So this week we will be taking a quick look at the Shkadov Thruster, one of simplest types
of Dyson Swarm to make, and a system for both moving things around the solar system and
moving entire solar systems.
This will be one of the shortest videos on the channel, which both ironically and non-coincidentally
will be followed next week by the longest video to date on the channel.
I’ll talk about that a bit more at the end in a quick channel update.
Here in the megastructures series we’ll be looking at Dyson Spheres for a while, and
there’s a reason why I don’t really consider them a megastructure in of themselves, even
though today’s example could actually be built as one single rigid structure.
As I mentioned in the Dyson Dilemma, the notion of any Dyson Sphere as a rigid body has always
been at odds with the original concept, which is a swarm of bodies orbiting the sun.
These could very from quite tiny individually, smaller than even modern satellites, to enormous
structures, all with varying purpose, and to me a Dyson swarm is a megastructure the
same way a metropolis is a building.
Though you could build a single large city-sized building, often called an Arcology, something
we’ll probably look at down the road.
I tend to regard even a minimal early dyson, basically one where you’ve hollowed out
the asteroids in the asteroid belt to build solar collectors around the sun, and rotating
habitats in those asteroids, as a Dyson.
You don’t mine asteroids for planetary consumption, except maybe of very rare and precious metals
as a small but lucrative side business.
You mine them to build other space-based industry.
And a lot of times that will be very thin power collectors, either direct solar panels
or mirrors.
This gets us to our Shkadov Thruster because one of the most obvious applications of all
those mirrors and panels is that light can be concentrated to produce thrust on objects
like spaceships.
Either direct thrust by pressure or by heating up gas inside them that gets expelled as rocket
thrust.
I’ve talked before about the concept of statites, ultra thin satellites that can remain
static in their location rather than orbiting, held up by radiation pressure, and this is
a major portion of the Shkadov Thruster because this notion of moving solar systems is usually
comes up for moving your own, especially to avoid hazard areas of the galaxy like near
a potential Supernova, but as we’ll see in a bit the better method is actually moving
those giant stars that can go supernova.
Now a Shkadov Thruster itself is a really simple concept.
Stars emit energy, both solar wind and light, but they do it omnidirectionally so they don’t
move.
If just one hemisphere of the star was lit up, it would begin to accelerate in the opposite
direction.
This is worth remembering for more than the Shkadov thruster because every object in space
radiates energy, waste heat, and if you make it emit more in one direction then another
it will get thrust from that.
Meaning a lot of our megastructures can be made into very slow spaceships simply by slightly
altering their geometry so they emit more light in one direction continuously.
Photons, the particle of light, are nothing but momentum, they have no mass
So if an object is emitting asymmetrically it will begin to have a net force on it equal
to the emitted power divided by the speed of light.
Or an acceleration equal to the emitted power divided by the mass of object and the speed
of light.
If we want to move a one kilogram of object at one gee, normal Earth gravity, we can emit
from that object, or let it absorb, 2.94 Megawatts.
Now since we usually think of most space stations as being about room temperature, 300K, they’d
emit about 460 Watts per square meter of exterior and probably weigh at least a ton per square
meter, making for an acceleration of less than a billionth of a gee even if you can
get it to be mostly asymmetric, making it a crummy acceleration method but possibly
enough for station-keeping, where you just need minor nudges.
But you can get twice the thrust by using a mirror instead.
A photon hitting and absorbing into something only gives you that one push, if it reflects
off it doubles that push, since it moves away with the opposite momentum it hit with, meaning
mirrors let you get twice the bang for your buck, or watt, and gives you 1.47 megawatts
per kilogram of object as your 1 gee power need.
That’s what makes laser sails more attractive, you can hit something with a mirror on the
back with concentrated light at a distance and give it a big boost to speed.
And a real big boost to speed when you consider that the sun puts out 3.8x10^26 Watts of power,
which means you could accelerate an object massing 1.3x10^18 kilograms, or 1.3 billion
megatons, at one gee constantly.
And just one megaton makes for a very large ship.
