Quarks are the subatomic particles thought to make up nearly everything we can see. Now it turns out they could be an illusion created by quantum trickery.
FINNEGANS WAKE has a reputation for being one of the most difficult novels in the English language. Written by James Joyce over 17 years, it blends invented words with real phrases in grammar-defying constructions. The final line ends mid-sentence – only for you to realise that the words that should come next are the ones at the book’s beginning. Some say it is Joyce’s attempt at recreating a dream. Others claim that it contains no meaning at all.
It might seem odd, then, that a nonsense word from this most ungraspable of books should have given its name to a particle known as the building block of reality: the quark. In modern physics, a quark is what you would find if you were able to take a piece of matter and cut it in half again and again until you could cut no more.
Quarks are as fundamental as anything can be. But they are also exceedingly weird. They have strange quantum properties known as flavour and spin. They crave each other’s company, clustering together in pairs or triplets. And they have a special sort of charge that comes not in the positive or negative variety, but in colours.
And now, in a twist to rival that of any experimental novel, it seems quarks may not actually exist. According to tantalising new research, they may instead be an illusion, the product of quantum trickery we don’t yet fully understand. Perhaps the absurdist origin of their name is apt after all. The search for reality’s foundations may turn out to be as meaningless and insubstantial as a half-remembered dream.
The hunt for matter’s most basic constituents is millennia old. The Greek philosopher Democritus coined a new word to describe fundamental units of matter: atomos meaning indivisible. While physicists today would agree with Democritus in principle, history has played a nasty joke on his terminology. Our modern understanding of atoms suggests that they are composed of particles called electrons that orbit a nucleus made of protons and neutrons. And those latter two are actually made of quarks (see “Nature’s lego bricks”).
It is tempting to think of these particles as tiny spheres whizzing around like balls on a snooker table, but we have long known that particles are more enigmatic than that. The problems began with light. For centuries, scientists disagreed over its nature, with some believing it was a steady stream of particles, and others calling it a wave. With the advent of quantum theory in the early 20th century, we were forced to accept the evidence that light can take on either form, depending on the situation. The same reasoning that had been applied to photons of light was soon extended to all other particles. Electrons, protons, neutrons, even quarks, can all be said to exist as waves as well as particles.
Things only got more muddled from there. We now know that under the right conditions, particles can be coaxed into doing something even weirder. Inside certain specially primed materials, electrons can merge into a two-dimensional sea where their individual identity is lost.
Out of this collective behaviour, strange particles emerge. They can be heavier than an electron, or only have a fraction of an electron’s charge, or have the opposite charge altogether. Think of a football crowd doing a Mexican wave: the stadium remains full of people, but an external observer sees a new phenomenon emerge that looks nothing like individual football fans. In particle physics, these apparitions are known as quasiparticles.
For all the nuance of this picture, however, it still makes physicists’ lives easier to think of particles as real objects that exist in the world. But that wasn’t the case when quarks were first conceived.
In the 1950s, more particles were turning up than physicists knew what to do with, barrelling towards us out of the depths of space or summoned into existence by particle colliders. It created a messy zoo of particles of different masses, charges and sizes that seemed impossible to corral. The insight that would resolve the chaos was developed separately by three researchers: Murray Gell-Mann, Yuval Ne’eman and George Zweig.
They noticed that many of these particles obeyed a symmetry, suggesting that they were all produced by different combinations of the same core ingredients.
Instead of treating the particles in the zoo as fundamental entities, Gell-Mann and the others invented a new set of particles one size smaller. With these quarks in place (Gell-Mann coined the name and got most of the credit – he was the only one to win a Nobel prize for his work), the mess of particle physics suddenly snapped into order. “The introduction of the idea of quarks was revolutionary,” says Tara Shears at the University of Liverpool, UK. “That idea of a similarity, or symmetry, in behaviour hinting at deeper structure is something we hold very current in research today.”
At first, no one was sure whether quarks were real particles or just a helpful organising idea. In a 1972 lecture, Gell-Mann warned his audience against invoking “fictitious objects in our models that end up turning into real monsters that devour us”.
There were two pieces of evidence to suggest that quarks were more than just monsters in the mathematics. First, physicists firing electrons at protons noticed that some bounced off at wide angles. This suggested that the electrons had hit something inside the proton – something like a quark. What’s more, Gell-Mann’s model indicated that certain combinations of quarks remained undiscovered. Like the gaps in Dmitri Mendeleev’s original periodic table of the elements, this gave the model predictive power. When the missing particles turned up as expected, the quark model’s acceptance was nigh-on guaranteed.
It was a moment of celebration, but much remained unclear. One of the major mysteries was why certain combinations of quarks flourished and others didn’t. You could, for example, pair a quark with its antiparticle to form a meson, or stick three quarks together to form a baryon, such as a proton or neutron. But you couldn’t easily produce a composite particle made of four or five quarks (see “Quarky quirks”), or ever get a quark on its own. Why was this?
The answer lies in a remarkable property of quarks known as colour charge, which bears no relation to the colours we think of in daily life. “Colour is something we’ve just picked to name it because it comes in threes,” says Freya Blekman at the Free University of Brussels in Belgium. Quarks of these different colours – called red, green and blue – can sit together because their colour charges cancel out, by analogy with the way different colours of light blend together to make white. Through the same logic, a quark and an antiquark could sit together assuming they had colour charges of red and anti-red. This also explains why single quarks don’t fall out of atoms in detectors: without their colour partners they are too unstable. “Quarks are always team players,” says Blekman.
By the end of the 1970s, we finally had what is still the most complete description of quarks and the force that binds them together: quantum chromodynamics (QCD), named for the colour charge that quarks possess.
