Questions About Quantum Theory: What's Wrong?

[Eye_in_the_Sky]

As far as I can figure out, classical probability is as subject to collapse as QM. That is, measurement simply tells us at that moment what is, whether an electron in a scattering experiment, or the price of IBM stock, a sales forecast, or what you will have for dinner in two weeks.

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". If "hidden variables" do not physically exist, then any 'induced' change in the quantum state-vector implies a corresponding physical change in the status of the system in question. ... I see no way around it (... except perhaps to 'deny reality', whatever that is supposed to mean).
______________

There's one thing I've never understood about the Schrodinger cat problem. It has nothing to do with QM, and everything to do with standard probability matters.

Again, this standpoint is consistent with a "hidden-variables" perspective. But from the alternative perspective (i.e. "hidden variables" do not physically exist), the Schrödinger-cat scenario has everything to do with Quantum Mechanics, and nothing to do with standard probability matters.

To see that this is so, consider – from the "no-hidden-variables" perspective – the following:

Suppose that a quantum system is in the state

|ψ> = (1/√2) [ |φ₁> + |φ₂> ] ,

where the states |φ₁> and |φ₂> are eigenstates of an observable which we can physically measure.

(Remember, we are assuming here that there are no "hidden variables". The state vector gives a "complete" characterization of the physical state of the system.)

Now, what do we want to say about this situation? Do we want to say that the quantum system is not at all actually in the said (physical) state of superposition, but that it is, in fact, in one or the other of the (physical) states |φ₁> or |φ₂> with probability equal to ½ ?

... Certainly NOT![/color]

From this perspective, then, the Schrödinger-cat scenario is a challenge to the following contention:

The quantum-mechanical state-vector description can be meaningfully applied to systems of arbitrary "size" and "character".

But the challenge is raised only in the context of no "hidden variables".
______________

Tony Leggett recently wrote a terrific article summarizing the so-called "measurement problem" of QM, and in particular, the Schrodinger Cat-type phenomenon (it is no longer a "paradox").[1] I strongly suggest people who insist that there is a "measurement problem" to read this, and his other paper in J. Phys. Cond. Matt. to look at the wealth of experimental observations and how they compare to what we know about what QM is saying.

[1] A.J. Leggett, Science v.307, p.871 (2005).

I recently read an essay of Leggett's written some time around the mid to late 80's in which he discussed the "Measurement Problem" with a great deal of care. In that essay, he suggested "the possibility that the complexity of a physical system may itself be a relevant variable which may introduce new physical principles." I am excited to find out what conclusions he has now reached some two decades later. Thank you for posting the above reference, ZapperZ (and also for the many others which you have posted).

... Can you be more specific about the J. Phys. Cond. Matt. paper?

 

[ZapperZ]

I recently read an essay of Leggett's written some time around the mid to late 80's in which he discussed the "Measurement Problem" with a great deal of care. In that essay, he suggested "the possibility that the complexity of a physical system may itself be a relevant variable which may introduce new physical principles." I am excited to find out what conclusions he has now reached some two decades later. Thank you for posting the above reference, ZapperZ (and also for the many others which you have posted).

... Can you be more specific about the J. Phys. Cond. Matt. paper?

The reference to the J. Phys. paper is one of the papers Leggett cited in his Science article. It is a more in-depth look at the Schrodinger Cat-type scenario, especially in light of the Delft and Stony Brook's recent experiments using SQUIDs.

Zz.

 

[Eye_in_the_Sky]

The reference to the J. Phys. paper is one of the papers Leggett cited in his Science article. It is a more in-depth look at the Schrodinger Cat-type scenario, especially in light of the Delft and Stony Brook's recent experiments using SQUIDs.

Zz.

Thanks!

 

[Stingray]

I disagree. This is because the quantities we measure are classical! Position, momentum, energy, etc. are all "classical" quantites that we inherited out of classical mechanics. QM is simply telling us what they are if we insist on using these quantities. (Again, square objects being forced through round holes).

Then what should be measurable? Any physical theory must describe what its variables mean in some (operational) sense. If those variables can only be described by mixing QM and classical mechanics, then this is itself a type of incompleteness. Or are there measurement types I'm not aware of?

Also, thanks for the reference. I'll take a look at it.

 

[dextercioby]

We can measure typical QM quantities:probabilities with which certain eigenvalues occur and (integral) cross sections...What else is there to QM...?

Daniel.

