Thousands of sensors spread across a square kilometer near the South Pole are tasked with answering one of the biggest unsolved questions in physics: Does quantum gravity exist? Sensors monitor neutrinos, particles that have no electrical charge and almost no mass, coming to Earth from space. A team from the Niels Bohr Institute (NBI) at the University of Copenhagen has contributed a method that uses neutrino data to discover whether quantum gravity exists.

“If quantum gravity really exists, as we believe, this would contribute to the unification of the two existing worlds of physics. Today, classical physics describes phenomena in our normal environment, such as gravity, while the atomic world can only be described by quantum mechanics.”

“Unifying quantum theory and gravity remains one of the most important challenges in fundamental physics. It would be great if we could contribute to this goal,” says NBI Associate Professor Tom Studtard.

Stuttard co-authored an article published in the journal Natural Physics. The article presents the results of a large study by the NBI team and their American colleagues. More than 300,000 neutrinos have been studied. But these are not the most interesting types of neutrinos originating from sources in deep space. The neutrinos in this study were created in Earth’s atmosphere when high-energy particles from space collided with nitrogen or other molecules.

“Looking at neutrinos originating in the Earth’s atmosphere has the practical advantage that they are much more common than their counterparts in space. We needed data from many neutrinos to validate our methodology. It is now complete, so we are ready to move on to the next stage, where we will study neutrinos from deep space,” says Stuttard.

Carelessly wandering the Earth

The IceCube neutrino observatory is located next to the Amundsen-Scott South Pole Station in Antarctica. Unlike many other astronomy and astrophysics setups, IceCube works best for observing space on the other side of the world, namely the northern hemisphere. This is because although neutrinos have the ability to penetrate our planet—even its hot, dense core—other particles will be stopped, so the signal is much cleaner for neutrinos coming from the northern hemisphere.

The IceCube facility is managed by the University of Wisconsin-Madison in the USA. More than 300 scientists from around the world participated in the IceCube collaboration. The University of Copenhagen is one of more than 50 universities with an IceCube center for neutrino studies.

Because the neutrino has no electric charge and is virtually massless, it is unaffected by electromagnetic and strong nuclear forces, allowing it to travel billions of light-years across the universe in its initial state. An important question is whether the properties of the neutrino actually change completely as it travels long distances, or whether small changes are still noticeable.

“If the neutrino undergoes the subtle changes we suspect, this would be the first convincing evidence of quantum gravity,” says Stuttard.

There are three types of neutrinos

Background information is needed to understand what changes in neutrino properties the team is looking for. Although we call it a particle, what we observe as a neutrino is actually the formation of three particles together, known as superposition in quantum mechanics.

Neutrinos can have three basic configurations (flavors, as physicists call them); these are electron, muon and tau. We observe which of these configurations change as the neutrino progresses; This is a truly strange phenomenon known as neutrino oscillations. This quantum behavior persists for thousands of kilometers or more and is called quantum coherence.

“In most experiments, coherence breaks down quickly. But this is not believed to be due to quantum gravity. It’s really hard to create ideal conditions in the laboratory. You need a perfect vacuum, but somehow a few molecules manage to get in there, and so on.”

“Instead, neutrinos are special in that they are not affected by the matter around them, so we know that if coherence breaks down it will not be due to flaws in the man-made experimental setup,” explains Stuttard.

Many of my colleagues were skeptical about this

When asked whether the results of the research have been published or not Natural PhysicsMeeting the expectations, the researcher answers: “We found ourselves in a rare category of scientific projects, namely experiments without an established theoretical basis. So we did not know what to do, but we knew that we could look for some general features that we could expect from the theory of quantum gravity.”

“Although we expected to see changes related to quantum gravity, the fact that we did not see them does not in any way exclude that they are real. When an atmospheric neutrino was detected in an object in Antarctica, it was usually passing through the Earth. This corresponds to a distance of about 12,700 km; it is a source of light coming from the distant universe.” “It is a very short distance compared to neutrinos. Frankly, quantum gravity, if it exists, requires a much larger distance,” said Stuttard, adding that the main purpose of the study was to create a methodology.

“For years, many physicists have doubted whether experiments could test quantum gravity. Our analysis shows that this is indeed possible, and with future measurements with astrophysical neutrinos, as well as more accurate detectors to be built over the next decade, we hope to finally answer this fundamental question.”