Is quantum theory really necessary?

[Andy Resnick]

I'm not sure what you mean by "Formally you could have say that classical liquids involve CM via exact hierarchy equations such as the BBGKY one...".

Continuum mechanics essentially begins with Cauchy's laws, along with the specification of jump conditions on boundaries. The origin of viscosity (or any constitutive relation, for that matter) is an open part of continuum mechanics, but if a constitutive relationship is given, mechanical behavior is completely specified by the governing equations.

My comment regarding velocity in QM comes from Page 4 of Landau/Lif****z, vol. 3. My goal is simply to illustrate that while some results are better explained via QM, some concepts are more straightforward in CM.

Or am I misunderstanding your point?

 

[Maaneli]

No, I don't actually buy that, because I've seen that already.

You've seen this paper already? What exactly do you object to about the claims of that paper? Are you denying now that even the simple barrier tunneling phenomena can be obtained in SED?

Stochastic electrodynamics have been used to give the "classical" description of the photoelectric effect as well.

I don't think that's true. You're probably thinking of the Lamb/Scully paper in which they derive the photoelectric effect for classical EM plane waves impinging on quantized matter. That's different from SED. The latter treats the entire system in a classical stochastic way.

The PROBLEM with this description is that it can only go so far, i.e. explain the ROUGH, naive phenomenon, but never, ever, the details. I've mentioned this already that while the photoelectric effect can be explained by it, a more intricate phenomenon such as angle-resolved photoemission, resonant photoemission, and multiphoton photoemission have never been described using stochastic electrodynamics. No attempt has even been made to use that formulation to describe those phenomena. So given that fact, which one would you prefer to use - the one that can only explain the simplest version of the family of phenomena, or the one that can explain all of them without exception?

Hmm you seem to be conflating a number of different issues. Also, I'm not sure what you mean that SED can only give the "ROUGH", "naive" phenomenon. Do you consider the following to be "rough", "naive" phenomena?:

Empirical agreements with predictions between SED versus QM and QED for linear systems such as for (1) calculations of ensemble averages of free electromagnetic fields, (2) systems of electric dipole simple harmonic oscillators (SHO), including the complicated situation of van der Waals at any distance, (3) all experimentally known Casimir/van der Waals type situations, (4) diamagnetism, (5) the retarded van der Waals forces between electric dipole oscillators at temperatures T = 0, (6) the repulsive Casimir-type force prediction between a perfectly conducting plate and an infinitely permeable plate, (7) the Unruh-Davies effect, and perhaps even more such phenomena?

Now, it is true that SED has not been applied to the more complicated condensed matter phenomena; and since that is the case, in fairness we simply cannot say for sure if it will work or not. No question it would be a very difficult (perhaps impossibly difficult), nonlinear problem though. Indeed I think that is why it has not been applied to these various emission processes - because at the moment it can only give the QM ground state for hydrogen, in lengthy numerical simulations! Don't bother to jump on this point though, as I never claimed that it can or should be able to do the things you suggest.

What could very well work for all those emission phenomena, is a semiclassical theory in which the classical ZPF of SED is used to replaced the second quantized ZPF of QED, or even replace the classical self-field effects of charged matter that is first or second quantized. The reasons to expect this is plausible is that the SED ZPF shares all the same statistical properties (N-point correlation functions) as the QED ZPF, when the latter is obtained from symmetric ordering of field operators in the Heisenberg operator equations of motion. Indeed Marshall and Franca have already shown that the classical ZPF gives the correct excited state decay rates for quantized matter. Also, Barut has shown that the classical self-fields of charged first or second quantized matter can also be used to replace the QED ZPF, and still account for all known QED effects in low orders of perturbations, including the photoelectric effect. So, given the fluctuation-dissipation theorem that applies to dissipative forces and stochastic noise, I would expect that replacing the classical self-fields with the classical SED ZPF, but keeping the matter first or second quantized, should be sufficient to explain the condensed matter phenomena you propose as a challenge. Though to my knowledge, no one has done much with this.

The same can be said with tunneling. Would you like to see if stochastic electrodynamics can actually get the density of states of the single-particle spectrum of a superconductor, or the phonon modes from the second derivative of the I-V curve?

As a matter of principle, it would be interesting to see how far this classical ZPF induced tunneling effect can be taken, even if it is a mathematically difficult problem. After all, I bet at one time no one thought that SED could correctly derive any of the phenomena on the list above that I presented.

