The Nature of Light - Particle, Wave - both?

Factual Questions
21

Maybe so, Ring.

But, I have been right over Ph.D’s in their own field a number of times. That doesn’t necessarily mean that I am correct this time, but I am fairly certain that I am. It doesn’t mean he is correct simply because he has an Ph.D… If he is correct and has a Ph.D. in physics as you say, he will certainly be capable of demonstrating to me that he is indeed correct.

Besides, there are many physicists that are not particularly strong in QED or QCD as they aren’t terribly relevant to what they are working on.

I have been studying Feynman’s work for quite a while (about 7 years). I have read and followed all his popular stuff. I am working on the full treatment of QED now. When I say that I am working on it, I actually mean that I am finding out what I need to know in more detail to be able to get through it. The math is positively ass kicking. At the moment I am working my way through Max Born and Emil Wolf’s “Principals of Optics” to get a more solid grounding in the classical treatment of light.

But, one thing that seemed quite appearent to me in most if not all the QED texts was the message to forget the dual nature of light. It is a nice stop on the way to understanding light, but looking at light strictly as particles is the way to really understand what is going on.

One of the main purposes of the “sum over” business is to provide a means for explaining patterns that are consistent with inteference in a particle setting (single photons passing through the apperatus at a time) when wave like interference plainly could not be occuring. (see double slit experiment above)

I’m no dummy either, and have enough background to likely follow any explanation that would demonstrate how/where my previous understanding is/was flawed.

Hopefully Bonzer will return and correct me if I got it wrong, I always learn more when I lose an arguement than when I win one.

22

Isn’t it simply that case, scotth, that you’re just saying the same thing in a different way; if all particles exhibit wavelike behaviour (interference, diffraction etc), then there’s nothing particularly special about photons (by which I’m supposing we mean a broader sense than just the visible spectrum).

The way I heard it explained was (something like) that a single photon doesn’t actually pass through one or the other of the slits, it passes through the entire apparatus as (something like) a ‘probability wavefront’ (forgive me if I’m mangling terms here), thus each photon (or whatever it really is when it is in transit) displays the behaviour that we might expect at greater volumes.

23

Interestingly you should be able to observe wave behaviour (e.g. diffraction) in tennis balls.

Wave particle duality is governed by De Broglie’s equation,

λ = h/mv

24

MC, I can’t quite read your formula, so I’m not sure what you put, but it may need a minor correction. For non-relativistic objects, the de Broglie wavelength is given by

lambda = h / mv

but more generally, it’s

lambda = h / p

25

Well, I wasn’t quite expecting a debate, but I find the fruits of the discussion interesting, so no real complaints as long as you play nice.

Artemis I appreciate your analogy - makes sense.

scotth and Bonzer, your debate directly applies to my query (which would make sense, give the question heading up this thread). My intent with the post was more to work from the assumption that light exhibited both particle and wave behaviors (thanks to posters who steered me towards this language). Within that assumption, I wanted to see if I understand - in its simplest form - how light propagates without ether (I stated it was through its constant transitioning between energy states) and how its wave/particle duality played into that propagation - I didn’t state this in the OP, but I am curious as to whether any one discrete energy state could be thought of as a light particle - so if a photon is moving in a wave-like pattern, each point on the wave where it assumes a discrete energy state would “qualify” as a particle…

Well, that was my going in thinking - the debate both supercedes it and clarifies (I think). If light is, as scotth asserts, particles, then my OP - or the intent behind it - is moot; if wave/particle duality holds sway, then it would be less correct to state that the each energy state qualifies as a particle and more correct to simply state that light can paradoxically exhibit particle and wave behaviors as it propagates - right?

Carry on and thanks for the help.

PS: Scott Dickerson, I couldn’t agree more about the unwieldy nature of words with regard to this and many other topics…but they are all we have on the ol’ SDMB, so I perservere.

