Warped Passages - Lisa Randall [144]
Furthermore, Green and Schwarz completed their work at a felicitous moment. Physicists had been searching unsuccessfully for theories that could extend the Standard Model to incorporate supersymmetry and gravity, and they were ready to consider something new. They could not ignore Green and Schwarz’s discovery of a supersymmetric theory that could potentially reproduce all the particles and forces of the Standard Model. Even though the additional structure of string theory was a nuisance, the superstring had succeeded where other potentially more economical theories had failed.
Two further significant developments soon ensured string theory’s inclusion in the physics canon. One was from the Princeton collaboration of David Gross, Jeff Harvey, Emil Martinec, and Ryan Rohm, who in 1985 derived a theory that they named the heterotic string. This word is derived from the word “heterosis,” which in botany means “hybrid vigor,” a term used to refer to hybrid organisms with properties superior to those of their progenitors. In string theory, a vibrational mode can move either clockwise or counterclockwise along the string. The name “heterotic” was used because waves moving to the left were treated differently from those waves moving to the right, and consequently the theory included more interesting forces than did the versions of string theory that were already known.
The discovery of the heterotic string was further confirmation that the forces that Green and Schwarz had discovered to be anomaly-free and acceptable in ten dimensions were truly special. They had found several sets of forces, including all of those that had already been shown to be possible in string theory, as well as another set of forces that had never before been discovered (theoretically) to be part of string theory. The forces of the heterotic string were precisely the new ones that Green and Schwarz had shown were free from anomalies. With the heterotic string, this additional set of forces, which could include those of the Standard Model, was shown not only to be a true string theory possibility, but one that could be realized explicitly. Physicists considered the heterotic string a real breakthrough in the attempt to relate string theory to the Standard Model.
There was one final development that cemented string theory’s prominence. This discovery dealt with the extra dimensions essential to the superstring. It is all well and good to show that superstring theory is internally consistent and embodies the forces of the Standard Model, but this is not very interesting if you are stuck with the wrong number of dimensions of space. Superstring theory stipulates ten dimensions. The world around us appears to contain only four (including time). Something needs to be done about the superfluous six.
Physicists now think that one answer might be compactification—rolled-up dimensions of an imperceptibly small size, as described in Chapter 2. At first, however, this curling up of extra dimensions didn’t seem the right way to treat the extra dimensions of string theory. The problem was that theories with rolled-up dimensions could not reproduce the important (and surprising) feature of the weak force discussed in Chapter 7: the weak force treats left-and right-handed particles differently. This is not a mere technical detail. The entire structure of the Standard Model relies on left-handed particles being the only ones that experience the weak force. Otherwise, few predictions of the Standard Model would work.
Although ten-dimensional string theory could treat left-and right-handed particles differently, it appeared that this would no longer be true once the six extra dimensions were rolled up. The resulting four-dimensional effective theory always contained neatly matched pairs of left-and right-handed particles. All of the forces that acted on left-handed fermions also acted on right-handed