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Light and Entropy

  1. May 20, 2004 #1
    let's see if anyone can answer the question of why c (lightspeed) a universal unyealding constant does not violate the laws of thermodynamics - especially entropy

    from what energy source do free photons in space draw the limitless power to travel at 299792458 m/s and no other speed?

    what causes the apparent optical illusion of light slowing down 25% to 225000000 m/s in a tank of water - apparently losing energy (since c is constant it must be an illusion after all) and returning to 299792458 m/s upon leaving the tank? (apparently gaining energy)

    what causes electromagnetic signals to slow down due to gravitational drag especially since gravity is supposed to propagate at lightspeed
    (since c is a constant this must be an illusion as well)

    why is light not able to escape a black hole?
    if lightspeed is a constant and gravity propagates at the speed of light
    why should light be stopped or slowed by gravity?
    causing it to be blue or red shifted

    if light is a wave how does it propagate in a vacuum?
    if light is a particle, why does it behave like a wave, causing interference patterns in Thomas Young's double slit experiment ?

    if a laser injected a single precisely perpendicular pulse between 2 parallel mirrors (assuming one mirror was a 2 way mirror) would the beam reflect for infinity at the speed of light - producing a perpetual floating light beam?
    (according to entropy the beam must lose energy)

    why does light travelling through a transparent solid cause the solid to heat up? as if the light were losing energy to the solid

    if light (a photon) has no mass how can it push a solar sail?
    Last edited: May 22, 2004
  2. jcsd
  3. May 20, 2004 #2


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    It doesn't draw any energy. Via conservation of mass-energy, it has ALL of its energy in [tex]h\nu[/tex]. If it draws any more energy, this conservation will be violated.

    If you study a bit of a field of physics called optical conductivity, you will learn that photons do NOT lose speed in a dispersive material. The scattering-absorption-reemission of the material is what appears to slow down light in matter. It is the GROUP velocity that is measured, and it is this velocity that is found to have slowed down in matter.

    Gravity warps spacetime metric. Light travels along such lines. Blackholes are thought to bend these spactime lines into itself. Thus, light follow these lines and never escape.

    Why would there be a problem of propagation whether it is a wave or a particle? It would be more unusual if something in free space doesn't move. What would make it THAT special that it shares the same reference frame with us?

    I deal with a high powered Ti-Sapphire laser with roughly 8 ps pulse length. If you can find a "perfect" mirror that can do 100% reflectivity, give me a call.

    Because it does! Solids ALWAYS have phonons, which is partly responsible for what makes something transparent, and something else opaque.

    Hint: solar sails works most efficiently if it has a metallic surface. Hint 2: how would you know there isn't another definiton of "momentum" that doesn't involved mass? p=hbar*k. Look that up.

  4. May 20, 2004 #3
    let's start with this one...

    assuming 100% reflectivity were possible
    would you have a perpetually reflecting beam without a source?
  5. May 20, 2004 #4


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    1. How would you have a beam already without a source?

    2. By simply allowing reflectivity, it implies an interaction of the light with the plasmon states of the metallic surface. Unless you want to insist that (i) electrons should have zero mass and (ii) they don't scatter of each other and the phonons within the material, then we can't just weely-neely assume 100% reflectivity, because those are the two things that have to happen FIRST for you to have that.

  6. May 20, 2004 #5
    you're not doing so well on this test

    maybe you should review your physics books a bit before taking on the question

    the question is...

    assuming 100% reflectivity (hypothetically) and a pre-injected perfectly perpendicular laser pulse between 2 perfectly parallel reflectors
    would (after this initial pulse) there be a perpetual beam reflecting between the mirrors without a subsequent source?

    it's not such a hard question, even a child should be able to answer it
  7. May 20, 2004 #6
    Yes, if you had a perfect reflector you would get a perpetual beam.

    You might get further if you drop the patronizing tone.
  8. May 20, 2004 #7


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    What do you think is a "reflectivity"? Light BOUNCING of a surface? If that is all that is, then why doesn't light bounces off ALL surfaces? Why does it bounce off metallic surfaces BETTER than dielectric surfaces? It is the interaction of the conduction electrons that participate in the retransmission of the incoming light AND the rest of the lattice recoiling via the phonons to preserve the conservation of momentum that ALLOWS for what you call "reflection" to happen! For there to be 100% reflectivity, you WANT zero electron mass, and for the object to be at 0K for there to be ZERO losses via the phonons.

    Unless you live in a different physical reality than I do, I haven't seen both of those criteria ever being fulfilled.

