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Since we know that when we watch a star, we are watching it in a state it was before when the light started travelling from it. So suppose, I travel from Earth to a distance of 1 light year and than watch back at Earth, what Earth will I watch, the present Earth or the Earth 1 year in the past?

Qmechanic
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Astro Falcon
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    You would have to wait a year if you could travel at the speed of light to see 1 year into the past (a little counterproductive, no?). If you can travel 1 light year in an instant then you don't need to wait. – DKNguyen Sep 22 '20 at 14:09
  • Related: https://physics.stackexchange.com/q/11940/2451 and links therein. – Qmechanic Sep 22 '20 at 14:28

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It always takes light in a vacuum 1 year to travel 1 light year, so if you were 1 light year away from anything, including Earth, you would see it as it was 1 year ago when the light you are seeing left it. In fact since light never travels instantaneously, everything you see has already happened even if only microseconds ago.

Adrian Howard
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  • But assume that I'm not travelling at the speed of light but a speed which can be achieved by current means, and than I watch back towards the Earth? – Astro Falcon Sep 22 '20 at 19:37
  • It is theoretically impossible for matter to travel at the speed of light. Any currently attainable speeds would not be great enough for noticeable relativistic effects. – Adrian Howard Sep 22 '20 at 19:57
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If you are 1 light year from Earth, and the Earth time is 1/1/2020 12:00:01 AM, then you are seeing Earth's light that was emitted on 1/1/2019.

If you are stationary with respect to Earth, then that light is "one year old" for you, and lags Earth's clock by one year. How you got there doesn't matter.

If you are moving at the time of detection, then it gets more complicated. If you are moving away from Earth at near light speed then the time on Earth approaches 1/1/2019, and the age of the light approaches zero even faster.

If you are moving towards Earth, then the time on Earth approaches 1/1/2021 when you see the light from 1/1/2019 in Earth's 1/1/2020. You might think that is in the future, but since it is outside your light cone, it cannot be time ordered. If you are 1 ly from Earth on 1/1/2020, then you cannot assign a unique time order to any event that occurs on earth between 1/2/2019 and 12/31/2020.

Basically talking about the time difference (and certainly the ordering) between space-like separated events is meaningless.

JEB
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