In the late 1960s theastrophysicistastrophysicist American theorist John Bahcall expressed his depression during a seminar in which he presented his theory of the flow of water neutrinosneutrinos produced by thermonuclear reactions that make the sun shine. His friend and colleague Raymond Davis, behind an experiment to detect this current at the Homestake gold mine in South Dakota (United States), now famous as Homestake experiment, had just shown that there were fewer neutrinos than predicted by Bahcall’s calculations. The scene took place at the legendary Caltech where the equally legendary Richard Feynman was based. The Nobel laureate in physics attended the seminar and Feynman tried to cheer up Bahcall by telling him that he too saw no errors in his neutrino flux calculations.


Olivier Drapier, researcher at the Leprince-Ringuet laboratory of the École Polytechnique, CNRS, talks to us about neutrinos, these particles of matter that can be used to study stars and the universe. © Polytechnic School

Particles that periodically merge into each other

Remember that these particles are neutral, have a small mass, and interact only very weakly with other matter particles. through the weak nuclear forceweak nuclear force and gravity. Neutrinos are therefore very penetrating. But in the 1950s the physicistphysicist The Italian Bruno Pontecorvo, inspired by the work of Murray Gell-Mann on quantum transformations due to oscillation between a particle called kaon zero in sound antiparticleantiparticleintroduces the basis for a similar oscillation phenomenon involving neutrinos converting into each other.

In the early 1960s, Lederman, Schwartz and Steinberger discovered that there is a second neutrino called muonic, while the first is called electronic. Pontecorvo’s theory allows us to consider that these neutrinos are continuously transformed into each other according to the laws of probability determined by the Quantum mechanicsQuantum mechanics. The discovery of a third neutrino, the tauonic one, makes it possible to generalize the Italian theory with oscillatory transformations in time between these three types of neutrinos.

It will prove to be the missing solution to the solar neutrino riddle. On the way between the Sun’s core and the Davis detector – designed to detect only electronic neutrinoselectronic neutrinos, the only ones that should come from the sun, some of these had been transformed into the other two flavors of neutrinos, as physicists say in their jargon. The neutrinos were all there in the end, but some were undetectable as part of the neutrinos Homestake experiment. The theory has since been confirmed by several experiments, such as the experiment called T2K in Japan.

In recent years, several researchers have developed a fascinating theory, a consequence of the existence of neutrino oscillations. It should make it possible to do what is sometimes considered an oxymoron quantum gravityquantum gravity experimental. Remember, a quantum theory of gravity should allow us to understand howUniverseUniverse observable began at the time of Big bangBig bang and how it generated matter and galaxies. It should also make it possible to unravel the mysteries of water evaporation black holesblack holes. However, quantum gravity is notoriously difficult to test experimentally.

A quantum space-time in turmoil

To understand the relationship between neutrino oscillations and quantum gravity, we must go back to the late 1950s, when the legendary John Wheeler began his search for a unified theory ofspace timespace timematter and forces, including quantum mechanics.

Wheeler then wondered about the effect of quantum fluctuations on the structure of space-time. Seeing analogies between the properties of space-time governed by Einstein’s equations of general relativity and those of fluid mechanics, he concluded that space-time must have an aspect offoamfoam or boiling water on the scale of PlanckPlanck (at lengths of the order of 10-35 M). Just as a rough ocean looks smooth from space, the geometry of space-time appears continuous and topologically simple only because the famous Shelf lengthShelf length is incredibly small compared to a atomatom D’hydrogenhydrogenwhose radius is 10-10 m environment.

But if we have a microscopemicroscope sufficiently powerful, for example with the particle beams of an accelerator, we would see appear turbulenceturbulence of quantum gravity. They would take the form of pairs of charged mini-black holes, neutral mini-black holes, and even mini-wormholes that continually appear and disappear into the void, such as the virtual pairs d |8c2e79f61651d25c416bd37623226a2e|-positronpositron responsible for l’effet Lamb.


A video presentation from IceCube for its 10th anniversary. To get a fairly accurate French translation, click on the white rectangle at the bottom right. English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Auto translate”. Choose “French”. © IceCube Neutrino Observatory

Today, members of the IceCube collaboration – more than 300 researchers involved in using a massive neutrino detector in the ice beneath the Amundsen-Scott Station at the South Pole in AntarcticAntarctic – just published an article in Natural physics which explains why they were able to study the effect of the foam structure of space-time, produced by pairs of virtual black holes, on the atmospheric neutrino flux measured by IceCube. The article, which is freely available at arXiv, specifies that the researchers were inspired by a theory of quantum gravity about these pairs of virtual black holes that we owe to Stephen Hawking.

Hawking had sought a more detailed description of Wheeler’s quantum foam using his work on quantum field theory in curved space-time applied to black holes. He concluded that virtual black holes could cause a quantum decoherence effect on the particle fields propagating in space-time, disturbed by pairs of virtual microscopic black holes that continually appear and disappear.

A sound that produces a decoherence of ‘quantum music’

The question is actually very technical. We say that the evolution of a quantum state is according to the famous Schrödinger equationSchrödinger equation is usually unitary, that is, it is done according to a mathematical law that has the property of unitarity. If we translate the idea a little into radio waves, we basically say: business suitbusiness suit The waves that describe a piece of music move in space and time, while the shape, coherence and therefore the entire information that defines the piece of music is preserved. A violation of unitarity causes a decoherence of the wave packet, which becomes noisy due to external disturbances, so that eventually we are no longer able to extract the original piece of music if the environmental disturbance is carried out over a sufficiently long path. the wave package. All that’s left is noise.

Some of IceCube’s neutrinos are made up of neutrinos produced by cosmic rayscosmic rays on high energiesenergies collide with moleculesmolecules from the high atmosphereatmosphere on the north Pole. The Earth then acts as a screen that filters out the common particles generated by these collisions. But because neutrinos are very penetrating and interact only weakly with other matter particles, they reach the IceCube detector at the South Pole. More than 300,000 of these neutrinos have been detected, which are fairly isolated from environmental disturbances. Oscillation theory predicts a ghostghost in energies and flavors of these very specific neutrinos measurable by IceCube, the analogue of the wave packet for an earlier piece of music.

But by applying the Hawking-inspired theory of quantum foam, which causes a perturbation of the environment, we should see a distortion of the spectrum more and more clearly as we consider more and more neutrinos that are more energetic and depend on the distance traveled in space is taken. which allows to accumulate the effects of quantum foam on the coherence of neutrino oscillations.

Ideally, we should have very high energy neutrinos, billions of which have passedlight yearslight yearsbut very few were detected with IceCube.

Yet the number of detected atmospheric neutrinos that have passed through Earth still allows us to put limits on the decoherence effect. Another way to see this effect is to consider that an evaporating quantum black hole only remembers the mass, the electric charge and the mass. cinematic momentcinematic moment particles of matter he had swallowed. Therefore, he does not remember the flavors of neutrinos he absorbed while evaporating, he has forgotten the music.

The pairs of virtual black holes, by swallowing a taste of neutrinos that disappears through evaporation, will spit out another that may be different and no longer corresponds to what we would expect due to a transformation of one taste into another through the usual oscillation process .

As it stands, no quantum decoherence effect has been found, but given the limitations of the experiment and the detector, this does not mean that a decoherence effect is not produced by quantum gravity. through the foam of space-time.

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