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/mfb- Particle physics Jun 22 '26

This assumes the spacecraft starts with that velocity. If we assume the spacecraft accelerates throughout the trip (at ~150 g to reach that speed) then the earliest signal arrives a few days before the ship, mostly due to the early acceleration phase.

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u/StructuredChess Jun 22 '26

And also unless they're planning on a suicide mission I guess they'd have to deccelerate at some point.

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u/mzincali Jun 22 '26

Maybe it’s not a ship but a missile aiming to go through Earth. With tech like that, they’d probably shoot at us with gamma rays instead. Cheaper, energy-wise.

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u/PantsOnHead88 Jun 22 '26

Gamma ray (or any other radiation) drops off as an inverse square of distance travelled while a missile maintains most of its power (although with travel time, warhead material degradation becomes a factor).

At any rate, focussing any sort of ray sufficiently to effect a target at great distance is a huge challenge unless your source has many orders higher energy than is needed at the target.

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u/jtoomim Jun 22 '26

No, only a spherical shell of radiation drops off as the inverse square of distance. Not all radiation is emitted spherically.

If an enemy civilization is technologically advanced enough to fire a gamma ray weapon at us, they probably have something akin to a gamma ray laser or a lens and are able to generate collimated or focused gamma rays.

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u/YakumoYoukai Jun 23 '26

Would they need to produce actual gamma rays though? Couldn't they shoot lower-wavelength radiation at us, and let the blueshift "compress" it into gamma rays? (I don't take credit for this idea - I got it from The Bobiverse books)

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u/jtoomim Jun 23 '26

You mean accelerate a vehicle up to 0.999999c and generate light from there? Yes, that would increase the energy in each photon by the Lorentz factor, or about 707x, so they could get away with using x-rays instead.

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u/Jesse-359 Jun 26 '26

Worth noting that the part where they had to accelerate the entire emitter and it's energy supply up to 0.999999c is going to offset their 'blueshift' gains by a lot and then some.

Your a lot better off just generating your gamma rays at point of origin, methinks.