# Learning at college about wave partile duality

1. Dec 11, 2005

### QueenFisher

i've been learning at college about wave partile duality, how some phenomena can only be explained using the wave theory, but for some background reading i decided to read QED and in the introduction feynman says light behaves like particles. can anyone enlighten me?

2. Dec 11, 2005

Staff Emeritus
This has been discussed on a number of recent threads but, because of the Feynmann QED reference, perhaps a further explanation is appropriate. We can OBSERVE photons (and other "particles") as waves or as particles .

This depends on the kind of experiment we do; if the experiment depends on the momentum of the photon, then it can be interpreted as a particle with momentum and energy. If it is an interference type experiment, like the famous double slit, then it is usually interpreted as a wave phenomenon.

But Feynmann saw a way to explain the slit experiment with a particle interpretation. He theorized that the particle has an amplitude which is a complex number (he never calls it that, but if you follow his rules for adding and multiplying his "little arrows", that is what they boil down to). The particle follows all possible paths in the experiment and you add up the amplitudes over the paths and most of them cancel each other out, and Presto! the interference comes out.

This does not mean Feynmann denied the wave interpretation or said that only a particle interpretation is necessary. His book is, after all, a description of the QED field theory, in which fields are the primary constituents, and particles in any guise are just quanta, bundles of field energy that come and go.

Last edited: Dec 11, 2005
3. Dec 11, 2005

### Ratzinger

Really? I say photons behave like waves between measurements, but behaves like particles WHENEVER observed. Double slit experiments only let us better infer photons wave-like behavior between measurements. There is no duality between experiments, there is quantum dynamics via Schrödinger eq. and there is quantum measurment via projection states into eigenstates.

4. Dec 11, 2005

Staff Emeritus
Quantum mechanics doesn't say what photons are like "between measurements" except to give them states in a Hilbert space which evolve unitarily. This is neither wave nor particle nor indeed anything defined in spacetime, but a mathematical model.

5. Dec 11, 2005

### QueenFisher

but he wrote 'I want to emphasise that light comes in this form - particles. It is very important to know that light behaves like particles, especially for those of you who have gone to school, where you were probably told something about light behaving like waves. I'm telling you the way it does behave - like particles.

so i'm still quite confused.

6. Dec 21, 2005

### huhuayu

I think the "particle" Feynmann means is not a classical partical.If we use Feymann's theory to explain all,a classical partical do also have infinite paths to go from A to B,and each path has a complex amplitude.But different from the quantuam world,the amplitudes vary so fast that at last only one path is possible,and all information about the phase loses.So,in classical world,partical is partical, wave is wave;while in the quantum world,they entangle together.
Thanks for criticism.