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“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. The American physicist Richard Feynman in 1965 said that this experiment ‘has in it the heart of quantum mechanics’. Werner Heisenberg, the German physicist and founding member of quantum physics, would often refer to this strange experiment in his discussions with others to ‘concentrate the poison of the paradox’ thrown up by nature at the smallest scales.
In its simplest form, the experiment involves sending individual particles such as photons or electrons, one at a time, through two openings or slits cut into an otherwise opaque barrier. The particle lands on an observation screen on the other side of the barrier. If you look to see which slit the particle goes through (our intuition, honed by living in the world we do, says it must go through one or the other), the particle behaves like, well, a particle, and takes one of the two possible paths. But if one merely monitors the particle landing on the screen after its journey through the slits, the photon or electron seems to behave like a wave, ostensibly going through both slits at once.”
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