COMMENT
DR KATIE MACK:
WE STILL HAVE A LOT TO LEARN ABOUT THE PROTON
Physicists investigating the subatomic particle’s properties quickly find themselves going down a rabbit hole of complexity
A proton should be one of the simplest objects in physics. It’s a basic building block of all atoms, or, alternatively, the simplest possible atom all by itself, since hydrogen (one positively charged proton plus one negatively charged electron) is still hydrogen when it’s ionised. Most of the atoms in the Universe are hydrogen, as are most of the atoms in your body. In fact, since electrons are tiny and weigh very little, it’s straightforward to conclude that you are mostly, specifically, protons.
Given all this, you’d think physicists would understand protons very well by now. You would be wrong.
If you ask your physics teacher what protons are made of, they’ll likely tell you protons are made of three smaller particles called quarks. Quarks come in six different types, or ‘flavours’: up, down, charm, strange, top, and bottom (they were named in the 1960s and 1970s), with up and down quarks combining to make protons and neutrons.
Since the up quark has a charge of +2/3 and the down quark has a charge of -1/3, the sums all work out if a +1-charged proton is two ups and a down (2/3 + 2/3 – 1/3 = +1) and a neutral neutron is two downs and an up (-1/3 -1/3 + 2/3 = 0).
So far, so good. But while the charges add up perfectly, the masses don’t. In particle physics, we usually measure mass in terms of energy (interchangeable via that old standard, E = mc2), and for this purpose we’ll use units of MeV, for Mega-electron-volts.
If you look up quark masses online you’ll find that the mass of an up quark is around 2MeV while a down quark is close to 5MeV. But those same sources will tell you the mass of a proton is a whopping 938MeV. Our sums are off by about 99 per cent.
“So how do we build a proton that weighs 938MeV out of three quarks that weigh a total of 9MeV, and a handful of particles with no mass at all? The answer is even more complicated than you might imagine”
Before we panic, we can ask, what else is in the proton? And we have a convenient answer: gluons! Gluons are the aptly named particles that carry the strong nuclear force, just as photons carry light – the electromagnetic force. Gluons are in the proton to hold the quarks together, so surely they must contribute something. But gluons have something else in common with photons: they’re entirely massless.
So how do we build a proton that weighs 938MeV out of three quarks that weigh a total of 9MeV and a handful of particles with no mass at all? The answer is even more complicated than you might imagine. For one thing, it’s not quite right to say there are three quarks in a proton. Really, a proton is a roiling quantum sea of an uncountable number of quarks, antiquarks and gluons, constantly shifting in and out of existence by transforming into one another. And those ethereal particles zipping around inside the proton carry kinetic energy, which, via E = mc2, gets us about 60 per cent of the 938MeV that we need.
The final piece comes from the energy of the strong nuclear force itself. The quarks are not merely bound by the strong force, but confined. This is different from gravity or electromagnetism, where the more separation you get, the weaker the attraction – you can, with enough effort, pull magnets apart, or accelerate a rocket away from Earth. But the strong force will just keep pulling.
There’s so much energy tied up in the force itself that even if you manage to pull two bound quarks apart hard enough to overcome their strong force attraction, the energy you have to put in to break that bond will spontaneously create two new quarks, one bound to each of the ones you just separated. Quarks do NOT like to be separated.
The energy inherent in quark confinement solves the proton mass puzzle, but the calculations of exactly how this term arises, and what its magnitude is, are incredibly complex, and the more you look into them, the more complex they become. Recent experiments have shown that protons can sometimes be observed containing charm quarks, which is particularly surprising, since charm quarks are more massive than protons are.
Measurements of the proton’s size have been controversial for decades: you get different answers depending on whether you measure it by scattering electrons off the proton or by watching the electron in a hydrogen atom pass right through the proton, which is a thing it does routinely, just on a normal day, because nothing at that scale is sacred at all.
With new, advanced computational techniques, we’re making progress. And the measurements are already incredibly precise. If we can unlock the mysteries of this most basic of atomic building blocks, we’ll be closer to understanding the fundamental laws that govern reality itself. Or maybe we’ll discover something even more bizarre hiding within it.
DR KATIE MACK
(@AstroKatie) Katie is a theoretical astrophysicist. She currently holds the position of Hawking Chair in Cosmology and Science Communication at the Perimeter Institute for Theoretical Physics.