If you’re looking to expand your population,
one way is to go out and colonize alien planets in other solar systems. Another is to just
build your own planets, but if you like having a lot of elbow room, nothing quite beats a
Ringworld. So today we are going to be looking at a type
of Megastructure called a Ringworld, a giant ring-shaped structure that goes all the way
around a star and contains vastly more living area than Earth. It was popularized in the
novel Ringworld, by Larry Niven, and unsurprisingly, this is our book of the month, sponsored by
Audible. You can grab a copy of Ringworld by using my link Audible.com/Isaac, or click
on the link in the description below. That gets you a FREE audio book and a 30 day free
trial of Audible. The novel came out in 1970, in Niven’s Known
Space series, and while that includes a lot of great short stories and other books, many
written before Ringworld, that novel became what that setting is best known for and spawned
several sequels itself and a few aborted attempts to bring it to film or television.
It’s not hard to see why either, there’s tons of other fascinating themes, aliens,
and technology in that novel and the series in general, I’d recommend reading it even
without the famous megastructure, but the sheer scope of a Ringworld captures the mind.
Since I will be discussing that object in detail today, I do want to emphasize that
how it functions isn’t the important part of that book, and the story has tons of other
fun elements I won’t be spoiling today. We’ve discussed Dyson Spheres in the past,
and how the rigid kind don’t work but you can do a Swarm of objects instead. The big
problem with a rigid sphere is that there’s no gravity on the inside of it, so everything
falls down into the Sun. Even if you spun the object to produce centrifugal force acting
as gravity, only near the equator would you have full gravity, and at the poles you would
have none. Niven suggests just going with that, an equatorial
slice of a rigid Dyson Sphere. It’s smaller than a Dyson, Swarm or Sphere, but unlike
the Swarm, which is physically possible, it has all of its land area connected, and unlike
the Sphere, which physically is not, this works, more or less.
Here is the basic concept, much like any rotating habitat. You take a big ring or cylinder,
spin it around quite fast, and those on the inside are shoved against it by centrifugal
force, or by their own inertia if you prefer. Since we are going to be spending a little
more time than normal on the physics and engineering aspect of things, I might as well go ahead
and address that. I reference spin-gravity and centrifugal force
here a lot, and so a lot of folks assume I bypass calling centrifugal force a pseudo-force
or imaginary force to save time. Which is partially true, but mostly not. Centrifugal
force is an inertial force, or a pseudo-force or fictitious force, in the sense that it
only appears real when you treat an accelerating object as stationary. Of course, in physics,
99% of the time we are actually doing just that, and every time you think of yourself
as standing still, sitting still, or stopping, you are too, because the surface of Earth
is non-inertial reference frame that has inertial forces acting in it.
But the bigger issue is that if someone says to a physicist, “Centrifugal Force isn’t
a real force, like gravity” they will not get nod of agreement, but more that grimace
we tend to reserve for when the correct answer is very hard to quickly explain. You see under
Einstein’s General Relativity, gravity is also an inertial or fictitious force. So saying
centrifugal force isn’t real, but gravity is, is like saying that the shadow a man casts
is not a real thing, but his reflection in a mirror is. You can make the argument neither
is real, or both are real, but for pretty much all practical purposes they are real
enough, and the same for gravity, or centrifugal force, which can be used to mimic gravity.
More importantly, if we do make a big cylinder, or ring, and spin them around quite quickly,
the apparent force holding you there is going to feel quite genuine to you, and if you jump
up from it you will fall right back down too, just like with normal gravity. In a rotating
frame of reference that is because centrifugal force is pulling you back down, to an observer
watching from outside, you didn’t really jump up, you jumped up a little while flying
forward in the same direction as the spinning ring, and ran back into it at about the same
place on it you left. This is our only current trick for generating
artificial gravity, they have the classic sci fi kind in the book too, but not so cheap
that you can cover planets with it. Now how much force or acceleration you feel,
how much ‘gravity’, is entirely dependent on two variables, how fast the thing is spinning,
either given in its actual velocity, or tangential velocity, in meters per second or miles per
hour, or its spin rate, rotations per minute, and how wide the thing is, it’s radius or
diameter. The default equation is that the acceleration is equal to the square of the
velocity over the radius, and you want that acceleration equal to 9.8 m/s² for Earth
gravity. This means that a ring that is 224 meters
in radius, and spins around twice a minute, having a tangential velocity of 47 meters
per second, or 105 miles per hour, will seem to have normal Earth gravity. Since spinning
around more than twice a minute can cause nausea, we usually consider this the safe
minimum size for any cylinder or ring meant for comfortable long term use by people.
