Quantum objects exist as clouds of possibilities, only manifesting as something definite when we look at them. The implication is that we make reality – but objectivity may yet be rescued

Physics 25 August 2021QUANTUM stuff, whether single atoms, electrons or photons of light, is notorious for seeming to be here, there and everywhere – and indeed everything – all at once. It exists as clouds of possibilities, manifested in a beast you can’t get around when contemplating quantum mysteries: the wave function.

On one level, the wave function is just a mathematical expression that lets you calculate the probability a particle will manifest in a particular location, say. The mystery is the way the maths says that, once you look at it, the wave function “collapses” to leave something definite we can all agree on. This creates the picture of the world that our classically trained eyes see. But how does the mathematics relate to the reality before the measurement – and what exactly, if anything, does the act of measurement change?

Erwin Schrödinger expressed the unease we might feel about apparently “making” reality when he mused about a cat inside a box that might or might not have been killed by a random quantum process inside it. Before you look, he asked, is the cat dead and alive at the same time?

The orthodox take on quantum theory, known as the Copenhagen interpretation, says yes: the maths adds up, so just shut up and calculate. “From a practical point of view, it works perfectly,” says Angelo Bassi, a theoretical physicist at the University of Trieste in Italy. “But from a fundamental point of view, why should the wave function collapse?”

Some physicists argue that it all makes complete sense if you think of the wave function as a way to predict what might happen. …