For nearly a century, physicists have relied on the Born rule to calculate the chances of different outcomes when they measure a quantum system. Yet despite decades of effort, nobody has managed to derive the rule from more basic mathematical principles without making assumptions about probability along the way.

Through new analysis published in Physical Review A, Jiaxuan Zhang at the University of Oxford has shown that five of the most prominent derivations of the Born rule all depend on one such assumption. Her work suggests that quantum probabilities can’t simply be extracted from a theory that contains none to begin with.