Is quantum theory really necessary?

[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.