Interstellar travel is horrible-what with the cramped quarters of your spaceship and only
the thin hull separating you from deathly cold and deadly cosmic rays. Much safer to
stay on here Earth with our gloriously habitable biosphere, protective magnetic field, and endless
energy from the Sun. But what if we could have the best of all worlds? No pun intended. What
if we could turn our entire solar system into a spaceship and drive the Sun itself around
the galaxy? Well, I don't know if we definitely can, but I don't know if we definitely can't.
Moving the Sun would be a prodigious technological feat—an act befitting a type II civilization on
the Kardashev scale—the same technological realm as building a star-encompassing Dyson sphere.
Although it’ll be several centuries at least before we can reach that level,
the actual physics of a stellar engine is easily explained in a short YouTube video in 2023,
and that’s what I’m going to do today. It’s really just Newton’s laws of motion. Newton’s first law
tells us that objects tend to coast at a constant speed unless acted on by an external force—say,
the mutual gravity of the rest of the galaxy holding our Sun in its orbit. But it’s also
possible for an object to change its speed through propulsion—ejecting a smaller part of itself
at a very high speed in the opposite direction to your desired direction of motion. Newton’s
second law says that every action has an equal and opposite reaction. If, say, a rocket blasts its
propellant, the act of blasting pushes the rocket forward. This is just conservation of momentum.
A stellar engine follows the exact same principle,
with the mild additional technical challenge of building a solar-system-scale structure to
do the job. Although it sounds impossible to move something as massive as the Sun,
which weighs more than 300,000 Earths, we have a ready-made energy source to do so: the Sun itself.
It was Fritz Zwicky who first came up with a scheme to move the Sun. Zwicky had a lot of wild
ideas—he was the first to ponder the possibility of dark matter, and predicted the existence of
neutron stars and gravitational lensing long before they were discovered. In 1961 he proposed
the first stellar engine. It was a giant particle accelerator in space that would shoot pellets
at the sun at close to the speed of light. With enough kinetic energy, these would ignite fusion
on impact and blast jets of matter off at high speed, pushing the sun. The Sun’s material would
be its own propellant and energy source, although we’d need to come up with at least enough energy
to power the particle accelerator. Now Zwicky never did the math on how much fuel this would
take or how fast it would go, and in his classic style he never returned to this idea. But the
proposal inspired humanity—or at least a few of us—to start thinking about how to move the Sun.
Although the next serious proposal was over two decades later. In the late 80s,
Soviet aerospace engineer Leonid Shkadov came up with a concept that doesn’t require any external
energy source, nor even a propellant. The Shkadov thruster relies on the fact that
light itself carries momentum. Each of those photons carries momentum away and
so pushes the Sun an infinitesimal amount in the opposite direction. The push is very tiny,
and photons are emitted evenly in all directions, so the Sun really stays put.
But what if the Sun could be coaxed a little more into shining more in one direction? Shkadov’s
proposal was to build a giant parabolic mirror made of a super thin foil the size
of a planet’s orbit. Photons hitting the mirror would be reflected back,
transferring momentum to the mirror in the process. If placed just right,
this outward radiation pressure would be perfectly countered by the inward pull of
the Sun’s gravitational field, enabling the mirror to remain in a fixed position relative to the Sun.
The mirror is a solar sail in the truest sense—powered by sunlight and pulling the
Sun itself. There are two ways to think about how this works. One is that we consider the Sun and
mirror as a single system which radiates light preferentially in one direction.
Conservation of momentum then demands that the system accelerates in the opposite direction.
Another way is to think of the mirror as a giant gravitational tugboat. In this interpretation,
Sun’s radiation does indeed cause the mirror to accelerate in the opposite
direction to the incoming photons. But the sail’s own gravitational field tugs the Sun
slightly so that our star accelerates with the sail. Same effect, different interpretations.
Positioning of the sail determines the direction of motion, which could in
principle be in any direction. You might want to be careful to keep the sail above or below the
planetary orbits however; otherwise you get roasted passing through the reflected beam,
or freeze passing through the shadow of the sail. But if you’re a Kardashev-II civilization,
it’s probably no big deal to manipulate the sail or the reflected beam to spare your planet.
Besides being careful not to fry the Earth, we otherwise don’t need to worry to much about the
planetary system. As long as the acceleration of our stellar engine isn’t too high,
the planets will just come along for the ride, held in place by the Sun’s gravitational field.
And for any plausible stellar engine, that acceleration is going to be pretty small.
For example, the acceleration produced by Shkadov thruster would be less than a trillionth of
earth’s surface gravity’s acceleration. It’s not so much a solar jetpack to zoom around the galaxy,
but rather a tool to just tweak a star’s existing galactic orbit. Over the course
of an entire galactic orbit this thruster could get a sunlike star a few tens of lightyears away
from where you otherwise would have been under the influence of gravity alone. In the best-case
scenario it would take a billion years to change the sun’s orbital speed in the galaxy by 10%.
But that could be enough to allow a long-lasting civilization to narrowly avoid a long-projected
collision with another star or a black hole. Or perhaps to pass alongside another star for
easy migration— even maybe an entire planetary transfer as their home star is starts to burn out.
