I always thought this was a cute physics joke and never really thought about it very deeply. Upon a small amount of consideration though you would think that the speed of 'dark' i.e. the lack of light would be equal to light's speed at vacating or illuminating a scene i.e. equal to c. In observation this is not 'totally' true. Due to quantum mechanics it could be calculated that the presence of light on a scene actually slightly precedes an illumination event and correspondingly the absence of light on a scene will succeed the illumination stop event.
Where does this go? It seems to go to the root of the time travel problem in that the speed of dark is not c but actually -c ... an interesting conjecture indeed in that it implies that dark would be an inverse time phenomena ...
Whether or not this is actually true it leads to some interesting ideas and thought experiments. Which came first, the light or the dark, depends entirely on your time vector.
Have fun and dust off those QM textbooks!
