# Is there a speed of darkness.

## Main Question or Discussion Point

there is the speed of light. just wondering, is there a speed of darkness. if so, what is it.

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## Answers and Replies

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brewnog
Gold Member
Well, darkness is simply the absence of light. Therefore 'darkness' will take the same time to reach something (following extinguishing of a light source) as light would (following illuminating of the source).

I suppose that if you were in an enclosure then there might be some optical equivalent of reverberation.

Danger
Gold Member
Yeah... it's called reflection; but it still is simply light moving at light speed.

krab
time traveller d said:
, is there a speed of darkness.
That's a funny one. Reminds me of something I used to tell the kids to get them to close the door at night: "Don't let the dark in."

absence of light is darkness

darkness is like the wagon tied to the back of the bike going light speed. when the bike goes by the wagon takes its place just as quickly. it has no definite speed because it is nothing. even though it technically goes at the speed of light, theres no way of telling because we cant measure how quickly nothing moves

we'll solve this problem right after we figure out the speed of silence

Tide
Homework Helper
The speed of darkness has been measured as negative 300,000 km/s. :)

deepak9191
darkness doesnt existbcoz light is present everywhere . remember that light is a wave and has no definite size and shape so it can be present everywhere . even the black hole is not devoid of it. its just a matter that in a black hole we are una ble to see light

deepak9191
wud u say that speed of silence in air is - 330m/s ?

Tide
Homework Helper
Deepak,

That was an attempt at humor! :)

ahrkron
Staff Emeritus
Gold Member
Since darkness is not a result of a physical process, but rather a name we give to a condition, I think there is no limit on its speed.

For instance, imagine a screen onto which you shining ten lamps. If you switch them off one by one, you can see "darkness" travelling from one side of the screen to the other.

Varying the timing for switching off the lamps, you can vary the speed of darkness from very slow to infinite (turning them all off at the same time), and all values in between.

can anyone define darkness? darkenesss is different for different conditions. we are just talking about visible region wat about infra red and others. i know all travel at same speed but their imactas on the surrounding differ.

siddharth
Homework Helper
Gold Member
I agree with ahrkron.
For example, If I shine a torch and wave my hand in front of it, the shadow which falls on a very very far away wall, can move extremly fast. The further away the wall is, the faster the shadow travels across the wall.
Isn't that right?

-Job-
Not exactly. Initially, the farther away the wall is, the faster the shadow will appear to move, but, eventually, the speed of "darkness" should stop increasing with the distance. If you had a really big hand and covered a star in a nearby galaxy with your hand, then as you waved your hand around you'd still have to wait for the light to travel to the "wall". So you can't make "darkness" have infinite velocity or be faster than light.
The relation between the "speed of darkness" and distance from the hand to the wall is somewhat bellshaped. Meaning that, initially, as the distance increases, the faster darkness will seem to move, until this distance reaches a point after which the darkness not only can't move any faster, but will start to move more slowly.

Also, on ahrkron's example, it wouldn't be possible to turn off all the lights at the same time if you have alot of lights (for instance a light year's distance of lights), there will always be a delay so i think you're making use of an invalid assumption.

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deepak9191 said:
darkness doesnt existbcoz light is present everywhere . remember that light is a wave and has no definite size and shape so it can be present everywhere . even the black hole is not devoid of it. its just a matter that in a black hole we are una ble to see light
I would like to take issue with this comment. Being a certified cave diver I have experienced complete and total darkness many times. When your light goes out a few thousand feet from the cave entrance is gets pitch black. There is certainly no visible light being emitted by the cave walls or the water.
As far as your black hole comment goes, I believe that is total speculation.
Darkness is a human concept to explain the absence of visible light. It doesn't mean an absence of EM waves, just absence of detectable visible spectrum light. Since it takes a certain number of photons to stimulate sight in humans, anything less than that number can be considerd total darkness, even if there is some light present.
As for the "speed" of darkness I think some people are reaching here. For something to have a speed it has to move relative to something else. Darkness surely doesn't move or propigate in any way. Seems that we are just debating semantics.

siddharth
Homework Helper
Gold Member
-Job- said:
Not exactly. Initially, the farther away the wall is, the faster the shadow will appear to move, but, eventually, the speed of "darkness" should stop increasing with the distance. If you had a really big hand and covered a star in a nearby galaxy with your hand, then as you waved your hand around you'd still have to wait for the light to travel to the "wall". So you can't make "darkness" have infinite velocity or be faster than light.
Even if I wait for the light to travel to the "wall", I still don't see why the shadow which is moving across the wall can't travel faster than the speed of light.

