Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: DeepestBlue on 19/04/2009 22:21:58
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Ive been pondering this all morning, and I'm no closer to a satisfactory answer, nor am I happy with the answers online. Here is a resume of my thinking:-
1 Photons travel at a fixed speed (the speed of light) by definition of relativity ergo the bending cannot be caused solely by the photon slowing down
2 One theory is that transparent materials absorp a photon and then emit an identical photon, almost immediately afterwards, thus creating a small delay and apparently slowing the photon down.
3 In less dense materials (e.g. air, and moreso in warm air) the photons encounter fewer atoms and so are absorbed and emitted fewer times
Ergo there is a theoretical mechanism for effectively slowing the photon down
BUT why would this mechanism STILL cause a change in direct of the photon??????
Look forward to reading your musings.....
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Nice question ;) I'm (mainly) a chemist, this isn't my field, but here's my stab at an answer.
1 Photons travel at a fixed speed (the speed of light) by definition of relativity ergo the bending cannot be caused solely by the photon slowing down
Not re-eally, the "speed of light" only applies to light travelling through a vacuum, light travelling through other materials travels slower.
2 One theory is that transparent materials absorp a photon and then emit an identical photon, almost immediately afterwards, thus creating a small delay and apparently slowing the photon down.
I think this is just an explanation for the observed fact of light travelling slower through materials which are not a vacuum.
3 In less dense materials (e.g. air, and moreso in warm air) the photons encounter fewer atoms and so are absorbed and emitted fewer times
Yup...
Ergo there is a theoretical mechanism for effectively slowing the photon down
BUT why would this mechanism STILL cause a change in direct of the photon??????
In wave terms, it's explained in an intuitively satisfying way by one end of the wave being slowed down as it goes into the higher refractive index material, see the explanation for the pyrex and oil kitchen science experiment here:
http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/making-pyrex-invisible/
You can see this happening to sea waves if you're on a cliff looking down on a beach...
I think this is one of those things where the particle explanation just isn't terribly satisfactory, a bit like the double slit experiment, which frankly makes my brain hurt (tho' I think I'm in good company there..):
http://en.wikipedia.org/wiki/Double-slit_experiment
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OK I think we are on similar wavelengths here....(soorry) so in effect the fact that light bends through the less dense strata of air is a direct consequence of the photon being an uncollapsed wave function...if it wasn't then it wouldn't bend...
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There will be no bending if the light hits the hot / cold interface at right angles, remember. The bending is 'strongest' when the light 'grazes' the interface. Snell's Law applies even when there is only a slight difference in velocity.
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Does that infer that the difference between reflection and refraction is that the wave function collapses in reflection, whereas it remains uncollapsed in refraction...the thought behind that is that if the wave function collapsed when it encountered an atom, the new photon would appear to come from that position, whereas in an uncollapsed form, the wave function could continue from its original source.....
Another thought...is the refracted wave function a physical manifestion of Richard Feynman's "Sum over Histories" in that it describes all the possible paths of a photon from a given point....Im an Architect, so I'm in deep water here....
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You can visualize the refraction process if you think of quanta of light as occupying a spacial area. Each quantum will first encounter the refraction zone at points within its spacial area as the points arrive at the zone. The first arrivals move at different speed so the quantum must adjust is direction of travel to maintain its internal structure.
The internal structure could be anything but we suspect that it is a sinusoidal amplitude variation of electric and magnetic force.
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Deep water, indeed. I would even question the statement "photons travel at the speed of light".
My view is that, as you can only say that there is a photon there during the interaction, the photon does not even need to exist between A and B. The photon interaction which occurs during scattering by a gas molecule cannot be a simple absorption / re-radiation due to electron / nucleus energy levels because atomic energy levels tend to be discrete at optical frequencies.
I feel that the absorption - radiation model makes life too complicated. As I have said so many times, trying to say "what actually happens" may be a bit naive if one uses a limited set of concepts to start with.
IS there anything wrong with a macroscopic treatment?
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The only reason I can think of to view the EM propagation process in its micro state is to try and understand the process.
As I have said so many times, trying to say "what actually happens" may be a bit naive if one uses a limited set of concepts to start with.
I like the idea that maybe nothing happens between the transmitting event and the receiving event. There's no way that I know of to challenge that notion.
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The only reason I can think of to view the EM propagation process in its micro state is to try and understand the process.
I like the idea that maybe nothing happens between the transmitting event and the receiving event. There's no way that I know of to challenge that notion.
Trying to understand the process is exactly what I set out to do!
