Is position not an observable of a photon?

[unusualname]

Yes, I guess by "observable" people usually mean a property that can be measured (without destroying the particle), but thanks for the links and clarifications on what is a very subtle and hard question Fwiffo and Demystifier.

btw, http://xxx.lanl.gov/abs/quant-ph/0609163 is an excellent review article on QM in general (I believe Demystifier is the author)

 

[Demystifier]

btw, http://xxx.lanl.gov/abs/quant-ph/0609163 is an excellent review article on QM in general (I believe Demystifier is the author)

Let me just say that a similarity between my name and the title of this article is not a coincidence.

 

[blenx]

…… Remember that a photon is an electromagnetic wave.

When talking about what a photon is, I always hear about different versions: Some says it is a short pulse, some says it is a particle, and some says it is a plane wave with determined momentum and polarization…… Different viewpoints make me very confused.

If a photon is just a wave, can we say that its position is where the wave amplitude has a maximum? Otherwise how can we know a photon is also moving at speed c?

If a photon is a particle, how large does it occupies in space? Can we specify its position from its scattered by other particles?

 

[Fwiffo]

This is your basic wave/particle duality. The photon has wave like properties, these properties obey the maxwell equations (i.e the equations for electromagnetic waves). However it is quantized (a particle property) so two photons cannot interfere with each other in the same way two electromagnatic waves can, this is (i'm not being precise here) the photoelectric effect.

This is true for any particle but the equations are different, electrons follow the schrodinger (or dirac) equations etc...

Confusing? I know!

 

[blenx]

This is your basic wave/particle duality. The photon has wave like properties, these properties obey the maxwell equations (i.e the equations for electromagnetic waves). However it is quantized (a particle property) so two photons cannot interfere with each other in the same way two electromagnatic waves can, this is (i'm not being precise here) the photoelectric effect.

This is true for any particle but the equations are different, electrons follow the schrodinger (or dirac) equations etc...

Confusing? I know!

I know any particle have the wave-particle duality. Contrast to photon, it's easy to understand that an electron locates at some point in space. However, wave-particle duality has not told us that whether a photon does the same and what the term "photon" is indeed referred to ---- a small energy packet locates at some point and moves at speed c or just an EM wave whose length is whether long or short, will disappear entirly when part of it interact with things such as an atom in the ground state. It seems that the term "quantum excitation of the EM field" has not made things clearer.

One more question, you said a photon is a wave, then can you write down the mathmatical expression of this wave to us?

 

[vanhees71]

Again, I cannot stress enough the importance to forget about classical notions when one talks about elementary particles, and photons are elementary particles and they are even farther away from any classical notion of "particles", because they are massless.

Photons are described by the (asymptotically) free single-particle Fock states of the quantized electromagnetic field, no more no less!

 

[jcsd]

Let me just say that a similarity between my name and the title of this article is not a coincidence.

I have to say I really enjoyed that article, well worth a read.

 

[Fwiffo]

One more question, you said a photon is a wave, then can you write down the mathmatical expression of this wave to us?

This is not as easy as it seems, It depends on which method you want to use to define the photon and which gauge you are working in. The simplest expression for a photon is |1> in fock space. If you want a "space-time" kind of wave you have a look at the paper "O. Keller / Physics Reports 411 (2005)" for example the equations in page 39.