Consecutive measurements on the same quantum system reveal that wavefunctions do not collapse*

Whether or not wavefunctions collapse in quantum measurements is often viewed as a metaphysical question because interpretations of quantum theory that differ in their assessment of the reality of wavefunction collapse have not made testably different predictions. We show that the standard collapse interpretation, and a formalism that uses the relative-state description of measurement in terms of von Neumann operations, differ in their predictions of the outcome of three or more consecutive measurements on the same system (while they agree about the outcome of one or two consecutive measurements). An experiment to distinguish between the two theories has been carried out using three consecutive (strong) quantum measurements of photon polarization, and shows unequivocally that the results are incompatible with wavefunction collapse, but perfectly in line with predictions using von Neumann (non-collapsing) measurements.