    If a child can answer your question, then why can't you answer it yourself?

  9. May 21, 2004 #8


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    In other words, you can not have your 100% reflectivity, unless you violate your favorite entropy law.

    "If I only had a little humility, I'd be perfect." - Ted Turner
  10. May 21, 2004 #9
    since you can't just answer the question with a simple yes or no

    I will answer it for you

    the answer is: NO

    it is not possible for a perpetual beam to reflect between 2 mirrors without a constant source

    it violates the law of thermodynamics

    there - that wasn't so hard was it?

    the patronizing nature of my tone is directly proportional to that of the tone I respond to - ZapperZ set the tone many posts ago

    I will go back to my normal tone....

    Last edited: May 21, 2004
  11. May 21, 2004 #10


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    It violates the law of thermodynamics, so what do we conclude about perfect refelectors then?
  12. May 21, 2004 #11
    the question was very hypothetical, since it would not be possible in reality

    it was as much a question of logic as of entropy

    it seems everytime a simple hypothetical question is asked

    for example: if a 200KG piano is accelerating through space at 1 g at what speed does the piano's relative mass reach 400KG?

    some brilliant physicist jumps in to enlighten us to the fact that pianos cannot accelerate through space (wow! who would have guessed?)

    respectfully - no, you would not get a perpetual beam

    this violates entropy since you cannot draw more energy out of a system than goes into it

    and it takes considerable energy for a laser beam to even exist
    perfect reflectors or not

    this is an excellent example of the difference between the modern scientific approach, and the classical (Natural Philosophy) approach

    the modern approach relies almost exclusively on a set of accepted paradymes
    and leaves logic out in the cold

    the classical approach relied heavily on logic and reason - as well as experimental trial and error

    which explains why the most prolific scientists lived centuries ago
    Last edited: May 21, 2004
  13. May 21, 2004 #12


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    What ????????????????????
  14. May 21, 2004 #13
    have you more to add to the discussion besides... what????????????
  15. May 21, 2004 #14


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    Okay, so even if you had 100% reflectivity, you don't get a perpetual beam ! Either the clauses in this sentence contradict each other, or I'm missing something. Where does the lost energy go ?
  16. May 21, 2004 #15
    the energy would be lost as soon as the laser source were cut off
    we must remember how a laser works

    Light Amplification by Stimulated Emission of Radiation

    this is not just a beam of light, it's amplified light
    which requires a high energy source (an inductor coil) to drive it

    as soon as this source is cut off, the laser beam is also cut off
    perfect reflector or not

    there is nothing for free according to entropy
  17. May 21, 2004 #16

    Doc Al

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    Are you claiming that the laser beam, having left the laser, still requires an energy source to continue to propagate?
  18. May 21, 2004 #17
    I generally find that anyone who must explicitly give the speed of light down to an integer value doesn't have a solid argument.
  19. May 21, 2004 #18
    to propagate.. no

    to perpetually reflect...yes

    oh? perhaps you think it's better (or maybe safer) to use the term 'c' rather than a precise value, since c can be any value we like within an equation

    299792458 m/s leaves nothing to interpretation, and forces accountability

    I was not even offering an argument, it was a question
    and just for the record, I never start an argument without a solid basis

    accepting something to be true as a paradyme does not make a solid argument, that approach is for the most timid and conservative scientists
    and it's a shortcut to thinking
    Last edited: May 22, 2004
  20. May 21, 2004 #19

    Doc Al

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    I asked:
    You answered:
    Then what was all this about?

  21. May 21, 2004 #20
    first of all it violates the laws of thermodynamics

    secondly it violates common sense

    and if these are not enough for you... perfect reflectors do not exist and cannot exist

    perhaps you should attempt the experiment yourself
    and report the results

    the best metallic mirror absorbs a certain amount of light instead of reflecting. for focusing a laser beam, that's a serious problem. metallic mirrors reflect almost all wavelengths of light; for lasers you only want to reflect specific wavelengths

    there's another kind of mirror that doesn't share these shortcomings. "Dielectric" mirrors use materials that selectively conduct electricity, stacked in a layer. If they're layered just right, they'll reflect almost all of a particular wavelength or wavelengths. Until recently, unfortunately, they've had one big drawback: they only reflect light that hits them at specific angles.

    but for this experiment dielectric mirrors would work just fine

    but you will have to inject the laser beam through a hole in one of the mirrors
    and very quickly shift the hole in the mirror to one side before the light is reflected back, which means that these 2 mirrors must be a great distance apart
    Last edited: May 21, 2004
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