Some physicists see great potential in superstring theory, while others reject it. What is the alternative to superstring theory?
The answer is brief: there is no alternative to String Theory. The reasons? String Theory is an ambitious one: it is supposed to contain all of the fundamental particles that we observe today, describe the reason why they have the interactions they do, include quantum mechanical theory of gravitation, explain the microscopic origins of cosmological observations like why we live in an expanding universe, what is dark energy/matter, etc.
There is simply no other theory that is as all-inclusive as well as having a chance of being consistent with the things we see to date. There are, however, alternatives to a quantum theory of *gravitation* (and hence of spacetime itself). The most notable one is the quantum geometry program (known in previous forms as "non-perturbative canonical quantum gravity" or "loop quantum gravity").
In summary, String Theory is certainly a rich one, with the potential of explaining in a single framework the phenomena on both small and large scales. On the downside, it is perhaps "too big", containing too many possibilities to tell us why we have the particular universe we do, and not any of the others allowed by String Theory. In addition, experimental predictions at the level of our technology are lacking.
Sean McReynolds, M.S., Physics Doctoral Student, Penn State University
'If you want to live a happy life, tie it to a goal, not to people or things.'