Our universe has been ruled by weirdness for at least six centuries. If the quantum effects in a new experiment aren’t genuine, but are somehow caused by past meddling, then that is how long ago it must have happened – a finding that makes would-be alternatives to quantum theory even more unlikely.
Two qualities seem to describe our everyday world: realism, the idea that things have properties which don’t vanish when we’re not looking; and locality, which means no influence can travel faster than the speed of light.
But the quantum effects we see on tiny scales defy these descriptions. The properties of particles aren’t set in stone until we measure them, and their states can be entangled – such that altering one affects the other much faster than light can travel between the two.
There are loopholes in quantum theory, though. David Kaiser at the Massachusetts Institute of Technology and his colleagues are trying to close them down – aided by starlight.
Their experiment exploits a standard test for locality: Bell’s inequality. It sets a limit on how often two entangled particles can end up in the same state just by chance – without quantum mechanics or some unknown “hidden variables” to guide them.
The first step is to create a pair of entangled particles – often photons of light – then fire them off in different directions. Usually, a random number generator determines at the very last moment which property of each particle to measure. The detectors used are far enough apart that the arriving particles can’t “cheat” and coordinate their states – unless they can signal each other faster than light.
If the measurements tally more often than allowed by Bell’s rule, then the particles aren’t governed by locality. Previous experiments have shown this consistently, and so backed quantum mechanics.