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  1. Standard member sonhouse
    Fast and Curious
    25 Mar '13 12:33
    http://phys.org/news/2013-03-ephemeral-vacuum-particles-speed-of-light-fluctuations.html

    This would have implications for multiverse theories I think. Such as other universes, if they are there, could have wildly different values for c, depending on how much mass that universe contains.
  2. 25 Mar '13 15:07
    Originally posted by sonhouse
    http://phys.org/news/2013-03-ephemeral-vacuum-particles-speed-of-light-fluctuations.html

    This would have implications for multiverse theories I think. Such as other universes, if they are there, could have wildly different values for c, depending on how much mass that universe contains.
    The speed of light has never been thought to be constant, it is the speed of light in a vacuum that was thought to be constant. If however vacuums are not all the same, as the article suggests, then it would not be surprising if the speed of light varies.
  3. Standard member Soothfast
    0,1,1,2,3,5,8,13,21,
    25 Mar '13 20:51
    Originally posted by twhitehead
    The speed of light has never been thought to be constant, it is the speed of light in a vacuum that was thought to be constant. If however vacuums are not all the same, as the article suggests, then it would not be surprising if the speed of light varies.
    Photons always travel at the same speed. Light "slows down" in air, liquid, and certain solids only in the sense that the photons become repeatedly absorbed and emitted by the electrons of intervening atoms. But at any moment when the photon exists it is moving with velocity c. I'm guessing you know all this, but I always thought it was somewhat misleading when physicists say light "slows down" in a medium -- misleading, at least, with respect to non-physicists.
  4. Standard member sonhouse
    Fast and Curious
    25 Mar '13 23:18
    Originally posted by Soothfast
    Photons always travel at the same speed. Light "slows down" in air, liquid, and certain solids only in the sense that the photons become repeatedly absorbed and emitted by the electrons of intervening atoms. But at any moment when the photon exists it is moving with velocity c. I'm guessing you know all this, but I always thought it was somewhat mislead ...[text shortened]... say light "slows down" in a medium -- misleading, at least, with respect to non-physicists.
    There are ways to make light stop dead. It is with near infinite refractive index material. It can be stopped then started back up, maybe be useful for quantum computers and quantum internet.
  5. Standard member Soothfast
    0,1,1,2,3,5,8,13,21,
    26 Mar '13 01:52
    Originally posted by sonhouse
    There are ways to make light stop dead. It is with near infinite refractive index material. It can be stopped then started back up, maybe be useful for quantum computers and quantum internet.
    Right. Of course, refractive index of a material is defined as c/v, where v is best described as the "average" speed of a photon traversing the material when you factor in the time during which the photon does not exist save in the form of an electron excited to a higher energy level.

    But, you know, I can make light (nearly) stop dead by putting on a black shirt.
  6. Standard member Kepler
    Demon Duck
    26 Mar '13 10:19
    Originally posted by Soothfast
    But, you know, I can make light (nearly) stop dead by putting on a black shirt.
    If we heated you up you'd radiate light at the correct speed as well.
  7. 26 Mar '13 19:36
    Originally posted by Soothfast
    Light "slows down" in air, liquid, and certain solids only in the sense that the photons become repeatedly absorbed and emitted by the electrons of intervening atoms. But at any moment when the photon exists it is moving with velocity c. I'm guessing you know all this, ....
    No, I most definitely do not know all that. In fact, it can't possibly be true can it? References?

    If it were true then:
    1. Why would the photons be emitted in the exact opposite direction from which they were absorbed?
    2. How do the photons manage to maintain their quantum entanglement throughout this process? eg. the two slit experiment works in air/glass or other mediums. Refraction is a direct result of this quantum effect.
    3. How do photons 'get absorbed' by an electron anyway? My understanding was that whenever an atom absorbs a photon, it changes the energy level of the electrons and it will later on emit that energy as photons, but that the absorption and emission was in very specific wavelengths of light. So why aren't these photons also emitted in specific wavelengths?
  8. 26 Mar '13 19:37
    Now I am curious as to how light reflects.
  9. Standard member Soothfast
    0,1,1,2,3,5,8,13,21,
    26 Mar '13 20:12 / 1 edit
    Originally posted by twhitehead
    No, I most definitely do not know all that. In fact, it can't possibly be true can it? References?

    If it were true then:
    1. Why would the photons be emitted in the exact opposite direction from which they were absorbed?
    2. How do the photons manage to maintain their quantum entanglement throughout this process? eg. the two slit experiment works in ai ific wavelengths of light. So why aren't these photons also emitted in specific wavelengths?
    When light reaches glass, or water, or some quartz crystal, some of it gets reflected, but the bulk of it enters the material along a trajectory that is bent to a degree directly proportional to the refractive index. I assume, though I could be wrong, that a transparent substance is transparent precisely because it allows photons in the visible spectrum to pass through along a straight line with minimal scattering. The photons will be continually getting absorbed by electrons along the way (the sense in which light "slows down" by virtue of having a punctuated existence), but quickly are emitted again along their original path.

