I probably don't know enough to answer questions involving QFT
Hi again, notknowing,
I think the answer depends upon what you mean by "exist". Perhaps because of the background sketched in a post I put up earlier today, I am comfortable saying that gravitons "exist" as a -theoretical concept- in the ("naive") canonical quantization of gtr, or that black holes "exist" as a concept in gtr, or that electrons or photons "exist" as concepts in conventional quantum field theory. I doubt whether it makes much sense to claim that any of these theories might be "true descriptions of nature" in any literal sense, however, and very few working physicists would make any such claim, I think. Yes, experimental physicists have the ability to "detect individual photons" or to "hold electrons in wells for extended periods of time", but philosophers can easily debunk claims that such statements are more than convenient shorthands for extremely lengthy and involved descriptions of bits of genuine "scientific knowledge".
If the suggestion that mundane theoretical concepts correspond at best to mere "convenient fictions" in Nature seems strange, ask yourself whether "the Sun" exists. If you think it does, ask yourself where the solar atmosphere ends. When does "an ion" stop being part of "the Sun" and become part of the "solar wind"? Now consider how ions are treated in quantum mechanics. Precisely where the heck is it "located"? You probably get the general idea--- it's not so easy to argue that we have much hope of defining concepts which we can confidently assert, even in the face of philosophical objection, "exist in Nature".
In general, it should be possible in principle to say whether some concept is realized in some theory, but I don't know how to claim persuasively that anything exists in Nature. Rather, we can claim that using our theory we can build mathematical models which seem to agree with what we can measure. Usually this involves a very long chain of reasoning using multiple theories (often not fully consistent with each other!) which results in speaking and thinking of electrons or whatever as "real objects". This seems to work fine for the purpose of doing both theoretical and experimental physics, but philosophers seem to have little trouble in deflating any grand claims that physicists (or mathematicians) have discovered any eternally and everywhere applicable insights into "how Nature really works" or "what really exists in Nature".
To which I would add that perhaps the single most interesting aspect of the scientific enterprise is that, while our theories are not "true" in the sense that it is true that 1+1=2 in the ring of integers, this "defect" doesn't seem to much matter either conceptually or technologically, as long as we have at least one good theory which can reliably and effectively treat some class of natural phenomena. I like to define mathematics as the art of thinking precisely about simple things without getting confused. And I like to define science as the art of "understanding" without "knowing". The best applications of mathematics are to science, and the best applications of science are to engineering. Since we can use our math to create scientific theories which allow engineers to design devices which work (and which would appear quite magical or even divine to less technologically adept civilizations, as per Arthur C. Clarke and "cargo cults"), I think that we can be confident that I can't be entirely wrong in asserting that apparently it is not necessary to -know- anything in order to -understand- something.
If your question is, "will the concept of a graviton appear in the long sought theory of quantum gravity?", then of course I can only guess, since it is generally agreed that at present there is no true quantum theory of gravity.
I am extremely inexpert in quantum field theory, by the way, so you should address any serious questions about gravitons to someone else.
Chris Hillman (hoping to stay out of the War of Strings)
