3 ms·

It is possible for an observer to accelerate uniformly from their own perspective for an indefinite period of time in special relativity (the motion traces out

by meric 12y ago

It is possible for an observer to accelerate uniformly from their own perspective for an indefinite period of time in special relativity (the motion traces out a hyperbola in spacetime asymptotic to the lightcone). The person undergoing the acceleration doesn't notice anything changing as time goes on. All of the relativistic slowing down and time dilation effects etc. that you mention are seen in an inertial reference frame looking at the accelerating ship. http://physics.stackexchange.com/questions/53587/if-a-space-ship-accelerated-constantly-would-its-astronauts-constantly-feel-the http://physics.stackexchange.com/questions/53587/if-a-space-... From the ship's frame, the acceleration would continue at the same rate. However, due to Lorentz contraction, the galaxy around the ship would appear to become squashed in the direction of travel, and a destination many light years away would appear to become much closer. Traveling to this destination at sub-luminal speeds would become practical for the onboard travellers. Ultimately, from the ship's frame, it would be possible to reach anywhere in the visible universe, before the ship has time to accelerate to light speed. http://en.wikipedia.org/wiki/Space_travel_using_constant_acceleration http://en.wikipedia.org/wiki/Space_travel_using_constant_acc... If you can build an engine that can constantly accelerate, without having to constantly throw fuel behind you, e.g. http://higherperspective.com/2015/03/nasa-engine.html http://higherperspective.com/2015/03/nasa-engine.html, an astronaut can potentially travel 4 light years in less than 4 years, from their point of view, because although from Earth's reference point of view they will always be less than c, as their speed approaches c from Earth's point of view, there will be time dilation between them and Earth. If from Earth's point of view they take 4 years and a bit to travel 4 light years, assuming acceleration to very close to c very quickly, you as an astronaut will experience the same time as a lot shorter, possibly as months. From the astronauts point of view, they can travel one light year in less than one year, if they can accelerate constantly fast enough without dying. Of course I haven't taken into account the amount of energy that needs to be involved, except to say, if you're talking about a one-way trip, the time factor can be overcome, through technology that isn't that impossible. If you know what's on the other side and it's possible can build a self-sustainable colony, colonising other planets is perfectly feasible from a psychological point of view given we have the technology we can accelerate close to c and decelerate from c, by achieving the time dilation effect.

dmichulke 12y ago
If you accelerate with g=9.81 m/s^2 then you'd need (300M/9.81) seconds which is ~30581040 seconds ~ 8495 hours ~ 354 days, so you could at least accelerate with a moderate 1g in approx. 1 year to the desired velocity. This calculation is probably wrong because Newtonian calculations are like that (wrong) when they get close to c. However, it deserves bonus points for + having exactly earth's gravity in the ship + travelling "up" all the time, making you feel like in a huge elevator. However, I'm not sure whether you'd consider 354d as fast enough :)
- meric 12y ago
  If you see my first two links you'll see from the astronauts point of view they can accelerate indefinitely with no penalty besides chance of colliding with matter on the way. Accelerating at 1g for two years and then decelerating at 1g for two years means you will travel 3 light years in 4 years. Acceleration beyond 1g will speed this up from the astronauts point of view. At 2g it would take only 2.5ish years to reach alpha centauri from the travellers reference.
aptwebapps 12y ago
The thing is, space isn't empty, so what happens when you run into a speck of dust at a significant fraction of c? Along with our sci-fi engines we'll need sci-fi shields.
jerf 12y ago
"If you can build an engine that can constantly accelerate... Of course I haven't taken into account the amount of energy that needs to be involved..." Yes, once you throw away all the hard problems, space travel is easy. Once you take them back into account, and seriously consider the problem as an engineering problem and not a science fiction problem, which, alas, many people have been sort of accidentally been trained to do because of the prevalence of science fiction, it starts looking really, really hard again, like, "The most plausible answer to the Fermi paradox is that star travel is impossible"-level hard. (Which, again, isn't my opinion... I think it just means it takes a "bit" longer (you know, two or three orders of magnitude, just a trifling detail) but assuming that all possible societies in all the universe are as impatient as modern-day humans is a very strong assumption. But it isn't such a crazy argument that it can simply be dismissed... real space travel is hard.)

by meric 12y ago

dmichulke 12y ago

If you accelerate with g=9.81 m/s^2 then you'd need (300M/9.81) seconds which is ~30581040 seconds ~ 8495 hours ~ 354 days, so you could at least accelerate with a moderate 1g in approx. 1 year to the desired velocity. This calculation is probably wrong because Newtonian calculations are like that (wrong) when they get close to c. However, it deserves bonus points for + having exactly earth's gravity in the ship + travelling "up" all the time, making you feel like in a huge elevator. However, I'm not sure whether you'd consider 354d as fast enough :)

meric 12y ago
If you see my first two links you'll see from the astronauts point of view they can accelerate indefinitely with no penalty besides chance of colliding with matter on the way. Accelerating at 1g for two years and then decelerating at 1g for two years means you will travel 3 light years in 4 years. Acceleration beyond 1g will speed this up from the astronauts point of view. At 2g it would take only 2.5ish years to reach alpha centauri from the travellers reference.

aptwebapps 12y ago

The thing is, space isn't empty, so what happens when you run into a speck of dust at a significant fraction of c? Along with our sci-fi engines we'll need sci-fi shields.

jerf 12y ago

"If you can build an engine that can constantly accelerate... Of course I haven't taken into account the amount of energy that needs to be involved..." Yes, once you throw away all the hard problems, space travel is easy. Once you take them back into account, and seriously consider the problem as an engineering problem and not a science fiction problem, which, alas, many people have been sort of accidentally been trained to do because of the prevalence of science fiction, it starts looking really, really hard again, like, "The most plausible answer to the Fermi paradox is that star travel is impossible"-level hard. (Which, again, isn't my opinion... I think it just means it takes a "bit" longer (you know, two or three orders of magnitude, just a trifling detail) but assuming that all possible societies in all the universe are as impatient as modern-day humans is a very strong assumption. But it isn't such a crazy argument that it can simply be dismissed... real space travel is hard.)