WHEN you hear the word “particle”, what image floats into your mind? Chances are you’re thinking small, and then some – like the tiniest billiard ball imaginable. Indivisible chunks of matter pinging off each other in the vast expanses of space, or jostling for position in a crowded chunk of stuff.
Chances are, too, you’re nowhere near the vision of particles painted by our best picture of how they work, quantum theory. This says that despite making up stuff that definitely has a size – ourselves, the paper or screen you’re reading this on – particles occupy a point in space precisely zero metres across.
While you’re chewing that one over, you might consider how quantum theory also allows these size-zero particles to occupy multiple places at once, or be “entangled” so the state of one becomes inextricably bound up with the state of another. But even that doesn’t prepare you for the latest assault on any common-sense conception of a particle that physicists have been preparing.
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Strike the billiard balls from the table. An alternative breed of shape-shifting particles can be split up, change their mass, and be combined with other stuff to make more than the sum of the parts. These particles don’t seem to exist in any way that makes sense, and yet we are increasingly bending them to our will. The results are reshaping technology, from superconductors to quantum computers — and helping us probe deeper into the fabric of reality than ever before.
It was Einstein who began to blur the lines of what we think of as the basic building blocks of reality. In 1905, he proposed that light – at the time largely considered to be a space-filling wave, given its ability to interfere with itself, diffract around corners and so on – could also be seen as particles. He later called these photons. It was the beginning of what came to be known as the “central mystery” of quantum theory: wave-particle duality. This principle says that neither a wave nor a particle is a perfect way to think of a photon of light, an electron orbiting an atom or any of the particles at the heart of the atomic nucleus. Sometimes they are best thought of as one, sometimes the other.
This duality was a profound insight. But it became even more profound when we began to apply it not just to particles knocking about in free space, but to the goings on within solid materials made of very many particles jostling around for position.
Take what happens when you set a flame under a lump of table salt. The individual atoms all start rattling around a tad more enthusiastically, setting up waves of vibrations. In 1932, the Soviet physicist Igor Tamm realised he could treat these waves as particles, mathematically at least. He called them phonons.
Phonons have since become a staple, helping us for instance to understand processes such as superconductivity, in which electrons flow through a material with zero resistance, and opening the way for devices that turn heat into electricity (see “Five particles that don’t exist – yet could change our world“). Because they emerge from the movements of more traditional particles, phonons are called emergent particles or quasiparticles. “Phonons are not actually real,” says , a physicist at Royal Holloway, University of London. “They are really just a way of simplifying a very complicated problem.”
You can think of a quasiparticle as a little like a Mexican wave at a sports match. The only physical movement is people jerking their arms up and down one after the other and yet something seems to be travelling through the crowd (see diagram): a movement of energy that amounts to a quasiparticle.
Since Tamm’s work, we’ve steadily discovered a whole zoo of such beasts roaming the interior of solid materials. Another species arises from spin, a quantum property that is the basis of magnetism. Spin works something like an arrow on an atom pointing north or south; when all the spins in a material are aligned, you have a magnet. But spins can also flip into and out of alignment, creating a Mexican wave effect that can be treated as a particle known as a magnon. Like phonons, magnons are proving practically useful, helping us to develop low-power “spintronic” computers that exploit atomic spins rather than currents of electrons.
Phonons, magnons and the like exist in a kind of twilight world: useful to work out how things work, but doubtful as entities in their own right. But these half-existing particles aren’t even the half of it. Quasiparticles can exist, it turns out, even when nothing is there.
That discovery first came in 1947, with a seminal moment for the history of computing. William Shockley, a solid state physicist at Bell Labs in New Jersey, and his team were trying to perfect the transistor, an on-off switch for electrical current. They were using semiconductors, materials whose atoms are deficient in electrons. It had been known for a decade or so that this would create gaps of nothingness, like the empty square in a sliding puzzle. But no one thought these “holes” were anything more than the absence of an electron. Shockley proposed that the hole was actually a particle in its own right, something like an electron that carried positive charge.
