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Measuring Light as a wave

  1. Apr 16, 2010 #1
    Light is conceived as a self-propagating perturbation of the EM field.

    This may sound like a silly question, but has this perturbation actually been measured/observed? Or is it accepted as the most parsimonious option that matches the evidence we have?

    My guess is that it is impossible to directly measure the ripple made by light due to QM limitations. Is that correct?

    One reason I ask is that, if this perturbation has been observed and we are confident that light is such a thing [or at least manifests as such a thing], then it appears the most accurate way of portraying wave/particle duality is "Light is a wave that exhibits some of the behaviors we attribute to particles (in particular quantization)."
     
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  3. Apr 16, 2010 #2

    Cthugha

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    See "Direct Measurement of Light Waves" by E. Goulielmakis et al. (Science 305, 1267 (2004))

    Also available without subscription here: http://www.attoworld.de/junresgrps/attosecond-dynamics/publications/Science305p1267_2004.pdf [Broken]
     
    Last edited by a moderator: May 4, 2017
  4. Apr 16, 2010 #3
    Thanks! I must admit that much of the discussion regarding how they took the snapshot is above my head, but this appears to be exactly what I was looking for!

    Thanks!
     
  5. Apr 16, 2010 #4

    ZapperZ

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    Er.. we use pick-up antenna all the time to detect such EM waves. I'm not sure if this is a mystery or unknown at all.

    If you can hook up your analog radio antenna to a scope, you have all the evidence that you need.

    Zz.
     
  6. Apr 16, 2010 #5
    I was referring specifically to visible light as investigated in the way I believe the article's writers did.
     
  7. Apr 16, 2010 #6

    ZapperZ

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    What makes visible light any more special than radio waves?

    Zz.
     
  8. Apr 16, 2010 #7
    Also, ZapperZ, for whatever it might be worth...I think a radio antenna measures the EM disturbance at a particular point... it does not give a "freeze frame" of the field at a particular time to show the wave packet in profile. My understanding from looking at the pretty pictures in the article is that this was accomplished by their setup.

    However, the above aside, I admit that the difference between an antenna picking up radio waves to show that an honest disturbance in the EM field occurs and what was done in the article is mostly a matter of scale. And I apologize for the dismissive tone of my last post.

    My real reason for asking this question is that, given that we have measured the disturbance, it seems wrong to continue to speak of light as merely "acting like a wave." Regardless of whatever other properties it may have, if light is definitely simply a self-propagating disturbance of the EM field, then it [in my view] people should not merely speak of it as "acting like a wave." It is a wave because it satisfies any reasonable definition of what a wave is. It just happens to be the case that it also exhibits properties we attribute to particles as well.

    I might propose instead describing light as the following:
    Light is a wave (it transmits energy across distance by local fluctuations in a field or medium that give rise to local fluctuations elsewhere, and these various "elsewhere"s lie along the direction of propagation) transmitting quantized energy, and its curious behavior can be best modeled by abstract particles (here used in the QFT sense of the term) that exhibit both classical-particle-like and wave-like properties.
     
  9. Apr 16, 2010 #8

    ZapperZ

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    I have no clue what you are getting at here.

    First of all, if this is a 'wave-particle' duality issue, you need to read the FAQ.

    Secondly, when I say 'wave', I was trying to invoke the usual pedestrian view of what a wave is. We talk about light wave because one can measure such oscillating E-field, be it at a particular location over time, or a snapshot of it at a particular time. This is true no matter what frequency we are talking about. I deal with RF spectrum all the time in our waveguides, and we have tons of detectors and pick-up antenna to see such EM wave. There's nothing special that happens in the visible range that doesn't happen in the RF range.

    Thirdly, ALL wave-like observations can be described via QM's photon picture. See the Marcella paper that I've referred to repeatedly on here.

    Zz.
     
  10. Apr 16, 2010 #9

    Cthugha

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    It is rather funny that a lot of people think there are differences and also within the history of science people considered radio waves different from visible light. When Hanbury Brown and Twiss developed the optical intensity interferometer and found photon detections at two detectors to be correlated for coherent and high intensity light and were able to calculate the angular size of Sirius from this data, a lot of scientists like Ferguson and Adam thought that the results were plain wrong, contrary to thermodynamics and violating the uncertainty principle. The same technique had been applied to astronomic radio sources by using radio intensity interferometers several years before and nobody had any doubts.
     
