Why Building an Artificial Planet Might Actually Work: Shell Worlds and Birch Worlds

Shell Worlds and Birch Worlds are some of the largest megastructures ever imagined, offering a vision of artificial planets that could support civilizations ranging from a single family to trillions of people. From hollow planets filled with hydrogen to colossal Birch Worlds built around supermassive black holes, these concepts push the limits of engineering, physics, and the future of space colonization.

In this episode of Entropy Rising, we explore how shell worlds could be constructed, why an advanced civilization might choose to build artificial planets instead of living on natural ones, and how these incredible structures compare to O'Neill cylinders, Dyson swarms, and other megastructures.

Along the way we discuss:

• How shell worlds generate Earth-like gravity
• Building planets around gas giants and black holes
• Birch Worlds and galaxy-scale civilizations
• Artificial gravity and orbital rings
• The engineering challenges of planet-sized structures
• Future space colonization and megastructures
• Black holes, Hawking radiation, and the far future of the universe
• Whether shell worlds could become interstellar generation ships

If you enjoy discussions about megastructures, black holes, interstellar travel, the Fermi Paradox, future technology, and the science shaping humanity's future, subscribe for new episodes every week.

Transcript

Cold Open On Planet-Scale Living

SPEAKER_01 0:00

This would be more living area than the civilization that built dicing swarms around every single star in their galaxy. Are we talking like a planet the size of like a city block? You hollow out the Earth and then fill it with something like hydrogen, which is abundantly common. Think about how many people could foot on a planet like that.

SPEAKER_00 0:17

Like several trillions.

SPEAKER_01 0:19

Hello and welcome to Entropy Rising, a podcast dedicated to discussing science and the future of humanity. I'm your host, Jacob.

SPEAKER_00 0:26

And I'm Lucas. And today we're going to be talking about Shell Worlds, a planet that we can build to any specification we want.

SPEAKER_01 0:32

Yeah, this is an interesting topic because, like you said, this is effectively one kind of going against everything we've said on the show, which is people won't live on planets, they'll live on a new cylinder, which was an oversimplification, of course. There is situations when you'd want to live on a planet, and Shell Worlds offer a really interesting example of that because these are effectively customizable planets. You can build them to be whatever you want. And in today's episode, we're going to talk about how you might make absolutely miniature ones that could just hold your family all the way up to truly massive structures that might be just the biggest thing you could possibly build given our known laws of physics in the universe. So this is an absolutely fascinating topic. It's really diverse. I think we're going to touch on a bunch of different stuff, and I hope it's going to be a good episode for everyone.

SPEAKER_00 1:17

Oh, I think it will be. There's a lot of possibility that goes into these, but I think that we should probably jump into the basics and like what makes a shell world a shell world.

SPEAKER_01 1:26

Yeah, no, that's absolutely fair. So I think the the name's kind of in the tin, right? So a shell world is effectively a planet, if you want to think about it that way, artificially made, where instead of being solid all the way down, it has a thin shell and the inside is hollow. Well, hollow or filled with a different matter that we're going to be talking about later. And there can be some advantages to building these, like we said earlier. You can make them at any size and really customize them to be what you want them

Tiny Shell Worlds And Lost Atmospheres

SPEAKER_01 1:54

to be.

SPEAKER_00 1:54

Wow. Any size. So now when I'm thinking of a shell world that can just house an individual family, are we talking like a planet the size of like a city block?

SPEAKER_01 2:04

There is a lower limit on how you can build these, which we're definitely going to be touching on a little bit later, how you would get the gravity at that level. But the issue with smaller shell worlds typically is going to be atmospheric stratification. You actually have your gravity drop off really fast with these smaller ones. So could you make one that small? Absolutely. You probably could, but it's not going to be an open world. You would need to have it be more like a contained planet to be able to hold your atmosphere in and have a pressurized atmosphere. So sort of like a shell donut. It wouldn't be a donut. It'd be a sphere that you could live on with gravity, but you would need to have like a glass, a secondary sphere around it or put domes on it. Oh, like a Fushiki. Yeah, but potentially you could make them quite small. Like is that the like levitation?

SPEAKER_00 2:44

Yeah, it's like it's like it's like the metal core inside of the glass. So it's like as you move it, it looks like it's not really moving.

SPEAKER_01 2:49

Oh, okay. Yeah, like that. Uh so maybe city block size is a bit small, but you could definitely get down to truly small sizes and be, you know, a couple of square miles, maybe um in the tens to low hundreds of square miles, which is quite small for a spherical planet.

SPEAKER_00 3:04

Okay. That definitely is cool. And we will get into like the sizing and scaling uh a little bit later for sure. But thinking about what the benefits are to a shell world, I mean, of course, you're the possibilities are endless, but what do you think would be the big things that you would be looking to benefit from with a shell

Why Hollow Planets Beat Raw Planets

SPEAKER_00 3:21

world?

