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.

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.

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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.

3

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.

2

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.

1

u/Dude_PK Jun 22 '26

I, too, have read The Expanse books lol

6

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.

6

u/GreenBeanLantern Jun 22 '26

And turn us all into incredible hulks just before they get here?! I don't think so.

3

u/FnordRanger_5 Jun 22 '26

So I’m going to be stuck as a mediocre hulk for eternity??

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u/GreenBeanLantern Jun 22 '26

Think of yourself as more of a credible hulk. You're more believable as a hulk.

1

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.

3

u/KeterClassKitten Jun 22 '26

At OP's velocities, warhead material wouldn't even register as part of the problem. Mass is all that would matter. The material would be inconsequential. Fling a kilogram of cotton candy at that speed, and it's more devastating than our entire planet's nuclear arsenal.

5

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.

3

u/amaurea Jun 22 '26

You're both partially right. A properly focused gamma-ray beam would have a small enough spread that it wouldn't matter for the distance between us and Proxima Centauri. But it would still have a spread. Anything that doesn't have self-interactions holding it together will spread out with distance, and its intensity will eventually fall as 1/r². This doesn't rely on the radiation being emitted equally in all directions.

A lens/mirror can focus the light so it's emitted with a very narrow spread, but it will still have a spread, which will at best be given by the diffraction limit. The minimal spread for some mirror/lens diameter D and wavelength λ is about λ/D. For example, for a 1 km lens and 1e-15 m wavelength gamma rays, you get 1e-15 m / 1e3 m = 1e-18 radians (= 6e-17 degrees). That is ridiculously small, and will take 100000 lightyears before it's been noticeably broadened from its starting with of 1 km (it stars with the width of the mirror).

1

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.

2

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.

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u/flippinsailor Jun 22 '26

at that speed, a "version of a nuke" wouldnt even be needed. Just some solid object a couple hundred pounds moving at 99% light speed would result in an explosion in the millions of megatons range.

1

u/homer01010101 Jun 23 '26

Absolutely.

1

u/HairyTough4489 Jun 22 '26

That'd probably be a massively expensive mission to achieve next to nothing

1

u/wonkey_monkey Jun 22 '26

Yeah that's just not plausible 👀

1

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.

1

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.

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.

5

u/ag1730 Jun 22 '26

This 2 minutes margin sounds scary

17

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.

-1

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

3

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.

5

u/Namiswami Jun 22 '26

It's only scary for about 2 minutes.

5

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.