 

[ZapperZ]

You've seen this paper already? What exactly do you object to about the claims of that paper? Are you denying now that even the simple barrier tunneling phenomena can be obtained in SED?

No, I have seen the argument being given that SED seems to be able to describe quantum tunneling.

I don't think that's true. You're probably thinking of the Lamb/Scully paper in which they derive the photoelectric effect for classical EM plane waves impinging on quantized matter. That's different from SED. The latter treats the entire system in a classical stochastic way.

Only if there's something different between that and "Stochastic Optics" the way Marshall and Santos did it. If they are different, then I meant the latter.

Hmm you seem to be conflating a number of different issues. Also, I'm not sure what you mean that SED can only give the "ROUGH", "naive" phenomenon. Do you consider the following to be "rough", "naive" phenomena?:

I am characterizing "photoelectric effect" as the "rough, naive phenomenon". It is "rough and naive" because

(i) it ignores the band structure effects of the material
(ii) it simplifies any matrix element effects of the coupling of the photons to the initial and final state of the photoemission process
(iii) it ignores the details of the energy and momentum spread of the emitted photoelectrons.

None of these things are in the standard photoelectric effect experiment. Yet, they are part of the detailed treatment of the photoemission phenomenon. That is why I characterized the standard photoelectric effect as "rough and naive". It is similar to estimating a cow to be a sphere. From very far, you can do many different types of approximation and arrive at almost the same answer. It is only when you get very close and look at the details that you can distinguish which is the more accurate description of it. I would think that, after so many years of proclaiming that there is another "alternative" to describing the photoelectric effect, they would have proceeded to the next step and gone beyond doing the spherical cow approximation and try to match the details already.

And that's also something I would expect out of any alternative tunneling description. Can it be used to derive, let's say, the Fowler-Nordheim model?

Zz.

 

[Maaneli]

Though I should add that I would expect significant modifications to SED to be made, if it is ever capable of handling condensed matter physics, let alone energy quantization in general.

 

[Maaneli]

I would think that, after so many years of proclaiming that there is another "alternative" to describing the photoelectric effect, they would have proceeded to the next step and gone beyond doing the spherical cow approximation and try to match the details already. And that's also something I would expect out of any alternative tunneling description.

Zz.

This is a fair point. But please keep in mind the technical difficulty of taking these logical next steps, and the time and man power devoted to advancing these alternative models is quite small, and so it is not too surprising that it has not been done yet!

 

[ZapperZ]

This is a fair point. But please keep in mind the technical difficulty of taking these logical next steps, and the time and man power devoted to advancing these alternative models is quite small, and so it is not too surprising that it has not been done yet!

Fair enough. However, because of that incompleteness, it cannot be considered as an "alternative". The Bohr model is not an "alternative" to the full QM treatment of an atom just because it has managed to match the most naive model of an atom but not the more complete, non-simplified observations, i.e. the details. That is the main point of my argument that I'm trying to get across.

Zz.

 

[Maaneli]

Fair enough. However, because of that incompleteness, it cannot be considered as an "alternative". The Bohr model is not an "alternative" to the full QM treatment of an atom just because it has managed to match the most naive model of an atom but not the more complete, non-simplified observations, i.e. the details. That is the main point of my argument that I'm trying to get across.

Zz.

I agree with you here. But I didn't claim it was a true alternative to the full QM treatment of an atom. That was David Lush's claim, I think.

 

[DaveLush]

I have to say, I regret naming this thread as I did. I only meant to start a discussion about whether QM is certainly a fundamental theory, or not.

I can only agree that currently and for the foreseeable future there is no alternative to QM in many regimes, essentially all regimes where it applies.

Personally I do hope to see quantum theory put on a firmer basis than the Heisenberg uncertainty principle. Seems to me it's a leap of faith to go from saying that one can't measure the position and velocity of a particle simultaneously to saying that they cannot even be simultaneously defined. I view the HUP as a contrived justification for how matrix mechanics is structured. Historically, the HUP was developed (published in 1927) after matrix mechanics (published in 1925), which is consistent with it being a justification of matrix mechanics after the fact. Matrix mechanics is a nice theory that explains a lot of things, and it is inherent in it that position and velocity are not simultaneously defined with arbitrary position. So fine, but that is no justification for claiming that there is a universal fact that they don't simultaneously exist.

If there is a more prosaic reason why quantum mechanics is true, I would like to know it.

 

[ZapperZ]

I have to say, I regret naming this thread as I did. I only meant to start a discussion about whether QM is certainly a fundamental theory, or not.

