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Scattering occurs in our atmosphere when light is absorbed by nitrogen, oxygen, etc. and is then emitted in multiple directions. It is the reason we see the sky as blue.
We know the speed of light slows down when it leaves a vacuum. Is this because of scattering? If so, does the speed of light really ever go below c, or does it just appear that way because the light is constantly being absorbed and emitted by molecules?
Order of Kilopi
Just appears that way for the reason you stated.Originally Posted by AonSao
Photons trying to make their way from the core of the sun, to the surface, take 17000 to 40000 years. They get absorbed by hydrogen nuclei and then are the re-emitted in a random direction. If that direction is back towards the center of the sun, the photon has lost ground! It will get re-absorbed, and then re-emitted, over and over, trillions of times. The path it follows is called a "random walk"
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How are you defining 'absorption' here? I wouldn't use that term (at least how I define it) in reference to Rayleigh scattering by molecules in the atmosphere.
Order of Kilopi
Nor, I think, is it an ideal word for the process by which light slows down in transparent media. There are no specific events that can be designated "absorption" and "emission" in that process: just a quantum-mechanical superposition of photons and atoms that results in a slower transmission of the photons.
Grant Hutchison
Order of Kilopi
Questions like this sound like they are about reality, but actually they require the selection of a particular theory of light to answer. If one selects our most accurate version of light propagation, one says that our theory can predict the likelihood of a photon showing up at various distances after various times, and the ratio of those distances to those times is extremely likely to be c in a vacuum, whereas in a transparent refractive medium (like water or glass or much lower densities) it is extremely likely to be less than c. The prediction involves a superposition like Grant said, it is a superposition of mathematical entities called "amplitudes" that we use to make the predictions, which basically tabulate all the possible things the photon could do, and keeps track of how they interfere with each other. That interference causes the slowdown, not so much because any photons are being absorbed, but more simply because they could be absorbed.
Or, one may adopt a classical treatment of light as an electromagnetic wave, and then the slowing in a refractive medium is a simple application of Maxwell's equations when the medium acts like it contains lots of little "dipoles", which are points with no net charge but acting like there is tiny charge separations between positive and negative charges that respond to the presence of the electromagnetic wave in a way consistent with Maxwell's equations. In effect the electromagnetic wave then becomes a sum of the original wave and one added by the dipoles in the medium, and the upshot is the combined wave propagates with a reduced phase velocity and a reduced group speed (two ways of talking about the speed of waves). Note the different tenor of this answer-- both fields I'm superimposing are completely "real" and measurable, there is no summing over "possible outcomes", it is a real sum of real fields that all show up in Maxwell's equations.
So the bottom line is, as is so often the case in science, these two answers do not sound at all alike and indeed describe two quite different "realities". The first is more accurate and more fundamental, but the second may be more typically used in practice-- that's also quite typical in science. But what about the "real" answer to the question? Gosh, why do we always have to imagine there is one of those?
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Because isn't that sort of the aim of science, to figure out what that is?
Everyone is entitled to his own opinion, but not his own facts.
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I used to think so, but am now resigned to the fact that when we talk of Planck-type scales, this is so beyond our capacity to explore, that we are like the blind men inspecting an elephant. One feels the trunk, the other leg, the other a tusk, the other the tail. Each is correct, but only is aware of a part of the elephant.
We just create "black box" type models, with no idea of what is inside the box... don't think we will ever know... unfortunately...
Order of Kilopi
Science certainly is not set up to achieve such a lofty and ill-defined goal. It is actually a much simpler enterprise if you look at it-- it is a process of generating unifying quantitative predictive capability to a point where we gain both a sense of understanding and a sense of mastery connected with our environment. That's just exactly what it is, and has always been. Yet for some reason, over and over throughout its history, its practitioners have imagined they were finding the answers to how things "really are". (Gravity "really is" a force, velocity "really is" a relationship with fixed space, wave functions "really are" the state of a particle, etc., the list just goes on and on). I have no idea why, this is quite obviously not part of the mission statement of science, nor has it ever been a good description of science's accomplishments (say, in the context of the OP question about whether or not light "really always travels at c"). Science is, and has always been, the art of practical approximation-- albeit with stunning accuracy in idealized applications.
Order of Kilopi
Is it really so unfortunate, though? Wouldn't the worst possible thing be to completely understand and predict why everything happens, would that not lead to a very mundane existence? I think we take for granted as humans that most of what happens we'll have no clue about, so we take pleasure when we do find something, say a hydrogen atom, that we feel we can understand. But Galileo once famously remarked words to the effect that anyone who thinks they understand everything must never have really discovered anything, since the act of discovery is always a humbling experience-- opening as it always does a new door onto deeper unknowns.We just create "black box" type models, with no idea of what is inside the box... don't think we will ever know... unfortunately...
In that same spirit, I've argued that it is a complete myth that the goal of science is to eliminate mystery. Instead, it is the goal of science to do what it has always done-- replace superficial mysteries with far more profound ones. So there is nothing unfortunate about mystery-- what is unfortunate is superficial mystery, when there are so much more amazing ones just around the corner to ponder.
