Small things, and a few very, very large things. The interstellar medium ain't much, around 10-20 hydrogen atoms per square meter. But it adds up.
Particles zip around at larges fractions of C all the time, but because they're so small there's not much to interact with them out there so the only thing to slow them down is space stretching beneath them.
And black holes, neutron stars, binary dwarf stars, a few of these are in orbits at are at appreciable digits of C, but shit, good luck stopping those with anything.
Like, if you're willing to wait, you can just leave ur deathstar on for a few million years and it'll start pushing the star & everything bound to it with it for the ride. Make Sol a mobile home and go arbitrarily fast. But if you're willing that long, .2 is pretty fast actually.
Small things, and a few very, very large things. The interstellar medium ain't much, around 10-20 hydrogen atoms per square meter. But it adds up.
Particles zip around at larges fractions of C all the time, but because they're so small there's not much to interact with them out there so the only thing to slow them down is space stretching beneath them.
And black holes, neutron stars, binary dwarf stars, a few of these are in orbits at are at appreciable digits of C, but shit, good luck stopping those with anything.
Like, if you're willing to wait, you can just leave ur deathstar on for a few million years and it'll start pushing the star & everything bound to it with it for the ride. Make Sol a mobile home and go arbitrarily fast. But if you're willing that long, .2 is pretty fast actually.