Warped Passages - Lisa Randall [139]
However, the quantum field theory description of gravity cannot be complete. No quantum field theory for the graviton can predict its interactions at all energies. When a graviton is as energetic as the Planck scale energy, quantum field theory breaks down. Theoretical reasoning demonstrates that extra graviton interactions that wouldn’t make a difference at low energies become important at high energies, but the logic of quantum field theory is not sufficient to tell us what they are or how to include them. If we incorrectly used a quantum field theory of gravity, ignoring the interactions that don’t matter at low energies, and attempted to make predictions for extremely energetic gravitons, we would conclude that graviton interactions occur with probability greater than one—something which is clearly impossible. At the Planck scale energy, or equivalently (according to quantum mechanics and special relativity) at the Planck scale length, 10-33 cm, the quantum mechanical description of the graviton obviously breaks down.
The Planck scale length, nineteen orders of magnitude smaller than the size of a proton, would be much too small for physicists to care about were it not for the fundamental issues that a more comprehensive theory can potentially address. For example, current theories of cosmology conjecture that the universe began as a tiny ball, a Planck scale length in size. But we have no understanding of the “Bang” of the Big Bang. We understand many aspects of the universe’s later evolution, but not how it began. Deducing the physical laws that apply to sizes less than the Planck scale length should shed light on the earliest stage of the evolution of our universe.
Furthermore, there are many mysteries about black holes. Important unresolved questions include what exactly is happening at the black hole’s horizon, the place of no return beyond which nothing can escape, and at the singularity, the place in the center of the black hole where general relativity no longer applies. Another unanswered question is how information about objects that fall into a black hole is stored. Unlike the gravitational force we experience, gravitational effects inside a black hole are strong, as strong as effects from objects with the Planck scale energy in ordinary flat space. We will never solve these black hole mysteries until we resolve the problem of finding a single theory that consistently includes both quantum mechanics and general relativity—a theory of quantum gravity on the Planck scale length, 10-33 cm. Black holes exemplify some of the questions about strong gravitational effects that will be resolved only by a quantum theory of gravity. String theory is the best known candidate for such a theory.
String Training
String theory’s view of the fundamental nature of matter differs significantly from that of traditional particle physics. According to string theory, the most basic indivisible objects underlying all matter are strings—vibrating, one-dimensional loops or segments of energy. These strings, unlike violin strings, say, are not made up of atoms which are in turn made up of electrons and nucleons which are in turn made up of quarks. In fact, exactly the opposite is true. These are fundamental strings, which means that everything, including electrons and quarks, consists of their oscillations. According to string theory, the yarn a cat plays with is made of atoms that are ultimately composed of the vibrations of strings.
String theory’s radical hypothesis is that particles arise from the resonant oscillation modes of strings. Each and every particle corresponds to the vibrations of an underlying string, and the character of those vibrations determines the particle’s properties. Because of the many ways in which strings can vibrate, a single string can give rise to many types of particle. Theorists initially thought there was only a single type of fundamental string that is responsible for all known particles. But that picture has changed in the last few years, and we now believe