For decades, physicists have been searching for a model of quantum gravity that would unify quantum physics, the laws that govern the infinitely small, and gravity. A major obstacle has been the difficulty of experimentally testing the predictions of the candidate models. But some of the models predict an effect that can be probed in the lab: a very small violation of a fundamental quantum principle called the Pauli exclusion principle, which determines, for example, how electrons are arranged in atoms.
A project at INFN’s underground laboratories beneath Italy’s Gran Sasso mountains looked for signs of radiation produced by such a violation in the form of atomic transitions forbidden by the Pauli Exclusion Principle.
In two articles published in journals Physical examination letters (published September 19, 2022) and Physical examination D (accepted for publication December 7, 2022) the team reports that no evidence of violation has been found, so far, ruling out some models of quantum gravity.
In school chemistry lessons, we are taught that electrons can only organize themselves in certain specific ways in atoms, which turns out to be due to the Pauli exclusion principle. At the center of the atom is the atomic nucleus, surrounded by orbitals, with electrons. The first orbital, for example, can only hold two electrons. The Pauli Exclusion Principle, formulated by Austrian physicist Wolfang Pauli in 1925, says that two electrons cannot have the same quantum state; thus, in the first orbital of an atom, the two electrons have opposite “spins” (a quantum internal property usually represented as an axis of rotation, pointing up or down, although no literal axis exists in the electron).
The happy result of this for humans is that it means matter cannot pass through other matter. “It’s all over the place – you, me, we’re based on Pauli’s exclusion principle,” says Catalina Curceanu, member of physics think tank, Foundational Questions Institute, FQXi, and chief physicist of experiments at INFN, Italy. “The fact that we can’t walk through walls is another practical consequence.”
The principle extends to all elementary particles belonging to the same family as electrons, called fermions, and was mathematically derived from a fundamental theorem known as the spin statistics theorem. It has also been confirmed experimentally – so far – to appear valid for all fermions in tests. The Pauli Exclusion Principle is one of the fundamental principles of the Standard Model of particle physics.
Breach of principle
But some speculative models of physics, beyond the Standard Model, suggest that the principle may be violated. For decades, physicists have been searching for a fundamental theory of reality. The Standard Model is great for explaining particle behavior, interactions, and quantum processes at the microscopic scale. However, it does not encompass gravity.
Thus, physicists attempted to develop a unifying theory of quantum gravity, some versions of which predict that various properties that underlie the Standard Model, such as the Pauli Exclusion Principle, can be violated under extreme circumstances.
“Many of these violations occur naturally in so-called ‘non-commutative’ quantum gravity theories and models, such as those we have explored in our papers,” says Curceanu. One of the most popular candidate frameworks for quantum gravity is string theory, which describes fundamental particles as tiny threads of energy vibrating in multidimensional spaces. Some string theory models also predict such a violation.
“The analysis we reported disfavors some concrete realizations of quantum gravity,” Curceanu says.
It is traditionally thought that it is difficult to test such predictions because quantum gravity will generally only become relevant in arenas where there is an enormous amount of gravity concentrated in a tiny space – think of the center of a black hole or the beginning of the universe.
However, Curceanu and his colleagues realized that there might be a subtle effect – a signature indicating that the exclusion principle and the spin statistics theorem have been violated – that could be detected in lab experiments on Earth.
Deep in the Gran Sasso mountains, near the town of L’Aquila, Italy, the Curceanu team is working on the main experiment VIP-2 (Violation of Pauli’s Principle). At the heart of the device is a thick block of Roman lead, with a germanium detector nearby that can pick up small signs of radiation emanating from the lead.
The idea is that if Pauli’s exclusion principle is violated, a forbidden atomic transition will occur in the Roman wire, generating an X-ray with a distinct energy signal. This X-ray can be picked up by the germanium detector.
The laboratory must be housed underground because the radiation signature of such a process will be so weak that it would otherwise be drowned out by the general background radiation on Earth from cosmic rays. “Our laboratory ensures what is called ‘cosmic silence’, in that the Gran Sasso mountain reduces the flow of cosmic rays by a million times,” explains Curceanu. This alone, however, is not enough.
“Our signal has a possible rate of only one or two events per day, or less,” says Curceanu. This means that the materials used in the experiment must themselves be “radio-pure” – that is, they must not emit any radiation themselves – and the device must be shielded from the radiation. mountain rocks and radiation from underground.
“What’s hugely exciting is that we can probe certain models of quantum gravity with such high precision, which is impossible to do with current accelerators,” Curceanu says.
In their recent papers, the team reports that they found no evidence that Pauli’s Principle was violated. “The FQXi funding was fundamental in developing the data analysis techniques,” says Curceanu. This allowed the team to set limits on the size of any possible breach and helped them constrain some proposed quantum gravity models.
In particular, the team analyzed the predictions of the so-called “theta-Poincaré” model and were able to exclude certain versions of the model at the Planck scale (the scale at which the known classical laws of gravity break down). Additionally, “the analysis we reported disfavors some concrete realizations of quantum gravity,” says Curceanu.
The team now plans to expand their research into other models of quantum gravity, together with fellow theorists Antonino Marcianò from Fudan University and Andrea Addazi from Sichuan University, both in China. “On the experimental side, we will use new target materials and new analysis methods, to search for faint signals to unveil the fabric of spacetime,” says Curceanu.
“What is extremely exciting is that we can probe certain quantum gravity models with such high precision, which is impossible to do with current accelerators,” adds Curceanu. “It’s a big step forward, both theoretically and experimentally.”
Kristian Piscicchia et al, Strongest Atomic Physics Bounds on Noncommutative Quantum Gravity Models, Physical examination letters (2022). DOI: 10.1103/PhysRevLett.129.131301
Kristian Piscicchia et al, Experimental test of non-commutative quantum gravity by lead VIP-2, Physical examination D (2022). journals.aps.org/prd/accepted/ … 182249cd253e38bf3406
Provided by the Fundamental Questions Institute, FQXi
Quote: Underground Italian lab searches for quantum gravity signals (December 19, 2022) Retrieved December 19, 2022 from https://phys.org/news/2022-12-underground-italian-lab-quantum-gravity.html
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