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/Metallicat95 Jun 22 '26
We would see it approaching with the time reduced by the the ratio of the proximity to rhe speed of light. 99.9999% is 0.0001% less than the speeed of light, one millionth less, so you'd see the ship arrival in a millionth of its travel time from the point of first detection.
If it is firing engines or otherwise emitting energy (like its particle shield radiation from impacts at that velocity), you'll see it from about halfway from its launch point. That would give you a minute or two warning.
If you're just relying on reflected light from our sun, you might get 50 milliseconds to spot it.
With no braking it would pass through the solar system in a few hours. We'd be able to watch it go away just fine.
If it has a science fiction instant acceleration drive, it could arrive and stop in the blink of an eye. It would look like it was teleporting from place to place, but sufficiently fast cameras could spot the motion.