# What is the speed of dark?
**URL:** https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824
**Category:** Factual Questions
**Created:** [July 7, 2007, 8:29am UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824 "2007-07-07T08:29:13Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@human\_extinction](https://boards.straightdope.com/u/human_extinction)
#### Post date: [July 7, 2007, 8:29am UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824/1 "2007-07-07T08:29:13Z")
What is the speed of dark? I’d like a bit of detail so I don’t just say “the opposite of the speed of light” next time someone asks me (even though that doesn’t seem technically correct). Thanks
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### Author:  [@ParentalAdvisory](https://boards.straightdope.com/u/ParentalAdvisory)
#### Post date: [July 7, 2007, 8:45am UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824/2 "2007-07-07T08:45:10Z")
Hmm, I like this question. Wiki turns up fiction novels and video games. Surely there’s something to be said of it scientifically.
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### Author:  [@chowder](https://boards.straightdope.com/u/chowder)
#### Post date: [July 7, 2007, 8:53am UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824/3 "2007-07-07T08:53:43Z")
I imagine it’s the same as that of light.
Switch light on…bright
Switch light off…dark
Both instantaneous
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### Author:  [@Stealth\_Potato](https://boards.straightdope.com/u/Stealth_Potato)
#### Post date: [July 7, 2007, 8:55am UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824/4 "2007-07-07T08:55:16Z")
Unless I’m mistaking your intent, it should be the same as the speed of light, correct?
If I turn off a lightbulb, the soonest time it would be possible for you (the observer) to detect that change would be precisely the time required for the last photons generated by the bulb to reach you. Thus, “dark” “propagates” at the speed of light.
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### Author:  [@chowder](https://boards.straightdope.com/u/chowder)
#### Post date: [July 7, 2007, 9:03am UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824/5 "2007-07-07T09:03:48Z")
Exactly
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### Author:  [@ParentalAdvisory](https://boards.straightdope.com/u/ParentalAdvisory)
#### Post date: [July 7, 2007, 9:16am UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824/6 "2007-07-07T09:16:37Z")
So if I am traveling at near the speed of light and turn my headlights off…
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### Author:  [@chowder](https://boards.straightdope.com/u/chowder)
#### Post date: [July 7, 2007, 9:21am UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824/7 "2007-07-07T09:21:01Z")
[QUOTE=ParentalAdvisory]
So if I am traveling at near the speed of light and turn my headlights off…
[/QUOTE]
You’d hit that tree off to the right
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### Author:  [@The\_Librarian](https://boards.straightdope.com/u/The_Librarian)
#### Post date: [July 7, 2007, 9:57am UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824/8 "2007-07-07T09:57:16Z")
Dark is much, much faster than light, it always gets there first and it can get out of the way effortlessly.
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### Author:  [@Alive\_At\_Both\_Ends](https://boards.straightdope.com/u/Alive_At_Both_Ends)
#### Post date: [July 7, 2007, 9:57am UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824/9 "2007-07-07T09:57:20Z")
You’d need to experiment with an electric dark bulb to be sure, of course.
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### Author:  [@glee](https://boards.straightdope.com/u/glee)
#### Post date: [July 7, 2007, 10:30am UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824/10 "2007-07-07T10:30:51Z")
[QUOTE=chowder]
I imagine it’s the same as that of light.
Switch light on…bright
Switch light off…dark
Both instantaneous
[/QUOTE]
Instantaneous? 😕
Actually under certain conditions, I can cycle faster than light:
The physicists have created conditions under which light travels at only 17 meters per second, which is about 38 miles per hour…
> **[Slow Light](http://www.jupiterscientific.org/sciinfo/slowlight.html)**
>
> Light Speed Reduction to 38 Miles per Hour
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### Author:  [@ParentalAdvisory](https://boards.straightdope.com/u/ParentalAdvisory)
#### Post date: [July 7, 2007, 10:33am UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824/11 "2007-07-07T10:33:34Z")
[QUOTE=chowder]
You’d hit that tree off to the right
[/QUOTE]
I knew it.
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### Author:  [@magellan01](https://boards.straightdope.com/u/magellan01)
#### Post date: [July 7, 2007, 10:38am UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824/12 "2007-07-07T10:38:34Z")
Do we have reason to believe that photons or waves of light would recede at the same rate at which they are propelled forward? It seems to simply be an assumption.
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### Author:  [@Broomstick](https://boards.straightdope.com/u/Broomstick)
#### Post date: [July 7, 2007, 10:40am UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824/13 "2007-07-07T10:40:38Z")
[QUOTE=magellan01]
Do we have reason to believe that photons or waves of light would recede at the same rate at which they are propelled forward?
