|Scale invariance means roughly speaking a theory is the same no matter how close one looks. This typically is not the case with things we observe, but theoretically a possibility that is compelling because of its simplicity that is the very absence of such a scale. A nice example for scale invariance is e.g. the Koch curve (pictured to the right), where one can zoom in arbitrarily and the structures just repeat. In contrast to this, if we zoom in on things around us, structures change considerably, from Quarks to Quasars.|
The presence of such unusual fields would, because they can not be understood as particles, lead to phenomenological consequences different from all other expectations. E.g. cross-sections would display an energy dependence that could not be explained with theories so far considered. This could be observable even if one had only a missing energy signal.
I did not really look into the details of Georgi's proposal, but I have to say the paper is pretty much ingenious. It's a very original work that outlines a scenario with a couple of well made arguments, points out the interesting phenomenology and ends with a list of further possibly interesting things to examine. It's a paper whose destiny it is to become a top-cite.
One month later, there was a follow up paper "Another Odd Thing About Unparticle Physics" (Howard Georgi, arXiv: 0704.2457) about the propagator of the Unparticle fields. After this, there started a steady flow of further investigations of the properties of such fields and their phenomenology for particle physics, astrophysics, cosmology, CP violation, lepton flavor violation, muon decay, or neutrino oscillations, Unparticle fields became supersymmetric, colored, got deconstructed and topped off - just to give a very incomplete list . The number of papers with Unparticle in the title up to now is
- March: 1
- April: 3
- May: 15
- June: 7
- July: 16
- August: 12
- September: so far 7
The Unparticles are a theoretical possibility that has observable consequences, a splendid example for physics beyond the standard model, and an excellent playground. One can set bounds on the model from the absence of evidence in observations made so far, which will occupy people for some more while. On the other hand there is not the slightest indication that these things actually exist. Unlike e.g. supersymmetry, extra dimensions or GUT models, which attract a lot of people because of their multi-faceted motivations, there is to my knowledge no motivation for Unparticles besides the mere possibility of such a scenario. Reasons to work on the topic then sound like "Recently there has been much interest in the possible existence of ..." or "The possibility of a non-trivial scale invariant sector [...] was advocated by Georgi...". A very similar peak of interest followed on the proposal of extra dimensional models in 98 and 99 (for more details see here).
Here we have the visionary standing on the sea-shore, whilst the great ocean of truth lies all undiscovered before him . A mission is planned to sail into the far distance, ready to leave the harbor in May 2008. The visionary points towards infinity and says: "It could be there is a civilization in the West. But it might be different from all we've imagined so far. Maybe they live on the bottom of sea, and what we will see of their cities are the tops of their skyscrapers."
Six months later, people have made detailed drawings of the undersea creatures, they have tried to find out how they would move, how they would live, how they would make love (and of course they would wear Levi's). And we would know their skyscrapers couldn't be too high because otherwise we had already seen them. And their music couldn't be too loud because otherwise we had already heard them. And their sex life couldn't be too wild because otherwise there was a lot of troubled water.
It kind of makes me wonder who's going to pick up the pebbles left behind.
See also: T. Siegfried's Why files.
 I apologize profoundly, but I did not read all of these papers, so please don't ask me for the details. For a more complete list, see here.
 "I do not know what I may appear to the world; but to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me."
~ Isaac Newton, In Brewster, Memoirs of Newton (1855), vol II, Ch. 27