 

[Haelfix]

"The Schrodinger Cat-type phenomenon (it is no longer a "paradox")."

Forgive me if I am a little bit skeptical about these sorts of matters. The paradox of Schrodingers cat ultimately is the problem of resolving when a quantum state becomes a classical one, and last I checked there is still wars going on in the measurement camp about these thorny issues. Taking to the extreme you end up with one of two scenarios.

1) Everything is quantum, classical behavior is just an emergent illusion. Ergo notions like the wave function of the universe become acceptable, despite their known enormous failure in various field theories and quantum gravity.

2) Something weird happens and there is some sort of phase change between quantum to classical mechanics. Perhaps its something not accounted for, or some tiny effect that only becomes important in the huge complexity of interactions. Eg people can't, under any circumstance, walk (tunnel) through walls.. The probability isn't just 1e-50502, but identically zero.

 

[ZapperZ]

Then what should be measurable? Any physical theory must describe what its variables mean in some (operational) sense. If those variables can only be described by mixing QM and classical mechanics, then this is itself a type of incompleteness. Or are there measurement types I'm not aware of?

The problem here is that ALL our measureables or observables are classical quantities. This is what I have been trying to stress all along. We have no other alternatives (so far). QM is our description of a world beyond classical physics USING classical physics concepts. When we do that, OF COURSE some things will simply make no sense based on our classical measurement. Our squares and cubes came through the round holes with the edges chopped off, and we struggle to still want to call them squares and cubes when they do not look quite like squares and cubes. They look funny to us. So we blame the round hole and forget all about the fact that we were forcing incompatible shapes through the round hole.

With this in mind, I do not worry about the "measurement problem", the schrodinger cat, the EPR-type measurement, the "collapsing" wave function, etc.. etc. It is what it is, and that's what Nature has decided to reveal herself so far. I have accepted and am aware of my prejudice of trying to describe nature using concepts that may not be accurate. I use the formalism, but pay more attention to the experiments. As far as I'm concerned, in the end, that is the only thing that matters.

Zz.

 

[Crosson]

ZapperZ, I like what you say here:

Unless we want to invent a new set of quantites and concepts, we're stuck with these classical ideas.

In my opinion this is the progressive approach we need to take towards QM. Take for example the particle-wave duality; what we need is a new concept, not a bunch of people saying "Sometimes its a particle and sometimes its a wave and that's just the way it is".

Energy and momentum observables are already predicted very well by QM, there is no reason to change this. We know energy and momentum are conserved quantities, and that they obey the quantized hamiltonian relationship. But unlike the state of affairs in classical mechanics, all we know in terms of experiment is that energies are proportional to the frequency of photons.

In terms of the thing we are describing, energy is not defined (without resorting to a circular definition relating momentum and energy, or the relation of "energy" to the frequency of emitted photons rather than the system we are talking about).

A separate criticism of QM which I have not brought up (because physicist seem to attack philosophers) is determinism. In my mind QM is a theory of observations, and so in cannot make claims concerning determinism. But many people embrace the idea that QM makes the universe indeterminate, and this is:

1) Not supported by experiment or quantum theory (as I read it).

2) Physically appaling, in that an indeterminate event is necessarily uncaused.

Because it is a theory of observations, QM involves indeterminate events and fails in not describing explicit causes.

Keep in mind that I know QM proves that the universe is indeterminate for an observer, but that does not rule out determinism. (indeed, it was clear from classical chaos that it would never be possible to actually predict the future this way)

I define determinism as: The present corresponds to only one future.

 

[ZapperZ]

"The Schrodinger Cat-type phenomenon (it is no longer a "paradox")."

Forgive me if I am a little bit skeptical about these sorts of matters. The paradox of Schrodingers cat ultimately is the problem of resolving when a quantum state becomes a classical one, and last I checked there is still wars going on in the measurement camp about these thorny issues. Taking to the extreme you end up with one of two scenarios.

1) Everything is quantum, classical behavior is just an emergent illusion. Ergo notions like the wave function of the universe become acceptable, despite their known enormous failure in various field theories and quantum gravity.

2) Something weird happens and there is some sort of phase change between quantum to classical mechanics. Perhaps its something not accounted for, or some tiny effect that only becomes important in the huge complexity of interactions. Eg people can't, under any circumstance, walk (tunnel) through walls.. The probability isn't just 1e-50502, but identically zero.

