Despite many decades of effort, quantum physics and gravitation remain separate: no consistent quantum theory of gravity exists. However, that doesn't mean we don't have hints about what such a theory would look like. In particular, many proposals that are wildly different in their details agree that there is a fundamental length scale—the Planck length—at which the force of gravity becomes as strong as the other forces. Close to the Planck scale, ordinary physics, including the famous Heisenberg uncertainty principle, breaks down. Exactly how this breakdown happens is presently (wait for it!) uncertain.

When theory is silent, experiment must step in. A new paper analyzed results from the AURIGA gravitational wave experiment to check for deviations from standard quantum mechanics in the vibrations of a massive metal bar at cryogenic temperatures. The AURIGA results showed no deviation from standard quantum physics, yielding an upper bound on the energy of quantum gravity modifications. The experimenters concluded that the theorists needed to get back to work so that the experimenters have a better idea of what to expect.

So far, experimenters and theorists haven't had much to talk about. This is in part because the Planck length is extremely tiny: 1.6×10-35 meters, compared with the 10-15 meter or so occupied by an atomic nucleus. The particle physics approach to probing short distances is to crank up the energy of collisions, but these energies are well beyond the capabilities of any collider—including the Large Hadron Collider (LHC).

As a result, researchers have turned to other methods to search for quantum-gravitational effects. One such technique involves observing extremely high energy astrophysical events, though these carry their own measurement problems. In the laboratory, the focus has been on collective behavior in very cold systems. (For one such proposal, see my earlier Ars Technica article.)

AURIGA is the Antenna Ultracriogenica Risonante per l'Indagine Gravitazionale Astronomica, or "ultra-cryogenic resonant antenna for gravitational-wave astronomy." The detector consists of an aluminum cylinder 3 meters in length and weighing approximately 2.3 tons. The whole apparatus is cooled to a few thousandths of a degree above absolute zero to minimize the effect of thermal vibrations of its atoms. When a gravitational wave from a cataclysmic astronomical event reaches the detector, the aluminum bar should vibrate at its resonant frequency, like a wine glass when a certain musical note is played.

(To date, no gravitational waves have been detected directly, either by AURIGA or the larger laser-based detectors such as LIGO.)

Although this is not its primary purpose, AURIGA's ultra-cryogenic operation temperature made it a good candidate for searching for quantum-gravitational effects. According to quantum physics, mechanical systems will vibrate even at absolute zero, exhibiting something known as zero-point energy. If the Heisenberg uncertainty principle is modified by quantum gravity, then the zero-point energy would also be changed—and it's possible we could measure its increase.

The present analysis found an upper limit for quantum-gravitational effects in the AURIGA data by determining the vibrational energy of the aluminum bar. This is accomplished by coupling the bar to sensitive electrical readout instruments, which were designed for the detection of gravitational waves.

The limit the researchers found was not very stringent: there's still a lot of room for deviation from quantum mechanics at the resolution provided by this experiment. The lack of stringency in these limits leaves open the question of whether AURIGA is sensitive enough to measure quantum gravitational effects. However, these results are a proof of principle, since this stands as the first use of a very macroscopic body to probe physics at the smallest physical scales.

As the researchers emphasized, theory needs to catch up now. While some quantum gravity schemes make specific predictions for deviations from the Heisenberg uncertainty principle, they are often based on idealized systems or single-particle measurements. Promising techniques, including the AURIGA experiment, harness collective phenomena: the behavior of many coupled atoms. Theorists need to catch up by taking into account many-particle effects and measurement processes before any experimental deviation from quantum mechanics could be interpreted.

Nature Physics, 2012. DOI: 10.1038/nphys2503 (About DOIs).