-Job- said:
The relation between the "speed of darkness" and distance from the hand to the wall is somewhat bellshaped. Meaning that, initially, as the distance increases, the faster darkness will seem to move, until this distance reaches a point after which the darkness not only can't move any faster, but will start to move more slowly.
Why is it going to be bell shaped? And why should it slow down suddenly?

Integral
Staff Emeritus
Gold Member
Darkness is a physiological state rather then a physical one. Darkness is determined by the sensitivity of the eye and varies with the species (the observer). To speak of it physically we would have to define intensity levels of the wavelengths in the visible spectrum. How can something so defined have a speed?

-Job-
I think it can be seen as a physical one. If i have a laser beam shooting against a wall and i temporarily obstruct it with my hand there will be a gap in the laser beam travelling at the speed of light (assuming no radiation gets through my hand).
Ok, to make this more scientific i did a drawing depicting the situation. While doing this i realized that it may be the case that the shadow can seem to move faster than light (even if i'm sticking to my original opinion). It does show the bell curve i was talking about.

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I would view "darkness" (e.g., lack of photons) the same as I view "death" (e.g., lack of life). For me, dead folks move at the speed set by the "arrow of time" (here I think of my dead parents, whose memory fades for me at a speed set by the arrow of time). Perhaps darkness moves at the same rate as death ? Thus, the example given by God_Am of "pure" darkness experienced in a cave, does not this experience move at the speed set by the arrow of time ?

Ki Man said:
absence of light is darkness
...
it is nothing
...
we'll solve this problem right after we figure out the speed of silence
I believe Ki Man has made the best contribution to this thread so far.

"Darkness" is the absence of light. I suppose you could think of the "speed of darkness" as being the speed with which light leaves a particular region of space, but it's not like darkness is a physical entity independent of light. When you speak of the "speed of darkness," that's really what you're doing...and honestly, it sounds pretty silly.

Also, if you view the "speed of darkness" in this regard (simply as the speed with which light leaves a region of space), then it's not really proper to say that you can make it any speed simply because you can wave your hand in front of a lamp and observe a shadow moving on some surface very far away at a speed faster than light. You can do this with light, too. Just wave a laser in the same arc at the same object, and you will observe the point of light to move faster than the speed of light could across the surface of the object. This doesn't mean that the light broke its own speed limit; it's a bunch of different photons (or different parts of the same wave, if you prefer that) hitting different parts of the object at very close instants in time. It would be similar to concluding light travelled faster than the speed of light as we know it simply because you saw two lightbulbs which were 1 lightyear apart flash within 1 second of each other.

EDIT: I wanted to mention this, as well. Although whenever we see light, it HAS a speed by its nature, whenever we observe darkness, it only has "speed" when we observe that light is also present (and therefore moving in some direction). So you can't speak of the "speed of darkness" when you have no light at all (which happens), which is another reason why this is a silly question.

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is there a speed of duh? threads like this are crazy. there is no such thing as a speed of darkness. darkness is no light.
If you get hit by a car you might ask how fast that car was going. You won't say "how fast was that car not hitting me going.

This reminds me of the http://64.233.187.104/search?q=cache:Y5PtgJL1KfEJ:www.netcom.com/~rogermw/darksucker.html&hl=en" [Broken]. Although the original author's site is gone, I managed to retrieve a cached version of this infamous crackpot theory.

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I posted something about the dark sucker as well I didn't even look for the original author's site though and probably took credit for the idea.

Tide said:
The speed of darkness has been measured as negative 300,000 km/s. :)
Wouldn't it still be positive 300 000km/s? Its moving in the same direction as the light is moving away from it, therefore both signs would be positive? If it was moving at -300 000km/s, wouldnt it be moving backwards? and then nothing would take the spot of the light or the darkness, which would infact be darkness itself! Omg, now i am confused, way to go.

It's really very simple: Darkeness is a concept/attribute related to light(or photons)
Now, since the actual propagation of darkness can not exist without the presence of photons defining that "darkness" the propagation of darkness is identical to the propagation of light, as it is immutably bounded.