Exploring Photon Trajectories in Double Slit Experiments

[Demystifier]

However, I would like to ask the Bohmian's about this experiment. I guess in the Bohmian picture, the pilot wave will diffract off both slits, but each single electron will take a well-defined trajectory through one slit to the screen. Is that correct? Also, in the Bohmian picture, are properties like charge associated with both the pilot wave and the particle, or just one or the other? It seems like charge would be a particle-only property, since charge is usually (always?) associated with localized detection events, but perhaps that is just a poor assumption on my part.

In Bohmian picture, position in space-time is the only property of the particle alone. All other properties depend on the wave function as well.

 

[DevilsAvocado]

@SpectraCat, Ken G, JesseM – thanks A LOT for your help on DCQE!

I will study DCQE in detail before opening my mouth on this topic again (and I’m sure some of you will appreciate this 'approach'... ), I’ll get back when I know what I’m talking about.

@Ken G – I appreciate very much the deep knowledge you (and JesseM) posses in QM, but I have to ask you about your 'classical viewpoint' and your reference to Maxwell's equations and the Correspondence principle.

The Correspondence principle only works for large quantum numbers, right? Thus, there is no way to explain in a 'classical interpretation' what’s going on with the single electrons and single photons in the double-slit experiment, right?

Anton Zeilinger shows single photons in the double-slit experiment:
http://www.youtube.com/watch?v=B4OA3cnoGIc&hd=1
https://www.youtube.com/watch?v=B4OA3cnoGIc

Single electrons in the double-slit experiment:
http://www.youtube.com/watch?v=FCoiyhC30bc&hd=1
https://www.youtube.com/watch?v=FCoiyhC30bc

Could we please make this clear once and for all that there is no way to describe what’s going on here in a classical way?

(Since there are others on PF who run this "Newton & God is back!" campaign quite aggressively...)

 

[SpectraCat]

In Bohmian picture, position in space-time is the only property of the particle alone. All other properties depend on the wave function as well.

Ok, but clearly the charge is localized on the particle when it is detected in BM, right? How does it get there if it is not carried by the particle all along? Does the Bohmian pilot wave "collapse" at detection in similar fashion to the wavefunction in standard QM?

Also, what makes the property of position so special that it has a special entity (the particle) reserved to carry it? Is this just a feature of the respresentation? I know that in standard QM, you can choose the representation (e.g. position or momentum) in which you want to write the wavefunction representing the quantum state. Is there an equivalent formulation of BM where momentum is carried only by the "particle" (or it's analog), and the rest of the properties depend on both the particle and the pilot wave?

 

[Demystifier]

Ok, but clearly the charge is localized on the particle when it is detected in BM, right? How does it get there if it is not carried by the particle all along?

Before attempting to answer it we must first agree on definitions. How do you DEFINE the charge of the particle?

Does the Bohmian pilot wave "collapse" at detection in similar fashion to the wavefunction in standard QM?

Not really. Concerning the issue of collapse, Bohmian pilot wave is more like MWI wave.

Also, what makes the property of position so special that it has a special entity (the particle) reserved to carry it?

Let me answer it by a question. What makes the property of position so special in classical mechanics?

 

[Ken G]

The Correspondence principle only works for large quantum numbers, right?

Correct, although the principle is really more general than that. What the principle really says is that the way classical systems couple to each other is in some important sense similar to how they couple to quantum systems, meaning that the intuitions we build up from classical/classical couplings are a kind of "legitimate witness" for quantum/classical couplings. In particular, there is a smooth transition from quantum/classical to classical/classical. This can play out in several ways, it means the classical form must be recovered for large quantum numbers of a single particle, or large occupations numbers (lots of particles), or even in the form of the equations as we take h-> 0. What's interesting about it is how we attribute the reason for the correspondence principle-- to a complete realist, the correspondence principle works because the classical world is really "made up of" the quantum world. To a Copenhagenite, there is no quantum world to "make up" a classical world, but classical couplings is how we understand everything, so the principle just expresses the fact that what we define as understanding is whatever comes out from the coupling to our classical approaches. If that doesn't transition smoothly, then something is fatally wrong with how we understand.

Thus, there is no way to explain in a 'classical interpretation' what’s going on with the single electrons and single photons in the double-slit experiment, right?

Not necessarily, the situation is not as "black and white" as it is often portrayed in that kind of language. Just because quantum information can get "averaged out" when we aggregate to the classical level does not mean it leaves no signature at the classical level. That's a common misconception I'm trying to correct here-- quite often there is a very clear classical signature of phenomena that people think of as "quantum." The most obvious example is quantum interference, whose classical signature is called an "interference pattern." Another example is quantum spin, whose classical signature is called "polarization" in the case of electromagnetic radiation.

Could we please make this clear once and for all that there is no way to describe what’s going on here in a classical way?

We can't make that clear because it isn't categorically true. The fact is, everything we understand about the quantum comes from measuring it in a classical way, that's the correspondence principle right there, so in some sense all our quantum understanding is "going on in a classical way." That's pretty much the Bohr mantra! But what we can say that what is going on in a classical way when we are looking at a quantum system is a bit bizarre, and may or may not have clear classical analogs, depending on what you are talking about. Certainly interference patterns are purely classical, so the only thing odd in those films is that the patterns get built up from discrete dots instead of as darker and darker patterns like adjusting the contrast in the movie. So we can say that we cannot classically explain why the pattern is built up from dots (that's the inherently 'quantum' part), but we would be silly to say we cannot classically explain the pattern, because an interference pattern is a perfectly classical signature of this process.

 

[SpectraCat]

Before attempting to answer it we must first agree on definitions. How do you DEFINE the charge of the particle?

The charge is the property that causes the "particle" to "feel" a force in an applied electric field. In the case of electrons, it is also the property which causes it to give rise to a measurable signal when it impacts the detector. How do you define it?

Not really. Concerning the issue of collapse, Bohmian pilot wave is more like MWI wave.

Can you please elaborate, I know even less about MWI than I do about BM?

Let me answer it by a question. What makes the property of position so special in classical mechanics?

I guess I don't really know what you are getting at. I guess it is "special" in CM because position indexes spatial coordinates and allows us to describe the relative orientation of objects (kind of a circular description I know) ... perhaps it even "defines" space itself in some context? This is the kind of foundational question that I don't have much experience thinking about .. my background in quantum comes from the chemistry side, so my training in foundational issues, and even CM , is rather weak I am afraid. That is one of the reasons I find PF so valuable.

 

[SpectraCat]

Not necessarily, the situation is not as "black and white" as it is often portrayed in that kind of language. Just because quantum information can get "averaged out" when we aggregate to the classical level does not mean it leaves no signature at the classical level. That's a common misconception I'm trying to correct here-- quite often there is a very clear classical signature of phenomena that people think of as "quantum." The most obvious example is quantum interference, whose classical signature is called an "interference pattern." Another example is quantum spin, whose classical signature is called "polarization" in the case of electromagnetic radiation.

There is no classical signature of the quantum phenomena themselves ... there is only a classical signature of the *average* quantum phenomena. That is the content of the Ehrenfest theorem. Also, I do not agree that we can only measure classical properties of systems, for example the quantum/classical distinction between spin and polarization is not necessary for single photons. Even in the quantum description, single photons have well-defined polarizations that can be measured without resorting to classical analogies. Thus polarization measurements on single photons represent quantum signatures, not the classical signatures.

We can't make that clear because it isn't categorically true. The fact is, everything we understand about the quantum comes from measuring it in a classical way, that's the correspondence principle right there, so in some sense all our quantum understanding is "going on in a classical way." That's pretty much the Bohr mantra! But what we can say that what is going on in a classical way when we are looking at a quantum system is a bit bizarre, and may or may not have clear classical analogs, depending on what you are talking about. Certainly interference patterns are purely classical, so the only thing odd in those films is that the patterns get built up from discrete dots instead of as darker and darker patterns like adjusting the contrast in the movie. So we can say that we cannot classically explain why the pattern is built up from dots (that's the inherently 'quantum' part), but we would be silly to say we cannot classically explain the pattern, because an interference pattern is a perfectly classical signature of this process.

