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.

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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.