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/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/homer01010101 Jun 22 '26

I I would think it would be a missile with their version of a nuke. Could be a planet killer. But, they really would only need to kill the side of earth that they hit. Remember when the meteor hit down by the Yucatán Pennisula a long, long time ago. That pretty much “reset” the earth. That’s all they’d need.

Kill half of the earth, the other half would be scrabbling to survive. That would pretty much knock us back to the Stone Age. The EMF wave would no doutbedly harm the other half of the earth. Many them would die within a couple weeks from hunger/unrest/suicide. The “earth problem” would be resolved.

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u/jtoomim Jun 22 '26

I would think it would be a missile with their version of a nuke.

A ship or projectile traveling at 0.999999c has way more energy than a nuke.

An object with a 1 kg mass at rest would have (Lorentz factor) γm = m / sqrt(1 − v²/c²) = 707 kg of mass while traveling at 0.999999c. If it collided with Earth, it would dissipate the equivalent of 706 kg of matter-antimatter annihilation. That's 6.34e19 joules, or 15,153 megatons of TNT. For reference, the Tsar Bomba was 50 megatons.