I am not sure what you mean here, there is no classical explanation for the diffraction of electrons in a double-slit experiment, so there is no "classical signature" for that process.
Classically you would predict that the double slit simply windows the ballistic trajectories of the electrons, which certainly doesn't give rise to an interference pattern. So I am not sure how can call that a "classical signature".

 

[Ken G]

There is no classical signature of the quantum phenomena themselves ... there is only a classical signature of the *average* quantum phenomena. That is the content of the Ehrenfest theorem.

Yes, but note this does not mean we cannot look for classical ways to understand the quantum phenomena. In fact, I claim that sometimes people tout a quantum understanding of some phenomenon, but when you analyze it, their understanding is just its classical signature. That was my claim about the ballyhoo on the "average trajectory from weak measurements" concept.

Also, I do not agree that we can only measure classical properties of systems, for example the quantum/classical distinction between spin and polarization is not necessary for single photons.

It is demonstrably true that we can only measure classical properties of systems, because all measuring apparatuses are classical systems. Ask yourself why that is-- why have you never seen a completely quantum system used as a measuring apparatus? It's because the entire concept of measurement is classical, and that's why we never measure anything but classical properties of any system. The "quantum properties" are always purely inferential, and this is pretty much the source of the need for "interpretations" of what those properties are.

Even in the quantum description, single photons have well-defined polarizations that can be measured without resorting to classical analogies.

Do it without using a classical measuring device. No? Then it's more then an analogy, it's the reality that is classical.

Thus polarization measurements on single photons represent quantum signatures, not the classical signatures.

Think more about the way a polarization measurement is actually done.

I am not sure what you mean here, there is no classical explanation for the diffraction of electrons in a double-slit experiment, so there is no "classical signature" for that process.

There certainly is a classical explanation for the diffraction of electrons! It is called "wave mechanics", and it applies to a classical ensemble of electrons (and yes, it came as a large surprise that it applied, but it is still a perfectly classical version). Do you think that if all we had access to were classical ensembles of electrons, not individual ones, we could never have derived the equations of quantum mechanics? Yes, we could have, and did. Tests of quantum mechanics are often done in the classical limit, most obviously in the case of lasers.

Classically you would predict that the double slit simply windows the ballistic trajectories of the electrons, which certainly doesn't give rise to an interference pattern. So I am not sure how can call that a "classical signature".

Ah, I see we have a language disconnect. By "classical", I mean "in the limit of large quantum numbers", or h->0 if you prefer. I do not mean Newton's laws! There is such a thing as classical wave mechanics, good examples being Huygen's principle and Maxwell's equations. This is indeed an unfortunate language ambiguity.

 

[SpectraCat]

There certainly is a classical explanation for the diffraction of electrons! It is called "wave mechanics", and it applies to a classical ensemble of electrons (and yes, it came as a large surprise that it applied, but it is still a perfectly classical version). Do you think that if all we had access to were classical ensembles of electrons, not individual ones, we could never have derived the equations of quantum mechanics? Yes, we could have, and did. Tests of quantum mechanics are often done in the classical limit, most obviously in the case of lasers.

Ah, I see we have a language disconnect. By "classical", I mean "in the limit of large quantum numbers", or h->0 if you prefer. I do not mean Newton's laws! There is such a thing as classical wave mechanics, good examples being Huygen's principle and Maxwell's equations. This is indeed an unfortunate language ambiguity.

That's not a language disconnect ... there is no explanation in all of classical physics (mechanics or electrodynamics) for the wave nature of electrons. There is no way to predict or model their wavelike properties using classical physics. There may be a mathematical analogy between the equations used to predict the behavior of waves in classical electrodynamics with those used to predict the wave-like behavior of massive particles in quantum mechanics, but that's not the same thing. You also cannot "take the limit as h-->0" in any physically meaningful context, because h doesn't go to zero, it has a well-defined finite value.

If you think that there is a explanation "in the limit of large quantum numbers" for the diffraction of electrons that does not involve first assuming their wave-like properties from quantum mechanics, then I would be very interested to see it.

 

[SpectraCat]

Also, I do not agree that we can only measure classical properties of systems, for example the quantum/classical distinction between spin and polarization is not necessary for single photons. Even in the quantum description, single photons have well-defined polarizations that can be measured without resorting to classical analogies. Thus polarization measurements on single photons represent quantum signatures, not the classical signatures.

It is demonstrably true that we can only measure classical properties of systems, because all measuring apparatuses are classical systems. Ask yourself why that is-- why have you never seen a completely quantum system used as a measuring apparatus? It's because the entire concept of measurement is classical, and that's why we never measure anything but classical properties of any system. The "quantum properties" are always purely inferential, and this is pretty much the source of the need for "interpretations" of what those properties are.

Do it without using a classical measuring device. No? Then it's more then an analogy, it's the reality that is classical.

Think more about the way a polarization measurement is actually done.

I have thought about it at considerable length. It is clear to me that polarization measurements on photons are quantum mechanical in nature, in the sense that the interactions with the optical elements preserve the coherence of entangled states. If this were not true then the quantum teleportation experiments would not be possible. The only classical aspect of these experiments is the "click" that registers the arrival of the photon at a particular detector, and that does not reflect the polarization of the photon, but only it's existence. The polarization of the photon is determined by correlating the click with the path taken through a polarizing beamsplitter.

 

[Ken G]

That's not a language disconnect ... there is no explanation in all of classical physics (mechanics or electrodynamics) for the wave nature of electrons.

There is no explanation in quantum mechanics for it either, that's not what is meant by an "explanation" in science. It's like you seem to think if we imagine lots of electrons, we have no idea why they diffract, but if we imagine electrons are discrete, suddenly diffraction makes perfect sense!

In science, an explanation is just a theory that works, that's all. We certainly do have a classical theory that works on the diffraction of large ensembles of electrons, it is called wave mechanics. We also have a quantum theory that works on the diffraction of large ensembles of electrons, it is called quantum mechanics. There is extremely little difference between them in the context of two-slit experiments, basically the dispersion is different but that doesn't even matter if you imagine a single wavelength/momentum in your initial condition. This is precisely the issue I am drawing out here, people have a lot of misconceptions about the differences between a "quantum" experiment and a "classical" experiment, it's like they have forgotten the correspondence principle as soon as the word "quantum" got mentioned.

There is no way to predict or model their wavelike properties using classical physics.

Of course there is, just use a wave theory. That's what you do in quantum mechanics too. Do you think that just because you imagine you are treating a quantum, that suddenly it's clear you should have a wave equation? You have a classical wave equation for the same reason you have a quantum wave equation: it is what works, period.

There may be a mathematical analogy between the equations used to predict the behavior of waves in classical electrodynamics with those used to predict the wave-like behavior of massive particles in quantum mechanics, but that's not the same thing. You also cannot "take the limit as h-->0" in any physically meaningful context, because h doesn't go to zero, it has a well-defined finite value.

Huh? There isn't a correspondence principle?

If you think that there is a explanation "in the limit of large quantum numbers" for the diffraction of electrons that does not involve first assuming their wave-like properties from quantum mechanics, then I would be very interested to see it.

Goodness gracious, there is only one explanation for why electrons diffract: they do it. It doesn't make a bit of difference if historically the equations that describe it were first found classically, as for photons, or first found quantum mechanically, as for electrons. Those equations are the same in the absence of dispersion, for example if you idealize the initial conditions as having a known momentum. Put differently, we could have had all the equations that quantum mechanics uses to describe electron diffraction even if we never became aware that electrons were particles. We would just call it the electron analog of Maxwell's equations.

 

[Ken G]

I have thought about it at considerable length. It is clear to me that polarization measurements on photons are quantum mechanical in nature, in the sense that the interactions with the optical elements preserve the coherence of entangled states.

