Why can an electron mass vary from zero to infinity?
  When a particle is in high speed or bound in the Coulomb potential of point nucleus, the variation of its mass can be ascribed to the variation of relative ratio of hiding antimatter to matter in the particle. At two limiting cases, the ratio approaches to 1. 
Guang-jiong Ni, Weimin Zhou, Jun Yan (Department of Physics, Fudan   University, Shanghai, P.R.China)