[Coda]

Hmm. I suppose that's not bad as a rough visualization.

At any scale that humans can meaningfully interact with without technological aid, the speed of light might as well be infinite, because any time elapsed for light to get from point A to point B is so small that it's not going to influence the significant figures of your measurements. The speed of electrical propagation is likewise beyond meaningful consideration. The speed of sound is even fast enough to be irrelevant in most cases. (And the speed of sound is a particularly relevant metaphor, because it determines how long it takes for the far end of an object to respond to a force applied to the near end.)

As for the English question: Both are grammatically correct, but they have different meanings. If you can swap around the adjectives without influencing the meaning, include the comma. If swapping them makes the phrase mean something different, then leave the comma out.

So for example, if you have multiple greasy films and you want to refer to the dark one, "dark greasy film" is correct. However, if you only have one film, and it happens to be both dark and greasy, then "dark, greasy film" and "greasy, dark film" are equivalent.

In the case of "Americans" the distinction is particularly significant. Are you referring to the set of middle-class Americans who are white, or are you describing a group of Americans as being both white and middle-class?