Scientists Test for Quantum Gravity

The tension between quantum mechanics and relativity has long been a central split in modern-day physics. Developing a theory of quantum gravity remains one of the great outstanding challenges of the discipline. And yet, no one has yet been able to do it. But as we collect more data, it shines more light on the potential solution, even if some of that data happens to show negative results.

That happened recently with a review of data collected at IceCube, a neutrino detector located in the Antarctic ice sheet, and compiled by researchers at the University of Texas at Arlington. They looked for signs that gravity could vary even a minuscule amount based on quantum mechanical fluctuations. And, to put it bluntly, they didn’t find any evidence of that happening.

To check for these minuscule fluctuations, they analyzed more than 300,000 detected neutrinos that IceCube had captured. IceCube is an impressive engineering feat, with thousands of sensors buried over one sq km in the ice. When one of the detectors is triggered by one of a hundred trillions of neutrinos passing through it every second, data on whether it was affected by any perturbations in the local gravity of that area can be collected.

Fraser discusses the neutrino detectors of IceCube.

Such massive data sets allowed for a very accurate reading—”over a million times more [accurate],” according to Dr. Benjamin Jones, one of over 300 physicists who worked on a paper detailing IceCube’s findings, which he described in a press release from the University of Texas at Arlington. Despite that, the researchers were still unable to find any evidence for those quantum fluctuations in the local gravitational field.

That’s not all bad news, though. Eliminating one possible explanation for quantum gravity could lead to work on others. Dr. Jones sees that prospect as he describes how his lab’s efforts are shifting to studying the mass of neutrinos themselves. Understanding more about these elusive particles certainly won’t hurt efforts to understand the overall physical model of the universe. Still, many scientists are likely disappointed by this newest failure to find a potential lead in the solution to a “theory of everything.”

For now, IceCube will keep collecting data, and scientists will continue to analyze it. But efforts to find a new theory of quantum gravity seem to be back at the theoretical drawing—which is a necessary step before they can be tested, no matter how fancy the detector itself is.

PBS Spacetime explains the idea behind quantum gravity.

Learn More:
IceCube Collaboration – Search for decoherence from quantum gravity with atmospheric neutrinos
UT – Scientists are Recommending IceCube Should be Eight Times Bigger
UT – IceCube Makes a Neutrino Map of the Milky Way

Lead Image:
IceCube Lab under the stars in the Antarctic.
Credit – IceCube/NSF

One Reply to “Scientists Test for Quantum Gravity”

  1. The IceCube test is not for quantum gravity in general but testing those theories that specifically quantize the spacetime metric. E.g. it doesn’t test a simplest effective quantum gravity field theory that quantize the gravitational field against a flat – or nearly so – background. [C.f. “Quantum gravity as a low energy effective field theory”, John F Donoghue (2017), Scholarpedia, 12(4):32997.]

    There are other ways to test gravity quantization.

    One of the perhaps clearest methods is looking for entanglement, which is hard since gravity is so weak.

    Another method, which was used to show that the electromagnetic field is a quantum, not classical field, is to show an Aharonov-Bohm phase effect [“Aharonov–Bohm effect”, Wikipedia]. That was recently shown to be the case for gravity as well. [“Physicists detect an Aharonov–Bohm effect for gravity”, Physics World, 25 Jan 2022.]

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