I haven't posted for quite a while now since I've been very busy with work. I'm also preparing a paper for a conference this summer, for which I've thrown aside many extra activities. I probably won't be posting again for another month or so.
In the meantime, I thought you might find interesting this great quotation from David Bohm on rest mass as inward movement, and light as pure outward movement:
In this connection it must be noted that every form of energy (including kinetic as well as potential) contributes in the same way to the mass. However, the “rest energy” of a body has a special meaning, in the sense that even when a body has no visible motion as a whole, it is still undergoing inward movements (as radiant energy, molecular, electronic, nucleonic, and other movements). These inward movements have some “rest energy” E0 and contribute a corresponding quantity, m0 = E0/c2 to the “rest mass.” As long as the energy is only “inward,” the rest mass remains constant, of course. But as we have seen, internal transformations taking place on the molecular, atomic, and nuclear levels can change some of this to-and-fro, reflecting “inward” movement into other forms of energy whose effects are “outwardly” visible on the large scale. When this happens, the “rest energy” and with it, the “rest mass,” undergo a corresponding decrease. But such a change of mass is seen to be not in the least bit mysterious, if we remember that inertial and gravitational masses are merely one aspect of the whole movement, another aspect of which is an equivalent energy, exhibited as a capacity to do work on the large scale. In other words, the transformation of “matter” into “energy” is just a change from one form of movement (inwardly, reflecting, to-and-fro) into another form (e.g., outward displacement through space).
It is particularly instructive to consider how, in this point of view, one understands the possibility for objects with zero rest mass to exist, provided that they are moving at the speed of light. For if rest mass is “inner” movement, taking place even when an object is visibly at rest on a certain level, it follows that something without “rest mass” has no such inner movement, and that all its movement is outward, in the sense that it is involved in displacement through space. So light (and everything else that travels at the same speed) may be regarded as something that does not have the possibility of being “at rest” on any given level, by virtue of the cancellation of inner “reflecting” movements, because it does not possess any such inner movements. As a result it can exist only in the form of “outward” movement at the speed c. And as we recall, the property of moving with the speed of light is invariant under a Lorentz transformation, so that the quality of the movement as purely “outward” does not depend on the frame of reference in which it is observed. (On the other hand, movements at speeds less than c can always be transformed into rest by a change to a reference frame with velocity equal to that of the object under consideration).
Bohm's insights here have significance when combined with Hans Jonas's insights on "mediacy."
David Bohm, The Special Theory of Relativity (New York: Routledge, 1996), 90-91.