string theory, as a fundamental theory, could be consistent with special relativity and quantum theory only if several conditions were satisfied. First, the world had to have twenty-five dimensions of space. Second, there had to be a tachyon—a particle that goes faster than light. Third, there had to be particles that could not be brought to rest. We refer to these as massless particles, because mass is the measure of a particle’s energy when it is motionless.
