Coherence - one or many waves?

[sci-guy]

When waves are said to be 'coherent', or in-phase, do they always act as a single wave, or do they maintain their individual identities?

 

[Naty1]

generally they are separate..in other words, if you change or remove one, the total is differenent...but that can depend on your definition and the many type of waves...there is spacial, temporal and spectral coherence for example...

See http://en.wikipedia.org/wiki/Coherence_(physics )

You might also find this intyeresting:
http://en.wikipedia.org/wiki/Noise_cancellation
Bose is one maker of earphone type noise cancellation...

 

[Andy Resnick]

When waves are said to be 'coherent', or in-phase, do they always act as a single wave, or do they maintain their individual identities?

Interesting question. I would say that fully coherent waves (that is to say, the mutual coherence function is 1) no longer have individual identities- they now form a single entangled state.

 

[Naty1]

Andy makes a good point..especially considering quantum mechanics...

My post was attempting a classical...everyday... perspective...

 

[sci-guy]

Thanks, that's pretty much what I thought; I've read that quantum coherence means you can define the entire system by a single wavefunction. I assume that is like saying it acts as a single wave.

But it seems to apply even on the classical level. If you have a wave generator in a water pool pumping out synchronized waves, the output flows as a single larger wave.

 

[sophiecentaur]

I have a problem reconciling this with what happens with a radio transmitting array with multiple driven elements. Do we have quantum entanglement there too?

 

[sci-guy]

Sorry, I don't understand the technical jargon -- are you referring to a common radio broadcast signal? If so, I'm curious about that too.

 

[sophiecentaur]

Any sort of radio transmission at any frequency will do. Many transmitting antennae consist of multiple elements (dipoles, for instance) which are fed from a common source. Imagine the transmitted signal or just a continuous single sine wave. Each element will be radiating the exact same signal (differing only in details of the precise phase and amplitude) and you will get an interference pattern which (usually) forms a directive 'beam' by careful choice of positions of the elements. This is totally the same as what happens a laser, in which the excited atoms release their energy in-phase with the wave that reflects up and down the cavity (stimulated emission).
As it happens, the photon energy of RF signals is exgtremely low and the vastly bigger numbers of photons can't be thought of as coming from individual atoms. But anything said about Quantum Coherence must surely apply, too, in some way.
I should like to hear how these two 'extremes' of the same phenomenon can be reconciled with a single model.
I know that RF is the 'poor relative' of light on these forums but my background is RF Engineering and I always like to square some of the Quantum Magic I have read with my own earthy experience. Stimulated emission is a minority even for light but coherent sources dominate in the RF world.

 

[sci-guy]

I'm no scientist, but since light and radio frequencies are all part of the same electromagnetic spectrum, it seems to me that the coherence phenomena should be the same in both, as you suggest.

 

[sophiecentaur]

Oh of course coherence means coherence for any system of waves - not just electromagnetic ones. My problem is the QM discussions around it which nearly always seem to involve excitation of an atom. I'd be interested to know where entanglement gets involved in an RF system with amplifiers and antennae.