Inside every proton, 10 neutron stars

The pressure inside a single proton is enormously greater than that found inside a neutron star, according to the first measurement of the internal mechanical properties of subatomic particles.

In a study published in the journal Nature, a team headed by nuclear physicist Volker Burkert of the Thomas Jefferson National Accelerator Facility in the US reports that quarks, the building blocks of protons, are subjected to a pressure of 100 decillion pascals at the centre of the particle – about 10 times the pressure at the heart of a neutron star.

Pressure inside the particle, however, is not uniform, and drops off as the distance from the centre increases.

“We found an extremely high outward-directed pressure from the centre of the proton, and a much lower and more extended inward-directed pressure near the proton’s periphery,” explains Burkert.

A proton is made up of three quarks, bound together by what physicists call the strong force. It is one of four fundamental forces that condition the universe. Two of these – electromagnetism and gravity – produce effects that govern macro-scale interactions. The other two, known as strong and weak, operate on a subatomic scale and determine nuclear reactions.

Obtaining detailed information about the internal mechanics of a subatomic particle has long been thought impossible, but Buckert and colleagues managed to do so, ironically enough, by combining modelling systems that rely on electromagnetism and gravity.

The researchers paired two theoretical frameworks to obtain their data.

The first concerned the distribution of partons – a term coined by physicist Richard Feynman to describe a method of modelling point-like entities inside protons and neutrons, namely quarks. Parton modelling allows researchers to produce a three-dimensional model of a proton as probed by the electromagnetic force.

The second framework involved gravitational form factors, which describe the scattering of subatomic particles by the classical gravitational field.

Combining the two approaches and applying the result to data obtained by using electron beams produced by a continuous beam accelerator at the Thomas Jefferson facility yielded world-first information.

“This is the beauty of it,” says co-author Latifa Elouadrhiri. “You have this map that you think you will never get. But here we are, filling it in with this electromagnetic probe.”

The findings are likely to generate great interest among other physicists.

“We are providing a way of visualising the magnitude and distribution of the strong force inside the proton,” says Burkert.

“This opens up an entirely new direction in nuclear and particle physics that can be explored in the future.”