How big is the proton? That seems like a straightforward question, but a clear answer is hard to come up with. Several experiments have reported measurements of the proton radius, but their values differ by 4%. The puzzle is sometimes framed in terms of the proton’s three-dimensional charge density, but this is a misconception, says Gerald Miller from the University of Washington, Seattle. In a new study, Miller shows how the proton radius can be defined in a unified way according to photon-proton interactions.
Read more at APS Physics