The inside of a proton endures more pressure than anything else we’ve seen

Cecile G. Tamura

The proton, one of the components of atomic nuclei, is composed of fundamental particles called quarks and gluons. Gluons are the carriers of the force that binds quarks together, and free quarks are never found in isolation—that is, they are confined within the composite particles in which they reside. The origin of quark confinement is one of the most important questions in modern particle and nuclear physics because confinement is at the core of what makes the proton a stable particle and thus provides stability to the Universe. The internal quark structure of the proton is revealed by deeply virtual Compton scattering, a process in which electrons are scattered off quarks inside the protons, which  subsequently emit high-energy photons, which are detected in coincidence with the scattered electrons and recoil protons. Here we report a measurement of the pressure distribution experienced by the quarks in the proton. We find a strong repulsive pressure near the centre of the proton (up to 0.6 femtometres) and a binding pressure at greater distances. The average peak pressure near the centre is about 10³⁵ pascals, which exceeds the pressure estimated for the most densely packed known objects in the Universe, neutron stars³. This work opens up a new area of research on the fundamental gravitational properties of protons, neutrons and nuclei, which can provide access to their physical radii, the internal shear forces acting on the quarks and their pressure distributions.For the first time, scientists used experimental data to estimate the pressure inside a proton.

Protons break the pressure record set by neutron stars, the incredibly dense dead stars that can form when a massive star explodes and its core collapses, squeezing more mass than the sun’s into a remnant the size of a city. The pressure in the proton’s center averages a million trillion trillion times the strength of Earth’s atmospheric pressure.

"A neutron star is a city-size celestial object, with a mass 1.4 times our sun's. They were once massive stars that ended in supernova, then collapsed into a small, dense core. These are the densest object in the universe, and a proton’s internal force is even stronger than that! Previously, scientists surmised a proton may contain intense pressure. Yet, this is the first time it’s been proven."

Protons are made up of smaller particles including quarks, which are electrically charged, and gluons, which transmit the strong nuclear force that holds protons together.

In the center of this ball of particles, Burkert and colleagues report, an intense pressure pushes outward. But this record-breaking outward force is kept in check by an inward pressure from the outer regions of the particle.

This pressure pattern parallels what happens in much larger objects: “In some sense, it’s looking like a star,” says physicist Oleg Teryaev of the Joint Institute for Nuclear Research in Dubna, Russia. Stars also have pressures that push outward in their centers, which counteract the inward pull of gravity.

We seem to forget that these forces are going on inside of us all the time, too.