Questions About Quantum Theory: What's Wrong?

[reilly]

caribou -- At the risk of sounding simple minded, I can't understand how you could ever accomplish your B scenario. I would be most appreciative if you could help me with whatever I'm missing. Thanks.

Regards,
Reilly Atkinson

 

[caribou]

At the risk of sounding simple minded, I can't understand how you could ever accomplish your B scenario. I would be most appreciative if you could help me with whatever I'm missing.

Think of it as a thought experiment involving an isolated two slit set-up isolated from the rest of the universe much like Schrodinger's cat. I wouldn't bother too much about trying to understand it from my apparently poor attempts at explanation, though, and suggest if you are really interested the books and papers of Robert Griffiths and Roland Omnes instead.

 

[Eye_in_the_Sky]

Previously, I wrote:

Yes, the notion of "collapse" can be applied to classical scenarios. However, in order to put quantum "collapse" on equivalent 'footing', one must be prepared to accept as true the physical existence of "hidden variables".

____________

I have no clue why I would need hidden variables to suggest a correspondence between classical and quantum notions of probability.

In a classical collapse scenario, information is gained, and so, the probability distribution "collapses". Nevertheless, the physical status of the system in question remains unchanged.

Consider, for example, a "blob" in phase space corresponding to a probability distribution representing the possible states of some mechanical system (or, more accurately, an idealization thereof). It happens that the physical state of the system is represented by a single point within the "blob". If we should gain more information about the system, the "blob" will then "collapse". But there is no corresponding change in the physical state of the system – that state is just the same phase-space point that it was before. Thus, relative to the "blob" the actual phase-point plays the role of a "hidden variable".

Now, in Quantum Mechanics, if one says there are no "hidden variables", then one is saying not only that the state vector |ψ> contains in it probability information, but also that |ψ> itself is the physical state of the system (or, more precisely, one is saying that |ψ> is in one-to-one correspondence with the physical state (or, at least, it is in one-to-one correspondence with those aspects of the physical state relevant to kinds of measurements we customarily perform)). This means that whenever |ψ> "collapses", the system in question itself undergoes a corresponding physical change. This is not the same kind of passive information gain to be found in classical "collapse" scenarios.

 

[elas]

The problem is not with QT but with the SM interpretation which experts admit is an "incomplete guess" I am trying to get a debate going on interpretation on:

http://www.multimedia.com.ro/webforum/viewtopic.php?p=102#102

 

[misogynisticfeminist]

The problem is not with QT but with the SM interpretation which experts admit is an "incomplete guess" I am trying to get a debate going on interpretation on:

http://www.multimedia.com.ro/webforum/viewtopic.php?p=102#102

your post is unscientific by saying "expert". Its easy as to how laypeople say that "experts" say that this is so and so, therefore it is so. The copenhagen interepretation may not be as philosophically appealing as more deterministic and causal interpretations but if you see, what are the viable alternatives to copenhagen? The closest i would think is Bohmian Mechanics but does BM have the power to solve a large number of physical situations as standard interpretations of QM?

Give me an interpretation which solves real-life problems and does not exhibit the indeterminism as Copenhagen and maybe I'll bite.

I'm with zapperz on this one and I go with the validity of the copenhagen interpretation. "commonsense is the set of prejudices collected by age 18".

 

[reilly]

Previously, I wrote:____________In a classical collapse scenario, information is gained, and so, the probability distribution "collapses". Nevertheless, the physical status of the system in question remains unchanged.

Consider, for example, a "blob" in phase space corresponding to a probability distribution representing the possible states of some mechanical system (or, more accurately, an idealization thereof). It happens that the physical state of the system is represented by a single point within the "blob". If we should gain more information about the system, the "blob" will then "collapse". But there is no corresponding change in the physical state of the system – that state is just the same phase-space point that it was before. Thus, relative to the "blob" the actual phase-point plays the role of a "hidden variable".

Now, in Quantum Mechanics, if one says there are no "hidden variables", then one is saying not only that the state vector |ψ> contains in it probability information, but also that |ψ> itself is the physical state of the system (or, more precisely, one is saying that |ψ> is in one-to-one correspondence with the physical state (or, at least, it is in one-to-one correspondence with those aspects of the physical state relevant to kinds of measurements we customarily perform)). This means that whenever |ψ> "collapses", the system in question itself undergoes a corresponding physical change. This is not the same kind of passive information gain to be found in classical "collapse" scenarios.

