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Author Topic: What is the EM field of a photon?  (Read 1624 times)

Offline Pmb

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What is the EM field of a photon?
« on: 21/05/2013 01:03:10 »
I read a comment in aother forum where someone mentioned the electrogmagnetic field of a photon. That level of QM is over my head so I asked some friends of mine who have a deep, solid knowledge of this. They all gave me different responses. I finally found something about it in a text I have that a friend of mine lent me. I've attached only the first page to this thread on the subject. There is only limited space and both files exceed it, unless I post another file in a seperate post. Let me know if there is any interest in reading it. It gives the column vector (which I've expressed as a row vector for obvious reasons) as

|Psi> = (Ex, Ey, Ez)

Has anybody heard of this thing before? I also hear people talking as if the photon can expressed as a superposition of eiigenstates and thus be a wave packet. Anyone know where that's proven? Thanks.


 

Offline imatfaal

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Re: What is the EM field of a photon?
« Reply #1 on: 21/05/2013 17:59:09 »
calling JP - calling JP.  Beyond my ken I am afraid
 

Offline JP

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Re: What is the EM field of a photon?
« Reply #2 on: 22/05/2013 21:18:18 »
We've had this discussion before of "what is a photon?"  That's a question you have to answer before you start trying to define a photon's wave function (which presumably would be related to the spatial distribution of its EM field).  The common definition is a Fock state (i.e. the smallest excitation of the EM field), and this state has no functional form that can be written over space.  It's commonly said that a photon has no wave function and this is what's meant.  A Fock state has precise spin (related to polarization) and momentum (and through momentum + zero invariant mass, you know its frequency and energy).  You can write single photon states which have imprecise momentum (and probably spin) which are restricted over some region of space, though those aren't usually called photons.  You can also approximate where photons might live over space if you know the source and receiver, for example.  This can be done because while a single Fock state can't be localized in space, many photons acting together are not in a single Fock state and can be localized. 

I probably over-complicated this.  At any rate, I have seen treatments that try to define wave functions of photons, but they aren't very convincing.  A good book on the generals is Optical Coherence and Quantum Optics by Mandel and Wolf.  There are plenty of others, but that's the one that happens to be on my shelf, and it covers much of this in detail.
 

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Re: What is the EM field of a photon?
« Reply #2 on: 22/05/2013 21:18:18 »

 

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