Is Light a Wave or a Particle?

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ZapperZ
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IS LIGHT A WAVE OR A PARTICLE?

Contributed by Marlon and ZapperZ.

In our ordinary world, “wave” and “particle” behavior are two different and opposite characteristics. It is difficult for us to think that they can be one of the same. Is light a particle or a wave? The simple, naïve answer to that is “both” or “neither”.

Light, or photon, was never defined as a “particle” the way we normally define a particle. Light is not defined to have a definite boundary in space like a ping-pong ball, or a grain of sand. Instead, light is defined as having quanta of energy. So the discreteness is not defined as discrete object in space, but rather in the energy it can carry. Already, this is not your regular “particle”, and should not be confused as such.

Secondly, in quantum mechanics, the description and properties of light has only ONE, single, consistent formulation, not two. This formulation (be it via the ordinary Schrodinger equation, or the more complex Quantum Electrodynamics or QED), describes ALL characteristics of light – both the wave-like behavior and the particle-like behavior. Unlike classical physics, quantum mechanics does not need to switch gears to describe the wave-like and particle-like observations. This is all accomplished by one consistent theory.

So there is no duality – at least not within quantum mechanics. We still use the “duality” description of light when we try to describe light to laymen because wave and particle are behavior most people are familiar with. However, it doesn’t mean that in physics, or in the working of physicists, such a duality has any significance.
 
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sophiecentaur
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The problems with that word 'particle' are with us because, imo, Richard Feynman used the term so much. He knew what he meant by it but I don't think he realised that Joe Public did not understand the strict way he used it. As his word is Law, with Science students, the misapprehension has persisted. If only he had decided to use a different word, we could have sorted this out, long ago.
As for the 'granular' nature of the Universe; that is still under discussion, afaik.
 
  • #3
From the original post denying a duality: "... in quantum mechanics, the description and properties of light has only ONE, single, consistent formulation, not two. This formulation (be it via the ordinary Schrodinger equation, or the more complex Quantum Electrodynamics or QED), describes ALL characteristics of light "

First, the Hamiltonian is an operator based on the kinetic and potential energies of a (rest mass) particle and Schrodinger used that as the basis of his equation. Since the photon has no rest mass the Hamiltonian does not apply. You cannot write a Schrodinger equation for a mass less photon!
Second, QED gives a "...complete account of matter and light interaction..." https://en.wikipedia.org/wiki/Quantum_electrodynamics QED is NOT a description of the in-flight photon; it is a description of the photon's interaction with matter. This conflates the photon with it interaction (termination).

The original posters might want to refine their arguments...

Physics pfan

BTW: I think Richard Feynman knew exactly what he meant when he used the term 'particle' He was a particle guy who used his path integral formulation to direct particles where they were supposed to go. Particle in, particle out...
 
  • #4
PeterO
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IS LIGHT A WAVE OR A PARTICLE?

Contributed by Marlon and ZapperZ.
I would be tempted to say NO.
In order to establish what light is, we first note (carefully) what light does.
We then consider things we are familiar with that also do that.
Some of the things light does, can be easily achieved with a wave: reflection, refraction, two waves passing through each other, interference.
Some of the things light does cannot be easily achieved with a wave - notably photo electric effect - but a particle model can help there.
So presumably light has to be something else?
While we are trying to find exactly what light is, we can use a wave model, and/or a particle model, to predict the behavior of light in most (all?) the cases normally encountered.
When we move to a sub-atomic world it can be difficult (light interacting with atoms or electrons) - but most people never find themselves there.
When light travels over great distances passing massive objects - like light from a distant source passing by a large mass in the universe - it can be tricky, but most people never find themselves there either.
In the every-day situation people find themselves in, the wave model can predict most (all) things they come across so to think of light as a wave will likely have you expecting the results you come across - Mirrors, lenses, rainbows, colours on the television.
 
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davenn
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In the every-day situation people find themselves in, the wave model can predict most (all) things they come across so to think of light as a wave will likely have you expecting the results you come across - Mirrors, lenses, rainbows, colours on the television.
But that doesn't make it a wave. It just means that under some conditions we can observe a wave like nature
 
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  • #6
PeterO
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But that doesn't make it a wave. It just means that under some conditions we can observe wave like nature
I didn't say it makes it a wave - in fact I started by answering the question "IS LIGHT A WAVE OR A PARTICLE?" by saying NO. (No it is not a wave and No it is not a particle).
Sound certainly behaves like a wave - and possibly/probably is a wave, but I am not sure anyone has actually SEEN sound to confirm that it is a wave.
 
