Sounding the Drums to Listen for Gravity’s Effect on Quantum Phenomena

Like jars of peanut butter, nature’s basic rules come in both smooth and chunky-style.

General relativity, which smoothly accounts for gravity and its effects on large objects ranging in size from galaxies to fly ash, describes events as continuous and deterministic so that every cause matches up to a specific, local effect. Chunky-style quantum mechanics, by contrast, accounts for the electromagnetic and nuclear forces and their effects on small things like atoms in a discontinuous, step-wise fashion.

Unlike smooth and chunky peanut butter, however, both of these fully validated conceptions of physical reality do not typically mix. At the borderline between large and small objects, the writ of one set of nature’s rules rather mysteriously falls away in favor of the other, a phenomenon that makes the ’mesoscale’-size range one of the least understood, least explored areas in physics. Yet research physicists led by Andrew Briggs at the University of Oxford, in the UK, are soon to start reaching into this terra incognita medio in hopes of finding clues that might help reconcile quantum mechanics with gravitation.

Quantum phenomena generally go unseen in the macro world, as nanoparticles’ delicate quantum properties almost always get smeared out through interactions with larger-scale objects. But under the right conditions odd effects like quantum superpositions—the ability for an atom to be in multiple energy states at once, for instance—can proliferate sufficiently to appear where we can observe them.

It’s no surprise then that recent advances in nanotechnology and cryogenic engineering have led Briggs, and his Oxford co-investigators, Edward Laird and Natalia Ares, to become interested in the quantum manipulation of macroscopic mechanical systems. They have constructed a bench-top lab apparatus that they hope will be able to achieve the extremely sensitive measurements needed to pick out gravity’s effect on quantum superposition—if it has one. Their project is funded by a grant of almost $115,000 from the Foundational Questions Institute.

The set-up involves tiny, but still macroscale, oscillators—extremely thin, millimeter-size membranes of stiff silicon nitride ceramic—placed inside a vacuum-filled cavity. The oscillators operate like "tiny trampolines or drums," says Ares. Once stimulated to vibrate, they ring true for many seconds like tuning forks, she says.

The plan is to cool the oscillators using a dilution-refrigeration system to just around 10 to 15 milliKelvin above absolute zero. At this low temperature, enough energy has been removed from the oscillators to make mechanical vibrations grow nearly quiet. In this quiet environment, the researchers hope to detect a competition between two effects: The electrical interactions between neighboring atoms favors creating a superposition, but gravity will try to collapse it. This provides a platform on which to study the interplay between gravitational and quantum physics.

Gravitational Kicks

The theory the team is testing is based on the work of team member Gerard Milburn, a quantum physicist at the University of Queensland, in Brisbane, Australia. It predicts that gravity forces the atoms’ superposition to collapse into definite states. "Because the gravitational force between atoms is so tiny, this effect would not yet have been detected," Ares explains. "This constant collapse of superpositions applies tiny random kicks to the atomic positions, which heats up the material."

The fact that the oscillators act as drums gives a way to sensitively measure the predicted heating. The temperature rise causes the drum to vibrate a tiny bit more, which in principle could be detected. "We hope to monitor the heating due to these gravitational kicks, although measuring that heat will be difficult because it will require an extremely low noise level," Laird says.

Even if the experiment is successful, Laird notes, the best way to check the results would be to carry out similar experiments in orbit, where there will be less background vibration.

Jörg Schmiedmayer, a quantum physicist at the Technical University of Vienna, in Austria, says it will be important to see if nano-mechanical systems can yield large enough gravitational effects to be measurable. "In most of these schemes, any quantum effects are destroyed extremely easily," says Schiedmayer. "Hopefully, their instrumentation will be sensitive enough to produce a clear-cut result."