    The only instance I can think of (in the realm of optics) in which photons are sent off in the opposite direction from whence they came is in the instance when they impinge a reflective surface at a ninety degree angle.

    It is true, I believe, that photons can only be absorbed by the electrons of a given atom only if they have the right energy level (or wavelength -- same difference).

    Concerning how photons are reflected, I'm not sure. I think it occurs when an electron impinges an atom with the incorrect energy. My guess is that reflection is not something that occurs in an absorption/emission scenario, but one of the mainstays of modern physics has lately been to find pathological exceptions to every rule so I wouldn't bet my life on it.
  10. 26 Mar '13 20:58
    OK, never having heard of any of this before, I did some quick research and came up with this random guy on the internet:
    http://www.physicsforums.com/showpost.php?p=899393&postcount=4
    He says it is not the normal atomic absorption emission but something about bonds.

    My question then would be why light travels slower in a gas.

    I have also given the whole emmision direction and quantum effects some thought and decided that it is all about quantum mechanics.
    When the photon is absorbed / interacts it does not leave a trace in the material. Thus its quantum nature is preserved.
    When the photon is emitted in any direction other than in a straight line to its absorption, it interferes with itself (travelling in all possible alternate paths). The result is of cause what we all know as the wave nature of light.
    At the boundary of different materials which absorb/emit photons with different intervals, the interference has the same effect as a wave being refracted.

    I guess reflection must work the same way, but I can't seem to figure out how.
  11. 27 Mar '13 04:58
    OK, I have some questions for any quantum mechanics experts out there.
    I know the wave nature of light is caused by the quantum effects of photons interfering with themselves.
    1. Can photons interfere with other unrelated photons or only with themselves? eg if you shine two beams of light from different sources do they interfere, and why?
    2. If yes to 1. then can beams of electrons do the same?
  12. Standard member Kepler
    Demon Duck
    27 Mar '13 16:19
    Originally posted by twhitehead
    OK, I have some questions for any quantum mechanics experts out there.
    I know the wave nature of light is caused by the quantum effects of photons interfering with themselves.
    1. Can photons interfere with other unrelated photons or only with themselves? eg if you shine two beams of light from different sources do they interfere, and why?
    2. If yes to 1. then can beams of electrons do the same?
    1. Yes, of course. Radio waves interfere and that's just light of a different wavelength.

    2. Maybe.....
  13. 27 Mar '13 17:23
    Originally posted by Kepler
    1. Yes, of course. Radio waves interfere and that's just light of a different wavelength.

    2. Maybe.....
    Very interesting. Any good layman type free sources on the subject you might know of? I am specifically interested in how interference works with respect to quantum mechanics, and how reflection happens.

    My understanding of the two slit experiment is that a photon (or other particle) takes all possible routes through the system. Then the probability of it hitting the screen at a given point depends on the sum of the probabilities of each route. The result is the wavelike banding called an interference pattern.
    It must be noted that the photon/particle never 'cancels itself out' or combines with itself to form a crest, but rather always interacts with the screen in one and only one spot. Only the probabilities combine or cancel out.
    I am curious as to how two photons going through the system can affect each others probabilities and create and interference pattern. Is it that the screen cannot actually know which photon is which? I am guessing this is the heart of it.
    But does this mean different wave lengths of light cannot interfere with each other?
  14. 27 Mar '13 17:46
    Originally posted by twhitehead
    OK, I have some questions for any quantum mechanics experts out there.
    I know the wave nature of light is caused by the quantum effects of photons interfering with themselves.
    1. Can photons interfere with other unrelated photons or only with themselves? eg if you shine two beams of light from different sources do they interfere, and why?
    2. If yes to 1. then can beams of electrons do the same?
    1. Yes.
    2. Yes. Atoms too, and any other particle(s) that might form coherent waves. For example, intereference patterns have been measured in Bose-Einstein condensates.
  15. 27 Mar '13 21:05 / 1 edit
    Originally posted by KazetNagorra
    1. Yes.
    2. Yes. Atoms too, and any other particle(s) that might form coherent waves. For example, intereference patterns have been measured in Bose-Einstein condensates.
    OK, I have given this some thought and it doesn't tie in with what I have heard about the two slit experiment.
    My understanding is that if you have a single beam, split into two, then recombined, you get an interference pattern.
    If however you monitor one of the paths, then the interference pattern disappears - because we now know which path each photon takes and the photons cannot interfere with themselves.
    But if, as you say, different photons can still interfere with each other, then why does the interference pattern disappear?
    What am I missing?