There are, however, features of the theory that may be open to examination even with our incomplete understanding. We may be able to test the theory's predictions of particular new particle species, of dimensions of space beyond the three we can directly see, and even its prediction that microscopic black holes may be produced through highly energetic particle collisions. Without the exact equations, our ability to describe these attributes with precision is limited, but the theory gives enough direction for the Large Hadron Collider, a gigantic particle accelerator now being built in Geneva and scheduled to begin full operation in 2008, to search for supporting evidence by the end of the decade.

Research has also revealed a possibility that signatures of string theory are imprinted in the radiation left over from the Big Bang, as well as in gravitational waves rippling through space-time's fabric. In the coming years, a variety of experiments will seek such evidence with unprecedented observational fidelity. And in a recent, particularly intriguing development, data now emerging from the Relativistic Heavy Ion Collider at the Brookhaven National Laboratory appear to be more accurately described using string theory methods than with more traditional approaches.

To be sure, no one successful experiment would establish that string theory is right, but neither would the failure of all such experiments prove the theory wrong. If the accelerator experiments fail to turn up anything, it could be that we need more powerful machines; if the astronomical observations fail to turn up anything, it could mean the effects are too small to be seen. The bottom line is that it's hard to test a theory that not only taxes the capacity of today's technology, but is also still very much under development.

Some critics have taken this lack of definitive predictions to mean that string theory is a protean concept whose advocates seek to step outside the established scientific method. Nothing could be further from the truth. Certainly, we are feeling our way through a complex mathematical terrain, and no doubt have much ground yet to cover. But we will hold string theory to the usual scientific standard: to be accepted, it must make predictions that are verified.

Other detractors have seized on recent work suggesting that one of string theory's goals beyond unification of the forces — to provide an explanation for the values of nature's constants, like the mass of the electron and the strength of gravity — may be unreachable (because the theory may be compatible with those constants having a range of values). But even if this were to prove true, realizing Einstein's unified vision would surely be prize enough.

Finally, some have argued that if, after decades of research involving thousands of scientists, the theory is still a work in progress, it's time to give up. But to suggest dropping research on the most promising approach to unification because the work has failed to meet an arbitrary timetable for complete success is, well, silly.

I have worked on string theory for more than 20 years because I believe it provides the most powerful framework for constructing the long-sought unified theory. Nonetheless, should an inconsistency be found, or should future studies reveal an insuperable barrier to making contact with experimental data, or should new discoveries reveal a superior approach, I'd change my research focus, and I have little doubt that most string theorists would too.