going to somehow survive the interstellar trip, those micrometeorites tearing it apart. So, you know, there's potential there, but there's a lot of engineering

challenges to solve. Yeah. This is a cool technology. Are the, are the little nanoships that breakthrough star shot is sending? Are they going to have like cameras and scientific instruments on them? Or are they just able to say, we're here and that's it? TBD, but I hope they're going to put some cameras on there. I mean, it'd be amazingly ridiculous to get something all the way there and take no data, right? Yeah, that would be a major bummer. We haven't talked yet about a method to me that seems perhaps likely something that we have some experience with, which is gravity slingshots, which were used in Aurora, Kim Stanley Robinson's book, to slow down on the way back to Earth. Is that's something that we could use to speed up to get there?

Absolutely. Gravitational slingshots are amazing. You know, you whizz near a planet, basically steal a little bit of its speed. We should dig into the physics of that in a whole episode because there's some subtleties there about like why you end up with a little bit of speed. And the short version of it is that you slow down the planet's rotation around the star, which is really cool. And we've done this. You know, Voyager 1, for example, one of our only interstellar probes, it swung around the sun.

But it's not going that fast, you know. it's traveling like a light year in about 18,000 years. So you can't get going to like interstellar speeds unless you're getting crazy close to the star and that's got its own dangers. So gravitational slingshots are a good way to boost your speed and you get going, but on their own, they're not going to provide the speed that you need to get to another star. Got it. All right. So let's let's wrap things up on the most sci-fi methods. So what can we do to like, can we like bend space or time to make this happen? Yeah, so far we're considering flat space. We're imagining, you know, light is going to take a few years to get there.

What can we do to approach the speed of light? But, of course, we know that space is not just flat. Space bends. It curves. And in the presence of mass, it can do all sorts of things. It can wiggle. Not something we fully understand, but, you know, we know that this kind of thing is possible. And so in the last few years or so people have been thinking about, like, could we actually build a warp drive?

This would be something that bends space so that effectively the journey is shorter. I mean, that's really what space curvature means that you're changing the relative distance between two points. So, hey, why not make Alpha Centaur really, really close? So that doesn't take as long to get there. And that's the idea of a warp bubble, that you, like, shrink the space between you and the star and expand the space behind you. And then when you get there, you pop out of this warp bubble and you're like, hey, that wasn't a big deal. How likely is it that these warp bubbles actually exist or could be created? That's the topic of hot debate.

On the positive side, there's a guy who's figure. out that warp bubbles do not violate general relativity, meaning like a warp bubble doesn't break any of those rules. That doesn't mean that a warp bubble could actually exist in the universe because you create something because it's allowed in the universe. Like everybody knows chocolate soufflays are allowed in the universe. It doesn't mean everybody knows how to make them. You have to have a very delicate process to get from no chocolate souffle to chocolate suflay, right? And there's lots of ways to fail. We don't have a recipe for building a warp drive. We just know that the resulting drive is not disallowed by general relativity. It could be that some step between no warp drive

and warp drive is disallowed or is impossible. And so if you could just like magically pop a warp drive into existence, then physics is cool with it. But we don't know if it's possible to create one in our universe. All right. Well, I'm going to hope that it is because that would make things really convenient. At the beginning of the show, you mentioned that in your favorite book, they just moved stars closer together. How would that work and why would everyone be dead? This is basically like the think big version of the solar sail, right? In this version, you build a huge sail like half the size of the sun. Yeah, that's right. Half the size of the sun. Wow. You wrap half of the sun in a big mirror that points back at the sun. Okay. And so you might

think, well, what's going to happen that mirror is just going to shoot off? It's a big solar sail. It gets pushed away by the sun, right? Well, you bring it close enough and you make it massive enough that it's gravitationally captured by the sun. So now basically the mirror in the sun are like a single gravitational object. The sun is shooting half of its photons to the mirror, but they get bounced back. So basically recaptured. And the other side, the sun is shooting off its photons out into space. So now it's like an ion drive.

So one half of the sun is basically captured and nullified. The other half is not. Now the sun can move, right? The sun is basically like a rocket. And so this could move the whole solar system. And Kelly should feel great about this because you could travel to another star without even leaving your house. You could like literally sleep in your bed at home and we could go visit Alpha Centauri. Move the whole solar system.

Okay. All right. So that sounds awesome. I love that you dream big. But like, so we, you know, we've talked before. If like suns get too close to each other, one of them gets like thrown off into the vastness of space, this feels like a hundred. This feels like a high risk method. Well, it's hard to turn a star.

