proton radius puzzle
... that different measurements of the size of the hydrogen atom nucleus when a muon replaces an electron is an unsolved problem in physics known as the proton radius puzzle?
... that different measurements of the size of the hydrogen atom nucleus when a <a href="./Muon" id="mwBGM" rel="mw:WikiLink" title="Muon">muon</a> replaces an electron is an <a href="./List_of_unsolved_problems_in_physics" id="mwBGQ" rel="mw:WikiLink" title="List of unsolved problems in physics">unsolved problem in physics</a> known as the <b id="mwBGU"><a href="./Proton_radius_puzzle" id="mwBGY" rel="mw:WikiLink" title="Proton radius puzzle">proton radius puzzle</a></b>?