This comic centers around a joke about Quantum superposition in physics - if you don't observe something, it has all possible states, not a specific one. It is a double-entendre with the word observe meaning both "look at" (physics sense) and "celebrate" (a holiday). One of the most famous examples on this is the Schrödinger's cat paradox.
In this comic, a Jewish physicist does not know when Christmas is. Being a physicist, he believes that since he doesn't observe Christmas, it therefore has no definite date.
Adding to the joke, the physicist's observation is actually valid for certain other holidays (such as Easter) that don't have a definite (i.e. annual) date. Because Easter's date seemingly bounces around at random, it could be said to exist in a superposition of all possible Easter dates, and as he doesn't observe (celebrate) Easter, the physicist would be unlikely to check which date it falls this year. Thus, he would be unsure of when the holiday is celebrated.
The title text refers to another principle in physics where the act of measuring something must also change it in some way. If one drops a thermometer into a mug of water, energy spent (or released) when heating (or cooling) the mercury in the thermometer changes the temperature of the water in the mug by a small amount. The only way not to interfere with the temperature of the water in the mug is not to measure it.
- [Cueball is standing behind a friend, who is sitting at a computer.]
- Cueball: Hey, will you be in town the day after Christmas?
- Friend: Couldn't say—
- Friend: I'm Jewish.
- Cueball: But... how does being Jewish keep you from knowing your plans?
- Friend: I know my plans—
- Friend: I just don't know when Christmas is.
- Cueball: Really? Why not look it up?
- Friend: Well, I'm also a physicist.
- Cueball: So?
- Friend: I believe that since I don't observe Christmas, it can't have a definite date.
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Interestingly, the comic text could be interpretted as dealing more with classical physics (only by observation do we actually know something) while the hover text is definitely referencing the fact that observing does something to a quantum system. 184.108.40.206 12:57, 17 May 2013 (UTC)
A bit of theology: Jews are still waiting for Christ to be born, which makes 'I just don't know when Christmas is.' even more true. 220.127.116.11 06:05, 29 October 2013 (UTC)
- Strictly speaking, we're waiting for the Messiah to be born. With any luck, he or she won't get crucified, so it won't be proper to refer to him/her as "Christ". 18.104.22.168 07:25, 23 November 2013 (UTC)
- Loosely translated, "Christos" means messiah in (transliterated) Greek. It's descended from "chriein" meaning "annoit". So, yes, Jews are still waiting for Christ. The proper term is still "messiah", though, as it is the equivalent Hebrew term (parallel etymologies even! See Merriam-Webster for both). Anonymous 18:26, 4 December 2013 (UTC)
- So you can't be Jewish and believe in the Messiah or you can be Jewish but not disblieve in the Law of Moses?
- Sheesh! Jewish scientists have it hard/unyielding/relatively impenetrable.
I used Google News BEFORE it was clickbait (talk) 21:24, 27 January 2015 (UTC)
Does the idea the title text references also apply to infrared thermometers? 22.214.171.124 (talk) (please sign your comments with ~~~~)
Yes, The water is giving off IR radiation, some of it is being reflected back- placing the thermometer will reflect either more or less than the environment around the water (i.e. walls of the room etc.) this will effect a change in the temperature. (this comment is xkcd 326 compliant)126.96.36.199
Since other people are obviously observing Christmas, its state should already be known, even if not known by Cueball. Schroedinger's cat depends on the fact that no one has observed the state of the cat. flewk (talk) 21:07, 8 January 2016 (UTC)
...You know, technically, he's kind of right. Christmas can have different dates depending on the culture, and if he does not celebrate any of them he has no reason to associate with any particular version. And it gets more complicated if he's aware of the whole thing where Christmas as we know it is suspected not to be Christ's actual birthday in the first place (IIRC, the current best estimates place it in April somewhere). --188.8.131.52 12:47, 16 November 2020 (UTC)