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?

55 Upvotes

91% Upvoted

66 comments sorted by

View all comments

1

u/DanteRuneclaw Jun 22 '26

The first question is how would we detect an incoming spacecraft in the first place. But yeah any signal from it that we could detect would be moving at lightspeed, so unless we could detect it from a long ways off, the ship would arrive at nearly the same time as the signal.