4 ms·

The reason I mentioned no returning of data is that it obviously is not plausible within 25 years. I appreciate you doing the mathematics, but you don't base i

The reason I mentioned no returning of data is that it obviously is not plausible within 25 years. I appreciate you doing the mathematics, but you don't base it on the 1g acceleration I mentioned, as far as I can tell.

drostie 15y ago
Yeah, sorry if that was misleading. I wrote something to this effect and then pulled it out: "You're sort of right -- since rapidities are linear in relativity one also has v/c = tanh(a t / c), γ = cosh(a t / c), and that's in principle an exponential growth, so if you can keep up a constant acceleration at this rate for 8 years, you could get to γ ~= 2,000. But here's why maintaining a constant acceleration for 8 years is pretty much totally unfeasible: getting a γ around 2,000 requires carrying 2,000 times as much fuel as spaceship." At some point I edited this out to focus on that main point without editing out the quote at the front. Sorry if that's confusing.
- lutorm 15y ago
  getting a γ around 2,000 requires carrying 2,000 times as much fuel as spaceship. You're way off, it requires much more than that. See my post.
  - drostie 15y ago
    Well, I'm stating a basic constraint of physics: you have to carry around this much energy and it will require this much mass. You're making a certain assumption about the kind of engine and its exhaust velocity, and finding that there's a problem with low exhaust velocities. That's mostly correct, but in principle there are probably fuel sources which could achieve an exhaust velocity near c -- I'm thinking in particular if you were carrying matter and antimatter, and annihilated them in an ultrahigh-Q cavity with a small aperture to let the light out in only one direction -- or some really crazy futuristic application of lasing. You will still not get better than carrying thousands of times the mass of your spaceship for the deceleration phase, due to a basic constraint of physics.

drostie 15y ago

Yeah, sorry if that was misleading. I wrote something to this effect and then pulled it out: "You're sort of right -- since rapidities are linear in relativity one also has v/c = tanh(a t / c), γ = cosh(a t / c), and that's in principle an exponential growth, so if you can keep up a constant acceleration at this rate for 8 years, you could get to γ ~= 2,000. But here's why maintaining a constant acceleration for 8 years is pretty much totally unfeasible: getting a γ around 2,000 requires carrying 2,000 times as much fuel as spaceship." At some point I edited this out to focus on that main point without editing out the quote at the front. Sorry if that's confusing.

lutorm 15y ago
getting a γ around 2,000 requires carrying 2,000 times as much fuel as spaceship. You're way off, it requires much more than that. See my post.
- drostie 15y ago
  Well, I'm stating a basic constraint of physics: you have to carry around this much energy and it will require this much mass. You're making a certain assumption about the kind of engine and its exhaust velocity, and finding that there's a problem with low exhaust velocities. That's mostly correct, but in principle there are probably fuel sources which could achieve an exhaust velocity near c -- I'm thinking in particular if you were carrying matter and antimatter, and annihilated them in an ultrahigh-Q cavity with a small aperture to let the light out in only one direction -- or some really crazy futuristic application of lasing. You will still not get better than carrying thousands of times the mass of your spaceship for the deceleration phase, due to a basic constraint of physics.