I'm not talking about the philosophical inferences about the true nature of polarization, what I am talking about here is simply that polarization measurements are always done with classical instruments (and why is that?), so you are definitely observing a classical phenomenon there, by definition. However, we do use the term "quantum phenomenon", which really means a "classical phenomenon that you only get when you involve quantum systems." Nevertheless, it is simply true that we had a classical concept of light polarization long before we had a quantum concept of photon spin. This is what I'm telling you-- phenomena that get called "quantum phenomena" often have perfectly classical analogs, which appear in the perfectly classical ways that those quantum phenomena get measured and studied. This is not a purely semantic issue, failing to recognize this fact causes people to leap to all kinds of incorrect assumptions about what you can and cannot do with classical models. Just set the point aside for now, and wait for examples to appear.

The polarization of the photon is determined by correlating the click with the path taken through a polarizing beamsplitter.

We didn't have to wait long! So this polarizing beamsplitter you claim to have placed in your apparatus, why do you call it a polarizing beamsplitter in the first place? How do you know it actually does that? You know it by its classical function, that's what defines the instrument in the first place.

 

[SpectraCat]

There is no explanation in quantum mechanics for it either, that's not what is meant by an "explanation" in science. It's like you seem to think if we imagine lots of electrons, we have no idea why they diffract, but if we imagine electrons are discrete, suddenly diffraction makes perfect sense!

In science, an explanation is just a theory that works, that's all. We certainly do have a classical theory that works on the diffraction of large ensembles of electrons, it is called wave mechanics.

I really don't know what you are talking about, to clarify the situation, please write down the classical formulation of wave mechanics that predicts the diffraction of "large ensembles of electrons".

We also have a quantum theory that works on the diffraction of large ensembles of electrons, it is called quantum mechanics. There is extremely little difference between them in the context of two-slit experiments, basically the dispersion is different but that doesn't even matter if you imagine a single wavelength/momentum in your initial condition. This is precisely the issue I am drawing out here, people have a lot of misconceptions about the differences between a "quantum" experiment and a "classical" experiment, it's like they have forgotten the correspondence principle as soon as the word "quantum" got mentioned.Of course there is, just use a wave theory. That's what you do in quantum mechanics too. Do you think that just because you imagine you are treating a quantum, that suddenly it's clear you should have a wave equation? You have a classical wave equation for the same reason you have a quantum wave equation: it is what works, period.

The Schrodinger equation is mathematically similar to a classical wave equation, but it is physically distinct. The important distinction is that the energy of a classical wave is related to its amplitude, whereas the energy of a quantum wave is related to its frequency.

Huh? There isn't a correspondence principle?

Of course there is, but it is a logical criterion about "the way things have to be", rather than a law of physics. You can rationalize it by using mathematical tricks like taking the limit as h-->0, but those are just tricks. It makes some physical sense because there is such a huge scale difference between the quantum world and the one we live in, so we don't notice the effects of h being finite when we hit a baseball. However, we use devices everyday that rely on h NOT being zero, such as photoelectric detectors, flash memory and LCD monitors. Thus the applicability of taking the limit of h-->0 is ONLY useful for rationalizing why quantum effects are not noticable in certain sub-systems of the world we live in, i.e. those governed by the laws of classical mechanics.

Goodness gracious, there is only one explanation for why electrons diffract: they do it. It doesn't make a bit of difference if historically the equations that describe it were first found classically, as for photons, or first found quantum mechanically, as for electrons. Those equations are the same in the absence of dispersion, for example if you idealize the initial conditions as having a known momentum. Put differently, we could have had all the equations that quantum mechanics uses to describe electron diffraction even if we never became aware that electrons were particles. We would just call it the electron analog of Maxwell's equations.

Nope, that is just not correct .. it's not a matter of having the equations, it's a matter of the significance of the terms in those equations. As I said above, the mathematical similarity between the classical and quantum mechanical wave equations does not equate to a physical similarity. What you seem to be missing in all of your classically based arguments is the fundamental importance of the uncertainty principle. That is where the "wave-like" properties of massive particles originate from (in the theoretical treatment anyway), and without it, you can never get from the equations of classical mechanics to the diffraction of electrons.

 

[unusualname]

We can't make that clear because it isn't categorically true. The fact is, everything we understand about the quantum comes from measuring it in a classical way, that's the correspondence principle right there, so in some sense all our quantum understanding is "going on in a classical way." That's pretty much the Bohr mantra! But what we can say that what is going on in a classical way when we are looking at a quantum system is a bit bizarre, and may or may not have clear classical analogs, depending on what you are talking about. Certainly interference patterns are purely classical, so the only thing odd in those films is that the patterns get built up from discrete dots instead of as darker and darker patterns like adjusting the contrast in the movie. So we can say that we cannot classically explain why the pattern is built up from dots (that's the inherently 'quantum' part), but we would be silly to say we cannot classically explain the pattern, because an interference pattern is a perfectly classical signature of this process.

This really is a ridiculous statement. Almost the opposite is true, everything we know about quantum mechanics comes from theoretical deductions and "guesses" based on the barest of experimental data (sometimes none, eg de Broglie waves, Dirac's antimatter deduction, Bose statistics etc)

Experiments are mostly testing the validity of the QM theory, precisely because it is spectacularly non-classical

Where did you get the idea that experimental results enable us to "understand the quantum". They just confirm that the (spectacularly non-classical) theory is not falsified.

 

[yoron]

Ken, reading you I get confused. Are you arguing that if there isn't a equivalence to a macroscopic explanation for a QM experiment, then it must be wrong? I haven't checked on your other thoughts admittedly, but my first impression was that you defined a photons path as impossible to define?

Weak measurements is a statistical approach over time, and as such they give us a statistical certainty, but they do not answer HUP:s definition, as I understands it. And to define a trajectory through that approach may in 'hind sight' seem beautifully plausible, but at each single measurement creating that thought up 'path' your measurement will answer to HUP.

To me it's like two 'realities', HUP being one, weak measurements being another. Both make sense from their own perspective, but they are not the same to me. And this need of defining a path, involving pilot waves for example? Why not accept the limitations we find when measuring, instead of filling in the space with ideas describing notions that's already questionable, distance and motion?

 

[Ken G]

I really don't know what you are talking about, to clarify the situation, please write down the classical formulation of wave mechanics that predicts the diffraction of "large ensembles of electrons".

First you need a classical measurable. Electron energy flux density will suffice (we could use electron number flux, but my point is that we never need to think of these things as particles at all to "understand" diffraction). Now you need a wave theory. Huygen's principle works fine. Let's simplify life and just get a theory that works for electrons of a given energy (which here means a given ratio of energy flux to mass flux). The wave equation with the v of that population of electrons will work fine, where v is found from timing experiments. Now we need a concept of frequency because it's a wave theory, and here we can leave the frequency as a free parameter that the interference experiment will determine. The wave equation describes the speed of signal propagation, Huygen's principle tells us how to handle the sources and the slits, and the frequency parameter gives us the interference we need. Every one of these is a 100% classical concept, remember that we are pretending we don't even know we have particles here. Now we put them together to calculate the energy fluxes everywhere subject to the free frequency parameter, compare to experiments, and poof, both the frequency parameter drops out, and the fact that we have what we would call a correct theory for electron diffraction, and all classical.

This will all work fine as long as the electron ensemble is prepared in a uniform way, say by using a fixed potential drop to accelerate the beam (which in our present quantum understanding would say gives "monoenergetic electrons"). If you object that the theory so far only handles one voltage, we can just repeat the whole experience for other values of the voltage, ultimately tracing out the dependence of frequency on voltage. Still purely classical, and now we have the "Green's functions" for a complete classical description of any type of electron diffraction experiment on a large ensemble of electrons, so long as we know how that ensemble was generated. All perfectly mundane classical physics, we can do the whole thing imagining we have an "electron field" and no electrons at all.

That we can always do all this depends on only one thing: the correspondence principle. I'm saying the correspondence principle could be reframed thusly: "If the experiment is done in the classical limit, it will be describably by a classical theory", or "any quantum theory gives birth to a classical theory in the limit of large occupation numbers."

The Schrodinger equation is mathematically similar to a classical wave equation, but it is physically distinct.