SPEAKER_01 3:21

Yeah, no, completely fair. So the biggest issue with a shell world and the biggest issue that actually a spinning habitat like an O'Neill cylinder or a ring world solves is where you get your gravity from. A shell world ultimately gets its gravity the old-fashioned way through raw mass. And because of that, you don't have any mass savings compared to a traditional planet. So, for example, an O'Neal cylinder, you could take the mass of Earth and make millions to billions, maybe not billions, but you can make millions of O'Neill cylinders and get so much more living area out of that same amount of mass because you're simulating that gravity by spin. But you really can't do that with a shell world, because ultimately if you have a planet that's the size of Earth and you want Earth-sized gravity, you you need the same mass as Earth. There's really no way around that. So when we talk about the benefits of shell worlds, it's not necessarily savings in the raw mass, but it is savings in what type of mass that you use. So a traditional planet, if you think about Earth, for example, is made up of a lot of relatively speaking, uncommon elements: iron, nickel, cobalt, these elements that we really want and that in the composition of the universe aren't as common. The universe is mostly hydrogen and helium. 99% of it, in fact, is hydrogen and helium. So one advantage of a shell world is what you use to get that mass. So, for example, you could imagine a situation where you hollow out the earth and then fill it with something like hydrogen, which is abundantly common. Just compress that in there until you have enough to simulate Earth gravity, and then you get to keep your planet and you get to have the same gravity on your planet, but you also get to use all of those less common materials to build other things that you might want to build, like O'Neill cylinders or Dyson swarms. So that's that's advantage number one. Advantage number two is it allows you to decouple the size of your planet from the surface gravity.

SPEAKER_00 5:17

I see. Yeah, so you would be able to scale it up just based off of the material that you're using to create your gravity in the middle of the planet.

SPEAKER_01 5:24

So yeah, exactly. So if you think about Earth, for example, let's say we wanted a planet twice as big as Earth. If it's equal density of Earth, then you're gonna have twice the surface gravity because the volume grows by the cube while the surface area grows by the square, so you get this linear relationship. Right. Which is a convenient thing to talk about. Linear equations are so much easier to talk about uh without having to check my math. But what that means is that for a given density, if you double the size of a planet, you get twice the surface gravity. And vice versa, if you half the size of the planet, you get half the surface gravity. Right. Now you can counteract that by changing the density. For example, if we wanted twice the size of the Earth with the same surface gravity, then we could use a material that's half as dense. And uh again, you could do the reverse too. If you wanted something half the size of the Earth with the same surface gravity, you could use a material that's twice as dense. So, for example, for something about moon size, that's really about as small as you can get while using Earth's gravity on a traditional planet, because already you need something as dense as tungsten, which is already a quite rare material, and vice versa, going twice the size of Earth, you need something that's half as dense as the Earth, which Earth's density is roughly 5.5 grams per cubic centimeter. So you need something that's about 2.25 grams per cubic centimeter, which is about twice the density of water, a little more. So you're gonna have to really struggle to find a material that's that density while also containing the strength needed to support a whole planet. Right. And this is why I was saying the surface gravity of a planet is more or less coupled with its size. Generally speaking, bigger planets are going to have more gravity and smaller planets are going to have less gravity. And we're a little bit limited in how we can change that with density because you ultimately will run into material constraints eventually.

SPEAKER_00 7:02

I could definitely see that.

Gas Giants, Pressure Support, And Stability

SPEAKER_00 7:04

Like even trying to incorporate, you know, gases to create that density, you can scale them up to create massive planets, but if you try and inject too much gas, it would make it volatile, wouldn't it?

SPEAKER_01 7:16

Yes, it would to a certain extent, but you need to inject a lot of gas to make a planet volatile. I mean, look at the size of Jupiter, for example. That's quite a large planet, and it's not yet big enough to have anything like deuterium fusion. So you could get away with injecting a lot of matter.

SPEAKER_00 7:30

Yeah. So let's say that we were building a shell world around Jupiter. What is Jupiter's gravity on the outside of its, I guess, its gas diameter?

SPEAKER_01 7:39

Putting me on the spot with numbers. Off the top of my head, I don't really remember, but actually let's change our mindset from Jupiter and look at Saturn, because Saturn does have a surface gravity that's about equal to Earth. Okay. So if we wanted to build a really big shell world using hydrogen as the material, Saturn would probably be the planet you'd want to build it around, not Jupiter.

SPEAKER_00 7:58

All right, I see. So then that gives us an idea. Saturn would be about as big as we could build around a gas giant made out of that specific gas.