But, of course you can go bigger. Take that same ring and make it twice as wide and gravity
will drop to half, make it 4 times wider and the apparent gravity will drop to a quarter
of what it was, 10 times wider, one tenth the gravity, and so on. To keep up the proper
gravity, you need to spin it faster. A ring 4 times wider will need the velocity to be
twice as high, again it is velocity squared over radius.
You don’t need to worry about nausea though, because even though it’s spinning twice
as fast tangentially, it now has four times the radius and circumference so it takes twice
as long to spin around, one rotation per minute, not two.
Now, to make something big enough it would wrap around an entire star, at about the distance
Earth is from the Sun, 1 AU or Astronomical Unit, and give it Earth gravity, would require
that it spin around not once or twice a minute, but about 40 times per year, every 9 days,
and that it have a velocity of about 1200 kilometers per second.
That’s one of those ridiculously huge numbers, sounds small compared to the speeds of light,
four-tenths of a percent, but it is also over a hundred times the escape velocity of Earth.
If you had a ring spinning like this, you could jump off the side and fly off into interstellar
space and arrive at Alpha Centauri in about 1000 years. It also means that anything that
smacks into it is going to do a lot of damage, because their relative velocities are a lot
higher than a meteor hitting Earth’s are. It is a speed at which a person, who weighed
86 kilograms or 190 pounds, slamming into it, would release exactly the same explosive
force as the Hiroshima bomb. Needless to say you want to have some powerful
anti-meteor defenses on such a thing, though since you need to clear out just about every
bit of rocky matter from your solar system to build one it maybe isn’t such a big an
issue. Of course, you could also build it out of
something very tough too, and you have to anyway, because spinning an object that fast
puts enormous strain on it. Whenever building a rotating ring, the force it is under in
terms of stress is the same as a suspension bridge with a length equal to the ring’s
circumference operating in the same apparent gravity. It’s fairly difficult with modern
materials to build a suspension bridge even a kilometer long, though most of that has
to do with other factors like wind that isn’t an issue here, and even stuff like carbon
nanotubes and graphene maxes out at about a thousand kilometer radius for a rotating
habitat. It’s also nice to have some margin for error and damage, so you don’t want
to go to the maximum. Plus everything you load inside that habitat,
all the dirt and air and water, weigh down on it just like a bunch of vehicles do on
a bridge. So unless you want the structural shell to be much more massive than the stuff
inside it, you have to make it even smaller than the theoretical limit the material allows,
which incidentally is the same breaking length we discussed in the Space Elevators episode
of the Upward Bound series. We don’t have any material that could even
vaguely permit a ring a whole astronomical unit in radius, so the Ringworld is usually
thought to be confined to fiction, but we’ll challenge that and discuss some options in
a bit. It is worth noting though, that this is why
so many of us who discuss this topic often prefer a giant swarm of smaller rotating habitats
instead, since their main disadvantage compared to the Ringworld is you can’t walk from
any given point on them to any other point in the swarm. Which is unfortunate, but not
really an inconvenience to any civilization capable of building such things anyway, and
as we saw in the Dyson Spheres episode, you can create a variant called a Rungworld that
still lets you walk around the whole thing, even if you might have to do occasional brief
stretches in low or zero gravity, though these can still have air and even water if you don’t
mind employing some pumps. The big disadvantage of rotating habitats
in general, the normal kind or the Ringworld, is the daylight. On a normal O’Neill Cylinder
you are spinning around every other minute, so you not only need an elaborate system of
mirrors to get the light inside the can, but wouldn’t want to see the outside anyway,
it’s probably rather unpleasant to see the sun rise and set every two minutes.
