## If there is a speed of light, does that mean there is also a speed of dark?

The "speed of light" refers to the time taken for a pulse of light to travel to a mirror and back divided into twice the distance to that mirror. (It has to be done that way because of the way time works in relativity.) [math]c = 2L/T[/math], where L is the distance to the mirror, and T is the time taken to return to the source/detector setup.

This speed will be different for different media, and for a vacuum, this speed is the same for all observers.

We can do this because light is a thing that can travel from place to place. Darkness, however, is not. There are no "darkness particles" that mysteriously fill the void between photons. Therefore talking about the "speed of darkness" in this way makes no sense. The closest you can get is to have a beam of light, which you switch off for a short time and then switch back on. Then you have made a "pulse of darkness" in the sense that darkness may be defined as "absence of light".

However, "obscurity" is not so much the absence of light as the absence of illumination.

Illumination happens when light is scattered from a surface. When all the walls of a room are illuminated, we say the room is lit. When it is very well lit, we say it is "filled with light". But you should realise that these are common-use terms and have no scientific standing.

A spot of light can be moved across a surface as fast as you like - and likewise you can move a spot of obscurity in the same way you can have a gap in a beam of light.

The upshot of this discussion is that darkness does not have a speed in the conventional sense, but may be seen as a speed, the same as the speed of light, in contrast to the surrounding light.

(Then there is the issue with variable refractive media.)

You can measure how cold it is by considering the lack of heat. Likewise you can measure the darkness by measuring the lack of light. We perceive light by the presence of photons, and darkness by the lack of photons. To any degree, the speed of dark is the rate of which light is removed from your sight. As a photon enters your eye, it stimulates your retina and sends a signal to your brain to perceive a flash of light. The rate of which you perceive this light is based on your brain communicating the information of the photon. If there's a photon, you perceive light. A continual amount of photons, your brain lets you perceive a well-lit room. A lack of photons, there are no photons allowing your brain to perceive objects around you. The speed of dark would hence be defined as the rate of which you perceive light to which you'd then perceive dark, which is an instantaneous transition (aside from glair in your eye) as there are no photons to continue your brain's perception of light. However, more specifically, the rate the perception of darkness would dissipate would be the refractory period of the optic nerve. This refers to there being no physical governance on "darkness" but rather the governance of the speed of photons, ie. the speed of light.

What is the speed of dark?

Of course the speed of dark is infinite, just as douG asserts in his answer!

And it is easy to prove, just as it is proved - again and again - that density is infinite at any "black hole singularity" as well as at the "initial singularity" at the beginning of everything![math]^{[1]}[/math] We performed this experiment at home, and the math indeed verifies the hypothesis:

I placed myself 15 meter from the only lightsource in a room, my wife controlling the lightswitch. We wired the switch so that when the light was switched off, it triggered an electronic timer to start, and that timer was controlled by a CCD, so when the darkness arrived at the CCD light sensor, it turned the photon counter off.

As a control, I used a stopwatch to measure the duration of time it took for the room to become dark when my wife turned the light off.

Lo and behold! Both the electronic timer and the stopwatch showed that no time at all had elapsed! Zero seconds! And as is well known with in the Quora pseudo-science community, 15 meter (the distance) divided by 0 seconds (the time) = 15/0 = ∞ ... the speed of dark is infinite!

QED

Footnotes

[1] Wikipedia matter-of-factedly confirms, as usual (bold emphasis added):

Gravitational Singularity: A gravitational singularity or space-time singularity is a region in space in which matter takes infinite density and 0 volume (basically infinitely dense and infinitely small).[math]^{[2]}[/math] [...] Gravitational singularities are mainly considered within general relativity, where density apparently becomes infinite at the center of a black hole, and within astrophysics and cosmology as the earliest state of the universe during the Big Bang. [...]

Interpretation

Many theories in physics have mathematical singularities of one kind or another. Equations for these physical theories predict that the ball of mass of some quantity becomes infinite or increases without limit.

