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Comment by phendrenad2 | Hacker News Reader

So we're traveling away from one another at 0.9c. I send a paper airplane in your direction at 500x the speed of light. You receive it and send it back (at 500c). Does it not arrive before I sent it, from my perspective?

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I’m still studying this, but IIUC that’s only a problem when you and the other party both launch the paper plane at 500c relative to your own reference frames, and (separate thing I might have misunderstood) “no preferred frame” is because Occam’s razor says GR doesn’t need one rather than because such a frame would be incompatible with observations.

(Again I may have misunderstood, but the frame of the CMB sometimes gets talked about as if it might be a preferred frame).

Even at 500x the speed of light it took some time for me to receive it, and then you receive it some time after, so no it does not arrive before you send it. You'll just have to wait ~500x that time to see the corresponding visual. For me it's not weirder than sending and getting back the paper airplane at 500x the speed of sound and waiting for the sound. But I surely miss something.

See, the formula for velocity is v=s/t. We know the distance the object has traveled, so it's the time it took to get there that gets adjusted so as the resulting velocity never exceeds the speed of light. At least that's my understanding of it. I might be entirely wrong.

Does this reverse time though? 0.9c is 90% of lightspeed, and for the sake of your example, 500c is just instantaneous.

If light is just information, wouldn’t it just be a matter of seemingly getting something very fast?

If you're traveling at 0.9c then there will be significant time dilation. Since velocity is relative, both parties will see each other as severely time dilated which can lead to somebody receiving a reply to a message before they send it.

But how do you know that you need to send a message or what if you don’t send a message? How can you get a reply?

Everytime I think I get it, I don’t!

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