A twist in the search for quantum gravity

A theoretical superconductor in which pairs of elementary particles circulate around a chiral axis displays quantum behaviours that could reveal the elusive secrets of quantum gravity, a study reported in JETP Letters suggests.

There are four known fundamental forces in physics – gravity, electromagnetism, and weak and strong elementary interactions – but only gravity defies explanation from a quantum perspective. Although Einstein’s general theory of relativity has served as a robust classical description of gravity that holds true under most circumstances, the classical theory breaks down under extreme conditions such as the centre of black holes where exotic quantum effects are thought to become dominant.

It has proved exceptionally challenging to study and theorize what ‘quantum gravity’ could entail because its effects only become apparent at extremely small length scales and high energies far beyond the capabilities of today’s particle accelerators.

Grigory Volovik, from Aalto University in Finland, and the Landau Institute for Theoretical Physics in Moscow, has proposed that an exotic chiral superconductor could realize some of the anomalies that might explain quantum gravity by reproducing the physics of the very early Universe.

“In the early Universe, elementary particles such as electrons and quarks had no mass,” explains Volovik. “This changed during the so-called ‘electroweak’ phase transition, when particles acquired mass. The massless ‘Weyl’ particles are chiral, meaning they have a preferred spin direction, either right- or left-handed with respect to the direction of motion. Although there are no Weyl particles around today, we can simulate chiral systems in which quasiparticles – analogues of elementary particles – obey Weyl dynamics.”

In his paper, Volovik considers two chiral systems: electrically neutral superfluid helium-3, and its electrically charged analogue as a chiral superconductor. In the neutral superfluid, the motion of quantized vortices produces conditions analogous to the primordial massless soup of the early Universe during the electroweak phase transition. This system manifests the theorized chiral anomaly – a quantum phenomenon required to explain the excess of matter over antimatter in our Universe.

Volovik suggests that the charged chiral superfluid, being a superconductor with Weyl quasiparticles, also manifests another type of chiral anomaly – a gravitational anomaly that is used to unify gravity with the weak and strong interactions in high energy physics.

“In such a chiral superconductor, the electric and magnetic fields act on these quasiparticles in the same way as the fundamental elements of the gravitational field,” says Volovik. “I propose that this gives rise to an effect analogous to the quantum gravitational anomaly, which could allow us to study gravity on its most fundamental level as a step toward the construction of a theory of quantum gravity.”