Quantum gravity, cosmic neutrinos could help us understand it

Quantum gravity

Modern physics rests on two great pillars, both of which were cast in the first half of the last century. Quantum mechanics, which with its laws describes the behavior of waves and particles on microscopic spatial scales, and general relativity, which explains the behavior of gravity in terms of a sort of deformation of space-time, the four-dimensional structure in which we are immersed. Separately, quantum mechanics and general relativity work perfectly, and both have been (and continue to be) experimentally verified with ever greater precision; the problem is that when physicists try to fit them into a single theoretical framework, to harmonize them into a more general theory that also describes gravity in quantum terms, things stop working. At the moment, the quantization of gravity is one of the most important unsolved problems in physics: which is why every small step in this direction is greeted with great enthusiasm by the physics community. This is what just happened to the IceCube detector, a huge device built in the ice of Antarctica to capture and analyze particles arriving from Space, the so-called cosmic neutrinos: the scientists working on the experiment (an international collaboration that also includes the 'Italy), in fact, have just shown that cosmic neutrinos can be used to infer information on the theory of quantum gravity. Details of the research were published in the journal Nature Physics.

Before trying to go into the details of the work - which are, as it is easy to guess given the theme, quite technical and complex - let's take a look at the state of the art. One of the currently most promising ways to build a theory of quantum gravity involves the existence of a sort of spacetime foam: this is a prediction common to many theoretical models, according to which space on microscopic scales would not be continuous, but it would have, in fact, a foamy nature. We can clarify this with an analogy: our current geometric description of space-time is somewhat reminiscent of the geometry of an ideal sheet, a sheet that responds to stresses (for example to a mass resting on it) by bending or becoming more taut. but still remaining smooth, or more precisely characterized by a continuous geometry. The "foam" hypothesis predicts that continuous geometry is only a first approximation, a rough image, of space-time: in a more accurate microscopic description, the sheet should be in a certain sense "porous", just like a foam. The porosity of this grain should change rapidly and dramatically as spatial distances get very small. And the problem lies precisely in this: the size of the elements that make up this foam is too small to be measured directly, and you must (try to) access it indirectly, for example by studying if and how other particles (such as photons) interact with it. Among others, Giovanni Amelino-Camelia, professor of theoretical physics at the University of Naples and one of the leading experts in quantum gravity, took care of it: "Up to 15 years ago - he told us - being able to observe the texture of space - time seemed impossible, now we have shown that it can be done. And we realized that the foam is even more impalpable than we thought ".

What does all this have to do with neutrinos and with IceCube? "Cosmic neutrinos - explained Piera Sapienza, researcher at the Southern National Laboratories of the Institute of Nuclear Physics (Lns-Infn) and one of the founders of Km3Net, a similar experiment (and in some way" competitor ") of IceCube, which studies cosmic neutrinos with a detector placed 3500 meters under the sea, off the coast of Capo Passero in Sicily - they are particles able to travel for very long distances and reach us with very high energy, bringing very valuable information on cosmic phenomena that otherwise they would be unfathomable. Today we know that there are three "types" of neutrinos, which we call "flavors": the electron neutrino, the mu neutrino and the tau neutrino, and that neutrinos can "oscillate" between these flavors, transforming one into the other. The “classical” theory foresees that the neutrinos that reach the detectors such as IceCube or Km3Net are equalized between the three flavors; if not, it would mean that there is an anomaly due to a physics that we do not yet know, including possible effects of quantum gravity ". In short, the idea is that quantum gravity could have an effect on the oscillations of cosmic neutrinos, and that we might be able to measure this effect. Or rather, only some types of quantum gravity: "The IceCube study - specifies Amelino-Camelia, who among other things conducted two IceCube neutrino analyzes (this and this) with a different approach than the article just published - has to do with the possible effect of a particular subgroup of quantum gravity models on neutrino oscillations, the so-called non-universal models, that is, those that predict that the effects of quantum gravity are different on different particles ".

Thus we arrive at the results: the new data (which are not very many: 60 events in over seven and a half years of observation, which gives the idea of ​​how difficult it is to study phenomena of this type) have not found anomalies in the equipartition of neutrinos potentially associated with new physics and in particular with quantum gravity effects. It may seem disappointing, but it is not at all: "Perhaps the most relevant aspect - says Sapienza - is a further demonstration of how cosmic neutrinos, normally used to investigate extreme cosmic phenomena, can also prove to be important in the study of physics. of particles, revealing itself to be a sort of 'bridge' between astrophysics and particle physics. Furthermore, an apparently 'negative' result is actually very important, because excluding something helps theorists to move on to another path, indicating a direction by exclusion ". Amelino-Camelia also shares the same opinion: "The study just published by IceCube - he concludes - is very significant, especially for the power of the experimental limit obtained on 'non-universal' effects".