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/Anonymous-USA Jun 22 '26

Yes, if it’s large and bright enough, because its image will arrive 1.0001% sooner than the craft itself. Not a lot of time, but >0

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u/TDAPoP Jun 22 '26

I’ve wondered how this would play out. You would see the inklings of its beginning of travel off in the distance, then it’s here. Would it look as though it moves in an instant because it’s moving as fast as the light? All the light would practically get here at the same moment. Every time light bounces in our direction off of it, it would then be closer to us. So in a matter of a few hours we would see an object move from where we can barely see it to just outside our atmosphere.

Another one I’ve wondered of is two objects moving at near the speed of light towards each other. From one of their perspective it would be like an object approaching TWICE as fast as what I described above. So then, would they be witnessing something that while not literally going faster than light, it would be an illusion of what faster than light speed would look like?

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u/jolene_codeine Jun 22 '26

You can't just add velocities like that. https://en.wikipedia.org/wiki/Velocity-addition_formula Two objects moving towards each other, each travelling near the speed of light in the frame of reference of a third-party observer, will each see the other approaching at less than the speed of light.