Suppose Ploni eats some meat. He then gets in a spaceship and accelerates to some significant percentage of the speed of light for a short trip into outer space, and then returns home. At the time when he returns home, exactly six hours have passed from the Earth's reference frame; however, because of time dilation, less than 6 hours have passed for him.

This is not an issue of who is right. On Earth, six hours have actually passed, whereas for Ploni, the amount of time has actually been less than six hours. Now that he has returned home, may he eat something with milk?

  • 1
    – Double AA
    Commented Aug 15, 2012 at 21:46
  • 5
    my gut (ha!) reaction would be that he has to wait until HE has passed 6 hours. The origin (IIRC) has to do with the time one would wait till another meal. Just because you change the clocks forward and "lose" an hour doesn't change the biological process. Is there responsa on how to deal with the time change vis-vis halachic time periods?
    – rosends
    Commented Aug 15, 2012 at 21:47
  • 11
    I like how the time and travel tags line up right next to each other.
    – Double AA
    Commented Aug 15, 2012 at 21:48
  • 1
    @Dan What about if he was on the spaceship when he ate the meat, and then he stopped, picked up a passenger, and continued going. He would pass 6 hours before the passenger would perceive him to pass 6 hours. Big maris ayin problem, no?
    – Daniel
    Commented Aug 15, 2012 at 21:53
  • 3
    my physics isn't good enough to comment on the possibility to teleconferencing at the speed of light while traveling near the SOL. Either way, maybe the passenger would just assume he's Dutch. We don't generally assess someone else's practice and draw conclusions without asking. "Dan" l'chaf zchus.
    – rosends
    Commented Aug 15, 2012 at 22:00

3 Answers 3


The short answer is no. Waiting between consuming meat and consuming dairy has nothing to do with how much time we perceive to have elapsed but with the experience of the person who consumed it. Spaceman Ploni, who decided to eat meat immediately prior to takeoff (a revolting thought), can still taste it when he returns to earth, despite the fact that his more sensible brother, who ate meat with him but who didn't board the space-ship, is now enjoying a milkshake.


  • Rambam, Hilkhot Ma'akhalot Asurot 9:28, which gives the reason for waiting six hours as being because of food that is still between the teeth (although see the Kesef Mishna there);

  • Tur, YD 89:1, which gives the reason as being because of the flavour of the meat that remains in the mouth.

  • 3
    That truly is a revolting thought. Especially considering the acceleration necessary to get up to a fraction of the speed of light and then back down to 0 in under 6 hours. Definitely makes more sense to be the identical twin brother.
    – Daniel
    Commented Aug 16, 2012 at 1:35

If he eats meat, gets onto a fast plane and flies east, and lands before six hours have elapsed for him, I don't think anybody would say that he can now eat dairy just because the clock shows a later time. For that matter, he doesn't get to jump the gun when switching to Daylight Saving Time. (But citation needed.) I would expect the same logic to apply to actual time dilation -- until it's been six hours (or whatever his custom is) for him, it doesn't count.

  • What about the situation I give in the comments above where 6 hours passes for him before it does for another observer?
    – Daniel
    Commented Aug 15, 2012 at 21:58
  • 1
    I think that's probably right. As a side note, I dont think your example is completely analogous because if you cross time zones, 6 hours have not actually passed, the clock just says 6 hours later. In my example, 6 hours have actually passed
    – Daniel
    Commented Aug 15, 2012 at 22:15
  • 1
    But in the case of time dilation, it appears to me that 6 hours elapsed on earth at the same time that 6 hours elapsed for him, just that he doesn't feel as if 6 hours elapsed
    – b a
    Commented Aug 15, 2012 at 22:29
  • 2
    @Daniel, ba makes a good point. As soon as you have multiple observers, you never really know what time it is or how much time has elapsed. But CYLPhysicist if this is a practical matter for you. :-) Commented Aug 15, 2012 at 22:34
  • 1
    @ba It's not that he doesn't feel as if 6 hours have elapsed. From his perspective, 6 hours has not elapsed, although you are correct that from the Earth's perspective, 6 hours has elapsed for him. @ MonicaCellio that's because how much time has elapsed is not an absolute fact. It depends on reference frame.
    – Daniel
    Commented Aug 15, 2012 at 22:52

According to the P'ri Chadash (89:6), six hours is not meant literally, and a person should just wait from one meal until the time for the next meal. In the winter, the P'ri Chadash writes, this could be approximately four hours. (Disclaimer: I haven't noticed many people follow this opinion in practice).

It seems reasonable to interpret the P'ri Chadash as understanding that you simply wait until the time for the next regularly scheduled meal. As such, let's say six hours passed on Earth (during the springtime, for example) and four hours passed for the astronaut. He might decide to eat at the scheduled meal time on Earth. Perhaps the P'ri Chadash would consider this acceptable.

A more immediate application of this question would perhaps involve an airplane trip from west to east (e.g. during spring). Let's say the flight is four hours and the destination is two time zones ahead the departure location. Perhaps the P'ri Chadash would allow the passenger to eat a dairy meal for supper in Chicago four hours after he ate a meat meal for lunch in Los Angeles.

Note also the reasoning given by the P'ri Chadash in 89:2, which suggests that there must still be a significant amount of time that elapses for the eater before he eats a dairy meal, so it is not possible to assume that the P'ri Chadash would permit any duration shorter than approximately four hours.

  • 1
    A novel interpretation and application, but I don't know anyone who does this.
    – Seth J
    Commented Jan 6, 2013 at 23:47
  • @SethJ - An example of someone who follows the P'ri Chadash?
    – Fred
    Commented Jan 7, 2013 at 5:52
  • Fred, first of all, without further confirmation from Hacham Gabriel, I don't see how that would necessarily follow. Second, I meant that I haven't ever heard of anyone eating based on what time zone he landed in. There's no relativity there, and even the P"Ch's logic doesn't lead in that direction. A traveler won't necessarily feel hungry at the time his hosts are eating, for example, if he just ate 4 hours ago.
    – Seth J
    Commented Jan 7, 2013 at 6:05
  • @SethJ I didn't say that anyone does or should make that jump, but the theory seems reasonable. The P'ri Chadash (89:2) gives as the reasoning for waiting that meat particles in the mouth should have time to dislodge or dissolve. He does not suggest that it takes exactly six hours, and I doubt he meant that the time it takes varies by season. Rather, a vague range of timespans are included in the general rule to wait from one meal until the next. It is reasonable that a short day would have as much of an effect on the time until the next meal whether it was due to the season or a plane ride.
    – Fred
    Commented Jan 7, 2013 at 6:16
  • 1
    @SethJ Read the P'ri Chadash. He says that "flossing" (well maybe not exactly flossing) alone isn't enough of a guarantee. As far as flossing and then splitting the difference timewise or something like that, I suppose לא נתנו חז"ל דבריהם לשיעורין.
    – Fred
    Commented Jan 7, 2013 at 21:25

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .