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Light - What exactly is happening?

  1. Jun 27, 2011 #1
    When a source begins to emit light, what exactly is occurring to produce an instantaneous velocity of c? If we're talking about quantized photons, would it be be appropriate to say there is zero acceleration? (I would think not because technically there is no change in velocity) Or if we refer to the light as a wave function, is the wave simply propagating with a velocity of c? Or if we use the term wavicle how is its behavior described?
     
  2. jcsd
  3. Jun 27, 2011 #2

    Dale

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    That is correct, light does not accelerate to c, it is always going at c from beginning to end.
     
  4. Jun 27, 2011 #3
    So what exactly is occurring? Is a wave propagating at c and what we see as light just some sort of EM disturbance? I understand how light is reflected off of objects and into our eyes, letting us see, but what is going on between the source and the destination?
     
  5. Jun 27, 2011 #4

    rede96

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    What a good question!

    From a physics point of view, I have often wondered exactly that. How do we see? From my limited understanding, we have to 'see' the object at the same time the photon enters the eye. Otherwise we could say that the photon carries the image.

    Anyway, look forward to getting the answer on this one.
     
  6. Jun 27, 2011 #5
    We actually see the object slightly after the light enters the retina. The retina picks up the light source and sends a nerve impulse through the optic nerve to the brain, where it is then perceived. I think about what I am perceiving and what is happening and they are largely different things. It causes me to constantly redefine what it means to see.
     
  7. Jun 27, 2011 #6

    Dale

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    Are you familiar with Maxwell's equations, and in particular vacuum solutions to Maxwell's equations?
     
  8. Jun 28, 2011 #7
    I'm somewhat familiar, but it's been years since I took the class that covered them. I'll read up and get back to you.
     
  9. Jun 28, 2011 #8

    HallsofIvy

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    Although you can, in quantum theory, talk about photons as particles, in relativity it is better to think of light as waves- "ripples" in the electromagnetic field. Such ripples immediately move at c just as, when you throw a rock into a pound, the ripples move immediately with whatever the wave speed is for that pond (it depends upon the depth). There is no acceleration because there is nothing "physical" actually moving with the waves.
     
  10. Jun 28, 2011 #9
    nobody knows: what we do observe is that as you stated light is instantaneous at speed c.
    So when you turn on a light bulb and an electron is excited to a higher energy state, when it falls to a lower energy (different quantized energy) it emits radiation.....light....but why and what is happening?? Quantum theory suggests quantum states but who ordered that??
     
  11. Jun 28, 2011 #10
    Ahhh there's the answer I was looking for. Amazing, we still don't know what light really is. I remember being fairly young (6 or 7) and trying to think of something that was truly 2D and I arrived on light shining on a wall; I guess something at the quantum scale is about as close to 2D as you can get.
     
  12. Jun 28, 2011 #11

    Dale

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    Well, if you want posts that protest complete ignorance then Naty1 is your go-to-guy. He asserts ignorance even when something is well understood.

    The fact is that there is no EM phenomena that QED doesn't accurately describe, so the claim that we don't know what is happening is wrong.
     
  13. Jun 28, 2011 #12

    Drakkith

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    What? Nothing about light or the particles at a quantum scale is 2d. And yes, we really do know what light is just as much as we know what an electron or proton is.
     
  14. Jun 28, 2011 #13
    I know it isn't actually 2D. I was saying something at the quantum scale is as close to 2D as you could get, in that it's extremely small/thin. I understand there is no such thing as a physical 2D space

    Guess it's time to read up on QED then. Would someone mind giving me the short version of what exactly happens in the transmission of an EM wave? I think the part that is confusing for me is that no medium is required.
     
  15. Jun 28, 2011 #14

    Dale

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    You certainly don't need QED for that, Maxwell's equations are entirely sufficient. The short version is that even without a medium an E field which is changing in space makes a B field which is changing in time, and vice versa.
     
  16. Jun 29, 2011 #15

    Drakkith

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    Would it be incorrect to say that the medium is spacetime?
     
  17. Jun 29, 2011 #16

    Dale

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    As long as you associate the "medium" only with geometric properties like distance and time and not with material properties like density and velocity.
     
  18. Jun 29, 2011 #17
    That clears things up; I was stuck thinking about something with a density.
     
  19. Jun 29, 2011 #18

    Drakkith

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    That is pretty much what I was thinking. :biggrin:
     
  20. Jun 30, 2011 #19
    Photons have no mass, so it is impossible for them to accelerate. This means as soon as they are created, the travel at 'c'.
     
  21. Jun 30, 2011 #20
    Doesn't the value of evaluating the Stress Tensor harken back to the Maxwell-Faraday concept that ultimately EM parameters are in some way suspended and propagate in space because there is some sort of tension there? If so, that seems more like a medium than nothingness (that nothing provides the basis for EM wave propagation - only the numbers in a mathematical equation) ?
     
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