Photons do not travel in straight lines

[bjacoby]

QM is a formalism, and it is a lot better than you give it credit for. For example, where is it wrong?

So if you have an issue with the QM explanation, it seems to me to be with the semantics and not the formalism or its application (of which you have provided no specific criticism).

The problem is not with a formalism which has obviously been tweaked to a high degree, but with the concept of "wrong". You say I take issue with mere "symantics". I say that words actually have meanings. I say logic and definitions have a role in science and elastic versions of these "explain" nothing.

No question that the formalism "works" to a degree, meaning that mathematical predictions can be obtained that approximate what is observed. But the fundamental questions of philosophy remain. What is a "photon"? If a particle has a certain meaning then one must reconcile that meaning with the particle nature of photons. A particle does not have a wave nature. And to say so is nonsense. A wave has a wave nature and to call that wave a particle is likewise nonsense. Words really do have meanings.

So the question here is not to disprove the results of the formalism, but to establish a logical philosophy that does not require magic, does not violate causality, or has a lot of mysterious undefined operations and terms. Obviously when one attempts to describe the formalism in words it doesn't work. There is a great wandering in the wilderness. The formalism works. The description of it does not!

Why is that? Well one reason in my opinion is that everyone wants to pretend that there are no mysteries here. Nobody is willing to say "We really don't know why this works as it does." And the end result is a myriad of "explanations" that include bogus ideas like "action at a distance", things that don't exist until you look at them, and "particles" that split and go through two holes at once and magically recombine in some fashion to give correct answers.

No. The problem is not in our stars or our semantics, but in your philosophy!

 

[bjacoby]

First of all: In almost all experiments you do not fire single photons at the slits. In most cases you do not even fire a well defined number of photons at the slits. Coherent light as well as thermal light does not have a fixed photon number, but some intrinsic photon number uncertainty.

The idea, that photons are logically traditional particles is wrong. The idea that photons are traditional particles going through just one of the slits is simply not compatible with experiments. However, contrary to what you say, qm does not predict photon particles to be dual entities switching back and forth between wave and particle behaviour. Probability amplitudes have wave character. This is true for any probability amplitude, whether it is about electrons, photons or buckyballs. Photons are quantized excitations of the em-field. If you think of photons as traditional (classical) particles, you deny the role of the underlying fields, which must go wrong as any basic text on QFT or quantum optics will show you.

First of all, I don't care about all experiments, only the one I'm talking about. Shine a laser at a dual slit. Put up a channel plate viewer to see single photons. Insert ND filters until photons are coming one at a time. How long do you want me to wait between photons?

Obviously you can't say the idea that photons are traditional particles is "wrong" since you really can't tell us what photons are, can you? It is clear, however, that photon propagation involves much more that simple traditional particles. What exactly do the words "quantized excitations of the EM-field" actually mean? Can you explain that for us in detail?

So what is wrong with the idea of saying that LIGHT is a combination of particles and waves of some type? Why is there a requirement that both must be somehow merged into a mythical wave-article?

We DO know that light is NOT waves. Energy transfer is too fast. And energy transfers are limited to certain values. Plus we know that the physical dimensions of these light "particles" are too small to in any way sense the presence of the other slit. The bottom line is exactly as I have stated it. Particles in the classical sense appear to be shooting along but have their trajectories modified by matter in some way so as to produce trajectory statistics that are wave solutions. (please note, however that these are NOT actual wave solutions which are mathematically continuous and differentiable, while our "diffraction" results are statistical distributions of points which are NOT continuous nor differentiable)

Maxwell noted long ago that energy (information) can only be transferred in two mechanisms: by particles or by waves. There are no other mechanisms. Light energy has long ago been determined not to be transmitted by waves for the reasons I stated.

So as you say, the logical assumption is that SOMEHOW a "wave" must also be involved in this operation. Of course you can't possibly tell me what exactly a "probability wave" is. But we all know that such formalism actually does give more or less correct predictions even if we can't explain why.

So why won't you admit you can't explain why? Then perhaps we can begin to speculate as what might be involved...

 

[Cthugha]

First of all, I don't care about all experiments, only the one I'm talking about. Shine a laser at a dual slit. Put up a channel plate viewer to see single photons. Insert ND filters until photons are coming one at a time. How long do you want me to wait between photons?