You have mentioned two separate aspects of the so-called Schrodinger Cat paradox. Again, most of the stuff that I have mention (and will mention here) are contained in Tony Leggett's paper, including the full treatment in his J. Phys paper.[1] I said that the Schrodinger Cat scenario is no longer a paradox because (i) it does truly occur at the QM scale.[2,3] These have been unambiguously verified.

The issue that's left, which is the 2nd part of your point, is why don't we see it at the macroscopic scale. I do not see this as another "paradox" because while it is still an active research area, we have plenty of indications of possible explanations for such dichotomy. We have seen how classical system emerges into the classical observation via careful and controlled decoherence.[4,5] Not only that, there are every indications that the emergence of "objective" properties come out of a "selective" destruction of quantum coherence, resulting in what is known as preferred pointer states.[6] This is what eventually results in what we perceive macroscopically as an "objective" and deterministic universe.

I am far from claiming this is a done issue. I just do not see this as being a "paradox" anymore as if these things simply have no explanation whatsoever. In fact, there exists many plausible explanations that connects the evolution from quantum states into classical observations. We just need more experiments to verify them convincingly.

Zz.

[1] A.J. Leggett, J. Phys. Cond. Matt. v.14, p.415 (2002).
[2] J.R. Friedman et al. Nature v.43, p.406 (2000).
[3] C.H. van der Wal et al. Science v.290, p.773 (2000).
[4] K. Hornberger et al. PRL v.90, p.160401 (2003).
[5]C.J. Myatt et al. Nature v.403, p.269 (2000).
[6] H. Ollivier et al. PRL v.93, p.220401 (2004).

 

[reilly]

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". If "hidden variables" do not physically exist, then any 'induced' change in the quantum state-vector implies a corresponding physical change in the status of the system in question. ... I see no way around it (... except perhaps to 'deny reality', whatever that is supposed to mean).
______________Again, this standpoint is consistent with a "hidden-variables" perspective. But from the alternative perspective (i.e. "hidden variables" do not physically exist), the Schrödinger-cat scenario has everything to do with Quantum Mechanics, and nothing to do with standard probability matters.

To see that this is so, consider – from the "no-hidden-variables" perspective – the following:

Suppose that a quantum system is in the state

|ψ> = (1/√2) [ |φ₁> + |φ₂> ] ,

where the states |φ₁> and |φ₂> are eigenstates of an observable which we can physically measure.

(Remember, we are assuming here that there are no "hidden variables". The state vector gives a "complete" characterization of the physical state of the system.)

Now, what do we want to say about this situation? Do we want to say that the quantum system is not at all actually in the said (physical) state of superposition, but that it is, in fact, in one or the other of the (physical) states |φ₁> or |φ₂> with probability equal to ½ ?

... Certainly NOT![/color]

From this perspective, then, the Schrödinger-cat scenario is a challenge to the following contention:

The quantum-mechanical state-vector description can be meaningfully applied to systems of arbitrary "size" and "character".

I have no clue why I would need hidden variables to suggest a correspondence between classical and quantum notions of probability. So, I would be most grateful to find out what I'm missing.

Because I've worked in the consulting business for years with probability and statistics, and in my younger years for some time as a particle theorist and teacher of QM, I've concluded from practical experience that the two probabilities are, generically the same. In fact, what made and makes sense to me is that the probabilities are statements about our knowledge, as, more or less, suggested by von Neuman and Wigner. The troubling collapse is a reflection of changes in our knowledge.

Subsequent to the long time it took me to come to this conlusion, I discovered that the great physicist, Sir Rudolph Peierls, agrees -- or, really I agree with Sir Rudolph -- with the knowledge interpretation. In Andrew Whitaker's book, Einstein, Bohr and the Quantum Dilemma(1996), Peierls " a theoretical physicist of massive achievements" In response to Bell's "Against Measurement" (1990), Peierls writes:

In my view the most fundamental statement of quantum mechanics is that the wavefunction, or more generally the density matrix, represents our knowledge of the system we are trying to describe.

Whitaker quotes more, but I shan't bore you, Whitaker's book is a review of the history of QM, particularly of QM interpretation -- Bohr to Bohm to Everett and more. The book is a very impressive piece of work.

Peierl's ideas make great sense to me. Again, quantum or classical, you don't know until you measure. Why, if we, say, see a boat sail out to sea, to vanish over the horizon, do we say we know the boat, in all probability, continues to sail after we lose sight? After all, we cannot see it. (Not a bad question for a PhD candidate's oral exam.)

