Neutron stars are under incredible pressure. Take a star twice the size of our sun, squish all that mass down to the diameter of a city, and you get a neutron star, the densest known object in the universe. So it comes as a bit of a shock to learn that protons — the particles inside every atom of everything you know, including stars themselves — contain pressures 10 times greater than that. How did it take us this long to find out?
The first measurement of a subatomic particle’s mechanical property reveals the distribution of pressure inside the proton.
Just Squeeze Me
Before we explain this new discovery, it's best to back up and explain just what's under pressure inside a proton in the first place. Protons exist in the nucleus of every atom in the universe. You know how every element has its own number on the periodic table? That's the number of protons contained in an atom of that element. But if you could zoom in even further past the inside of an atom to the inside of a proton, you'd see even more particles. Those are called quarks and gluons, and they're what's known as elementary particles, since they can't be broken up any further. Quarks and gluons are just two types of elementary particles, and there are others you're probably more familiar with: the electron, for instance.
When scientists say that protons are under pressure, it's the quarks they're talking about. Inside the proton, those elementary particles withstand pressures of 1035pascals, according to the new paper published in Nature. That's roughly 1030 times the pressure you experience at sea level here on Earth. But protons are incredibly tiny, and quarks even more so. How do we know what kind of pressure they're under?
Who's Down With GPD
We know because of a particularly brilliant strategy conceived by Volker Burkert, Latifa Elouadrhiri, and Francois-Xavier Girod at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility. See, in 1966, a physicist named Heinz Pagels theorized that we might be able to probe the mechanical structure of a particle like the proton by shooting it with a beam of gravitons to find what's known as its gravitational form factor. The graviton is a hypothetical particle thought to be responsible for the force of gravity. The only problem here is "thought to be" — we've never actually detected a graviton, and since gravity is the weakest force in the universe, it's unlikely that we ever will. As you might imagine, then, shooting a proton with a beam of gravitons to probe its structure is a pretty far-fetched idea, and even Pagels himself didn't think it would happen.
But recent research has shown that you can substitute a more realistic measurement for a particle's gravitational form factor. To get that measurement, called the generalized parton distribution or GPD, scientists perform another heavily acronymed process called deeply virtual Compton scattering, or DVCS, which involves sending an electron into a proton to exchange energy with one of its quarks. The proton then releases that energy, which the scientists analyzed to produce a 3D image of its electromagnetic (but not mechanical) structure. The Jefferson Lab researchers used what they knew about the similarities between electromagnetic probes, which are real, and gravitational probes, which are hypothetical, to convert that analysis into information about the proton's mechanical structure — and therefore, the pressure inside of it.
Now that we know that protons are under an immense amount of pressure, what does that tell us? Well, considering that protons are inside every atom in the universe, it tells us a whole lot. Everything new we learn about them tells us something new about matter. And as common as the proton is, there's still a lot to discover. According to Gizmodo's Ryan F. Mandelbaum, scientists are still puzzling over its size, its decay process, and its other fundamental properties. When you think about the fact that this little particle makes up every bit of you, the more we know about the proton, the better.
For a more fundamental understanding of the particles that make up our universe (and the forces that govern them), check out "The Cosmic Machine: The Science That Runs Our Universe and the Story Behind It" by Ph.D. physicist Scott Bembenek. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.