It may be one of the building blocks of all matter, but we still have a lot to learn about the proton

It may be one of the building blocks of all matter, but we still have a lot to learn about the proton

A proton should be one of the simplest objects in physics. It is a basic building block of all atoms, or, alternatively, the simplest atom possible on its own, since hydrogen (a positively charged proton plus a negatively charged electron) is always hydrogen when ionized.

Most atoms in the Universe are hydrogen, as are most atoms in your body. In fact, since electrons are tiny and weigh very little, it’s simple to conclude that you are mostly, specifically, protons.

Considering all of this, you would think that 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 probably tell you that protons are made of three smaller particles called quarks. Quarks come in six different types, or “flavors”: up, down, charm, weird, up and down (they were named in the 1960s and 1970s), with up and down quarks combining to form 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 if a +1 charged proton is two ups and one down (2/3 + 2/3 – 1/3 = +1) and a neutral neutron is two lows and one high (-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 this older standard, E=mc2), and for this we will use units of MeV, for Mega-electron-volts.

If you look up quark masses online, you will find that the mass of an up quark is around 2 MeV while a down quark is close to 5 MeV. But these same sources will tell you that the mass of a proton is 938 MeV. Our sums are skewed by about 99%.

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Before we panic, we can ask ourselves, what else is in the proton? And we have a practical answer: gluons! Gluons are the aptly named particles that carry the strong nuclear force, much like photons carry light – the electromagnetic force. The gluons are in the proton to hold the quarks together, so surely they must be contributing something. But gluons have something else in common with photons: they are entirely massless.

So how can we build a proton that weighs 938 MeV out of three quarks that altogether weigh 9 MeV and a handful of particles with no mass at all?

The answer is even more complicated than you might imagine. On the one hand, it is not entirely correct to say that there are three quarks in a proton. In reality, a proton is a bubbling quantum sea of ​​countless quarks, antiquarks, and gluons, constantly moving in and out of existence by transforming into each other. And these ethereal particles that walk around inside the proton carry kinetic energy which, via E = mc2gives us about 60% of the 938 MeV we need.

The final piece comes from the energy of the strong nuclear force itself. Quarks are not simply 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, separate magnets or accelerate a rocket away from Earth. But the mighty force will keep pulling.

There is so much energy tied to the force itself that even if you manage to pull two bound quarks apart strong enough to overcome their strong attraction, the energy you had to put in to break that bond will spontaneously create two new quarks, one bound to each of the ones you just separated. Quarks don’t like to be separated.

The energy inherent in the confinement of quarks solves the riddle of proton mass, but the calculations of exactly how this term appears and its magnitude are incredibly complex, and the more you examine them, the more complex they become.

Recent experiments have shown that one can sometimes observe protons containing charmed quarks, which is particularly surprising, since charmed quarks are more massive than protons.

Measurements of the size of the proton have been controversial for decades: you get different answers depending on whether you measure it by scattering electrons from the proton or by watching the electron of a hydrogen atom pass through the proton, which it done regularly. just a normal day, because nothing on this scale is sacred at all.

Thanks to new advanced computing techniques, we are making progress. And the measurements are already incredibly accurate. If we can unlock the mysteries of these most basic atomic building blocks, we will be closer to understanding the fundamental laws that govern reality itself. Or maybe we’ll discover something even more bizarre lurking there.

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