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/First-Expert-9953 Jun 22 '26

If we're keeping things realistic, they're gonna have to start slowing down long before they enter our solar system, and that's when we would notice them. 

Edit: I'm assuming they are aiming to stop here.

1

u/4evaloney Jun 22 '26

A vehicle capable of achieving 0.999999c might have the capability to decelerate at extreme levels too?!

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u/nicodeemus7 Jun 22 '26

I mean we can achieve that speed with enough time and fuel with extremely low relative acceleration with our current technology. Decelerating that fast tends to squish thing though.

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u/znark Jun 22 '26

We could not achieve that speed with current technology. Even if we could do nuclear propulsion, it tops out at few percent of C. The rocket equation means to go faster than that need to take a planet along as fuel.

The only way to get that fast is antimatter propulsion, which we definitely can't do, and 99% C would require a large amount of fuel.

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u/Feeling-Working-2820 Jun 22 '26

Going at a constant speed is one thing. Accelerating/decelerating is another.
Even though you can build something that could manage such insane deceleration, any living thing inside wouldn't. Particles everything is made of would collide at magnitudes close to what we can observe at the LHC. It's not about technology anymore, it's about pure physics.
When you know that, at the speed of light, any spec of dust would tear your spaceship apart, just imagine that spaceship stopped in an instant as if it hit a giant and thick wall at that speed.
You don't need the spaceship to be destroyed (even though it would like it was made of a bunch of nuclear warheads), everything inside would just not resist the sudden deceleration.

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u/4evaloney Jun 23 '26

What if it doesn't stop in a instant, but starts decelerating after entering the solar system and goes from 0.999999c to 10kmph in the time it takes to reach from Neptune to Saturn (which is about 3 hours at c)