Consider the famous Schrodinger’s cat paradox, a thought experiment in which a cat in a box simultaneously exists in two states (this is known as a ‘quantum superposition’). According to quantum theory it is possible that the cat is both dead and alive – until, that is, the cat’s state of health is ‘measured’ by opening the box.

When the box is opened, allowing the health of the cat to be measured, the superposition collapses and the cat ends up definitively dead or alive. The measurement has disturbed the cat.

This is a property of quantum systems in general. Perform a measurement for which you can’t know the outcome in advance, and the system changes to match the outcome you get. What happens if you look a second time? The researchers assume the system is not evolving in time or affected by any outside influence, which means the quantum state stays collapsed. You would then expect the second measurement to yield the same result as the first. After all, “If you look into the box and find a dead cat, you don’t expect to look again later and find the cat has been resurrected,” says Stephanie. “You could say we’ve formalised the principle of accepting the facts”, says Stephanie.

Corsin and Stephanie show that this principle rules out various theories of nature. They note particularly that a class of theories they call ‘discrete’ are incompatible with the principle. These theories hold that quantum particles can take up only a finite number of states, rather than choose from an infinite, continuous range of possibilities. The possibility of such a discrete ‘state space’ has been linked to quantum gravitational theories proposing similar discreteness in spacetime, where the fabric of the universe is made up of tiny brick-like elements rather than being a smooth, continuous sheet.

As is often the case in research, Corsin and Stephanie reached this point having set out to solve an entirely different problem altogether… >*continue*<