In the comment section of a newspaper article reporting on the 2015 Nobel Prize for Physics, which was awarded for work on neutrino oscillation, I found the following joke:

"I'm sorry, we do not serve neutrinos", says the barman.

A neutrino walks into a bar.

What physical phenomenon does this actually allude to? The structure of the joke seems to imply some sort of time-reversal. I realize explaining a joke kind of defeats the purpose of the joke, but I'm just curious what the point is here.

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    $\begingroup$ Now reverting to the original "tachyon" joke $\endgroup$ – user56903 Oct 7 '15 at 10:30
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    $\begingroup$ This is a meta-joke, in that if you never heard the tachyon bar joke this one won't make much sense. BTW, the "official" neutrino joke goes: Neutrino comes into a bar//bartender says "can I get you a drink?"//neutrino says "No thanks, I'm just passing through" (try the veal; I'll be here all week) $\endgroup$ – Carl Witthoft Oct 7 '15 at 12:45
  • $\begingroup$ Oh I like that :D $\endgroup$ – TellMeWhy Oct 13 '15 at 11:43

This was a reference to the apparent measurement that neutrinos travel faster than light. FTL travel can be used to travel back in time (though the procedure for doing so is somewhat involved).

Sadly the apparent superluminal speed turned out to be due to experimental errors: a fibre optic cable attached improperly, which caused the apparently faster-than-light measurements, and a clock oscillator ticking too fast. I say sadly because the result would have been tremendously exciting if it had proved to be correct. Still, grandfathers everywhere are probably quite relieved.

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    $\begingroup$ It was cleared up that the measurement was in error, some bugs in the hardware contributed the result. $\endgroup$ – anna v Oct 7 '15 at 10:28
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    $\begingroup$ Indeed. John, especially since this is on HNQ I think it would be worth editing into the answer that the measurement was incorrect. $\endgroup$ – David Z Oct 7 '15 at 13:24
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    $\begingroup$ since this is on HNQ - aha, I wondered why such a trivial answer was attracting so many upvotes. I'm not sure the the hot network questions list is an asset for the Physics SE. Anyhow, I've extended the answer as requested. $\endgroup$ – John Rennie Oct 7 '15 at 16:03
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    $\begingroup$ Sadly? Violating causality overturns every last piece of science we have. As in, there would be no more science as we know it. It would be the end of predictability and locality -- anything from anywhere in the entire universe, past, present, or future, can and with incalculable probability will warp in to upset your experiment. $\endgroup$ – user10851 Oct 8 '15 at 6:12
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    $\begingroup$ @ChrisWhite - if neutrinos turn out to be able to go back in time, then to say "there would be no more science as we know it" seems a bit histrionic. Surely it would just be another paradigm shift? You could imagine going back in time 100 years (no pun intended) and say the same about "all these new confusing results coming out of this new science of quantum mechanics, which seem to overturn everything we know about the universe". Generally, physicists long to be surprised - that's the most exciting thing that can happen. Time travelling neutrinos would be a great suprise. $\endgroup$ – Max Williams Oct 8 '15 at 7:51

protected by Qmechanic Oct 8 '15 at 7:27

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