I can only agree that currently and for the foreseeable future there is no alternative to QM in many regimes, essentially all regimes where it applies.

Personally I do hope to see quantum theory put on a firmer basis than the Heisenberg uncertainty principle. Seems to me it's a leap of faith to go from saying that one can't measure the position and velocity of a particle simultaneously to saying that they cannot even be simultaneously defined. I view the HUP as a contrived justification for how matrix mechanics is structured. Historically, the HUP was developed (published in 1927) after matrix mechanics (published in 1925), which is consistent with it being a justification of matrix mechanics after the fact. Matrix mechanics is a nice theory that explains a lot of things, and it is inherent in it that position and velocity are not simultaneously defined with arbitrary position. So fine, but that is no justification for claiming that there is a universal fact that they don't simultaneously exist.

If there is a more prosaic reason why quantum mechanics is true, I would like to know it.

I must say that I don't quite understand the logic of your objection. It seems to rest entirely on something that is not fundamental - the HUP. It may surprise you that the HUP is merely a consequence, not the cause. You may want to look, instead, to the basic postulates of QM and what is some time known as the "First Quantization" of QM. It deals with the commutating relations between two observables.

Secondly, and I believe I've mentioned this already (maybe not in this thread), there's nothing to prevent you from making a measurement the position, and a measurement of the momentum, of a single particle with arbitrary accuracy that is limited to the technology at hand. It is just that after you make that first measurement, how well you can predict the 2nd measurement depends on the accuracy of your first measurement. Refer to the example I gave for a single-slit diffraction. You can't say something to the effect that you don't like HUP, when the most blatant example of HUP at work is the single slit diffraction that is staring at you right in the face. Physics cannot be falsified based simply on a matter of tastes!

Zz.

 

[Zacku]

I'm not sure what you mean by "Formally you could have say that classical liquids involve CM via exact hierarchy equations such as the BBGKY one...".

Continuum mechanics essentially begins with Cauchy's laws, along with the specification of jump conditions on boundaries. The origin of viscosity (or any constitutive relation, for that matter) is an open part of continuum mechanics, but if a constitutive relationship is given, mechanical behavior is completely specified by the governing equations.

Actually I am a lot more used to statistical mechanics than continuum mechanics. And I have seen many derivations of continuum mechanics equations or principles from CM first principles using statistical mechanics, that's what I meant.
It is, for example, well known that Navier-Stockes equation in hydrodynamics is only an order one solution of the Boltzmann equation in the time relaxation approximation. And there exist many ways to derivate more or less rigorously this Boltzmann equation from CM (through the BBGKY hierarchy for example).
I was just saying that the same approaches exist in QM and give the same results in the classical limit.

My comment regarding velocity in QM comes from Page 4 of Landau/Lif****z, vol. 3.

I admit that I'm not very familiar with the concept of velocity in QM (except in solid physics) so I will trust you for this part.

My goal is simply to illustrate that while some results are better explained via QM, some concepts are more straightforward in CM.

I totally agree with that. But, according to me, there is a big difference between a better suited model and a wrong one.
I argue that the best classical model that you can do in CM will have the best form for the action (or lagrangian description) to describe the phenomenon you want to explain. As I remember, this Lagrangian approach is very used, even in continuum mechanics.
If you are not in the CM range of validity (comparison between \hbar and the order of magnitude of the action,say) then, this best classical model is wrong and you have to add QM corrections that are, in principle, measurable.

Or am I misunderstanding your point?

I don't know...

P.S : excuse me for the english.

 

[Andy Resnick]

Actually I am a lot more used to statistical mechanics than continuum mechanics. And I have seen many derivations of continuum mechanics equations or principles from CM first principles using statistical mechanics, that's what I meant.
It is, for example, well known that Navier-Stockes equation in hydrodynamics is only an order one solution of the Boltzmann equation in the time relaxation approximation. And there exist many ways to derivate more or less rigorously this Boltzmann equation from CM (through the BBGKY hierarchy for example).
I was just saying that the same approaches exist in QM and give the same results in the classical limit.

I admit that I'm not very familiar with the concept of velocity in QM (except in solid physics) so I will trust you for this part.

I totally agree with that. But, according to me, there is a big difference between a better suited model and a wrong one.
I argue that the best classical model that you can do in CM will have the best form for the action (or lagrangian description) to describe the phenomenon you want to explain. As I remember, this Lagrangian approach is very used, even in continuum mechanics.
If you are not in the CM range of validity (comparison between \hbar and the order of magnitude of the action,say) then, this best classical model is wrong and you have to add QM corrections that are, in principle, measurable.