SPEAKER_01 8:06

More or less. And yeah, that specific gas being hydrogen, and there's a reason for that. So I touched on this a while back, but gas giants kind of have a maximum size before adding more gas doesn't really make them any bigger. Because what ends up happening is the gravity generated from that gas starts to compress the planet. And Saturn really is the maximum size you can get to that while still having a reasonable amount of gravity close to Earth. Once you start going bigger than Saturn, the planet doesn't really grow bigger any faster. And because of that, the surface gravity starts to get stronger and stronger and stronger. Just because, again, the gravity drops off with the square of the radius. So if you're getting more gravity, but your radius isn't growing, you effectively end up getting you're limited on your surface gravity. Now for Jupiter, you you could theoretically build a shell around Jupiter with Earth gravity. It would just have to be much bigger than the planet itself. And the reason that I wanted to refrain from doing that is because if you were to imagine using Saturn, what you could potentially do is you could actually use the pressure of the gas to put an outward force on your shell, and that way you're holding it up against gravity. So that's kind of a convenient thing with smaller gas giants like Saturn. Potentially you might even go smaller than Saturn. That way the radius needed for Earth-like gravity is kind of within the atmosphere of the planet, and you could actually compress the gas and again use that to help support the shell. So you actually have a passively supported structure.

SPEAKER_00 9:32

Yeah, that is a great point because they they suffer from the same issues that ring worlds have, where if there is no outward force, then there's nothing to really hold it in place and keep it from kind of like collapsing in on itself, right?

SPEAKER_01 9:44

Ring worlds have the opposite problem. Ring worlds have too much outward force because they're spinning. Shell worlds would not spin.

SPEAKER_00 9:50

I see.

SPEAKER_01 9:50

So shell worlds will naturally want to collapse in on themselves because they have to resist effectively Earth-like gravity. And you can imagine this being about the same thing as building a ring around Saturn, and you would need that to support itself. And the math works out that building a ring around something is about the same as building a suspension bridge. So imagine the forces involved in building a suspension bridge at Earth's gravity, the radius of Saturn. Quite a monumental task, unless you have some type of active support, like using the planetary's own atmosphere to support that structure. And that's the advantage of these kind of smaller shell worlds. So especially something Earth size up to Saturn's size, you can effectively pressurize the hydrogen inside of that shell and use that to support the shell itself, which gives you Earth-like gravity while resisting those collapsing forces. You will still have to stabilize the core. And especially when you get to smaller shell worlds like the ones on Earth, a lot of that hydrogen is going to be compressed to the point of being metallic hydrogen. And you would actually end up with a situation where you're actually not dealing with compressive forces of the shell collapsing on itself, but it's more like a balloon. It's having to resist these expansive forces of this highly compressed hydrogen. Now, one of the ways you could get around that is you actually might play with the thickness of your shell to find the right point where the weight of the shell perfectly cancels out with the passive pressure from the hydrogen. And I think I was looking at Earth, and if you wanted to use Earth-like density, it's something like 50 to 100 kilometers thick, which is about as thick as Earth's crust is in general, which means for Earth at least, that means you could basically scoop out everything beneath the Earth, reinforce the shell, pump it full of pressurized hydrogen, and that would be a fairly stable, passively supported system and a shell world. And that gives you effectively a whole Earth's worth of material to play around with. Just to put that in perspective, Earth has more metals than the entire asteroid belt combine. So we're quite a metal-rich planet. That's not something to sneeze at, and something I could see an advanced civilization definitely wanting to have access to while preserving their home planet. Didn't work out well for Krypton. Is that what happened? Yes.

SPEAKER_00 11:51

So that's the thing. They kept mining their planet until it collapsed in on itself.

SPEAKER_01 11:56

Well, they obviously did not get these forces to cancel out correctly.

SPEAKER_00 12:00

Yes, they didn't pump in hydrogen.

SPEAKER_01 12:02

Yeah, exactly. Fools. So yeah, that's one of the huge advantages of these, is that you can use up the material resources of your planet while keeping it safe. And by the way, 50 kilometers, the thickness of our crust is way deeper than we've ever drilled or interacted with at all. You would really only have to worry about that at the deepest parts of our ocean, the Mariana Strength. But even that, you got plenty of room underneath it. You're in a pretty safe spot.

SPEAKER_00 12:25

Yeah, that is a massive amount of material, though. We've covered some crazy mega structures in our past episodes, but I feel like when you're talking about the scale that these can get up to, these might be the biggest.

SPEAKER_01 12:37

Yeah, they absolutely might. And we're definitely going to be diving into some of the really big ones later in the episode. But I did quickly want to touch on before we dive into that, uh, some other reasons you might build shell worlds other than just scooping out the insides of your planet. One of the biggest things I would say that actually makes me think that humans would build shell worlds is the fact that we might actually already have a lot of that hydrogen sitting around that we want to store in general. I don't know if you remember when we talked about our star lifting episode where we were talking about how you actually mine a star. But in that episode, I mentioned you might pull a bunch of hydrogen off of a star and then keep it basically in artificial gas giants as a reserve for use in later. If you already have a big ball of hydrogen, why not build a shell around that and make a whole new planet for yourself? And again, a lot of these balls of hydrogen will be in that Saturn-size range. So you've already got this huge mass of material that you're not using and you want to keep it safe for a long period of time. It seems like an obvious choice to go ahead and build a shell around that and increase your living area by quite a large amount.

