New research proves theoretically that the Weak Equivalence Principle can be violated by quantum particles in gravitational waves — the ripples in spacetime caused by colossal events such as merging black holes.

As well as resolving a long-standing debate in quantum theory, Quach’s findings could lead to the development of advanced new materials, including fluids with infinite conductivity and zero viscosity. These could be used as advanced gravitational wave detectors and may even lead to devices which can mirror gravitational waves and harvest their energy. Quach based his approach around a principle named ‘Fisher information’ — a way of measuring how much information an observable random variable carries about a particular unknown parameter. Here, the random variable describes the position of a quantum particle in a gravitational field, while the unknown parameter is its mass. If the WEP were obeyed, the Fisher information should be zero in this case.

Through his calculations, Quach rewrote an equation describing the WEP for freely falling quantum particles, to incorporate their Fisher information. He showed that while these particles obey the WEP in static gravitational fields, their trajectories can indeed give away information about their mass when they pass through gravitational waves. For the first time, the calculation precisely characterises how the WEP can be violated by quantum particles, and provides key insights for future studies searching for the violation through real experiments.

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