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I just find it useful to consider the progress through space of massive things as being qualitatively different than the progress through space of massless things. The distinction between "motion" and "propagation?" In the context in which I'm speaking here, it's purely a semantic one. Change back to light-seconds and seconds, and c is once again sensibly and sanely equal to one. But that's just because you're working in a stupid system of units. If you use units other than the correct ones - for instance, if you decided to be stupid and use the meter and the second - you'd find that the numerical value of the speed of light is some oddball number. It's equal, in other words, to one, when you use appropriate units for distance and duration. In our universe, that speed of propagation is equal to one light-second per second. Instead, it just continues propagating as fast as it can, because nothing interferes with it. Well, in the vacuum, light doesn't interact with anything, so it's not delayed at all. Glass is denser than air, so light is delayed more frequently as it propagates through glass than when it propagates through air. ![]() In essence, every time light interacts with matter, it gets delayed slightly. Light that propagates through glass, for example, does so at a rate slower than the rate at which it propagates through air. The rate at which light propagates depends on what its propagating through. ![]() (I reiterate that what I'm describing right now is philosophy rather than physics, so please bear that in mind.) Instead, it propagates. Light, on the other hand, does not "move" in the same sense. Just take it, for the moment, as an axiom. Again, there are geometrical reasons for this that I won't bother to elaborate on right now. The speed of a thing that moves can never be measured, by anything else that moves, to be greater than c. Of course we know that "speed" is only meaningful when described relative to something, so that naturally raises the question of relative to what is that speed limited to less than c? The answer is relative to anything. Things that move are limited, for a variety of reasons involving geometry that I won't bother going into right now, to speeds less than the speed of light. It's a subtle distinction, and one that's frankly not all that well grounded either in theory or observed fact, but it helps focus the mind. The constant threat of defeat adds importance and tension to every action.This becomes a bit clearer, at least to me, when you stop thinking of light as something that moves and start thinking of it instead as something that propagates.
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