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Speed of water and relation to light

  1. Sep 30, 2012 #1
    Fill a bowl with water, lay a piece of fabrics over the bowl, one end touching the water and the other end laying down on the table. Due to the capillarity effect the water will follow the fabrics and after a while it will be all wet. From this little experiment we can deduce that water travels, at a speed that depends of the type of fabrics, and has a maximum speed in each type of fabrics. We can even extrapolate that the speed of water in nylon can be something like 1 cm/minute. Silly isn't it? But its apparent speed is dictated by the medium that carries it.

    That brings my questions related to light.
    1. Could it be possible that light has an infinite speed but it is slowed down by the medium where it travels? Isn't there a vacuum permeability in the vacuum of space that would limit its speed?
    2. (my favorite option) Is it possible that light doesn't move at all? Like the water in the previous analogy? That it just dissipates and follow the fabrics of space, even empty ones, as water in nylon? That would supply a different explanation of why light slow down in water and speed up when it get out.
     
  2. jcsd
  3. Sep 30, 2012 #2
    Hi Serge, welcome to physicsforums!

    The generally accepted model for light propagation in relativity is a wave model. However there are variations in the interpretations, as there are problems with details; in practice what people do is know how to calculate based on rules from wave mechanics. In that context it's easier to point out how your suggestions conflict or agree with currently used models:

    - the speed of light in vacuum is a local constant determined by the vacuum and when light travels through water, it is effectively delayed due to interactions with the water molecules. See:
    https://www.physicsforums.com/showthread.php?t=613481
    https://www.physicsforums.com/showthread.php?p=3772953

    - concepts as "vacuum" and "fabric of space" can be used with relativity theory.

    However, this forum is not meant for discussing personal possible new theories (see Rules at top of the page). As you can see in the linked threads, refraction is very well explained by current scattering theory.
     
    Last edited: Sep 30, 2012
  4. Sep 30, 2012 #3

    mfb

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    It has a fixed, constant speed in vacuum, for all observers. This is not possible if the vacuum would be some medium and light would travel relative to that medium.

    It can travel from A to B, that is usually called "movement".

    Water moves in your (wrong) analogy.
     
  5. Sep 30, 2012 #4
    In fact, it has been known since 1904/1905 that this is possible and some of the early relativists related to such a wave model*. The problems with specific wave models do not depend on relativity. To mention a basic problem: As far as I know, a spherical transverse light wave emitted from a light source is an impossibility.

    * see for example:
    https://www.physicsforums.com/showthread.php?t=631954
    https://www.physicsforums.com/showthread.php?t=590601
    https://www.physicsforums.com/showthread.php?t=561128
    (and many more)
     
    Last edited: Sep 30, 2012
  6. Sep 30, 2012 #5

    Dale

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    Why is that a problem? That is just another way of saying there is no monopolar EM radiation. Why is it a problem that there is no monopolar EM radiation?
     
    Last edited: Sep 30, 2012
  7. Sep 30, 2012 #6

    mfb

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    You can construct some co-moving ethers and whatever, but then you give up the concept of a preferred reference frame, and get the same predictions as SR (maybe just with a more complicated framework). Otherwise, you get predictions different from SR, which are inconsistent with experiments.
     
  8. Oct 1, 2012 #7
    That's right; I simply corrected an erroneous claim. However, we were next both inaccurate. When I wrote that "the problems with specific wave models do not depend on relativity", I was only thinking of detailed mechanisms and not about what you call, "some co-moving ethers and whatever". While a Lorentz ether gives the same predictions, it is held that pluralistic "co-moving ethers" can't work as a model for SR (the prime argument was stellar aberration).
     
  9. Oct 1, 2012 #8
    I think that you mean something else. I merely referred to an idealized classical spherical uniform light source that supposedly emits transverse spherical waves around it. Did you ever try to draw that in 3D perspective? I did and if I'm not mistaken, then that is a geometric impossibility. However, if I am mistaken, then I'll be delighted to find out how it can be done! :tongue2:
     
  10. Oct 1, 2012 #9

    Dale

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    It is impossible, I just dont know why that is a problem according to you.
     
  11. Oct 1, 2012 #10
    I wish to reformulate my question. Why is there a maximum speed of light in vacuum? is it because it won't go faster or because it can't?
     
  12. Oct 1, 2012 #11

    mfb

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    Light does not have a will, or "tries" to do something.
    It has a specific speed (that is an observation), and you can derive this based on the Maxwell equations, for example.
    The laws of physics are the same in all reference frames (another observation), this makes sure that the speed is the same for all frequencies and all directions.
     
  13. Oct 1, 2012 #12
    According to Maxwell's theory on which SR was based, it's a property of vacuum, so that light can only propagate at the speed of light - similar to sound which propagates at the speed of sound.
    See: http://en.wikipedia.org/wiki/Vacuum_permittivity
     
  14. Oct 1, 2012 #13
    Impossible means that such a basic classical model is plain wrong - simply due to self contradiction, without any need to introduce QM. I'm not aware of a more severe form of "problematic". :biggrin:
     
  15. Oct 1, 2012 #14

    Dale

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    It is only a self contradiction if the model predicts it and it is impossible, but Maxwells equations do not predict a spherical transverse wave. The lowest order of radiation allowed by Maxwells equations is dipole.

    Since it is not predicted the fact that it is impossible is not a problem.
     
  16. Oct 2, 2012 #15
    So i can suppose that if light doesn't travel faster, it is not because it won't, but because it can't. It is possible to assume that light would go faster if it wasn't of the permittivity of vacuum?
     
  17. Oct 2, 2012 #16

    Dale

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    Can you think of an experiment which could distinguish between "it won't" and "it can't"? If not, then the concept is philosophy, not science, and doesn't belong here. If so, then please describe the experiment and we can analyze what SR would predict.
     
  18. Oct 2, 2012 #17
    Good point! Assuming that that's correct, I'm afraid that this is not sufficiently recognized. A quick internet search produces on the one hand claims in courses such as
    "light consists of [..] fields that travel through space as transverse waves", e.g.
    http://ubpheno.physics.buffalo.edu/~dow/lectures/phy102/ch24_print.pdf ,
    but also claims like "Spherical EM wave" and "The E.M. wave has a spherical wave front", e.g. both on slide 7 in http://www2.warwick.ac.uk/fac/sci/physics/current/teach/module_home/px384/lecture_05.pdf
    and not to forget the typical SR computation based on "consider a pulse of light that [..] propagates as a spherical wave" such as in
    http://www2.warwick.ac.uk/fac/sci/physics/current/teach/module_home/px384/lecture_05.pdf
    and of course "let a spherical wave be emitted" in
    http://www.fourmilab.ch/etexts/einstein/specrel/www/
     
  19. Oct 2, 2012 #18
    If vacuum permittivity and permeability are reduced, then also c is increased. In fact, as measured with a reference system on Earth, the speed of light is slightly reduced near the sun. That was predicted by GR and confirmed by experiments.
    I found nice and clear explanations plus animations here:
    - http://www.astro.ucla.edu/~wright/deflection-delay.html
     
    Last edited: Oct 2, 2012
  20. Oct 2, 2012 #19
    But aren't they actually talking about aggregate collections of photons isotropically emitted from a symmetric source. Not a single wave in any real sense.
     
  21. Oct 2, 2012 #20
    It looks to me that they are not talking about photons but about spherical waves just as they state; for sure Maxwell wasn't thinking of photons and neither was Einstein implying photons in his SR paper.
     
    Last edited: Oct 2, 2012
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