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?

  • 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 6, 2023 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 6, 2023 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? Dec 6, 2023 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 6, 2023 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 Dec 6, 2023 at 20:24


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