Warped Passages - Lisa Randall [142]
However, the predictions of string theory did not agree with the SLAC experiment’s results. Strings would never lead to the dramatic scattering that only a hard, compact object could cause. Because only pieces of the strings would interact at any given time, strings would collide more softly. This quiet, relatively undramatic, scattering was the death knell for the string theory of hadrons. However, from the vantage point of quantum gravity it looked like it could be a very promising property.
In a particle theory of the graviton, the graviton interacts far too strongly at high energies. A better theory would be one in which energetic gravitons don’t interact so fiercely. And that is what happens in a string theory of gravity. String theory, which replaces pointlike particles with extended strings, guarantees that the graviton interacts much less dramatically at high energies. Strings—unlike quarks—have no hard scattering processes. They have more “mushy” interactions that take place over an extended region.24 This property means that string theory could potentially solve the problem of the graviton’s ridiculously high interaction rate, and correctly predict high-energy graviton interactions. Strings’ softer high-energy collisions were another important indicator that a string theory of gravity might be correct.
In summary, superstring theory contains fermions, force-carrying gauge bosons, and the graviton—all the types of particle we know about. It doesn’t contain a tachyon. Furthermore, superstring theory includes a graviton whose quantum description potentially makes sense at high energies. String theory looked like it could potentially describe all known forces. It was a promising candidate theory of the world.
The Superstring Revolution
Superstring theory was an extremely bold step, even to solve a problem as deep as quantum gravity. A string theory of gravity predicts an infinitely large number of particles beyond those we know. Moreover, string theory is extremely difficult to analyze with computations. What a steep price to pay for solving the problem of quantum gravity: a theory with infinitely many new particles and a potentially intractable mathematical description. Working on string theory in the 1970s required individuals who were either very determined or somewhat crazy. Scherk and Schwarz were among the very few who negotiated this risky path.
After Scherk’s untimely death in 1980, Schwarz persevered with string theory. He collaborated with another (perhaps the only) convert at that time, the British physicist Michael Green, and together they worked out the consequences of the superstring. Schwarz and Green discovered a bizarre feature of the superstring: it makes sense only in ten dimensions, nine of space and one of time. In any other number of dimensions, unacceptable vibrational modes of the string give rise to manifestly nonsensical predictions, such as negative probabilities for processes involving modes of the string that should not exist. In ten dimensions, all the unwanted modes are eliminated. A string theory in any other number of dimensions made no sense.
To clarify, the string itself extends along a single spatial dimension and travels through time. Those were the two dimensions that Ramond had studied when he first discovered supersymmetry. But just as we know that a pointlike object—which has no extent in any spatial dimensions, and therefore has zero spatial dimensions—can move about in three dimensions of space, a string—which has one spatial dimension—can move around in a space with many more dimensions than it itself possesses. Strings could conceivably move around in three, four, or more dimensions. Calculations indicated that the correct number (including