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nivlark t1_j6ukdl0 wrote

Reply to comment by Okonomiyaki_lover in Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science by AutoModerator

From either ship's perspective, the Earth is approaching (at the same speed an observer on Earth measures the ships to be travelling at), and the other ship is approaching at a slightly faster speed (which you can calculate with the relativistic velocity addition formula).

But from the ship's perspective, the distance from it to the Earth at any given time is smaller than the distance from the Earth to the other ship. The other ship is approaching faster, but it has more ground to cover. These effects cancel, and the two ships arrive at the same time.

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Okonomiyaki_lover t1_j6uyjs6 wrote

So from ship a's perspective, they are

  1. closer to earth than ship b and

  2. the speed of ship b appears to be faster than ship a?

Ok so at a significant % of c the distance appears shorter... But if you stop does the distance appear to increase as you slow down?

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nivlark t1_j6v0vqr wrote

Ship A thinks they are standing still - again, both the Earth and Ship B are moving towards them.

The difference in distances is not because of the speed of the ship, it's because of the speed of light. Ship A receives light from Ship B that started travelling some time ago, when B was further away. As the two ships approach, the time lag decreases until it vanishes when they meet - which means that from A's perspective, time onboard B is running fast.

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Okonomiyaki_lover t1_j6v76i9 wrote

Ok so it's more about ship b's progress is compressed and then ship a is essentially watching the ffwd version of the information coming to them.

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