String theory is on the ropes. After decades of prominence as the key to physics’ elusive “theory of everything,” challengers say the hypothesis is unraveling.

Why? Because there haven’t been experiments to prove it — and there don’t seem to be any on the horizon.

“The interplay with experiments is essential, and string theory just doesn’t have that,” says physicist Lee Smolin, author of The Trouble With Physics: The Rise of String Theory, the Fall of Science, and What Comes Next
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Schwartz, one of the fathers of modern string theory, replies by e-mail that experiments will verify string theory in the future. The big question is how much energy an experiment would have to pound into a collision between particles to reveal strings, he adds. Many physicists hope that Europe’s Large Hadron Collider facility will offer some answers, starting in 2007.

Ultimately, Carroll says, “the only way for someone to kill string theory will be to come up with a better one.”