It originally seemed that light had the property of energy but no rest mass and matter had the property of mass but no rest energy, until Einstein concluded that light had a relativistic mass and matter had a rest energy that could be released in nuclear bombs. It became apparent that mass and energy were somehow related.
Mass was originally defined as weight but this was later changed to be called gravitational mass. Newton’s discovery that a mass needed a force to accelerate it led to the different definition of inertial mass. They are different because a weightless object in space still needs a force to move it, i.e. it has inertial mass even though it has no gravitational mass. If momentum is defined as mass times velocity, a massless photon should have no momentum but solar sails move when the sun shines on them and photons are bent by the gravity of the sun. This led to another definition upgrade as a photon with no rest mass was said to gain relativistic mass as it moves to give it momentum.
Light was originally seen as pure energy, where Planck’s relation E = hf defined the energy E of a photon as its frequency f multiplied by Planck’s constant h. The last chapter interpreted this relation in processing terms. If a photon is one quantum process spread out, its energy transfer was the processing transfer rate at the node. Since the throughput per node reduces as more nodes share the process, a photon’s energy decreases as wavelength increases. Conversely as wavelength decreases, fewer nodes running the same process each get a bigger share so photon energy increases with frequency. If matter is made of photons running at the highest possible frequency, it follows that the inherent energy of matter relates to the energy of those photons.
Einstein’s equation E=mc2 does for matter what Planck did for light, defines its energy. In 1905 he deduced that the energy of matter is its mass times the speed of light squared, and atom bombs confirmed this, but it has never been clear why mass relates to light at all. If mass is an inherent substance, why does its energy refer to the speed of light?
In quantum realism, an electron is many photons repeatedly colliding in many channels. Each channel contains the equivalent of a photon with a one node wavelength, whose energy by Planck’s relation is Planck’s constant times the speed of light divided by one Planck length. If Planck’s constant is one quantum process transferred over a Planck length squared per Planck time, substituting for Planck’s constant in Planck’s relation gives Einstein’s equation for mass and energy. Quantum realism thus derives Einstein’s equation from the conclusion that matter is light condensed.
PS. In this model, the speed of light c=LP/TP, for LP Planck length and TP Planck time. A photon’s energy EP=hP.c/l, for hP the energy of one quantum process transfer, c the speed of light and l the wavelength. In an electron l is one node, so EP=hP.c/LP. If mass m is the program that repeats, h transfers m over a Planck length square every cycle, i.e. hP=mp.LP.LP/TP. Substituting gives EP= mp.LP.c/TP, or EP=m.c2. This derivation doesn’t prove E=mc2. Einstein did that based on how our physical world behaves. It just finds this model consistent with Einstein’s equation.