The Strong Force Comes Into Focus
Scientists now have theories and experiments to describe how the strong force behaves across its full range of energy and spatial scales.
The strong force, one of the fundamental forces of the universe responsible for the existence of matter, has been notoriously challenging to explain through theory or experiments. At short distances, the strong force is, well, strong, stronger than the electromagnetic force that would cause same-charge protons to repel each other. Gluons are the force carrier of the strong force that bind matter particles together, the glue of matter, if you will.
Quantum Chromodynamics (QCD) is the theory that describes the strong force. It’s complicated by the gluons interact with other gluons.
Recent theoretical advances and new experimental data have finally come together to explain a key parameter of QCD, called the coupling constant. While the other forces’ coupling constants have been measured to impossible levels of certainty, the coupling constant of the strong force has only been measured to less mind-boggling degrees of certainty, particularly in domains of very high energy, a numerical range ominously called Terra Damnata (forbidden territory).
About 99% of the mass of the visible universe—the material universe of atoms and the like, comes through the strong force. The Higgs boson provides the small but critical remaining 1%.
The mass tied up by the strong force comes primarily from atoms. Most of the mass in atoms is tied up in protons and neutrons. Electrons, by contrast, are exceedingly light. Now, you would logically think that the constituent matter particles that make up protons and neutrons, called quarks, account for most of their mass, but you’d be wrong. Quarks also have very little mass of their own, so what gives? Where is all the missing mass?
The recent revelations about the strong force coupling constant help answer that question. This is the cool part: At the scale of proton, quarks gather clouds of force-carrying gluons around them, generating most of the mass of the proton. This is profound. The binding energy of the strong force is responsible for nearly all the mass we see around us, because mass and energy are all part of the same construct through E=mc². The Higgs boson contributes only the small remainder that the fundamental particles like quarks and electrons possess on their own.
There are lots of practical implications that could follow this improved scientific understanding of the structure of matter, especially how matter behaves at extremely high temperature and pressure.
Knowing the behavior the strong force coupling constant at long distances also means that QCD is the first full quantum field theory that yields only finite results across it’s operating energy domain. This might seem like just a mathematical feather in the cap to most of us. But all the other known quantum theories yield unresolvable, infinite quantities at very high energies. Despite being one of the most difficult forces to understand, the model of the strong force may now lead to analogous advances in understanding the other forces, potentially leading to finite resolutions. Finite results provide definitive limits and solid answers to big questions.
QCD optimists think the recent revelations might even be applied to reveal whether string theories that require higher dimensions of spacetime are necessary to explain the universe or whether our solidly four-dimensional spacetime is good enough. Time will tell. Regardless, physicists have new theoretical territory to explore.