With a spin doublet, superposition, according to Peierls, just says there are two possibilities. To work backwards seems to me to be an exercise in futility. Why should nature follow our conceits? I always thought science and physics were about nature, not about man's preconceived notions, whether ego driven or not.


If you don't like QM as it is, find a better way. Explain the electron microscope, or semiconductors some other way. So far, nobody has come close, and I find that very telling -- even though I'm not so foolish as to maintain there can't be a better way.

I posed a set of questions a post or two above. Why has no one dealt with them? I find that rather odd, given the lofty thoughts about the inadequacies of QM. QM ain't the only problem.

I remain a servant of Nature.

Reilly Atkinson

 

[Locrian]

1) Not supported by experiment or quantum theory (as I read it).

2) Physically appaling, in that an indeterminate event is necessarily uncaused.

I see no supporting evidence for the first, and the second is simply wrong. There is no reason that an indeterminate event is uncaused.

 

[ZapperZ]

ZapperZ, I like what you say here:



In my opinion this is the progressive approach we need to take towards QM.

Puhleeze. I'm the one hindering progress in physics, remember?

Take for example the particle-wave duality; what we need is a new concept, not a bunch of people saying "Sometimes its a particle and sometimes its a wave and that's just the way it is".

This is just plain wrong. Where exactly in QM is there this "duality"? Really now!

Such things are only used when it is being described to the general public! This duality is NOT part of QM! There is only ONE description of light and matter, not two, not three. Every single so-called "wave" behavior can be fully and consistently described via the SAME description that describe the "particle" behavior. There are no dichotomy. The dichotomy only comes in because CLASSICALLY, those two are different beasts and are incompatible with each other.

Again, this is another clear example where we are forcing the classical picture onto the QM domain. Our insistence that "wave" and "particle" are separate descriptions is rearing its ugly head, while we ignore the fact that QM has no such separation!

Energy and momentum observables are already predicted very well by QM, there is no reason to change this. We know energy and momentum are conserved quantities, and that they obey the quantized hamiltonian relationship. But unlike the state of affairs in classical mechanics, all we know in terms of experiment is that energies are proportional to the frequency of photons.

You are not describing QM in general here. You are describing only "special cases" here. Energy and momentum are not always conserved, especially when you deal with virtual particles. "Quantized" hamiltonian isn't automatic! Quantization is a direct result of the boundary condition. Energy, momentum, position, etc are NOT quantized for a free particle, for example. And it is certainly continuous in the conduction band of solids.

A separate criticism of QM which I have not brought up (because physicist seem to attack philosophers) is determinism. In my mind QM is a theory of observations, and so in cannot make claims concerning determinism. But many people embrace the idea that QM makes the universe indeterminate, and this is:

1) Not supported by experiment or quantum theory (as I read it).

2) Physically appaling, in that an indeterminate event is necessarily uncaused.

Because it is a theory of observations, QM involves indeterminate events and fails in not describing explicit causes.

Keep in mind that I know QM proves that the universe is indeterminate for an observer, but that does not rule out determinism. (indeed, it was clear from classical chaos that it would never be possible to actually predict the future this way)

I define determinism as: The present corresponds to only one future.

Again, you haven't cited any experimental observations. All you have done here is cite your distaste. I have posted a reference to a paper relating a decoherence process with an emerging pointer states. I suggest you read that.

Zz.

 

[reilly]

There are a few things worth remembering about Quantum Physics, to return to my mantra, which are evident upon reading history, and getting the facts straight.

Classical theory in the face of
Blackbody radiation
Photoelectric effect
Atomic Spectra,
Electron diffraction

was rendered totally impotent. Classical ideas failed to do the job. Nature failed to agree with our usual modes of perception and description. That was fact 100-75 years ago. In spite of revisionist attempts, classical theory and concepts still fail to describe many phenomena. ZapperZ hit this right on the head.

Nature has posed us with phenomena that appear to be logically impossible, beyond the scope of our language. JJ Thomson discovered the electron as a particle. Some years later, Davisson and Germer discovered electron diffraction. How can this be? We still grapple with this seeming contradiction, there appears to be no way around it. So, indeed the formulation of QM is grounded in experimental evidence. How, in their right mind would could anyone invent QM out of nothing?

Nature has wired us to have certain perpetual mechanisms, which give us direct experience of very little of Nature. That our logic, intuition and knowledge do not extend well into parts of Nature that are beyond our perceptual boundaries, is not surprising, and has been discussed by many. So to our normal way of thinking, Nature can be very weird. It's the job of science to accept this empirical weirdness, and try to deal with it, and if some of our concepts are found wanting, well that's that.