26

I’m not going to get into arguments with PhDs here, but my understanding is that you’ll get the answer you expect. If you’re testing for particle-like aspects, you’ll get results that conform to a particle. If you’re testing for wave-like aspects, you’ll see waves. The nature of the test is part of the universe being tested. The duality is so fundamental that it is meaningless to talk about one or other term as if they have independant meaning. As John Griffin explained (in In Search of Schrodinger’s Cat), you’ll be better off talking about slithy toves.

So, my answer to the thread is …
yes

[hijack]
is g8rguy a deliberately chosen name, or a mistype - should have been gr8guy (much more meaningful)???
[/hijack]

27

No, it is not.

And it’s not a wave, either.

The terms “particle” and “wave” have no meaning on the quantum level; that is, in fact, one of the most counter-intuitive aspects of quantum mechanics. To attempt to view quantum phenomena using the semiotic tools we’ve devised to describe the macroscopic world is an excercise in futility. Our everyday language doesn’t have terms to accurately describe the true nature of matter/energy.

Ultimately, if you want to understand quantum dynamics, you’ve got to learn to think in mathematical terms, and not using linguistic metaphors. Language will mislead you in this area.

28

Neither. IMNA Physicist - IMHO One of the things common in Relativity and Quantum Mechanics is “observer created reality”. So the photon (or light) existence is meaningless unless and until you try to measure it or it has some effect on you.

And the moment you detect it, its properties change. If you try to see it as a wave it appears as a wave and if you see it as a particle it appears as a particle.
Why is light like that ? There comes the Anthropic Principle of Physics - If it were any different then we would’nt be here to ask that question :smiley:

Believe me or not, I have heard that answer to many of the questions.

29

[response to hijack]
No, g8rguy is delibarately chosen, as if there’s one thing I’m not, it’s great
[/response]

That said, for what it’s worth, I do know where scotth is coming from, and most of the field theorists I’ve talked to prefer to regard photons as particles, when pressed to choose particle or wave. Of course, what they would really rather do is simply say that light is an excitation of the Maxwell vector field, but most people don’t find that a very satisfying answer for some reason. :wink:

30

If you look at his location, the name makes perfect sense.

Haj

31

Not at all Mangetout. Stay with me a minute, and I will try to make my objections clear.

Is gravity a force between two objects as Newton said or is it a distortion of space like Einstein says?

In many instances, they give the same answer to a very high degree of accuracy. When things get hairy, Newtonian gravity begins to give increasingly wrong answers. Einstein’s gravity continues to work like a champ.

Einstein’s view of world appears to be the correct one. But, that doesn’t make Newton’s view un-useful. Spacecraft are launched, eclipses are predicted, and they all happen as planned using nothing but Newtonian equations. It is much less time consuming and intuitive to work with Newtonian physics. But, the answers will be more accurate using Einstein’s methods.

The point is that these systems are fundementally different from a “philosophical” point of view. The underlying mechanisms, the causes if you will, are different. Keep this idea in mind.

Light has an interesting history in our understanding of it. It has gone through many revolutions.

Our understanding of the fundemental nature of light has changed many times over the years. It was not too long ago when the light as waves became pretty much the universally endorsed theory. This came about when interference and other things were discovered with light that previously had only been understandable in terms of waves.

That understanding held until around 1900. The photoelectic effect was a big problem, and led to the beginnings of quantum mechanics. QM forwards the idea of the dual nature of both light and matter. QM explained many more things, but still had its short comings.

During the 40’s and 50’s QED got serious. QED completely explains every thing in nature except two things: Gravity and the forces at work inside the nucleus of atoms. Think about how much that encompasses. This includes all of chemistry, mechanics, optics, magnetism, and on and on and on. And within our ability to measure, it numerically predicts expected results for all of these things completely accurately. QED gets rid of the wave/particle duality. It is fundementally different philosophically. Light is particles. All the events that were previously understood as wave like are now understood in terms of particle action.