SPEAKER_00 13:39

Now you're kind of talking my language where we are already creating these essentially gravity balls that that we can live around. So we might as well build stations around them being the shell worlds. Uh that that actually provides much more efficiency and makes more sense. Maybe the first thing ever to challenge our um O'Neal cylinders.

SPEAKER_01 14:01

I think so. And again, I just think that's like if you're an advanced civilization, anyways, and you've already got these massive gas depots that you're building, why not take advantage of that matter and use it for something useful? And I think when I'm talking about shell worlds, a lot of people think, you know, it sounds ridiculous. Why not would just build a planet at that point? But that's my favorite case for these, is you've already got a planet's worth of mass and hydrogen sitting around doing nothing. This seems like a good use case for it. And also, if you're in a fusion economy, that's a lot of fusion fuel sitting right beneath your feet. So not only are you building a planet, but you've got enough fuel to run the civilization on that planet for millions of years. That's kind of awesome. That might be the way. Yeah, absolutely. And I mean, realistically, from the surface of the planet, you would have a really hard time telling it's a shell. There's basically nothing that would give it away. It could be built exactly like Earth. You could just have that thin crust on it, have trees, have forests, have mountains. Um, the only way you would know that it's not a shell is through things like seismic events, right? Like earthquakes, for example, basically send sound waves all the way through our planet. And we can use that to measure our core, and we see them bouncing off the crust, and that's how we know that we're not living in a shell world. But other than that, there's effectively nothing that would give it away.

SPEAKER_00 15:12

Yeah. It would essentially be like a a safer, more regulated planet that we would be able to control. And really only made possible in that way by, you know, we've already expanded and we have all these extra resources to, you know, build entire surface living areas on top of these um structures.

SPEAKER_01 15:29

Yeah, but I mean, if you already built one out of Earth, then you've got a bunch of material to work with.

SPEAKER_00 15:33

So I guess that's true. Now these start going hand in hand, right? You're harvesting the in the entire inside of our planet. You could build four or five of these. Probably more. Probably more.

SPEAKER_01 15:43

But yeah, let's actually for a moment though talk about some of the bigger ones we could build because I do want to discuss those as well.

Layered Shells, Gravity Math, Heat Limits

SPEAKER_01 15:49

One key thing is that you don't need just one shell. You could potentially layer these and kind of get like a Matrioska world going on, where each layer is maybe 50 kilometers from the one below it, simulated with uh an artificial blue sky, kind of like what you would do with an O'Neill cylinder. And now we're really talking about increasing your living area of your civilization by orders of magnitude. And the cool thing too is you can actually adjust the thickness of each shell to provide a little more mass to bump the gravity of the shell above it, because like I said, gravity drops off with the radius of distance. So if you build bigger and bigger shells, they're gonna get weaker and weaker gravity, unless the shell below it has a little more mass to counteract that. And conveniently, this is a theorem that's been proved. I'm the math is crazy. But if you're inside of a spherical shell, all gravity cancels out. So the shell above you won't affect the gravity below it, but the shell below you will add mass to your surface gravity.

SPEAKER_00 16:47

What a phenomenon. I was thinking in my mind while you were talking about it, I was like, well, wouldn't the shell above lessen the gravity of the one below?

SPEAKER_01 16:54

But that's uh Yeah, it all completely cancels out. And the best way I can think about it like this is if you're in the center of a spherical shell, it's pretty easy to see that everything would cancel out. But imagine what happens as you start moving to one of the walls. Basically, yes, as you get closer to that wall, the matter that you're getting closer to now pulls you to it stronger because, like I said, gravity drops off with distance, and as you close that distance, the matter will attract you stronger. But comparatively speaking, there's now more matter behind you in the rest of that shell. And there's a whole proof and there's a paper about it, but effectively it all cancels out.

SPEAKER_00 17:27

Think about how many people could foot on a planet like that, like several trillions.

SPEAKER_01 17:31

Yeah, you could do a lot. Really, the only limiting factor with these kind of matriosco worlds is heat. It's what we talked about with our Yuka Monopolis episode is how do you actually get rid of all that heat? And so that's going to be basically the only thing limiting you for how many shells you could put into these. And we're gonna definitely talk about black holes later, but if you build these around a black hole, maybe you can actually dump waste heat into that black hole. We're not sure if you can do that, but if you could, that basically removes the only constraint you have around size.

SPEAKER_00 17:59

Yeah. I mean, being able to turn the heat generated from a surrounding planet into more energy to continue feeding the thing that you're surviving on is a win-win. Just acting as a battery. Absolutely.