Initial Singularity: The initial singularity was a singularity of infinite density thought to have contained all of the mass and space-time of the Universe before quantum fluctuations caused it to rapidly expand in the Big Bang and subsequent inflation, creating the present-day Universe. The initial singularity is part of the Planck epoch, the earliest period of time in the history of the universe. [...] General relativity is used to predict that at the beginning of the Universe, a body containing all mass, energy, and spacetime in the Universe would be compressed to an infinitely dense point.

Big Bounce: [...] One of the main problems with the Big Bang theory is that at the moment of the Big Bang, there is a singularity of zero volume and infinite energy. This is normally interpreted as the end of the physics as we know it; in this case, of the theory of general relativity.

[2] I find it infinitely amusing that the Wikipedia scientist wiki author considers a volume of zero extent[math]^{[3]}[/math] to be "basically, infinitely small" ...

[3] This can of course also be interpreted as the scientific fact that nothing is infinite.

In summary: The speed of abstract objects is unbounded.

For a more technical description, read on below.

The speed of any reference frame attached to non-physical objects (such as shadows) is not bounded by [math]c[/math].

Attaching a reference frame to a laser point that is swept-e.g. by rotating a high-powered laser pen-across the surface of the moon, for example, this point can move with the velocity [math]2c[/math], or some other multiple. This example would be bounded by the rotation angle speed (in radians per second) of the sweeping motion, times the distance to the surface.

However, if we attach a reference frame to something else such as "the point where a wave currently touches the beach," there is no bound on how fast it can move along the beach; using a common assumption, that space is continuous.

The point of contact between wave and beach will move rather slowly in the first image, compared with the second image, granted that the wave moves with the same speed in both instances. This is due to the angle between the approaching wave and the beach, which can be made as small as we want, resulting in the point of contacting being unbounded in speed:

It isn't meaningful to speak of the "speed of darkness". It's nonsensical, like asking "what is the square root of cows?" This is despite the phrase "speed of light" being meaningful and well-defined!

The reason is that you are conflating two distinct senses of the word "light".

"Light" has two meanings: a phenomenological meaning, and a physical meaning.

Phenomenologically, "light" means "how it is during the daytime, or when you open the curtains, turn on the lamp, etc."

Physically, "light" is a handy synonym for "electromagnetic radiation in the visible spectrum". When we say "speed of light", that has a physical meaning, because we are using this phrase to mean "vacuum speed of electromagnetic radiation".

"Dark", however, is really just a phenomenological term. It means, "how things are at night, or when you close the curtains or turn off the lamp, etc."

So there is no "speed of dark." That just doesn't make any sense.

Arguably there is a physical meaning for "dark", but in this sense it just means "absence of light". To speak of a physical speed of propagation for the absence of something doesn't have any obvious general sense of useful meaning. Just as indeed it would be meaningless to speak of "the speed of the presence of lightness" - however, once again that's not what "speed of light means", it means the propagation speed of an electromagnetic wave in space.

Phenomenological "light" is due to the presence of light radiation (physical light). Darkness, however is not caused by the presence of "dark radiation" (there is no such thing), but by the absence of light.

Cheers, hope that helps... :-)

(It seems like this question gets asked about once a week on Quora... I feel some merge-related activity coming on...)

I feel, you sir, will love v sauce... Unless you saw it and decided to ask it here as well.

The speed of light is known as c and is given the value of roughly 2.998*10^8m/s

The speed of light is a speed, but it's a distorted form of speed as light always travels the same speed relative to EVERYTHING. If you are travelling at 99.9% of the speed of light, someone watching the event will still see the light travel the speed of light away from you. This is a brief intro to special relativity. Enjoy :p

The speed of dark however is... undefined, as nothing physical or spectral is even moving. All it is, is the absence of light. If we for example treated a shadow as a travelling object, The ‘speed' of the shadow simply depends on how fast one can move something emitting light from a dark section. It has no set ‘speed'. Furthermore, it can appear faster than the speed of light. No relativity is concerning it. As... this it is nothing. In mathematics, you can't just apply speed and direction as a vector to nothing and get something.

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