Is the QM axiom for measurements misleading?

[lalbatros]

Was the QM postulate for measurements misleading?

Interactions and their understanding is the central topic of QM. So it is for CM too.
The interaction between a QM system and a (macroscopic) Classical system is for sure an extremely interresting subject. Particularly when we want to describe it with the languages of both QM or CM.

However, I have the feeling that the axiom for the wave packet reduction is not only useless but also very misleading, specially in its wording. It has -maybe- been introduced to make a clean presentation of QM. But so many have stick to the letter of this axiom that it has been the source for interpretation and re-interpretation for QM. Just as if some hidden truth had to be discovered. Of course, interpretation has been useless till now. And fortunately few people believe that the human brain has a special relationship with atoms.

What are your ideas?

Michel

 

[kvantti]

"The equations of physics that model the time evolution of systems without embedded observers are sufficient for modelling systems which do contain observers."
- The MWI

http://en.wikipedia.org/wiki/Many-worlds_interpretation

 

[lalbatros]

kvantti,

Sorry, but really I cannot be satisfied by a fantasy solution to a non-existing problem.
Why indroducing this useless MWI?

The evolution equation is valid from microscopic to macroscopic. The wave packet reduction can be understood on this basis. What more is needed?

Maybe one would like to avoid randomness, but that's not possible (as far as we know). So, in the most optimistic point of view, the MWI describes the randomness that we are unable to avoid, a sort of catalog of what might be possible. But giving a reality to this catalog of "the possibilities" is a useless abstraction.

The measurement process is nothing more that an interaction and would proceed even if the experiment was build on purpose or by mere chance. What is called a "measurement process" when it occurs in a lab is still a measurement process when it occurs in the chlorophyll of a leaf, in my eye, or anywhere. Who could pretend that Stern & Gerlach sort of interactions can only occur in a laboratory? "Measurement process" is really a bad wording that should be replaced by interaction (between microscopic and macroscopic). This vocabulary is really the source of the debate.

I would like to introduce a comparison. Let's consider the well-know "http://en.wikipedia.org/wiki/Safety_engineering" " analysis. This methodology is much used in nuclear engineering to create a list of possible defects for a nuclear reactor and their probability. The analysis considers all possible failure and their eventual accumulation.
My question is then: why should the nuclear engineers introduce a kind of MWI theory? Why should they consider parallel worlds where Tchernobyl doesn't happen or where it happens with a worse outcome? And what would be the use of the theory and what could it explain about nuclear safety?

So, in summary, I find it interresting to study the interaction between quantum systems and macroscopic system, but I really see no use for interpretations, on the contrary.

Michel

 

[Farsight]

lalbatros: I think "particle" is the cause of a lot of problems here.

 

[kvantti]

lalbatros:

http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm

How yould you interpret the results of this experiment? In my point of view, the MWI is the only choice to describe this experiment physically.

You can always go with the "shut up and calculate", but then you're missing a big part of the mystery of QM.

The mathematical formulation of QM makes amazingly accurate predictions, and yet you really can't explain it physically; there's just different interpretations.

Btw. the "fantasy solution" is considered to be THE solution by most (but not all) physicist for explaining quantum mechanics. Copenhagen interpretation was the first one, but it doesn't have to be the right one. It could be that the MWI isn't the right one either, but it sure does get rid of the "paradoxes" of QM...

 

[lalbatros]

kvantti,

My preoccupation is not to discuss the MWI vs the CoI or another attitude toward the strangeness of QM. My preoccupation goes to the reasons why this debate occurred and if it really has a meaning. And also, maybe, the criteria to decide about this question.

My interrest in this question started really in the 80's. I was very excited by reading the famous compilation "Quantum Theory and Measurement, by JA Wheeler" (and also the essay by Bernard d'Espagnat). A few years earlier I had to learn the postulates of quantum mechanics. The "measurement postulate" proved very convenient to avoid lengthy explanations about the interaction between microscopic and macroscopic system (kind of shut up!). But this postulate can be dropped easily, as I realized only much later (reading Landau & Lifchits) (kind of nothing to say!). I also come close to conclude that this debate started from a misundertanding around the word "measurement" (and also the word "observable" !). What is left then for discussion and interpretation?

I admit however a difficulty. The wave function is the irreductible object of QM. This is to be contrasted to statistical physics where the probabilities are derived from more fundamental objects (say the coordinates). The irreductibility of the wave function is difficult to swallow for the "classical beings" that we are.

Finally, I guess that all interpretations agree on the on the outcome of all QM experience. So why the hype then?

I don't want to convince, but to learn and to test my opinion.

Michel

 

[kvantti]

However, I have the feeling that the axiom for the wave packet reduction is not only useless but also very misleading, specially in its wording.

