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Resonance and Coherence

  1. Sep 18, 2012 #1
    Could someone please concisely explain the difference between resonance and coherence?
     
  2. jcsd
  3. Sep 19, 2012 #2

    I could take a whack at it, but I'll refer you to Wikipedia instead for a consensus view.
     
  4. Sep 19, 2012 #3

    sophiecentaur

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    Perhaps you could explain where you find the two terms indistinguishable. Coherence describes the spread in phases between a number of waves but resonance describes the phenomenon by which an oscillator can be excited by an incident wave or signal. They seem quite distinct, to me.
    Take Imaloser's advice and read the two Wiki definitions carefully; you may have come across the terms in some context where there's an implication that they have more in common than they do.
     
  5. Sep 19, 2012 #4
    I apologize for my ignorance, as my understanding is quite limited, but isn't a harmonic oscillator tracing out a wave pattern in spacetime, so can't resonance be viewed as a wave interacting with an incident wave? Wouldn't there be interference and some degree of coherence/decoherence in such a system? Then wouldn't resonance be referring to coherent interference in spacetime?
     
  6. Sep 20, 2012 #5

    sophiecentaur

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    My problem with this is that you seem to be wanting to 'force' two words to be the same thing. The two terms are often used in connection with the same phenomenon, of course, but that's a different matter. Perhaps you have read about some phenomenon and what is written may have caused you to associate the terms more closely together than is normal. I think you need to read around and see how the two terms are used in general and that may resolve your apparent confusion.
    You need coherence between two waves for standing waves to occur and, given the right conditions, you can get 'resonance' with a standing wave pattern but in this case, the incident wave is from the same source as the other wave (the reflected wave). You can also produce a standing wave / interference pattern from two independent sources of identical frequency. These would be called coherent sources but in practice, this is only achievable at RF frequencies or lower with very stable oscillators. I doubt that even the levels of coherence achievable with lasers would allow this to be done optically - although someone may be able to quote an example. But that is not resonance - it's just stable interference between two sources. For resonance, the same wave travels back and forth many (thousands, even) times - if you do something to upset the self-coherence of the waves involved (changing the length of the paths involved by vibrating the reflecting ends, for instance) then the resonance will break down.
    I could go on . . . .:smile:
    In a standing wave, the range of wavelengths (where coherence comes in) involved will affect the 'sharpness' of the standing wave. To get a resonance on a string requires a range of excitation frequencies which are close to the natural modes of the string. Where there are no losses, the frequencies have to be exact.
     
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