Is a tabletop search for Planck scale signals feasible?

Quantum gravity theory is untested experimentally. Could it be tested with tabletop experiments? While the common feeling is pessimistic, a detailed inquiry shows it possible to sidestep the onerous requirement of localization of a probe on the Planck length scale. I suggest a tabletop experiment which, given state-of-the-art ultrahigh vacuum and cryogenic technology, could already be sensitive enough to detect Planck scale signals. The experiment combines a single photon’s degree of freedom with that of a macroscopic probe to test Wheeler’s conception of “quantum foam,” the assertion that on length scales of the Planck order, spacetime is no longer a smooth manifold. The scheme makes few assumptions beyond energy and momentum conservations, and is not based on a specific quantum gravity scheme.

Figure 1

The suspended block and the photon’s path (dotted line) starting from the single-photon emitter E, passing through a $\mathrm{\text{divergent}}+\mathrm{\text{convergent}}$ lens system (to spread the pulse) and, after transiting the block, passing through a second convergent lens that focuses the pulse onto the single-photon detector $D$.

Figure 2

Setup of suspended blocks showing (dotted lines) the alternative paths for the photon. E is the single-photon emitter; $D$ and $D^{\prime }$ are the single-photon detectors. BS denotes the beam splitter, and M the mirror. DL is the fiber optics delay line, and EB are the electronics that trigger $D$ and $D^{\prime }$ through cable C. The optical elements to widen the beam before and focus it after each block are left out for clarity. In the real experiment, the blocks would hang side by side.