You can forget the "wave packet" and stick to particles only. The "wave packet" is just a way to calculate probabilities. But you can also calculate the probabilities the following way:

http://en.wikipedia.org/wiki/Path_integral_formulation

The path integral formulation or "the sum over histories" formulation plays a central part in the formulation of quantum field theory, which is the fundamental theory of reality.

If you want to study quantum interactions or "measurements", start with decoherence:

http://en.wikipedia.org/wiki/Quantum_decoherence

 

[Careful]

However, I have the feeling that the axiom for the wave packet reduction is not only useless but also very misleading, specially in its wording. It has -maybe- been introduced to make a clean presentation of QM. But so many have stick to the letter of this axiom that it has been the source for interpretation and re-interpretation for QM. Just as if some hidden truth had to be discovered. Of course, interpretation has been useless till now. And fortunately few people believe that the human brain has a special relationship with atoms.

What are your ideas?

Michel

Come on, what a terrible question . There are plenty of people who have given this lots and lots of braintime (see papers of Schrodinger, Wigner, Barut, 't Hooft, Toffoli...) and they all come up with different stories. The latter is normal since you are asking questions about processes you cannot directly acces through experiment. So, my answer is ``yes, R should be taken with the right amount of salt'' but any decent alternative is physics for the future.

Careful

 

[lalbatros]

kvantti,

I like the path integral picture and I find studying the decoherence very interresting. These are the kind of path I would favour instead of any kind of interpretation (and specially MWI, I confess). Looking at QM from other point of view or in new situation is really the right job for physicists, I believe.

In addition, path integral theory has yield a high return already and can really be considered a a fruitful point of view? Similarly, the analysis of decoherence is directly related to experience as well as understanding QM and one can only hope the best from such topics. could we say the same from the MWI ? for the CoI, we can at least give it the credit of history as well as some pedagogical utility.

Careful,

I agree totally with you. I think it is not a wrong direction to try reducing QM to something more decent. Hidden variable was a valuable attempt indeed. I guess this way is not closed.


Michel

 

[kvantti]

Yes, the path integral formulation is mostly thought to be the formulation of QM; so many things just get easier with it (especially with the quantum field theory).

The thing with the path integral formulation is that, in my point of view, it avoidably leads to the MWI. Why else would you have to calculate all the possible paths of a particle if the particle wouldn't actually travel along all those paths?

According to the MWI, the particle actually does travel along all the possible paths; one path in one universe.

I'm sorry that I bring up the MWI all the time, but I just recently understood its reasoning and found it to be logical. Before that I hated to think that our universe wouldn't be the only one... but instead of one universe, we have one multiverse... according to the MWI, that is.

Heres another site about the sum over histories formulation:

http://www.einstein-online.info/en/spotlights/path_integrals/index.html

 

[lalbatros]

kvantti,

I would like to understand, but unfortunately I don't see what the MWI explains and how it could tested experimentally and how it could lead to anything new in the quantum theory.

We can contrast that with the Hidden Variable Theory. The main success of this HVT attempt were:

that it could be tested experimentally,
that it failed to contradict QM,
it allowed us to reduce the number of possible interpretations
that it restricts the directions for other interpretations
that it was a first operational model to try an explanation for QM​

Now, let me try a comparison with electromagnetism. The fathers of EM had complicated mental pictures to represent the electromagnetic fields. These pictures were essentially analogies with fluid mechanics and led to the famous and lengthy discussions on the nature of "ether". Today, the EM fields are taken as irreductible objects. Nobody feels a need to discuss an explanation altough a more general theory can be considered.

Compare this situation in EM theory with that of elementary QM. Assume that the wave function (or state vector) is the irreductible object needed to describe the world at the microscopic level. I think that nothing else would be needed. In particular, the dark discussions about the postulate for the "reduction of the wavepacket" is totally useless. It is enough to study the interaction between a microscopic system and a macroscopic system (eventually a measurement device) to get the at the rule stated in the postulate. Why then would we need anything else and specially the MWI (purely formal physics I think).

I admit however that QM might be reduced to something more convenient for our human minds and that it is interresting to think about it. It could also be reduced to something less convenient for our minds. Both options might bring new physics. Finally, it may well be that physics cannot be reduced to something more confortable, then we need to cope with it.

Michel

 

[selfAdjoint]

Yes, the path integral formulation is mostly thought to be the formulation of QM; so many things just get easier with it (especially with the quantum field theory).

The thing with the path integral formulation is that, in my point of view, it avoidably leads to the MWI. Why else would you have to calculate all the possible paths of a particle if the particle wouldn't actually travel along all those paths?

According to the MWI, the particle actually does travel along all the possible paths; one path in one universe.

