To prove gravity is quantum, use a laser, not a mass

The mystery of the possible quantum nature of gravity could be solved with lasers – a major contrast with the conventional approach of using masses, say ANU physicists.

Quantum mechanics’ success in describing light and particle behaviour in other forces suggests that gravity, too should be quantised – although this notion has some detractors, notably high-profile physicists such as Albert Einstein and Roger Penrose.

The ANU team’s paper in Physical Review Letters proposes experiments that could for the first time resolve this problem, said Zain Mehdi, a PhD student in the Department of Quantum Science and Technology, who led the research.

“It provides a pathway to working out the true nature of the universe,” he said.

To date, no experiments or theory have been devised that can satisfactorily describe the quantum nature of gravity. One reason for this is that the quantum of gravity, the graviton, would be so small as to make it basically impossible to measure directly, said Dr Simon Haine.

“You’d need apparatus 37 orders of magnitude larger than LIGO,” he said.

“That would equate to an interferometer of the scale of galaxies, with components so massive that they would probably collapse and form black holes.”

A precision quantum measurement specialist, Dr Haine discussed the problem with Mr Mehdi, who was tutoring in general relativity. Together they came up with a different approach, Mr Mehdi said.

“A lot of the people working on this come from the field of quantum information, and they’re not familiar with how to measure small effects.

“We started by asking what signatures a quantum gravity field would produce, and looked at which signature might have the highest likelihood to produce a measurable result,” he said.

Surprisingly, the two concluded that light held potential. Although photons have no mass, they do have energy density that interacts with gravity.

“Light would seem to be the worst probe, because it interacts so weakly with gravity,” Dr Haine said.

“But with light we do know how to control the quantum state better than any other particle.”

The team devised a system using an elongated Mach Zehnder interferometer with two parallel arms close to each other, to maximise the gravitational interaction between the photons in each arm.

While the interaction would not be strong enough to bend the beams, the effect of the beams on each other should be detectable. Most importantly, the output of the interferometer would be different if gravity is quantised, and the field was in a superposition of states, than if gravity is classical.

Quantum squeezing would also enable the quantum effects to be magnified, by employing it in the opposite way to that used in LIGO, in which it is used to reduce quantum noise.

“With an optical system you can repeat the measurement and build up the statistics, to reveal nuances of the interaction,” Dr Haine said.

The fundamentally relativistic nature of light would also give the experiments access to information that low energy masses could not: the team believe they could distinguish whether the graviton has spin zero or two.

“The experiment is futuristic – if you showed the numbers to an experimentalist today they would say it’s crazy – but it is possible,” Mr Mehdi said.

The team calculated that the laser power required would be around 100 megawatts circulating in an optical cavity. With a high finesse cavity that could be achieved with a 100-kilowatt laser, a figure that is possible with today’s technology. However the noise requirements are stringent, said Mr Mehdi.

“You’d need a very quiet laser. The precision of the experiments you would need to perform with this much laser power is out of this world.”

Insights into the nature of gravity would address a fundamental mystery of quantum mechanics, Dr Haine said.

“It’s the measurement problem. Is it quantum all the way down? Or is there something classical at the bottom of it all to get us out of the problem that everything could be in a superposition – which is uncomfortable for a lot of people.”

Reference
Signatures of Quantum Gravity in the Gravitational Self-Interaction of Photons

Zain Mehdi, Joseph J. Hope, and Simon A. Haine

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.240203

Australian National University