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Light speed niavety?

  1. Dec 23, 2004 #1
    Light speed niavety??

    Light speed naivety??


    The speed of light - c is a fundamental constant, at least in vacuum.
    However, it appears that c varies and slows down at the interface between different density materials ie: air to water. But why is this?

    If a) photons travel between particles and b) the space between particles is empty then the only reason why c slows is because of the interaction with said particles. If this is the case then why doesn't c slow to nothing whilst travelling and interacting with particles during it's 15 billion light year journey from the edge of the Cosmos to our eyes?

    If a) is correct, but b) is not because of the various energy emissions and elemental forces the photons encounter (ie:- do the photons lose energy?) in the less than empty space between particles, then does c vary because of it's interaction with them?

    Or is it a combination of factors and if so what are they and what determines their effect/s?

    If anyone can answer this query could they e mail me at:
  2. jcsd
  3. Dec 23, 2004 #2


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    In materials photons are adsorbed, then remitted. Thus the "slowing" of light in materials. Photons always travel at c. After adsorption, when remitted they may be remitted as several different photons at different frequencies, but they will always propagate at c.


    [tex] c = \sqrt {\frac {\epsilon_0} {\mu_0} [/tex]

    it is more correct to say that the fundamental constants are [itex] \epsilon_0[/itex] and [itex]\mu_0[/itex].
  4. Dec 29, 2004 #3
    Thanks for the info. Please excuse me if the question seemed simple, I'm not a physicist nor am I a mathematician, nevertheless I'm fascinated by the concepts of cosmology, physics and relativity. Unfortunately they aren't subjects which people usually have much interest in, so they don't crop up in general conversation. This means that I don't get much chance to discuss them in detail with anyone, especially with someone who understands more than me so I can ask questions and learn.

    I found this site whilst exploring the net and visit it when I can. I just wish I was more intelligent and could study and investigate the subjects more thoroughly.

    What a pity so many people are scared of the subjects and think it clever and funny to be ignorant about science generally. But it is their loss and they miss so much. With understanding comes an even greater appreciation of the world around them, which becomes even more beautiful.
  5. Dec 29, 2004 #4


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    This wasn't answered:
    There are very few particles in space, so there is very little slowing. Also, you seem to be under the impression that the more distance, the slower the light gets - not so.
  6. Dec 29, 2004 #5
    I found a mistake by Integral! Your formula is wrong.

    It's actually:

    [tex] c = \sqrt {\frac {1} {\epsilon_0\mu_0} [/tex]

    I'm willing to give you the benefit of doubt and assume you made an error is Tex.
  7. Dec 29, 2004 #6


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    No, the error was mine, I did not check a reference or the units. You are absolutly correct! That is what I get for working from a faulty memory. Sorry :smile:
  8. Dec 29, 2004 #7


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    your "faulty" memory only confused what is the speed of propagation vs. what is the characteristic impedance of propagation. if i were king of the world, i would have Maxwell's Equations rewritten to replace epsilon_0 and mu_0 with c and Z_0. c and Z_0 are simply another way to represent it.

    r b-j
  9. Dec 30, 2004 #8
    Not at all

    Methinks you got the wrong end of the stick.
    Whilst I agree that the particle density in space (which is mostly vacuum) is very low, it isn't zero. Over a period of 15 billion years, even in vacuum, individual photons will encounter and interact with many particles during the journey to our eyes or photo sensitive equipment.
    As I stated in my reply to Integral, I'm not a classical physicist nor a mathematician. Nevertheless I'm intrigued as to how much energy a photon will loose by this process during such a journey and what effect entropy will have.
    Also I presume that if an estimate can be reasonably made of the number and type of particles encountered during this journey, it is possible to calculate how this will affect the energy loss and speed of each photon?
    Further, will these interactions cause the photons to deflect and refract (as short wavelength blue photons do when they encounter molecules, mainly nitrogen, in the earths atmosphere, which is why the sky appears blue) and therefore make it nigh impossible to accurately locate the original source of the photons?

    Additionally, what effect (if any) will quantum vacuum energy have on the photons?
  10. Dec 30, 2004 #9


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    Actually, the density of intergalactic hydrogen is so low, 4e-34 kg/m³, That a single photon would interact with less than 3000 hydrogen atoms over that 15 billion year journey. In contrast, A photon can interact with over 3 million atoms passing through a mere 1 meter of air. Since the delay caused by passing through one meter of air is insignificant, so would the delay caused by passing through 15 billion lightyrs of intergalactic space.
  11. Dec 30, 2004 #10


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    Berty, you implied that the more material the light goes through, the slower it goes, ie thicker glass makes for a higher index of refraction. That isn't the case. There is also no loss of energy in the transmission of a photon.
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