I'm sorry that I bring up the MWI all the time, but I just recently understood its reasoning and found it to be logical. Before that I hated to think that our universe wouldn't be the only one... but instead of one universe, we have one multiverse... according to the MWI, that is.

Heres another site about the sum over histories formulation:

http://www.einstein-online.info/en/spotlights/path_integrals/index.html

Feynmann "The particle follows all the paths and you add up all the amplitudes and everything cancels out except the classical path". Cancels out. No real paths in sidebar realities.

 

[kvantti]

Well, David Deutch claims that the only way to explain the operations made by a quantum computer physically is the MWI. No one has put a serious argument against him.

In another https://www.physicsforums.com/showthread.php?t=123610" setAI wrote:

as the reference to the poll indicates- it is MWI- since this poll the MWI has been experimentally verified http://www.quiprocone.org/Protected/Lecture_2.htm - since around the year 2000 the Everett interpretation has been confirmed as the only EXPERIMENTALLY VALID interpretation of QM- all other [non multiverse[ interpretations no longer fit with observations- as a result we now have the field of quantum computers- which are only predicted and described by the Everettian MWI-

Check out the video for more info on quantum computing. As you can notice, David Deutch explains what happens in quantum computer using the multiverse concept. Theres no pro-Copenhagenist who has done the same... as far as I know.

Feynmann "The particle follows all the paths and you add up all the amplitudes and everything cancels out except the classical path". Cancels out. No real paths in sidebar realities.

Yes, that is the most probable path of the particle, but it isn't the only one that is observed. And according to the MWI, the paths cancel out because the particle interferes with itself in the multiverse. And that is what is being calculated: the particle (or wavefunction of the particle) interfering with itself.

 

[CarlB]

lalbatros:

http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm

How yould you interpret the results of this experiment? In my point of view, the MWI is the only choice to describe this experiment physically.

The heart of the problem is the comment in the above link:

Time 6. Upon accessing the information gathered by the Coincidence Circuit, we the observer are shocked to learn that the pattern shown by the positions registered at D0 at Time 2 depends entirely on the information gathered later at Time 4 and available to us at the conclusion of the experiment.

The position of a photon at detector D0 has been registered and scanned. Yet the actual position of the photon arriving at D0 will be at one place if we later learn more information; and the actual position will be at another place if we do not.

The error that the author is making is to divide the experiment up in the middle. This is not physical. All that we can be certain is that before the experiment, when the events are all in the future, we have an initial condition that we can control. After the experiment, when the events are all in the past, we can examine the final state that was measured. We cannot halt time and examine the state of the system between time 2 and time 4.

The MWI purports to provide a physical explanation for the state of the system between time 2 and time 4. While it is true that the Copenhagen interpretation refuses to give a physical interpretation to this intermediate state, it is not true that MWI is the only version of QM that gives an interpretation. Bohmian mechanics, for example, also gives one.

The only way we can "halt" time and examine the states between time 2 and time 4 is to make measurements of the state at some time. We do this by making a measurement, that is, we arrange for a sequence of events which will result in a set of final states that we can distinguish. But that activity in itself is a measurement and doing this would change the whole system. No, the only physical thing we can do with experiments is set them up and much much later, examine the results. For this reason, the initial and final states are the only states that it makes any sense to relate to physical conditions.

We known almost nothing about how time really works. To assume that there is a continuous sequence of states between the initial and final states is to make assumptions about time that we have not and cannot prove. It is quite possible that some other, simpler and more natural, method of describing how this experiment will arise, one that does not need the fantasy of the MWI. I believe that such a description does exist, and arises from making time a much more complicated thing than it usually is. MWI instead assumes a very simple model for time.

You can always go with the "shut up and calculate", but then you're missing a big part of the mystery of QM. The mathematical formulation of QM makes amazingly accurate predictions, and yet you really can't explain it physically; there's just different interpretations.

I agree with you here, but I would be remiss if I didn't say that the reason we are told to "shut up and calculate" is because the alternative appears to be a waste of time. The MWI has not explained anything about the universe that was not already known. The MWI does not give a calculation for the masses of the neutrinos, or for the weak mixing angle, or for any other physically measurement. So far, MWI has not proved itself useful. Then again, Bohmian mechanics has not proved itself useful either. Shut up and calculate.

Btw. the "fantasy solution" is considered to be THE solution by most (but not all) physicist for explaining quantum mechanics.

I've heard this unlikely statement made before. There must have been some obscure poll taken over a very small number of physicists. MWI is barely even mentioned in the textbooks and no one has earned a Nobel prize from it.

Carl

 

[Hurkyl]

As you can notice, David Deutch explains what happens in quantum computer using the multiverse concept. Theres no pro-Copenhagenist who has done the same... as far as I know.

