Summary
Strong force is 100 trillion trillion trillion times stronger than the force of gravity. Scientists don’t know precisely how strong the strong force is. The theory describing how it works is so complicated we can’s use it to make direct calculations. The strong force accounts for the origin of around 99 percent of the mass in the visible universe.
If αs is a constant, how can it change? The answer has to do with the concept of quantum loops, also known as vacuum polarization. For most of the forces, the couplings change slowly with distance. For the strong coupling, however, the change is huge.
The problem is that αs grows too quickly. Before we can reach a fermi, αs becomes too big for the standard calculation method to be applicable. This is why the (not very) long-distance domain became Terra Damnata. It was crucial to determine what αs does at a distance.
The Bjorken integral manages to filter out most of the multiquark processes and separate out effects on individual quarks. It turns out, this calculation of αs would not have worked with almost any other type of nucleon data. The result, published in 2010, was gratifying: their αs matched Deur’s experimental data exquisitely.
Deur published his first results on αs in 2005, almost 20 years ago. Until this point, theorists had employed two parallel strategies to use QCD’s equations of motion. The “top-down” approach tried to predict the properties of gluons. The "bottom-up" approach aimed to use directly measurable quantities to infer αs.
The CEBAF Large Acceptance Spectrometer took some of the measurements that helped to define the strong force at a scale never possible before. The key finding is that as the distance grows greater, the coupling stops growing, and the inconstant constant becomes constant once more.
