String theory is a revolutionary way of describing the universe. It says that all aspects of our universe -- the particles that make up matter and the forces that act on those particles -- consist of tiny strings of energy. According to string theorists, these imperceptibly small strings, or loops, of energy hold the key to understanding and explaining the behavior of everything from the smallest particles in the universe to its largest bodies.

But many physicists, including some string theorists themselves, recognize that the strings their theory hangs on are the greatest stumbling block to the acceptance of this radical idea. Far smaller than any subatomic particle identified so far, strings may be too minuscule to ever be seen or detected, which leads to the question: How can string theory be tested?

To be scientifically useful and valid, a theory must make predictions that are testable. Confirming a prediction lends support to a theory; negating a prediction suggests the theory may be wrong. Unless such tests can be performed, an idea is simply philosophical, not scientific. Without confirmation that the mathematical theory based on strings explains ideas that were not understood before, string theory may never be fully accepted as a scientifically valid concept.