The difficulty is that if nothing happens between the transmitting event and the receiving event , there would be no reason for light to move sideways during the refraction process. However it does do, and therefore a process is occur ing which alters the light path
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The difficulty is that if nothing happens between the transmitting event and the receiving event , there would be no reason for light to move sideways during the refraction process. However it does do, and therefore a process is occur ing which alters the light path
It is easy to visualize the refraction process if you imagine photons as each occupying a spacial area. The part that encounters the slowing media first bends the path.
Richard Feynman explained the process differently. According to Feynman, light takes the path of least time. When propagation speed is different between media, light negotiates along the path of shortest time.
But this can't dispel the notion of nothing moving between sender and receiver. The sending atom could be advertising for a receiver by broadcasting over a wide area, then the first receiver found instantly gets the packet. This would explain the shortest time thing.
Feynman's book was aptly named Surely you're joking
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The only reason I can think of to view the EM propagation process in its micro state is to try and understand the process.
I like the idea that maybe nothing happens between the transmitting event and the receiving event. There's no way that I know of to challenge that notion.
Trying to understand the process is exactly what I set out to do!
The difficulty is that if nothing happens between the transmitting event and the receiving event , there would be no reason for light to move sideways during the refraction process. However it does do, and therefore a process is occur ing which alters the light path
The very simplest wave construction shows precisely how the direction changes at an interface (not a 'sideways' movement). The change of velocity, coupled with continuity of fields at the interface and the constancy of frequency all give the well known Snell's Law. So the wave model satisfies more or less all you would want. If the ENERGY alone is quantised then the model is still consistent with measurement. The only time that the 'photon' needs to come into the picture is during the interactions at the terminals. The implications of SR are that the energy lost by one system and the energy gained by the other system 'coincide' in the timeless existence of the photon. I do have a bit of a problem with the actual length of time which the interactions take; they could well take different times, depending upon the details of the two systems. It could need some degree of 'slop' in the systems.
Vern- do I detect a slight note of irony there? I think that it may well be impossible to challenge, validly, the idea in conventional terms.
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The only reason I can think of to view the EM propagation process in its micro state is to try and understand the process.
I like the idea that maybe nothing happens between the transmitting event and the receiving event. There's no way that I know of to challenge that notion.
Trying to understand the process is exactly what I set out to do!
The difficulty is that if nothing happens between the transmitting event and the receiving event , there would be no reason for light to move sideways during the refraction process. However it does do, and therefore a process is occur ing which alters the light path
The very simplest wave construction shows precisely how the direction changes at an interface (not a 'sideways' movement). The change of velocity, coupled with continuity of fields at the interface and the constancy of frequency all give the well known Snell's Law. So the wave model satisfies more or less all you would want. If the ENERGY alone is quantised then the model is still consistent with measurement. The only time that the 'photon' needs to come into the picture is during the interactions at the terminals. The implications of SR are that the energy lost by one system and the energy gained by the other system 'coincide' in the timeless existence of the photon. I do have a bit of a problem with the actual length of time which the interactions take; they could well take different times, depending upon the details of the two systems. It could need some degree of 'slop' in the systems.
Vern- do I detect a slight note of irony there? I think that it may well be impossible to challenge, validly, the idea in conventional terms.
Love it...and yes you are quite right to pick me up on the sloppy use of "sideways"...What you describe in terms of the Snell model is fine...I have no problem with being able to measure or predict what would happen under refraction. My problem occurred when I began asking why...dangerous question that....Im going to have to do more reading...and possibly purchase some more Paracetamol!
Thank you all for pointing me in a few directions...isnt it wonderful that something so commonplace can really challenge exactly what light is!
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A little light diversion?
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Vern- do I detect a slight note of irony there? I think that it may well be impossible to challenge, validly, the idea in conventional terms.
No irony intended; I am genuinely fascinated by the notion that the propagation of light may be instantaneous and only the negotiation between the sending and receiving atom is done in time limited by c. It is a notion that at first seems so ridiculous that it can be disposed of immediately; however, after a day or so of thought, there is still no disposition.
After thinking about it for awhile, I keep getting images of Schroedinger's cat, for example. Then the thought; how does a loose neutron know when to initiate beta decay? And, why does the act of observation change the beta decay interval? Could it possibly be that the neutron needs a receptor for some of its decay products? If so, would that lend credence to the original question?
The fascinating part is that something that should be easy turns out not to be so easy [:)]
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It is a notion that at first seems so ridiculous that it can be disposed of immediately; however, after a day or so of thought, there is still no disposition.