Of course, many measurements, particularlry on large systems induce small changes in the system. But that's not the issue. The issue is "before and after", whether a coin toss, winning in poker, ascertaining the temperature of bath water -- who wants to injure their child with water that is too hot. While you may have some notions about the temp of the bath water -- a Baysean situation -- you don't know until you measure -- with your hand or foot, or with a thermometer. The water temp does not change, but your head does -- you go from "I don't know" to "I know". That's collapse.

QM state vectors,based on the famous complete set of measurements, apart from a phase factor, by definition, give a complete description of the system at hand. That's as basic as it gets. Two issues: does the measurement change the measured system, and what happens to your head? Yes it certainly can, think of scattering experiments. Just like the classical situation, your state of knowledge changes -- see the work of Nobelist Sir Rudolf Peierls on a knowledge based interp. of QM.

I still don't get the need for hidden variables -- in my dissertation I used QED to compute radiative corrections for various electron-nucleon scattering experiments, which helped map out the electromagnetic structure of nucleons. Should I be worried that I didn't use hidden variables? Are the nucleon form factors in danger? And, all the time I thought the most difficult problem we faced was how to deal with deuteron structure in relativistic terms -- deuterons being, then, the best choice for neutrons as targets.

Regards,
Reilly Atkinson

 

[reilly]

caribou -- Why bring up something which you do not understand well enough to explain? I do not feel that it is my responsibility to go dig out something which you bring up, discuss, and then bail out. I won't be so bold as to tell you what to do, but I'm sure that you can figure that out. Sorry to be more frank than I usually am.
Regards,
Reilly Atkinson

 

[reilly]

Bye.
Reilly

 

[elas]

your post is unscientific by saying "expert". Its easy as to how laypeople say that "experts" say that this is so and so, therefore it is so.

The quotes are in the introduction,the experts are listed in the footnotes. (such as Baggott, Veltman, and Griffiths). I do not say "therefore it is so", I state my reasons for saying it's time for a debate on the need for change.

 

[misogynisticfeminist]

your post is unscientific by saying "expert". Its easy as to how laypeople say that "experts" say that this is so and so, therefore it is so.

The quotes are in the introduction,the experts are listed in the footnotes. (such as Baggott, Veltman, and Griffiths). I do not say "therefore it is so", I state my reasons for saying it's time for a debate on the need for change.

There are in fact many experts who are not satisfied with the copenhagen interpretation of QM, and quite a number of them are great physicists (philosophers don't count). Examples are John Bell and Penrose. It could be due to my ignorance, but I do not know as yet, any theory which could rival the predicting power of standard copenhagen.

True, certain great physicists right now and in the past might not agree with copenhagen and rest assured that there is certain debate going on within established physicists in the physics community. But eminent physicists disagreeing with copenhagen is not a call for us to ditch it.

We ditch copenhagen only if we find another interepretation which can predict physical phenomena as well, if not better than copenhagen and (hopefully) consistent with other theories (GR, for example).

 

[caribou]

caribou -- Why bring up something which you do not understand well enough to explain? I do not feel that it is my responsibility to go dig out something which you bring up, discuss, and then bail out. I won't be so bold as to tell you what to do, but I'm sure that you can figure that out. Sorry to be more frank than I usually am.

Well, I do not feel that it is my responsibility to go spend hours and hours trying to compress conceptually difficult information from many books and papers into a few posts in a forum for free, particularly as what I had already written was dismissed by people who quite plainly don't understand what they're dismissing.

To quote Robert Griffiths on consistent/decoherent histories on page 368 in the final chapter of his book Consistent Quantum Theory, he sums up what I said about quantum theory in this way:

The principle of unicity does not hold: there is not a unique exhaustive description of a physical system or a physical process. Instead, reality is such that it can be described in various alternative, incompatible ways, using descriptions which cannot be combined or compared.

That's what Gell-Mann, Hartle and Omnes also say. It's the modern version of Bohr's principle of complementarity. It's what you end up with if you really try and understand quantum theory. And it applies to the two slit experiment, as well as to a great many other things, like EPR.

A failure of others to understand what they are talking about is no loss to me whatsoever. I also gain nothing from their successful understanding. So, for me, this discussion in this thread is most definitely over.

