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by trhway 1846 days ago
>On Earth, 10 hours into the mission, mission control sends a message to the spaceship. Because of time dilation, the spaceship receives the message only 6 hours into the mission, from their perspective.

And from their perspective - "for every 10 hours that pass on the spaceship, only 6 will appear to pass on Earth." - the message was sent at 3:36 of the Earth time. They immediately send a response message which immediately arrives at 3:36 of the Earth time from their perspective. No paradox so far.
1 comments
AnimalMuppet 1846 days ago
There's a paradox if the message replies to the first message, because the reply arrives before the original message is sent.
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trhway 1846 days ago
>because the reply arrives before the original message is sent.

from which POV? From the ship's POV - the message is sent at 3:36 Earth time , and the immediately sent reply is received at 3:36 Earth time.

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AnimalMuppet 1846 days ago
From Earth's POV.
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trhway 1846 days ago
From Earth's POV - the message is sent at 10 of Earth time which is 6 of the ship's time from the Earth's POV, the reply sent and received immediately at 6 of the ship's time which is 10 of the Earth time from the Earth POV.
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weavejester 1846 days ago
There's no absolute reference frame here.

From the perspective of someone on Earth, when it's 10:00 on Earth, it's 6:00 on the spaceship.

But from the perspective of someone on the spaceship, when it's 6:00 on the spaceship, it's 3:36 on Earth.

From the ship's reference frame, Earth is in the past; from Earth's reference frame, the ship is in the past. If you have superluminal communication, the Earth can send a message to the ship's past, which can then send a response to Earth's past.

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trhway 1846 days ago
>From the ship's reference frame, Earth is in the past; from Earth's reference frame, the ship is in the past.

no. 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. The 3:36 of "slow time" isn't "less" than ("past" of) the 6:00 of "fast time". 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".

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drran 1846 days ago
If you use a mobile phone, you can reply to the second message (by network) before the first message will reach you (by air), so you can break causality and travel back in time!
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