This simple result has great significance for the philosophy of physics because it makes a clear separation between the experimentally prover) Doppler effect and mass increase, and the controversial length and time changes. It adds further experimental evidence of the wave structure of matter.
Not surprisingly, the origins of relativity are found in the quantum behavior of matter and the way in which energy (light) is exchanged between two charged particles (wave resonances). The relativistic mass increase is a result of the Doppler changes of frequency of the advanced and retarded spherical waves from the electron. These spherical waves produce the correct mass increase, regardless of whether the two electrons are moving together or apart. This unusual combined Doppler change results from the symmetry of the advanced and retarded waves. This mechanism is also responsible for the de Broglie wavelength, also in direction of relative electron motion.