# Light is only a photon - no wave by feynamn?

1. Apr 16, 2012

### jd12345

I have been studying light and learnt that light has a dual character. But then i saw one of the lectures of Feynman out of my interest - http://vega.org.uk/video/programme/45

He says light is not at all wave and he sticks to the photon concept.
But now we describe light pretty much both as a wave and particle but he didnt. So was he wrong or did i miss anything?

2. Apr 16, 2012

### ZapperZ

Staff Emeritus
3. Apr 16, 2012

### jd12345

okay - according to today's physics we have found that the rules( quantum physics ) of the behaviour of light
But if anyone has seen the video http://vega.org.uk/video/programme/45
Feynman says that light is a particle. He doesnt give it a dual character or anything like we do today. He just sticks to the particle theory

4. Apr 16, 2012

### Dickfore

I should further add that the photon is a massless particle, i.e. it is always ultra-relativistic. One consequence of this is that one cannot ascribe a position as an observable for the photon. The role of a wave-function is given to a field operator that creates/annihilates a photon at a particular space-time point. Mathematically, this field operator coincidences with the 4-potential $A^\mu(x)$.

As for classical waves, such as radio-waves emitted from an antenna, they are best described by so called coherent states.

5. Apr 16, 2012

### Dickfore

Op, what is referred to as a particle in Modern Quantum Field Theory are excitations of the vacuum corresponding to simple poles in the 2-point correlators. These have a definite energy-momentum relationship (called a dispersion relation), remain stable for a definite time, and have various characteristics (mass, spin, El. charge, color, etc.). Even classical fields, such as the electromagnetic field, have single-particle excitations. For example, the photon is the one for the EM-field. Usually, the fields are named according to the name of the elementary particle that is their excitation.

These particles are not the billiard balls from Classical Mechanics. Feynman, as the co-founder of one of the more successful Qunatum field theories, Quantum Electrodynamics (QED), was certainly aware of this. Thus, he referres to particles in the above sense.

But, the video that you are referring to describes Quantum Mechanics of a single particle. In this case , one may construct a wave equation that describes the evolution of the particle. This wave is the wave of De Broglie's wave-particle duality hypothesis. It corresponds to the semi-classical (non-quantum) limit of the equation of motion for the quantum field, in an approximation where decays and collisions of the particle with others may be neglected, or the vacuum is different (the above coherent state).

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