The three valence quarks of a proton contribute to its spin, but so do the gluons, sea quarks and antiquarks, and orbital angular momentum as well. The electrostatic repulsion and the attractive strong nuclear force, in tandem, are what give the proton its size, and the properties of quark mixing are required to explain the suite of free and composite particles in our Universe. (APS/ALAN STONEBRAKER)

Ask Ethan: Where Does A Proton’s Mass Come From?

The whole should equal the sum of its parts, but doesn’t. Here’s why.

The whole is equal to the sum of its constituent parts. That’s how everything works, from galaxies to planets to cities to molecules to atoms. If you take all the components of any system and look at them individually, you can clearly see how they all fit together to add up to the entire system, with nothing missing and nothing left over. The total amount you have is equal to the amounts of all the different parts of it added together.

So why isn’t that the case for the proton? It’s made of three quarks, but if you add up the quark masses, they not only don’t equal the proton’s mass, they don’t come close. This is the puzzle that Barry Duffey wants us to address, asking:

What’s happening inside protons? Why does [its] mass so greatly exceed the combined masses of its constituent quarks and gluons?

In order to find out, we have to take a deep look inside.

The composition of the human body, by atomic number and by mass. The whole of our bodies is equal to the sum of its parts, until you get down to an extremely fundamental level. At that point, we can see that we’re actually more than the sum of our constituent components. (ED UTHMAN, M.D., VIA WEB2.AIRMAIL.NET/UTHMAN (L); WIKIMEDIA COMMONS USER ZHAOCAROL (R))

There’s a hint that comes just from looking at your own body. If you were to divide yourself up into smaller and smaller bits…