And you trace that to its quantum nature? What in the Schroedinger equation jumps up and says "I'm a quantum"? Nothing.

The important distinction is that the energy of a classical wave is related to its amplitude, whereas the energy of a quantum wave is related to its frequency.

Again you seem to be saying there is no correspondence principle. But there is.

You can rationalize it by using mathematical tricks like taking the limit as h-->0, but those are just tricks.

Labelling something a "trick" does not make it untrue. If a procedure spawns a classical theory, then voila, you have a classical theory. I'm sorry if you feel you got tricked in the process. The Schroedinger equation for electrons spawns a nonrelativistic classical theory simply by taking the limit of h-->0 keeping h/m a constant, all it has is a different dispersion relation from the dispersionless wave equation we are most used to seeing.

However, we use devices everyday that rely on h NOT being zero, such as photoelectric detectors, flash memory and LCD monitors.

All of which is irrelevant to anything I've said. Note I never said "there is no such thing as a quantum effect that has no classical analog." What I did say is "many quantum effects do have classical analogs that we take advantage of all the time, especially when testing quantum mechanics, yet many people seem to be unaware of this fact."

Nope, that is just not correct .. it's not a matter of having the equations, it's a matter of the significance of the terms in those equations.

Again, nothing we are talking about has anything to do with the philosophical interpretation that we commonly give to quantum phenomena. The point I'm making is quite a bit simpler than that: all quantum theories spawn classical theories in the classical limit, and such classical analogs can be very useful for understanding a wide array of what we call quantum phenomena, in stark contrast to what is often said about many kinds of quantum weirdness. Most quantum weirdness is simply the fact that it seems weird to us that a wave theory applies to quanta, but since wave theories are classical, we should not say that it's weird because there's no classical analog, we should say it's weird because there is a classical analog and we didn't expect that.

As I said above, the mathematical similarity between the classical and quantum mechanical wave equations does not equate to a physical similarity.

And that just doesn't make any sense. There is no distinction between "mathematical similarities" and "physical similarities", the mathematical is all we have to understand the physical.

What you seem to be missing in all of your classically based arguments is the fundamental importance of the uncertainty principle.

Ah, the uncertainty principle-- another 'quantum phenomenon' with a perfectly classical analog. Comes up in Fourier transforms of classical fields all the time, thanks for bringing that up.

[qutoe]That is where the "wave-like" properties of massive particles originate from (in the theoretical treatment anyway), and without it, you can never get from the equations of classical mechanics to the diffraction of electrons.[/QUOTE]Well, I just told you how you could do just exactly that. Remember, we arleady agreed that "classical" does not mean "Newton's laws."

 

[Ken G]

This really is a ridiculous statement. Almost the opposite is true, everything we know about quantum mechanics comes from theoretical deductions and "guesses" based on the barest of experimental data (sometimes none, eg de Broglie waves, Dirac's antimatter deduction, Bose statistics etc)

I don't even know what you're talking about, you're saying quantum mechanics is some kind of data-poor area of physics? That's just baloney, quantum mechanical predictions confront a spectacular amount of extremely accurate data all the time. I don't even see how this wrong point would be relevant if it were right!

Where did you get the idea that experimental results enable us to "understand the quantum". They just confirm that the (spectacularly non-classical) theory is not falsified.

I guess you see some distinction in those two sentences, but I'm afraid I do not.

 

[Ken G]

Ken, reading you I get confused. Are you arguing that if there isn't a equivalence to a macroscopic explanation for a QM experiment, then it must be wrong?

Certainly not, had I meant that I would have said that. I'm saying that any quantum mechanical theory spawns a classical theory that works on the classical limit. It's really very simple, it's just the "correspondence principle." By "classical theory" I mean one that takes advantage of purely classical constructs, and could have been developed by physicists who were never even aware they were dealing with quanta. It's kind of an accident of history when exactly that happened, and when it did not.

What I am not saying is that there isn't any such thing as quantum behavior, I'm saying there is a kind of myth about all kinds of quantum behavior that "have no classical analog", when in fact they very often have fairly obvious classical analogs, which has a lot to do with why they could have been, or were, first encountered in a classical theory. Two-slit diffraction being a prime example, the uncertainty principle being another. The "quantum weirdness" in the HUP is not at all that is has no classical analog, it is simply that its classical analog is something we didn't expect to apply to quanta. That's the quantum weirdness of it, it is that the classical analog is not the classical analog we expected, not that there is no classical analog. Just look at the language of quantum mechanics, do we have a "quantum function"? No, we have a wave function. Waves are classical concepts, so this is another perfect example of the relevance of classical analogs in what gets sold as "purely quantum" behavior.

Why not accept the limitations we find when measuring, instead of filling in the space with ideas describing notions that's already questionable, distance and motion?

This sounds like a different issue, like you are inserting an alternative interpretation to quantum mechanics. Any interpretation is fine by me as long as:
1) we don't take it too seriously,
2) it generates interesting ontological structures to ponder, and
3) it gets the predictions right.

 

[unusualname]

I don't even know what you're talking about, you're saying quantum mechanics is some kind of data-poor area of physics? That's just baloney, quantum mechanical predictions confront a spectacular amount of extremely accurate data all the time. I don't even see how this wrong point would be relevant if it were right!

I guess you see some distinction in those two sentences, but I'm afraid I do not.

QM is a theoretical model of the world which required a sensational paradigm break from classical physics, the very basic essentials were proposed by de Broglie and Heisenberg for purely theoretical reasons.

I have understood from you this week that the recent Nature article where the QM wave function is "measured" has nothing of merit that a classical analysis could not explain.

I think you are confusing the correspondence principle with something other than what it is, in particular it doesn't say that anything we measure has a classical description/explanation.

 

[Ken G]

QM is a theoretical model of the world which required a sensational paradigm break from classical physics, the very basic essentials were proposed by de Broglie and Heisenberg for purely theoretical reasons.

Yes, I believe we can all agree on that.

I have understood from you this week that the recent Nature article where the QM wave function is "measured" has nothing of merit that a classical analysis could not explain.

That is my opinion, although I laid out the map for how my claim could be falsified: simply describe a meaningful difference between the outcome of the "average trajectory" plot and a classical Poynting flux streamline diagram. No one has yet attempted to claim there is a meaningful difference in those figures, yet they seem perfectly willing to believe there is without any evidence for that belief. Simple skepticism demands that we ask for evidence of a difference, and an argument that the difference is somehow "quantum" in nature.

I think you are confusing the correspondence principle with something other than what it is, in particular it doesn't say that anything we measure has a classical description/explanation.

At no time did I ever say any such thing. What I did say is that every quantum theory spawns a classical one that describes the classical limit, I said that the classical theory could have been discovered without the quantum theory, that this often did happen in fact, and when it did not it's just an accident of history. I also said one ramification of this fact is that much of the behavior that appears in a one-at-a-time type quantum experiment has a classical analog that appears in the many-at-a-time classical limit. I also said that all this is an example of the correspondence principle.

 

[unusualname]

That is my opinion, although I laid out the map for how my claim could be falsified: simply describe a meaningful difference between the outcome of the "average trajectory" plot and a classical Poynting flux streamline diagram. No one has yet attempted to claim there is a meaningful difference in those figures, yet they seem perfectly willing to believe there is without any evidence for that belief. Simple skepticism demands that we ask for evidence of a difference, and an argument that the difference is somehow "quantum" in nature.

I was referring to http://www.nature.com/nature/journal/v474/n7350/full/nature10120.html

At no time did I ever say any such thing. What I did say is that every quantum theory spawns a classical one that describes the classical limit, I said that the classical theory could have been discovered without the quantum theory, that this often did happen in fact, and when it did not it's just an accident of history. I also said one ramification of this fact is that much of the behavior that appears in a one-at-a-time type quantum experiment has a classical analog that appears in the many-at-a-time classical limit. I also said that all this is called the correspondence principle.

Yes but the classical analog is truly misleading. In fact it turns out that the only reason classical physics works the way we once thought is precisely because the quantum "one-at-a-time" explanation is primary. There are no classical waves, they emerge from much more sophisticated QFT, that you have this backwards is almost comical.