However, a Shkadov thruster is not quick enough to avoid imminent disaster.
If we project that our system is going to wind up in the 100 light year kill-zone
of an impending supernova, we’re going to need more hustle than the Shkadov thruster provides.
Now if you still have any doubt about the incredible value of YouTube science shows,
it could be that one saves our species. The Kurzgesagt channel commissioned the next
advance in stellar engine technology back in 2019 resulting in the Caplan thruster—a
scheme which could potentially grant a thousand times the acceleration of the Shkadov thruster.
This design starts with a Dyson sphere - or perhaps a Dyson swarm—which is basically many
mirrors that either orbit the Sun or float in place on its radiation.
This structure reflects sunlight back to a specific point on the solar surface. This
raises the temperature, increasing the flow of solar wind emerging from that spot. But
itself this results in a very weak transfer of momentum. But now we add our thruster.
One or more large, probably electromagnetic collection cones gather this ejected solar
material and channel it into a fusion reactor, where the helium is fused into carbon and oxygen.
This produces an enormous amount of energy, and that energy is used to eject the fusion
products out the back as a propellant, and also to send a beam of hydrogen back into the Sun.
This is a variation on a sci-fi concept called the ‘fusion candle’, used to drive a gas giant around
in space. It’s also related to the Bussard ramjet, which collects interstellar matter and fuses it to
propel a spacecraft—only here the Sun is providing the matter, and also becomes the spacecraft.
In principle, you could imagine a huge number of these thrusters collecting the solar wind
and using it to power the jets, all orbiting the sun and only firing once they’re in position.
An added advantage of this method is that it actually increases the lifespan of the
star. Lower mass stars live longer than higher mass stars. An advanced civilization might
actually want to remove some of the mass of its home star in order to eke out a few more billion
years. The process of doing this by reflecting energy back onto the stellar surface is called
star-lifting. Well, why not use the ejected mass to also drive your star around the galaxy.
And with methods like this, it could one day be possible to radically change the Sun’s
orbit—perhaps even reverse it's orbit to fly-by and colonize many planetary
systems —or even escape the Milky Way’s gravitational field for a trip to Andromeda.
By the way, there is an option if we aren’t satisfied with merely changing the Sun’s orbit
or wandering the local intergalactic neighborhood. If we were to use the Caplan thruster or something
like it to burn not just the helium but also the hydrogen, it’s plausible to reach speeds
up to 10% that of light. That would enable us to catch up to far more distant galaxies before
they recede from our reach due to the expanding universe. On the other hand, burning the hydrogen
instead of throwing it back into the Sun would eventually drain our star of its mass, until it
faded into a dim brown dwarf star. Basically a very, very, very fast moving mega-Jupiter.
To build a stellar engine of any sort we’re going to first need to master massive-scale
astro-engineering. We’d probably need to build self-replicating mining and construction robots
to disassemble entire planetary bodies to build these things. That’s several centuries
away at a minimum, and probably more like thousands of years if we get there at all.
But that doesn’t mean it’s not worth thinking about this stuff now. For one thing,
understanding what’s possible in the future might tell us how to detect
other civilizations that have already reached these frankly ridiculous technological levels.
Ever since Freeman Dyson first thought up his famous sphere,
dozens of observational searches have been done, sometimes they've looked for the excess infrared
radiation that would leak out of the shell, and sometimes looking for weird flickering from
orbiting panels obscuring the star from time to time. All have turned up empty.
In the same spirit, scientists have calculated how a star might dim if a Shkadov thruster is
rotated over it, perhaps as aliens look to change the direction of their star.
Others have looked into the Gaia space telescope data for any inexplicably fast stars that might be
zipping around the galaxy. Those searches have also come up empty, unsurprisingly.
But that doesn’t mean artificially accelerated stars don’t exist—just that they don’t exist in
sufficient numbers or with sufficiently high speeds or deviant orbits to be spotted yet.
So can stellar engines ever be built? We don’t know. But the fact that it’s even a maybe is
pretty awesome. It's not something we should spend a lot of effort on right now. We should
probably focus more on the pressing matter of saving the planet. But here's another reason to
save it--so in the far future we can pimp it and the rest of the solar system into the sweetest
ride in the Milky Way. And then, from the comfort of home explore the distant reaches of spacetime.
Before I go, I'd like you to know we’re that having a problem with the YouTube algorithm
triggered by our comment response live stream a few weeks ago. Our subsequent episodes haven’t
been reaching nearly as many members of the Space Time community and we need your help!
The two best ways to help are 1. click on the bell icon and
select “ALL” notifications not “Personal” notifications. By doing this you can make
sure you’re always getting notified of every new episode of Space Time on your
home page. And 2. Join the early gang and watch the episode as soon as you can when
you see it. Early viewership is one of the best ways to keep the algorithm happy and
helps spread the episode to the rest of the Space Time community. We love the community
we’ve built and want to make sure as many people as possible see each episodes and can participate
in all the great discussions in the comment section. Thank you for your continued support!