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.