Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    Staff: Mentor

    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

    User Avatar
    Gold Member

    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

    Staff: Mentor

    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

    User Avatar
    Science Advisor

    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

    Staff: Mentor

    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

    User Avatar
    Staff Emeritus
    Science Advisor

    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

    Staff: Mentor

    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

    User Avatar
    Staff Emeritus
    Science Advisor

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

    Dale

    Staff: Mentor

    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

    User Avatar
    Staff Emeritus
    Science Advisor

    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) ?
     
  22. Jun 30, 2011 #21

    Drakkith

    User Avatar
    Staff Emeritus
    Science Advisor

    I don't believe the propagation of an EM wave requires any tension in a medium. The fields themselves are alternating back and forth, not any medium.
     
  23. Jun 30, 2011 #22
    But if there is no stress, any local concentration of energy or any other parameter couldn't exist - it would immediately diffuse into the background, wouldn't it? There would be nothing to support a finite speed of propagation or diffusion.
     
  24. Jun 30, 2011 #23

    Drakkith

    User Avatar
    Staff Emeritus
    Science Advisor

    Stress of what? Why would stress be required? This isn't a wave like in water or air. It is composed of fields. The electromagnetic field of an electron needs no medium to work. Why would a photon?
     
  25. Jun 30, 2011 #24
    Actually the wave equations are not limited to the propagation of fields but pertain to potentials and energy, though energy indirectly because it flows in different directions than the movement of the fields.

    In considering the movement of a photon, its transit will be the path of least energy resistance according to the Langrangian or Hamiltonian, won't it? If there were no stress of some type propelling it, and no type of relaxation of that stress which determines its path, then what would otherwise cause it to form and move where it moves? Sure, other physical models might work also, but to not entertain any model is pretty much saying nothing about the causation of EM propagation. And saying nothing is not particularly good science in my opinion.
     
    Last edited: Jun 30, 2011
  26. Jul 1, 2011 #25
    Well, that's one viewpoint. It might seem that the medium is not required to exist, because the determination of electromagnetic field propagation doesn't require it. I have always found it interesting how some prefer to believe fields extend themselves by themselves unto themselves thru the nothingness. Sounds more akin to magic IMO. The fact that the medium is not required to define EM field propagation does not lead the medium doesn't exist. So while it is true that a photon's location or speed is determinable w/o description of any medium, it is an assumption (at best) that an electron (or photon) needs no medium to work. In fact, it is more likely that fields exist only because the medium exists, and that the nature of fields (in part) defines the properties of the very medium. But then, that's just my opinion.

    From what I've read, both Maxwell and Einstein believed in a medium. Since neither could prove its existence and thus define it, they both left it unsaid ... except that Einstein stated any aether was superfluous to his theory. That doesn't mean it does not exist though, nor that it is more likely that it doesn't exist. Maxwell left it at fields alone, and Einstein left it at space and time, because they are measurable and thus definable. I'd submit that everything that exists is of the very medium. If no medium, the electron could not exist let alone work. But then, I haven't been able to prove it either, so :)

    GrayGhost
     
    Last edited: Jul 1, 2011
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook




Loading...