 

[reilly]

caribou -- You miss the point. For example, I've been paid as a professor and researcher, I've paid when I was a student, and more recently as I run a consulting business I've for free studied, read, written, assimilated, taught, and thought about QM, for 40+ years. If I do not understand something, then I say so at the outset. If people have difficulty in understanding what I say or do, I try hard to correct that, as I think it is my responsibility, as a physicist, to help people understand me. Spending hours pouring over material is a large part of what physics is about.


Griffith, Gell-Mann et all do not have the truth and the light all to themselves. By suggesting that if you truly try to understand QM, you end up in their camp, is to do great disservice to many hard working, brilliant physicists who think otherwise. What you say is simply not so, brilliant as Griffith, and Gell-Man are. Over the years I've been privileged to hear V. Fock (of Fock Space) and Norbert Weiner, Victor Weisskopf, Robert Oppenheimer, Fritz Rohrlich, J.H. Van Vleck, Wigner and Felix Bloch discuss the interpretation of QM. Copenhagen worked for most of these gentlemen, and I what I mean by Copenhagen is pretty much based on Born's notion of probability -- nothing fancy, just practical. Wigner talked about a more knowledge based interpretation, an idea taken further by Sir Rudolf Peierls, and one to which I subscribe. And, of course, they are of the generation that made QM work. I will simply say, that one's appreciation of the nuances and difficulties of interpreting QM are greatly enhanced by reading the masters, as many as possible, and, of course, by doing QM. Born/Copenhagen is alive and well, as are other approaches to QM's interpretation.
Regards,
Reilly Atkinson

 

[caribou]

Okay, I'll say a little more then...

Griffith, Gell-Mann et all do not have the truth and the light all to themselves. By suggesting that if you truly try to understand QM, you end up in their camp, is to do great disservice to many hard working, brilliant physicists who think otherwise.

Then those who think otherwise had better come up with a theory other than quantum mechanics. The reason is that the decoherent histories approach simply follows quantum mechanics to its logical conclusion. It essentially changes nothing about the underlying theory. And as we know, the underlying theory appears to be very good indeed.

Decoherent histories is about dealing directly with the consequences of quantum mechanics and not trying to escape from difficulties by bringing in non-physical ideas like wave function collapse or external observers that cause more problems than they solve.

By following the theory, other interpretations that do so similarly then can't help but be included in decoherent histories. Copenhagen is simply an anthropocentric special case in decoherent histories and many-worlds is just a literal way of interpreting the predictions in decoherent histories. I was amused when we had a survey here in which we could vote for which of the these three "different" interpretations we believed in.

Gell-Mann has been notorious throughout his career for his refusal to say anything in print unless he's absolutely sure and yet he calls the approach "the modern intepretation of quantum mechanics". I think it should be obvious of the reason for that now. It's not about adding another interpretation, it's about ending interpretation in most senses.

So I disagree I am doing "a great disservice to the many hard-working, brilliant physicists" who think otherwise than decoherent histories. I believe there are physicists more brilliant again, and I'd include Bohr as much of his intuition and insights appear to have been correct.

That decoherent histories is about the standard theory and only the standard theory and not yet another half-thought-through addition to it is the reason why I'm studying it with such interest and think it's important.

Anyway, interpretation is obviously of little or no practical importance. I only mention anything in this thread to make people aware of something I find very interesting, so I'm not going to write anything more than something brief on the subject. Like I suggest, people should look it up if they also find it interesting.

 

[reilly]

Caribou -- I'm happy for you, and sad for the rest of us. I guess we've been mistaken all along. I used practical Copenhagen in my Ph.D thesis, perhaps I should redo my work, and my various published papers. Given your expertise, what do you think? I suspect I'm not the only person in the Forum facing such difficulties. Do we need to come up with, as you state it, a theory other than QM in order to save our status as physicists, active or retired? I ask because of your imperative, "no doubt about it" directive to develop this other theory. And all this time I thought I had a reasonable clue about what I was doing. And worse yet, your directive very much invalidates most of the physics from the 1920s until recently. Looks to me like an impending crisis.
Reilly Atkinson

 

[Chronos]

Reilly, you can lead a horse to water, but you can't lead water to a horse.

 

[elas]

QM are greatly enhanced by reading the masters, as many as possible, and, of course, by doing QM. Born/Copenhagen is alive and well, as are other approaches to QM's interpretation.

