Actually, I deal more with pre-energy “subspace” but your question has got me thinking a little on this matter (mass).
The term “mass” in physics is a little ambiguous but essentially relates to either inertia or gravitational effect. Something is called “massless” when it has neither of those properties even though it is still given a “mass equivalence”. Technically, metaphysically, nothing can exist without the property of inertia because that term merely means “reluctance to change”.
But if we consider that a photon is a spinning entity that travels forward at the speed of light (by definition), then any actual spin posses an interesting conclusion. If something is moving forward and also spinning, that which is spinning within is actually traveling faster than the over all object. Assuming the spin was at the speed of light and the object was also traveling at the speed of light, the inner wave would represent an affect that was changing at a rate of sqrt(2) = 1.414 times the speed of light. I haven’t gotten far enough along with my project to determine whether subspace maximum travel speed is greater than that of photon travel, but I’m getting more suspicious every day.
But the idea of energy came from the notion of how much effort it takes to accomplish “work” or to cause any degree of change. It began, from the equation F = ma because the change being considered was the total effort required to move something against gravity, or any constant acceleration. Once anything reaches any particular speed, given no resistance, by definition it will continue without effort. So only issues of changing something’s speed were of interest.
Forces are always applied between two things rather than merely “from” something as many tend to think. So the issue is how much force for how long of a time is required to overcome the inherent inertia (reluctance to be changed) within 2 objects in order to accomplish a degree of changing in state within a degree of time, a velocity per time.
The reluctance is an issue of the inner wave already traveling at a maximum value, so of course the speed of light, or more properly, the speed of affect, would have to come into play in determining the actual amount of inertia or mass each object had in the first place. Then it all becomes a question of how much effort is going to be required to force against said inertia enough to cause a separation at any particular rate. “F = ma” was measured to be pretty exact, but of course it depends on how exactly the terms are defined and in regards to what.
If you multiply the reluctance to change times the speed of changing, you get a value related to mc. But that assumes no acceleration and basically describes the momentum of an object, inertia in relative motion. But anything already in motion isn’t requiring any effort. To get a measure of how much effort it takes to get anything up to a speed, we have to multiply that momentum concept times the speed involved, and thus "Effort = mc^2".
But all of that is merely referring to energy between objects due to their inertia, not how much potential is within an object to yield that effort if released to do so.
Inside any particle, we have the reluctance to change that is already traveling at a maximum change rate, c’ . But it is also curving into a sphere or bundle and thus accelerating by virtue of constant change in direction rather than linear acceleration. And that changing in direction would be occurring at the max speed of change as well. Thus within any particle, we have the reluctance to change, the inertia or mass times its internal speed of changing times its speed of turning =>
Inner Effort = m*c’*c’ = mc^2. Again, assuming that the speed of affect is the speed of light or close to it.
I’m sure Einstein didn’t take that route at all in his derivation, but it seems that he would have been right in his conclusion merely from a superficial analysis of what is going on. Eventually, if I can complete my project, I can tell you far more precisely exactly what is going on and exactly how much “energy” is related to any inertia (although to relate it to mass would require physics to get their definitions more coherent and precise).