James' World 2

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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 α_s (“alpha s”), is by far the least understood of these quantities. The precision of the best measurements of α_s 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 α_s in this range is scarce. Until recently, no one had made any experimental measurements of α_s at this scale. Theoretical predictions for its value were unhelpful, covering the entire span from zero to infinity.

The strong force’s might makes it difficult to study in lots of ways. The theory describing how it works, called quantum chromodynamics, is so complicated we can’t use it to make direct calculations or precise predictions. One of the reasons for this complexity is that the carrier of the strong force—a particle called the gluon—interacts with itself. Electromagnetism, in comparison, is simple because its carrier, the photon, is chargeless. But the gluon carries the strong force’s version of charge, called color, and its self-interactions quickly get out of hand. So despite its importance to nuclear physics and building the material world, the strong force is not unconditionally loved by researchers. Instead many look at the domain where the strong force is truly strong as a “Terra Damnata,” a realm to avoid at all costs.

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Deena So’oteh

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