# Photon classical or nonclassical object?

1. Feb 25, 2012

### waterfall

Do you consider "photon" as belonging to classical physics or non-classical physics? When Einstein discovered the photoelectric effect and conjectured light was composed of photons. He didn't say it in the context of quantum field theory where the photon is excitation of the electromagnetic quantum field.

2. Feb 25, 2012

### dx

'photon' is concept that is outside the frame of classical description.

3. Feb 25, 2012

### lugita15

You can have a classical particle theory of light, like Newton did, or you can have a classical wave theory of light, like Maxwell did. The fact that neither kind of theory seemed sufficient to explain all the experimentally known properties of light is what necessitated the grand framework of quantum mechanics. The double slit experiment could be explained by waves, and the photoelectric effect could be explained by particles, but neither could easily explain both.

4. Feb 25, 2012

### waterfall

I know but when Einstein proposed the photon, it was as classical particle theory of light. He didn't actually say it is the quanta of the electromagnetic field as mentioned in the 1920's. But now we know photons are quanta of the EM field. Therefore must we use Einstein classical definition or QFTs when we mentioned about the photons in general?

5. Feb 25, 2012

### lugita15

Of course nowadays we think of photons in terms of QED, not the way Einstein thought about them. Pretty much all Einstein knew was that electromagnetic energy was quantized.

6. Feb 26, 2012

### Jano L.

In fact Einstein did not use the term "photon". He would say "light quantum". And he said he do not understand what it really is; it was just an heuristic hypothesis that allowed him to make some calculations, but with no clear definition of what light quantum is.

Planck did not even used "light quanta" in his work on thermal radiation. He just assumed the laws of emission involved some hidden mechanism which would make the emission from the material oscillators (atoms, molecules) to begin when their energy was multiple of $\hbar \omega$. But in his view the radiation did not consist of any quanta.

The word "photon" was introduced by G. Lewis in 1926. His meaning was different from that of Einstein. He thought photon can be bound to atom, can be interchanged between them and their total number was conserved.

The most used meaning of the word photon is "the energy of em. field that corresponds to one step in a ladder of eigenstates of the Hamiltonian of the field".

Photon of the quantum field theory is a picturesque expression describing some formulae of the perturbation theory. This is entirely different meaning from that of Einstein or Lewis.

A slightly different meaning sometimes used is "the quantum state of the field with all oscillators in ground state, except one and this one is in its first excited state." This is not to be thought of as a point-like particle; the eigenstates involve big (even infinite) space.

In simple introductory textbooks, they usually mean "energy of light wave $\hbar \omega [\itex] where omega is the frequency of that wave, which gets absorbed or emitted by the atom during a transition between two states with energies that fulfill [itex] E_2-E_1 = \hbar \omega$.

In quantum optics, people use the word with the meaning "non-classically behaving clicks of detectors".

These meanings hardly seem to be the same. It seems (please correct me if I am wrong) there is no universal definition of the word "photon".

So beware the word "photon". It seems best to avoid it; no physical theory is really based on it. In non-relativistic quantum theory, there are just individual particles of matter (electrons, protons,...), their wave-function and the classical electromagnetic field. In quantum field theory, there are only quantum fields (electron, electromagnetic , ...).