To know the history of science is to recognize the mortality of any claim to universal truth. ~Evelyn Fox Keller
It may be that the blush is off the rose of string theory. According to an article at Wired, string theory is under attack, and scientists are starting to look elsewhere for their grand unification theories (GUTs).
Peter Woit of Columbia University has written “Not Even Wrong: The Failure of String Theory”; Lee Smolin has a new book with the rather cumbersome title, “The Trouble With Physics: The Rise of String Theory, The Fall of Science, and What Comes Next.” Of course, I've had my say several times, including here, which contains a quote from Mr. Smolin: “When it comes to extending our knowledge of the laws of nature, we have made no real headway [in 30 years] ... It's called hitting the wall."
And therein lies the problem. Since the 1970's, string theory has been a hot commodity, despite the unsettling fact that a) it doesn't make testable predictions, and b) the complex equations that do come out of string theory have multiple solutions, which certainly helps the theory conform with existing theories and data, but makes it a less than precise way to attempt to describe the quantum world, much less the relativistic universe.
So, if string theory is losing favor at last, what next? According to the article, we have:
Twisters. A while back, Roger Penrose of Oxford University invented a mthematical tool called “twisters.” Twister theory is actually a restructuring of string theory that stays in the comfy four dimensional world with which we are comfortable. That's nice, but it sounds very much like someone has just lopped off the more unacceptable parts of string theory, generating more solvable equations, but that's all. There appears to be no cohesive theory coming out of the math so far.
Loop Quantum Gravity (LQG). According to the Wired article, the physics bookies are betting that, if string theory does fizzle, this baby is going to be the Next Big Thing. LQG is coupled with something called Spin Foam Theory, which ought to make it suspect right now. The cuter the theory's name, usually the less likely that it will amount to anything. At any rate, the Loopy Foam Theory utilizes “loops of field lines”, which sound suspiciously like stringy sorts of things. The idea is that, at quantum levels, Einsteinian spacetime breaks down, a distinct possibility. While no one has seen down to Planck distances (the extremely small length that defines the quantum world), it's generally felt that it's a seething froth of particles coming and going in that superpositional world of multiple probabilities all existing at once.
The trouble is that for LQG to make it as a GUT, conventional spacetime ought to fall out of the equations when you move above the Planck limit. So far it doesn't.
Interestingly, fans of the theory are looking at the GLAST satellite for support in its measurements of the cosmic background radiation. What makes this interesting is that string theorists are look to the same satellite to provide proof of the additional dimensions that they need to support the brane model.
Causal Dynamical Triangulations (CDT). Don't like strings? How about tiny little tetrahedra? This theory breaks spacetime down into these regular solids. Why? You got me, especially since it looks like this theory has a problem generating testable hypotheses, just like string theory.
Non-commutative Geometry. Basically, this theory admits that the Uncertainty Principle is right. Thanks, but we knew that. What it does is use non-commutative methods (in other words A+B does not equal B+A) to describe the spatial geometry in which the Principle operates. Now this may be a lovely mathematical exercise, and, according the story, it has played a part in string theory and LQG. Since neither of these seem to work, it's hard to see where NG is going to take us, but perhaps it will provide a tool to develop a theory that, at the least, has a reasonable chance of being viable.
No Unified Theory. That's right; maybe there is no single set of equations that can reconcile the quantum world with the relativistic universe. After all, scientists have been trying for around 90 years now, and they seem no closer now than they were when Einstein intoned that “God does not play dice.” Would this be so tragic? After all, if we have two sets of rules, one for the very large and one for the very small, and those rules work quite well, is it so necessary to reconcile them?
Necessary? Perhaps not, but it is desirable. For one thing, a unified theory might go a long way toward explaining what went on during that annoying period very early in the Big Bang where the known laws of physics break down. It would also help in defining mysterious aspects of the universe like dark energy and gravitational repulsion. These are questions that are basic to our understanding of the cosmos, so these are not just annoying trifles.
And, it continues to look as though any theory had better be in excellent agreement with Einstein's theories, because, despite frequent pronouncements of the pending demise of General Relativity, observational evidence continues to support it. After three years of observing a double pulsar system (and how odd can that couple be), a team at the Jodrell Bank Observatory has presented data supporting Einstein's predictions to within 0.05 percent. They test aspects of gravitational redhift, Shapiro delay (pulses being delayed by the warping of spacetime), and gravitational radiation and decay (loss of energy due to gravity waves).
When the Loopy Foam Theorists can match that, call me.