# If you travelled at exactly the speed of light, what would the stars look like behind you? [closed]

If you were in an Alcubierre drive bubble travelling at exactly 1 c, I wonder what it looks like if you looked back, to the direction you come from. If you went faster-than-light you'd obviously see nothing behind you, just pitch black. When travelling at 2 c, I think that the visibility border would be 90 degrees to your sides. But if you went at 1 c, the light beam that reached you from behind is still with you, so does it look like if time was frozen behind you?

I know that when travelling near, at, or above light speed the stars before you would turn blue and the ones behind you red, if still visible. I guess the faster you'd fly the more would the colors shift, until the stars before you appeared red, while close to and behind you nothing would be visible.

• I found this: gamelab.mit.edu/games/a-slower-speed-of-light It sort of answers my question. I can much recommend it. May 8, 2021 at 8:56
• I think questions about theoretical toys are probably off-topic (certainly I'd be much more worried about the time machines that pour out of the Alcubierre metric...). But there have been at least two papers about this I think.
– user21103
May 8, 2021 at 10:15
• The Alcubierre metric gives you time machines (ie you can travel into your own past). If that's possible then causality is pretty much dead. So people tend to assume that metrics which give rise to such things are unphysical. The Alcubierre metric is almost certainly directly unphysical since it violates energy conditions as well, but any similar metric would be considered by many people (including me) to be unphysical. We may be wrong, and causality may actually be dead.
– user21103
May 8, 2021 at 14:53
• Why was my question downvoted and closed, but not other questions in these tags? May 8, 2021 at 15:08
• We have a couple of hundred posts on the Physics stack on the Alcubierre drive that you might like to browse. As you can see, such questions have a fairly high probability of getting closed. physics.stackexchange.com/search?q=alcubierre May 8, 2021 at 21:13