  11. Apr 16, 2010 #10
    I found this post a terribly rude one that was completely uncalled for.

    I was simply saying that since visible light has been shown to be a perturbation in the EM field that matches any reasonable definition of a wave, people should not merely say it exhibits the properties of one. Whatever particle properties light possesses should not make someone feel bashful about saying, flat-out, "light is a wave."

    I don't see why it was so difficult for you to glean that from my earlier posts.
     
  12. Apr 16, 2010 #11

    jtbell

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    Keep in mind that many people consider questions about what something "really is" versus "behaves like" to be not physical in nature, but instead, metaphysical or philosophical. Especially when when dealing with quantum physics, for which there is no universally accepted interpretation of its mathematical structure.
     
  13. Apr 16, 2010 #12
    Yes, but there is a difference between putting something into a category ["combustion is a chemical reaction"] versus describing the essence of something ["combustion is the emission of phlogiston."]

    The term "wave" has a reliable meaning that can be applied to a form of energy without exhausting the meaning of that object. We can say "sound is a wave" without having describe what sound really is. Saying "sound is a wave" refers to its movement by local fluctuations in a field/medium that give rise to local fluctuations elsehwere along the line of propagation. It doesn't pretend give the essence of sound but describes one aspect of it. Just like saying "Florence Olafson is my mother" does not pretend to give the essense of who Florence Olafson is.

    Objections to the statement "light is a wave," in my opinion, come down not to arguments over the nature of light or QM in general, but rather conflict over the meaning of what it means to be a wave. While waves may tend to have certain properties and tend not to have certain other ones, to make the having of those properties versus not having those properties a determiner of whether something gets to be said to "be a wave" is to put the cart before horse.
     
  14. Apr 16, 2010 #13

    ZapperZ

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    I did! And it still doesn't say why it has to be confined to "visible light".

    However, since you posted this in the "quantum physics" section, presumably you are trying to see why this would make sense quantum mechanically. So if you want to say that "Light is a wave", then you cannot simply say that without accounting for various other properties and observations that cannot be reconciled with light being the classical wave that you are proposing, such as the photon antibunching phenomenon and the which-way experiments. I would also add that the phenomena of angle-resolved photoemission, multiphoton photoemission, and resonant photoemission have ZERO description using classical wave picture of light.

    Can one detect the oscillating E and B field in EM wave? Sure one can. But in other circumstances, such wave picture simply breaks down. This, one cannot ignore when one wants to make such bold declaration as "Light IS a wave"!

    Zz.
     
  15. Apr 17, 2010 #14
    Would you be as opposed to the statement "Light travels/propagates as a wave"?
     
  16. Apr 17, 2010 #15
    I don't agree at all. I thought it was a great response.
     
  17. Apr 17, 2010 #16

    ZapperZ

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    Would YOU buy such a statement after reading something like B.J. Pearson and D.P. Jackson, Am. J. Phys. v.78, p.471 (2010)?

    Again, you seem to be ignoring all of the evidence here. You provided NO explanation on why you would want to say such a thing in light of the phenomena that I've mentioned.

    Zz.
     
  18. Apr 17, 2010 #17
    Actually, I've explained EXACTLY why I am saying such a thing, yet you appear to have no interest in grasping the semantic content of my posts.

    Are you claiming that in the experiments performed in the reference [which I have requested, it not being available immediately through my university's library] that, when the source of a light provides essentially one photon at a time [the "spontaneous parametric down conversion" described in the abstract] that there is no local fluctuations in the EM field between the source and the receiver? My guess is that you are not saying that, and my guess is that such a claim is impossible to make due to QM limitations in any event.

    You appear to be taking the statement "light is a wave" or "light travels as a wave" and, against all my efforts to the contrary, read into them something that simply is not there.

    Saying "light travels as a wave" is in no way equivalent to saying "Modeling light as a wave allows us to predict (or at least explain) the behaviors light exhibits."
     
  19. Apr 17, 2010 #18

    ZapperZ

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    But saying light travel "as a wave" isn't something to be said in a cavalier manner! This is because Maxwell equation clearly has an EXPLICIT description of what that means! A wave is a wave! You can't simply redefine what it is without invoking a new mathematical formalism! That's why I wanted you to tell me how you are able to reconcile those experiments with such a picture!

    Zz.
     
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