Orbital Rings As Active Structural Support

SPEAKER_01 18:12

Now you can also build these bigger. And actually, two things I want to touch on at once, if that's okay. Uh, it's both on building these bigger and how you would build multiple shells of these and have them be stable. And I'm gonna bring into this the concept of an orbital ring, which is not a topic we've discussed on the show yet, and something we will be making an episode about down the road because they're fascinating. But orbital rings basically are a way that you can actively support an orbital structure. And my quick explanation on this is imagine that you are in orbit of a planet, and if you add energy to that orbit, right, you try to speed up, you're not going to move around that planet any faster. What actually happens is you move into a higher orbit. Make sense? Yeah. So how could you go faster without moving into a higher orbit? Well, the answer is you would need to apply basically a downward force on yourself as you were going around the planet, and you would have to do that constantly. And that's the only way you could go faster than the orbit you're in without moving to a higher orbit. Right. So now imagine that you build this solid ring with magnets on it, and then below that you have material orbiting faster than its orbital velocity. And by doing that and speeding that matter up, you're actually applying an outward force on that ring that counteracts the inward force of gravity. And that's effectively how you get an orbital ring as an actively supported structure. Interesting. So what matter would you be using to push out on the ring? You could use anything. I think one of the most convenient ones for me to think of is just plasma. Just take a bunch of ionized gas because that responds to a magnetic field, anyways, and just move it really, really fast inside of your ring. And then you could build a stationary ring outside of that. And the stationary ring, like I said, would not be orbiting. It would contain this orbital material inside of it. And that would actively support the stationary ring. And you could use anything. You could use plasma, you could use iron, you could use literally anything. It doesn't matter what the material is, as long as it has mass. The more mass it has, the slower it has to move faster than its orbital velocity. That's an I mean a pretty incredible idea.

SPEAKER_00 20:08

That would that would be centripetal force.

SPEAKER_01 20:10

Right, basically, yeah. So it wants to move out into a higher orbit. So that's the idea of an orbital ring, something we're gonna do episodes on. But imagine you built a bunch of these orbital rings. Right. So you build a bunch of them and then you connect them together with a lattice and build a surface on it. Well, now you have a shell world that's actively supported by the orbital material inside of it. And if you really wanted to get fancy and you're using a plasma, these could effectively be gigantic tokamaks and you could actually be harvesting some other energy for nuclear fusion as well.

SPEAKER_00 20:36

And you could just scale that to the, I mean, you're only limited by how big you could build the orbital rings or how much material you could pump into it, I guess.

SPEAKER_01 20:44

Yeah, there's really not much of a limit on any on the size. It's just a matter of how fast can you speed up that material inside of it, because ultimately the speed of light is a limit, and how big is your imagination, I guess. So this is how you could actually build shell worlds around something like Jupiter, for example. And you can do that with these shells being supported by a bunch of orbital rings within the shell itself. And that way you're not relying on the pressure of the gas, but the orbital motion of the material confined within these rings.

Birch Worlds And A Compact Galactic Empire

SPEAKER_01 21:11

That's pretty incredible. Yeah, so this would give you basically shell worlds around planets like Jupiter, or you could go even bigger and build them around black holes, which is the idea of something like a birch world.

SPEAKER_00 21:22

I've read a lot about birch worlds, and I know that they are some of the largest megastructures that have been conceived in science fiction. When you think about them, they are structures that can encompass an entire black hole, but they also can encompass an entire supermassive black hole or an entire galaxy, even. So when thinking about these structures and just how massive they could be, I just don't even know what the use of something that size could even be, you know, I can't even conceive of it.

SPEAKER_01 21:47

Yeah. They are absolutely massive. And actually, do you know who coined the term Birch Planet? Who? Isaac Arthur. I didn't realize he actually coined it. That's how much sway his show has. I love his show. But yeah, I was uh I watched the episode a while back. And I was like, oh, this he came up with the name for this, which is really cool. Uh, but he denoted that name for any shell world effectively built around a supermassive black hole. And these can scale from the supermassive black hole in the center of our galaxy, or the kind of theoretical maximum is if you have a black hole that's about a light year in radius, which would consume most of the matter in an entire galaxy, you could then build a birch world around map and would have Earth-like surface gravity and be absolutely massive. I mean, this world is really hard to comprehend how large this is. This would be more living area than a civilization that built Dyson Swarms around every single star in their galaxy. Especially if you built a couple of different shells around this. So you don't need just one shell, you could potentially build several different shells around this massive black hole. And like I said, that would give you more living area than building Dyson swarms and Dyson spheres around every single star in your galaxy. But to answer your question, why? Why on earth would you want something this massive? And I think the answer to that is beautifully simple. One of the biggest issues with Galactic Empire is that it is impossible to stay in contact with the entire empire. But if you build a birch planet, you're only one light year in radius. That means your entire civilization, an entire populace bigger than a K3 civilization, is within a year of communication from each other. And a year of communication is very doable. This is something that empires of old have done even more extreme for a long period of time. So now you're actually talking about having true K3 civilizations that could function and work as a society. And that to me is probably the biggest reason why you would potentially want to build something as massive as a true birch world.