Determinism? I think David Hume pretty much destroyed that concept a few hundred years ago. (Read and learn)

Yes, we could use new concepts. The question is: how do we formulate them?

I guess that what's wrong with QM is that it does not easily fit into the usual modes of thinking. It is weird because what it describes is weird.

Regards,
Reilly Atkinson

 

[caribou]

I've been reading a bit about interpretation and I've got to say that I agree with a lot of what Crosson is saying.

Perhaps the most interesting attempt at interpretation is the "decoherent histories" approach (closely related to the "consistent histories" approach). What that does is takes quantum mechanics seriously and see what the theory actually says and where it leads in a modern way.

One of the things you end up with is the modern equivalent of complementarity -- like for example wave/particle duality -- of the most extreme kind when you try to describe a series of events. You are limited to talking about what is consistent or inconsistent.

A good example of this is in the two slit experiment:

A) If you choose to describe where the particle hits the screen, you can't describe which slit the particle goes through.

or

B) If you choose to describe which slit the particle goes through, you can't describe where it hits the screen.

And that is that. Trying to combine both the A and B descriptions makes no sense. Another good example is particle spin version of EPR in which not realizing that there are lots of other ways of describing the events makes many people think there is some influence traveling from one particle to another.

(Roland Omnes is someone whose books I think I'd recommend here as he has some expertise in this area.)

That there are different incompatible descriptions of the same events is mind-boggling. And these, of course, in quantum mechanics are events without causes. And these could be events without causes that split the universe into different universes as well. But that is apparently where quantum mechanics leads us.

It's like some kind of nightmare... but the theory will give the right answers if asked the right questions.

I think you can see why I might agree we need a better theory. I'm not giving up hope that we can think of something a bit less insane.

You know, suddenly determinism doesn't sound so bad anymore.

 

[ZapperZ]

One criticism that I will level off regarding your comments here is that you should leave room for the possibility that you understood QM wrongly. If you start off with the wrong premise, then of course you will not get this right. It is unfair to criticize or comment about anything, not just QM, when you start off with the wrong understanding of what it is.

I will illustrate:

I've been reading a bit about interpretation and I've got to say that I agree with a lot of what Crosson is saying.

Perhaps the most interesting attempt at interpretation is the "decoherent histories" approach (closely related to the "consistent histories" approach). What that does is takes quantum mechanics seriously and see what the theory actually says and where it leads in a modern way.

One of the things you end up with is the modern equivalent of complementarity -- like for example wave/particle duality -- of the most extreme kind when you try to describe a series of events. You are limited to talking about what is consistent or inconsistent.

A good example of this is in the two slit experiment:

A) If you choose to describe where the particle hits the screen, you can't describe which slit the particle goes through.

or

B) If you choose to describe which slit the particle goes through, you can't describe where it hits the screen.

This is incorrect. First of all, in (B), I can describe where it hits the screen equally well as where it went through the slit (your usage of the phrase "choose to describe" is something I haven't seen in physics). It is the interference pattern that goes missing when I unambiguously can determine which slit the object passes through. You get two gaussian peaks, rather than the regular interference pattern. There's nothing to prevent me from making a full description of anything.

And that is that. Trying to combine both the A and B descriptions makes no sense. Another good example is particle spin version of EPR in which not realizing that there are lots of other ways of describing the events makes many people think there is some influence traveling from one particle to another.

Again, your "sense" isn't perfect, and it EVOLVES. These things may not make sense to you, but they make perfect sense to me because I've admitted my prejudice in forcing nature of confine herself into my classical concepts of "position" and "wave" and "particle", etc. (square objects through round holes).

We cannot, and should not do physics simply via a matter of tastes. Go through this thread carefully and you will see that in every single instance of people making a so-called challenge to QM, it is done due to some personal preferences. I have said this way in the beginning of this thread. I do not understand why people continue think this is a valid way to challenge anything in physics, not just QM.

And things are made worse when people make the wrong statement about QM in the first places, such as "wave-particle duality". Everytime someone says that that is what QM is describing, I tend to think that person has never studied QM formally. I mean, where in QM is there such a thing as two separate descriptions of wave-particle? You cannot expect to be able to make a coherent comment about any subject when you only have a superficial understanding of it. QM cannot be learned and understood via its interpretation! You cannot get the full taste of a food via its description by someone else!