Can you still get useful answers to questions with previous theories? Sure! But our fundemental understanding of what light is has changed. A guy designing a telescope generally uses nothing but classical optics and considers light in terms of waves. That isn’t what is really going on, but it is much simpler to work with and gives useful answers.

Light as waves, and light as a wave/particle duality are useful steps on the way to understanding it more deeply and are still extremely useful for day to day engineering in the same way that Newtonian gravity is still what you will find implemented in any ballistic computer. Fundementally, it is a flawed understanding though. That doesn’t destroy its utility.

QED says light is particles. Period. For at least 40 years, QED has provided answers that agree with experiment perfectly, and with no exceptions what-so-ever.

32

Jeez, scotth that came across as compelling - thanks for the analogies.

I would appreciate other physicists or at least afficionado’s to comment if you get a chance. Obviously Bonzer it would be really helpful to get your perspective - I am hoping that this is more of a language thing than a philosophy thing, but “Light is particles” vs. “light exhibits wave/particle duality” seem pretty different.

Is there a book regarding QED that a layman could read?

33

I would enthusiastically endorse QED by Richard Feynman.

As I am learning this stuff on my own, and not being guided by instructors or really have anyone to even ask questions of, I found this book to be simply invaluable. I would have never understood any of the stuff in the complete treatments that I have gotten so far without the mental framework provided by this book.

One of the brilliant things about this book (IMHO at least) is that when he shows you how something works in a simplified manner, he goes to great pains to point out that what you are learning at that moment is a simplified (incomplete) explanation and that there will be a more detailed and better explanation coming later.

Anyway, I have a number of books on QED and they fall into one of three categories.

  1. A popularization that never really leaves feeling like you got it.
  2. A detailed account of what is really going on, minus the mathmatically techniques to really perform useful calculations in a reasonable amount of time (Feynman’s QED is the only book I have found in this category).
  3. College texts that teach almost entirely from the working the math point of view with very little (or not enough for me, at least) explanation of what the heck this juggling of figures really represents, or why exactly you should do it that way.

The Feynman QED book really gives one an understanding of what is happening, and makes the books in category 3 tremendously more intelligible.

34

scotth, be aware that the reason that the books in category 3 are written with lots of math and little phyiscal interpretation is that what they are trying to teach you how to do is to use the theory; they make no claim on teaching interpretation of the theory.

And it’s right that they don’t, because one of the dirty little secrets of QED is that while using Feynman diagrams and interpreting them and all the rest can provide some insight and really quite accurate numbers, the fact is that it’s not actually right. The reason we do it that way is that it’s all we can really do; it’s a matter of convenience and of getting good answers, not of actually solving the problem correctly.

35

John Baez and Greg weeks are physicists of some renown. Here is what they think with regard to the particle/wave concept of the photon in QED.

36

You keep saying that QED says that light is particles - it does not. That is a simplification used in popular books that attempt to “explain” quantum theory to laypersons (generally unsuccessfully). It’s no more accurate than saying “an electron is a negatively-charged particle orbiting an atom’s nucleus” (a phrase you’ll often see in introductory chemistry textbooks), when in fact, an electron in an atom is better described as a vector field centered on the atomic nucleus. g8rguy’s statement “light is an excitation of the Maxwell vector field” is accurate, but that phrase doesn’t conjure up an image in people’s minds the way "particle’ or “wave” does.

The reason those textbooks in your category 3 don’t attempt to tell you “what is going on besides the math” is because in fact nothing IS going on besides the math. Quantum theory is purely a mathematical construct, and when you try to use visual or verbal analogies to “make sense” of it, you will be mislead. Our primate minds, which were designed to solve the problems encountered while swinging in trees, are not constructed to intuitively “get” quantum mechanics in the same way we can “get” Newtonian physics (to offer just one example). What you’re doing is the equivalent of asking “how much candy can we mine from the Big Rock Candy Mountain?”; the question is meaningless, even though it LOOKS like it should have a sensible answer. The “stuff” of the universe examined on the quantum level doesn’t fit into any categories that we are familiar with on the macroscopic level, such as “particle”, “wave”, “matter”, or “energy”.

