Is Light a Particle or a Wave: What’s the Real Answer in Modern Physics?

[Vitani11]

We've learned Q&M in modern physics but I need to make sure I'm getting this concept right...

So the photoelectric effect demonstrated the fact that a photon (wave) can knock an electron out of a metal, which could only happen if a photon was a particle. So much for photons being waves. I get that part. I'm trying to think of this the other way around. So if an electron was shot at a photon (wave) and caused it to deflect, that would have to mean that an electron was a wave because the only way a wave can deflect is by some sort of wave interference. So much for electrons being particles??

 

[mfb]

Light a particle or wave

Neither. It has some properties that are typically associated with classical particles, and it has some properties that are typically associated with classical waves, but it is a quantum object -- not a wave and not a particle. The same applies to electrons and everything else.

 

[dextercioby]

An introductory course in modern physics contains the typical misconceptions some of the PF users are always trying to fight. It teaches students the obsolete model of Bohr, the obsolete idea of wave-particle duality, emphasizes incorrectly that you need a particle description of light to explain the photoelectric effect and describes the Compton effect as a billiards problem. Then if students get lucky to get a good graduate course from their uni, they should have the chance of washing off all these misconceptions. Or not.

 

[ZapperZ]

We've learned Q&M in modern physics but I need to make sure I'm getting this concept right...

So the photoelectric effect demonstrated the fact that a photon (wave) can knock an electron out of a metal, which could only happen if a photon was a particle. So much for photons being waves. I get that part. I'm trying to think of this the other way around. So if an electron was shot at a photon (wave) and caused it to deflect, that would have to mean that an electron was a wave because the only way a wave can deflect is by some sort of wave interference. So much for electrons being particles??

Your conclusions here are puzzling.

What if I shoot electrons at a metal, and electrons also comes out, just like the photoelectric effect. Will you now go back to considering electrons as "particles"?

The concepts of "waves" and "particles" as used in QM are NOT the same ones that we know of in our ordinary world. The sooner you make this realization, the less will be your tendency to want to force nature into behaviors that it doesn't follow.

Zz.

 

[Vitani11]

Your conclusions here are puzzling.

What if I shoot electrons at a metal, and electrons also comes out, just like the photoelectric effect. Will you now go back to considering electrons as "particles"?

The concepts of "waves" and "particles" as used in QM are NOT the same ones that we know of in our ordinary world. The sooner you make this realization, the less will be your tendency to want to force nature into behaviors that it doesn't follow.

Zz.

Yes I would go back to calling them particles but I would also call them waves?

 

[vanhees71]

We've learned Q&M in modern physics but I need to make sure I'm getting this concept right...

So the photoelectric effect demonstrated the fact that a photon (wave) can knock an electron out of a metal, which could only happen if a photon was a particle. So much for photons being waves. I get that part. I'm trying to think of this the other way around. So if an electron was shot at a photon (wave) and caused it to deflect, that would have to mean that an electron was a wave because the only way a wave can deflect is by some sort of wave interference. So much for electrons being particles??

Sigh :-(. Planck was wrong when he said that wrong theories die out one day since the following generation are taught the right theories right away.

The photoelectric effect does NOT demonstrate the quantization of the electromagnetic field but only the quantization of the electron. The photoelectric effect can be understood by investigating the question what happens to a bound electron when it interacts with a classical electromagnetic wave:

https://www.physicsforums.com/insights/sins-physics-didactics/

Further, according to the socalled "modern quantum theory" (i.e., the theory developed by Heisenberg, Born, Jordan, Schrödinger, and Dirac in 1925/26) there's no such thing as a wave-particle duality but a consistent description in terms of quantum theory.

Last but not least, you should first learn modern non-relativistic quantum theory (which deals with a lot of real-world physics, including large parts of atomic, molecular, and solid-state physics) and then attack the more puzzling relativistic case and massless "particles" (i.e., quanta) like photons.

 

[Simon Bridge]

Yes I would go back to calling them particles but I would also call them waves?

... the consensus is to call them "particles" in the sense of "particle physics" ... but what is meant by the word has changed since the term "wave particle duality" was coined.
The difference between this phenomena and the elephant is that there is no third term to call it.

 

[Simon Phoenix]

So the photoelectric effect demonstrated the fact that a photon (wave) can knock an electron out of a metal, which could only happen if a photon was a particle

I can't really add much to the excellent answers you've already been given. The problem with the statement you made here is the word only. As Vanhees has pointed out this is simply not true. Thinking of the photon as a 'particle', i.e. a sort of very, very tiny billiard ball, certainly affords one possible explanation - but ultimately it's not necessary to do so in order to explain the effect.