Zz.

 

[caribou]

ZapperZ,

I think you should also leave room for the possibility that I've understood something correctly but that I could be doing a much better job of describing it. I'm sorry if that's the case, as I think it probably is.

Yes, I agree that descriptions A and B in the two slit experiment work together just fine when measurement or interaction is allowed but this isn't allowed in the situation I mean.

What I am referring to involves only the usual set-up of the two slit experiment leading solely to the interference effect and not any other set-up which allows for measurement or interaction around the slits. This set-up I mean features no destruction of the interference effect for particles.

Then looking at this set-up with the interference effect in the theory, we can get from it basically two equally valid but incompatible descriptions of the experiment, A and B.

A is useful for describing where on the screen the particle could be detected but at the cost of not being able to describe which slit the particle could go through.

B is useful for describing which slit the particle could go through but at the cost of not being able to describe where on the screen the particle could be detected.

Obviously, we choose to use A. And that's the standard description in that we just say that if we didn't measure it in some way, then assigning a slit to the particle's journey makes no sense.

B describes the particle at the slits with no detection and it also makes the screen appear as a macroscopic quantum superposition state! It's fairly useless in any practical sense but B exists and is a valid alternative description.

Some alternative descriptions are useful, however, as in EPR they reveal a human prejudice for realism of particle properties which quantum mechanics doesn't have. Knowing that there are other ways of describing the particle properties of EPR other than just the standard one of wave function collapse is very interesting indeed.

The decoherent/consistent histories of Gell-Mann, Griffiths, Hartle and Omnes is the theory I'm describing. Their books and papers will obviously give much better details. Griffiths has part of his book and some questions and answers at his website as well:

http://quantum.phys.cmu.edu/CQT/

 

[ZapperZ]

What I am referring to involves only the usual set-up of the two slit experiment leading solely to the interference effect and not any other set-up which allows for measurement or interaction around the slits. This set-up I mean features no destruction of the interference effect for particles.

Then looking at this set-up with the interference effect in the theory, we can get from it basically two equally valid but incompatible descriptions of the experiment, A and B.

A is useful for describing where on the screen the particle could be detected but at the cost of not being able to describe which slit the particle could go through.

B is useful for describing which slit the particle could go through but at the cost of not being able to describe where on the screen the particle could be detected.

But this is, I'm sorry to say, bogus! You are claiming that by KNOWING which slit the particle goes through, you still did not destroy the interference effects. Can you cite an experiment that has shown this? Because if you can, I would nominate you for the Nobel Prize.

Obviously, we choose to use A. And that's the standard description in that we just say that if we didn't measure it in some way, then assigning a slit to the particle's journey makes no sense.

B describes the particle at the slits with no detection and it also makes the screen appear as a macroscopic quantum superposition state! It's fairly useless in any practical sense but B exists and is a valid alternative description.

Valid by whose standard? Where has this been proven to be valid? Can you show me published experiments that has shown this to be valid?

When I solve an electrostatic problem using an image charge, the solution inside a conductor exists, but it doesn't mean this is a physical solution! Thus, just because an "alternative" exist, doesn't mean it has any connection to reality, especially when there are no experimental verification that shows such a thing exist.

Some alternative descriptions are useful, however, as in EPR they reveal a human prejudice for realism of particle properties which quantum mechanics doesn't have. Knowing that there are other ways of describing the particle properties of EPR other than just the standard one of wave function collapse is very interesting indeed.

As far as ALL the EPR-type experiments have shown, ALL the results have been compatible with what QM has predicted. So how can you point out that these experiments contain things that "quantum mechanics doesn't have"? Point out exactly where in these experiments are indications that there are things that QM doesn't have.

The decoherent/consistent histories of Gell-Mann, Griffiths, Hartle and Omnes is the theory I'm describing. Their books and papers will obviously give much better details. Griffiths has part of his book and some questions and answers at his website as well:

http://quantum.phys.cmu.edu/CQT/

I know about consistent histories. What you described is NOT consistent histories especially with your 2-slit scenario, nor are consistent with what consistent histories are claiming, especially regarding the EPR-type experiments.

Zz.

 

[caribou]

But this is, I'm sorry to say, bogus! You are claiming that by KNOWING which slit the particle goes through, you still did not destroy the interference effects. Can you cite an experiment that has shown this? Because if you can, I would nominate you for the Nobel Prize.

Did I say knowing even once? No. I said describing. A lot.