I am sorry to hear that. There are lots of other experiments, which tell us a lot more about the properties of light, but I will see how far I can get without citing them. However I would like to stress that using a ND filter in laser beam path does not give you single photons. You just damp the amplitude of the coherent state. The photon number uncertainty of a single photon (or other Fock) state is 0, while the photon number uncertainty of a coherent state is always on the order of the mean photon number. If you really use single photons, for example one out of two photons of an entangled beam you will not see the double slit pattern because the light you use is too incoherent. However, this is not central to your concern, I suppose, but one should keep in mind that having single detector clicks does not assure that you have single photons. By the way, if you are really just interested in the double slit, you might find this paper interesting:
http://arxiv.org/abs/quant-ph/0703126
It is a preprint of T.V. Marcella Eur. J. Phys. v.23, p.615 (2002) and

Obviously you can't say the idea that photons are traditional particles is "wrong" since you really can't tell us what photons are, can you? It is clear, however, that photon propagation involves much more that simple traditional particles. What exactly do the words "quantized excitations of the EM-field" actually mean? Can you explain that for us in detail?

Well, how deep is your understanding of QM? I could tell you stuff about QM formulations of harmonic oscillators and stuff, but that would not help much, if you do not have some basic knowledge in QM, QFT and quantum optics. If you are a layman, let me boil down all of that to a simple "a photon is the sum of its properties". Besides that, this forum has a great FAQ article (in the general physics subforum) on wave-particle-duality and why there is not really one in QM. You might want to read it.

So what is wrong with the idea of saying that LIGHT is a combination of particles and waves of some type? Why is there a requirement that both must be somehow merged into a mythical wave-article?

Well, QM usually does not really do that. QM has one single description from which both particle and wave behaviour can be explained. QM always deals with probability amplitudes. This is where the wavelike aspect comes from. However it does not only apply to electrons, photons and such stuff, but theoretically also to cars, trees and such stuff. The reason why you do not see tree interference in double slit experiments is a matter of decoherence. This is also where the intuitive classical description goes wrong. The classical particle point of view is usually what you have after decoherence: Some discrete and definite position. However in QM the particle aspect boils down to discrete and quantized photon numbers. This is not the traditional meaning of a particle.

We DO know that light is NOT waves. Energy transfer is too fast. And energy transfers are limited to certain values. Plus we know that the physical dimensions of these light "particles" are too small to in any way sense the presence of the other slit. The bottom line is exactly as I have stated it. Particles in the classical sense appear to be shooting along but have their trajectories modified by matter in some way so as to produce trajectory statistics that are wave solutions. (please note, however that these are NOT actual wave solutions which are mathematically continuous and differentiable, while our "diffraction" results are statistical distributions of points which are NOT continuous nor differentiable)

Energy transfer is too fast? This is nonsense. That the energy transfer is limited to quantized values is of course true. This is what constitutes particles in QM. Your statement about the physical extents is however wrong. If you wanted to define some spatial extent for light, the closest thing you get is the coherence volume, which is the volume inside which photons are indistinguishable. By the way this is also the quantity, which defines whether there is a double slit pattern. If both slits are inside of one coherence volume, you will see interference. Otherwise you will not.
By the way, note that I never said that photons are waves. You just do not get around seeing wavelike behaviour because everything shows wavelike behaviour in QM.

Maxwell noted long ago that energy (information) can only be transferred in two mechanisms: by particles or by waves. There are no other mechanisms. Light energy has long ago been determined not to be transmitted by waves for the reasons I stated.

Well, you should start with the very basics of QM. As I said before, the wavelike aspect, which is often mentioned is a consequence of QM itself, not of the photons.

So why won't you admit you can't explain why? Then perhaps we can begin to speculate as what might be involved...

Going back to the deepest reasons of course I do not know why. However, this is not physics and just not interesting. I also do not know, why gravity works as it works on the deepest level, why there is electromagnetic force and why there is magnetism. You can always ask "why" on a deeper level and get to the final speculative answers

a) because god made it that way.

b) because the natural constants accidentally have exactly such values that our world turns out to be the way it is.

c) because the flying spaghetti monster arranged it.

So - to me - a photon is the sum of its properties. And to me the simplest model explaining all of its properties is the model I adopt. So to me it is a quantized excitation of the em field.

 

[bjacoby]

1. I just knew I was going to get into trouble for using the words ND filter! Let's just say or equivalent. While I'm using the double slit (or single slit for that matter!) as an example there are a lot of interesting subjects here having to do with coherence, self-confined beams and the like. I am purposely trying to not turn this into a discussion of the details of high-powered mathematics so that lurkers are not lost. On the other hand, I am trying to keep such fundamentals in mind so that I am not saying things that are demonstrably wrong.