37

Artemis:

“To attempt to view quantum phenomena using the semiotic tools we’ve devised to describe the macroscopic world is an excercise in futility. Our everyday language doesn’t have terms to accurately describe the true nature of matter/energy.”

Yup!

Everyone:

One little point I don’t want to let slip by is the question of the relevance of “interpretation”, which really addresses the relevance of metaphysical philosophy.

“It’s undoubtedly tempting to believe (the hope has ocurred to most of us) that all the philosophical arguments about quantum mechanics might be resolvable merely by focussing on field theory instead…” sayeth Bonzer; and I would guess the “us” is the community of active physicists. It would be an error to suppose, however, that metaphysical philosophers are in the position of having put their money on one horse or another. The dominant “it’s both, and neither” view is philosophically delicious and quite elegant, to such a degree that philosopher Hilary Putnam has suggested that we revise our standard treatment of basic logic in light of wave/particle duality, rather than force the universe into the Procrustean bed of our own preconceptions.

If one interprets QED as denying that light has, necessarily, a wave nature as well as a particle nature–I’d be interested in learning how the interference-pattern phenomenon is explained, and on what basis the “particle-ists” discount the possibility that some ultracomplex field kinematics might give a particle-like appearance to what could also be interpreted as wave-overlays.

But set all that aside–

have we established PRECISELY what is to be meant by the terms “particle” and “wave”?

38

Damn, that’s a nice piece of late-Feynman exegesis - certainly better than my quote. On the other hand, I’m trying not to be dogmatic here. The fact that he changed his mind on the matter is largely irrelevant - the key point is that these issues are arguable. This isn’t Great Debates; WordMan wanted clarification on his understanding and, in those circumstances, it’s hardly useful to present an extreme position as the final word, at least without some explanation.

Actually, I think this is irrelevant. The argument that particles are detected as particles, full-stop, (be they photons, electrons or whatever) can be made in vanilla quantum mechanics. One might even argue that Einstein’s position in 1905 was just this. It’s also a position that’s certainly been held by a minority in the years since. For instance, there’s the example of the de Broglie-Bohm pilot wave interpretation. This is a purely quantum mechanical theory (it’s notoriously difficult to generalise to QFT) in which particles are just that that, always. And it’s utterly consistent. But that doesn’t meant that it’s a remotely popular position.

Incidentally, I certainly second the suggestion that WordMan read as much Feynman (of whatever date) as he can lay his hands on

39

I quite agree here. My problem is that I couldn’t figure out how to summarize the 70 odd pages of text that really convinced me of this into something that I could type in here in a reasonable amount of time. In fact, I can’t see a way to condense the material hardly at all.

I will offer another quote from the same book (QED, by Richard Feynman) that reinforces the light as a particle view and a couple others that will answer some of the other concerns. This one comes after the first run through of sum over histories. He is about to get into them in more detail. For example he demonstrates why/how figuring partial reflections with sum over histories gets the exact same answer when only considering the surfaces of the glass as when considering that the light interacts with the entire thickness of the glass.

This quote really tells the reader what exactly QED is (Maxwell’s theory made compatible with Quantum Mechanics).

This is particularly for some of Artemis’s objections, and I found the book quite lived up to this quote.

Of course, none of that settles the OP.

But, in my opinion, the first two chapters of QED gives the most lucid answer to WordMan’s original question that I have ever seen. I strongly encourage him to pick that book up.

This is asking alot, but… It would be of great interest to me to get Bonzer to read the first 70 pages of this book and point out any specific failings for me. This would be of great value in selecting more material to learn.

40

In addition to my earlier questions, which seem to me ever more urgent, there’s this:

Why is it particularly important to establish exactly what the talented Mr. Feynman really thought–as opposed to trying to establish more directly what sort of consensus presently obtains in the relevant portion of the community of physicists, and why?