Professor Gene Hecht – author of the leading Optics textbook and Adelphi Professor of Physics – has us talking about fundamental physics yet again, potential energy, mass, relativity, etc. [It all seems so simple until Professor Hecht poses a gedanken experiment I can’t answer.] Read his comment on his 2016 essay on Relativity, potential energy, and mass:

http://iopscience.iop.org/article/10.1088/1361-6404/aaf5e1/pdf