Event

Gravity is usually regarded classically, obeying Newton's law or Einstein's equations. Here I will show that, when the gravitational field is treated quantum-mechanically, the classical trajectories of freely falling objects are subject to random fluctuations, akin to the jitters of Brownian motion. Intuitively, this random motion, or “noise,” can be thought of as coming from the bombardment of the falling object by gravitons. The statistical properties of this fundamental noise are precisely calculable using techniques developed by Feynman and Vernon. Incredibly, graviton noise may lie within the sensitivity of table-top gravitational wave detectors that are already under construction; if detected, it would provide our first experimental evidence that gravity — and thus spacetime itself — is also quantized. When these results are extended to congruences of geodesics, we find that the quantum fluctuations of spacetime give rise to an additional term in the Raychaudhuri equation which, however, does not appear to prevent the occurrence of singularities.