This is not at all how you would measure the pressure inside of a proton.
Illustration: Department of Energy/Jefferson Lab

The pressure inside the particles that make up every atom in the universe could be greater than the pressure inside the densest stars, according to a new measurement.

Scientists at Jefferson Lab in Virginia calculated the pressure using the lab’s Continuous Electron Beam Accelerator Facility, or CEBAF, and some tricky mathematics. The measurement will mainly be useful for fundamentally understanding these particles’ nature. The calculation is pretty mind-boggling.

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“Neutron stars are some of the densest objects we know of in the universe,” Volker Burkert, Jefferson Lab Hall B leader, told Gizmodo. “It’s an order of magnitude bigger than that. It could be the record observation of a pressure on Earth.”

The researchers calculated the pressure faced by the quarks that make up protons at 1035 pascals, equalling 1030 times the pressure at sea level, according to the paper published in Nature.

Ascertaining that pressure required a series of mathematical steps, beginning with another innate set of properties of the proton, called its gravitational form factors. These form factors get their name because they can only be directly measured by protons interacting with gravitons. Scientists have never observed a graviton, but two photons can serve as a proxy.

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The CEBAF measures the values by shooting electrons at protons in liquid hydrogen, resulting in an electron, a proton, and those two photons. Rather than collide with the entire proton, the electrons in the experiment collide with individual quarks through a process called “deeply virtual Compton scattering.” These already-published measurements provided the gravitational form factors needed to calculate the pressure.

So, what do you do with knowledge of the proton’s ridiculously high internal pressure? First, it’s interesting to know more about perhaps the most important particle to life, since without protons, there would be no atoms and no humans. And there’s still lots to know about protons: Just last week, another team at the Jefferson Lab measured a fundamental property of the proton, called its weak charge, for the first time.

Aside from that, scientists measure two different values for the proton’s radius based on the experiments they perform. It’s a frustrating inconsistency when it comes to understanding a property as basic as a particle’s size. This latest research could offer a new way to measure the proton’s radius based on how the pressure is distributed inside the particle, explained study author and Jefferson Lab physicist Francois-Xavier Girod. It can also motivate theorists to try to understand the very nature of the proton, things like why it doesn’t decay the way a neutron does.

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And things go even deeper than that: Quarks, the smaller pieces that make up protons and neutrons, can never exist on their own—they’re always “confined.” The pressure faced by quarks inside the proton illustrates their permanently social behavior, and could perhaps provide insight into a fundamental mechanism for this so-called quark confinement, said Mu-Chun Chen, a professor of physics and astronomy at the University of California, Irvine, who was not involved with the study.

More precise experiments to measure further properties of the proton are on the horizon, which would reduce some of the uncertainty currently faced by the researchers. But it’s worth appreciating that it takes a lot of effort to keep together the things that, well, keep you together.

[Nature]

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