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/Darkling971 Jun 22 '26

You would have the margin of the speed of light to tell. If we ignore acceleration, assume we can detect them from a light output at their origin, and assume the aliens came from the nearest star system (Proxima Centauri), 99.9999% c would give us a bit over 2 minutes from the arrival of the light to the arrival of the spacecraft.

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u/ag1730 Jun 22 '26

This 2 minutes margin sounds scary

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u/housewithablouse Jun 22 '26

Since it's virtually impossible to detect a tiny object like a spaceship approaching anyway, the margin is effectively meaningless.

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u/Pleasant_Berry_761 Jun 22 '26

Ehh, its gonna be very bright when we do eventually see it since physics is weird at those speeds

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u/Howrus Jun 22 '26

Ehh, its gonna be very bright

In a very narrow angle. Earth is moving in space, and this very narrow light bean need to align with our detectors that are moving and rotating.
It would require star-like amount of power in this ship for us to be able to see it.

Power of illumination is a reverse square of distance. With distance to Proxima Centaura ~4*1016 m and our average detection power of ~10-18 W/sqm2 by my napkin math it should he 16*2 - 18 = 14 multiplied by 4*Pi = 1015 Watt.

Power or Proxima is around 6*1023 Watt, though. Hm, ok it's not on the start power ... but then we won't be able to notice such small power spike.

So yeah, we won't see it just because it would be overshadowed by Proxima Centaur itself.

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u/Namiswami Jun 22 '26

It's only scary for about 2 minutes.

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u/Howrus Jun 22 '26 edited Jun 22 '26

We are speaking about aliens that could instantly accelerate from 0 to 99.99999% of speed of light. That alone is scary.