My character is traveling aboard a spaceship through inter-galactic space (it has left the Milky Way). The ship is traveling at the speed of light or so and has been for the last six thousand years. When she looks outside through a window, what does she see?
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1$\begingroup$ You might want to read "Tau Zero" by Poul Anderson for a story that includes intergalactic travel at ultrarelativistic speed. One thing you should bear in mind is that we can barely see the nearest external galaxies without a telescope, so your character wouldn't see much. Aberation will tend move what is observable toward the direction of motion & doppler shift will make galaxies in the direction of motion look bluer & galaxies in the opposite direction look redder $\endgroup$– Jim BaergFeb 12, 2020 at 5:04
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$\begingroup$ You probably won't have windows on such a ship, as a 1 gram dust speck travelling at 0.99 c will hit with 130 kilotons of force. And near-lightspeed is sort of an improbably high speed, as the intergalactic medium will cause some drag at those velocities, so you will have to constantly expend more and more fuel, while you could just accelerate and coast if you went at 0.1c or something instead. $\endgroup$– Richard SmithFeb 12, 2020 at 20:44
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A ship traveling at the speed of time suffers from time dilation to the point where it is standing still for the people in the ship. Thus no time passes, and therefore nothing is seen. (If that sounds impossible, it's because traveling at the speed of light is also impossible. But this answer is still true if you're traveling at an absurdly high fraction of c.) To the passenger, it appears as if the ship had teleported from one patch of space to the next.
answered Feb 12, 2020 at 3:28
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$\begingroup$ Perhaps I'm misunderstanding something, but would this not mean that the passenger would see a kind of 'freeze-frame' of what space looked like as the ship hit lightspeed, as opposed to nothing at all? $\endgroup$ Feb 12, 2020 at 3:38
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5$\begingroup$ @cyber101: No, it's not really a freeze frame, because time stops passing for the observer. If you suddenly jumped to the speed of light, traveled a few hundred million light years, and suddenly dropped back to a 'normal' frame of reference, it would seem as though no time had passed at all: You were here, and then you were there, in the blink of an eye. A few hundred million years would have passed for me, sitting back here on earth, but not for you. $\endgroup$ Feb 12, 2020 at 4:44
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$\begingroup$ I understand now - thanks for explaining! $\endgroup$ Feb 12, 2020 at 5:52
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1$\begingroup$ Unless you have magic instantaneous acceleration, you'll see something as you accelerate and decelerate, but yes, the middle bit will seem to happen "instantly". (This makes me wonder how the ship knows when to start slowing down...) $\endgroup$– MatthewFeb 12, 2020 at 15:54