 

[Ken G]

I was referring to http://www.nature.com/nature/journal/v474/n7350/full/nature10120.html

Oh that one, that wasn't the subject of any thread nor did I ever give it careful consideration, except to point out they could not actually be measuring the real and imaginary parts of the wavefunction independently because all wavefunctions have an arbitrary global phase. That might be an interesting thread, but note we did have a thread on the "average trajectory" paper, and I made extensive comments on that.

Yes but the classical analog is truly misleading. In fact it turns out that the only reason classical physics works the way we once thought is precisely because the quantum "one-at-a-time" explanation is primary.

How does the first sentence follow from the second? A classical analog is not misleading if you understand what it is-- a classical analog.

There are no classical waves, they emerge from much more sophisticated QFT, that you have this backwards is almost comical.

"There are no classical waves." Interesting. But you do think there are "quantum waves?" You don't think they "emerge" from some more fundamental theory too? This is just science, folks. You apparently do not really understand what I'm saying.

 

[unusualname]

"There are no classical waves." Interesting. But you do think there are "quantum waves?" You don't think they "emerge" from some more fundamental theory too? This is just science, folks. You apparently do not really understand what I'm saying.

I'll let you off the perhaps embarrassing posts you made about the Nature paper.

But this statement, wow, so you're now claiming that QM itself is not fundamental, and that helps what you've been arguing above?

QM is the WHOLE OF PHYSICS AS FAR AS WE KNOW, there is little doubt about that, the biggest effort by the most brilliant people is to make it explain gravity.

But it's pointless to mix this great intellectual endeavour up with silly discussions about the correspondence principle and interpretations.

 

[SpectraCat]

First you need a classical measurable. Electron energy flux density will suffice (we could use electron number flux, but my point is that we never need to think of these things as particles at all to "understand" diffraction). Now you need a wave theory. Huygen's principle works fine. Let's simplify life and just get a theory that works for electrons of a given energy (which here means a given ratio of energy flux to mass flux). The wave equation with the v of that population of electrons will work fine, where v is found from timing experiments. Now we need a concept of frequency because it's a wave theory, and here we can leave the frequency as a free parameter that the interference experiment will determine. The wave equation describes the speed of signal propagation, Huygen's principle tells us how to handle the sources and the slits, and the frequency parameter gives us the interference we need. Every one of these is a 100% classical concept, remember that we are pretending we don't even know we have particles here. Now we put them together to calculate the energy fluxes everywhere subject to the free frequency parameter, compare to experiments, and poof, both the frequency parameter drops out, and the fact that we have what we would call a correct theory for electron diffraction, and all classical.

This will all work fine as long as the electron ensemble is prepared in a uniform way, say by using a fixed potential drop to accelerate the beam (which in our present quantum understanding would say gives "monoenergetic electrons"). If you object that the theory so far only handles one voltage, we can just repeat the whole experience for other values of the voltage, ultimately tracing out the dependence of frequency on voltage. Still purely classical, and now we have the "Green's functions" for a complete classical description of any type of electron diffraction experiment on a large ensemble of electrons, so long as we know how that ensemble was generated. All perfectly mundane classical physics, we can do the whole thing imagining we have an "electron field" and no electrons at all.

That we can always do all this depends on only one thing: the correspondence principle. I'm saying the correspondence principle could be reframed thusly: "If the experiment is done in the classical limit, it will be describably by a classical theory", or "any quantum theory gives birth to a classical theory in the limit of large occupation numbers."

That is not an equation .. it's a phenomenological description of how might one generate an equation ... but as they say in the South, "many a slip 'twixt the cup and the lip" ... so please write down the equation.

However, based on what you write above, I get enough of the sense of what you are saying to understand that what you describe is NOT a classical theory of electron diffraction based on fundamental principles, but rather only a phenomenological model for rationalizing the observation of the experimental result of electron diffraction. If you recall, I asked for the classical equation that *predicts* the phenomenon of electron diffraction.

Finally, your use of the correspondence principle in this discussion is a red herring as far as I can tell. The fact is that you will get the same experimental results for an electron beam, whether it is firing electrons one at a time in a carefully controlled way, or as a continuous flux. So the phenomenological approach you are taking has nothing to do with "electron energy fluxes" or the use of a "large ensemble of electrons". I still want to see that equation though ...

And you trace that to its quantum nature? What in the Schroedinger equation jumps up and says "I'm a quantum"? Nothing.

If you really think that, then I guess you don't understand the Schrodinger equation very well. First of all, the Schrodinger equation is not even a wave equation. A classical wave equation equates the second time derivative of a scalar function representing the wave to the scaled Laplacian of that scalar function. The Schrodinger equation relates the FIRST time-derivative of the (scalar) wavefunction to the scaled Laplacian of the wavefunction. Furthermore, the square root of -1 appears on the RHS of the Schrodinger equation, whereas all the constants on both sides of a classical wave equation are real.

Again you seem to be saying there is no correspondence principle. But there is.
Labelling something a "trick" does not make it untrue. If a procedure spawns a classical theory, then voila, you have a classical theory. I'm sorry if you feel you got tricked in the process. The Schroedinger equation for electrons spawns a nonrelativistic classical theory simply by taking the limit of h-->0 keeping h/m a constant, all it has is a different dispersion relation from the dispersionless wave equation we are most used to seeing.

Again, you are making claims about mathematical equations .. it would be much more helpful to see the mathematical derivation. The only context in which I have studied quantum equations in the limit as h-->0 is in statistical mechanics, where it was used to derive the classical limit of the quantum mechanical partition function. If you could provide a reference which goes through the derivation you describe for the Schrodinger equation, I would like to review that. I will try to derive it myself, but I am unlikely to have time for a careful treatment of that in the near future (unless it is trivial, but I haven't seen my way to a trivial answer yet).

Speaking of time .. I am out of it .. I will address the rest of your points later.

 

[Ken G]

But it's pointless to mix this great intellectual endeavour up with silly discussions about the correspondence principle and interpretations.

Translation: you don't understand what I'm saying at all. However, I have seen many posts on this forum that suggest I am being understood by others, so I am perfectly content.

 

[unusualname]

Translation: you don't understand what I'm saying at all. However, I have seen many posts on this forum that suggest I am being understood by others, so I am perfectly content.

Yes Ken G you had (in the past) discussions in a restricted environment that seem to make sense to you and (maybe) the other participants. But you haven't had someone like me to clarify the situation and in fact point out that you aren't making sense.

 

[DevilsAvocado]

Ops... that’s a long discussion and a lot of words... but I don’t think it’s necessary to reply on every sentence, because:

The Correspondence principle only works for large quantum numbers, right?

Correct

There is no 'escape route' around this, and the only logical conclusion must then be:

The Correspondence principle does not work for (small or) single quantum numbers.​

Why is this so 'hard' to spell out...?

 

[Ken G]

If you think that obvious point is in any way relevant to what I said, it is hopeless that you will understand what I did say (which is that many quantum phenomena can be better understood by noticing their classical analogs). Indeed, I've mentioned several classical analogs in this thread, I explained how they could have been arrived at even without knowing that we are dealing with quanta (like two-slit diffraction patterns), and still you cling to the empty claim that this somehow doesn't make sense.

Maybe you can answer this: just what do you think is inherently "quantum" in a two-slit diffraction pattern? I'm all ears.

Meanwhile, I'll just have to hope that someone else was lurking in this thread, someone who actually did wish to understand the correspondence principle, and who might glean the importance of these words: every quantum theory spawns a classical theory that has to work in the classical limit. That's what "classical analog" means, for those watching at home.

 

[Ken G]

Yes Ken G you had (in the past) discussions in a restricted environment that seem to make sense to you and (maybe) the other participants. But you haven't had someone like me to clarify the situation and in fact point out that you aren't making sense.

If you think something I said didn't make sense, you are welcome to quote it again, and I'll explain it to you again. But just because you don't understand something, and completely misconstrue it every time you try to sumamrize it, doesn't mean it didn't make sense when it was actually said.

