Italian underground laboratory searches for signals of quantum gravity

Italian underground laboratory searches for signals of quantum gravity

The Gran Sasso underground laboratory with low radioactivity. Photo credit: Massimiliano De Deo, LNGS-INFN

For decades, physicists have searched for a quantum gravity model that would unify quantum physics, the laws governing the very smallest, and gravity. A major obstacle has been the difficulty of experimentally testing the predictions of candidate models. However, some of the models predict an effect that can be studied in the laboratory: a very small violation of a fundamental quantum theorem called the Pauli Exclusion Principle, which governs, for example, how electrons are arranged in atoms.

A project carried out in the INFN’s underground laboratories beneath the Gran Sasso Mountains in Italy has been looking for signs of radiation produced by such injury in the form of atomic junctions prohibited by the Pauli Exclusion Principle.

In two articles appearing in the journals Physical Verification Letters (published September 19, 2022) and Physical Check D (accepted for release December 7, 2022), the team reports that no evidence of a breach has been found so far, ruling out some quantum gravity models.

In school chemistry lessons we are taught that electrons in atoms can only arrange themselves in a very specific way, which turns out to be a consequence of the Pauli exclusion principle. At the center of the atom is the nucleus, surrounded by orbitals containing electrons. For example, the first orbital can only hold two electrons. The Pauli Exclusion Principle, formulated by the Austrian physicist Wolfang Pauli in 1925, states that no two electrons can have the same quantum state; So in the first orbital of an atom, the two electrons have oppositely directed “spins” (an intrinsic quantum 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 man is that matter cannot pass through other matter. “It’s ubiquitous – you, me, we’re based on the Pauli exclusion principle,” says Catalina Curceanu, a member of the physics think tank, the Foundational Questions Institute, FQXi, and lead physicist on the experiments at INFN. Italy. “The fact that we can’t cross walls is another practical consequence.”

The principle extends to all elementary particles belonging to the same family as electrons, called fermions, and was derived mathematically from a fundamental theorem known as the spin statistics theorem. It has also been confirmed experimentally – so far – and appears to hold for all fermions in tests. The Pauli exclusion principle is one of the core theorems of the Standard Model of particle physics.

violation of the principle

But some speculative models of physics that go beyond the Standard Model suggest that the principle could be violated. Physicists have been searching for a fundamental theory of reality for decades. The Standard Model is excellent for explaining the behavior of particles, interactions and quantum processes on a micro scale. However, it does not include gravity.

Therefore, physicists have attempted to develop a unifying theory of quantum gravity, some versions of which predict that various properties underpinning 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 that we have examined in our work,” says Curceanu. One of the most popular candidates for quantum gravity systems is string theory, which describes fundamental particles as tiny vibrating filaments of energy in multidimensional spaces. Some string theory models also predict such a violation.

“The analysis we report contradicts some concrete findings of quantum gravity,” says Curceanu.

It has traditionally been thought that such predictions are difficult to test, as quantum gravity usually only becomes relevant in arenas where a vast amount of gravity is concentrated in a small space – think the center of a black hole, or the beginning of the universe.

However, Curceanu and her colleagues recognized that there might be a subtle effect – a signature that the exclusion principle and the spin-statistics theorem were violated – that could be picked up in laboratory experiments on Earth.

Deep under the Gran Sasso mountains, near the town of L’Aquila, in Italy, the Curceanu team is working on the lead experiment VIP-2 (violation of the Pauli principle). The heart of the apparatus is a thick block of Roman lead with a nearby germanium detector capable of detecting small signs of radiation emanating from the lead.

The idea is that when the Pauli exclusion principle is violated, a forbidden atomic transition takes place within the Roman lead, producing an X-ray beam with a distinct energy signal. This X-ray beam can be picked up by the germanium detector.

cosmic silence

The laboratory must be located underground because the radiation signature of such a process is so weak that it would otherwise be drowned out by the general background of cosmic rays on Earth. “Our lab provides what is called ‘cosmic silence,’ in the sense that Mount Gran Sasso reduces the flux of cosmic rays by a million times,” says Curceanu. But that alone is not enough.

“Our signal has a potential rate of just one or two events per day or less,” says Curceanu. This means that the materials used in the experiment must themselves be “radio-clean”, i.e. they must not emit any radiation themselves, and the apparatus must be shielded against the radiation from the mountain rock and the radiation from the subsoil.

“What is extremely exciting is that we can study some quantum gravity models with such a high level of precision, which is impossible with today’s accelerators,” says Curceanu.

In their most recent papers, the team reports finding no evidence of a breach of the Pauli Principle. “The FQXi funding was fundamental to the development of the data analysis techniques,” says Curceanu. This allowed the team to set limits on the size of possible injuries 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 rule out some versions of the model on the Planck scale (the scale at which the known classical laws of gravity break down). In addition, “the analysis we report contradicts some concrete findings of quantum gravity,” says Curceanu.

The team now plans to extend their research to other quantum gravity models with fellow theorists Antonino Marcianò of Fudan University and Andrea Addazi of Sichuan University, both in China. “On the experimental side, we will use new target materials and new analysis methods to look for faint signals to uncover the fabric of spacetime,” says Curceanu.

“What is extremely exciting is that we can study some models of quantum gravity with such high precision, which is impossible with today’s accelerators,” adds Curceanu. “This is a big leap, both from a theoretical and an experimental point of view.”

More information:
Kristian Piscicchia et al, Strongest Atomic Physics Bounds on Noncommutative Quantum Gravity Models, Physical Verification Letters (2022). DOI: 10.1103/PhysRevLett.129.131301

Kristian Piscicchia et al, Experimental Test of Noncommutative Quantum Gravity by VIP-2 Lead, Physical Check D (2022). … 182249cd253e38bf3406

Provided by the Foundational Questions Institute, FQXi

Citation: Italian Underground Laboratory Searches for Quantum Gravity Signals (2022, December 19), retrieved December 20, 2022 from

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