Scientists have tested the relationship between gravity and quantum superposition

Physicists have tested the hypothesis that gravitational fluctuations in space-time can destroy quantum superposition and explain why large objects do not behave like particles. An experiment in the Gran Sasso underground laboratory did not reveal the predicted radiation. About why the result limits one of the theories of quantum decoherence, see the "Izvestia" material.

How gravity was linked to the disappearance of quantum effects

Quantum mechanics allows for the existence of particles in a superposition — a combination of several possible states simultaneously. However, this behavior is not observed in the usual macroscopic world. The process by which quantum effects disappear is called decoherence.

One hypothesis suggests that gravity itself may be involved. In the 1960s, the Hungarian physicist Fridjes Carojhazi proposed a model according to which space-time experiences inevitable small fluctuations — any random deviations of any quantity from its average or equilibrium value. They, in turn, could gradually destroy quantum superpositions.

Later, this idea was developed and refined by other researchers. The new work was aimed at experimentally verifying a generalized version of the Carohazi model. The study was published in the New Journal of Physics.

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What they were looking for deep under the Gran Sasso Mountain

It is impossible to directly detect the alleged space-time fluctuations. However, theory predicts their indirect effect: they should cause electrically charged particles to randomly move and accelerate. As a result, extremely weak electromagnetic radiation should occur.

To detect such a signal, the researchers conducted an experiment at the Gran Sasso National Laboratory in Italy. It is located under 1.4 km of rock, which protects the equipment from a significant portion of background radiation, including cosmic rays.

A detector with a high-purity germanium crystal about the size of a coffee mug was used for measurements. It was additionally protected with layers of copper and lead. The data was collected for 62 days, after which the expected background was excluded from the results and the remaining signal was compared with the predictions of the Carohazi model.

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Why didn't physicists detect the expected signal?

After processing the data, the researchers did not detect the radiation predicted by the model being tested. Thus, the experiment did not confirm the assumption that the gravitational fluctuations described in it lead to the observed effect.

The absence of a signal does not mean that gravity has nothing to do with quantum decoherence at all. The result excludes a specific version of the mechanism proposed in the framework of the Carohazi model and sets stricter limits for such theories. According to the researchers, the absence of the expected signal is in itself a significant result. Now physicists need to look for other explanations of how quantum mechanics and gravity are related.

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What does the experiment change in the search for quantum gravity?

The Carohazi model is based on the assumption of a fundamental limitation of the accuracy with which an object's position can be determined or distance measured. Similar ideas about minimum length arise in other approaches to combining quantum mechanics with gravity, including string theory and loop quantum gravity.

The authors of the study note that experiments already make it possible to verify some predictions of theories that were previously considered practically inaccessible to observations. Increasing the sensitivity of measuring devices can expand the possibilities of such checks.

Thus, the work does not solve the question of the nature of quantum decoherence, but it allows us to exclude one of the variants of its explanation. The next step will be to test other models that relate quantum effects to the fundamental properties of space-time.