wiki wrote:
String theory remains to be confirmed. No version of string theory has yet made an experimentally verified prediction that differs from those made by other theories. In this sense, string theory is still in a "larval stage": it is not a proper physical theory. It possesses many features of mathematical interest and may yet become important in our understanding of the universe, but it requires further developments before it is accepted or discarded. Since string theory may not be tested in the foreseeable future, some scientists[7] have asked if it even deserves to be called a scientific theory; it is not falsifiable in the sense of Popper.
For example, while supersymmetry is now seen as a vital ingredient of string theory, supersymmetric models with no obvious connection to string theory are also studied. Therefore, if supersymmetry were detected at the Large Hadron Collider it would not be seen as a direct confirmation of the theory. More importantly, if supersymmetry were not detected, there are vacua in string theory in which supersymmetry would only be seen at much higher energies, so its absence would not falsify string theory. By contrast, if observing the Sun during a solar eclipse had not shown that the Sun's gravity deflected light by the predicted amount, Einstein's general relativity theory would have been proven wrong.
On a more mathematical level, another problem is that, like many quantum field theories, much of string theory is still only formulated perturbatively (i.e., as a series of approximations rather than as an exact solution). Although nonperturbative techniques have progressed considerably – including conjectured complete definitions in space-times satisfying certain asymptotics – a full non-perturbative definition of the theory is still lacking.
Philosophically, string theory cannot be truly fundamental in its present formulation because it is background-dependent: each string theory is built on a fixed spacetime background. Since a dynamic spacetime is the central tenet of general relativity, the hope is that M-theory will turn out to be background-independent, giving as solutions the many different versions of string theories, but no one yet knows how such a fundamental theory can be constructed. A related problem is that the best understood backgrounds of string theory preserve much of the supersymmetry of the underlying theory, and thus are time-invariant: string theory cannot yet deal well with time-dependent, cosmological backgrounds.
Another problem is that the vacuum structure of the theory, called the string theory landscape, is not well understood. As string theory is presently understood, it appears to contain a large number of distinct vacua, perhaps 10500 or more. Each of these corresponds to a different universe, with a different collection of particles and forces. What principle, if any, can be used to select among these vacua is an open issue. While there are no known continuous parameters in the theory, there is a very large discretuum (coined in contradistinction to continuum) of possible universes, which may be radically different from each other. Some physicists believe this is a benefit of the theory, as it may allow a natural anthropic explanation of the observed values of physical constants, in particular the small value of the cosmological constant. However, such explanations are not usually regarded as scientific in the Popperian sense.
03 Mar 2007, 9:37 pm