SPEAKER_00 23:44

Beautifully put. I mean, we're talking about a population that spans, you know, like 30 zeros. It's really hard to copy that population. Quite a few people. And I guess that would be one of the only real ways to maintain an empire of just like that populace. Yeah, absolutely.

SPEAKER_01 24:01

There's really no way I could think of to have that many more people in a more compact area. That's really pushing the limits of physics.

SPEAKER_00 24:07

And that year is your maximum. That's just if one pole was trying to communicate with the other one. Of course, as other poles try to communicate with each other, you know, across shorter distances, it becomes less and less.

SPEAKER_01 24:18

So if you have a one light year radius, you actually have to go around the circumference. So you might be a few years because you can't go straight down through the black hole. Oh, because there's a black hole in the way. Right. So you'd have to communicate around the circumference. But again, you're only adding a few years, and you can actually get around even that kind of because the innermost layer of this birch world is going to be suffering some pretty gnarly relativistic effects. Nothing that stops this from working, but you're going to have time dilation of about a third. So you're going to experience one third time, which means that if you wanted to communicate with somebody on the other side of the planet and you wanted to stay in faster communication, you could potentially send your message and then go and slow down your clock and then come back up, and the message will be there even faster from your perspective. My brain hurts a little bit.

SPEAKER_00 25:02

That is awesome though. It crests into the unconceivable, just like the size and scale and why that would be used, but it definitely makes sense as being one of the only structures that you could use to maintain a galactic empire.

SPEAKER_01 25:16

Yeah.

SPEAKER_00 25:16

And it does use about a galaxy's worth of mass.

SPEAKER_01 25:18

So it's truly a galactic empire.

SPEAKER_00 25:20

Yeah. I mean, like, like what would have you would have harvested everything in the galaxy to create this structure. And then it's just you and the supermassive black hole.

SPEAKER_01 25:30

Effectively, yeah. And if you can dump heat into that massive black hole, then these things could be truly hard to detect. And this potentially could be a Fermi paradox solution of maybe if there is a galaxy that contains smart enough life forms, they just collapse their whole galaxy into a supermassive black hole, build this birch world around it, and all of their waste heat goes into the black hole. So there's no way we could ever detect them. Obviously, this still would rely on life being fairly rare because we see a lot of galaxies. But maybe in a few billion years, when all the stars die out or move away due to uh the expansion of the universe, our future civilizations might think that's what happened. Everyone went into these birch world planets because their night sky would be completely dark.

SPEAKER_00 26:09

That actually makes a good point. Yeah. If you can't see other galaxies anymore, then what is to stop you from thinking that you are encapsulated by an ancient civilization that just had you are so minuscule to them that you know you're just lucky that you were not harvested millions of years ago.

SPEAKER_01 26:25

Or thinking that all the other civilizations went into these birch worlds.

SPEAKER_00 26:28

Yeah.

SPEAKER_01 26:29

It's not really a Fermi Paradox solution because we obviously still see galaxies, but it is something that could lead to more advanced civilizations choosing to go inward instead of expanding outward and crossing the intergalactic medium if they could do that. And truly, crossing intergalactic space is a whole other can of worms compared to interstellar space. Uh Andromeda's like what, 2.5 million light years away. So it's not a quick trip by any imagination. So it's hard to imagine an empire spanning across that regardless, but this would definitely deter them, I think, even more, knowing that this is an option.

SPEAKER_00 26:59

Oh, yeah, absolutely. And then I guess another benefit of this would be it is a great way to combat uh entropy. You know, as the universe starts to die from heat death, black holes will be the only remaining thing that we can survive around. And we did do an episode on this previously, but if you are maintaining your planet off of the energy produced by a supermassive black hole, it would be reasonable to think that you could ride that wave up until, you know, maybe.

SPEAKER_01 27:25

There is an issue with that, is that the larger you make a black hole, the less hawking radiation it emits. And when you get to these massive supermassive black holes, I mean, for example, even our own supermassive black hole in the center of our galaxy, much less something that was an order of magnitude larger than that, uh, they are actually colder than the background radiation of space. They emit so little hawking radiation that they're growing just by background energy. So when you're talking about these supermassive black holes, uh, not only are they emitting almost no energy, but they actually would be more of an energy sink than anything. So you would have a really, really hard time actually harvesting useful amounts of energy around them, at least through Hawking radiation. I mean, there's some other tricks for getting energy out of black holes, but this is the same discussion we had in our ancient civilizations episode, which is that there's this gap between when the last star dies and when black holes start emitting useful radiation, and that's really hard to make it by. But you are right, this would be a great way to sequester a lot of mass, and eventually this would shrink to the point of emitting useful amount of radiation. But of course, now you've got this issue of this giant birch world, uh, and the the black hole has shrink. So you would have to shrink your birch world as you went.

