# Could a primitive sapient civilization survive on a planet being pulled along by a shkadov thruster?

From my understanding shkadov thrusters, they use huge mirrors built on the scale of a dyson sphere, to reflect all the radiation of a star in one direction, accelerating the star as an enormous star-ship.

As the acceleration is initially slow, the planets around the star will stay in orbit (I think); my question is, can a primitive civilization survive on one such planet, or will the stresses of being pulled along at a decent percentage of lightspeed (I've heard anywhere from 10% to 80%, so I'd appreciate it if someone who knew could clarify it for me) drive them to extinction?

• would the Dyson sphere contain the planet (heat death) or is the planet outside (super cold and dark until flash-fried while flying over the opening)? – bukwyrm Mar 29 '19 at 16:04
• The planet would be outside the mirror sphere, but possibly within a smaller Dyson swarm built around the planet itself, like a shell-world or matryoshka planet. All answers I've seen indicate cold and heat would be problems for the planet, so I've changed it up in the setting, and am probably done with this question. – Ushumgallu Mar 29 '19 at 17:06
• Concerning the lighting issues on the planet. Just keep it outside the Dyson swarm and builed a high orbit/geostationary orbital ring arround it. Then install a huuuuuuuuge floodlight up there. Or use a small asteroid or moon as a structural base. Power can be beamed in form the star, if that is the chase a mirror array will keep the planet warm and lit. Otherwise fusion fuel can be importet from the sun. No issues there. As for acceleration and speed related problems I suggest watching the latest SFIA episode, "planet ships". youtu.be/oim7VvUURd8 – TheDyingOfLight Mar 29 '19 at 18:36
• @TheDyingOfLight watched that yesterday, and I wish it had come out before I asked this question, cause it basically answers it. However, the problem is still how a primitive civilization would survive, and I guess they would be a primitive civilization living in the ruins of a more advanced one. – Ushumgallu Mar 29 '19 at 23:42
• If the advanced civilisation wants the primitive one to survive there a numerous other possibilities. Should the advanced civilisation have gone fully digital, minduploading the primitives into a simulation of their world is most plausable. Mentioned somewhere in the Alien Civ series, "Smug Aliens" maybe? Or peel the crust of the planet like an orange skin and relocate it into a cylinder habitat. Or decapitate all the primitives and freeze them. Then revive them when and wherever it isn't a hassle. Both options are from "Evacuating Earth". – TheDyingOfLight Mar 30 '19 at 4:46

You are assuming that the acceleration won't be a problem disturbing planetary orbits and also that the planet won't be overheated by the reflected radiation. (The former passes my "gut feel" test; the latter doesn't. It appears to me that the mirror contributes a healthy fraction of the planet's incident light, which would probably heat it significantly. This needs to be carefully calculated. Making the mirror big enough certainly helps. And it might be possible to tweak the mirror to minimize the extra light on the planet.)

Concerning the effect of high speeds, there is no effect from the speed because it will never move very fast. See the Wikipedia article which estimates that a Shkadov drive on the Sun would result in a 20 kps speed after a billion years of accelleration...

Speed alone has no effect, anyway. It's the matter that you ram into that may prove a problem. But not with a Shkadov drive!

• Unless the mirror's rim is aligned to the planet's orbital plane, the planet is going to spend some time in the mirror's shadow, totally blocking out any light from reaching it. Life would need to handle very long, very cold and dark winters. – Cadence Mar 27 '19 at 22:45
• @Mark Huh, for some reason I figured that a shkadov thruster could be accelerated really fast; I think i may have heard it on SFIA; but, as you pointed out, there is the problem of being fried. I'll try and do more research into the topic. – Ushumgallu Mar 27 '19 at 23:00
• @Cadence It's also possible that the mirror is outside of the planet's orbit. – Muuski Mar 27 '19 at 23:23
• @Muuski Fair point. I suppose if you're gonna build megastructures, there's no reason to think small... – Cadence Mar 27 '19 at 23:30
• Outside or inside, you have a potential problem. If it's inside, it all depends on where you want to go. If you want to go perpendicular to the plane of the solar system, as long as the mirror wasn't too big, both the beam and the shadow of the mirror would miss the planets. But if you wanted to go any other direction, the planets would spend some time in the beam and some time in the mirror's shadow. If the mirror is outside the planet's orbit and large enough to use half of the sun's output, you'd always be in the beam. The bigger the mirror, though, the less intense the beam is. – Mark Olson Mar 28 '19 at 0:17

Your mirror is probably shaped like an ellipse with the star at one focal point and the other focal point at infinity. This means the rays reflecting off of it are parallel. Think about it this way: imagine the light is going in the opposite direction, from space in parallel lines to hit the mirror and reflect back to the star. How big is the planets shadow on the mirror right now? Now think about how much radiation comes from the star and hits that area. That's how much extra radiation the planet is getting exposed to. The good news is that if the mirror is far enough away then the answer is not all that much. So I guess this was a long way to say, depends on how big your mirror is.

• Your general assumption is correct. However, the shape of the reflective sheet is closer to a perfect sphere (excluding planetary gravitation effects). The reason is that the sheet is not in orbit, but rather stationary (points in the same direction, to provide propulsion): The solar wind's pressure offsets the star's gravity. There may be some distortions caused by solar flares, as the star's magnetic field affects the distribution of solar particles and hence the solar wind's pressure is not uniform. – Christmas Snow Mar 28 '19 at 14:16
• When I was in high school, about the time ink was invented, "an ellipse with the star at one focal point and the other focal point at infinity" was called a parabola. – Zeiss Ikon Mar 28 '19 at 18:48

Shkadov thrusters will not give you relativistic speeds. For the sun the acceleration is around $$6.5\cdot 10^{-13}$$ meter per second squared. That will not reach relativistic speeds over a biosphere lifetime. The trick is to use it to steer towards close encounters with other stars, bumping up the speed drastically through gravity assists.

Up until that point there is no problem, except for shadowing and reflection making the solar input variable. But if the planet is orbiting orthogonal to the travel direction it would see a constant amount of sunlight.

• If you are going for a slingshot off another star, keeping planets in stable orbit is probably not a viable goal. – Michael Richardson Mar 28 '19 at 21:38
• The typical time before a slingshot is about 4-8 million years, so there is time. Even when you do the slingshot orbits can survive if they are in the right phase and distance, but this depends a lot on whether the designers planned for it - if it just happens randomly one should expect disruptions. – Anders Sandberg Mar 28 '19 at 22:02

The acceleration will be VERY slow. This can be inferred by comparing the luminosity of a star to the mass of the star. Our sun has a luminosity of ca. $$4 \cdot 10^{26}$$ Watt and mass of $$2 \cdot 10^{30}$$ kg. That's one-five-thousands of a Watt per kg. Strapping a flashlight to your back would accelerate you many times faster.

Reflected radiation need not be a problem. If the mirror is well inside the orbit of the planet and at an angle to the planet's orbit, the reflected light will never hit the planet. If it is well outside, it will not add much to the normal solar radiation when the planet passes through the reflected light, since the light must be reflected as parallel photons to achieve the greatest effect and hence be no more concentrated than the light that hits it (which will be weaker than what normally hits the planet). At worst, as smaller mirror in the planet's L2 Lagrangian point would deflect the light.