The O’Neill Cylinder’s much bigger brother, the McKendree Cylinder, which is 100 times
wider, takes the square root of 100, or 10 times longer to spin, about every 20 minutes.
As I mentioned, the Ringworld itself spins around every 9 days, but since its light source
is inside it, the sun does not rise or set or even move, it stays in the middle of the
sky, all day, every day, all the time. This is an irritating feature, and one that
can be addressed, but it is worth noting that for any given simulated gravity strength there
will be exactly one ring-radius that fits a specific day length. For Earth gravity and
day length, 24 hours, that would be a ring 1,857,000 kilometers in radius, or just under
12 million kilometers in diameter, and it would need to spin at 135 kilometers per second,
not the 1200 of the Ringworld. To simulate Martian gravity and day length, which is about
40% of Earth’s gravity and just a little longer than our day respectively, would require
only about 40% of the speed and radius. For any given planet, with a given surface gravity
and day length, there will be exactly one radius and spin rate that can mimic it.
We call this a Banks Orbital, and it is the Ringworld’s little brother, first popping
up in the novel Consider Phlebas by Iain M. Banks, book 1 of his Culture series, which
I’d also recommend. They are hundreds of times bigger than Earth in land area, not
millions like the Ringworld, though that’s still a lot of living room.
What is neat about these, is that if you go for a thick ring, rather than a big cylinder,
you can set it in normal orbit around a sun, but slightly cocked on its axis, so that it
spins around once a day and gives you a normal day/night cycle. Indeed if you give it a decent
tilt like Earth has, it can even have seasons, though you will get a big eclipse every year
and the seasons won’t change with how far north or south you are.
For the Ringworld, you need to instead use sun squares, another inner ring with dark
and clear patches that spins relative to the Ringworld to move those patches overhead once
a day to produce night, otherwise it’s eternal noon-time sun. Now in the book, this means
simple dark, then light, very little transition time, but if I were building one, I’d have
a single solid ring where even the clear patches had material there to block more harmful frequencies
of sunlight, and I’d not have just clear or opaque, but translucent areas to simulate
the dimmer light of mornings and evenings. Indeed they wouldn’t be translucent or opaque
either, but reflective, so I could bounce that light to some energy collector.
Another aspect of Dyson Shells or Ringworlds, is that while we always say 1 AU from the
Sun, and of course that distance would be different for other stars, we would actually
want them further out. Earth’s surface area is not twice our cross-section of light that
we get from the sun, but 4 times as much, because we’re a sphere not a disc.
If you don’t need the energy coming off that star for other things, which you really
do not since the ring is not a full shell, so there’s plenty more sunlight to use,
then you would actually want to go bigger yet, and instead of having that inner ring
having opaque, translucent, or reflective segments, have it be made of a lot of lenses
and prisms that concentrated light into bands or spots instead. It still lets you simulate
day night cycles, but let’s you use all of that light, and also let’s you vary the
colors coming down on a spot, more red for mornings, less light at certain times of the
year, more or less light at certain latitudes to simulate north and south polar regions
versus tropics, rather than a mono-climate. So while in the book these are sunshade squares,
I will simply call this inner ring the light ring, and it can have power collectors on
it too, sticking up further north or south, along with radar and lasers to help blow up
meteors. We’d want more on the outside edges too,
but the actual shell has a few features of note also. First, a Ringworld takes an insane
amount of mass to build, it has over a million times the land area of Earth, and matter isn’t
cheap, so you would want to have dips and rise in the outer shell to let you do deep
oceans and tall mountains without using tons of mass and needing an even stronger shell.