SPEAKER_00 28:36

Yeah, like uh taking a leaf out.

SPEAKER_01 28:39

Yeah. Like uh, oh yeah, like one of those tapes. But now that we've talked about the truly massive, why don't we talk about the minuscule?

Black Hole Cores For Small Worlds

SPEAKER_01 28:47

Yeah, no, I'd I'd definitely like to hear more about how that would function.

unknown 28:50

Yeah, absolutely.

SPEAKER_01 28:51

So black holes will allow us to make the biggest possible structures, but they also allow us to make the smallest because I kind of alluded to this already. The moon is about as small as you can reasonably go while using traditional matter, and it's already maxing out the densities of things like tungsten, which is, you know, the famously dense material people like to buy cubes made out of. So if you want to make a planet smaller than the moon that has Earth-like gravity, the only thing you can really turn to at that point is a black hole. And that might be one of the ways, if we ever did want to terraform our moon, that that would have to be one of the things we look at to give it earth gravity is putting a black hole in the center of it and using that to simulate the surface gravity. Of course, we would also need to move the moon a little further away from the earth if we did that uh to adjust for the tides, but not too much. I think you only need to add about five percent of the mass of Earth's mass to the moon, uh, because there's a smaller radius. Yeah. For something the diameter of the moon, I believe you need a total of 7% of Earth's mass to have Earth-like gravity at the surface. I think the moon's closer to like 3%, so you need about 5%. That makes sense. Something like that. Uh pulling these numbers off the top of my head. 6'25. So you need something like four to five percent of Earth's mass added to the moon uh to achieve that. And a black hole would be an absolutely fantastic way to do that. But if you want to go smaller to the moon, then you really need to start looking at black holes because that's the only way you're actually gonna get the density you need to do that. Now, I did allude to this earlier. So let's talk about the challenges of miniaturizing. I think the moon would be fine. You could build a moon-sized planet with Earth-like gravity and have an open atmosphere with relatively no issues. But as you go smaller and smaller and smaller, you do start to run into a few issues. One of the issues you're gonna run into is that the gravity starts dropping off faster and faster and faster as you get to a smaller and smaller black hole. And this is always the issue with smaller black holes. It's the tidal forces, right? Like a supermassive black hole, you can fall on the event horizon, you'll be fine for a very long time because the difference between the gravity from your feet to your head is almost not measurable. But when you get these really tiny black holes, they have a much sharper concentration gradient of gravity. And so you start getting to a point once you get so small that you actually don't have gravity extending far enough away from the planet's surface to trap and contain an atmosphere that gives you Earth-like pressure, even if it's the same surface gravity. So, like right now on Earth, for example, if you go out to low Earth orbit, which is about how far you would want your air column to extend to, you only lose about two to three percent of gravity. But if you imagine on a tiny kind of shell world, right, going out that far, you might be at half gravity, a quarter gravity, 10% gravity. And that's just not going to be enough strength to contain your atmosphere. You might just float away. Well, exactly. The issue is you might not float away, but your atmosphere surely will. It'll be stripped really fast by solar winds, and you just effectively wouldn't have the pressure gradient you need. And even to make it worse than that, because the gravity is dropping off as you move away, you would actually need even more air per unit of surface area than we have on Earth just to get that same uh surface pressure because you don't have as much gravity further out pushing down.

SPEAKER_00 31:58

Yeah, it's cool being able to use black holes for gravity on these smaller stations, but at that point, an O'Neill cylinder just becomes more of an efficient.

SPEAKER_01 32:07

Yeah, I definitely think so. But that's not to say you can't build them. It's just that you would have to put a dome around them.

SPEAKER_00 32:12

Yeah, yeah, because you all of your atmosphere would be stripped and escaped. It's like you said in the in the beginning, just make the little floating Fushigis, which I think would be uh definitely a cool look. We're all just in these encapsulated glass balls or you know, acrylic or whatever we would make them out of. Just floating around. You get your own. It's like it would be like owning a private island, like something for the rich.

SPEAKER_01 32:34

Yeah, absolutely. I mean, I could definitely see it. It's gonna be something like a private island owning it for the rich. Uh, that's definitely a fun one to think about. Although there is another interesting consequence, though, as you go smaller, and this I think actually might be a genuine benefit into going smaller. And this ties back into what I was saying earlier about black holes. As you go smaller, the hawking radiation they produce increases. And so you will eventually hit a good point where the black hole in the center of your tiny world produces a useful amount of energy, and you can harvest that energy and use it to power your world. And the nice thing is, depending on the size, it might produce that energy for hundreds of millions of years. So not only do you have a planet that has a useful amount of surface area, maybe the surface area of a large state like Texas, but it's also producing enough energy to sustain itself for maybe even billions of years. And probably one of the most efficient ways you could ever harvest energy from matter, which is a black hole.

