Megastructures (Original Summary Version)

**Megastructures** is the first episode of SFIA.

## Transcript

Welcome. This video, we'll be covering various types of megastructures and artificial worlds and constructs which you'll be familiar with from science or science fiction. We'll be covering everything from NEO low horizon orbital types which might have a dozen or so people inside it, up to things the size of planets or even the size of solar systems like a Dyson Sphere.

For that reason we have an ad-hoc classification system which we will be using that roughly mirrors stellar classifications. We'll go over that in just a moment. First though let me add a quick thank you and plug for Steve Bowers who provided the majority of the images will be using here and the Orion's Arm project. Orion's Arm is a shared sci-fi setting that is also a wonderful resource for looking into a lot of the other megastructures in detail and many of the sci-fi subjects which are covered in the Encyclopedia Galactica. They also have one of the most comprehensive collections of links for writers and sci-fi fans in terms of tools and maps that you can use for your own interests. So that's a site definitely worth checking out - orionsarm.com.

So let's go ahead and move into our classification system. First type is types.

### Types

* A: Artificial Gravity
* N: Not Applicable
* R: Rotating Gravity
* S: Spherical or Classic Gravity
* Z: Zero or Microgravity

Where artificial gravity is concerned, this is the situation you're most familiar with from science fiction films. There is simply no way to produce films in zero gravity or microgravity, so in TV and film they typically just have artificial gravity of some sort. We won't be using too many examples of that but it is worth noting as a class that we'll see a few of.

We also have a lot of situations where it simply won't apply, like the Lofstrom Launch Loop. The gravity that's there is really not related to the object or there's simply no reason why we care about the gravity there, like a giant space gun or railgun. It has nothing to do with the purpose of the object.

The one we'll spend the most time on is rotational gravity. This is a spin gravity case where the experience of gravity is felt by the fact that you're being pinned to the floor by centrifugal forces. We have an awful lot of examples of that we'll be going over one at a time but it probably makes up the bulk of all cases. We are also going to try to avoid getting too math heavy but there is an equation that's relevant for rotating habitats that matters. The acceleration, which for earth gravity will be 9.8 m/s2, that you would want to have to simulate gravity requires that you spin the station at a given velocity based on its radius and that's the velocity squared divided by the radius.

Our next type is spherical or classic gravity. This won't always be just a planet or an asteroid, but it is the most well-known example. This is straight, real gravity that you're experiencing. So this does not necessarily have to be caused by a classic planet. There are a number of situations you can create where you would have gravity from mass but the planet is not the same size as Earth or might not have a terrestrial natural origin. These come in as nearly as many shapes and sizes as the ring spinning habitats do and we'll be looking at quite a few of these.

Then our last category, which is simply zero gravity or microgravity. These are circumstances where the habitat, the space station or the artificial world just does not have any significant amount of gravity felt by the people inside. Besides the space station you could have habitats full of a large inflated balls full of air.

### Size

Our next category that we'll be looking at for our classification system is size. These things again can range from the size of the space station all the way up to things as big as the solar system. So we have a set order of magnitude scale to look at this. We'll give a number N where N is ten to the nth power in terms of its width in meters. So N=1 would be 10 meters, N=2 would be 100 meters, N=3 will be 1000 meters, N=6 would be a million meters and so on.

So type and size will be combined as follows: Z1 would be a zero gravity environment about 10 meters across like the space station a large rotating habitats several miles long would be a R4 something the size of the earth and almost all of the planets you normally associated as being a planet that you could live on would be a S7 indicating a spherical gravity and is about ten thousand kilometers or 10 million meters across and then an A11 would be something with artificial gravity about the size of Earth's orbit around the Sun, like a Dyson Sphere.

### Habitability

Our last category is habitability. Now, for all purposes we're just going to say that one hectare, which is a 100×100m area (about the size of a acre) is about how much space you need for one person to grow all their food and have some forested garden area. It's a very approximate figure but Earth has about 14 billion hectares of land including unusable stuff like deserts and polar regions and about 7 billion people or roughly 1010. This conveniently gives us the roman numeral X for 10 and we use that same 10n system so X would be what it would be for earth and other planets and S7x would be your typical planet that was fully populated.