OK....hanging on in there here...if a photon travelled instananeousy, and the process of interacting with the receiving atom was c, then the time for interaction would have to depend on the distance from the sending atom to the recipient atom...taking it to extremes, a photon from a distant star would have reach our eye x million years ago, only thing is I suspect my eye wasn't around then.....I'm still finding it a fruitful exploration though!
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Yes; I thought of that; and about the far reaches of the universe. But light that reaches us from great distances has probably been helped along by electrons. Empty space is thought to contain about one electron for each two cubic meters. A few folks cite the elasticity of these intervening electrons as the mechanism that produces the observed red shift in distant galaxies.
I don't imagine the negotiation at speed c, and instant transfer of energy as being reality. It probably is not; but it solves the puzzle of beta decay being observer dependant.
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I agtee with SC in that we cant really be sure if photons 'move' as the only thing we see is its 'interactions'. That, on the other hand, have nothing to do with lights possible speed. In our spacetime everything will be seen the same no matter how the 'path' may be created. As for the reflections I find them no stranger than the way light reflects from a mirror.
"The puddle mirage (which can also be seen on warm days in more temperate climates) starts with simple physics. A shallow layer of sizzling air lies on the surface of the hot pavement. When light from the sky encounters this superheated layer it's bent, or refracted, upwards toward our eyes. The effect is very much as though a mirror were laid flat on the road. We see an image of the bright sky, and even upside down images of cars and cacti.
We can't catch up to the water because light entering the hot air layer close to us isn't refracted upwards steeply enough to reach our eyes. The mirage looks like a puddle because when we see sky and automobiles apparently reflected off the ground, our brains insist on interpreting the scene as something familiar. In everyday experience, the most common reflective object we see on the ground is a body of water."
Hot air contains lots of water molecules, and, those will reflect light. If it is like this you could expect the reflection to be a little hazy in a wobbling sort of way, that as the water molecules is 'dancing' in the air, contrasting them to molecules creating the glass in your mirror, as well as the quicksilver surface in where the molecules are rigidly bound in latices. So I think you can see it as 'bouncing' photons too.
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Hot air contains lots of water molecules, and, those will reflect light.
Steady on there. It is not simple reflection. It's refraction and the way the waves are affected is based on dielectric constant variations. You can do ray trace calculations on the basis of refraction / TIR where there is a gradient in dielectric properties and predict just where the light will go. The added complication of small scale interactions of photons and atoms is one step too far for the brain to cope with because it involves statistics, essentially.
If you're going to get a handle on this I think you have to forget the specific instances and how they may relate to the microscopic. There are definitely two stages. One is the macroscopic / wave treatment (which works far too well to dispute). The other is to explain the way in which the macroscopic relates to the microscopic / QM mechanisms. There need not be a clash of philosophies.
I have seen too many secondary teachers trying to explain phenomena like convection and sound in terms of air molecules -getting their knickers totally in a twist - when the appropriate explanations for both of them are totally macroscopic. We deal with 'layers' in all the rest of Science (and life) and they work. We don't do Fermi statistics when we solve an electric circuit problem - we zoom out or in as appropriate.
The idea of one photon interacting with systems spaced wide apart in distance and time doesn't imply that the photon (or the quantum of energy) travels instantly. It travels at c and it is just the information that the photon has been 'accounted for' by an interaction with the receiving atom has to be transmitted 'instantly' throughout all space so no other atom can get its hands on that particular quantum of energy. If we consider that the photon cannot 'experience' time then there is no serious problem with that idea.
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This wasn't directed to me specifically, I presume SC?
"The idea of one photon interacting with systems spaced wide apart in distance and time doesn't imply that the photon (or the quantum of energy) travels instantly. It travels at c and it is just the information that the photon has been 'accounted for' by an interaction with the receiving atom has to be transmitted 'instantly' throughout all space so no other atom can get its hands on that particular quantum of energy. If we consider that the photon cannot 'experience' time then there is no serious problem with that idea."
As this is what I believed myself to express too, although in other words (knowing that you too thought so btw:)
As for the rest, i kind of agree with you, it was a non-thought-through example, with me using molecules reflecting photons.. Not very smart at all :) But you have that idea, describing them as having 'springs'. I will have to look into that to see if i can make a better description of how I think. If seen as waves everything you say is correct, but seen as 'objects' the solution will have to differ, don't you agree? Anyway, I'll definitively have too look into it some more before trying to describe it. For the moment I'm kind of buzzy, but I'll be back :)