LIGHT ON THE DISCWORLD

There are (at least) four forms of light on Discworld:

•    common light — to see things by. Its speed was measured to be equal to that of sound by the Ephebian philosopher Febrius in his "Give me a shout when you see it, OK?" experiment involving two hills, a lantern, and an assistant with a very loud voice. Splits into eight colours (ROYGBIV+ octarine) in a strong magical field.
•    meta-light — to see dark by. Widely used in the moving picture industry.
•    dark light — absorbed by Uberwaldian Deep Cave Land Eels: light within darkness. Heaver than normal light so mainly found under the sea or in deep caves in Uberwald.
•    light fantastic — not strictly light at all but the opposite of light: the light that lies on the far side of darkness.

Also related is
•    dark — not the absence of light, as it is in Roundworld, but a thing in its own right. Relates to light as knurd relates to drunk (in which case Roundworld ‘dark’ is ‘sober’). Thanks to The Science of Discworld Chapter 22 we know that dark is a privative.
    The speed of dark is known to be greater than that of light. It must be, otherwise the dark wouldn’t be able to get out of the light’s way.
    This led to an epic discovery by the famous* Troll mathematical physicist Big Al Onestone, who originally worked as a clerk in a patient office†.

* Well, he was famous in Slakki
† A doctor’s waiting room, so named because you wait in it, and have to be patient.

He became famous when he devised the:
THEORY OF SPECIAL, GENERAL, AND
TOTALLY INDISCRIMATE RELATIVISM

Define
               c = speed of light ~ 600 mph
                  d = speed of dark ≥ 601 mph
According to Big Al Onestone’s TOSGATIR each contributes to the inherent energy of matter:
               EL = mc2      light energy
               ED = -md2   dark energy ✌

✌ Note the minus sign.

We know that d > c as already mentioned. So when “the soft yellow light of dawn flow[s] over the landscape like the caress of a gentle lover, or, as some would have it, like golden syrup” we observe a curious phenomenon, unremarked by anyone until Big Al started asking stupid penetrating questions:
    As dawn flows over the Disc, the gap between light and dark regions will grow at a rate
               d –c > 0
This is both a problem and a major insight.

INSIGHT
    This is why a round world makes no sense. On a sphere, the dark would pass round the back and eventually catch up with the light. Physicists do not yet understand what happens at that point, but they suspect that a singularity develops.
    It is a pity this was not discovered earlier as it would have saved a lot of difficulty with Omnianism.
light moving across the Disc
WHITE: light.
BLACK: dark.
SPOTTY: Well, that's an interesting question:

PROBLEM
    What is in the gap?

Onestone hypothesized that the gap is occupied by a second form of dark, which he called twidark.
               b = speed of twidark ~ ? mph
What is b? There are various theories.
•              b = 0. Twidark is left behind when dark moves, and is removed again when the light arrives.
•              b is variable. Its speed depends on what it is relating to. At a twidark/light boundary, b = c. At a twidark/dark boundary, b = d. So one of the fundamental constants isn’t.
    The disadvantage of this theory is that it does not require a third or fourth type of dark to fill the gaps between twidark and light or twidark and dark — which would be far more fun.
•              b = umpty. This theory is favoured by the Bursar.