The strongest force in the universe is called, aptly, the strong force. We never get to witness its fearsome power because it works only across subatomic distances, where it binds quarks together inside protons and neutrons and joins those nucleons into atomic nuclei. Of the four basic forces of nature, the strong force is by far the most potent—it’s 100 trillion trillion trillion times stronger than the force of gravity. It’s also the most mysterious.
Despite knowing roughly how it compares with the other forces, scientists don’t know precisely how strong the strong force is. The other three forces—gravity, the electromagnetic force and the weak nuclear force (responsible for some radioactivity)—are much better measured. The strength of electromagnetism, for example, denoted by its “coupling constant,” has been measured with the same precision as the distance between New York and Los Angeles, to within a few hair breadths. Yet the strong force’s coupling constant, called αs (“alpha s”), is by far the least understood of these quantities. The precision of the best measurements of αs is 100 million times worse than that of the electromagnetic measurement.
Even this level of (un)certainty is known only in the simplest domain of the strong force theory, at very high energies involved only in some of the rarest and most extreme events in nature. At the lower energies relevant to the world around us, the strong force earns its name by becoming truly intense, and concrete information on αs in this range is scarce. Until recently, no one had made any experimental measurements of αs at this scale. Theoretical predictions for its value were unhelpful, covering the entire span from zero to infinity.
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