String Theory, Black Holes and Quantum Gravity
As the leading candidate for a theory of quantum gravity, string theory promises to unify two of the great achievements of twentieth century physics: quantum mechanics and general relativity. In doing so, string theory provides new insights into the physics of gravity at very short distances. Many of the most exciting advances in the last several years have involved applications of string theory to the physics of black holes, in extreme settings where our intuitive ideas about space and time break down. The theory group at SLAC has been at the forefront of research in this area. For example, last year I, along with several collaborators, demonstrated that string theory can resolve several long-standing conceptual puzzles involving these mysterious objects.
Quantum mechanics and general relativity are undoubtedly two of the crowning achievements of physics. Quantum mechanics describes nature at very small length scales, such as the atomic or subatomic scales relevant for particle physics at SLAC. General relativity is a theory of gravity; it successfully describes the motion of very massive objects, like planets, galaxies or even the entire universe. Both quantum mechanics and general relativity have been been experimentally verified repeatedly over the course of decades. Nevertheless, the two theories are fundamentally incompatible.
General relativity is a classical theory, which describes the evolution of space-time in a smooth, continuous manner. In quantum mechanics, however, space-time is anything but smooth: it is governed by the same counterintuitive quantum rules that govern the motion of atoms. So either quantum mechanics or general relativity must be modified in some basic way, to give a theory of "quantum gravity." The search for this theory is one of the most exciting and difficult problems in theoretical physics today.
String theory is the leading candidate for a theory of quantum gravity. The basic constituents of string theory are extended objects (known as strings, or branes) rather than the point-like objects typically used to describe fundamental particles. The implications of this simple ideathat particles are extended objects rather than infinitesimal pointsare quite profound. String theory turns out to include gravity, and provides an extension of Einstein's theory of general relativity that is consistent with rules of quantum mechanics.
In addition (as Bogdan Florea described in a recent SLAC Today article) it contains the basic ingredients of the standard model of particle physics. So string theory has the potential to describe the fundamental forces of particle physics along with gravity in a unified and self-consistent manner.
So how can we test string theory? Unfortunately, this is currently impossible with even the most powerful particle accelerators. But there are many interesting theoretical problemssuch as those involving black holeswhere these ideas can be put to the test. Black hole physics provides an ideal theoretical laboratory for the study of quantum gravity, because a black hole is both very massive (so that general relativity is important) and very small (so that quantum mechanics is important).
String theory includes a series of higher order corrections to general relativity, which dramatically alter the space-time structure of certain black holes. For example, the simplest types of black holes in string theory are composed of a single excited string. However, these objects can not be described in the standard theory of general relativity: they contain what is known as a "naked singularity," where the theory breaks down and stops making sensible predictions. However, as I demonstrated in collaboration with Atish Dabholkar and Renata Kallosh, and elaborated in later work with Veronika Hubeny and Mukund Rangamani, the situation changes dramatically once string-theoretic corrections are included. These higher order terms convert this apparently singular object into a smooth solution without a naked singularity. So string theory resolves an apparent singularity of classical general relativity. This is just one example of the successes of string theory as a theory of quantum gravity. More recently, in work with Jon Hsu and Alessandro Tomasiello, and in an ongoing project with Paul Aspinwall and Aaron Simons, we have further studied various mathematical aspects of black holes in string theory. Many more surprises are sure to come as we continue to explore the quantum structure of space-time in string theory.
Above black hole image courtesy of the BBC