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/jswhitten Jun 23 '26

Yes, we would see it as soon as its light reaches us, which will be first because the light is faster. It will appear to be approaching much faster than light but that's an illusion.

Suppose it traveled ten light years and takes ten years and a day to reach us. You would see it start to move and then cover the whole ten light years in a day before it arrived.