You know, if you realize you're going in the wrong direction, this is harder to turn than like one of those U-Hauls that are already pretty hard to turn. Super hard, yeah. Yeah. There's a lot of ways this could go wrong. You know, maybe a safer method is to use a black hole instead because black holes. You physicists, wouldn't it be safer if we used a black hole? I like you just laugh in my face at that one.

I'm sorry. I mean, a black hole also converts mass into energy, right? Black holes evaporate. They have energy stored inside them because of their mass, but they generate hawking radiation, which is energy. So put a black hole next to a mirror, and in the same way, it'll generate propulsion in one direction, right? If it's gravitationally captured the mirror,

and so that's just absorbing that radiation, now you have a black hole, which is turning its mass into radiation and pushing your ship. So you don't have to move a whole star. Now you have a black hole powered spaceship. And of course, there's technical problems here like, how do you actually make the black hole? We don't know how to do that. Maybe you could find a black hole. We don't know where any of them are. Also, there are some risks involved in having a black hole on board your ship. Please sign these waivers. All right. So I'm going to ask myself the question. Which one of those methods do I think I would

use if I was forced to go on a ship to Alpha Centauri. I think that I would agree with you about the solar sail technique, but what would make me nervous is that somebody else is in control of the propulsion. And so, like, I imagine that keeping a giant laser constantly powered is got to take a lot of energy. And so what if one day, you know, the Americans are like, you know what, we're not powering this laser anymore? It's just too expensive. And then what do you do? Yeah, well, there's much you can do. You know, the other issue with the solar sail is slowing down. Like, it gets you up to near the speed of light using that big laser. Then you have to use the destination sun to slow down, but it is no laser there to give you that boost. And so you got

to, like, combine that with gravitational slingshots. It's tricky. Yeah, um, there's no way I'm getting on any of these ships ever. But I encourage all of you to, like, please go, explore the universe and tell us all about it. Send me a postcard. All right. So I think we've surveyed some of the technologies that might get us to other stars. And my take is that there's some promise here. Obviously, there are kinks to be worked out. And Kelly and her legal team need to be put at ease about the black hole on board, you know, or moving the entire sun. Do we need to take a vote before we do that? I'm not sure. But to me, these feel like solvable problems. And, you know, we're young as a species technologically. In 100 years and 1,000 years, we'll have figured this out. And maybe even by that, then somebody will have built an EM drive. Well, and to surprise everybody, I am actually optimistic about some of this stuff. And I feel like these are really interesting problems that we have to solve.

And imagine the other things that we're going to learn along the way or the new technologies will come up with as we try to make some of these methods work. I think it's all very exciting. All right, but before you get too excited, next episode, we're going to talk about all the other tricky problems you would have to solve to get humans to another solar system. Giving birth on board, is it okay to have children born between planets? Embryos, robots, cryogenics, radiation, all that good stuff.

So tune in next time for Kelly to throw a wet blanket on your interstellar dreams. That's what I do. So well. Oh, hey, ouch. All right, tune in next time, everyone. Daniel and Kelly's Extraordinary Universe is produced by I-Heart Radio. We would love to hear from you. We really would.

We want to know what questions you have about this extraordinary universe. We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you. We really mean it. We answer every message. Email us at questions at danielandkelly.org. Or you can find us on social media. We have accounts on X, Instagram, Blue Sky,

and on all of those platforms, you can find us at D and K Universe. Don't be shy. Write to us. I was diagnosed with cancer on Friday and cancer-free the next Friday. No chemo, no radiation, none of that. On a recent episode of Culture Raises Us podcast, I sat down with Warren Campbell, Grammy-winning producer, pastor, and music executive to talk about the beats, the business, and the legacy behind some of the biggest names in gospel, R&B, and hip-hop.

Professionally, I started at Deadwell Records. From Mary Mary to Jennifer Hudson, we get into the soul of the music and the purpose that drives it. Listen to Culture Raises us on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. The U.S. Open is here, and on my podcast, Good Game with Sarah Spain. I'm breaking down the players, the predictions, the pressure, and of course the honey deuses, the signature cocktail of the U.S. Open. The U.S. Open has gotten to be a very wonderfully experiential sporting event. To hear this and more, listen to Good Game with Sarah Spain,

an IHeart women's sports production in partnership with Deep Blue Sports and Entertainment on the IHeart Radio app, Apple Podcasts, or wherever you're get your podcasts. Brought to you by Novartis, founding partner of IHeart Women's Sports Network. Why are TSA rules so confusing? You got a hood of you on take it all! I'm Manny.

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Listen to Betrayal Weekly on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. This is an IHeart podcast.