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Where does the waves energy go when destructive interference occurs?

  1. Jul 8, 2011 #1
    For example, when two light waves completely cancel each other out, there is the black on an interference pattern, indicating absence of light.

    What happens to the photons? It seems they cease to exist.

    Also, How does this relate (or not) to the "Conservation of Energy" law?

    Any insights appreciated.

    -Gil
     
  2. jcsd
  3. Jul 9, 2011 #2
    Two light waves cannot completely cancel eachother out.It's true that there are regions where there is an absence of light ,the so called regions of destructive inerference but these are accompanied by regions where the light is more intense the so called regions of constructive interference.The energy lost in the destructive regions has been shifted across to the constructive regions and as usual energy is conserved.
     
  4. Jul 9, 2011 #3
    OK, thanks for the info.

    To expand on this: what about just two individual photons destructively interfering with each other in a confinement or vacuum?

    And making things even more confusing to me, the event of a single photon interfering with itself.
     
  5. Jul 10, 2011 #4
    According to Heisenberg, photons don't have position. While they're on the fly, it's best to visualize them as a wavefunction, not as little grains. Or... by QED, an infinite number of virtual photons take all possible paths, where some cancel after superposition and others reinforce.

    Back to your original question. During absorption of radio waves, the receiving antenna emits a wave out of phase with the incoming wavetrain. The two are superposed and produce an interference pattern having a central node. This produces a downstream shadow in the electromagnetic wavetrain. Net energy goes missing. This energy winds up in any resistive load attached to the antenna terminals (or it ends up in the receiver front-end transistor.)

    The above is a case where photons were destroyed by interference and not simply moved around as a diffraction pattern.

    From a semi-classical standpoint, the above antenna description might apply to photons being absorbed by atoms. However, I've not yet encountered any text which describes it this way. Suppose the atom emits a wavetrain which superposes with the incoming wavetrain (both having the same wavelength.) An interference pattern is produced, a pattern with a central node. The atom casts a shadow, and the missing energy ends up driving an electron to a different orbital. Pretty cool, eh? But not necessarily matching reality!
     
    Last edited: Jul 10, 2011
  6. Jul 10, 2011 #5

    Drakkith

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    Staff: Mentor

    While I don't think wbeaty's explanation is incorrect, I'll just say that I don't like the description of "energy going missing" and all that. The energy in a photon is transferred to whatever absorbs it. Whether that is by emitting an out of phase wavetrain or whatever, is beyond me.
     
  7. Jul 10, 2011 #6
    I go along with Drakkiths description in that the "energy in a photon is transferred to whatever absorbs it".If photons are sent one at a time through a Youngs slit arrangement then these photons will be detected one at a time at the screen/detector.As time passes the overall pattern observed approaches that of the standard "Youngs fringes pattern" observed with a more intense light source.The end results are pretty much the same but when separate photons are considered the analysis looks at the probabilities of each photon arriving at a particular location.
     
  8. Jul 10, 2011 #7
    Say it this way then: During absorption, the EM-wave energy goes missing.

    Hmmmm. Actually what has vanished is the Wakalixes! :) (re. Feynman chapter "Judging books by their covers!")

    Acoustic version: the sound goes missing. Optical version: the light goes missing. It's destroyed upon absorption.
     
  9. Jul 10, 2011 #8

    Drakkith

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    Staff: Mentor

    Missing implies that you don't know what happened to it in my opinion, hence why I don't like that term.
     
  10. Sep 1, 2012 #9
    I've seen a polarized beam of coherent light split and recombined out of phase and canceled out without constructive interference.
    Physisists like to cling to the notion that energy can't be destroyed because if not they would have to admit it could be created and everything we "know" might be wrong...
     
  11. Sep 1, 2012 #10

    Dale

    Staff: Mentor

    That can only happen in the presence of matter, in which case the energy goes into the matter.


    Because of that and also because there is no evidence to support the notion that it can be destroyed and plenty of evidence to support the notion that it can't.
     
  12. Sep 1, 2012 #11
    The energy of the canceled photons could become bosons or quarks or some other type of tiny particles. But a similar experiment in water waves illustrates that the waves can be canceled out complely by adding additional energy to a wave to completely eliminate it.
     
  13. Sep 1, 2012 #12

    Dale

    Staff: Mentor

    Please post a credible reference or stop speculating. What you describe cannot happen without an interaction with matter where the energy goes.
     
  14. Sep 1, 2012 #13
    Bose sound cancellation head phones are a great example of using energy to cancel out unwanted waves.
    I've demonstrated this personally in a wave tank full if water and seen it demonstrated with an interferometer.
     
  15. Sep 1, 2012 #14

    Dale

    Staff: Mentor

    Do you have any reference which has done a careful measurement of the energy of a Bose headset and found that energy was not conserved? If not then it is safe to say that the headset either causes constructive interference elsewhere or acoustic energy goes into the headset itself and is dissipated there.
     
  16. Sep 1, 2012 #15
    In the case of the sound cancelling headphones, so called "experts" blindly claim the enery is converted to heat and dissipated, but this is not the case.
    When conducting similar tests on waves through other mediums such as water or light the heat generated comes no where near the amout of energy lost when destructive interference occurs.
    Rather than looking for refferences, I suggest conducting experiments to witness the results yourself as I have.
     
    Last edited: Sep 1, 2012
  17. Sep 1, 2012 #16
    I agree. Here's their description - this is a summary of a 6 page explanation.

    http://electronics.howstuffworks.com/gadgets/audio-music/noise-canceling-headphone5.htm
     
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