This shots our handy guide and we'll be using this classification only very approximately because a hunter-gatherer civilization, for instance, would be about too low on the chart. They don't need one hectare per person; they need around 100. Alternatively, with very intensive hydroponics or aeroponics and artificial lighting, you can easily get 100 people fed off of one hectare, and you could have sci-fi settings, for instance, or futures of humanity, where you don't even eat food – you run on electricity. So it's an approximate scale and X conveniently is about the population of a planet.

We'll only be using this for when you'd expect the place in question to be mostly self-sufficient as a kind of artificial planet, even when they're not terribly huge themselves. In a lot of situations, the habitability index just doesn't apply. For those where it does, the classification will look a lot like this:

* Z1i would be the space station – it's got very few people on it.
* R4v would be a situation about 100,000 people
* S7x, again being Earth, would be an example of a place that could handle some billions of people
* A11xix, the case for Dyson Sphere, it's tens of billions of billions of people, who would live in that kind of circumstance, with the same space that you would have on Earth

### Rotating gravity

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So since the Artificial and Not Applicable categories don't really have any kind of specific shape and come in so many different varieties, we'll start with the rotating gravity ones as specific examples. It's also important to understand that these things do not necessarily have to appear from the outside as a cylinder. When you're dealing with a ship or when you're dealing with a space station, they are cylindrical in terms of parts that have gravity; they can have a non-rotating shell outside them. You can actually go ahead and embed them right into an asteroid, for instance.

The thing that comes up sometimes with this is your hear people say "Let's hollow out an asteroid and spin it". You would never actually do that. The gravity that these things are under gets higher the further you are from the axis of rotation. Most asteroids are loosely held together balls of gravel. They have a very weak surface gravity and if you actually spun the entire asteroid at speeds to produce artificial gravity for humans, they would fly apart. Nor do you need to spin them; you just need to make sure there was a few metres of room between the actual spinning object and some shell you've constructed inside the asteroid.

The other thing of note with these is that you can only build them as wide in terms of radius or diameter as something that can actually handle that force on Earth would be able to handle. It's like a suspension bridge that's as long as the circumference of the station. Many materials just cannot handle miles and miles long circumferences.

Now let's actually get into all specific examples. This is Von Braun's space station from 1952 – one the original designs and one that is pretty familiar looking. It's not a very large object; it's less than 100 meters across and it's got room for less than a hundred people on it.

NASA did quite a few of these kind of mockups in late 50s and early 60s. This is a hexagonal rotating one, designed to be inflated from the early 60s.

It wasn't very long before people started envisioning much larger places; places where people would actually live, as opposed to tight confined spaces you'd find at a place of business, a post in the Antarctic, that sort of thing. One of the first of these is the 'Stanford Torus'. There's also the 'Bernal Sphere' which is another one from that era and these are designed of the assumption that you're using steel to make them.

These are examples generally known as 'O'Neill Cylinders', named for Gerard O'Neill, and work on an assumption that you're powering these things with the light of the local sun, that they're some kilometers across or miles across, and that they could be home to many thousands, tens of thousands or hundreds of thousands of people.

One example is a lot of people be familiar with from science fiction is the Babylon 5 space station and TV show. These work on the assumption you just cannot build a space station, more than about 10 kilometers in diameter, before it's going to start ripping itself apart, from normal metals.

Here's another example from the city of Enoch project (you can check out their Facebook page).

At this point we have to go into objects that are much bigger, that can only be built out of things that could not be conceived until the mid 90s, when we discovered carbon nanotubes and graphene. Theses are substance you hear a lot about and what let us think about ideas like space elevators. At this point in time, you're no longer discussing things that a few thousand people can live on, something the size of a city, maybe a metropolis. You're looking at things that are size of large countries. There's a picture of Great Britain; it is actually to scale there. Great Britain is about a thousand kilometers from top to bottom; this thing is about 500 kilometers wide. It's about 2,000 kilometers in diameter, so it is actually pretty close to the size of the eastern United States in terms of total land area.

