With his general theory of relativity, Einstein showed that gravity emerges beautifully from the curvature of space and time. But Einstein’s theory is only classical: a quantum theory of gravity is expected to reveal the properties of quantum spacetime. Here I will show that when gravity is treated as a quantum field theory, the trajectories of free-falling objects become 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.
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