# Wave Particle Duality

1. May 22, 2006

### tennis freak

hey guys i was wondering if you knew any thing that helped to prove the wave-particle duality theory of light. any help would be greatly appreciated

2. May 24, 2006

### mathman

Basic "proof" is experimental.

Particle behavior is shown by the photoelectric effect (Einstein, 1905 - this is what got him the Nobel prize).

Wave behavior is shown by the double slit experiment - the effect shows up even when the source is emitting one photon at a time!

3. May 24, 2006

### ZapperZ

Staff Emeritus
You may also want to read our FAQ in the General Physics forum.

Zz.

4. May 24, 2006

### Mattara

5. May 27, 2006

### pmerrill

So a followup question, in the double slit, since diffraction takes place, how does the photon know that the slit is there, considering it does not touch it?

6. May 27, 2006

Staff Emeritus
Some photons go through one slit, some through the other; that is, there are two eigenstates and the collapse goes to one or the other randomly. The interference pattern comes when you look at a whole series of photons, where classical physics would produce two spots, one behind each slit, what actually happens is the diffraction pattern showing that overall, the photons interfered with each other.

If the photon was a wave, this would be a natural phenomenon; see Huygen's construction. In the photon case of course you can frequently think of it as an EM wave; the problem comes with matter particles like electrons, which act the same way.

7. May 27, 2006

### Farsight

8. May 27, 2006

Staff Emeritus
Good catch, Farsight. Why don't you start a thread on the Bell State Quantum Eraser and explain what you think happens and why it confuses you?

9. May 27, 2006

### Ernies

Quite right, PF, mentor. But a more fundamental question is how, with a system in which a quantum can travel two paths, does the introduction in one path AFTER the time allowble for the quantum to pass affect the results? I don't recall the reference, but it is at least five years old.

Ernies

10. May 27, 2006

### mathman

One point that should be stressed concerning the double slit experiment is that the diffraction pattern shows up when the source is emitting one photon at a time, so you don't have interference between phtons, only interference of a photon with itself. Similar results can be seen with an electron source, one at a time through the double slit.

11. May 27, 2006

### pmerrill

So that implies that a photon is a non localised entity. The double slit implies that the photon goes through both slits at the same time. Correct? Does this mean that a photon is non localised all the time or that it "magically" splits when it goes through the double slit and recombines after?

If it is so non localised why wouldn't it interact with other photons all the time? Would it be pointless to say "one photon", when in reality it's a probability distribution that is spread over the entire universe but happens to collapse to a point when it "interacts" with the environment?

Quantum physics $%$#@#\$!!!

12. May 27, 2006