In 2003 Carlo Rovelli wrote a paper entitled "A dialog on quantum gravity" (Int.J.Mod.Phys. D12 (2003) 1509-1528, http://www.arxiv.org/abs/hep-th/0310077). There, in the form of a dialog between professor Simp, a high energy physicist, and Sal, a graduate student who has decided to study loop gravity, Rovelli gives a "State of the Union" (read Unification) account of theoretical physics. . . .
Simp – It is not the fault of the theoretical physicist if the path of the natural evolution of the research has lead to a theory which is very complicated.
Sal – And if it was the fault of the theoretical physicist? I suppose when you say “the path of the natural evolution of the research” you mean the line that goes along Fermi theory, QED, SU(2)×U(1), QCD, the standard model, and then grand unified theories, the revival of Kaluza-Klein, supersymmetry, supergravity, . . . strings. . .
Simp – Yes.
Sal – But what if this “path of natural evolution” has taken a wrong turn at some point. Seems to me there is precise break along this path.
Simp – What do you mean?
Sal – Dirac predicted the positron, and it was found. Feynman and friends developed a calculation method for photon-electron interactions, and it works to devastating precision. Weinberg Glashow and Salam predicted the neutral currents and they were found, and the W and Z particles and Carlo Rubbia found them, precisely where predicted, just to name some . . .
Simp – So?
Sal – And then?
Simp – Then what?
Sal – Then the Veneziano formula predicted a very soft high energy behavior of the amplitudes, and nature was not like that. The grand unified theories predicted proton decay at some precise scale, and proton decay was not found where expected. Kaluza-Klein theory, revived, predicted the existence of a scalar field that was searched by Dicke, and not found. Supersymmetry predicted the supersymmetric particles and these were not found where repeatedly annunciated. Extra dimensions did not show up where recently suggested by string theory. . .
Simp – But the proton may take a bit longer to decay, the masses of the supersymmetric partners may be higher . . .
Sal – Of course, they “might”. Everything is possible. But the cut between the previous fantastic sequence of successful predictions right on the mark, and, on the other hand, the later series of unsuccesses is striking. Before, experimental particle physicists were always smiling and walking like heroes: looked like God was reading Phys Rev D and implementing all suggestions of the theorists. Nowadays, thanks god they are still busy figuring out aspects of the standard model, because all the new physics that theoreticians have suggested wasn’t there . . .
Simp – Theory has always made wrong predictions.
Sal – Yes, but also right predictions, and those are missing, after the standard model.
Simp – It is because energies of new predicted physics are too high.
Sal – Not at all. There have been plenty of predictions that were well within reach. They just were wrong.
Simp – So, what do you make of this?
Sal – That perhaps Nature is telling us that our path of theoretical research has taken a wrong turn, at some point . . .
Simp – This is not a proof.
Sal – Of course. The fact is that we do not know.
No one doubts that theoretical physics is in a bit of a slump. The only issue is, whether it is on the wrong track, or simply short on juice for particle colliders to provide elaborate theories designed to solve the existing puzzles. The ending of this bit of dialog is correct. "The fact is that we do not know."