I don't know...

P.S : excuse me for the english.

I think we are in agreement, for the most part. My quibble is your comment that "the best classical model that you can do in CM will have the best form for the action (or lagrangian description) to describe the phenomenon you want to explain". There are many phenomena for which there is no Lagrangian (or Hamiltonian), becasue of dissipative processes. Sure, certain terms can be put in by hand, but that's different from having a first-principles derivation.

Sure, CM has limited validity- as does QM. CM at least extends to general relativity.

 

[reilly]

Once again: QM is absolutely necessary, and will be until an alternative theory yields all QM results -- hydrogen to Higgs; crystal lattice structure to superconductivity; radioactive decay to transistors, and on and on. QM might be likened to a tapestry or mosaic; getting a few squares right amounts to a curiosity and nothing more. To compete with QM requires a huge number of results identical to QM, and, to date, there's no alternative theory that can even begin to compete.
Regards,
Reilly Atkinson

 

[Zacku]

I think we are in agreement, for the most part.

So do I.

My quibble is your comment that "the best classical model that you can do in CM will have the best form for the action (or lagrangian description) to describe the phenomenon you want to explain". There are many phenomena for which there is no Lagrangian (or Hamiltonian), becasue of dissipative processes. Sure, certain terms can be put in by hand, but that's different from having a first-principles derivation.

I thought you would say something like that, which is right but depends fundamentally of the scale of study.
Indeed, continuum mechanics is a coarse grained mechanics and you can't solve the problem at this scale with only CM first principles based on the long range (fundamental) interactions known in CM. You have to add effective interactions such as dissipation in the medium to match what is observed. These interactions, I agree, aren't well discribed in a variational approach and, practically, this is not the best way to treat a problem in continuum mechanics. BUT I think that we also agree about this point because, microscopically, these effective interactions come from an average of a more fundamental interparticle interactions that can be discribed by a variational approach.

That said, it is obvious that a lot of macroscopic phenomena only explained, for the moment, by QM theory can also be explained in CM by an appropriate effective modeling that will take into account, partly, quantum effects as one can see in the debate above about SED.
Nevetheless, I want to underline the fact that this "explanation" is on the one hand due to the QM (theory) whose effects are valid whatever the phenomenon under study and on the other hand due to an effective modeling that can only (a priori) handle the phenomenon for which it has be made and nothing else, which is a big difference, in my opinion.

CM at least extends to general relativity.

That's true but I hope it's only a matter of time...

 

[Andy Resnick]

<snip>

I thought you would say something like that, which is right but depends fundamentally of the scale of study.
Indeed, continuum mechanics is a coarse grained mechanics and you can't solve the problem at this scale with only CM first principles based on the long range (fundamental) interactions known in CM. You have to add effective interactions such as dissipation in the medium to match what is observed. These interactions, I agree, aren't well discribed in a variational approach and, practically, this is not the best way to treat a problem in continuum mechanics. BUT I think that we also agree about this point because, microscopically, these effective interactions come from an average of a more fundamental interparticle interactions that can be discribed by a variational approach.

<snip>

Can I just say it's a breath of fresh air to speak with someone else who understands a balanced approach to science?

Maybe it's becasue I just read Lee Smolin's "Trouble with Physics" book, but lately I have very little patience for scientists (not just physicists) who frenetically generate data and papers containing insipid results, all the while claiming (without presenting evidence) that longstanding unsolved problems have been trivially solved some time ago.

 

[Zacku]

Can I just say it's a breath of fresh air to speak with someone else who understands a balanced approach to science?

You're welcome .

Maybe it's becasue I just read Lee Smolin's "Trouble with Physics" book

I have to read this book too.

 

[reilly]

That said, it is obvious that a lot of macroscopic phenomena only explained, for the moment, by QM theory can also be explained in CM by an appropriate effective modeling that will take into account, partly, quantum effects as one can see in the debate above about SED.

Please show us "an appropriate effective model"

Regards,
Reilly Atkinson

 

[Zacku]

Please show us "an appropriate effective model"

Regards,
Reilly Atkinson

I haven't one and I'm not very interested in this kind of work actually.
I just wanted to point out the fact that it was not impossible, in principle (I somehow exaggerated with the word "obviously"), to make effective models that take into account quantum effects for specific cases.

However, the term "appropriate" is quite subjective, according to me, and even the Bohr model can be appropriate in some extent to explain hydrogen spectroscopy although it is fundamentally wrong in term of microscopic understanding.