It’s a good idea to keep your oceans fairly shallow and make your mountains hollow or
full of something like aerogel too. Second, your typical Ringworld should have
two huge mountain ranges that extend above the atmosphere at each rim, because you need
to have walls there to keep the air from spilling out. Once it leaks over the side it is gone,
because even though these things are far more massive than a typical planet, and have a
decent gravity well, they are spinning far faster than their own escape velocity. This
makes it quite handy to land or launch ships moving at fast interplanetary speeds from
them, or even slow interstellar speeds, but it means those air particles are going to
zip away, right out of the solar system, and indeed the galaxy eventually, it’s that
fast. So you want walls to keep the air in, and you might as well stylize them as mountains
a few hundred kilometers tall. You might even want to keep concealed vacuums in them to
suck air back down and further minimize the leakage.
Adding machinery to artificial planets always seems to bug some folks, but no megastructure
lacks them, they are always there, automated or not, and Ringworlds are not actually in
stable orbits so they do need corrective thrusters on top of an impressive point defense system.
You can probably use light, rather than fusion or chemical rockets to provide the thrust
to keep the ring stable, it is fairly stable over the short term, but you still need thrusters
for corrections. I like to think that in the interests of robustness,
the builders would use simple technologies like light to keep things going and probably
some other form of relatively simple system for corrections too. As we’ve discussed
before, you can use light as a propellant. That is one thing I do get a kick out of though,
this notion of some advanced species building something like this then falling back to primitive
technology so they can’t maintain it. That’s vaguely plausible on a regular old Earth-like
artificial planet, but when you’ve got a million times the living area, even if you
fell back to hunter-gather technology and population densities, you still have many
trillions of people, and even primitive agriculture should get you close to a quadrillion people
total. Even following a collapse, you would think
technology would be prone to catching on in a few places here and there and then spreading,
and if you have some collection of kingdoms somewhere just hitting the industrial era,
in a tiny corner of the ring just a few hundred times the size of Earth, they ought to be
fielding an awful lot of scientists and inventing technology again awful quickly, and once you
have light speed communications from phone and radio again, you could easily have a modern
era civilization with a trillion professional scientists working to re-invent technology
that they have examples of all over the place. Dark Age megastructures are fun in fiction,
but not terribly plausible. I do get asked a lot what the inside of rotating
habitats look like, and the answer is that it varies a lot, depending on their size,
in the smaller ones the sky looks like your neighbor’s backyard. The horizon curves
up and wraps overhead and back down. That is one of the reasons I generally suggest
lighting them from the inside and simulating a sky through brute force technology, in other
words stick another cylinder inside it and paint it blue, or go a bit more elaborate
with holograms or TV screens simulating the right look and lighting.
For one as big as a Ringworld though, you don’t see the horizon rising up, and the
other side of the ring will look like a blurry blue green thing, since at those distance
continents are smaller than a dot in the highest resolution a human can see. With a simple
telescope you could see them though, of course the sun is rather in the way of a clear view.
But, as to the horizon, the curvature is so small that there just isn’t one. It will
eventually be broken up by the terrain or by the air itself. On the sea or a very flat
area, or seen from a great height so land isn’t in the way, it would seem like a hazy
band where sky blends into earth or sea, probably with a red tint, like a perpetual sunset.
I suspect you’d probably have a lot of smaller mountain ranges dividing areas up too, lots
of hills and valleys are a good way on any larger rotating habitat to remove the appearance
of the weird horizon. Now we are normally only looking straight
up through about ten kilometers of air, in the mornings and evening the light is coming
in at an angle so it passes through a lot more air, thus the reddish color near the
sun rainbowing outward. Here, the reflected light of the rest of the ring has to pass
through a lot of air to get to your eyes from the parts near you on the ring, so it will
re-emerge like a giant rainbow arch across the sky from over the non-horizon once the
amounts of air in between you and it, both by your and by its position, drops to enough
to allow clear vision. So people on the ground will see this more like a giant glowing bridge
across the heavens, though your inner light ring will interfere with that too, depending
on how close to the ground it is. Get up on a tall enough mountain, and you
might be able to tell it’s a ring, and if folks have telescopes and communicate with
folks decently far away, their maps of that sky bridge are going to start making it very
obvious they live on a big ring that bridge is part of, not a big flat earth with a bridge
over it, same as we realized we live on a big flat planet, that just seems flat close
to it, but is curved over very big distances. When it comes to weather, overall it’s fairly
similar, at this kind of scale the issue of being on a ring that is spinning to make gravity
versus a sphere that has gravity, and spins to produce its weather, is not too big of
a difference. The important thing though, is you do want to have mountains ranges and
go for relatively normal sized continents and seas, rather than trying to make continents
a hundred times bigger than Eurasia or oceans a thousand times the size of the Pacific.
