It's been nearly 25 years since the European Muon Collaboration made a startling discovery: only a portion of a proton’s spin comes from the quarks that make up the proton. Thee revelation was a bit of a shock for physicists who had believed that the spin of a proton could be calculated simply by adding the spin states of the three constituent quarks. This is often described as the "proton spin crisis." Tags: particle physics - Charles Daney