So there is a Dark Side of the Moon.
No, you have to ask what happens if you are traveling in reverse at near the speed of light and kill the headlights.
In that case you hit the tree off to your left
It’s entirely possible for a shadow to move faster than the speed of light. Of course, that’s not a thing, being pretty much “absence of thing”. Would that qualify under the terms of the OP?
Can you explain, or am I being whooshed?
I’ll take a try.
suppose that you’re standing 100 feet away from a wall, shining a bright wide floodlight on it.
parallel to the wall, 20 feet away from you, some superhero comes running by at 0.96c He’s big enough to cast a shadow on the wall, though it’s probably very hard to make out. How fast does the ‘shadow’ he casts move?
This isn’t as easy to sort out as I thought, actually, because I was first modeling it as if the light was travelling instantaneously compared to mister super-flash, which obviously won’t do. But I wouldn’t be surprised if it works out that the shadow STILL seems to ‘move’ across the wall faster than C. Anyone capable of taking over from here? Or am I barking up the wrong tree?
Well if you take illumination (or what I prefer to call, the “anti-shadow”) rather than shadow, then imagine a “laser”. Now let’s set up a large semi-reflective diffusing flat square surface that is one so-called “light year” on each side. Let’s call our surface “Martha” and put “Martha” ten so-called “light years” away from our “laser”, oriented in a plane such that our “laser” is on the normal. Now, pointing the “laser” straight at “Martha” you can produce a spot somewhere on “Martha”. Now imagine your standard issue generic clumsy scientist, let’s call him “Martha 2” who, while trying desperately to hold on to a cup of coffee, bumps into the “laser” and pivots it forty-five degrees off from where it was originally aimed. As such, “Martha 2” just made a “laser” spot move much faster than c on the surface of “Martha”.
(Edit: :eek: Note to self – do not post before morning coffee)
Dark is always there, though. You just don’t see it because light is in the way. It doesn’t “move” in, because there is nothing to move. It’s the light moving out. What appears to be a moving shadow on a wall is actually the seperate parts of the wall losing their illumination sequentially.
You aren’t being whooshed at all! Light speed and fingers
It was established in The Neverending Story II that the speed of darkness was in fact much faster than the speed of light. Sadly, they wouldn’t say how much faster, though it was used to describe the rate of speed at which the nightmarish mechanoid “giants” travelled after burrowing underneath the ground.
What’s that look you’re giving me? It is pretty dark under the ground, I’ll have you know.
Dark has no frequency or wavelength, so therefore has no speed and is merely the absence of light.
Photons are unidirectional in a vacuum. It seems to me that dark recedes at the same speed at which light light advances.
Three points:[ol][]Photons don’t “‘die’ a slow, llingering death”: they are emitted from a source (in normal terrestrial environment, from an electron dropping to a lower energy state), travel along a “straight” geodesic at c, and then are absorbed by another body (again, likely to be an electron at a lower energy state that is subsequently elevated). As a photon moves at c, it experiences no sense of motion through time; from the perspective of a photon, its life is one instant, continuous path from A to B. Indeed, for all a photon can know, it may as well be going from B to A. Kind of boring, really. And yes, light receeds, just as it advances, exactly and precisely at c, not withstanding any mediums (including the Bose-Einstein condensate cited by glee) which absorb and reradiate it giving it a much slower apparent speed. If you were somehow able to make “dark[ness]” propagate faster than light, you’d violate the principle of causality, not to mention that you’d have to have some means of destroying energy.[]There is no “dual nature” of light. Photons (and other fundamental particles) are not “sometimes a particle and sometimes a wave,” or simultaneously both, or whatever. What they are is some kind of fundamental construct that has properties that resemble both a classical particle and a classical waveform. They don’t switch back and forth between one state and another except as the consequence of the mathematical formalisms used to represent them in quantum mechanics. There is no duality, no paradox, no problem except that our puny, classically-trained minds can’t really cope with something that is entirely unlike our conventional expererience with the decoherent macro-level world.[*]There is no “speed of dark[ness]”; darkness (insofar as we can say anything definitive about it at all) is exclusively the quality of absense of light; asking what its propagation speed would be is like asking what the “speed of vacuum” is; likewise, we can only answer that question in terms of the speed of what would otherwise occupy that volume.[/ol]Stranger
Stranger On A Train,
Thanks, for the input.
