Are photons particles or waves?

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Main Question or Discussion Point

I've heard of photons being described as a wave/particle duality. But what evidence is there that individual photons behave like anything other than a particle? I can see how photons en masse can display wave/particle characteristics, but what evidence is there that any individual photon possesses such a duality?
 

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  • #2
phinds
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Photons are not particles (in the classical physics sense) nor are they waves. They are quantum objects that will exhibit wave-like behavior if that's what you measure for or particle-like behavior if that's what you measure for.

"wave-particle duality" has been deprecated for many decades.
 
  • #3
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Are photons particles or waves?
Neither. They are objects in quantum mechanics. There are situations where they behave similarly to classical particles, and situations where they behave similarly to classical waves.
but what evidence is there that any individual photon possesses such a duality?
You can perform all experiments with individual photons.
 
  • #4
OmCheeto
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Neither. They are objects in quantum mechanics.
...
Now you tell me...... :oldgrumpy:

Years ago, I learned that a high energy photon could create "actual" particles.*
So I lightly studied their properties for the next 38 years, collecting evidence, that might explain what they were.

Had I known back then, that I would never understand quantum mechanics, I'd probably have spent more time fishing.

*At that time, I was only aware of the positron-electron pair. It wouldn't be until about a month after I arrived here at physicsforums, 30 years later, that I learned that photons can create proton-antiproton pairs, which kind of ruined my crackpotian theory on the "true" nature of photons. :oldsmile:
 
  • #5
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You can perform all experiments with individual photons.
But is there an experiment that you can perform on an individual photon in which it would be correct to describe the results as displaying wavelike characteristics? Or are wavelike characteristics only applicable to a group of photons?
 
  • #6
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You can even do Young's double split experiment with single photons and still observe the diffraction pattern on the back screen. Thus, you can describe the photons as waves. Yet, if you tried to measure which slit the photon was going through, you lose this diffraction pattern on the back end that would suggest waves. Rather, you would see two discrete "clumps" of illuminated spots on the screen where photons hit, thus making you conclude they act like particles.

This is sort of the duality you mentioned; depending on whether or not you measure photons going through slits in Young's double slit experiment, you can treat photons as either waves or particles. But, as others said, you cannot simply say a photon is a wave or is a particle, but simply a quantum object that displaces both characteristics.
 
  • #7
edguy99
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Sure, wavelike characteristics are applicable to individual photons. Individual photons will diffract around corners like a wave would. Individual photons will interfere with other individual photons seeming to reinforce each other or cancel each other out.
 
  • #8
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Usually light is of dual nature, wave as well as particle. Youngs double slit experiment is an experiment that proves wave nature of light and photoelectric effect proves particle nature of light. But main drawback of wave theory was consideration of presence of eather medium by Huygen
 
  • #9
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But is there an experiment that you can perform on an individual photon in which it would be correct to describe the results as displaying wavelike characteristics? Or are wavelike characteristics only applicable to a group of photons?
I said "all" experiments, that includes the double-slit experiment for example.
There are single-photon sources where you can be sure to have exactly one photon. And diffraction still works with them.

But main drawback of wave theory was consideration of presence of eather medium by Huygen
What does that mean?
 
  • #10
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I believe what he means in regards to the consideration of the presence of an eather is that all waves travel in some medium. It was reasonable to say that if every other type of wave travels through a medium, light must as well. However, looking out in space, near vacuum, there is no such medium. I believe he is just referring to the old question of: "If light is a wave, what is it waving in?"

The key here is that the answer is nothing! Light (photons) do not need a medium to travel through to be described as a wave - unlike every other type of wave. They can have these "wave-like characteristics" in vacuum!
 
  • #11
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They don't need a medium to move like other waves because they aren't waves. Assuming that photons are anything like waves on water is as faulty as assuming that they are particles. They are quantum mechanical and only have to obey the laws of quantum mechanics, which are often counterintuitive. I like to think of them sort of like kinks in a field.
 

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