How a sunbeam split in two became physics’ most elegant experiment, shedding light on the underlying nature of reality
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.
When microscopic entities have the option of doing many things at once – like that metaphysical baseball – they seem to indulge in all possibilities. Such behaviour is impossible to visualise. Common sense fails us when dealing with the world of the quantum. To explain the outcome of something as simple as a particle encountering two slits, quantum physics falls back on mathematical equations. But unlike in classical physics, where the equations let us calculate, say, the precise trajectory of a baseball, the equations of quantum physics allow us to make only probabilistic statements about what will happen to the photon or electron. Crucially, these equations paint no clear picture about what is actually happening to the particles between the source and the screen.
It’s no wonder then that different interpretations of the double-slit experiment offer alternative perspectives on reality. For example, in the late 1920s and early ’30s, some physicists made the startling claim that a particle going through two slits has no clear path or indeed no objective reality until one observes it on a screen on the other side. At a gathering of physicists and philosophers at the Carlsberg mansion near Copenhagen in 1936, the Dutch physicist Hendrik Casimir recalled someone protesting: ‘But the electron must be somewhere on its road from source to observation screen.’ To which Niels Bohr, one of the founders of quantum mechanics, replied that the answer depends on the meaning of the phrase ‘to be’. In other words, what does it mean to say that something exists? One philosopher in the group that day, the Danish logical positivist Jørgen Jørgensen, retorted in exasperation: ‘One can, damn it, not reduce the whole of philosophy to a screen with two holes.’
Yet it is extraordinary just how much of quantum physics and philosophy canbe understood using a screen with two holes – or variations thereof. The history of the double-slit experiment goes back to the early 1800s, when physicists were debating the nature of light. Does light behave like a wave or is it made of particles? The latter view had been advocated in the 17th century by no less a physicist than Isaac Newton. Light, Newton said, is corpuscular, or constituted of particles. The Dutch scientist Christiaan Huygens argued otherwise. Light, he said, is a wave – the name given to the vibrations of the medium in which the wave is travelling. For example, a wave in water is essentially the way water moves up and down as the wave propagates. Huygens argued that light is vibrations in an all-pervading ether.
In the first years of the 19th century, the English polymath Thomas Young seemingly settled the debate. He was the first to perform an experiment with a ray of sunlight, a sunbeam, through two narrow slits. On a screen on the other side, he observed not two strips of light – as you’d expect if light is made of particles going through one slit or the other – but a pattern of alternating bright and dark fringes, characteristic of two sets of waves interacting with each other.
Fig 442 from Thomas Young’s ‘Lectures’ published in 1807 detailing his original ‘two-slit’ experiment. Courtesy Wikimedia.
Imagine an ocean wave hitting a coastal breakwall that has two openings. New waves spread out from each opening and head toward the coast. These waves eventually overlap and interfere – at some places constructively (where the crest of one wave meets the crest of another), and at some places destructively (the crest of one wave encounters the trough of another). In Young’s experiment, he saw similar interference. The fringes that he observed had bright regions indicative of constructive interference and dark regions typical of destructive interference.
This view of light as a wave gained strong mathematical support when the Scottish physicist James Clerk Maxwell developed his theory of electromagnetism in the 1860s, showing that light, too, is an electromagnetic wave.
That would have been the end of story – if not for the birth of quantum physics, which began with the German physicist Max Planck’s argument in 1900 that energy comes in quanta, or tiny, indivisible units. Then, in 1905, Einstein studied the photoelectric effect, in which light falling on certain metals dislodges electrons; the effect can be explained only if light is also made of quanta, with each quantum of light analogous to a particle. These quanta of light came to be called photons.
Now, the double-slit experiment gets maddeningly counterintuitive.[…]
is is an award-winning journalist and former staff writer and deputy news editor for New Scientist in London. His work has appeared in Nature, The Wall Street Journal and National Geographic News, among others. His latest book is Through Two Doors at Once (2018). He lives in Bangalore in India and Berkeley in California.