Alternatively, mirrors bouncing that sunlight can be quite small, tin foil thicknesses or
less, quite possibly a lot less since some materials like graphene might only need to
be a few atoms thick to make a nice mirror, and it could be very close to the sun without
melting especially since its reflecting most of that light, not absorbing it, so you could
get away with placing these mirrors where the sun is a lot brighter per square meter
of reflecting material, and use less mirrors.
A few tons of cheap mirrors could easily provide the light to accelerate a few tons of spaceship.
And those spaceships don’t have to move directly away from the mirror or sun, they
can bounce that light to move at angles.
You can also place mirrors farther out in the solar system on or gravitationally anchored
to heavy objects like planets and reflect light off them and back into the system to
let ships move toward the sun, not just away from it.
So we get our first motivation for a simple early dyson, mirrors are cheap, especially
super-thin mirrors in space.
You could make whole planet sized ones out of a very small asteroid, and they’re nothing
but cheap thin material with a tiny computer and a bit of mechanism to let you twist or
distort the mirror.
And if they’re thin enough we don’t even need to put them into an orbit, they can just
hover right over the star like a piece of paper over an air vent, using the light and
the solar wind to stay afloat.
That’s the concept of a statite, a static satellite, which I’ve discussed before.
And the interesting thing about these is that their thickness doesn’t depend on how far
away they are from a star, since gravity, as well as light and solar wind, all fall
off inverse-square of distance, the same statite can hover in by Mercury or out by Jupiter.
This thickness, linear density, of a statite, varies by star, not distance, since bigger
stars put out way more light and solar wind then smaller ones and it rises a lot faster
than gravity does.
A star 10 times as massive as our own puts out ten times the gravity but several thousand
times the light for instance, letting you have a statite several hundred times heavier
per unit area.
You can also link statites together physically since they don’t orbit, I render it graphically
this way, because its easier, though you probably would never build a truly rigid sun-encompassing
shell.
You could though, so ironically the easiest Dyson to build, one of these thin sheet statites,
can be made as a rigid shell, whereas normally we consider that impossible for more substantial
Dyson Spheres.
This doesn’t take much mass either, even the smallest planet is more than sufficient
and it is unlikely there are many stars that don’t have enough rocky material floating
around them for this.
Now the Shkadov Thruster is the obvious if extreme application, we build a hemisphere
instead of a sphere around a sun and let the light bounce off it, pushing the sun.
The mirrors stay in place because gravity holds them there.
You could also do this with an orbiting swarm, by having those mirrors perpendicular to the
light on one side of the sun, and twist parallel to the light when they are in the direction
you want to leave open so the light passes by them unreflected
This approach has some advantages in letting you build more robust objects and better aim
your light too, and you also have a hybrid stage, object that are still thin enough to
get significant push from light but too thick to be held up by it, which can have abnormally
slow orbits for their distance from the sun, a concept not unlike Lagrange points, which
can be very handy if you’ve got solar collectors you want to keep in line with Earth.
Lagrange points work by causing a slightly faster or slower orbit around a primary body
by the slight decrease or increase in gravity from a second body, and this is a similar
concept, though by using light instead of secondary gravity, I don’t think this object
has a name yet so I will tentatively coin the term ‘lagites’ as hat tip to lagrange
points and lagging orbital periods.
I also haven’t seen any papers on the matter giving it a serious detailed treatment so
that’s probably fair game for any grad students or postdocs looking for a nice paper to knock
out.
Getting back to Shkadov Thrusters, they’re a fairly simple concept, but we have to ask
ourselves why you’d build one.
Because they’re slow.
You won’t be able to reflect all that light so using our straight non bounce method probably
gives us our best quick approximation and the sun emits 3.8x10^26 Watts but it also
masses 2x10^30 kilograms.
So pumping that into our equation gives us… ouch, 6.3x10^-13 m/s2.
Okay, but stars live a long time, and a year is over 30 million seconds, so our delta-v
in a year is… oh crumbs, 20 millionths of a meter per second, okay so it will take 50,000
years to get up to a meter a second, walking speed.
But in 50 million years that would be a kilometer per second and that is actually a lot at the
galactic scale, since those are the kind of timelines and speeds that will let you dodge
stuff.
Aim your star like you were trying to break orbit from the galaxy and you could sail off
eventually, though we orbit the galactic core at about 200 km/s.
