How a sunbeam split in two became physics’ most elegant experiment

Anil Ananthaswamy in Aeon:

Imagine throwing a baseball and not being able to tell exactly where it’ll go, despite your ability to throw accurately. Say that you are able to predict only that it will end up, with equal probability, in the mitt of one of five catchers. The baseball randomly materialises in one catcher’s mitt, while the others come up empty. And before it’s caught, you cannot talk of the baseball being real – for it has no deterministic trajectory from thrower to catcher. Until it becomes ‘real’, the ball can potentially appear in any one of the five mitts. This might seem bizarre, but the subatomic world studied by quantum physicists behaves in this counterintuitive way.

Microscopic particles, governed by the laws of quantum mechanics, throw up some of the biggest questions about the nature of our underlying reality. Do we live in a universe that is deterministic – or given to chance and the rolls of dice? Does reality at the smallest scales of nature exist independent of observers or observations – or is reality created upon observation? And are there ‘spooky actions at a distance’, Albert Einstein’s phrase for how one particle can influence another particle instantaneously, even if the two particles are miles apart.

As profound as these questions are, they can be asked and understood – if not yet satisfactorily answered – by looking at modern variations of a simple experiment that began as a study of the nature of light more than 200 years ago. It’s called the double-slit experiment, and its findings course through the veins of experimental quantum physics.

More here.