But the 'Bishop Ring' is just the tip of the iceberg of what you can do with graphene and nanotubes. The 'McKendree Cylinder' is the next biggest one and it's actually several thousand kilometers long and as wide as the 'Bishop Ring'. Here you have an object that's capable of housing potentially nearly a planetary population; that have the ix population for habitability. You could fit a billion people on this thing quite comfortably. It's typically several thousand kilometers long; the thing is enormous and you can put multiple levels in there as well. We typically picture these just having that one level, open sky, but they could be many, many levels high. In this picture, that is not a close-up of the 'McKendree Cylinder' relative to a planet; that's actually about the scale of a 'McKendree Cylinder'. Again, its many thousands of kilometers long. It does not necessarily need to have those light absorption discs; that's just the assumption that you don't have some power source that could let you simulate sunlight.

Now, no matter what you do there are certain limitations on how big you can make these things using classic materials, but you can still keep scaling up as one solid object. An example of that is a 'Rungworld', where each of those little rungs on that apparent ladder going around a circle could be something size of an 'O'Neill Cylinder' or a 'McKendree Cylinder', so that thing could be the size of several planets or can wrap around an entire star.

One orbital habitat that's of special note is the 'Banks Orbital' named for Ian M. Banks, the late and great author of the Culture series. This is an orbital that specifically the necessary radius to produce Earthlike gravity and have a day length, brought by its own natural sun that it's in orbit around, that is exactly the same as a day on there. For Earth, that's several million kilometers across. These things are typically the size of a couple hundred Earths when they are set up that way; you can make them a lot thicker though. But that's always going to be unique size; for a given day length and a given gravity, there's only going to be one exact size that can make that configuration.

Next, we have a very popular one – the Ringworld, first again popularized by Larry Niven in the book titled that. This is an orbital habitat that is placed around a star, typically at the distance from it that Earth is, and it's going to be about the size of a couple of million Earths.

Less well-known but from the same author is a 'Topopolis' or a 'Spaghetti World'. This is a thin rotating habitat that's simply very, very long. You can wrap it around a star several times and so long as it's much longer than it is wide, it has no problem spinning even when you curl it around, just like a rope. so you can spin it around a star and these can be pretty much any length you want.

Somewhat like the 'Rungworld' or the 'Topopolis' example is a Polyhedral habitat, which is sometimes called 'Bucky Habitat'. These can be very, very large, wrapped around entire stars, but they are essentially a long connection of cylinder habitats, whether they're McKendree-sized objects or the 'O'Neill Cylinder'-type objects. And again, McKendree objects are usually thousands of kilometers long, whereas O'Neill Cylinders are usually in the tens of kilometers long.

### Spherical gravity

We're moving on to the next class – spherical gravity. I want to talk about a hybrid example. A lot of times, when you're walking with a Moon base or a Martian base – some place where gravity exists and is significant, but isn't what you want – you tend to be stuck with that in fiction or examples, but you don't actually have to be. You can combine rotational gravity and spherical/classic gravity. Much like a washing machine when you put water in, it will spin and form a parabola, you can arrange to have the sides of your habitat slope like that, so if you spin the habitat, it will combine the gravity that's caused by the local mass and by spinning to produce a gravity equal to that of Earth. And if you're on a vacuum environment like the Moon, there's no real reason why you wouldn't do that. A bit of a hassle, a little energy intensive, but not actually all that bad.

On next major category is spherical/classic gravity. Now, this is a much bigger category that you initially think. We're not just talking about large moons or classic planets; we're also including shell worlds. examples of this would be where you construct the world around a large object, like a black hole (which could be smaller than a planet or larger because it's not official black hole) or around the gas giant or even just compressed gas.

This isn't strictly limited to super-powerful materials that allow you to make a rigid shell the size of multiple planets. This requires though that you use something called active support. The easiest way to think of active support would be a garden hose that was connected to a pump at both ends. Once you turn the pump on, the hose will immediately spring into a circle. You can even do this with magnetically propelled small particles moving mass stream, so that you can create rigid objects in a circular form that are just huge in size. We call this active support because no matter how efficient you make this system, there's going to be some energy loss, so you're going to have to constantly be adding power and assist to hold your planet together (or rather hold it apart, so it doesn't fall together). This can be done; it's perfectly scientifically possible. It's a little bit ahead of us right now because it's just huge and energy consumptive. But it is one alternative to space elevators. You could make one of these rings that was hovering just above the atmosphere of the planet and just connect shorter tethers a few miles high to it.