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You take me too literally. I was trying to explore what hapens to light when it is stopped being generated. Regarding the propogation of dark, I leave open the possibility that in propogates faster than light because light takes energy: work. Whereas darkness is the natural state of affairs, requiring nothing.
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Agreed. I was using dual in the sense of “bi”. No fluctuation intended.
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I see your point, and from a literal/technical interpretation, I agree. But “speed of darkness” is just another way to express the recession (fading, withering, exit) of light. If it takes X time to go from total darkness to Y level of illumination, how long does it take to go from Y level of illumination back to total darkness?
X – the time it takes from total darkness to Y level of illumination is not bounded by the speed of light. If a whole bunch of photons hit your receptor at once reaching your measurement Y then it’s instantaneous.
The time it takes from Y level of illumination to total darkness is not bounded by the speed of light. If a whole bunch of photons hit your receptor at once reaching your measurement Y and then there were no more incoming photons your next measurement is going to be 0 and you’ve reached total darkness instantaneously.
However, I think you’re trying to ask something different. Let’s take an air hose analogy – your level of illumination is “pressure of an air stream on some surface” and the rate at which the pressure changes has nothing to do with the velocity of air coming out of the hose. If the air hose is turned or shut off then the pressure will change (with a delay that IS dependent on the speed of the stream) at the same rate as you’re able to turn the stream off.
groman,
Let me try this:
l
l
l
Light source >
l
l
l
^
Observer
The dotted line represents a solid wall with one opening. If I start in darkness, then turn the light on, may area will be illuminated at the speed of light. Correct. Now I turn the light off, how quickly will I get back to darkness? If a light beam is a packet of photons and travels like the laser gun shots in star wars, I can see how the light would be observable at the speed of light and darkness would be reinstated at the same rate. But what happens to a packet of photons that is shot across an infinite, empty dark universe? Does it travel forever in its initial form? Or does it degrade as it travels through infinity? Just to be clear, I’m thinking that the “packet” of photons was sent from its source when the light source was turned on for an instant and turned off.
Does this make any sense. I am thinking it through. But I thiink the answer to those last questions would be illuminatiing 
as to the initial question.
Egad! My diagram didn’t work. It’s supposed to be a light source to the left, a wall with a slit, and an observer to the right of the wall looking upward. Hope that helps.
Think of water from a hose. When you shut the hose off the end of the water stream advances at the speed of water and when the end passes you there is no more water.
There is no such thing as “propogation of dark.” There is nothing to propogate, or indeed, of which to measure displacement or speed. Dark[ness] can only be defined as the state in which a locality has no light within it. If your question is, “How fast can a volume that is filled with light become evacuated?” the answer is the time it takes for photons photon to exit the region, i.e. the path distance divided by c. The time it takes for light to evacuate a region about a single light source that is suddenly turned off (assuming that all emission is simultaneously terminated) is the radius of the volume about the source divided by c; before that, the last emitted photons are still jetting their way to the exerior of the volume and thus it is not “dark”. (We’re assuming that the volume is otherwise evacuated and that no interactions or forces are distorting the free path of the photons).
Asking “What is the speed of dark?” is akin to asking “What is the top speed of a Lamborgotti Fasterossa XT550?” It doesn’t exist, so there is no answer to this question.
Stranger
Here’s a simpler question that goes to what I am trying to explore: do photons deteriorate in any way? If I shoot a light particle across an infinite and empty universe, will it—its luminosity— ever “deteriorate” on its own?