This helps make sure storms can’t build up over huge distances and that water evaporating
on an ocean can get deep into a continent. Indeed, when making your own landmasses, by
and large big chains of big snaky islands and shallow seas is probably best. You might
be able to make continents a hundred times bigger than Eurasia, but you probably want
to keep most of them the size of England or smaller, gives you a lot more coastal real
estate and while I’m sure you would want some deserts and tundras, I don’t think
you would want as much of them as we have on Earth, percentage-wise.
These things have tons of space, but there’s no point being wasteful with it, build mostly
the land you like and use smaller proportions of the kind you don’t. If you are low on
space, make the ring wider, or build another at a different angle. Multiple Ringworlds
cocked at angles can form a Dyson Sphere. Again these things also take a lot of mass
to build, depending on how wide you want to build one, north to south, and how deep you
want to make the land. You could disassemble all your own planets and even mine out neighboring
solar systems to build one, but as we’ve discussed before, most of our solar system’s
heavy elements are in the Sun and there’s more than enough there to build one if you
can get Starlifting working. Okay, so those are the basics of a Ringworld,
and you might be asking why I even covered this in detail when I said earlier we had
no material strong enough that we could ever make them from, and I did say we have a couple
tricks for that. You don’t necessarily need one though, there
is a variation of this Niven explored in another novel called Smoke Ring, that was a naturally
occurring object, but the megastructure version is just a giant glass donut around a star
orbiting at normal speeds with an atmosphere inside. No gravity, but you could stick some
smaller rotating habitats inside it, and if you like flying and don’t care for gravity,
it works without needing super-materials. I think Peter Hamilton included one in his
Commonwealth saga too, another good series. But if we want gravity, again we do have some
tricks. The first one is that we might one day learn how to make such materials. We have
a concept called magmatter, that is a hypothetical material you might be able to make if magnetic
monopoles turn out to be possible. This could permit matter that is ridiculously stronger
than even stuff like graphene. We also have to keep in mind that normal materials have
their strength based on the strength of electromagnetic bonds between atoms.
The forces inside atomic nuclei are different and stronger, and for that matter all the
cool materials we make are based on protons and neutrons, made out of up and down quarks.
There are 4 other types of quarks and someone might figure out how to mass manufacture them
and make stable stuff out of them someday. For that matter, when we say normal matter
it’s worth remembering that dark matter is actually normal matter, since it makes
up most of it. Not really fair to call it exotic when it is the majority, and we know
next to nothing about its properties. Now, what we do know about it makes it very unlikely
you could build anything out of it, it’s incredibly weak interactions with everything
else include other dark matter is about it’s only known characteristic, but it’s worth
remembering in the sense that we haven’t finished exploring all the options yet, and
even graphene and carbon nanotubes are only a generation old.
However, we do have an option inside known physics and materials. We’ve talked a lot
on the channel about Active Support Structures, and how you can use them to make space elevators
for instance if you can’t find a material strong enough. Instead of a super strong material
you hang down from orbit, able to hold its own weight, you use a stream of fast moving
matter to push and hold a structure up, like holding something aloft by hitting it with
a stream of water from a hose below or a piece of paper floating over an air vent.
You can’t quite do that trick with rotating habitats. However, we can use a trick a lot
more like the orbital ring, another megastructure and active support device we have looked at.
