Evrim Yazgin
Cosmos science journalist
A method which connects gravitational theories to interactions of the smallest particles of matter inside protons has revealed one of the three unknown measurements of protons.
Nuclear Strong Force vs. Gravity
The answer to the new measurement lies in gravity.
Gravity is one of the four fundamental forces of nature – gravity, electromagnetism, and the strong and weak nuclear forces. Gravity is also the weakest of the four.
Gravity – to have a significant effect – requires objects with enormous mass. That’s why gravity is associated with the massive scales seen in the universe of moons, planets, stars and galaxies.
On the other end of the spectrum, the most powerful force in nature is the nuclear strong force. This “glues” nucleons (protons and neutrons) together, as well as binding together subatomic particles called quarks to make protons, neutrons and other exotic particles.
The nuclear strong force is 1043 (a number with 43 zeroes) times stronger than gravity. But it only works on scales 1015 (a thousand million million) times smaller than a metre.
One of the fundamental problems in modern physics is how to marry the effects of gravity with the other forces which would become in effect a grand theory of everything.
Until such a unifying theory is discovered, physicists have to come up with new and ingenious ways of seeing the interplay between gravity and the world of the ultra-tiny.
Peering inside the proton
Now nuclear physicists at the US Department of Energy’s Thomas Jefferson National Accelerator Facility have, for the first time, revealed a snapshot of the distribution of the strong force inside the proton.
“At its peak, this is more than a four-ton force that one would have to apply to a quark to pull it out of the proton,” explains Jefferson Lab Principal Staff Scientist Dr Volker Burkert. Burkert adds that this is just an illustration of the strength of the force, but nature wouldn’t actually allow a single quark to be removed.
The research, published in the Reviews of Modern Physics journal, details only the second mechanical property of the proton to have been measured. Nuclear physicists call this the measurement of the proton’s shear stress.
The proton’s internal pressure was measured in 2018. Its mass distribution (physical size) and angular momentum remain unmeasured.
Next up, the researchers hope to determine the proton’s mechanical size for the first time.
“Now, we can express the structure of subnuclear particles in terms of forces, pressure and physical sizes that also non-physicists can relate to,” says Burkert.
Co-author, Dr Latifa Elouadhriri agrees, adding: “In my view, this is just the beginning of something much bigger to come. It has already changed the way we think about the structure of the proton.”
