# Light = Particle or Wave?

1. Jul 20, 2004

### futb0l

What does the world currently believe? and also Why?

2. Jul 20, 2004

### Thinkmarble

According to quantum mechanics a photon is neither a wave nor an particle, it has some properties which can be likened under specific circumstances to the properties of the classical objects wave and it has some properties which can be likened to the properties of the classical object wave, but it also has properties which can not be explained by either.
It is best to think about photons (and everything else for that matter) a "new" kind of object.

And why does the "world" believe it ?
Simply because it works.

3. Jul 20, 2004

### ZapperZ

Staff Emeritus
There appears to be a strong and continuing miconception about light/photons as far as this so-called "duality" is concerned. Let's get ONE thing straight here - Quantum mechanics does NOT have two separate descriptions of light for when it behaves as a "wave" and when it behaves as "particles". PERIOD! It has one, and only one, consistent description for light, and that's that.

Now, after reading that, would one still want to consider light as having a "wave-particle duality"?

From the way I see it, the continuing misconception here is due to the ambiguity of the quality used in the question. We apply our classical ideas of what "wave" is, and what a "particle" is. A particle, like a grain of sand, has a definite boundary in space, i.e. a grain of sand doesn't appear spread out that it's exact shape and boundary are vague. Thus, it has what we classically define as a particle. A wave, on the other hand, can spread out over space.

Now, a photon, as a particle, was NEVER defined this way! A photon description in QM is NOT defined as having an exact shape and boundary in space. It is defined as clumps of energy. So in energy coordinates, it has definite "points", but it has no definite "size" in real space! This isn't your classical particle.

Having said that, the most common explanation for the "wave-particle duality" is that light behaves as waves in experiments such as the double slit, and behaves as particles when we do things like the photoelectric effect. Now, the fact that it is EASIER to describe an observation using one type of description while describing another observation using another type of description does NOT mean that they can't be described using ONE consistent discription. Most people often do not realize that one CAN describe interference effects (a typical wave phenomena) using photons![1] In fact, such technique CLEARLY explain diffraction patterns, and how the uncertainty principle is clearly at work. We don't normally subject students to such things because it is MORE involved than using the simple wave description. But we should not fool ourselves into thinking that the photon picture cannot be used to arrive at the idential phenomena that once thought can ONLY be described using the wave picture.

Again, one needs to learn QM and realize that there are no separate description for this wave-particle duality illusion. It is only a duality based on our pre-existing prejudice that something must either be a wave, or a particle. This "duality" thing only appears to be a major "issue" in pop-sci books and articles. It is a non-issue in QM texts.

Zz.

[1] T. Marcella, Eur. J. Phys., v.23, p.615 (2002).

4. Jul 20, 2004

### Galileo

Light is neither a wave nor a particle.
You cannot account for certain experiments when holding on to a corpuscular interpretation of light, and the same is true for a pure wave interpretation. Instead of that the new theory which explains these phenomona with fastastic agreement is quantum theory.

Furthermore, this is not a peculiarity of light. Also electrons, neutrons and so on are neither wave nor particle. So when we refer to particles, we dont mean anything corpuscular or wavelike, but rather a physical entity which exhibits both properties (they cannot be seperated).
I guess this is where the term 'duality' comes from. I dont like the term since it sounds like they act like waves on a summerday and like particles when it's raining. It's simply not so. There is a unified and logical way to describe the behaviour of particles, but we must abandon the thought of it being a wave or particle.

5. Jul 20, 2004

### Les Sleeth

You seem a bit distressed to find out there are still people left who don't know everything about physics. I've thought that maybe PF should have a forum catagory called "Ask All the Stupid Questions You Want!" for those of us who want to understand, but don't yet and so are bound to have misconceptions.

My stupid question would be, is it possible a photon is oscillating between two conditions, one of which exhibits wave-like behavior, and the other which exhibits particle-like behavior?

Last edited: Jul 20, 2004
6. Jul 20, 2004

### ZapperZ

Staff Emeritus
Distressed??!!

I'm not distressed. I had to use CAPS to make sure there is a clear emphasis on the main point of what I'm trying to get acrossed. This type of question appears repeatedly on here and elsewhere. I'm hoping that at the very least, people will pay attention to one or two sentences of what I wrote, so at least the effort I spent would not have been in vain. I have no delusion that this would stop the very same question from popping up again.

Secondly, why would a photon "oscillates" between two separate behavior when in QM, there is no separate behavior?

Zz.

7. Jul 20, 2004

### JohnDubYa

Particle. No experiment has ever been performed on a photon that demonstrated wave properties to my knowledge.

