Knocking on Heaven's Door - Lisa Randall [161]
Einstein became more mathematically inclined after mathematics helped him finally complete his theory of general relativity. Since mathematical advances were crucial to completing his theory, he had more faith in theoretical methods later in his career. Looking to Einstein won’t resolve the issue, however. Despite his successful application of mathematics to general relativity, his later mathematical search for a unified theory never reached fruition.
The Grand Unified Theory proposed by Howard Georgi and Sheldon Glashow was also a top-down idea. GUTs, as they were known, were rooted in data—the inspiration for their conjecture was the particular set of particles and forces that exist in the Standard Model and the strength with which they interact—but the theory extrapolated from what we know to what might be happening at very distant energy scales.
Interestingly, even though the unification would happen at an energy much higher than a particle accelerator could achieve, the initial model for a GUT made a prediction that was potentially observable. The Georgi-Glashow GUT model predicted that the proton would decay. The decay would take a long time, but experimenters set up giant vats of material with the hopes that at least one of the protons inside would decay and leave a visible signal. When that didn’t happen, the original GUT model was ruled out.
Since that time neither Georgi nor Glashow has chosen to work on any top-down theory that makes such a dramatic leap in energies from those we can directly access in accelerators to those so far removed that they might have only subtle experimental consequences—or likelier still, not any. They decided it would just be too ridiculously unlikely to make a correct guess about a theory so many orders of magnitude away in distance and energy from anything we currently understand.
Despite their reservations, many other physicists decided that a top-down approach was the only way to attack certain difficult theoretical issues. String theorists chose to work in a netherworld that isn’t clearly traditional science but has led to a rich, if controversial, set of ideas. They understand some aspects of their theory, but they are still piecing it together—looking for the key underlying principles as they go along and develop their radical ideas.
The motivation for string theory as a theory of gravity didn’t come from data, but from theoretical puzzles. String theory provides a natural candidate for the graviton, the particle quantum mechanics tells us should exist and communicate the force of gravity. It is currently the leading candidate for a fully consistent theory of quantum gravity—a theory that includes both quantum mechanics and Einstein’s theory of general relativity, and that works at all conceivable energy scales.
Physicists can use known theories to reliably make predictions at small distances, such as the inside of an atom, where quantum mechanics plays a big role and gravity is negligible. Because gravity has such feeble influence on atomic-mass particles, we can use quantum mechanics and safely ignore gravity. Physicists can also make predictions about phenomena at large distances, such as inside galaxies, where gravity dominates predictions and quantum mechanics can be ignored.
However, we lack a theory that includes both quantum mechanics and gravity—and works at all possible energies and distances. In particular, we don’t know how to calculate at enormously high