Another example of natural gravity would be a shell world – in this case a matryoshka doll-type situation – where it's just layer after layer of concentric spheres around each other. You'd have to artificially light those and you'd have large pillars that helped hold up the sky, in a very Atlas-like example. Here, each layer either has a slightly higher gravity as it gets further out or, if you space them out far enough, you can keep it at the same stable gravity by just having a different distance to each layer. You want to take a moment to avoid confusing this with a Matryoshka brain, which is another type of construct we'll discuss later on; you'll hear this term a lot more often than the 'Shell World' example – we'll come back to that later on.

Another example would be a 'Hoop World' or 'Torus World' or 'Donut World', call it what you would. In this example you'd have something that was about 10 times the size of Earth. So long as the hoop's diameter is much wider than that of the actual ring itself you're going to have perfectly normal gravity on the surface.

### Microgravity

Our next category of microgravity habitats. These can come in almost any type or ???, the size of space station or Space Shuttle, all the way up to self-contained worlds just full of air. This is another artificial world that was popularized by Larry Niven. Him as well as Dr. Freeman Dyson and Paul Birch were some of the guys who came up with almost all these ideas. The 'Smoke Ring' example -- he actually has a large, loosely held together atmosphere around an entire star. In another, large donut shape, but there's also self-contained varieties such as the 'Edersphere' where there is advertising on the outside of it, but inside you have a low-gravity environment, where you can breathe and where it's a warm temperature, but you'd have to have artificial lighting and, again, it's zero gravity.

Some of our oddballs are more popular examples, like 'Disc Worlds'. Here's an example one that's a couple thousand kilometers across; this where you live on a flat Earth (one side or both sides). They're big enough they could have their own gravity, but we usually think this is something with artificial gravity. Probably the best-known fictional example of this is the Discworld by Terry Pratchett (great series a fantasy parody novels) as well as the 'Alderson Disc'.

Now, normally an 'Alderson Disc' is not considered a really great one to use. People just like it because it's very large; it's surrounds an entire star. But it's pretty much always in a constant perpetual haze of twilight and does not get a lot of light. You can get around this and the way that you would usually do that would be to float mirrors over your star that will cause it to have a day and night cycle.

This is where we get into another one of those concepts will be using a little later on, called a 'Statite'. That's short for a static satellite. This is something that, much like a solar sail, is pushed on by sunlight in order to move. In this case though, it's hung exactly at the distance where the solar pressure and the gravity equal out for that object, so that does not fall into the Sun or get further away. With statites you can float mirrors over the Sun; one big one or hundreds of little tiny ones.

### Stellar engines

We are now going to move on to the class that are called 'Stellar Engines' and the classic example this is a 'Dyson Sphere'. Again, a Dyson Sphere is a couple billion times bigger than Earth in terms of land surface. Although a real one would probably be a little bit further out from the Sun than Earth is (because of that day night cycle, where you might have some statite mirrors around to actually mimic that), but Earth has two sides: one dark, one light. If you only get daylight the entire time, as far away as Earth is from the Sun, you're going to melt pretty quickly. You need to be a little bit further out than that.

Fundamentally, a 'Dyson Sphere' is just not really a stable object in of itself (it can even drift into its own star) and needs artificial gravity, so we usually talk about a 'Dyson Swarm' instead. This is identical to a 'Dyson Sphere' except it's composed of endless millions and billions or even trillions of rotating habitats or 'Statites'. The other aspect of a 'Dyson Swarm' is it doesn't have to be complete and you can build up in stages. So you could have a 'Dyson Swarm' that only was 1% coverage or absorption of light and build that out of local materials a lot easier than you can an actual 'Dyson Sphere', where you're probably not going to have enough real matter to make good, thick shells. 'Dyson Swarms' are also a little bit unstable, so we will also occasionally talk about a 'Jenkins Swarm' which is a sort of a very large donut around a star - of various swarm objects rotating bodies ???. These don't follow circular orbits though, they follow a slightly elliptical path that brings them a little bit closer and further away from the Sun and you'd probably use that to simulate your seasons.

