The unification of quantum theory and gravitation remains one of the most outstanding challenges in fundamental physics today. One mystery is the quantum nature of spacetime—a fusion of the three dimensions of space and the fourth dimension of time—and whether it is subject to the randomness seen in other quantum theories, resulting in fluctuations at very small distances and times.
The oscillation between flavor states—electron, muon, and tau—of tiny, nearly massless particles called neutrinos is a quantum process, where any perturbations induced by quantum gravity would distort the neutrino flavor composition. Because neutrinos only interact through the weak force and gravity, they are largely isolated as they propagate through space. This isolation allows quantum coherence to be measured over macroscopic distances, acting as natural interferometers for probing the structure of spacetime.
In a paper published today in Nature Physics, the IceCube Collaboration presents a study that tested for neutrino decoherence as a result of small fluctuations in spacetime caused by quantum gravity. No evidence of neutrino decoherence was observed, but the world’s strongest constraints on neutrino-quantum gravity interactions were set, significantly improving on past limits in a range of scenarios.
A large high-energy sample of neutrinos detected by the IceCube Neutrino Observatory, a cubic-kilometer array embedded in ice, was used to probe for small fluctuations in spacetime that may emerge from quantum gravity. Specifically, the researchers looked at atmospheric neutrinos in the energy range of 0.5-10 TeV, using simulated and real data, and at distortions of observed energy- and direction-dependent neutrino flavor compositions due to spacetime fluctuations.
“Our results are over a million times stronger than the previous ones in well-motivated parts of the parameter space,” says Benjamin Jones, an associate professor at the University of Texas at Arlington (UTA), who co-led the analysis.
The analysis was also co-led by UTA PhD student Akshima Negi, assistant professor at the Niels Bohr Institute Tom Stuttard, and Jones’s graduated PhD student Grant Parker, who was the main analyzer and author of the study .
The search for decoherence as a result of quantum gravity is the latest in a series of high-energy tests for new physics delivered by IceCube, including searches for sterile neutrinos, nonstandard neutrino interactions, and neutrino decays. With the promise of a larger dataset and more precise measurements with the IceCube Upgrade and IceCube-Gen2, the search for evidence of new physics beyond the Standard Model will continue.
“In the area of quantum gravitational decoherence, the IceCube constraints are now sufficiently strong that it is unlikely that any neutrino experiment will surpass their sensitivity in the near term,” says Jones. “As such, tests of gravitational decoherence will need to focus on other particles, such as electron, photon, or atom interferometry.”
+ info “Searching for Decoherence from Quantum Gravity at the IceCube South Pole Neutrino Observatory,” IceCube Collaboration: R. Abbasi et al. Published in Nature Physics (2024), DOI:10.1038/s41567-024-02436-w, arXiv