Evrim Yazgin
Cosmos science journalist
Scientists are going to extreme lengths – and places – to try and understand the fundamental nature of the universe.
Physics as we know it doesn’t seem to agree with itself. Gravity and quantum mechanics are two pillars of modern physics, but they don’t gel well together. One theory which suggests that there is a way of marrying the two worlds of physics is called “quantum gravity.”
“Today, classical physics describes the phenomena in our normal surroundings such as gravity, while the atomic world can only be described using quantum mechanics,” says Tom Stuttard from the University of Copenhagen’s Neils Bohr Institute (NBI). “The unification of quantum theory and gravitation remains one of the most outstanding challenges in fundamental physics. It would be very satisfying if we could contribute to that end.”
Stuttard is co-author of a paper published in Nature Physics which suggests that neutrino data from the IceCube Neutrino Observatory at the South Pole might be used to find evidence for quantum gravity.
More than 300,000 neutrinos have been studied at IceCube. These aren’t neutrinos from outer space, but those created in Earth’s atmosphere.
“Looking at neutrinos originating from the Earth’s atmosphere has the practical advantage that they are by far more common than their siblings from outer space. We needed data from many neutrinos to validate our methodology. This has been accomplished now. We are ready to enter the next phase in which we will study neutrinos from deep space,” Stuttard explains.
Neutrinos are sometimes called “ghost particles” because they have no electrical charge and are nearly massless, meaning they don’t interact with the electromagnetic field or the nuclei of atoms. They can travel billions of lightyears through the universe largely unbothered.
“If the neutrino undergoes the subtle changes that we suspect, this would be the first strong evidence of quantum gravity,” says Stuttard.
While the search hasn’t yielded results that suggest the existence of quantum gravity yet, Stuttard and his colleagues remain hopeful. Their top priority has been to prove that experiments could one day prove the existence of quantum gravity.
“For years, many physicists doubted whether experiments could ever hope to test quantum gravity. Our analysis shows that it is indeed possible, and with future measurements with astrophysical neutrinos, as well as more precise detectors being built in the coming decade, we hope to finally answer this fundamental question,” Stuttard says.
Future experiments which look at neutrinos from space, rather than atmospheric neutrinos, could answer the question once and for all.
“While we did have hopes of seeing changes related to quantum gravity, the fact that we didn’t see them does not exclude at all that they are real,” Stuttard notes. “When an atmospheric neutrino is detected at the Antarctic facility, it will typically have travelled through the Earth. Meaning approximately 12,700 km – a very short distance compared to neutrinos originating in the distant universe. Apparently, a much longer distance is needed for quantum gravity to make an impact, if it exists.”