We mentioned the 'Matryoshka Brain' earlier. This is an object that is basically a 'Dyson Sphere'/'Dyson Swarm'. Instead of it being for people to live on, you're using all their collective power to run a single, massive supercomputer. Where a 'Dyson Sphere' is almost incomprehensibly huge in terms of how many people it can support, a 'Matryoshka Brain' is almost incomprehensibly huge (even more so) in terms of how many calculations it can perform. Especially because it's assumed to be done by computer much more advanced in terms of micro sizing than our current ones are. You'd expect an object like this to, if it was treated as a human mind, to be one that was trillions of trillions of trillions of times faster than a human.

The next category of 'Stellar Engines' are the ones designed to remove mass from the star – 'Starlifting Swarm' or 'Starlifting Ring'. This is basically how you'd go about killing a star but isn't really weaponizeable. This requires a lot of infrastructure to do. Basically, it relies around using the magnetic field that a star has and setting up a ring or swarm around it that is going in the opposite direction to basically rip matter off and cause it to emit at the northern and southern poles of the planet. The magnetic field is very intense and by setting up a ring around it that spins and is a superconducting magnet, you can tear off gigatons of matter from that star, potentially reducing it down to nothing eventually. Big stars are much brighter and live a lot less time. They also burn a much smaller percentage of the hydrogen so it's reasonable to say that you might take a larger star than our yellow sun and strip matter away from it, so that lives longer. Or even a yellow sun to strip it down to be a red dwarf, which live for many hundreds of billions or trillions of years and use up almost all their hydrogen, so they could be seen as much longer lasting and more efficient engines.

Now if you actually want to move a star or solar system, you have what's called the 'Shkadov Thruster'. This is basically a half-mirror around a star and you often have a lot closer to the star than you would a 'Dyson Sphere' because you just need to make sure it's not so close it doesn't melt. This would be a 'Statite' or swarm of 'Statites'. The lights pushes on it from one direction and it pushes it back in the opposite direction. The light from the star goes out in one general direction and this causes a force to be pushed on the mirror and the star and the entire solar system slowly accelerating up to high speeds. It takes millions of years to move it any significant distance, billions of years to move it at a galactic scale, but this does actually work just fine. It should be noted that the rate at which it accelerates is related to the luminosity and mass of the star. The brighter it is, the faster it moves. The heavier it is, the the slower it moves. But stars actually raise their luminosity, or how bright they are, much faster than their mass, on an order of about the cube or fourth power. So a very bright star (such as one that could go supernova) is an ideal candidate to hit with a 'Shkadov Thruster' because you can move it very quickly. Alternatively, a red dwarf will take a very long time to accelerate up this way.

Those are your 'Stellar Engines'. Now, that's not the absolute end of things – there are many constructs that have been thought up that are even bigger on the galactic scale. The only one I'm going to take a moment to mention is a one of the examples Paul Birch has given, where you take a galactic-sized black hole (we're talking about black hole here of many millions of solar masses, like the kind you find in the center of a galaxy) and you build a shell around that, like the super mundane 'Shell Worlds' we discussed, and then you put another shell around that and another shell around that and another shell around that. Obviously, these have to be actively supported. You can put in so many layers that you're not just talking about a 'Dyson Sphere' here. You talk about something that's almost as big to a 'Dyson Sphere' as a 'Dyson Sphere' is to the Earth. And you can actually live inside this thing! You could have endless layers of 'Shell Worlds', trillions of Earths-worth of area to live on. And what's interesting is at the lowest levels, time is actually running slower from the distortion effect (though there's no tidal forces tearing you apart) than it is at the highest level. So at the lower levels time runs slowly and at the higher levels it runs the normal time. And this is, to the best of my knowledge, the biggest construct you could ever make that would have human habitability by normal people. I'm a little partial to it, not just because it's a so immensely huge, even compared to a 'Dyson Sphere', but because it's right at the center of the galaxy and seems like it would be an amazingly cool capital for a galactic empire.

So that's we're going to go ahead and finish off. There's a lot of megastructures we did not even touch on. I would certainly encourage you to research more of those. But these are most of the general ones that you'd expect to ever really see used multiple times in different settings or in science in general. Anyway, I hope you've enjoyed this and thanks for watching!