Signal detected that behaves like a graviton, the much sought after gravity particle

Space

Editorial of the Technological Innovation Website – 04/02/2024

Grviton

Physicists have obtained the first experimental clue that can be interpreted as the most sought after of all particles in the Universe, the graviton.

The signal, which physicists call collective spin excitation, was generated by “entities” called chiral graviton modes. And a chiral graviton mode appears to be similar to a graviton, an elementary particle yet to be discovered and which, according to some theories, gives rise to gravity, one of the fundamental forces of the Universe, whose cause remains mysterious.

It all seems too uncertain – and – but the possibility of studying graviton-like particles in the laboratory could help fill critical gaps between quantum mechanics and Einstein’s theories of relativity, resolving a major dilemma in physics and expanding our understanding of the Universe.

The team discovered the excitation – a spike in the data – in a type of condensed matter called fractional quantum Hall effect liquid (FQHE). FQHE liquids are a system in which electrons interact strongly; Despite its name, the material is a two-dimensional semiconductor subject to high magnetic fields and low temperatures. These liquids can be described using quantum geometry, giving rise to mathematical concepts that apply to the tiny physical distances at which quantum mechanics influences physical phenomena.

Theories already indicated that chiral graviton modes could emerge in FQHEs when they are hit by light, but until now no one had managed to come up with an experiment capable of demonstrating this experimentally.

“Our experiment marks the first experimental confirmation of this concept of gravitons, postulated by pioneering work in quantum gravity since the 1930s, in a condensed matter system,” said Lingjie Du, from Columbia University, in the USA.

Grviton analogue

One of the most used techniques to study quantum phases in solid-state systems is called low-temperature resonant inelastic scattering, which measures how light particles, or photons, scatter when they hit a material – the way this scattering occurs reveals the underlying properties of the material.

The team adapted this technique to use circularly polarized light, in which photons have a specific spin. When polarized photons interact with a particle such as a chiral graviton mode, which also rotates, the sign of the photons’ spin changes in a stronger way than if they were interacting with other types of modes.

After spending three years building a laboratory to enable this unprecedented experiment to be carried out, the team observed physical properties consistent with those predicted by quantum geometry for chiral graviton modes, including their spin-2 nature, characteristic energy gaps between their ground states and and excited and dependence on so-called filling factors, which relate the number of electrons in the system to its magnetic field.

And, the theory predicts, all these characteristics are also typical of gravitons, as yet undiscovered particles that are predicted to be the carriers of the force of gravity. And, because they are so similar, chiral graviton modes can function as an analogue of gravitons – gravitons themselves would be the result of quantized metric fluctuations, in which the fabric of space-time is pulled and stretched randomly in different directions.

Study other phenomena

Although the “particle” is just an analogue of the graviton, the theory behind the team’s results may connect two subfields of physics: High-energy physics, which operates on the largest scales in the Universe, and condensed matter physics, which studies atomic and electronic materials and interactions.

The team now intends to work with higher energies and use other quantum materials where quantum geometry predicts the emergence of properties due to collective excitations, as happens in superconductors, for example.

“For a long time, there has been this mystery about how long-wavelength collective modes, such as chiral graviton modes, could be investigated in experiments. We have provided experimental evidence that supports the predictions of quantum geometry,” said Liu.

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