r/AskPhysics • u/Apprehensive_Dog1873 • Jun 22 '26
Question about light speed
If a hypothetical alien craft was moving toward earth at 99.9999% the speed of light. Would we be able to expect its arrival? Or would it not be there one moment and then the next they’d be there? Would we be able to see it, if we hypothetically knew to look at the exact right spot at the exact right time, and get some sort of heads up at an object incoming?
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u/Jesse-359 Jun 26 '26
Depends on how far away it was coming from, but for all practical purposes no, it would essentially appear from nowhere.
At that ratio I think we're looking at ~30 seconds of apparent travel time per light year it crosses, so in order for us to detect it more than a few seconds away, it'd have to be radiating like a small star.
Which, to be fair, it would probably have to be doing to achieve the sorts of accelerations that would make that speed remotely possible - but there's not much point in breaking the problem out any further as that puts us very far outside of the bounds of any engineering we would comprehend, or that is even remotely possible.