The origin of spin

Xiang-Song Chen, Huazhong University of Science and Technology
March 26, 2013, 3:30 pm

The nucleon spin problem, namely how the nucleon spin originates from the spin and orbital motion of quarks and gluons, reveals unexpectedly that our understanding of such basic notions as spin, orbital angular momentum, and gauge symmetry may be insufficient. We explain that the same issue actually applies also to electrons and photons, and can be addressed unambiguously by experiments. Some such experiments even exited already, indicating that the photon spin and orbital angular momentum behave distinctively and cannot be universally expressed as the first moment of the Poynting vector. We propose new experiments to pin down the origin of spin.