I'm by no means a physicist theoretical or otherwise, so please do excuse any things that I may be ignorant of (likely a very long list).

If by whatever mechanism a traveller were able to travel back in time from a point on the earth and say you were able to travel back 100 years in time.

Would there be anything to observe at all? Would the earth even still exist at that place as an event in space and time ? Not that it would disappear or vanish mind you but would it be somewhere else assuming that the traveller is the one moving back in time and not everything in the universe? If there was nothing observable , would it be the result of only having moved to that point in space where the earth was 100 years ago relative to where it would be today?

User John Rennie informed me that:

In physics time travel means a closed timelike curve, and while most of us believe these do not exist there's no reason why following one would make the Earth disappear.


closed as unclear what you're asking by Bosoneando, rob, John Rennie, user36790, Jon Custer Sep 29 '16 at 12:43

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    $\begingroup$ Hi MrNexy - you're asking what would happen in a scenario that is not known to be possible. Its like asking: 'if you were holding a marble, would it be red or blue?' Red? Blue? We can't know because its just made up. The answer to your question is disappointing: there's no answer physics can give for nonphysical situations like the ones you and I have brought up. $\endgroup$ – anon01 Sep 28 '16 at 7:29
  • $\begingroup$ This is of course true, I hope you'll forgive me for indulging in a hypothetical. $\endgroup$ – MrNexy Sep 28 '16 at 10:10
  • $\begingroup$ I think you missed my point. $\endgroup$ – anon01 Sep 28 '16 at 12:21
  • $\begingroup$ Reading through your comment again, I may have done. Could you clarify for me ? Kind Regards MrNexy $\endgroup$ – MrNexy Sep 28 '16 at 14:16
  • $\begingroup$ Simply this: it is meaningless to talk about the consequences of flawed premises. Can unicorns run faster than horses? This is a meaningless question as unicorns don't exist. In your case, you're asking about the consequences of time travel - a physics unicorn, so to speak. $\endgroup$ – anon01 Sep 28 '16 at 14:41

First, you realize that there isn't ANY a mechanism allowing to travel back in time, so a real answer doesn't really exist.

But, I were to go ahead and assume that such a mechanism existed, then I would imagine that the answer would most probably be: If the mechanism would send the traveler back 100 years into the past without changing his position and momentum (which is also tricky, because: 1. with respect to what? 2. the universe is expanding), then the traveler would most likely find himself thrown into empty space, since every celestial body in the universe, including the earth, is always moving.

  • $\begingroup$ On the overarching point you are indeed correct. Thank you kindly for indulging me. With regards to the point following that, in such a hypothetical situation that's what I was thinking. I apologise for the waste of time, it was certainly entertaining for me. Kind Regards MrNexy $\endgroup$ – MrNexy Sep 28 '16 at 10:19
  • $\begingroup$ It would stick to the Earth unless the machine used enough energy to free it from the Earth's gravitational field $\endgroup$ – user56903 Sep 28 '16 at 12:39
  • $\begingroup$ Dirk, please elaborate, because I see no reason why a gravitational field should automatically make you change your position and momentum when your are traveling backwards in time. $\endgroup$ – Yuval Weissler Sep 28 '16 at 14:22

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