In one sense it's not entirely obsolete to think in terms of 'particles' and 'waves' - because this can sometimes give us useful heuristic insights - but one must be very careful to recognise this is only an incomplete (and sometimes incorrect) way of trying to picture what's going on. These heuristic pictures can help give us 'back of the envelope' pointers but they should not be confused with the full and complete treatment using the formalism. And to be honest these heuristic pictures and 'back of the envelope' calculations can occasionally give us (perhaps surprisingly) the 'right' answers - but they're no substitute for doing it properly

I kind of like these heuristic pictures, because as a physicist I feel I want a bit more than just a page of formalism - I want some picture that at least partially connects however incompletely - but they have to be used with extreme caution! Ultimately one has to go beyond 'classical' intuition (involving things like waves and particles and the like) and re-wire our brains to develop a 'quantum' intuition.

 

[vanhees71]

The "visualization" of a photon as a little massless billiard ball is simply wrong. A photon, as a massless "particle" with spin 1 doesn't even permit to define a position operator. A classical particle picture is flawed to begin with. The correct "picture" is given by QED (relativistic quantum field theory).

 

[Simon Phoenix]

The "visualization" of a photon as a little massless billiard ball is simply wrong. A photon, as a massless "particle" with spin 1 doesn't even permit to define a position operator. A classical particle picture is flawed to begin with. The correct "picture" is given by QED (relativistic quantum field theory).

I pretty much agree with all that, but I would slightly qualify things to say that there are certain times where quantum objects behave to all intents and purposes as if they were little billiard balls. It's my understanding, for example, that the particles in LHC collision experiments can be pretty much thought of as little billiard balls in a lot of respects (but obviously not all). Yes, technically speaking it's wrong to say they ARE little billiard balls.

Yes the classical particle picture is flawed, yes it's technically incorrect to visualize a photon as a teensy-weensy little classical particle, but unless one is a raging purist I don't really see the problem with having a heuristic picture, provided one is fully cognizant of its limitations.

 

[vanhees71]

If the heuristic picture is utterly wrong and misleading, I'd rather not use it, and that's the case with having a picture as a little billiard ball. Almost allways a picture as an electromagnetic wave is more appropriate, although also not completely right for real single-photon Fock states, and in fact by definition a photon is defined as a single-photon Fock state of QED.

 

[Simon Phoenix]

If the heuristic picture is utterly wrong and misleading.

Yet there are many historical examples where such heuristic pictures have given essentially the right answers (photoelectric effect, Compton scattering, spontaneous emission, etc) - so whilst I am essentially agreeing with your technical points I wouldn't describe such pictures as entirely misleading! They may be 'utterly wrong' from a technical viewpoint yet, curiously, they yield the correct results. That's kind of interesting I feel.

I'd like your thoughts on why it should be that these admittedly flawed heuristic models can actually be so powerful. I totally agree with you that really everything has to be done properly, but do you have any insight as to exactly why these heuristics can be so successful? I don't.

 

[vanhees71]

Often there's no explanation to why flawed heuristic ideas work so well. In the case of the photoelectric effect it's simply because the first-order perturbation theory is such a good approximation, and the radiation corrections from the quantization of the em. field is so negligibly small in this case.

 

[bhobba]

So the photoelectric effect demonstrated the fact that a photon (wave) can knock an electron out of a metal, which could only happen if a photon was a particle. So much for photons being waves. I get that part. I'm trying to think of this the other way around. So if an electron was shot at a photon (wave) and caused it to deflect, that would have to mean that an electron was a wave because the only way a wave can deflect is by some sort of wave interference. So much for electrons being particles??

They are neither:
http://www.physics.usu.edu/torre/3700_Spring_2015/What_is_a_photon.pdf

But you have to start somewhere.

Even good old classical mechanics is up the creek without a paddle.

Law 1 follows from law 2 which is definition. The physical content is in law 3 which is equivalent to conservation of momentum. But Noethers Theorem says this is nothing more than spatial symmetry. Whats going on? Start a new thread of you really want to know - my point here is in physics you continually refine what you learn.

Thanks
Bill

 

[zonde]

Yet there are many historical examples where such heuristic pictures have given essentially the right answers (photoelectric effect, Compton scattering, spontaneous emission, etc)

I would add to the list coincidences for downconverted photons in parametric down conversion. Two downconverted photons (in simpler case they are with certain polarization so they are not polarization entangled) are registered with two detectors within the same time window. So this is a typical particle like behavior. For waves one could expect that interactions between emission and detection could change the wave so that peaks would change the places along the wave. Coincidence rates for real imperfect cases however are consistent with classical particle type model where individual photons are considered lost in transit or not detected.

