Physicists have carried out an experiment that has ruled out one of the old theories explaining why quantum effects, such as the famous Schrödinger's cat, which is both alive and dead at the same time, are not observed in everyday life. Over two months of measurements in a laboratory deep beneath a mountain in Italy, scientists failed to detect the faint radiation that the theory predicted. According to it, gravity destroys quantum states.
The phenomenon that explains why particles can behave according to the laws of quantum mechanics, while the large objects we are used to follow the laws of classical physics, is called decoherence. "One of the deepest questions in modern physics is why the strange quantum behavior that governs atoms and elementary particles disappears in the macroscopic world we see every day," says Catalina Curceanu, scientific director and spokesperson of the VIP collaboration at the Italian National Institute for Nuclear Physics.
In the 1960s, the Hungarian physicist Frigyes Karolyházy suggested that spacetime itself constantly undergoes tiny fluctuations, that is, random oscillations. According to his model, these disturbances gradually destroy quantum superposition, the state in which an object can be in several places or states at once. The idea linked quantum mechanics with Einstein's general theory of relativity, which describes gravity as the curvature of spacetime.
Although these supposed fluctuations cannot be measured directly, they should have a noticeable consequence. They would force charged particles to undergo tiny random accelerations, causing them to emit very weak electromagnetic radiation. Detecting this radiation, or proving its absence, is a way to test the theory.
The experiment was conducted at the Gran Sasso National Laboratory, the largest underground laboratory in the world for fundamental research. It lies beneath 1.4 kilometres of rock, which shields sensitive instruments from cosmic rays. The researchers used a crystal of pure germanium the size of a coffee mug, surrounded by layers of copper and lead. After analysing data collected over 62 days and subtracting the expected background noise, they found no signal of the kind predicted by the Karolyházy model. The results are published in the journal New Journal of Physics.
"The absence of a signal is itself an important scientific result," says Catalina Curceanu. The measurements rule out the tested version of the Karolyházy model and place new constraints on theories that explain quantum decoherence through fluctuations of spacetime.
The results do not prove that gravity plays no role in this process. Other gravitational mechanisms remain possible, and the question of how quantum mechanics and general relativity are connected is still unresolved.
By ruling out one of the oldest and most natural models of decoherence driven by gravity, this work narrows the search for a theory describing the interplay between gravity and quantum mechanics, and brings us one step closer to understanding one of the deepest mysteries of fundamental physics.
— Catalina Curceanu, lead author of the study
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