There we had a ring spinning around the earth at greater than orbital speed, with magnets
on it over or around which something stationary to the Earth hovered. Their net momentum,
spinning section and stationary section, was kept the same as if the entire thing uniformly
moved at orbital velocity. You can do this same trick with a Ringworld,
by having a stationary ring just outside it, or even slowly counter-rotating. It does have
to be way more massive though, but it could be mostly hollow and full of cheap hydrogen
and helium. Makes a nice protective barrier too. The ring wants to rip apart from all
the centrifugal force on it, same as a suspension bridge wants to rip apart from all the gravity
on it. But if you stick pylons under the bridge, you can make it longer. That somewhat defeats
the point of a suspension bridge, but that hardly matters for the Ringworld, we want
all that speed for making spin gravity. So it can spin around terribly fast, trying
to rip itself apart, while being magnetically shoved inward by the outer ring. Since the
Ringworld is moving 1200 km/s, 40 times faster than the Earth orbits the sun, the outer ring
needs to be much more massive to balance out the momentum, but hydrogen and helium are
quite a lot more abundant than the heavier elements we want to build the ring from anyway.
Besides being the only way to make one of these with known materials, an outer non-spinning
ring provides a nice way to keep the structure from being punctured, which would drain all
its air out eventually, or ripping itself apart if structurally compromised. Though
in terms of features, you might use chains of mountain ranges to act as interior air
walls so only one area would drain of air if punctured, and have tunnels through those
with airlocks, and tunnels to the outside from there too.
For my part, I think the Rungworlds we looked at in the Dyson Spheres episode make a lot
more sense to build than Ringworlds do, and they are of the same scope and can be made
to be contiguous so you can walk, or at least float in some places, from one section another.
Still there is something truly awesome about the concept of an enormous single planet you
could walk or swim all the way around, a million times larger than our own planet, which is
hardly small. I think that’s part of what makes the journey to and around the place
in the book and its sequels so engaging. Niven never hesitates to make up advanced technology
in his novels either, but where he does, he makes it clear that he is and how it works
and what its limitations are. Otherwise, he tends to keep his science very
accurate, and where he misses the mark it is almost always because the novels aren’t
too recent, Ringworld itself was published 47 years ago and science has progressed since
then, though Niven is still actively writing as he approaches 80, and has produced no shortage
of excellent books, and while he is good about remembering the science part of science fiction,
he does weave some fascinating characters and stories.
Ringworld ties for my favorite by him, the other being A Mote in God’s Eye, which I
consider one of the best handled examples of first contact with aliens in fiction. Niven
writes fascinating aliens who are actually alien in appearance and manner, and we get
to meet a few of them in Ringworld too. Again, it is our SFIA book of the month, and
is available on Audible, and you can pick up a free copy today - just use my link, audible.com/isaac,
or click on the link in the description below, to get a FREE audiobook and 30 day trial,
That’s audible dot com slash I_S_A_A_C. I’m certain you will enjoy that story, but
if not, you can swap it out for free for any other book at anytime, and it’s yours to
keep whether you stay subscribed to Audible or not.
Ringworld is a great way to immerse yourself into one of the most thought-provoking sci-fi
settings with dozen of novels and short stories. Let me know what you think of it in the comments
below and let me know what book we should listen to next.
Next week, we will be celebrating the 100th Episode for the channel, which coincidentally
is also the third anniversary of the first episode, by returning to the Alien Civilizations
series for a look at the Zoo Hypothesis, the Fermi Paradox solution that argues that aliens
avoid contact with primitive civilizations, and some examples of it like the Star Trek
Prime Directive, in “Smug Aliens”. The week after that I will be teaming up with
John Michael Godier to look at the opposite case, where you intentionally contact and
even alter technologically primitive species, like making smarter dolphins with hands, in
“Uplifting”. For alerts when those and other episodes come
out, make sure to subscribe to the channel, and if you enjoyed this episode, hit the like
button, and share it with others. Until next time, thanks for watching, and
have a great week.
Cortana, what am I looking at?
THAT is another Halo...
The guys voice was annoying at first and then became almost hypnotic.
So that's what Homestar Runner is doing these days, narrating youtube videos.
For the unaware: https://www.youtube.com/watch?v=L0nuQ5o2DYU
You wanna get flood? Cause THATS HOW YOU GET THE FLOOD.
No vid he uploads will ever be able to best his Starlifting video because it's the technology that makes everything else in the series possible.