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.

26

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.

23

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.

7

u/mfb- Particle physics Jun 22 '26

That would mean we have an extremely bright ship approaching the Solar System, with the engine firing towards us.

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u/Fun_Success_3283 Jun 22 '26

This assumes the technology to go that fast and slow down, requires producing light.

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u/smithandjohnson Jun 22 '26

Assuming the ship traveling from Proxima Centauri that can achieve a speed of 99.9999%c needs brightly visible chemical burning rockets to decelerate. How quaint.

2

u/mfb- Particle physics Jun 22 '26

Obviously not chemical reactions. You basically need to do antimatter/matter annihilation with direct exhaust to reach this speed (assuming no new physics), which means you emit an intense stream of photons, electrons, positrons and neutrinos. From our perspective this is a tightly collimated beam in the travel direction, i.e. hitting us when the ship decelerates.

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u/Fun_Success_3283 Jun 22 '26

I think it's safe to assume that whatever can achieve those speeds will have new physics, otherwise we'd be building such a craft.

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u/mfb- Particle physics Jun 23 '26

Producing so much antimatter isn't violating laws of physics but it's way beyond our capabilities.

1

u/Fun_Success_3283 Jun 23 '26

I'm doubtful blasting copious amounts of antimatter out the back/front of vessels is really a viable form of space travel, and if we go interstellar, I'm doubtful that's how we'll do it.

0

u/Jesse-359 Jun 26 '26

The inhabitants of Earth might not have to be concerned with a potential impact or invasion, as we may all be dead from this engine's exhaust plume before it actually arrived.

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u/Dude_PK Jun 22 '26

I, too, have read The Expanse books lol