
#1
Apr1612, 08:16 AM

P: 260

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
Apr1612, 08:27 AM

Mentor
P: 28,800

You might want to read the FAQ subforum in the General Physics forum. In particular, read this:
http://www.physicsforums.com/showthread.php?t=511178 Zz. 



#3
Apr1612, 08:49 AM

P: 260

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
Apr1612, 08:52 AM

P: 3,015

Light is only a photon  no wave by feynamn?
I should further add that the photon is a massless particle, i.e. it is always ultrarelativistic. One consequence of this is that one cannot ascribe a position as an observable for the photon. The role of a wavefunction is given to a field operator that creates/annihilates a photon at a particular spacetime point. Mathematically, this field operator coincidences with the 4potential [itex]A^\mu(x)[/itex].
As for classical waves, such as radiowaves emitted from an antenna, they are best described by so called coherent states. 



#5
Apr1612, 09:12 AM

P: 3,015

Op, what is referred to as a particle in Modern Quantum Field Theory are excitations of the vacuum corresponding to simple poles in the 2point correlators. These have a definite energymomentum 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 singleparticle excitations. For example, the photon is the one for the EMfield. 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 cofounder 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 waveparticle duality hypothesis. It corresponds to the semiclassical (nonquantum) 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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