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Shape of light?

  1. Feb 14, 2010 #1
    Does light have a definite shape? (this may be stupid question but from many days this question is hanging in my brain

    Thanks
     
  2. jcsd
  3. Feb 14, 2010 #2
    Light doesn't have a shape in the same sense a tennisball has shape (sphere).

    Light is neither wave nor particle but both. Confusing eh? :smile:
     
  4. Feb 14, 2010 #3
    yes confused,it's not a partice nor a wave,but how is this possible
     
  5. Feb 14, 2010 #4
    Truth be told, we don't exactly know what light is. It both demonstrates particle properties (see photoelectric effect) and wave properties (see diffraction), though it is neither one of them singularly and yet both simultaneously.
     
  6. Feb 14, 2010 #5
    Because we as humans have metaphorical ways of thinking about how objects behave imo. We have a model of how a particle and a wave behave and light behaves as both to us experimentally. I dont see a problem with this as we made these models up to begin with because they work.

    It just helps us to understand and predict how light behaves a little better. So its a decent model. We make up the concepts to help us understand nature. If the models start falling short in certain situations, we change them to fit the new situations.
     
  7. Feb 14, 2010 #6

    Dale

    Staff: Mentor

    I tend to disagree with this type of statement. We have a very accurate theory called QED that completely describes the behavior of light. AFAIK there is no indication that QED fails to accurately predict any aspect of light's behavior. In other words it is both accurate and complete.

    As far as I am concerned, that means that we do know what light is. There is a philosophical sense in which we cannot know what anything is, but from a scientific standpoint I think that solipsism-type argument is completely pointless. The problem is not that we don't know what light is, it is just that we have a lot of linguistic baggage.
     
  8. Feb 14, 2010 #7
    The only problem with QED is that is treats light as a particle.
     
  9. Feb 14, 2010 #8

    Dale

    Staff: Mentor

    So, why is that a problem as long as it accurately predicts the behavior?
     
  10. Feb 14, 2010 #9
    There is nothing wrong with QED, it does as you say, accurately predicts the behaviour. Is just that we can also think of light in the classical sense, as a wave. Let me make this clear, there is no problem that light can be described as a wave or a particle. The problem is classifying it singularly as one or the other, when it is both or neither.
     
  11. Feb 14, 2010 #10

    Dale

    Staff: Mentor

    The description in QED is completely unified. There is no dichotomy between wave and particle in QED and all of the behaviors are predicted in a single framework.
     
  12. Feb 14, 2010 #11
    I am not arguing about the validity of QED, its probably the success story of the 20th century in physics. The problem is that we classify light as particles and waves when we know it cannot be just a wave nor just a particle, it is either both simultaneously. Or it is neither and something we have yet to discover.
     
  13. Feb 14, 2010 #12
    I think that closely questioning the way we think helps us immensely when modeling falls short. It forces us to break new ground in math or to take a very diff. view when our metaphorical world based on senses (largely sight as humans) fails us as an accurate descriptor.

    I would agree that the arguement you gave is pointless. For us predicting is a type of knowing.
     
  14. Feb 14, 2010 #13

    Dale

    Staff: Mentor

    I think you are missing my whole point. There is a lot of linguistic baggage associated with the English words "particle" and "wave", neither of which in common-usage adequately expresses what light is. This is not the case with the math of QED which accurately and completely characterizes light in one single unified way with no disparity.

    You say that light cannot be "just a wave nor just a particle", but that is not correct. It is a particle, but in the full sense of the QED usage of the word "particle". In QED particles have a single unified framework which shows diffraction, interference, reflection, and all of the other traditional "wave" aspects of light's behavior as well as scattering, quantization and all of the other traditional "particle" aspects of light's behavior. This single unified concept is called a "particle", but just like you can understand the difference between "banking to the right" and "banking with CitiBank" you should be able to understand the differences between a "classical particle" and a "quantum particle".

    Light is a particle which behaves as described by QED.
     
  15. Feb 14, 2010 #14
    Aye, I am not disagreeing with you. But I think you are missing my point, light is a phenomenon which cannot only be described as a particle as described by QED but also as a wave in classical physics. Both areas of physics can explain what the other one can't so it is fruitless to say the QED is the definitive theory on light. Here is my argument: The fact that we have this disparity in classifying light means we are unable to singularly point at one theory and say that is the correct one. We simply don't know. Due to this disparity, I ask you a question to which I invite your answer, what is light?
     
  16. Feb 14, 2010 #15

    russ_watters

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    That isn't correct. QED is a comprehensive theory of light. It doesn't have the limitation you are saying it has. There is no aspect that it can't explain while the wave theory can.

    DaleSpam is right - the way I see it is this entire issue is based on the English language not having a one word description for what light is (other than saying it is just light). It's not a wave and not a particle - it is light!
    Again, the wave theory is wrong, the particle theory is wrong, QED is right. [fixed spelling mistake]
    Light is light.
     
    Last edited: Feb 15, 2010
  17. Feb 14, 2010 #16
    First you say light is light, then you say QED is light, then light is light again. Which is it to be? I have posed a purely philosophical question to which you have provided a lousy answer. C'mon guys you can do better than this.
     
  18. Feb 15, 2010 #17

    russ_watters

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    Spelling mistake fixed. That should have been "QED is right".....
    I'm not really concerned with the philosophy here, which I consider to be just argumentative and pointless. What I am concerned with is your assertion that QED can't describe all of light's behavior. This assertion is just plain factually wrong.
     
    Last edited: Feb 15, 2010
  19. Feb 15, 2010 #18
    No, light does not have a definite shape. Normal object maintain their shape because of different atoms bonding together in some way and 'holding on'. Light particles do not have any such bonds with each other.
     
  20. Feb 15, 2010 #19

    sas3

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    I came across this http://blogs.discovermagazine.com/cosmicvariance/files/uploads/gallery_photon.jpg" [Broken]once when looking for the answer to that same question I thought there was an article with it but I do not see it now
     
    Last edited by a moderator: May 4, 2017
  21. Feb 15, 2010 #20
    your all discussion made me to know about Quantum Electrodynamics it's amazing

    thanks a lot
     
  22. Feb 15, 2010 #21

    Dale

    Staff: Mentor

    This is wrong. QED does not have any such limitation. If you disagree then please specifically identify which aspect of light's behavior classical physics can describe that QED cannot.

    Your argument is flawed because there is no such disparity, QED is accurate and complete. We can point at QED and say that it is the correct theory because it accurately describes all the observed behaviors of light. I have said it already, but I am glad to repeat my answer to your question: Light is a particle as described by QED.

    I think that you have a serious misunderstanding of what QED is. I would recommend this series: http://www.vega.org.uk/video/subseries/8
     
  23. Feb 15, 2010 #22

    f95toli

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    QED does not assume that light is a classical particle; a photon in QED is strictly speaking a localized excitation of the vacuum. We usually refer to these excitations as "particles"; but that does not mean that photons are small "balls of light" or anything like that.

    And, and has already been pointed out, all "wave-like" phenomena can be described by QED.
     
    Last edited: Feb 15, 2010
  24. Feb 15, 2010 #23

    madmike159

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    The problem with QED is its described using maths beyond most people. Since we can't see how exactly light behaves we come up with analogies about it which aren’t right.
     
  25. Feb 15, 2010 #24

    Dale

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    The same is true of Newtonian mechanics. That is hardly a failure of a theory, but rather a failure of the education system.
     
  26. Feb 15, 2010 #25

    madmike159

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    Yes, but Newtonian mechanics applies to objects we can easily rever dyer.
     
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