 

[Demystifier]

The charge is the property that causes the "particle" to "feel" a force in an applied electric field. In the case of electrons, it is also the property which causes it to give rise to a measurable signal when it impacts the detector. How do you define it?

I will use your definition - the cause of the force. But in Bohmian mechanics, the cause of ALL forces on particles is the wave function. Instead of dealing with various fields such as gravitational and electric field, you need only one "field" - the wave function. In this sense, there is only one force on particles - the quantum force caused by the wave function. But form such a point of view, the charge (i.e., the cause of electric force) does not have a fundamental meaning. It is a useful concept only in the classical limit of forces on particles, and for forces on the wave functions.

Can you please elaborate, I know even less about MWI than I do about BM?

It is explained in many places. See e.g.
http://en.wikipedia.org/wiki/Bohm_interpretation#Collapse_of_the_wavefunction

I guess I don't really know what you are getting at. I guess it is "special" in CM because position indexes spatial coordinates and allows us to describe the relative orientation of objects (kind of a circular description I know) ... perhaps it even "defines" space itself in some context? This is the kind of foundational question that I don't have much experience thinking about .. my background in quantum comes from the chemistry side, so my training in foundational issues, and even CM , is rather weak I am afraid. That is one of the reasons I find PF so valuable.

My point is that the "speciality" of the position observable in BM is fully analogous to that in CM. Therefore, to postpone controversy as much as possible, it is much better to first answer that question within CM, without referring to QM. For basics of CM see e.g.
http://en.wikipedia.org/wiki/Classical_mechanics

 

[SpectraCat]

First you need a classical measurable. Electron energy flux density will suffice (we could use electron number flux, but my point is that we never need to think of these things as particles at all to "understand" diffraction). Now you need a wave theory. Huygen's principle works fine. Let's simplify life and just get a theory that works for electrons of a given energy (which here means a given ratio of energy flux to mass flux). The wave equation with the v of that population of electrons will work fine, where v is found from timing experiments. Now we need a concept of frequency because it's a wave theory, and here we can leave the frequency as a free parameter that the interference experiment will determine. The wave equation describes the speed of signal propagation, Huygen's principle tells us how to handle the sources and the slits, and the frequency parameter gives us the interference we need. Every one of these is a 100% classical concept, remember that we are pretending we don't even know we have particles here. Now we put them together to calculate the energy fluxes everywhere subject to the free frequency parameter, compare to experiments, and poof, both the frequency parameter drops out, and the fact that we have what we would call a correct theory for electron diffraction, and all classical.

This will all work fine as long as the electron ensemble is prepared in a uniform way, say by using a fixed potential drop to accelerate the beam (which in our present quantum understanding would say gives "monoenergetic electrons"). If you object that the theory so far only handles one voltage, we can just repeat the whole experience for other values of the voltage, ultimately tracing out the dependence of frequency on voltage. Still purely classical, and now we have the "Green's functions" for a complete classical description of any type of electron diffraction experiment on a large ensemble of electrons, so long as we know how that ensemble was generated. All perfectly mundane classical physics, we can do the whole thing imagining we have an "electron field" and no electrons at all.

That we can always do all this depends on only one thing: the correspondence principle. I'm saying the correspondence principle could be reframed thusly: "If the experiment is done in the classical limit, it will be describably by a classical theory", or "any quantum theory gives birth to a classical theory in the limit of large occupation numbers."

And you trace that to its quantum nature? What in the Schroedinger equation jumps up and says "I'm a quantum"? Nothing.

I already responded to this section (and you have not yet responded to that post), but here I would like to emphasize the non-relevance of the correspondence principle to this whole line of discussion. As you point out, the correspondence principle deals with systems in the limit of large quantum numbers, and explains why objects with macroscopic sizes and masses can be described with arbitrary accuracy by classical mechanics. So, in the example above, what is different in terms of quantum numbers about the case you describe for a "large ensemble of electrons", and the case where electrons are sent through the double slit "one-by-one". The use of the word "ensemble" suggests that it is statistics, rather than the correspondence principle, that are relevant to your case, but of course the statistical behavior is unchanged.

On the other hand, for sufficiently high electron fluxes, there likely WILL be a difference in the experimental results, due to space charging effects. That is, the electrons will interact with each other as they pass through the slits, and that will affect the diffraction pattern. In fact, it might be that such interactions would destroy the interference pattern altogether. However in any case, a phenomenological theory to explain those results would necessarily be different from a correct quantum theory.

All of which is irrelevant to anything I've said. Note I never said "there is no such thing as a quantum effect that has no classical analog." What I did say is "many quantum effects do have classical analogs that we take advantage of all the time, especially when testing quantum mechanics, yet many people seem to be unaware of this fact."

I don't think it is irrelevant .. I raised those examples to point out that there are macroscopic systems that rely on quantum effects, which interact on the same scale with macroscopic systems where quantum effects can be ignored. The point being that taking the limit as h-->0 to get the classical limit of a theory doesn't prove anything unless you have confirmation from experiment that the result is correct. I maintain that taking the limit as h-->0 for the case of electron diffraction will give you nonsense, at least in terms of the experimentally observed results .. I have yet to see any evidence from you to the contrary.

Again, nothing we are talking about has anything to do with the philosophical interpretation that we commonly give to quantum phenomena. The point I'm making is quite a bit simpler than that: all quantum theories spawn classical theories in the classical limit, and such classical analogs can be very useful for understanding a wide array of what we call quantum phenomena, in stark contrast to what is often said about many kinds of quantum weirdness. Most quantum weirdness is simply the fact that it seems weird to us that a wave theory applies to quanta, but since wave theories are classical, we should not say that it's weird because there's no classical analog, we should say it's weird because there is a classical analog and we didn't expect that.And that just doesn't make any sense. There is no distinction between "mathematical similarities" and "physical similarities", the mathematical is all we have to understand the physical.

There is a huge difference between mathematical and physical similarities. One can write all sorts of mathematical equations that have no physical significance. The most obvious example of this is the importance of physical dimensions in physical equations ... units are irrelevant in math, while they are of paramount importance in physics. This point is relevant to the current discussion as well, since the physical dimensionality (i.e. relationships between units) of the Schrodinger equation is different from that of a classical wave equation. I already pointed this out in the context that for a classical wave, the energy is proportional to the amplitude, while in quantum mechanics, the energy is proportional to the frequency of the wave. That is not a trivial difference, and in fact is what gives rise to much of the "quantum weirdness" you mentioned.

Ah, the uncertainty principle-- another 'quantum phenomenon' with a perfectly classical analog. Comes up in Fourier transforms of classical fields all the time, thanks for bringing that up.

So what? That wasn't the point of my comment at all .. the point is that in those classical equations, it does not involve the complementarity of momentum and position, which provides the fundamental physical basis for deriving all of quantum mechanics. The correspondence principle ONLY applies in cases where the effects of the uncertainty principle can be ignored due to the large scale of the problem. That will never be true for electrons under any circumstances where electron diffraction can be observed.

 

[Ken G]

On the other hand, for sufficiently high electron fluxes, there likely WILL be a difference in the experimental results, due to space charging effects. That is, the electrons will interact with each other as they pass through the slits, and that will affect the diffraction pattern.

I'm not familiar with the term "space charging", but perhaps you are alluding to a subtle but interesting element of the correspondence principle here. Classically, the interference is in the electric fields, so there's no reference to photons and no need to worry if photons interfere with themselves or with other photons. Quantum mechanically, it seems different, because it seems like if a photon interferes with itself or another photon should be a physically significant fact. But the classical solution obeys a principle of superposition, so what is the principle of superposition a classical analog of if the individual photons think interfering with themselves is something different from interfering with each other?

Although you'll probably resist following this argument once again, perhaps even conclude I'm again embarrassing myself by trying to explain it to you, but here we have yet another perfect example of the correspondence principle and the power of classical analogs. The superposition principle is indeed the classical analog of something quantum, it's just something rather subtle that you might not know about. Photons are indistinguishable, so they don't actually have their own wavefunctions, that's just a kind of heuristic simplification we get away with if we know what we are doing. In a more complete description, there is only one wavefunction for all the photons, and the superposition principle is the classical analog of this fact.

