Quarks are one of the basic building blocks of matter, making up the protons and neutrons that are found in the nuclei of atoms. But a new model suggests that these subatomic particles may not be so fundamental after all, but instead just a quantum illusion.
The quark model was proposed independently in 1964 by physicists Murray Gell-Mann and George Zweig. At first, it was just a model. But in 1968 the first proof of the existence of quarks was discovered at the Stanford Linear Accelerator Center in California.
Quarks have a number of quantum properties — many of them strange — such as flavor, spin and color. In everyday matter, quarks exist only as pairs or triplets, but a team at the Large Hadron Collider near Geneva, Switzerland, recently detected a collection of five quarks, or pentaquark.
Because quarks have an affinity for combining with other quarks, individual ones are never seen. This might suggest that they were fictitious to begin with.
But early on, physicists saw signs that quarks were real — such as electrons bouncing off at wide angles when fired at protons, which hints that the electrons had hit something like a quark. Later, physicists detected pairs or triplets of quarks that had been predicted by the quark model — giving a boost to the “realness” of quarks.
Scientists also developed a description of the force that binds quarks together, known as quantum chromodynamics (QCD). This gets its name from the quark property of color (aka “chromo”), which is related to the strong force, one of the four fundamental forces in nature.
Quarks don’t actually have color. It’s a term physicists chose because that property comes in threes — in this case red, green and blue. With light, red, green and blue combine to form white. Likewise, when these three quark color charges combine, they cancel out. But the color charges can also cancel out when a quark pairs with its antiquark — such as red and anti-red.
QCD answered some questions about quarks, but as Joshua Howgego recently describes in New Scientist, this method sometimes required physicists to spend years making one complex calculation.
Physicist Gerard ’t Hooft got around this by getting rid of the parts of the equations that described quark color. This worked, but it also meant that quarks could have an infinite number of colors, rather than the three initially proposed.
However, in order for a collection of quarks — known as a baryon — to be stable, their color charges need to cancel each other out. So a baryon with an infinite number of quarks would also have an infinite number of colors. Because each quark has a property called spin, when you combine an infinite number of quarks, you get a baryon with an extremely high spin.
The quick method of calculation developed by ’t Hooft didn’t work well with this type of high-spin particle. So along comes string theory — a model that attempts to unify physics of the very large (relativity) with the very small (quantum). String theory allowed quarks to have part of their usual spin, writes Howgego. Physicists found that this type of fractional spin also worked with QCD.
Last year, Zohar Komargodski at the Weizmann Institute of Science in Israel came up with a new approach that would allow both an infinite number of quark colors and fractional spins. This is where quarks start to lose some of their “realness.”
Komargodski proposed that a baryon with a high spin — due to a large, or infinite, number of quarks — would flatten into a quantum foam with only two dimensions. From this foam, a quark with fractional spin would emerge. Howgego writes that in this case the quark is not really a fundamental particle, but instead is “a consequence of the quantum foam’s behaviour.”
You are unlikely to run into a high-spin baryon in your everyday life, but Howgego writes that these particles do exist in nature, such as in the very high pressures of the interior of a neutron star. Some physicists are trying to use Komargodski’s work to help them describe the properties of neutron stars.
So does this mean that quarks, once the building block of matter, are not real? According to Howgego, Komargodski “still thinks quarks are real, fundamental objects,” even though they may sometimes have weird — or weirder — behavior. But other physicists think that the quark may lose its “realness” in certain environments, as in the very dense matter of a neutron star.