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Physicists have conclusively proven that if a spaceship leaves Earth and travels fast enough, then turns around and comes back, a person in the spaceship experiences a much shorter time than people on Earth - a few days for the astronaut could be years on Earth.

The usual answer for people who are travelling outside zones where normal z'manim make sense (more locally in space or even to the Arctic Circle) is to use the times from where they set off from. However, this probably doesn't make much sense here as they could be in a situation where they don't have sufficient time do daven before the next z'man arrives, and Shabbos could be eg. every few minutes!

Is there any source for Halachic reasoning as to what such a traveller should do?

Moses Supposes
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  • I believe you would keep the zmanim of the city from which you departed Earth from, i.e. Cape Canaveral, Florida; or Houston, Texas. – ElonMusk Dec 06 '23 at 19:26
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    @ElonMusk what he is saying is that that point doesn't make sense at relativistic velocities. The clock on the ship and the one at Houston would go out of sync because, from the ship's POV, the clock at Houston would be running faster. At 0.999c, that might be "days per second" – Rabbi Kaii Dec 06 '23 at 19:35
  • @ElonMusk Unless you mean that he continues to track those zmanim in his personal timeline in terms of what he would have experienced them as if he hadn't left? – Moses Supposes Dec 06 '23 at 20:01
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    @MosesSupposes It seems very likely that that's the answer. What else could one do? It also seems very unlikely that this point has been addressed in Halachic literature, as human travel at speeds great enough to produce practically-relevant subjective deviation with respect to the rate of Earth's rotation is not likely to happen within the lifetime of any living rabbis. – Isaac Moses Dec 06 '23 at 20:18
  • @IsaacMoses I wasn't expecting a direct answer, but I thought that there may be something that it could be extrapolated from. I agree, that does seem like the most likely best answer – Moses Supposes Dec 06 '23 at 20:24
  • @MosesSupposes Yes. I believe that's the psak given to people too close to the North Pole to have normal zmanim, that they should use the zmanim of the closest "normal" city. I've heard that the same rule would apply to spacefarers. – ElonMusk Dec 06 '23 at 20:49
  • @ElonMusk the problem is that in this case if you follow what their time actually is then you could be hitting Shabbos eg every few seconds – Moses Supposes Dec 06 '23 at 21:39
  • @IsaacMoses Unless one of those rabbis achieve time travel. – shmosel Dec 06 '23 at 21:59
  • @MosesSupposes Ah, I hear what your saying. I think this is a very deep question. – ElonMusk Dec 07 '23 at 00:48
  • @mosessupposes The analogy is spiritual travel. Say you're doing more Avodah, devotions. You have more downs and ups. This is a velocity of frequency and not straight line travel. You are leap time-space distance from others bc you've traveled in the soul dimension the central one (the Keter Malkhut axis see S. Yetzira.) But it's not something you can measure empirical or physical necessarily. But the emotional maturity should reflect this travel. You know "old souls" or very mature people not in age? – Nissim Nanach Dec 07 '23 at 02:27
  • I just saw a similar idea of pulling yourself out of time on Star Trek the next generation episode Journey's End, Wes meets the Traveler who explains you pulled yourself out of their time and everyone else is Frozen because you've accepted the challenge to free your mind and begin a new existence – Nissim Nanach Dec 07 '23 at 02:53
  • Where the traveler really gets into trouble is when they return to Earth having just put away their menora, while the community they're rejoining is starting Selichot. – Isaac Moses Dec 08 '23 at 14:11
  • @IsaacMoses Or far worse - "you missed moshiach and techiyas hameisim"! - Does that mean he'd be the only one to die then? – Moses Supposes Dec 08 '23 at 14:12
  • That would be like the experience of Moshe Rabbeinu's family, the only Jews at the time to miss Yetziat Mitzrayim and possibly Ma'amad Har Sinai. – Isaac Moses Dec 08 '23 at 14:15
  • @IsaacMoses So maybe I should post a new question - "if Moshe Rabbeinu travelled on a spaceship at relativistic speeds and missed Moshiach, would he say 'Oh, not again' when he came back?" :-P – Moses Supposes Dec 08 '23 at 14:22

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