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IS LIGHT A WAVE OR A PARTICLE?
Light is composed of particles called photons. Each photon is a discrete packet of electromagnetic energy.

So there is no duality – at least not within quantum mechanics. We still use the “duality” description of light when we try to describe light to laymen because wave and particle are behavior most people are familiar with. However, it doesn’t mean that in physics, or in the working of physicists, such a duality has any significance.
Consider the quantum double slit experiment. If the there is no duality then how can the light pass through one slit and also interfere with itself on the opposite side creating the interference pattern? If you shot 20,000 photons at the slit one at a time, the interference pattern is still there.
 
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davenn
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Light is composed of particles called photons.
Gosh, I don't know how many times this has to be stated on the forum to get people away from bad science ?!

There is no duality ... period ! end of story !

Light/photons are not particles

Yes, they are quantum packets of energy

In experiments, depending on the experiment, light can be observed to have particle-like behaviour .... that doesn't make it a particle

conversely, in other experiments, light can be observed to have wave-like behaviour.... that doesn't make it a wave


Dave
 
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There is no duality ... period ! end of story !
In earnest I should not have used the double slit to support my argument.

In experiments, depending on the experiment, light can be observed to have particle-like behaviour .... that doesn't make it a particle
conversely, in other experiments, light can be observed to have wave-like behaviour.... that doesn't make it a wave
I agree with you here. People choose a particle when it suits a solution to their problem and a wave when it suits another solution. Your argument interested me enough to read further.

Each photon is a discrete packet of electromagnetic energy.
I won't be using the duality - particle or wave - (definition?) anymore.

----------------------------------------------------------------------------------------------
As a side note, I should probably get my old lecturer to drop it too.
 
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But that doesn't make it a wave. It just means that under some conditions we can observe a wave like nature
In everyday practical application, we can treat gravity like a force. That doesn't make it a force.
 
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  • #11
davenn
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In everyday practical application, we can treat gravity like a force. That doesn't make it a force.
irrelevant to the topic
 
  • #12
entropy1
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So a photon is a discrete packet of energy, or: a photon is energy? Waves carry energy, particles carry energy.
Consider the quantum double slit experiment. If the there is no duality then how can the light pass through one slit and also interfere with itself on the opposite side creating the interference pattern? If you shot 20,000 photons at the slit one at a time, the interference pattern is still there.
If we fire a photon at the left slit, how does it go through the right slit? How does it go through both slits?

A photon is an excitation of the EM field, so why isn't it true that it is a wave until it is measured as a particle?
 
  • #13
Vanadium 50
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Did you read what ZapperZ wrote?
 
  • #14
DaveE
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I think this whole duality issue is simply an artifact of how we teach physics, and the confusion created when really useful approximate models are confused with ontological questions about what a photon is.

Frankly, I think this is a philosophical question, not really physics. Physics is great at describing what things actually do and there are some great models to describe why. But, that is all, IMO. What is a photon? What is an electron? I'm not sure these are questions for physicists.
 
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  • #15
sophiecentaur
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The duality thing is such old fashioned thinking.

'People' like to be told what things "actually are" and so they just lap up the "er, it's both things" so they feel flattered when they feel they understand and that they can cope with this Zen - sounding / paradoxical idea. It's far too hard to understand for most folks so there will always be wrong ideas peddled in public.

But you have to remember that most of today's teachers were taught that idea (by other teachers). There's no point getting cross about it. In a few decades it may be sorted out.
 
  • #16
Vanadium 50
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In a few decades it may be sorted out.
Doubtful. "Relativistic mass" exited the mainstream scientific discussion in two years, yet more than a century later...
 
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  • #18
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MARCH 2, 2015

The first ever photograph of light as both a particle and wave
by Ecole Polytechnique Federale de Lausanne

https://phys.org/news/2015-03-particle.html
That is both a cool visual and an ingenious and subtle experiment... but when you read the details of how the experiment worked and what that photo is showing, you will come to two conclusions:
1) phys.org headlines are not trustworthy.
2) it’s not helping any with this FAQ

If we’re going to discuss this experiment it’s probably best to start a new thread (or look for one of the threads we already have about it) instead of grafting new discussion onto this one.

This thread is closed.
 
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