8. Jul 20, 2004

### Les Sleeth

Logic and observation will explain why. Notice how many members we now have. In a couple of weeks, look again. More new members! A great many of them are students, ranging from grade school to graduate school. Some of us are laypersons simply interested in how the universe works. People come here excited to find an opportunity to learn, and hoping others who do know will be charitable. In an ideal world they/we might read past posts and find the answers to their questions. But that isn't nearly as much fun as having someone to personally interact with, especially if you want to ask follow-up questions. So I think part of the deal if you are going to help out at PF is patiently explaining things again and again.

In the past I've asked experts if they think it would be a good thing if everybody learned something about physics. If so, then it is obvious there are going to be different levels of understanding and interest, which means different levels of teaching and more patience (when it comes to patience, I'm not talking about enduring stubborn insistance on maintaining one's own theories, or coddling lazy minds unwilling to do a little research). I love the subject myself, but since it isn't my area of expertise, I don't have time to study it with the thoroughness required to be a physicist.

This isn't my forum, so I suppose if everyone thinks it should be sort of an exclusive club for current and future physicists, then that's that. If so, then maybe one day PF will offer a forum area called "Physics for Dummies." :tongue2:

Well, that's what I was asking you, the expert. You yourself said "light behaves as waves in experiments such as the double slit, and behaves as particles when we do things like the photoelectric effect." I'll try to explain what I was thinking.

My question was asking if a photon's dynamics might include a photon oscillating between being a "clump" as you say, and opening up a bit to temporarily lose its clump integrity. That might establish phases which when observed one way reveal wave-like properties, and when observed another way reveal particle-like properties.

One reason I wonder about that is because of how background radiation has "stretched" along with the expansion of the universe since the big bang (at least, according to Marcus). Obviously radiation that is more expanded now was less expanded before, so right there is the possibility that radiation is oscillating between expansion and contraction in coherence with the historical and present universe.

Last edited: Jul 20, 2004
9. Jul 20, 2004

### ZapperZ

Staff Emeritus
You read wrong. What I said was...

"Having said that, the most common explanation for the "wave-particle duality" is that light behaves as waves in experiments such as the double slit, and behaves as particles when we do things like the photoelectric effect."

I said it was a "common explanation" that points to the existence of this so-called duality. But if you read the whole posting from where that came from, I insisted (repeatedly) that there is no such thing as a "duality". How can it when there is only ONE consistent description of light within QM? That was the whole point of my response. I do not know how else to make this any clearer. I have one description of light that can describe ALL the phenomena that it exhibits. So how can I answer your question anymore on the "duality" of light, when there isn't any!

Zz.

10. Jul 20, 2004

### Thinkmarble

Then please explain interference effects under the assumption that photons are particles.
After you have done so explain the fact that photons have (AFAIK) a spin of 1*.

*this can actually not be explained by assuming that photons are particles or waves but rather only by assuming that they are in the new** category of quaobs (quantum objects).
There is one description (qunatum theory) which can explain all three things (photoelectric effect, interference, spin).
So saying that there are on thing ignores two third nature, saying that there is a duality leads to mushy brains and still ignores one third of nature.
**Can something be new when it is 60-70 years old ?

11. Jul 20, 2004

### Les Sleeth

Well, you do seem to acknowledge behavioral differences (and I am not implying behavioral differences indicate duality), and then seem to contradict yourself in the following two statements:

". . . light behaves as waves in experiments such as the double slit, and behaves as particles when we do things like the photoelectric effect."

". . . why would a photon "oscillates" between two separate behavior when in QM, there is no separate behavior?"

I don't believe I did read you wrong. I don't think there is any duality, and never have. I am not confused about your explanation in the slightest, I am NOT ( ) questioning your explanation, we are not disagreeing about duality.

From your point, I lept to wondering what is happening at the quantum level that might make it appear there is duality there. My point was to ask: what is causing the different observed behaviors (i.e., double slit/photoelectric), all falling under the single reality of QM?

I asked if it might be that a photon is continously phasing in and out of particleness; or, looked at from the opposite direction, might it be continuously phasing in and out of waveness. Since we know a photon (and all particles) are intense oscillators, why couldn't a photon be something singular which simply alternates between being more "clumped" and more spread out? Might that not account for the APPEARANCE ( ) of duality? And the reason we see the differences in the double slit experiment and the photoelectric effect is because each of those processes accentuates a particular phase.

Last edited: Jul 20, 2004
12. Jul 20, 2004

### ZapperZ

Staff Emeritus
You need to read that these are observations that classically are defined to be of two different nature [I have mentioned this in the subsequent posting]. I then tried to emphasized that this is due to our prejudice from classical physics that these need to be of different nature. Nothing in QM indicates that these are of different beasts. When I can use one formulation to describe both sufficiently, then they are not "dual", they are "singular" in nature.

"phasing in and out" of a photon (at least a real one as opposed to a virtual one) is not described in QM, at least it is not needed to explain the photoelectric effect, Compton scattering, diffraction, and two-slit interference. Besides, why do we need to impose this phasing in and out when QM already has a statisfactory description of this?