[/QUOTE]
Um… because the speed of light through a given medium is always the same regardless of the frame of reference of the observer?
That’s why they call it a “constant” - because it never changes no matter how you look at it.
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### Author:  [@magellan01](https://boards.straightdope.com/u/magellan01)
#### Post date: [July 7, 2007, 10:50am UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824/14 "2007-07-07T10:50:57Z")
[QUOTE=Broomstick]
Um… because the speed of light through a given medium is always the same regardless of the frame of reference of the observer?
That’s why they call it a “constant” - because it never changes no matter how you look at it.
[/QUOTE]
I don’t think that answers the question. Your explanation talks about light. Light propels itself forward from a source at a constant rate, yes, but what happens when you turn the source off—and a what rate does that occur—resultiing in total darkness? I could see it being either faster than light, as there is no energy at work trying to create something. Or I could see it being slower than light, if photons “die” a slow, lingering death. Also, the dual nature of light may add to the complexity. Does it stop being a stream of particles at the same time it ceases behaving as a wave?
I certainly don’t know, but I do think it is an interesting question.
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### Author:  [@Meurglys](https://boards.straightdope.com/u/Meurglys)
#### Post date: [July 7, 2007, 11:05am UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824/15 "2007-07-07T11:05:46Z")
[This](http://www.amazon.com/Speed-Dark-Elizabeth-Moon/dp/0345481399/ref=sr_1_1/104-9434968-4566312?ie=UTF8&s=books&qid=1183806202&sr=1-1) is the Speed of Dark! 😉
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### Author:  [@chrisk](https://boards.straightdope.com/u/chrisk)
#### Post date: [July 7, 2007, 11:12am UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824/16 "2007-07-07T11:12:22Z")
Here’s my attempt:
There is no speed of dark, because dark doesn’t really exist. Dark is a word for a concept, whereas light refers to something that actually exists. Therefore ‘the speed of dark’ is fairly meaningless - like asking what blue tastes like. 😃
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### Author:  [@chowder](https://boards.straightdope.com/u/chowder)
#### Post date: [July 7, 2007, 11:28am UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824/17 "2007-07-07T11:28:53Z")
[QUOTE=chrisk]
Here’s my attempt:
There is no speed of dark, because dark doesn’t really exist. Dark is a word for a concept, whereas light refers to something that actually exists. Therefore ‘the speed of dark’ is fairly meaningless - like asking what blue tastes like. 😃
[/QUOTE]
Dark exists, of course it does…it’s pretty dark down the bottom of a mine without any light to… erm… light it.
And blue tastes like blueberries, gawd everyone knows that 😃
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### Author:  [@KneadToKnow](https://boards.straightdope.com/u/KneadToKnow)
#### Post date: [July 7, 2007, 11:44am UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824/18 "2007-07-07T11:44:08Z")
Long ago and far away, there was a children’s book about a boy who met the people who live on the moon, and learning the answer to this question was the cutting edge of moon-people science.
This constitutes all I remember about that book or its sequel. I would give money to be able to read them again, as the are just about the only children’s books I remember actually reading as a child.
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### Author:  [@Harmonious\_Discord](https://boards.straightdope.com/u/Harmonious_Discord)
#### Post date: [July 7, 2007, 11:51am UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824/19 "2007-07-07T11:51:48Z")
Darkness is the lack of light energy in the spectrum we see, so the space may be occupied by something moving slower than light, the speed of light, or faster than light and still be darkness. Slower than light being regular matter. The speed of light being the spectrum we don’t see. Faster than light being particles like tachions.
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### Author:  [@Baffle](https://boards.straightdope.com/u/Baffle)
#### Post date: [July 7, 2007, 1:31pm UTC](https://boards.straightdope.com/t/what-is-the-speed-of-dark/410824/20 "2007-07-07T13:31:18Z")
[QUOTE=magellan01]
I don’t think that answers the question. Your explanation talks about light. Light propels itself forward from a source at a constant rate, yes, but what happens when you turn the source off—and a what rate does that occur—resultiing in total darkness? I could see it being either faster than light, as there is no energy at work trying to create something. Or I could see it being slower than light, if photons “die” a slow, lingering death. Also, the dual nature of light may add to the complexity. Does it stop being a stream of particles at the same time it ceases behaving as a wave?
I certainly don’t know, but I do think it is an interesting question.
[/QUOTE]
The photons move at the speed of light until they hit something and are either reflected or absorbed (becoming heat). When you turn off the light switch, it stops sending the photons, but the photons that were already sent continue on until they are reflected or absorbed. They don’t ‘recede’.
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