Steven Carlip works on one of the fundamental unsolved problems in modern theoretical physics, the effort to combine general relativity and quantum mechanics into a consistent quantum theory of gravity. Carlip is a generalist: rather than focusing exclusively on a single research program such as string theory, he looks for "windows" into quantum gravity, relatively simple settings, models, and approximations that may offer insights without requiring a complete theory. Among his current areas of research are:
* Looking at quantum gravity in fewer than four spacetime dimensions as a test bed for approaches to the full quantum theory; * Trying to understand the quantum gravitational basis of black hole thermodynamics; * Investigating "causal dynamical triangulations," a discrete lattice model of quantum gravity that shows considerable promise; * Exploring the structure of spacetime at the smallest scales, and studying the possibility of "spontaneous dimensional reduction" from four to two dimensions; * Searching for experimental tests that might at least tell us whether gravity is really quantized at all.
You can read more at his personal web page.