And that would be 10 billion years of thrust to get to those kind of delta-v’s.
Putting that in terms of distance, off from where you would have been otherwise, a million
years will get you only about .036 light years away, or 13 light days, but that’s enough
to avoid dangerous approaches to other solar systems, and in 10 million years that would
be 3.6 light years, and in a hundred million years, 360 light years, and in a billion years,
36,000 light years, enough to be anywhere in the galaxy you want, with a full ten billion
year solar life time letting you cover 3.6 million light years and trucking along at
about a thousandth the speed of light.
Honestly not a great spaceship, but other stars live longer right?
Well, yes but the problem is the longer lived ones also put out way less power.
A star half our sun’s mass accelerates twice as fast from the same force, but it only puts
out about 4% the power of our sun does so it will accelerate even slower, 8% as fast.
But they do live a lot longer, so for instance our neighbor, Barnard’s star, would only
accelerate at 2.4% the speed our sun would, but it also ought to live a couple trillions
years, so it might only accelerate at a fortieth the speed but it gets to do it about 250 times
longer and end with about 6 times the velocity.
But the other direction, more massive, works out pretty well.
An O or B-type Giant Star, a potential supernova candidate, let’s say good old Spica 250
light years away, outmasses our sun by a factor of ten, meaning it needs ten times the power
for the same thrust, but it’s also emits about 12,000 times as much power as our sun.
So it can pull off 1200 times the acceleration.
These kinds of stars often live only a few million years, but after a million years as
a Shkadov Thruster you could have moved it 1200 times as far too, 43 light years, 172
light years in 2 million years, more than enough to get it out of your neighborhood,
and Spica is only a B-type star, not a Big O-type.
You can shove those away much faster, especially since you can also bounce your solar wind
out not just your light, giving you more thrust.
But here’s the key concept, if you’re afraid a supernova is going to hit your civilization
in a million years, it’s actually easier to move that star, not your own, especially
since you’d have to move all the solar systems you colonized too, and there are thousands
of normal stars you’d want to colonize for every supernova candidate in a given volume
it would threaten.
Now you might be thinking you’d want to direct that supernova candidate somewhere
close but not too close so you could harvest the metals it will make when it explodes,
after all, a real Dyson one thick with rotating habitats, not just a thin shell Shkadov Thruster,
takes a lot more metal than most solar systems have lying around, but honestly it wouldn’t
likely be worth it, especially since one of the things you can do with all that energy
from your swarm of thin solar collectors is some serious alchemy.
I mentioned in the Dyson Dilemma video that while fusion of lighter elements into heavy
ones is a nice way to gain power, but still currently beyond us even for the simplest
hydrogen to helium transmutation, if you don’t mind losing power in the process you can make
heavier elements much easier.
And you would, after all, have a lot of power lying around to run your supercolliders.
We’ll examine that more as we continue our look at Dyson spheres in future videos, but
we’ll stop here for today.
The Shkadov Thruster for moving stars, and its simpler version, simple thin solar collectors
for sucking up energy or propelling spaceships around or out of your solar system, is where
solar englobement begins.
Next week it is back to the Fermi Paradox to revisit the basics in what I’m dubbing
the Fermi Paradox Compendium as opposed to just Solutions 2.0.
That was supposed to be ready this week, but then it is a remake of the previous record
holder for longest video and I thought it would be shorter, unfortunately there was
a lot of material I wanted to cover so it’s got a runtime of 70 minutes and only about
a third of the video is done, which as these things go would make it about 70% complete,
usually researching, preparing, writing, and recording the script take about half the time
or less to make these videos, but that still meant doing this video was faster than finishing
that one.
I’m still deciding if I want to keep to a weekly release schedule, intermixing shorter
and longer videos, or just fall back to an every other week schedule.
We also have a poll this week for which of the selected topics you want me to hit next,
the queue is very full of topics but I’d appreciate knowing which you’d prefer I
cover first.
As always, questions and comments are welcome, if you’re new to this channel you can try
out some of the other videos, and if you enjoyed the video, don’t forget to like and share
it.
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Thanks for watching, and we’ll see you next time.