What is often overlooked is the most important but subtle aspect of something like the 2-slit experiment. That the interference effect is NOT really an intrinsic property of the object in question (i.e. not of the photon, electron, neutron, C60 molecule, etc), but rather due to the SUPERPOSTION OF PATH! If you scale your experiment according to the object in question so that it is of the order of the deBroglie wavelength, then the object is IRRELEVANT! The superpostion of all the possible paths thru the 2-slit is what is causing the interference! The object that is passing thru the 2-slit is incidental and irrelevant once the length scale has been taken into account. Again, this is clearly described in QM, and I have cited Marcella's paper that shows this in painful detail. It is even clearer if one uses the Feynman path integral approach.

Zz.

13. Jul 20, 2004

### Les Sleeth

Actually I said phasing between being more spread out (wave-like) and being more "clumped" (particle-like).

14. Jul 20, 2004

### ZapperZ

Staff Emeritus
That too.

Zz.

15. Jul 20, 2004

### reilly

Not only is there duality in QM, but in classical physics as well: to wit, just think about the macroscopic treatment of alternating current. The idea of duality arose because of the apparent conflict between Thomson's particulate electron and the phenomena of electron diffraction - when such diffraction was found, it was a major mind blower. Electrons do sometimes behave as particles, sometimes as waves -- like in electron microscopes. The samething holds for light -- interference is wave-like, Compton scattering is particle like. Why deny our senses?

Certainly the background radiation observed as a relic of the Big Bang is nicely thought of as standard wave-like radiation. Bohr and his colleagues worried about this duality, and its implications for the conceptual apparatus of physics and philosophy -- tricky stuff. How coulkd this really weird stuff be?

Zz is correct in that there is a unifying perspective about all of this -- it stems from Quantum Field Theory, which easily allows both a field and a particle perspective. That is, QFT makes it possible to flip back and forth from photon states to field states all with in the same basic fornalism. But the math is highly sophisticated and formidable -- see, for example,Optical Coherence and Quantum Optics, by Mandel and Wolff, everything you wanted to know and didn't want to know about photons.

Think of duality as pragmatic -- sometimes you use your wave "glasses", sometimes your particle "glasses" to understand your measurements--you choose what works best for you. For the layman, focus on the physics, learn the history. Physics is the cumulative effort of many generations of physicists, what the old guys saw and thought is very important to understanding physics today.

Regards,
Reilly Atkinson

16. Jul 20, 2004

### JohnDubYa

Single photons do not exhibit interference effects.

Is there a law of nature that particles cannot have spin?

17. Jul 21, 2004

### ZapperZ

Staff Emeritus
Actually John, unless I misread you, while you do need a lot of photons (or electrons or neutrons, etc) to be able to DETECT the interference effects, the interference pattern from, let's say, a typical 2-slit expt. is in fact the interference of a single photon with itself! In other words, 2-photon, 3-photon, etc interference aren't the same as single-photon interference.[1] That's why I mentioned earlier that what is significant here isn't the photon, electron, neutron, etc, but rather the superpostion of all the possible paths that ONE single object can take.

Zz.

L. Mendel, Rev. Mod. Phys., v.71, p.274 (1999).

18. Jul 21, 2004

### Les Sleeth

Thanks Reilly, I was thinking of QFT (what little I understand about it) when I posed my question. Probably no one knows the answer to my question. It seems what we have are methods of observation, math which accounts for the behavior, but no precise explanation of exactly what is going at the quantum level on that gives the two sorts of observations.

I like the "glasses" analogy because that is related to what I am asking. I'm wondering if the two different glasses we use are like polar filters which allow only one side of a particle's two hypothetical phases (i.e., my hypothesis) to appear. Likewise, when you say "to flip back and forth from photon states to field states," it sounds like what I am envisioning.

19. Jul 21, 2004

### ZapperZ

Staff Emeritus
I think I know what you are getting at here. Unfortunately, as you can see from the previous post, it can easily be misinterpreted or misrepresented. The biggest obstacle in this issue is trying to convey the meaning of the mathematics. QFT (and QED) makes NO provision for classical fields. Therefore, classical fields (and thus, "waves") essentially do not exist and are meaningless in QFT.

Then what are we left with? I hate to say we have a "particle field", because that again can be misinterpreted (refer to my discussion what classical particle is and why this is NOT what a photon is). Can I get away with saying a "quantum field" without grossly misrepresenting the mathematics? Maybe. If we stick by that, then I would not want to leave the impression that photons can flip back and forth between two apparently different perpective.

Zz.

20. Jul 21, 2004

### ZapperZ

Staff Emeritus
I'm sorry, but I was under the impresson that you were asking for what we already know about photons and how they are described within the theory that works. I did not realize you were trying to offer an EXPLANATION for why they work, which isn't contained in the theory. I thought such things are usually confined to the Theory Development section and I typically do not wish to entertain such querries.

Zz.