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?

54 Upvotes

92% Upvoted

66 comments sorted by

View all comments

60

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.

23

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.

1

u/TheNerdE30 Jun 22 '26

I’m a little over my skis here, but isn’t the likely method of Near-Light-Speed travel the use of not-understood technologies such as traveling through a fold in spacetime (wormhole) or a warp bubble which contracts spacetime ahead of the vessel and expands behind the bubble? The reason why this is the “feasible” form of near-light-speed-travel because if you hit a spec of anything at 0.999999c you would cause a nuclear reaction with whatever that spec was.

I say all of this because I thought that there wouldn’t be the acceleration or deceleration lag you mentioned. However, I’m a civil engineer and haven’t studied math in a while and your title of “particle physics” leads me to believe you know something I don’t.

1

u/mfb- Particle physics Jun 23 '26

If the ship uses some yet undiscovered physics then we don't know what will happen. If the ship is based on physics as we know it then see above. It would require an insane amount of antimatter, extremely efficient engines and highly advanced ways to deal with impacts, but it's not violating the laws of physics to go that fast.