Experiments to test quantum gravity have become a little easier

Is gravity a quantum phenomenon? This has been one of the major open problems in physics for decades. Along with a British colleague, Anupam Masudar, a physicist at the University of Groningen, proposed an experiment that could solve the problem. However, we need to study two very large entangled quantum systems in free fall. In a new treatise, headed by a third-year bachelor’s degree, Mazumdar shows how to reduce background noise to make this experiment more manageable.

Three of the four fundamental forces in physics can be explained in terms of quantum theory. This does not apply to the fourth force (gravity) explained by Einstein’s general theory of relativity. Experiments previously designed by Masudar and his colleagues may prove or disprove the quantum nature of gravity.


A well-known result of quantum theory is a phenomenon called quantum superposition. In certain situations, a quantum state can have two different values ​​at the same time. Takes the electrons irradiated by the laser beam. According to quantum theory, photon energy from light can be absorbed or not absorbed. When it absorbs energy, the spin of the electron changes. This can cause the magnetic moment to go up and down. As a result of quantum superposition, the spin is both up and down.

These quantum effects occur in small objects such as electrons. By targeting the electrons of a specially constructed miniature diamond, you can create superpositions on much larger objects. The diamond is not only small enough to maintain this superposition, but large enough to feel gravity. This property is used by experiments. Two of these diamonds are placed next to each other in free fall, thus offsetting external gravity. This means that they interact only through the gravity between them.


And then another quantum phenomenon occurs. Entanglement means that when two or more particles are generated in close proximity, their quantum states are linked. For diamonds, if one is spinning up, the other intertwined diamond must spin down. Therefore, the experiments are designed to determine if entanglement occurs in pairs during free fall when gravity between diamonds is the only way to interact.

“But this experiment is very challenging,” explains Mazdal. When two objects are very close together, there is another possible mechanism of interaction, the Casimir effect. In a vacuum, this effect allows two objects to attract each other. “The size of the effect is relatively large and you need to use a relatively large diamond to overcome the noise it produces.” The first thing you need to do to reduce this noise is to make your experiment easier to handle. It was clear from. Therefore, Masudar wanted to know if it was possible to shield the Casimir effect.


He passed this question on to Thomas van de Kamp, a third-year physics student. “He came to me because he was interested in quantum gravity and wanted to do a bachelor’s thesis research project,” says Mazdal. Van de Camp began to tackle the problem when most regular classes were interrupted during the spring blockade. “In a very short time, he presented his solution as described in our treatise.”

This solution is based on placing a copper conductive plate about 1 mm thick between two diamonds. The plate shields the Kashmir potential between them. Without the plate, this possibility brings the diamonds closer together. But with plates, the diamonds are not attracted to each other, but to the plates between them. Mazdal: “This removes a lot of noise from the experiment because the Casimir effect removes the interaction between the diamonds.”


Calculations performed by Van de Kamp show that the mass of two diamonds can be reduced by two orders of magnitude. “It may seem like a small step, but it makes the experiment less demanding.” In addition, other parameters such as the level of vacuum required during the experiment are also less demanding due to the Casimir effect shield. Become. Mazumdar states that further updates to the experiment may occur in the near future, including a contribution from a bachelor’s degree student, Thomas van de Kamp. “So his six-month project brought him the co-authorship of two treatises. This is a very amazing achievement.”


Thomas W. van de Kamp et al, Quantum gravity witness via entanglement of masses: Casimir screening, Physical Review A (2020). DOI: 10.1103/PhysRevA.102.062807

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