Now, we can certainly break the wavefunction up into what are called "subspaces", and if we do that cleverly (like lumping all the photons with the same energy and spin together), we can pretend that there is one wavefunction for that subclass. But the members of that subclass have a definite phase relationship, so can interfere, and we cannot safely break up the class any further unless we intend on keeping track of that ability to interfere. That fact is just precisely what the superposition of electric fields is the classical analog of. Which gets us to the real question: what significance do you thnk this has, because it certainly doesn't support your point that there are not useful classical analogs here.

I don't think it is irrelevant .. I raised those examples to point out that there are macroscopic systems that rely on quantum effects, which interact on the same scale with macroscopic systems where quantum effects can be ignored.

Since my point is that classical analogs are useful in understanding these systems, you would of course conclude they are not if you don't understand them. So saying they are not merely underscores the point that you don't understand them, it is not a logical argument that there are no such useful classical analogs. I've given the analogs, proved their existence via the correspondence principle, shown how to get them, and explained why they are useful. All you are saying translates to "but I don't understand, so I guess they are not useful after all." Yet they are.

The point being that taking the limit as h-->0 to get the classical limit of a theory doesn't prove anything unless you have confirmation from experiment that the result is correct.

You just keep confirming that you don't understand the correspondence principle. That's all this post is-- recurrent repetition of all the different ways you don't understand that principle. I'm not sure I can explain it any better than I have, it may require someone else, or else you can just not understand that principle.

I maintain that taking the limit as h-->0 for the case of electron diffraction will give you nonsense, at least in terms of the experimentally observed results .. I have yet to see any evidence from you to the contrary.

Then you haven't been watching very carefully. I already laid out in complete detail exactly how quantum mechanics diffraction calculations spawn a purely classical version of the same theory. Since you didn't get it before, there's little point in repeating, so I'll try a different tack. You seem to think the problem is with a classical theory of electron diffraction, but I presume you have no problem with a classical theory of photon diffraction, yes? So why do you think there's a classical analog to photon diffraction, but not one to electron diffraction? Is there something special about the electron quantum that blocks the correspondence principle for that particle? How about neutrinos, will they be the kind of particle that has a correspondence principle, or the kind that doesn't?

Of course the real answer is that experiment has confirmed many times that all particles we know of satisfy a correspondence principle, so the classical theory we can make for them does in fact agree with experiments in the classical limit.

There is a huge difference between mathematical and physical similarities.

I'm sorry, that's just more nonsense, no relevance to anything being said here.

The correspondence principle ONLY applies in cases where the effects of the uncertainty principle can be ignored due to the large scale of the problem.

One more statement that exposes you just don't understand the correspondence principle. The uncertainty principle is an excellent example of the correspondence principle, if you understand the connection between the bandwidth of any classical wave and its localizability. Are you familiar with the classical notion of a Fourier transform? You might want to start your discovery of the correspondence principle there.

 

[SpectraCat]

I'm not familiar with the term "space charging", but perhaps you are alluding to a subtle but interesting element of the correspondence principle here. Classically, the interference is in the electric fields, so there's no reference to photons and no need to worry if photons interfere with themselves or with other photons. Quantum mechanically, it seems different, because it seems like if a photon interferes with itself or another photon should be a physically significant fact. But the classical solution obeys a principle of superposition, so what is the principle of superposition a classical analog of if the individual photons think interfering with themselves is something different from interfering with each other?

For crying out loud, I have been talking about ELECTRONS .. not photons! You keep confusing the two, and there are huge differences between them. Of course photons don't have a "space charging" effect, but electrons, which is what this whole side discussion is about certainly do. They are like charged particles, so if their density in a region of space is sufficiently high, then the repulsive interactions between them give rise to pertubations of the "trajectories" that are clearly not included in when space charging effects are negligible, as in the one-by-one example.

Although you'll probably resist following this argument once again, perhaps even conclude I'm again embarrassing myself by trying to explain it to you, but here we have yet another perfect example of the correspondence principle and the power of classical analogs. The superposition principle is indeed the classical analog of something quantum, it's just something rather subtle that you might not know about.

The superposition principle is neither classical nor quantum .. it is a mathematical principle that applies for any linear system.

Photons are indistinguishable, so they don't actually have their own wavefunctions, that's just a kind of heuristic simplification we get away with if we know what we are doing. In a more complete description, there is only one wavefunction for all the photons, and the superposition principle is the classical analog of this fact.

I have no idea what you are trying to say with the above ... the superposition principle has nothing to do with photons per se. It applies to the case of photons for the reason that they don't interact. As I explained above, for electrons the superposition principle does not hold if the electrons have non-negligible interactions with other charged particles. If it did, then solving electronic structure problems would be a WHOLE lot easier.

Furthermore, the superposition principle for photons does not seem completely analogous to the correspondence principle. The superposition principle preserves the phases of the waves being added, so that for coherent photons of the same frequency, the classical EM wave obtained through the superposition principle has the same phase as the individual photons that make it up. If they are not coherent, then you get interference effects that cancel some of the amplitude and shift the phase, but the phase is still well-defined. The correspondence principle is all about the "loss" of phase information (and thus coherence) as the scale of a problem becomes macroscopic. It explains why we do not observe phase effects (like interference) for macroscopic objects, although the phase information is still there at the quantum scale of course. So, if the superposition principle for photons is analogous to the correspondence principle, what information about the quantum scale is "lost" as you increase the occupation numbers of the different states of the quantum field (which is how I understand quantum numbers in relation to photons).

Now, we can certainly break the wavefunction up into what are called "subspaces", and if we do that cleverly (like lumping all the photons with the same energy and spin together), we can pretend that there is one wavefunction for that subclass. But the members of that subclass have a definite phase relationship, so can interfere, and we cannot safely break up the class any further unless we intend on keeping track of that ability to interfere. That fact is just precisely what the superposition of electric fields is the classical analog of. Which gets us to the real question: what significance do you thnk this has, because it certainly doesn't support your point that there are not useful classical analogs here.

Since my point is that classical analogs are useful in understanding these systems, you would of course conclude they are not if you don't understand them. So saying they are not merely underscores the point that you don't understand them, it is not a logical argument that there are no such useful classical analogs. I've given the analogs, proved their existence via the correspondence principle, shown how to get them, and explained why they are useful. All you are saying translates to "but I don't understand, so I guess they are not useful after all." Yet they are.

You have not done any such thing .. the discussion we have been having is for ELECTRONS, not photons. As you say, classical descriptions of electromagnetic radiation can be obtained by simply adding up photons according to the superposition principle. I am not an expert on QFT, but as I understand it, this works because photons are massless particles, and the quantum fields describing them are highly analogous to the classical descriptions of EM radiation.

None of that applies to diffraction of massive particles like electrons, which is what spawned this entire side discussion. As I said at the beginning, there is no classical theory that predicts the diffraction of electrons. You tried to argue to the contrary, but have not yet responded to my detailed criticisms of your argument, nor provided the equation that you say can be generated to describe that phenomenon based on "wave-mechanics".

You just keep confirming that you don't understand the correspondence principle. That's all this post is-- recurrent repetition of all the different ways you don't understand that principle. I'm not sure I can explain it any better than I have, it may require someone else, or else you can just not understand that principle.

I certainly understand the correspondence principle as it applies to non-relativistic quantum mechanics. I have demonstrated that throughout my comments on this thread. You have not yet supplied a convincing argument why any of my points are incorrect.

Then you haven't been watching very carefully. I already laid out in complete detail exactly how quantum mechanics diffraction calculations spawn a purely classical version of the same theory.

Not really complete detail, since you never supplied the equation that I asked for. I tried to write it from your description, but I couldn't see how to do it in the few minutes I had to devote to trying. Therefore I asked you to do it .. for whatever reason, you seem unwilling to do it. Therefore I can only conclude for now that my initial suspicion was correct, and the equation cannot be written.

