Quantum mechanics, the theory governing the microscopic world, is famously counterintuitive. A particle can exist in a superposition of multiple states, such as different positions, until a measurement is performed. At that point, the wavefunction describing that particle appears to ‘collapse’ to a single outcome. This puzzle lies at the heart of the measurement problem, famously illustrated by Schrödinger’s cat, suspended between life and death until observed. The XENONnT detector, which was designed to be sensitive to rare physics events, has tightened constraints on one family of possible solutions to the measurement problem, known as ‘collapse theories.’ The work, which was partially funded by FQxI, was reported in Physical Review Letters in March 2026. Image credit: XENON Collaboration.