The proton isn't a solid ball; it's more like a tight tangle of quarks and gluons. Gluons are the particles that carry the strong force — a kind of glue that holds quarks together. But this glue doesn't just connect; it presses.
In the J/ψ-007 experiment at Jefferson Lab, a beam of photons was aimed at protons. Sometimes a photon, colliding with a proton, would produce a J/ψ particle, which then decayed into a pair of muons. By measuring how often this happened at different angles, physicists reconstructed how gluons are distributed inside the proton.
It turned out that gluons are concentrated closer to the proton's edge and create compressive pressure. This pressure prevents quarks from flying apart: it is the force that keeps the proton intact. The results matched supercomputer calculations, where physicists model quark and gluon behavior from first principles.
Now we have direct experimental confirmation of how gluons shape the proton's internal structure. This is important not only for understanding the nature of matter but also for future experiments at new accelerators, where protons will collide at even higher energies.
The pressure inside a proton is billions of times greater than the pressure at the center of a neutron star, the densest known object in the universe.
In Liu Cixin's novel "The Three-Body Problem," aliens create sophons by unfolding protons into two-dimensional structures, but even there, it's not mentioned how gluon pressure keeps them from falling apart.