2. You are obviously dying to tell me to go read a freshman textbook. However, suffice it to say that I studied graduate level QM under R. Mills (yes THAT Mills) and Yang. I know you find it difficult to understand how someone so properly indoctrinated would go bad and start questioning the gospel, but it happens. The only difference between then and now is that a few more practical problems have been calculated using QM. Back then the hydrogen atom was about it. Harmonic oscillators and energy wells are just mathematical exercises. They are not practical problems, even though everyone pretends that they are. I guess my problem is I need to upgrade to the latest version of QM...

3. I can see we actually ARE having problems with semantics here. I am assuming that the "on" in photon implies it's a particle. You are saying that the word "photon" is just a word like "turtle" that implies all the properties of the object. Thus your word is a name for the entire object and mine implies certain properties of an object which while associated with additional properties may or may not have those additional properties be part of it's own.

4. So I say there is no requirement that a "light particle" be a merger of both particle and wave properties and you say, yes it does because QM derives both these properties from some vague probabilistic formalism. But the fact is that QM is NOTHING but formalism. It does not and probably cannot describe what a light particle "is". It only predicts how it is likely to behave to some degree.

5. So where we stand is you are defending a vast and complex formalism of epicycles because it nicely explains the motion of the planets. And I am suggesting that perhaps one needs a new way to look at things that simplifies everything by a change of point of view.

6. But while you note that wave-like behavior is a property of QM, light is nevertheless a fooler because in the classical macroscopic sense it looks all the world like a high-frequency version of Maxwellian EM radiation. And in fact "electromagnetic spectrum charts" have always portrayed it as such. But if one looks close enough it's obvious that this idea is wrong. As I said before, energy transfers are too fast for light to be merely high frequency waves in the luminiferous aether. And there are other problems as well as we both note. But wave characteristics do persist. And they transfer to statistics. And we find they take the form of 'probability waves" whatever that might be. Just what is the medium that "imaginary probability waves" propagate in? Obviously the standard answer is that these waves need no medium just as no EM wave needs a medium. All these waves propagate quite nicely in "nothing at all". But that is a philosophical paradox like talking about the properties of "nothing at all". One cannot logically separate properties from the object that possesses those properties. It's nonsense. Particles on the other hand transmit energy and information from one place to another with no philosophical conundrums. Which is why I suggest the need for a higher point of view.

7. Let me say I am not asking "why" as you suggest. I'm asking "what logical model?". You on the other hand are suggesting that QM formalism is some kind of model. In spite of it's ability to predict to a degree, I suggest that there are too many unexplained definitions (such as probability waves) and illogical constructs (waves without media) to qualify as a model. You can accept it as useful, but don't call it a model.

As for the original question if photons travel in straight lines, I suggest that they do unless they interact with matter or fields and are deflected. Which oddly enough is a property of classic particles.

As for "why" I kind of like your spaghetti monster theory!

 

[DrChinese]

4. So I say there is no requirement that a "light particle" be a merger of both particle and wave properties and you say, yes it does because QM derives both these properties from some vague probabilistic formalism. But the fact is that QM is NOTHING but formalism. It does not and probably cannot describe what a light particle "is". It only predicts how it is likely to behave to some degree.

5. So where we stand is you are defending a vast and complex formalism of epicycles because it nicely explains the motion of the planets. And I am suggesting that perhaps one needs a new way to look at things that simplifies everything by a change of point of view.

So where is the criticism? How is QM a "vast and complex" formalism? If simplification can give us something, where is the simpler model? You imply that everyone else is in the "Earth is the center of the Solar System" camp and you are in the "Sun is the center" camp. Great. As far as I know, everyone here is open to a better theory and everybody here is interested in seeing scientific progress. But it takes a lot more than words, and a lot more than desire. And certainly a lot more than empty assertions of "logic".

The only person trying to play games with the definition of a photon seems to be you. Your criticism basically is: it must be either a particle or a wave and not both. A strange assertion indeed for someone with your background.

 

[Cthugha]

1. I just knew I was going to get into trouble for using the words ND filter! Let's just say or equivalent. While I'm using the double slit (or single slit for that matter!) as an example there are a lot of interesting subjects here having to do with coherence, self-confined beams and the like. I am purposely trying to not turn this into a discussion of the details of high-powered mathematics so that lurkers are not lost. On the other hand, I am trying to keep such fundamentals in mind so that I am not saying things that are demonstrably wrong.

