The secrets of spacetime revealed - on your workbench!

Laser source? Check. Lenses? Check. A suspended block of glass? Check. A single-photon detector? Check. Supercooling apparatus? Check. Congratulations: put that kit together, and you might be able to help resolve some of the most fundamental questions of quantum physics.

OK, it might not be within reach of the backyard scientist, but the fascinating idea proposed in this Arxiv paper is that structures at the very smallest scale might be probed with a tabletop apparatus.

Let’s start by describing the problem: at the very finest scales, quantum mechanics predicts that what looks like a “smooth” universe of spacetime isn’t. In 1955, American theoretical physicist John Wheeler noted that long-established principles of quantum mechanics (like Heisenberg’s uncertainty principle) meant that at Planck scales, spacetime itself inherited the uncertainty of the quantum world. He coined the term “quantum foam” to describe this.

The problem is that while the “foamy” nature of quantum spacetime is predicted in the theory, it’s hard to test. As the paper notes, “detection of non-smooth spacetime texture at around the Planck scale is unfeasible with an elementary particle as probe. If we tried to localize the particle to the required scale, the uncertainty principle would require that we give the particle … energy of at least 10¹⁹ GeV; this is many orders of magnitude beyond what foreseeable particle accelerators can supply.”

The paper, by Jacob D. Bekenstein of the Racah Institute of Physics, Hebrew University of Jerusalem, proposes a startling workaround using the energies accessible to a tabletop research laser.

A simple view of Berkenstein's experiment: the photon emitter (right), lenses, and the glass block.

Source: http://arxiv.org/abs/1211.3816

Here’s the idea: fire a single photon at a dielectric block, and see whether the photon traverses the block or not. Photons interact poorly with matter – but they do interact. If the photon moves the block by more than the Planck distance (1.616199 × 10^-35 meters), it will pass through. If the photon can’t move the block that distance, it won’t pass through, and the detector won't see it.

To take into account other uncertainties I won’t try to describe in full, Berkenstein proposes using classical mechanics to predict the number of photons that would be passed or reflected by the block on multiple runs. If the statistics gathered in the experiment differ from the classical prediction, he says, spacetime must be “lumpy” at the scale the experiment measures.

Bear with me, we’re nearly there: the final issue is how to “tweak” the scale at which the experiment might test spacetime for “quantum foam”. That’s relatively simple: use blocks of varying size, because they will absorb a different amount of momentum from the fired photons.

The blocks themselves have to be cooled close to absolute zero to minimise thermal noise in the experiment. ®

Bootnote: As an aside, the experiment takes advantage of yet another bit of quantum strangeness. It proposes using a single photon – but the lenses I mentioned at the top are designed to focus the light wave so that it’s the same size as the block. Wave-particle duality in practice! ®