 

[vanhees71]

This is not so clearly a particle-like behavior either. Classical electromagnetic waves can also come in well-peaked form or in any form for that matter. That's how radio waves work transmitting a signal. One should also note that the "location of the registered photon" is rather the "location of the registration event", i.e., the interaction of a photon with the massive macroscopic photodetector which of course has a defined position observable (within a certain finite resolution).

 

[quantumfunction]

We've learned Q&M in modern physics but I need to make sure I'm getting this concept right...

So the photoelectric effect demonstrated the fact that a photon (wave) can knock an electron out of a metal, which could only happen if a photon was a particle. So much for photons being waves. I get that part. I'm trying to think of this the other way around. So if an electron was shot at a photon (wave) and caused it to deflect, that would have to mean that an electron was a wave because the only way a wave can deflect is by some sort of wave interference. So much for electrons being particles??

Particles don't exist just fields. Semantics sort of leads to these conclusions because when people hear particles they think of something classical like particles of sand or particles of salt. Here's a ver thorough paper on this subject from Art Dobson.

There are no particles, there are only fields

Quantum foundations are still unsettled, with mixed effects on science and society. By now it should be possible to obtain consensus on at least one issue: Are the fundamental constituents fields or particles? As this paper shows, experiment and theory imply unbounded fields, not bounded particles, are fundamental. This is especially clear for relativistic systems, implying it's also true of non-relativistic systems. Particles are epiphenomena arising from fields. Thus the Schroedinger field is a space-filling physical field whose value at any spatial point is the probability amplitude for an interaction to occur at that point. The field for an electron is the electron; each electron extends over both slits in the 2-slit experiment and spreads over the entire pattern; and quantum physics is about interactions of microscopic systems with the macroscopic world rather than just about measurements. It's important to clarify this issue because textbooks still teach a particles- and measurement-oriented interpretation that contributes to bewilderment among students and pseudoscience among the public. This article reviews classical and quantum fields, the 2-slit experiment, rigorous theorems showing particles are inconsistent with relativistic quantum theory, and several phenomena showing particles are incompatible with quantum field theories.

https://arxiv.org/abs/1204.4616

Here's a really good video that also explains this.



Here's a quote from Werner Heisenberg:

“I think that modern physics has definitely decided in favor of Plato. In fact the smallest units of matter are not physical objects in the ordinary sense; they are forms, ideas which can be expressed unambiguously only in mathematical language.”
― Werner Heisenberg

Again, I think the confusion is a product of semantics and a lack of understanding of QFT as Dobson stated in his paper.

 

[vanhees71]

I think one of the largest sources of confusion is philosophy. They always claim that there is something "not understood", but in fact it is. The only thing that's not understood is the QT of gravity. Otherwise QT is a comprehensive description of all hitherto observed phenomena. I think it's utter nonsense to claim that QT isn't understood. A physical theory is understood if you know how to apply it to describe observations/experiments in nature and when the predictions are in accordance with the observations. If this is the case, as for QT, then there's nothing left to be understood (at least not for a physicist).

 

[Simon Phoenix]

A physical theory is understood if you know how to apply it to describe observations/experiments in nature and when the predictions are in accordance with the observations.

If a student is asked, without calculation, to write down the equation of the line described by the locus of points | z- a | = | z - a*| and cannot do it, but can do it algebraically, would you say that the student had 'understood' the description of complex loci? Personally I wouldn't, but I don't really see the difference between this and your statement above - simply being able to apply a technique does not really equate to 'understanding' for me :-)

 

[vanhees71]

Now I don't understand what you mean that the student can do it algebraically but hasn't understood the problem? How else should he/she do the problem, if not algebraically?

 

[Simon Phoenix]

How else should he/she do the problem, if not algebraically?

That is left as an exercise for the student

 

[vanhees71]

LOL.

 

[mfb]

A physical theory is understood if you know how to apply it to describe observations/experiments in nature and when the predictions are in accordance with the observations. If this is the case, as for QT, then there's nothing left to be understood (at least not for a physicist).

We knew that before decoherence got studied. As an example, do you think all the time spent on investigating decoherence (with more research still ongoing) is/was wasted?

 

[vanhees71]

No, why should it have been wasted? Decoherence explains why macroscopic bodies behave classical to a very good approximation. It's physics, not philosophy!

 

[mfb]

So you agree there were things not fully understood at the time where we could (in principle) calculate everything? Okay. That was my point. "I can calculate everything in my experiments" does not imply "I fully understand everything".