Ok, I work in a field, where the small difference between a damped coherent beam and a real single photon might matter, so I always put special emphasis on this point. However, as you know, what you are doing, let's move on.

2. You are obviously dying to tell me to go read a freshman textbook. However, suffice it to say that I studied graduate level QM under R. Mills (yes THAT Mills) and Yang. I know you find it difficult to understand how someone so properly indoctrinated would go bad and start questioning the gospel, but it happens.

No, I just wanted to know, on which level we can discuss.

The only difference between then and now is that a few more practical problems have been calculated using QM. Back then the hydrogen atom was about it. Harmonic oscillators and energy wells are just mathematical exercises. They are not practical problems, even though everyone pretends that they are. I guess my problem is I need to upgrade to the latest version of QM...

Well, with the arrival of lowdimensional semiconductor structures like quantum wells, wires and dots, energy wells have indeed become practical problems.

3. I can see we actually ARE having problems with semantics here. I am assuming that the "on" in photon implies it's a particle. You are saying that the word "photon" is just a word like "turtle" that implies all the properties of the object. Thus your word is a name for the entire object and mine implies certain properties of an object which while associated with additional properties may or may not have those additional properties be part of it's own.

Ok, there are a lot of other "on"s out there and usually this indeed means that the things mentioned are particles or quasiparticles. No problem with the assumption that "on" means a particle. However we might not agree on what a particle is...

But the fact is that QM is NOTHING but formalism. It does not and probably cannot describe what a light particle "is". It only predicts how it is likely to behave to some degree.

Sure, that is what models do. But what "is" a photon to you besides all the properties one can measure? Any model can only predict behaviour and does not tell us more.

5. So where we stand is you are defending a vast and complex formalism of epicycles because it nicely explains the motion of the planets. And I am suggesting that perhaps one needs a new way to look at things that simplifies everything by a change of point of view.

Go ahead. If you find another model that predicts the outcomes of QM and is easier or needs less assumptions, you are very welcome to publish it.

As I said before, energy transfers are too fast for light to be merely high frequency waves in the luminiferous aether.

I don't get your point. The maximum energy transfer rate happens at c. I do not see, where this is too fast for waves traveling at c.

And there are other problems as well as we both note. But wave characteristics do persist. And they transfer to statistics. And we find they take the form of 'probability waves" whatever that might be. Just what is the medium that "imaginary probability waves" propagate in? Obviously the standard answer is that these waves need no medium just as no EM wave needs a medium. All these waves propagate quite nicely in "nothing at all".

Well, empty space is different from nothing at all from a philosophical and physical point of view. Why do you need another medium?

But that is a philosophical paradox like talking about the properties of "nothing at all". One cannot logically separate properties from the object that possesses those properties.

To quote Glauber's Nobel lecture again:
"It is worth recalling at this point that interference simply means that the probability amplitudes for alternative and indistinguishable histories must be added together
algebraically."

If you like to, you can consider this as a property of our empty space (and time).

As for "why" I kind of like your spaghetti monster theory!

Hehe, everybody does. ;)

7. Let me say I am not asking "why" as you suggest. I'm asking "what logical model?". You on the other hand are suggesting that QM formalism is some kind of model. In spite of it's ability to predict to a degree, I suggest that there are too many unexplained definitions (such as probability waves) and illogical constructs (waves without media) to qualify as a model. You can accept it as useful, but don't call it a model.

Well, I do not see the problem with probability amplitudes and do not see the logical fallacies you see. The more philosophical approach to which picture one should have on what probability amplitudes are, is a matter of the interpretation of QM, which I suppose is what you would consider as a model. However, there are plenty of interpretations, from Kopenhagen over Bohm to multiple worlds. Therefore to me, QM is definitely a model. Models explain and predict stuff you can measure. What you call logical model is in my eyes a question of the interpretation of QM. And there are indeed still plenty of people out there trying to make heads or tails out of the philosophical implications of the fundaments of QM. You will find plenty of discussions on the pros and cons of Kopenhagen/Decoherence/Bohm/MWI around here. However my favorite interpretation comes from N.D. Mermin. His "in a nutshell" version of his interpretation always makes me laugh:

"My complete answer to the late 19th century question "what is electrodynamics trying to tell us?" would simply be this: Fields in empty space have physical reality; the medium that supports them does not.

Having thus removed the mystery from electrodynamics, let me immediately do the same for quantum mechanics: Correlations have physical reality; that which they correlate, does not."