A muon has about 207 times more energy than an electron. It seems that there must be some difference between these two particles that produces this difference. The standard model says that fundamental particles are “excitations” of their respective fields. Does this imply that there is structure to each particle that defines its energy, charge, spin and whether it is matter or anti-matter? Einstein and Wheeler both have given quotes which indicate that fields (and ultimately matter) are states of spacetime. If you believe that there is internal structure, what characteristics must the structure have? If you believe in point particles with no internal structure, then what determines the difference between particles? If you believe in vibrating one dimensional strings, then what are these strings made of and what is the difference between an electron and muon?