Speed of Dark

My local community radio station gets a little freeky this time of year. They are a great bunch of guys and gals.

Well, the other day they asked the question: What is the speed of dark. I came up with two possible answers:

1-Darkness has no speed, it is the absence of light.
2-286,282.397 Miles/Second. The speed of the receding light.

Well, they announced that the speed of dark was slightly faster then the speed of light. I kinda want to say BS to this. But first I thought I’d check with u all.

Are there any physicists out there that can shed any light on this??


Darkness being the absence of light clearly means that the “speed of dark” and the speed of light are exactly identical.

Isn’t there a (thought) experiment whereby an object’s shadow actually moves faster than c ? I believe I remember reading that it doesn’t violate Special Relativity because it is incapable of transmitting any information faster than c. If you stretch the definition of “dark” to be a shadow, maybe that qualifies.

Yes, it is possible for a shadow to move at more than the speed of light, but it is also possible for a spot of projected light to move at more than the speed of light for exactly the same reasons - the spot of light (or the shadow, which is directly equivalent to everywhere that the spot of light isn’t) is made up of a succession of reflected photons - no single photon exceeds c.

Picture it this way; I have a high-pressure water hose and a wall; I use the hose to squirt water at the wall, I now turn the hose rapidly so that the jet of water moves sideways across the wall, wetting new areas.
The speed at which new areas are being wetted can exceed the speed of water in the jet itself, but at no point is any part of the water jet moving faster, in fact, the ‘sideways moving jet’ is largely an illusion - all that is happening is that the jet is ceasing to squirt in one direction and beginning to squirt in another.

I used to explain Mange’s light/dark spot travelling faster than light by a line of weights falling consecutively, or a “domino rally”: It looks like something is moving at a certain speed, but this is an illusion. The illusion is shattered by having the weights fall in a random order, or knocking all the dominoes over simultaneously.

Or how about this:

Take a laser pointer and aim it at the sky, then draw a line accross the sky with it. Imagine that somehow your laser beam isn’t absorbed by the athmosphere, or stopped by a celestial body, and that there is a big wall at the end of the universe. The red dot that will appear on that big wall will move a lot faster than c, even though no single photon will.

Nice one (and much easier to understand than the water jet).

Importantly, there is no casuality, the movement of the spot from point A to point B on some surface cannot be used to trnasfer information between the two (to do that point A would have to transmit information to the source, which the source would have to transmit to B which clearly takes longer).

Darkness is the absence of light, in the same way that cold is the absense of heat.

Light is an object that can be measured and has. Darkness is a void, not an object and has no measure to measure by.

By the way, who discovered that light travels 186,000 miles per second anyway?

The first person to estimate the speed of light was Olaf Roemer, in 1675. He was a bit off in his measurement, figuring about 226 663 000 m/s while the actual value is 299 792 458 m/s.

There was an interesting article about Roemer and his measurement of light in the American Journal of Physics about 20 years ago or so. It surveyed several books giving accounts of his work, and found that they gave vastly different results. Then they went back to Roemer’s original work and, they claimed, found that Roemer hadn’t actually calculated a value at all! (I find this hard to believe – I figure that he must at least have figured a range of values, but I haven’t gone back to look through Roemer’s work). In any event, they claimed that assertions about Roemer’s value are all untrustworthy.

Roemer, at the very least, proved that light has a finite velocity (he did it by measuring the time lag between the expected appearance of Jupiter’s satellites and the actual appearance), something that had been in question for a while, at least since Galileo’s attempt to measiure the speed of light between two mountains failed. AFAIK, the undisputed values for measuring the speed of light first occurred in the 19th century, with experiments like Fizeau’s, using rotating mirrors.

(We had students duplicate this experiment for an optics class I taught)

As far as the “speed of dark”, there was once a hack article in an optics journal about the “darkon theory of light”, which suggested that there are actually particles of negative energy, called “darkons”, that carry dark. You can measure the speed of darkons by measuring the rate of travel of an interruption in a row of darkons, which can be called a “photon” by the uninformed. So the speed of a darkon is the same as the speed of light.

The most dramatic example of dark moving faster than light is the movement of shadows within Hubble’s Variable Nebula.

Here is an animation
And here is a more detailed description of the shadows and the nebula responsible.

SF worldbuilding at

I do not like posting a message and seeing that it did not get posted…

I never thought about the object emitting the light moving… Would this effect the speed of dark??

Dark doesn’t have a speed, because dark isn’t something that exists by itself. It’s merely a side effect.

Think of it this way: When you turn off the light in a room, you describe it as being dark once photons are no longer reflecting off of surfaces in the room to your eyes. This happens right after the last of the photons leaving the light bulb reach your eyes. Since those photons reach you by traveling at c, the “dark” has spread throughout the room at the heels of the photons at exactly the same speed (c). Thus “dark” can be said to always travel at the speed of light.

But remember that “dark” isn’t something that can be measured by itself. “Dark” is merely where photons aren’t.

I’d hazard a guess that this radio station wasn’t asking about the speed of dark as a scientific question, but rather for fun. Didn’t Terry Pratchet, author of the Discworld novels, once have a character comment that the only think in the universe faster than light was dark, because it has to get out of the way of light?

And how do you explain it nowadays? 'Cause I didn’t understand a single word of your “old” explanation.

No, that’s not true, but I did have to think about a bit before I got your analogy. The thing is though, with your domino explanation, isn’t information moving (at the rate of the collapsing front) – that information being that the first domino (or one in between (if one was to nipick)) had been knocked over?

Your right, Great unwashed, the domino analogy doesn’t work, as you have what is essientially a wave travelling through the dominoes, so there is a causal relationship between two dominoes falling over wheras with a light spot moving from point A to B there is no casual relationship between the two points.

Great! It’s ages since my right.

So is there a theoretical maximum to how fast the point of light (of the laser pointer) moves on the wall at the edge of the universe? or what is the maximum speed of the dominoes falling?


Firtsly there’s no wall at the edge of the universe and indeed no edge to the universe. There’s no lmit to the speed of the spot as the wall it is shining on can be arbitarily far away. The limit on the dominoes falling is c.