QCD isn’t perfect. For one thing, using it to calculate the most complex physics can be incredibly time-consuming. “A calculation can take us three years from start to finish,” says Ruth Van de Water at the Fermi National Accelerator Laboratory near Chicago. This is why, she says, the properties of many collections of quarks haven’t yet been calculated using full-on QCD. Instead they have been done using less sophisticated models that don’t account for every interaction a quark might have. “Our knowledge of QCD is a bit like trying to grasp what an elephant looks like by feeling some small part,” says physicist Howard Georgi at Harvard University. “One approach may describe the trunk without difficulty but do a really bad job on the ears.”
This isn’t a new problem. As far back as the 1970s, physicist Gerard ‘t Hooft was searching for a way to make QCD more tractable. He made a bold compromise on accuracy, essentially discarding the parts of the QCD equations that described colour. This made for a tremendous simplification, says Van de Water, allowing you to do calculations on the back of an envelope.
When ‘t Hooft tried it, he found that it reproduced the properties of mesons with surprising accuracy “That was pretty exciting,” says Georgi.
But Gell-Mann’s monsters were about to bite. Setting the colour term aside freed quarks from needing to have three colours. Instead, they could have any number of colours you liked – even an infinite number. And because a collection of quarks can attain stability only if all colours are equally represented, an infinite number of colours means baryons with infinite numbers of quarks.
“An infinite number of colours means an infinite number of quarks. This has consequences”
This had consequences. Every quark has a quantum property called spin. Multiply the number of quarks and, crudely speaking, you multiply the amount of maximum spin. In extreme cases, when all the quarks have their spins aligned, the resulting baryon has so much spin that ‘t Hooft’s model struggles to describe it. That’s true not only of particles in ‘t Hooft’s imaginary, infinite colour world, but for some particles in the real world, such as the unusual delta++ baryon, which consists of three up quarks with aligned spins.
The resolution to this has come from an unlikely place: string theory, a framework to unify the relativistic physics of the very large with the quantum physics of the very small. In the early 2000s, string theorists started noticing that their equations allowed quarks to do something bizarre. Under certain circumstances, they could take on a fraction of their usual spin. This was something that had never been seen in experiments, or predicted by QCD. It seemed like another mathematical monster. Then a few years ago, people began to see that QCD could describe quarks with fractional spin too.
Now, the quark story might be about to change far more substantially. Last year, Zohar Komargodski at the Weizmann Institute of Science in Israel saw a possible way to bring all of the disparate quark ideas together: using the infinite colour model of ‘t Hooft, but giving the quarks freedom to take on fractional spins. Physicists admit that his work shows ingenuity and skill – but it is also extremely complex. “I would like to understand it a little better myself,” says Georgi.
What everyone agrees on is that it breaks new ground. Whereas ‘t Hooft’s model couldn’t explain exotic, high-spin particles, Komargodski’s model does just that. “The picture [it paints] is completely different,” says Georgi. Instead of a three-dimensional cluster of quarks jostling for position, it says that the high spin pulls the baryon into a two-dimensional pancake of quantum foam, out of which emerges the quark with fractional spin.
Reality’s shaky foundations
It is very much like the way Mexican waves emerge in a stadium, or quasiparticles appear from a collection of electrons. In other words, it implies that the quarks in these particles aren’t fundamental at all, but a consequence of the quantum foam’s behaviour. “It’s like a new state of matter, or a new state of quark,” says Komargodski. We have always known that quarks are intensely strange beasts, almost inexplicable in everyday terms. “But Zohar’s quasiparticles are completely different, they’re nutty,” says Georgi.
They may also be extremely useful. When ‘t Hooft developed his simplification in the 1970s, no one was worried about its failure to calculate the properties of high-spin baryons like the delta++. That is because they tend to exist only in exotic environments like the super-pressurised interior of neutron stars. But today, neutron stars are at the centre of one of the hottest areas of physics: gravitational wave astronomy.
Over the past few years, the LIGO collaboration has detected the gravitational waves created when colossal objects in space collide, including instances of a black hole gobbling up a neutron star. These signals have given us a new window on the cosmos, but the glass is a little frosted. We can’t deduce much about neutron stars from signals apart from their mass, principally because we have no theory with which to describe the intensely pressurised matter they are made from.
Or at least we didn’t until now. “What Zohar has proposed is extremely exciting because it is relevant in these stars,” says Mannque Rho at the Institute of Theoretical Physics in Paris, France. He is trying to develop Komargodski’s work into a tool that could be used with gravitational wave signals to produce an equation describing more about the neutron stars, including their diameter and density.
The practical applications are only part of the story. Komargodski’s work also raises profound questions about the nature of quarks. If there are circumstances under which quarks seem to be emergent rather than fundamental, does that mean that all quarks are little more than abstractions? If so, what is reality really made of?
Perhaps surprisingly, Komargodski himself still thinks quarks are real, fundamental objects. He likens the situation to the odd behaviour of electrons: although there are some situations in which they take on weird properties, that doesn’t mean we need to bin the concept of electrons entirely. “But everybody has their own opinion,” he says.
Rho sees it differently. “The fundamental nature of the quark essentially loses its meaning in a highly correlated system like dense matter,” he says. “Quarks are not fundamental any more, I think.” Perhaps this shouldn’t come as a surprise. Most physicists think that the standard model of particle physics doesn’t capture the full truth about reality, not least because we don’t know why it is like it is. Quarks may represent another rung on the ladder of reality, but we haven’t reached the bottom yet. We may be right back at the beginning.