# Particle wave Duality

1. Sep 5, 2004

How would you prove that light is a particle rather than a wave? I understand that it is affected by black holes which proves it has mass but are there any other proofs?
I'm sorry if this has been answered somewhere before.

2. Sep 5, 2004

### HallsofIvy

Staff Emeritus
Do you understand the title you put on your post? The whole point of "particle wave duality" is that it is impossible to prove that light is a wave or a particle because it is not, strictly speaking, either. At the quantum level, the distinction between the two disappears. You do not "understand that it is affected by black holes which proves it has mass" because that does not prove light has mass. The photo-electric effect gives evidence that light sometimes has to be treated like a particle and diffraction gives evidence that it sometimes has to be treated like a wave.

3. Sep 5, 2004

### veryyoung

actually as of now some physicists believe that light is fundamentally a particle but due to it's extreme speed causes waves in everything including space. sorry if this has been disproven already i am not always informed of these things but if it has been please tell me.

4. Sep 6, 2004

### ahrkron

Staff Emeritus
It does not work that way.

As HallsofIvy mentioned, light does not behave entirely as particles, and neither as waves. It has a "dual" nature, well described by the math of QM.

The problem with your description ("a particle but due to it's extreme speed causes waves in everything") is that it tries to use our intuitive ideas of "waves" and "particles" all the way down to the explanation at the quantum level, but it turns out that those concepts (waves and particles) are not well suited for the description of quantum interactions.

5. Sep 6, 2004

### Gonzolo

It is mostly the photoelectric effect that suggests light is a stream of particles called photon, each with energy E = hf. Light thas no rest mass. Only relativistic mass, which is different than the mass people are familiar with.

6. Sep 6, 2004

### somy

well I have a question:
I know that there is a probiility function that we interprete as a wave. (or maybe I'm wrong!)
Now what is the relation between this and the light.(or maybe there is no relation!)
thanks a lot.
Somy

7. Sep 6, 2004

### da_willem

The Compton effect is also an example where the particle approach to light yields the correct equation.

8. Sep 6, 2004

### ZapperZ

Staff Emeritus
Rather than rumble on and on with this, I will just quote the abstract from this paper...

J.J. Thorn et al., Am. J. Phys., v.72, p.1210 (2004).

Zz.

9. Sep 6, 2004

### marlon

Somy, the connection between particle-like-behaviour and wave-like-behaviour is given by the famous deBroglie-relations...

E=hv and v is the frequency
p=h/l and l is the wavelength

regards
marlon

10. Sep 6, 2004

### jjalexand

I'm not really sure it is impossible for it to only be a wave. Perhaps some waves could have non-linear reactions that made their interferences/collapses look just like particle. Also, two exactly opposite waves could perhaps meet and cancel out. Would this look like the collapse of the wavefront? Would this meeting take place at a particular point/vector in time and space? Could that look like a particle, stationary/travelling?

11. Sep 6, 2004

### da_willem

This is a subject not treated in normal (nonrelativistic) quantum mechanics, so I am kind a curious too. If light is also described by a wavefunction, is this the same (has it the same properties) as a particle wavefunction?

12. Sep 6, 2004

### humanino

No. You can consider the potential vector A as a "wavefunction" for the photon, but this is plagued with difficulties : the photon can certainly be very well localized in momentum space, but cannot be localized at all in position space. The position of the photon is ill-defined, because this guys kind of spend no-time in a spot but go visit an infinite continuum of spots : the ray.

13. Sep 6, 2004

### Fredrik

Staff Emeritus
In quantum mechanics, every physical system is described by a wavefunction, or equivalently, by a vector in a Hilbert space. You're asking if a photon wavefunction has the same properties as the wave functions of other particles. They obviously don't have exactly the same properties. If they did, they wouldn't be different species of particles. But they have a lot in common, like the usual probability interpretation.

14. Sep 7, 2004

### somy

Dear Fredrik:
Can you explain it more???

15. Sep 7, 2004

### Gonzolo

Instead of always talking about things having a dual particle-wave behavior, some have coined the term "wavicle", which would include just about anything imaginable.

16. Sep 8, 2004

### Fredrik

Staff Emeritus
I'm not sure I can. This is actually rather difficult stuff. I'm afraid you won't really be able to understand it until you've studied quantum field theory. But if you have a more specific question, then maybe I'll be able to answer it.

17. Sep 8, 2004

### nrqed

There is major difference: light is described by a quantum field (which allows the number of photons to vary). That's very different from nonrelativistic wavefunctions. This is the subject of quantum field theory. It's a huge :surprised subject so I won't get into it for now.

Pat

18. Sep 8, 2004

### marlon

What exactly do you mean in your post ?

Are you saying that relativistic wave-functions do not occur in QFT ???

What is, according to you this difference between quantum-fields and non-relativistic wavefunctions???

All fields in QFT are quantized (that's the intention of QFT,second quantization...) and they can be relativistic or not...

regards
marlon