Order of Kilopi
I still think it unfortunate. It would not, IMHO, imply we can predict why everything happens. The Laplace deterministic model is long dead and buried by Quantum Mechanics. It is to satisfy our innate curiosity as to what is actually going on at the Planck level...
Order of Kilopi
Certainly, Laplace determinism was just one more foolish chapter in our ongoing failure to understand the consistent message of scientific history.Why is it necessary to insert the word "actually" in that sentence? Why can't we just do what it is that we always do, which is satisfy our curiosity about more superficial mysteries by discovering deeper and more interesting mysteries? If it's mystery all the way down, just as it always has been, why do we imagine it is different this time, and insert words like what is "actually" happening? What part of the scientific method inspires that misuse of scientific thought?It is to satisfy our innate curiosity as to what is actually going on at the Planck level...
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Because for me, there is no more interesting mystery than what is really going on at the Planck level. (you probably won't like the word "really" either...)
Order of Kilopi
Yes, so what it is is what it was, though what it was is a little different in content but not different in the true defined nature of science in its investigation of nature.
Using the scattering example, what it was... Newton and Clausius held that the blue sky was a result of light interference from water droplets (Newton) or water bubbles (Clausius). The intensity of the color of the sky, in their model, varied as the inverse square of the wavelength of light.
John Tyndall considered a scattering model due to large particles of moisture in the sky, but lacked a defined mechanism. Tyndall knew, however, that any model of explanation must include the fact that skylight is polarized.
John William Strutt (third Lord Rayleigh) in 1871, stirred by Tyndall and using Stokes and Maxwell ideas (though not the subsequent electromagnetic ones), deveoloped a mechanical model for scattering. Here it is, so skip it if you don't want the more nitty gritty.
Imagine a small particle -- one that is smaller than the wavelength of light that will be hitting it -- suspended in space and within the elastic aether. Since it was already known that light was a transverse wave at the time (mid-1800's), Strutt reasoned that the particle would begin to bobble (my word) but that the oscillating bobble action of the particle would only send secondary waves in directions that were not in the direction of the bobble action itself. In other words, the waves that would be generated in the direction of the bobble motion of the particle would be longitudnal waves, which are not light waves. The greates propogation likelihood would be in the direction of the incident light, coming or going, with declining likelihood as the angle approached the bobbling direction.Yet his theory did not address the actual observered saturation level of the blue from skylight. He assumed that multiple scattering did not take place, and, apparently, the computational tools necessity to resolve the differences did not exist until computers came along.
He used polarized light for simplification, then he addressed unpolarized light as the incident light and demonstrated how polarization is the necessary result from "scattering".
He then developed a mathematical model that showed, per his article, "the ratio of the amplitudes of the vibrations of the scattered and incident light varies inversely as the square of the wavelngth, and the intensity of the lights themselves as the inverse fourth power."
He also conducted experiements with light through prisms to determine the actual intensities of scattering to test his theory.
Maxwell praised this 1871 publication, which overturned Newton, Clasius, and others.
About 10 years later, and after Maxwell's em theory was strongly validated by Hertz creation of em waves that were also light waves, Strutt modified his theory to consider the particle to act as an oscillating dipole. But this did not change the propogation pattern much.
We know time flies, we just can't see its wings.
Order of Kilopi
Correct-- for it has absolutely no meaning at all. If one wants to use science to inform words like 'really", one must then accept the way science works. That means you must be talking about the unification of what you don't know-- not the elimination of what you don't know. That simply isn't science.Because for me, there is no more interesting mystery than what is really going on at the Planck level. (you probably won't like the word "really" either...
Let's just look at what you mean by the "Planck level". This is not an observational limit, we are nowhere near that as our observational limit. It is simply the place where we know our theory breaks down. The theory probably doesn't even make it to the Planck level before it breaks down, judging from history, but we know that it does break down at the Planck level. But we have reached the limit of our observations, and that was all science was ever intended to do. It is downright unscientific to imagine anything beyond what we can observe to hold well, it's a kind of personal fable whether it is consistent or inconsistent. I hear that you wish we had a consistent personal fable, lots of people are looking for that and there's nothing wrong with it, but science is something different-- when it's true to itself.
Order of Kilopi
Actually, my desire has nothing to do with science. I am well aware of its limitations. It just happens to be the only tool we have. The fact that our science, GR and QM, break down at the Planck length, is a characteristic of our models. If we find a grand unification theory, this limit might be redefined, and once again, when another "better" model appears, like and endless series of chinese boxes.
I am not looking for a personal fable, I would like to know what is really down there, knowingly full well, it is most probably impossible.
Order of Kilopi
All right then, that's another matter.Yeah, I think that's a fair characterization.The fact that our science, GR and QM, break down at the Planck length, is a characteristic of our models. If we find a grand unification theory, this limit might be redefined, and once again, when another "better" model appears, like and endless series of chinese boxes.
Ah, the quest for life, the universe, and everything! It has a storied history, that quest. Some poke fun at it, others devote their lives to it, and much gets said on all sides! You are right that I'm only talking about science, because that's the only piece that does not diverge into an infinitude of personal ideas.I am not looking for a personal fable, I would like to know what is really down there, knowingly full well, it is most probably impossible.
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