So what do the masters say, I give a few quotes:

Ordinarily the procedure is to guess a form for the interaction and compare the resulting theoretical calculations with the experimental data.
"Introduction to elementary particles"
David Griffiths

Quantum theory emerges largely unscathed, only serving to reinforce the point that the theory remains the most powerful framework for explaining observations of the quantum world, but its orthodox interpretation continues to offer little in the way of understanding in terms of underlying physical processes. "Beyond measure"
Jim Baggott

There is one truth the reader should be fully aware of. Trying to explain something is a daunting endeavour. You cannot explain the existence of certain particles much as you cannot explain the existence of this universe. In addition, the laws of quantum mechanics are sufficiently different from the laws of Newtonian mechanics which we experience in daily life to cause discomfort when studying them. Physicist usually cross this barrier using mathematics: you understand something if you can compute it. It helps indeed if one is at least capable of computing what happens in all situations. But we cannot assume the reader to be familiar with the mathematical methods of quantum mechanics, so he will have to swallow strange facts without the support of equations.
"Facts and mysteries in elementary particles"
Martinus Veltman

If we separate 'interpretation' from 'theory' then the interpretatation is both incomplete and unsatisfactory, only the theory is satisfactory, or do I misread the masters?

 

[ZapperZ]

Caribou -- I'm happy for you, and sad for the rest of us. I guess we've been mistaken all along. I used practical Copenhagen in my Ph.D thesis, perhaps I should redo my work, and my various published papers. Given your expertise, what do you think? I suspect I'm not the only person in the Forum facing such difficulties. Do we need to come up with, as you state it, a theory other than QM in order to save our status as physicists, active or retired? I ask because of your imperative, "no doubt about it" directive to develop this other theory. And all this time I thought I had a reasonable clue about what I was doing. And worse yet, your directive very much invalidates most of the physics from the 1920s until recently. Looks to me like an impending crisis.
Reilly Atkinson

Reilly,

It's even worse for me. As a lowly experimentalist, all I've been doing throughout my carreer have been to "shut up and calculate". I had very little time, nor inclination to make any philosophical interpretation - mainly because I care more about what I can show physically, and the fact that philosophical interpretation on this has not produced any significant contribution to the advancement of physics.

Zz.

 

[reilly]

elas -- Your list is just a tad short. First, you might consider, Einstein, Bohr, and the Quantum Dilemma, by Andrew Whitaker, goes from Planck to Bohm, Bell, Everett, and Legget. There are those of us, who even consider Bohr, Heisenberg, Schrodinger , and Born, as masters, not to mention Pauli, Weisskopf, Oppeheheimer, Dirac, Wigner, Pais, Peierls, Penrose, Weinberg, Feynman, Schwinger, Tomanaga, Dyson, Landau, Sakarov,and Einstein, and many more. These are all great physicists, hardly parochial figures,who have spent many years thinking about QM. Why some even figured out how to use QM to explain atomic spectra, electrical resistance, magnetism, basic nuclear structure, slow neutron scattering, and, if I'm not mistaken a few more phenomena. Note that they certainly do not agree on everything. (There are superb discussions in Pais' bio of Bohr, Kemble's old QM text -- remarkably sophisticated for the late 1930s and today -- Bohm's QM text.) And, there are quite a few of us who have indeed read at least some of the work of all these masters -- I'll admit that I've read very litle of Sakarov.

By the way, the centuries-old culture of physics , like in many fields, says: start by studying and sometimes emulating the masters: do this to pay your dues, and eventually to find your own voice, and to make your own reasoned and informed judgements. If after that you choose to follow Griffith et al, fine.

Zapper Z -- I commend you for your stoicism in the face of having to choose to care about physical phenomena. I worked with quite a few experimentalists who had the same view, and greatly benefited from that experience. It's a tough life isn't it.

Regards, Reilly Atkinson

 

[dextercioby]

That Sakarov is a relative to Andryi Sakharov,the russian physicist who designed the first russian thermonuclear device...?

Daniel.

 

[ZapperZ]

Reilly,

May I suggest you add another one to your list of distinguished physicists? John Bardeen. If there's anyone who examplifies the "shut up and calculate" practice, it's him. And it gave him the distinction of being the only person to earn 2 Nobel Prizes in physics.

I wrote an essay on him in one of my Journal entry:

[09-01-2004 08:16 AM] - The most influential physicist.

I could have easily titled it as the most overlooked and under-appreciated physicist. Everyone should read his biography written by Hoddeson et al.