To make what I mean as clear as I can, if we set up the two-slit experiment in a box completely isolated from the rest of the universe and arranged the experiment to build up an interference pattern over time then the descriptions A and B are alternative but incompatible ways to describe a single run of the experiment with a single particle.

We would not know but we could describe events in ways like A and B.

Valid by whose standard? Where has this been proven to be valid? Can you show me published experiments that has shown this to be valid?

A valid alternative description extracted from the theory. In practice, retaining macroscopic interference effects is obviously not easy due to decoherence but the description B is every bit as valid as Schrodinger's cat being described as being in a superposition.

When I solve an electrostatic problem using an image charge, the solution inside a conductor exists, but it doesn't mean this is a physical solution! Thus, just because an "alternative" exist, doesn't mean it has any connection to reality, especially when there are no experimental verification that shows such a thing exist.

As I understand it, the descriptions like A and B are coarse-grained and are collections of fine-grained descriptions which almost always interfere and don't say anything useful. Descriptions such as A and B are coarse-grained versions used to make sense of the experiment in human terms and to allow the prediction of the probabilities that fine-grained descriptions don't allow.

When we describe a particle as going through a slit or arriving at a point on the screen, we are using human concepts to coursen and make sense of something that at a fine-grained level is hard to make sense of.

As far as ALL the EPR-type experiments have shown, ALL the results have been compatible with what QM has predicted. So how can you point out that these experiments contain things that "quantum mechanics doesn't have"? Point out exactly where in these experiments are indications that there are things that QM doesn't have.

I was talking about the human preference for realism in particle properties as being the flaw and as quantum mechanics not following this human preference, not anything in quantum mechanics being flawed.

I know about consistent histories. What you described is NOT consistent histories especially with your 2-slit scenario, nor are consistent with what consistent histories are claiming, especially regarding the EPR-type experiments.

I've spent some time working to understand the fundamentals of decoherent/consistent histories. There are a lot of technical issues about the theory that I've still to resolve in my mind but I'm fairly sure I understand the fundamentals well enough.

The existence of a vast array of incompatible descriptions of the same events in quantum mechanics is what Gell-Mann has called the "Protean" nature of the theory, after the mythological prophet Proteus who resisted being asked to prophesize and would change shape and had to be held still to be made to predict the future, this being a reference to choosing a single description to make logical predictions in QM.

 

[ZapperZ]

Did I say knowing even once? No. I said describing. A lot.

To make what I mean as clear as I can, if we set up the two-slit experiment in a box completely isolated from the rest of the universe and arranged the experiment to build up an interference pattern over time then the descriptions A and B are alternative but incompatible ways to describe a single run of the experiment with a single particle.

There are already such experiments that collect low intensity photons going through a 2-slit system. What is different or new with what you are asking for?

Secondly, where exactly is the decoherence effects acting on light passing through 2 slits?

Thirdly, how do you expect to detect, or "build up" an "interference pattern over time" without having the intefering agent interact with anything since it is "isolated" from the rest of the universe?

I do not see any "incompatibility" at all. All I see is misinterpretation of what QM is describing. At best, it is the insistence that our classical concept should work in such cases. This is an a priori criteria that has no ab initio proofs that it should be valid.

A valid alternative description extracted from the theory.

What theory? You are "extracting" a description from QM? If QM is all that is needed to describe a phenomena, and you are "extracting" something MORE from it, doesn't this mean (i) you are producing something that has no experimental observation (after all, if there is, QM can be proven to be incomplete), and (ii) that you are doing nothing more than speculating?

In practice, retaining macroscopic interference effects is obviously not easy due to decoherence but the description B is every bit as valid as Schrodinger's cat being described as being in a superposition.

It is EASY. Superconductivity is the clearest manifestation of coherence effects. I've mentioned Carver Mead's paper from PNAS in another thread that clearly indicated such sentiments. So now, apply your "B" to it such as in the Stony Brook and Delft's SQUID experiments.

As I understand it, the descriptions like A and B are coarse-grained and are collections of fine-grained descriptions which almost always interfere and don't say anything useful. Descriptions such as A and B are coarse-grained versions used to make sense of the experiment in human terms and to allow the prediction of the probabilities that fine-grained descriptions don't allow.

When we describe a particle as going through a slit or arriving at a point on the screen, we are using human concepts to coursen and make sense of something that at a fine-grained level is hard to make sense of.

I will not even pretend that I understand what you are saying here. Coarse-grained? Fine-grained?

