## How Would Interstellar Weaponry Work?

In their latest video, Kurzgesagt takes a break from their more serious topics to consider a scenario from the realm of science fiction: interstellar combat. Using technology that is theoretically available to us here on Earth, could a more advanced civilization some 42 light years away destroy our planet without any warning? They outline three potential weapons: the Star Laser, the Relativistic Missile, and the Ultra-Relativistic Electron Beam.

Here’s what I don’t understand though: how would the targeting work? In order for an alien civilization to hit the Earth with a laser from 42 light years away, it has to not only predict, within a margin of error of the Earth’s diameter, precisely where the Earth is going to be, but also have a system capable of aiming across 42 light years of distance with that precision. Is this even possible? How precisely do we know where the Earth is going to be in 42 years? And if you’re aiming at something 42 light years away, if you move the sights a nanometer, how much angular distance does that shift the the destination by? And how much does the gravity of matter along the way shift the trajectory and is it possible to accurately compensate for that? Maybe this should be their next video…

Joshua Gooden

Finally! Something on which I can contribute.

My envelope math, and this could be very wrong, gives a negligible difference, 1e-12 km, which isn't much at all. Nanometers are really tiny.

However, an angular change of, say, 0.00001 degrees would be an error of 69,000,000 km at 42 light years, which is about half the distance to the sun.

To hit the earth from 42 light years away, you need 9.19×10⁻¹⁰ degrees of accuracy to catch some part of the earth. Not sure the best way to calculate what would constitute a full "broadside," but it would be really hard to hit us at that distance.

To your point about gravitational effects, yeah, lensing would cause issues, interaction with particles in the interstellar medium could deflect it, and definitely magnetic fields in interstellar space would cause some deflection as well, not to mention the heliosphere itself.

Joshua Gooden

That said, it's a huge part of smorpian culture to bluster as part of their negotiations, so this is largely just saber rattling over the disputed Trappist-1 planets

Kevin Miller

The frustratingly mediocre "Star Trek: Generations" has a few great moments in it, and one of them is the Stellar Cartography scene, where they work out that the villain is killing stars to adjust the course of an object through space. It's not exactly hard sci-fi ("killing" a star wouldn't change its gravity well, I think) but it's harder than Star Trek normally dabbles in.

Taco

That final bit on staying quiet reminded me of The Dark Forest, the second book in Liu Cixin's Three-Body Problem, which is a great read if you're into this stuff.

Lorem Ipsum

The smorpian culture could use pigeons as the heart of their targeting system, similar to some targeting experiments here on Earth. This is base on several assumptions: 1) smorpian pigeons live at least 42 years; 2) smorpian pigeons are capable of light speed travel (or space travel, in general, and survive the whole trip); 3) smorpian pigeons will target Earth in exchange for food pellets.

@djschwin

@Taco That is definitely what I was thinking as well. The Dark Forest ended up being my favorite book in that series.

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