Quantum mechanics’ probabilistic nature has puzzled researchers for more than a century. A new analysis shows that ridding the theory of this odd characteristic may be even harder than anticipated.

Infamously, quantum objects can exist in combinations of seemingly mutually exclusive states, such as in the Schrödinger’s cat thought experiment, where the animal is in a mix of states that makes it seem both alive and dead, and also neither. Left undisturbed and isolated, the cat remains mixed, but if it is observed – if an experimenter opens its box, say – it abruptly reduces to just one, unmixed state.