It turned out to be the crucial step. Only by treating holes as independent entities could you understand how they flow through areas rich in electrons without getting filled and disappearing. The result was the kind of semiconducting transistors, formed by junctions of materials rich in electrons and holes, that still flip and switch in their billions inside computer chips today.
Since then, we’ve learned that an electron and a hole can be made to combine – in which case, you don’t get zero as you would if you added +1 and -1, but a whole new type of particle, the exciton. Plants got there long before us: we now know the exciton is integral to photosynthesis (see “Five particles that don’t exist – yet could change our world“).
So, particles that don’t exist and yet do; that are made of nothing at all and yet can be added to something else to make more: is there any limit to how elastic we can make the idea of a particle?
“The particles are just smoke and mirrors, handy mathematical tricks and nothing more. Or are they?“
One red line might be the idea that a particle is something that has a set mass. But even that is breached by what happens within a material such as graphene. Graphene is much heralded for its superlative properties of electrical conduction. That’s down to its structure of carbon sheets just one atom thick, which forces its electrons to interact with one another. Rather like cyclists in a peloton, they end up whizzing along faster as a collective than they can individually, at close to light speed.
The way we explain that is by saying graphene is still populated by electrons – but electrons that have almost no mass. It’s a mathematical trick, to be sure, similar to the one Tamm used when he dreamed up phonons. But unless you accept that graphene’s electrons have pared down masses, the material’s amazingly good electrical conductivity is hard to explain.
And graphene is just the beginning. We are coming to realise that we can engineer solids in all sorts of ways to produce particle-like effects unseen in nature. Take Majorana fermions, particles with no charge and no energy that are their own antiparticles. Predicted in the 1930s by the Italian physicist Ettore Majorana, these particles have never been seen in the wild, aside from unverified suggestions that neutrinos might be their own antiparticles.
But we can now make them in the lab. In 2013, Leo Kouwenhoven and his team at the University of Delft in the Netherlands set up a Mexican wave among electrons squashed into a one-dimensional wire with little room to manoeuvre, and discovered that the electrons that peeled off at either end of the wire acted as a Majorana pair. One potential use for these particles is as “bits” in a future super-powerful quantum computer.
Then there are Weyl fermions. Last year, three research groups reported making these long-hypothesised particle pairs. These mirror-image particles are made from underlying waves of spin that are a bit like right or left-handed knots, a property that might add another dimension to spintronic computing.
For all their usefulness, you’d be forgiven for still thinking quasiparticles are so much smoke and mirrors: handy mathematical tricks, but nothing more. There’s still a clear distinction between them and the particles that are the building blocks of matter – the sort you can bash together a particle accelerator.
Perhaps not. In May, at the University of Regensburg in Germany and his collaborators unveiled the first . They took a semiconductor called tungsten diselenide and used laser pulses to split apart excitons inside it into their constituent electron and hole. Then, reversing the pulses, they could smash the two bits into each other (Nature, vol 533, p 225).
The excitons behaved as you would expect real particles to behave. The electron and the hole mutually annihilated, producing a photon, just as an electron does when it hits one of its antimatter equivalents, a positron, at high speed.
Huber reckons the equipment could crack open other quasiparticles, too. Top of his list are trions, charged quasiparticles made from two electrons and one hole. Unlike excitons, it should be possible to accelerate these and dash them into each other as complete entities. Experiments like this, he says, might help find the point where quasiparticles stop acting as if they were real. Because their behaviour is behind so many mysterious but useful phenomena, from photosynthesis to quantum computing to superconductivity, he thinks it could be a fruitful quest. “My dream is to use quasiparticle acceleration to track down some of the key enigmas of modern-day physics,” he says.
So what, after all, is a particle? That’s a question that’s been bugging us ever since Einstein first proposed light could be made of particles. In all likelihood it will continue to bug us. Perhaps ultimately we’ll have to accept that it’s all just maths, and equations are all that can guide us in understanding the workings of matter. But if we want a deeper answer than that, chasing particles that don’t exist into the cracks between atoms might just be our best shot.
This article appeared in print under the headline “Holes in reality”
Article amended on 20 September 2016
Superconductivity is now properly defined
