# Particle wave Duality

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.

Homework Helper
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.

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.

Staff Emeritus
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veryyoung said:
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.

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.

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.

somy
HallsofIvy said:
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.

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

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

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

While the classical, wavelike behavior of light (interference and diffraction) has been easily observed in undergraduate laboratories for many years, explicit observation of the quantum nature of light (i.e., photons) is much more difficult. For example, while well-known phenomena such as the photoelectric effect and Compton scattering strongly suggest the existence of photons, they are not definitive proof of their existence. Here we present an experiment, suitable for an undergraduate laboratory, that unequivocally demonstrates the quantum nature of light. Spontaneously downconverted light is incident on a beamsplitter and the outputs are monitored with single-photon counting detectors. We observe a near absence of coincidence counts between the two detectors—a result inconsistent with a classical wave model of light, but consistent with a quantum description in which individual photons are incident on the beamsplitter. More explicitly, we measured the degree of second-order coherence between the outputs to be g^(2)(0) = 0.0177±0.0026, which violates the classical inequality g^(2)(0)>=1 by 377 standard deviations.

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

Zz.

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

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?

da_willem
somy said:
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

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?

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.

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da_willem said:
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?
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.

somy
Dear Fredrik:
Can you explain it more?

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.

Staff Emeritus
Gold Member
somy said:
Dear Fredrik:
Can you explain it more?
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.

Homework Helper
Gold Member
da_willem said:
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?

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 subject so I won't get into it for now.

Pat

marlon
nrqed said:
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 subject so I won't get into it for now.

Pat

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