Zz.

 

[reilly]

Daniel and Zz -- I'm pretty sure that my "Sakarov" is indeed Sakharov the great. And, funny, after I posted I realized Bardeen should be on the list, as well as Von Neumann. Bardeen rocks dude, as some might say today. But, hey, what do we know?
Regards,
Reilly

 

[dextercioby]

Well,Mr.Atkinson,if u mentioned a mathematician,it would be fair to mention all 3 of them:von Neumann,Wigner and Weyl...

Daniel.

P.S.If u mentioned Sakharov,it would be fair to include Kurchatov,as well,after all,in the Manhattan project,a dozen of brilliant physicists worked and people remember all their names (or at least should),but in the russian version,besides Kurchatov & Sakharov,i really doubt any westerner knows other names...

 

[Crosson]

You cannot explain the existence of certain particles much as you cannot explain the existence of this universe.

In some sense this quote is correct; it is foolish to seek some ultimate explanation that leaves no further questions i.e. to begin we must assume certain things.


Young's classical double slit experiment established without a doubt that light had wave like properties. A generation of physicists grew up accepting this wave nature of light as a fundamental assumption (experimental fact).

Because of physics we now have a reason why light has wave properties. Although this reason requires assumptions of its own, it is quite satisfying.

QM is a great theory, just like the work of Young and Fresnel. But anyone who says the experiments of QM can't be explained any other way, is shortsighted at best.

 

[dextercioby]

QM is a great theory, just like the work of Young and Fresnel. But anyone who says the experiments of QM can't be explained any other way, is shortsighted at best.

Okay,i understand the concept of evolution is science,but WHY would we seek an alternative theory (to QM,to GR,to SM) to account for experimental results which are in agreement with actual (partial,c'est vrai ) theories.

What i understand from your last post is that someone,in order not to be called "shortsighted" should desparately search for a new theory which would account,let's say for the first 11 sign.digits of $g_{el}$...

I cannot follow that logics.We must search for theories which would COMPLEMENT QM (& GR),not replace them...



Daniel.

P.S.Or if they do replace them,at least be able to reproduce the same results obtained by QM & GR at least with the same accuracy.

 

[Stingray]

Okay,i understand the concept of evolution is science,but WHY would we seek an alternative theory (to QM,to GR,to SM) to account for experimental results which are in agreement with actual (partial,c'est vrai ) theories.

Besides being consistent with experiments, any physical theory must also be logically self-consistent (of course, sufficiently elaborate experiments would reveal logical inconsistencies, but that doesn't mean that they are practical to perform). Ignoring everything else that has been discussed in this thread, it is clear that QM has serious difficulties when applied to spacetime geometry. This is a logical inconsistency in the theory that must be resolved despite the lack of any current experimental problems.

I also wonder how exactly the Copenhagen interpretation would be applied to quantum geometry... How would quantum cosmology work? As far as I know, these sorts of issues are what prompted Gell-Mann and Hartle to develop their ideas. Unfortunately, I haven't ever gotten around to reading their papers.

From another point of view, finding alternative formulations of the same theory has been useful historically. In classical mechanics, we had Newton's "interpretation," Lagrange's, Hamilton's, etc. Each of these is particularly suited to different types of problems, and lends itself to somewhat different types of intuition. Also, these ideas were fundamental for the development of QM. Even within QM, we have the Schrodinger and Heisenberg representations, as well as Feynman's path integrals, etc. While technically equivalent, it would be ridiculous to claim that it is only important to learn one of these formulations.

Reilly, your sarcastic comments towards caribou are a bit strange. I think we all agree that copenhagen is extremely useful for a wide variety of situations. That doesn't mean that other ideas are pointless (either pedagogically or as a matter of principle). They obviously must reproduce the usual results in all experimentally tested situations. It would be particularly elegant, for example, if Born's rule were to emerge as a limit of something else. This would have experimentally measurable consequences, although we might be quite far from being able to measure them.

 

[dextercioby]

Besides being consistent with experiments, any physical theory must also be logically self-consistent (of course, sufficiently elaborate experiments would reveal logical inconsistencies, but that doesn't mean that they are practical to perform).

All partial theories developped and worldwide recognized so far are...

Ignoring everything else that has been discussed in this thread, it is clear that QM has serious difficulties when applied to spacetime geometry.