I was talking about the human preference for realism in particle properties as being the flaw and as quantum mechanics not following this human preference, not anything in quantum mechanics being flawed.

Then what's the problem? If the whole purpose of this is to "pacify" and make QM palatable for human consumption, then this is more appropriate to be in the Philosophy section. Physics has no such demands.

The existence of a vast array of incompatible descriptions of the same events in quantum mechanics is what Gell-Mann has called the "Protean" nature of the theory, after the mythological prophet Proteus who resisted being asked to prophesize and would change shape and had to be held still to be made to predict the future, this being a reference to choosing a single description to make logical predictions in QM.

What "incompatible descriptions"? You just said that QM isn't wrong. In physics, if I have incompatible descriptions of anything, that theory is suspect. Wave and particle descriptions are incompatible descrptions in classical mechanics of the same entity. That's why classical mechanics is suspect when we get into a scale where an entity can exhibit such properties. QM, instead, has NO such distinction, and has only ONE description of BOTH classical wave and particle behavior.

Again, even after reading your "A" and "B" scenario, where are these "incompatible descriptions"?

Zz.

 

[caribou]

I'm going to skip some of your questions as, although I believe I can answer them, I suspect there will be more questions following my answers. Then more questions, then more answers. And so on. If you really want to know, you should go to the source, and that source is the work of Gell-Mann, Griffiths, Hartle and Omnes.

I will not even pretend that I understand what you are saying here. Coarse-grained? Fine-grained?

That's the decoherent histories of Gell-Mann and Hartle which is closely related to the consistent histories of Griffiths and Omnes. As detailed a description as possible (a "fine-grained" description) in quantum mechanics doesn't usually allow for the assignment of probabilities but if we ignore the details, times, etc. that are irrelevant (a "coarse-grained" description), quantum mechanics can give the usual probabilities.

Gell-Mann gives a nice little introduction to the idea in his popular science book, The Quark and the Jaguar. Coarse-graining is just an explanation of what we do to extract useful probabilities from the fundamental physics.

Then what's the problem? If the whole purpose of this is to "pacify" and make QM palatable for human consumption, then this is more appropriate to be in the Philosophy section. Physics has no such demands.

The problem here is that with the EPR paradox, people are thinking the argument is over locality -- when it's actually about realism -- and this makes them think quantum mechanics and special relativity are arguing with each other and that we have faster-than-light effects when we don't.

It's important that people know there is nothing "wrong" with special relativity and there's nothing "wrong" about quantum mechanics, there's just some concepts in quantum mechanics that are not classical and are difficult and abstract to the human mind. It's either this or let people believe in physical effects and theoretical problems which don't exist.

What "incompatible descriptions"? You just said that QM isn't wrong. In physics, if I have incompatible descriptions of anything, that theory is suspect. Wave and particle descriptions are incompatible descrptions in classical mechanics of the same entity. That's why classical mechanics is suspect when we get into a scale where an entity can exhibit such properties. QM, instead, has NO such distinction, and has only ONE description of BOTH classical wave and particle behavior.

Again, even after reading your "A" and "B" scenario, where are these "incompatible descriptions"?

This central idea of "incompatible frameworks" is very difficult and even specialists in interpretation often misunderstand it. Both Gell-Mann and Griffiths have lamented this. Gell-Mann has even accused them of deliberate misunderstanding. I myself even know one person who is convinced Gell-Mann doesn't understand EPR and is talking nonsense.

It's strange, then, that when Gell-Mann and Hartle described their work at a lecture at Caltech, Richard Feynman stood up and said he "agreed with every word" they said. Then again, that's perhaps no surprise as the decoherent histories approach I'm referring to has some of its basis in on-off discussions over decades between Feynman and Gell-Mann. But I guess this person I know who says it's all nonsense must be far smarter than Feynman, Gell-Mann and Hartle, eh?

I certainly hope you are more open-minded than he was, and not for my sake either.

It's conceptually tough. If it was easy then it wouldn't have taken so long to sort out quantum mechanics and need the insight of people like Gell-Mann and Feynman to do it.

Anyhow, on the issue of incompatibility, Griffiths has a paper called "Choice of Consistent Family, and Quantum Incompatibility" that may help:

http://xxx.lanl.gov/abs/quant-ph/9708028

Of course, if you are not interested then fair enough. I'm not keen to continue this discussion as I'm not gaining anything from it, I just thought people might be interested.

 

[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.