Coming here from another star system in a reasonable time would require more energy than they'd get by turning every planet in the solar system to energy with perfect efficiency.

... I'm not sure that's right. If you're in the convert matter to energy game, you can go a long way with just a little bit. That c^2 bit of e=mc^2 is obscenely massive, and most importantly, since your kinetic energy is 1/2mv^2, efficiency and relativistic limitations aside, to get something going close to the speed of light you need to convert roughly the same amount of mass to energy for propulsion.

There's some nuance there, but even 100:1 means you could get a ship the size of Manhattan traveling at near light speed for a good sized asteroid of mass. No need to fuck with planets.

[–] mech@feddit.org 3 points 16 hours ago (2 children)

The rocket equation is a bitch.
If you want to accelerate close to c, brake at earth, then do the same thing again on the way back, you'll need to carry a planet mass worth of fuel at launch, just to propel a small craft.
And then you still haven't done anything productive in our solar system.

[–] OwOarchist@pawb.social 5 points 14 hours ago

The rocket equation assumes you need to bring all your fuel with you.

But with things like laser-propelled light sails, you might be able to propel the ship with energy the ship doesn't need to carry.

[–] MalReynolds@slrpnk.net 4 points 14 hours ago* (last edited 14 hours ago) (1 children)

Look at Bussard Ramjet. Why would you carry your fuel with adequately advanced technology (let alone zero point quantum stuff)

Shields are much more interesting near c.

[–] mech@feddit.org 3 points 14 hours ago (1 children)

Shields are another thing: How do you protect your hull from the protons that hit it at relativistic speed?

[–] sunnie@slrpnk.net 3 points 10 hours ago

Any problem is solvable with unlimited energy at your disposal. Protons are charged so you divert them with an electric field. If you’re using a ramjet for propulsion, you collect them to fuse. The incoming velocity could be a benefit there.