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According to this video, faster than light travel is analogous to time travel. Does this mean that if I could instantly teleport 1 light year away, and then sent a laser pulse to my friend, my friend would interpret the event as happening in the past?

Would it take him a year to see the pulse? Or would he see it immediately and interpret that the event occurred 1 year ago? The video I mentioned seems to suggest that my friend would interpret the event as happening in the past, I wanted to confirm.

What do they mean when they say that faster than light travel analogous to time travel?

https://www.youtube.com/watch?v=HUMGc8hEkpc

Qmechanic
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Marc DiNino
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    Questions of the form 'if < impossible thing> what would happen' are generally offtopic. –  Dec 21 '17 at 20:57
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    Possible duplicate of How does faster than light travel violate causality? – Rococo Dec 21 '17 at 21:27
  • Usually, people do not mean that you travel back in time, but just that you look back in time. In the same way that you see 8 min back in time when looking at the sun, because it takes sunlight 8 min to reach Earth. Of that same reason you will also never hear anyone claim to look into the future this way. – Steeven Dec 21 '17 at 22:56
  • -1 If you are going to quote an extended source (a video, or a multi-page document) you ought to identify where in that source the relevant material or claim is to be found. It is unhelpful to expect your readers to search through the whole of your source to look for the statement(s) you are referring to. – sammy gerbil Feb 24 '18 at 15:55

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It depends what you mean by "instantly."

A working definition of teleporting instantly is that your departure and arrival are simultaneous. But simultaneity is relative in special relativity, so what is instantaneous to one observer is not to another. To all other observers, you would disappear and then reappear in the past or the future.

The answer to when the light reaches your friend depends on whose definition of simultaneity you use. If you use your friend's, then the light will show up one year after you left. But using another definition of simultaneity it could instead appear immediately or more than a year in the future.

Emilio Pisanty
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Chris
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