Quantum Foam and Quantum Gravity Phenomenology

Abstract

Our understanding of spacetime has undergone some major changes in the last hundred years. Before last century, spacetime was regarded as nothing more than a passive and static arena in which events took place. Early last century, Einstein's general relativity changed that viewpoint and promoted spacetime to an active and dynamical entity. Nowadays, many physicists also believe that spacetime, like all matter and energy, undergoes quantum fluctuations. Following John Wheeler, many of us think that space is composed of an everchanging arrangement of bubbles called spacetime foam, a.k.a. quantum foam. To understand the terminology, let us follow Wheeler and consider the following simplified analogy which he gave in a gravity conference at the University of North Carolina in 1957. Imagine yourself flying an airplane over an ocean. At high altitude the ocean appears smooth. But as you descend, it begins to show roughness. Close enough to the ocean surface, you see bubbles and foam. Analogously, spacetime appears smooth on a large scale, but on sufficiently small scales, it will appear rough and foamy, hence the term “spacetime foam.” Many physicists believe the foaminess is due to quantum fluctuations of spacetime, hence the alternative term “quantum foam.” If spacetime indeed undergoes quantum fluctuations, the fluctuations will show up when we measure a distance (or a time duration), in the form of uncertainties in the measurement. Conversely, if in any distance (or time duration) measurement, we cannot measure the distance (or time duration) precisely, we interpret this intrinsic limitation to spacetime measurements as resulting from fluctuations of spacetime itself.