Since you didn't get it before, there's little point in repeating, so I'll try a different tack. You seem to think the problem is with a classical theory of electron diffraction, but I presume you have no problem with a classical theory of photon diffraction, yes? So why do you think there's a classical analog to photon diffraction, but not one to electron diffraction? Is there something special about the electron quantum that blocks the correspondence principle for that particle?

Yes, as I have explained multiple times in detail on this thread. The correspondence principle ONLY applies in the limit where quantum uncertainty can be ignored, since electron diffraction occurs purely due to quantum uncertainty effects, it cannot be explained by any theory in the classical limit obtained according to the uncertainty principle.

As you say, that works differently for photons, and the classical description of EM-radiation turns out to predict the same results as the full quantum treatment .. I already admitted I don't completely understand why that is true, but I do know that the fact that photons have no rest mass is essential to the explanation.

How about neutrinos, will they be the kind of particle that has a correspondence principle, or the kind that doesn't?

Good question .. I believe the question of the neutrino mass is an open area of research. If they are massless, then I would expect

Of course the real answer is that experiment has confirmed many times that all particles we know of satisfy a correspondence principle, so the classical theory we can make for them does in fact agree with experiments in the classical limit.

I'm sorry, that's just more nonsense, no relevance to anything being said here.

Well, I find your comment above nonsensical (as well as somewhat insulting), because I took care to explain precisely why my comment was relevant to the context of this thread, relating it specifically to the difference in dimensionality of the Schrodinger equation and the classical wave equation. As I already commented in another post, it appears that you don't really understand the Schrodinger equation very well, at least in terms of how it relates to classical wave equations.

One more statement that exposes you just don't understand the correspondence principle. The uncertainty principle is an excellent example of the correspondence principle, if you understand the connection between the bandwidth of any classical wave and its localizability. Are you familiar with the classical notion of a Fourier transform? You might want to start your discovery of the correspondence principle there.

Wow .. now the Heisenberg uncertainty principle is an example of the correspondence principle? And you say *I* am not understanding the physics in this thread? That is really putting the cart before the horse. The correspondence principle can be perhaps be rationalized in terms of *one of* the limiting behaviors of the Fourier transform relationship between position and momentum in QM, in that in the limit of highly localized particles, the momentum distribution gets very broad, and thus includes high-k momentum states that are close to classical description. In addition, the interference between the various momentum states washes out the spatial coherence of the position-representation of the wavefunction, so it can appear highly localized in position space. However, if the mass of the quantum system is small, a highly-localized wavepacket will not remain localized, but will rapidly spread as the wavepacket propagates. Thus the full realization of the correspondence principle (i.e. localization of the particle over essentially infinite timespans) requires high mass, as well as the broad momentum distribution.

Anyway, the point is that the HUP is much more fundamental to quantum mechanics than being "just and example of the correspondence principle", as you seem to be suggesting. Furthermore, this point applies directly to the example we have been discussing, since I have pointed out several times (and you have not addressed or acknowledged), that the correspondence principle does not apply for electron diffraction, because the phenomenon requires NOT being in the limit of high quantum numbers where the quantum wavelength of the electrons can be ignored.

 

[Ken G]

For crying out loud, I have been talking about ELECTRONS .. not photons! You keep confusing the two, and there are huge differences between them.

Yes, there are interparticle forces that could affect the classical limit, but I'm talking about the simplest classical limit for understanding diffraction. One might call it "collisionless hydrodynamics". So the hydro of ideal gases. That's the "classical limit" I'm referring to for the purpose of applying the correspondence principle. It's really plasma physics, so it's an idealization because there are lots of additional plasma modes there too, all of which are classical limits of electron-proton quantum interactions. But for the purposes of analyzing the classical limit of two-slit diffraction, we can ignore those complications. Fortunately the "classical limit" sets in long before the densities get high enough to have to worry about DeBye screening and so forth, but perhaps it would be best to imagine the two-slit diffraction pattern of a quantum that was just like an electron but had no charge, and let me ask you this: do you think a particle just like an electron but with no charge has the same, or different, quantum mechanical description of its diffraction? There's not much point in the rest until you can answer this one. But this needs clarification:

Wow .. now the Heisenberg uncertainty principle is an example of the correspondence principle?

My point about the HUP was that the HUP, which is a quantum principle, has a very clear classical analog, via the correspondence principle. Now do you think that's true, or don't you?

So let's just concentrate on these three questions, they are the heart of the matter:
1) Do you think an electron with no charge would show the same diffraction pattern? Yes or no please.
2) Do you think the HUP has an obvious classical analog, which can be used to understand the HUP better, and the existence of this analog is a perfect example of the correspondence principle? Yes or no please.
3) Do you think this statement is true: "every quantum theory can be used to spawn a classical theory, that invokes only classical concepts (including no quanta), that will always work in the classical limit of large occupation numbers of that same quantum system, and what's more, it is essentially an accident of history as to which was discovered first, the quantum theory or it's classical analog." Yes or no please.

Answer these, then we can see whether what I've been saying makes sense. In contrast, what does not make sense is this statement you just made:

The correspondence principle ONLY applies in the limit where quantum uncertainty can be ignored, since electron diffraction occurs purely due to quantum uncertainty effects, it cannot be explained by any theory in the classical limit obtained according to the uncertainty principle.

Translation: you don't understand the correspondence principle. Just think about photons, please-- unless you believe that electron diffraction occurs purely due to quantum photon uncertainty effects, but photon diffraction occurs for some other reason! Is that what you think? (I guess that's question #4. You can't learn the correspondence principle without answering those 4 questions.)

One more thing: here is another false statement by you about the correspondence principle:

The correspondence principle is all about the "loss" of phase information (and thus coherence) as the scale of a problem becomes macroscopic.

I regret having to be the one to tell you, but that's just completely wrong. The correspondence principle applies perfectly well in situations where phase is preserved, all it requires is large occupation numbers. Look it up. Think about a laser beam, and Maxwell's equations.

 

[yoron]

The correspondence principle makes sense if we define our macroscopic reality as hinging on what we see from a QM perspective.

Assuming that we have a causality chain that not only develop in the arrow of time but also according to some principle of limits of observation. To create such an assumption it seems as we introduce a question of 'size' too? As if we had something not only defining a classical causality, but also at 'some right angle of thought' :) also somehow related to its 'absolute size, or relative might be a better word?', as defined from where we observe macroscopically.

A little like this perhaps, when we come down to QM the chunks of 'time' we measure shrinks, not only the 'sizes' of whatever we measure, but also its duration? So 'size' and 'time'?

You made me look it up Ken, and it was interesting reading.

"According to the selection rule interpretation, Bohr's correspondence principle is best understood as the statement that each allowed quantum transition between stationary states corresponds to one harmonic component of the classical motion. More precisely, Bohr's selection rule states that the transition from a stationary state n′ to another stationary state n″ is allowed if and only if there exists a τth harmonic in the classical motion of the electron in the initial stationary state; if there is no τth harmonic in the classical motion, then transitions between stationary states whose separation is τ are not allowed quantum mechanically. "

But Bohr seems to have changed his definition from where he first defined it as "the radiation is not a result of the accelerated motion of the electron in its orbit, but rather of the electron jumping from one stationary state to another; and rather than giving off all of the harmonic “overtones” together, only a single frequency, ν, is emitted, and the value of that frequency is given by the Bohr-Einstein frequency condition. The spectral lines are built up by a whole ensemble of atoms undergoing transitions between different stationary states, and these spectral lines, though they exhibit a pattern of regularity, are not evenly spaced—except in the limit of large quantum numbers."

This is a very interesting definition to me, especially as we seem to have defined a 'shape' to that 'electron' as perfectly round. That statement, as compared to the idea of a electron as representing a probability function until measured, collides in my head. Myself I still see it that way. and when it comes to 'classical limits', it always makes me think of the way we classically define a 'perfect space' as being null and 'empty'. Which is a correct statement to me macroscopically.

Classical limits is another definition I will need to look up.
Bohr's Correspondence Principle. I will have to reread it a couple of times, for sure :)