| > The "when it's 6:00 on the spaceship, it's 3:36 on Earth" doesn't mean "From the ship's reference frame, Earth is in the past". It is just different speed of time. It amounts to the same thing. From the perspective of Earth, time travels slower on the ship than on Earth. From the perspective of the ship, time travels faster on the ship than on Earth. Superluminal communication allows you to abuse the difference in reference frames to break causality. Put it another way: when it's 6 on the ship, what time is it on Earth? It depends who you ask. Mission control would say it's 10. The people on the ship would say 3:36. Both are correct. > Say you have 2 clocks, and took one on a spaceship ride. When brought back it will be showing say 6 why the twin clock which didn't left the Earth would be showing 10 - the ship clock wouldn't be in the "past" as both clocks are on the table in front of you in the "present". In your example, the ship needs to change velocity in order to get back to Earth and compare the clocks. The ship is not always at rest in any reference frame; it either needs to stop and turn around, or accelerate to catch a retreating Earth. It just so happens that in this scenario, whatever reference frame you choose, the ratio between the time on the ship and the time on Earth works out to be the same. But this changes if the ship doesn't need to alter it's velocity. If the ship is travelling at 0.8c away from Earth, then it's equally valid to say that the Earth is travelling 0.8c away from the ship. It is therefore equally true to say: * For every 10 hours on Earth, 6 hours pass on the ship * For every 10 hours on the ship, 6 hours pass on Earth In some reference frames Earth's time progresses faster than the ship; in other reference frames it progresses slower. If you're able to pass around information between reference frames instantaneously, then you can pass information backward through time. |