Warped Passages - Lisa Randall [145]
In 1985, Philip Candelas, Gary Horowitz, Andy Strominger, and Edward Witten recognized the significance of a more subtle and complicated way to curl up the extra dimensions, namely a compactification known as Calabi-Yau manifolds. The details are complicated, but basically Calabi-Yau manifolds leave a four-dimensional theory that can distinguish left from right and potentially produce the particles and forces of the Standard Model, including the parity-violating weak force. Furthermore, rolling up the extra dimensions into a Calabi-Yau manifold preserves supersymmetry.* With the Calabi-Yau breakthrough, superstring theory was in business.
In many physics departments, superstring theory superseded particle physics, and the superstring revolution was more like a coup. Because superstring theory incorporates quantum gravity and could contain the known particles and forces, many physicists went so far as to think of it as the ultimate theory that underlies everything. Indeed, in the 1980s string theory was dubbed the “Theory of Everything” (or “TOE”). String theory was more ambitious even than Grand Unified Theories: with string theory, physicists hoped to unify all forces (including gravity) at an energy higher even than the energy associated with GUTs. Even without any observations that supported string theory, many physicists decided that string theory’s potential for reconciling quantum mechanics and gravity was reason enough to support its claim to prominence.
The Endurance of the Old Regime
If string theorists are right, and the world is ultimately composed of fundamental oscillating strings, must all of particle physics then be abandoned? The answer is a resounding “No.” The goal of string theory is to reconcile quantum mechanics and gravity at distances smaller than the Planck scale length, where we believe that a new theory takes over. Therefore, in conventional string theory (as opposed to the variants suggested by extra-dimensional models), a string should be about the Planck scale length in size. That tells us that in conventional string theory, the differences between particle physics and string theory should appear only at this tiny Planck scale length or, equivalently, at the ultra-high Planck scale energy, where gravity is expected to be strong. This size is so tiny, and this energy so high, that strings would in no way obviate the particle description at experimentally accessible energies.
For energies below the Planck scale energy a particle physics description is in fact quite adequate. If a string is so small that its length is undetectable, the string might as well be a particle; no experiment could tell the difference. Particles and Planck-length strings are indistinguishable. The string’s one-dimensional extent is just as invisible to us as the tiny curled-up extra dimensions we considered earlier. Unless we have instruments that can handle sizes of order 10-33 cm, such a string is much too small to see.
It makes sense that string theory and particle physics look the same at achievable energies. The uncertainty principle tells us that the only way to study small distances is with high-momentum particles, which are very energetic. Therefore, without sufficient energy, you have no way of seeing that the string is long and skinny, rather than pointlike.
In principle, we could find evidence to support string theory by searching for the many new particles it predicts—the particles that correspond to the many possible oscillations of the string. The problem with this strategy is that most new string-induced particles would be extremely heavy, with a mass as big as the Planck scale mass, 1019 GeV. This mass is huge compared with the mass of particles that have been detected experimentally, the heaviest of which is about 200 GeV.
The extra particles that would arise from the oscillations of the string would be so heavy because the string’s tension—its resistance to stretching that determines