In the phenomenon of quantum superposition, the quantum state of a particle is indeterminate and it can assume several states at the same time. Only a measurement causes the superposition to collapse and determines the state of the particle. This superposition forms the basis for quantum computers and many other quantum physics applications. It has already been demonstrated in atoms, ions, molecules and even metal lumps made up of thousands of atoms.
But why doesn’t this overlay also work with “normal” objects? “Why the quantum behavior that determines atoms and elementary particles disappears in our everyday macroscopic world is one of the most fundamental questions in modern physics,” explains senior author Catalina Curceanu from the Italian National Institute for Nuclear Research.
The thought experiment of Schrödinger’s cat: The animal sits in a box with radioactive material and poison. If a nuclear decay occurs, she dies. But unless you look, her condition is undetermined – she is dead and alive at the same time. — © Dana Kulbayeva/ iStock
Is spacetime to blame?
The Hungarian physicist Frigyes Károlyházy postulated a possible cause in the 1960s: He suspected that gravity causes the superposition in the macroscopic world to collapse. Specifically, he assumed that the space-time defined by Albert Einstein was “spongy” at the smallest scale and exhibited tiny fluctuations. These “erode” the coherence of the quantum superposition, especially for larger objects, and lead to a collapse of the coherence.
However, it is unknown whether these fluctuations in space-time really exist because they cannot be proven directly. But there could be indirect evidence of this effect – and this is what the physicists around Curceanu have now been looking for. According to the theory, charged particles should react to these fluctuations and move, releasing extremely weak electromagnetic radiation. However, this is so weak that it gets lost in the background noise under normal conditions.
Measurements under 1400 meters of rock
But there is a place where this telltale radiation should be measured: the Gran Sasso underground laboratory in the Alps. The underground halls of this particle research laboratory are shielded beneath 1,400 meters of rock. “The natural shielding provided by the rock makes it one of the quietest places on earth – well suited to detecting extremely rare physical phenomena,” explains Curceanu.
For their experiment, the physicists used a measuring apparatus insulated by several layers of lead and copper. Its center was a highly pure germanium crystal. The researchers recorded the emission spectrum of the crystal over 62 days. Through additional measurements, they determined the background noise in the measuring block – also from the materials used for shielding and measurement – and subtracted this from the measured radiation values.
Missing signal refutes Károlyházy’s theory
The result: After deducting all disruptive factors and background radiation, there was nothing left. The signal predicted by Károlyházy’s theory was not found. “This lack of signal is an important scientific result,” explains Curceanu. Because if the collapse of the overlay is caused by gravity and the peculiarity of spacetime assumed by the theory, then this should show up in these measurements.
“Our experiment thus refutes the decoherence mechanism postulated by Károlyházy, even in its extended Gaussian form,” report the physicists. Although this does not mean that gravity is ruled out as the cause, it can still be a decisive factor in the collapse of the superposition in macroscopic objects. But the results now rule out many gravity-based theories and further narrow the field of possible explanations.
And now?
“Our work narrows down the search for a theory that describes the interaction between gravity and quantum mechanics,” says Curceanu. “This brings us another step closer to solving one of the deepest mysteries in fundamental physics.” Among other things, gravitational time dilation is still in the running as a possible cause of decoherence. Physicists already presented this explanation in 2015. So far, however, the technical requirements for testing this are missing.
Source: Nicola Bortolotti (University of Rome) et al., New Journal of Physics, 2026; doi: 10.1088/1367-2630/ae774c