Oh! I finally see what you’re asking! I am hoping Stranger or other physics-minded dopers will fix any science goofs I make
A) Light and illumination are not properties of space that has photons traveling through it – light is the actual photons and illumination is your perception of those photons that you interacted with – those photons have been most likely reflected by some object you see and definitely absorbed by your retina. The photons you or your environment did not interact with will continue on.
B) Photons do not generally decay on their own. They will just continue traveling on in empty space. That space with photons will not be illuminated by any definition of illuminated I can think of. For something to be ‘illuminated’ it would have to absorb and re-emit light with a change of direction, phase, polarization, wavelength or something else.
C) However, an infinite and empty universe has no luminosity or illumination. If you just throw some photons in there they will just continue on forever. However, the universe without something to absorb those photons does not actually have any property you can call “lightness”, “luminosity” or “illumination”. If you throw in an observer, the only reasonable way this observer would perceive those photons is by absorbing them (possibly re-emitting new ones).
D) Reasonable? Ok, there’s the unreasonable way – photons, although massless and chargeless do have a total energy. As such they are subject to gravitational effects. You could theoretically build a detection device that interacts with your stream of photons using gravity only. This, while invariably affecting their path would not cause them to be absorbed, but allows for a method for detecting their presence.
Can a light ray be imagined as a series of “bullets” (photons) that emanate from the source and hit your eyes one after another, assuming you are looking at the light source?
So when the light is turned off, then doesn’t that last photon streak past you at c, and then it’s dark? So I’d say the speed of dark = the speed of light = c.
The short and clean answer to your implied question–by which I believe you mean, “Will it’s energy ever deteriorate on its own?” is “No,” because energy doesn’t disappear. There are some conjectures that the cosmological redshift is not due to, or at least not in its entirity, expansion of spacetime, and that some or all of the frequency shift is due to photons losing energy to curved spacetime as it expands, but even hypothetically these occur on astronomical timescales–billions of years–and the whole business is not widely accepted by astrophysicists and cosmologists at large. Even if it were true, on everyday scales of space and time there would be no measurable decline in the energy of a photon.
To be pendantic about your question, however, I have to point out that an individual photon has no luminosity; it can’t radiate away other photons, or indeed, even change without involving an interaction with another particle. You also couldn’t shoot a single light particle across an empty universe; to conserve momentum, you’d have to have two equal energy particles moving in opposite directions. As a practical matter, once you’ve emitted the photon it is totally gone to you, because you can never catch up with it, so you won’t have any knowledge about what it does or how it changes once you’ve released it. It is outside the so-called “light cone” of your personal Universe, and you’ll never meet it again, just like that red-haired girl that seemed to smile at me at the grocery store.
Ahhh, it appears you poor unelightened types have never heard of “Dark Suckers”
Most people incorrectly call these items “Light bulbs”… Nothing could be farther from the truth.
A lightbulb actually functions by creating a negative quantum gradient field where darkons (A particle of darkness) are instantly attracted. When you turn a darksucker on, it instantly sucks in all the “Darkons” in the area into its “Antipolar Feynmann Scalar”, and the result is that the area becomes illuminated.
I’m wondering if there’s something in this diagram that reveals the way you’re thinking about this. If you understand the light source as emitting straight through the slit, there’s no way an observer off to the side can detect the light. At least not the ‘main beam’, that is the light that proceeds straight to the right (see below). Nothing can interact with it except something that’s in its path. Perhaps you were thinking of observing it as somehow changing it, but you can’t (from the side).
If you’ve ever seen a laser pointer, you know you can’t see the light unless something or someone blocks it, reflecting the light toward you. Also, you can ‘see’ a light source in another room when you’re in a darkened room because of the light reflected off other objects in that room or the room you’re in.
Clarification on ‘main beam’ : There will be some diffraction (spreading out of the light from the slit) which may be observable, depending on the size of the slit and the position of the observer. This occurs at the slit, and is of minor concern. (Though interestingly enough, this happens to any matter/energy that passes through an opening. At least in a theoretical sense, all objects have wave-particle duality - read on de Broglie for more.)
On the topic of post formatting, you can preserve spaces by using the ‘code’ tags (curiously the # symbol if you use those), like so:
[code]
put stuff you want spaced properly in here
[/code]
Technical help: To produce ASCII (keyboarded) diagrams which do not get “edited” by VBulletin code, so the lines and spaces stay where you want them, enclose the diagram in ‘CODE’ and ‘/CODE’ coding (much like quotes, with square brackets). It preserves a monospace font and multiple spaces (which VB condenses to single spaces).
Thanks. I incorrectly used the vernacular of the simple minded -“what is the speed of dark?”- as a short hand for the true description: “How fast can a volume that is filled with light become evacuated?”
Several years ago, someone came up with the tongue-in-cheek "Darkon Theory of Light’, which held that photons didn’t exist – darkons (quantized units of darkness) did. You could measure the speed of darkons by measuring the speed of the interruption in a flow of darkons – whayt other people would call a photon. Therefore, the darkons travel at the same speed as this mythical “light”.
I don’t know if this is from the same people, but it proceeds in about the same way:
working from the standpoint of “dark” rather than “light” might be like working with negative rather than positive charges in electrical circuit analysis. Either approach gives the same result.
That’s one key facet of science. Nature does its thing irrespective of how we analyze it.