That is because the axioms of the nonrelativistic QM do not use the notion of spacetime.In fact,they're so abstract,that even the physical space (euclidean in Newtonian physics) is taken outta the picture...

This is a logical inconsistency in the theory that must be resolved despite the lack of any current experimental problems.

There is no logical inconsistence.QM doesn't explain gravity and spacetime,because it wasn't built to do it and,incidentally,every attempt to quantize gravity following the receipt given by the 6 axioms has failed so far.

I also wonder how exactly the Copenhagen interpretation would be applied to quantum geometry... How would quantum cosmology work? As far as I know, these sorts of issues are what prompted Gell-Mann and Hartle to develop their ideas. Unfortunately, I haven't ever gotten around to reading their papers.

From another point of view, finding alternative formulations of the same theory has been useful historically. In classical mechanics, we had Newton's "interpretation," Lagrange's, Hamilton's, etc. Each of these is particularly suited to different types of problems, and lends itself to somewhat different types of intuition. Also, these ideas were fundamental for the development of QM. Even within QM, we have the Schrodinger and Heisenberg representations, as well as Feynman's path integrals, etc. While technically equivalent, it would be ridiculous to claim that it is only important to learn one of these formulations.

Reilly, your sarcastic comments towards caribou are a bit strange. I think we all agree that copenhagen is extremely useful for a wide variety of situations. That doesn't mean that other ideas are pointless (either pedagogically or as a matter of principle). They obviously must reproduce the usual results in all experimentally tested situations. It would be particularly elegant, for example, if Born's rule were to emerge as a limit of something else. This would have experimentally measurable consequences, although we might be quite far from being able to measure them.

I know that I'm nitpicking,but if u decide to talk about QM,at least use its terminology properly.E.g.Schrödinger,Heisenberg & interaction (a.k.a.Dirac-Tomonaga-Schwinger) picture(s) .

Daniel.

 

[Stingray]

QM doesn't explain gravity and spacetime,because it wasn't built to do it and,incidentally,every attempt to quantize gravity following the receipt given by the 6 axioms has failed so far.

My point exactly. I was being a little loose with my wording. Fundamental physics as a whole is not logically self-consistent, and QFT/QM is obviously a part of this (I've been saying QM even when I mean QFT - sorry about the confusion). It is not very meaningful to say that QFT is just a stand-alone mathematical structure.

By the way, I don't understand your statement that the axioms of nonrelativistic QM are independent of spacetime. As I've learned them, they include Schrodinger's equation. What does that time derivative mean without a notion of spacetime (Newtonian or otherwise)? Can nonrelativistic QM be formulated on a classical curved background? I know QFT can, but you have to start from a completely different viewpoint than the one in textbooks. There might still be lingering issues as well. I don't know.

 

[ZapperZ]

Besides being consistent with experiments, any physical theory must also be logically self-consistent (of course, sufficiently elaborate experiments would reveal logical inconsistencies, but that doesn't mean that they are practical to perform). Ignoring everything else that has been discussed in this thread, it is clear that QM has serious difficulties when applied to spacetime geometry. This is a logical inconsistency in the theory that must be resolved despite the lack of any current experimental problems.

What does it mean to say something is "logically inconsistent"? What is logically inconsistent about QM? That it is built on a set of axiom that cannot be derived via First Principles? That it isn't built logically like mathematics?

Why does "difficulties when applied to spacetime geometry" implies "logical inconsistencies"? Does difficulty in applying BCS theory to High-Tc superconductors implies logical inconsistency of BCS theory, even when it is the MOST verified theory of a phenomenon in history? If we were to extend QM to include GR, does it then make it "logically consistent" when it wasn't before?

Zz.

 

[caribou]

Caribou -- I'm happy for you, and sad for the rest of us. I guess we've been mistaken all along. I used practical Copenhagen in my Ph.D thesis, perhaps I should redo my work, and my various published papers. Given your expertise, what do you think? I suspect I'm not the only person in the Forum facing such difficulties. Do we need to come up with, as you state it, a theory other than QM in order to save our status as physicists, active or retired? I ask because of your imperative, "no doubt about it" directive to develop this other theory. And all this time I thought I had a reasonable clue about what I was doing. And worse yet, your directive very much invalidates most of the physics from the 1920s until recently. Looks to me like an impending crisis.

I have no idea how you arrived at these conclusions, as my post suggests nothing even remotely like that.

