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In the phrases containing the word quantum, like quantum mechanics, quantum information, quantum computation, quantum field theory,etc. is quantum a noun or an adjective? I mean, is it really mechanics /field theory/etc. of quantum or a mechanics/field theory/etc that has some property (like the word classical in classical mechanics)?

The word quantum itself is a noun, but sometimes like in the phrase quantum information it seems to be an adjective (like classical information), and sometimes like in quantum mechanics it's ambiguous.

(motivation:I have problem translating it to other languages)

user215721
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    Why do you want to translate it? Is there some language which does not already have a word or phrase meaning "quantum mechanics"? You certainly do not need to preserve the parts of speech in translation. In French, it's la mécanique quantique, and quantique is clearly an adjective. In German, it's die Quantenmechanik, which is a noun (I believe composed of two nouns, and meaning "the mechanics of quanta"). – Peter Shor Feb 13 '14 at 00:45
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    Nouns are routinely used adjectivally. There are several discussion of this on the site, such as this one. – bib Feb 13 '14 at 00:50
  • @bib: In English, yes. But not necessarily in other languages. – Peter Shor Feb 13 '14 at 00:50
  • @PeterShor And this site is about English – bib Feb 13 '14 at 00:51
  • @PeterShor Actually there are some phrases, and I want to choose the most correct one. – user215721 Feb 13 '14 at 07:02
  • "Most correct" in your language may not be "most correct" in English. The "correct" translation of quantum mechanics into French is la mécanique des quanta. Ngrams shows that this was in use for a few years after the word die Quantenmechanik was coined, but has since been completely supplanted by la mécanique quantique. – Peter Shor Feb 14 '14 at 15:16

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Etymologically, "quantum" is the neuter singular form of the Latin word meaning "how much." As used in English in physics, it's a noun. It refers to one indivisible unit, e.g., the photon is the quantum of light, and e is the quantum of electric charge. The plural is "quanta," which is the neuter plural form, used in English as a plural noun.

In phrases like "quantum mechanics," "quantum state," and "quantum information," it's a noun being used to modify a noun, like "elephant gun" or "sandwich shop." The antonym is "classical." (Sometimes people use "classical" in contradistinction to "relativistic" as well, but that's incorrect. Special relativity and general relativity are classical theories.)

Although phrases like "quantum mechanics" are completely set in stone by tradition, there are alternatives such as "quantal" and "quantum mechanical." E.g., people do often write "quantum mechanical behavior" rather than "quantum behavior." "Quantum theory" refers to the abstract theoretical framework (e.g., it states that wavefunctions are complex numbers), while "quantum mechanical theory" implies the nonclassical version of a particular theory, e.g., the liquid drop model of the nucleus is classical, but there is also a quantum mechanical theory of nuclear physics.

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Etymologically "quantum mechanics" was the English translation of the German word "die Quantenmechanik", which was coined by a group of physicists at the University of Göttingen, including Max Born, Werner Heisenberg, and Wolfgang Pauli. (See Wikipedia.) In the German, "Quanten" (the plural of "Quant") is a noun meaning "quanta", and "Quantenmechanik" would have meant "the mechanics of quanta". So when the term "quantum mechanics" was introduced, the word "quantum" was a noun.

Peter Shor
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  • When they finally crack computing based on *quanta, it'll be interesting to see if anyone comes up with a way to do the same with [qualia](http://dictionary.reference.com/browse/qualia). ("To infinity and beyond!"* :) – FumbleFingers Feb 13 '14 at 03:20
  • How about quantum field theory or computation? Quantum here is certainly an adjectives, right? – user215721 Feb 13 '14 at 07:11
  • It certainly seems like an adjective in "quantum computation", and I believe it is one in "quantum field theory" as well. – Peter Shor Feb 13 '14 at 10:58
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http://en.m.wikipedia.org/wiki/Noun_adjunct

It is a noun being used to describe another noun. In "quantum field thory" both "quantum" and "field" are being used as noun adjuncts. See the link. In some other languages an adjectival form would be required.

neededandwanted
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