SPEAKER_00 33:27

That definitely brings it into a new light because being able to have, you know, essentially infinite energy in an area like that, it would be like, why wouldn't a civilization that's reaching the heat death of the universe just build a bunch of those?

SPEAKER_01 33:40

Yeah, exactly. And maybe before the heat death, that's really far away. But I could definitely see if you are able to build these microscopic black holes, which is always a question, we don't know if you can, then this could be a great way to build a bunch of little planetary civilization. I don't even know what you want to call them, like these little bubbles of life. You could keep them going for almost ever. And you might even be able to feed these black holes with matter over long periods of time and keep them at that useful size. And that could genuinely be a way a civilization could try to live past the end of the last star and into the useful area where supermassive black holes start shrinking and producing useful energy. That does tie back into us saying with these larger planets that are sitting on massive stores of hydrogen. You could imagine that you have those and you're slowly harvesting the hydrogen from those and using it to power these black holes in your many civilizations as well.

SPEAKER_00 34:28

The mini bubbles go to the big bubble to collect the hydrogen and then they redisperse. Yes, exactly. Oh man, sounds awesome.

SPEAKER_01 34:36

Yeah,

Shell Worlds As Starships And Farewell

SPEAKER_01 34:37

absolutely. And actually, I just thought of it though, but these smaller ones could maybe be a useful way to make interstellar spaceships. I mean, you get to live on a planetary surface. Don't get me wrong, you still have a lot of stuff you have to do to protect yourself from the interstellar media, but do you really care if it takes a thousand years to send your planet to another star, right? Like this isn't a colony ship anymore. This is a whole planet. And so you really might not care about the timescales here.

SPEAKER_00 35:02

Yeah, no, I mean, you're you're already drifting through space around our sun, might as well be drifting through space to a place you actually have to go.

SPEAKER_01 35:08

Yeah, and you would need to probably protect the surface in order to survive the interstellar media. So it already makes sense. You would need to build a giant shell around it, and you could paint an artificial sky, put an artificial sun, and from the surface, you would never even realize this isn't a naturally occurring planet, except for you're drifting to a new star.

SPEAKER_00 35:28

Yeah. And then, you know, you have to go to the hydrogen bubble every once in a while.

SPEAKER_01 35:32

Well, that's the sun. That's the new star you're going to. It's not like you're going to run out of fuel in the joining. You've got hundreds of millions of years potentially of fusion fuel.

SPEAKER_00 35:39

Yeah. Oh man, that would be uh well, I'd say it would be awesome, but for somebody living on there, I guess you would you wouldn't even know. Yeah, it wouldn't even matter.

SPEAKER_01 35:47

And the cool thing is this might be, I'm kind of imagining it how a lot of coral reproduces, right? They just kind of throw their offspring out to the ocean and the host doesn't care. It's just seeding the whole ocean with its uh offspring and hoping something sticks. This could be a similar thing to that. Maybe you have a civilization that decides they want to expand and they just keep building these things and shooting them out, and you're good to go.

SPEAKER_00 36:09

It's just like some kind of reproduction protocol built into the planet itself, and the people on it just continue living. And then as it just continues to produce more and more miniature black holes that then build planets around them.

SPEAKER_01 36:22

Yeah, and this could be a great way to ensure that your civilization lasts for effectively ever. I mean, you're you've got so many fail-safes at that point.

SPEAKER_00 36:29

As long as long as it could. I mean, I don't know, Jake. We might have solved it.

unknown 36:33

Yeah.

SPEAKER_01 36:33

Now, will that be the same even species as you by the time it reaches another star? Who knows? But uh, it's still a cool concept and something I don't see talked about a lot with these shell worlds, using them as actual spaceships.

SPEAKER_00 36:44

We would have thought the that the bubble men would would take over the universe. But I think that's all I have on Shell Worlds, you yeah. No, I mean that was uh that was a crazy topic to think about. I've all I've only ever really explored in the Bert's world, so being able to learn about the actual utility of these miniature ones is was pretty crazy.

SPEAKER_01 37:02

Well, hey, I'm glad it was an interesting topic for you. I hope it was an interesting topic for you, the viewer. And if you like this, consider subscribing on whatever your podcast platform of choice, giving us a like or a comment if you can. That really helps us grow in the algorithm. And of course, if you really love the show, you can support us on Patreon. Also, next week, we are gonna be diving into the science of Project Hell Mary. Different type of episode. We've never done this type of episode before, but I think it's brings up so many fascinating topics on different evolutionary paths that species can take, that civilizations could take, and raises the question of could we actually handle something like astrophage? So I hope it's gonna be an interesting topic. And if it sounds like one to you, please consider joining us next week.

SPEAKER_00 37:40

Hope to see you guys there.

SPEAKER_01 37:42

Take care. Bye bye.