The Copenhagan interpretation works just fine in most respects, as long as you don't ask certain questions that many never ask. These are questions, for example, like how a theory with "external observers" is supposed to apply to the universe as a whole or how wave functions can "collapse" with no interaction.

These issues don't matter in most situations but Copenhagen is simply inadequate for something like quantum cosmology, hence Hartle's interest in the clarification of quantum mechanics for the field he and Hawking helped found in a modern sense.

Decoherent histories is simply about clarifying following quantum mechanics by following quantum mechanics and seeing where it leads.

 

[caribou]

Reilly, you can lead a horse to water, but you can't lead water to a horse.

Instead of talking of horses and water, how about you talk about instantaneous wave function collapse over galactic distances with no interaction causing the collapse?

It's not a time-consuming challenge. Just sum up in a few words what you think of it.

 

[Chronos]

Instead of talking of horses and water, how about you talk about instantaneous wave function collapse over galactic distances with no interaction causing the collapse?

It's not a time-consuming challenge. Just sum up in a few words what you think of it.

That is no more mysterious than any other wave function collapse. Observation is the interaction causing the collapse.

 

[Stingray]

Why does "difficulties when applied to spacetime geometry" implies "logical inconsistencies"?

To put it most succintly, it is logically inconsistent to claim that GR and QFT (+standard model) together form a fundamental description of our universe. On a purely mathematical level, either one of these theories works just fine without the other. But experiment has shown that we can't throw out either of them (at the appropriate scales). We also can't combine them in any consistent way (so far). It is therefore obvious that fundamental physics needs to be extended.

I am calling this a "logical inconsistency" because we are arriving at the problem without any direct experimental evidence. The only sense in which experiment is involved is that both GR and QM are essentially our favorite (minimal) extrapolations of all known experiments (at least we think that they are consistent with everything).

Does difficulty in applying BCS theory to High-Tc superconductors implies logical inconsistency of BCS theory, even when it is the MOST verified theory of a phenomenon in history?

BCS theory has never been claimed to be fundamental physics. So no, there is no logical inconsistency implied by it being difficult to apply to high-Tc superconductors. It is just a limit to the model (apparently - I'm far from an expert).

Now, you might object that both GR and QM are also just models, and that labelling them "fundamental physics" is just arrogant semantics. I can argue against this, but there isn't much point. The statement that there are known problems with {GR + QFT} stands, and you can call them whatever you like.

If we were to extend QM to include GR, does it then make it "logically consistent" when it wasn't before?

Yes.

 

[caribou]

It's even worse for me. As a lowly experimentalist, all I've been doing throughout my carreer have been to "shut up and calculate". I had very little time, nor inclination to make any philosophical interpretation - mainly because I care more about what I can show physically, and the fact that philosophical interpretation on this has not produced any significant contribution to the advancement of physics.

Then it seems your lack of time and inclination to study interpretation means there might be a slight chance you were aware that lecturing me on what consistent histories does and does not say could be taking a risk on the off-chance you hadn't fully understood the theory.

Well, I've studied the subject part-time for a couple of years and, as just one example, I would never think "incompatible" meant contradictory. However, as I have said before, specialists in interpretation are making errors and, in fact, that's one of those that they make, so it would be extremely unfair to hold anyone else to account. So I didn't and I don't.

That's why it's not a big deal to me that you were trying to "correct" me on something I know doesn't need correction. If it was obvious then it wouldn't have taken decades to be found in the quantum mechanics. Only Bohr with his genius for insight anticipated it.

But I don't want to talk about this anymore. It's not important for most physics anyway. We can agree to disagree and leave it at that.

 

[Locrian]

It's not important for most physics anyway.

Actually, can you give any instance in physics in which it is important? It's total lack of importance is part of the point other posters are trying to make.

 

[caribou]

That is no more mysterious than any other wave function collapse. Observation is the interaction causing the collapse.

I see.

What about this problem of "interaction-free measurement" in which if a particle has a wave function which means it could be detected at A at time 1 or at B at time 2, we know by simple logic that if there was no interaction at A at time 1 then the particle will later be detected at B at time 2.

The wave function collapsed at time 1 because nothing happened. Or it collapsed at time 1 because we observed that nothing happened.

Either way, the wave function collapsed without any physical interaction.

Now that seems a bit strange to me.

The physicists whose work I was describing have found that wave function collapse is simply a mathematical shortcut and not a physical effect.

Now that makes more sense to me.

What do you think?