Probably because it's trivial.

The computer begins in a quantum state. It evolves to another quantum state. End of story. The only difference between the Copenhagen and MW interpretations here is what happens after the computer's done running.

To a Copenhagenist, all a functioning quantum computer proves is that no measurement occurred during the computation.

 

[Tipi]

Hi,

Maybe one would like to avoid randomness, but that's not possible (as far as we know). So, in the most optimistic point of view, the MWI describes the randomness that we are unable to avoid, a sort of catalog of what might be possible. But giving a reality to this catalog of "the possibilities" is a useless abstraction.

But then, QM can not describe an individual system? In fact, the reality of the catalog is given by an ensemble of measurements. And the theory gives no reality to a single measurement... I will not ask Why we obtain this result in that single measurement?, but I will ask : Why not to try to explain the result of an individual measurement, why?

giving a reality to this catalog of "the possibilities" is a useless abstraction

How we explain interference without giving a certain reality to the catalog?

According to the MWI, the particle actually does travel along all the possible paths; one path in one universe.

Is MWI really talk about particles on paths in particular universe of the multiverse? Your word's sound like Bohm's...

Then again, Bohmian mechanics has not proved itself useful either. Shut up and calculate.

What do you think of that:

http://arxiv.org/abs/quant-ph/0406173


I'm new here, so, nice to meet you guys!

Tipi

 

[CarlB]

What do you think of that:

http://arxiv.org/abs/quant-ph/0406173

The paper gives a Bohmian interpretation with trajectories for the Klein-Gordon equatoin. I think that the paper is brilliant and have saved it to my pile of papers that I am going to reference or reread someday. Thanks for bringing it up.

A question addressed is what happens when you insert a measurement in the middle of a propagation of the Klein-Gordon equation. This reads directly on the topic we've been discussing here, namely the MWI interpretation and the question of the existence of the interim states in between quantum measurements. What the author points out is that if you arrange for the particle to be measured at some time t, then you can't very well expect it to keep going forward in time, then come backwards in time back through time t, and then go forwards in time through time t again. Therefore you can't measure the same particle three times at one time t. He ties this up with the negative probability density problem for the Klein-Gordon equation beautifully.

The paper is a beautiful exposition, but I don't see how it will find a difference with standard QM. What my intuition says is that you will end up wanting to write the measurment problem in field theory terms and then his interpretation will give the same results as usual.

I should mention that despite my pointing out that Bohmian mechanics doesn't give anything over the usual interpretation, I am a great admirer and sort of believer in Bohmian mechanics. I think Bohmian mechanics is closer to the truth than the standard Copenhagen version which is only slightly less silly than MWI. Given the three theories at this time, as long as I were calculating, I would use the Copenhagen version. For my own purposes, I believe in a modification of Bohmian mechanics where the particle and wave are the past and future of the particle, where "past" and "future" are with respect to an observer. The addition to Bohmian mechanics I have to add is that I think that there is a continuous mapping from the wave to the particle trajectories. To explain this is long and difficult because on the face of it it appears to be incompatible with multiple particle wave functions, etc.

One of the interesting parts about the paper is how he looks at Bohmian mechanics' insistence on a preferred frame of reference from a relativistic point of view. I happen to not believe in relativity, except as a calculational convenience, an extremely accurate approximation, so in this case I prefer the usual Bohmian mechanics version. It turns out that not being a "true believer" in relativity is equivalent to a virulent form of leprosy as far as your relations with other physicists go, so I've been working on quantum mechanics written in finite degrees of freedom where the issue of the structure of spacetime can be avoided. But you can only go so far with this sort of retreat. I'm writing an introduction to quantum mechanics from the density matrix point of view (i.e. with spinors derived from density matrices instead of vice versa), but I'm restricting myself to finite degrees of freedom just to avoid the reference frame issue. Later, I figure on writing a follow up that deals with the more general case, and I'll be reading and referencing papers by Hrvoje Nikolic.

As in the case with relativity, I think that QM is not the truth, but instead is an extremely accurate method of making calculations. But not being a "true believer" in quantum mechanics is a fairly mild disease, maybe equivalent to a bad case of acne.

Carl

 

[kvantti]

To a Copenhagenist, all a functioning quantum computer proves is that no measurement occurred during the computation.

What is a "measurement" then? How does a "measurement" differ from any interaction that collapses the wavefunction? Wouldn't the wavefunction of a photon collapse when interacting with a beamsplitter?

And what do Copenhagenists have to say about http://xxx.lanl.gov/abs/quant-ph/9906007" paper? Deutsch claims, and shows, that if we chop something with the Occam's razor, quantum physics becomes a local theory. If this is true, wouldn't all non-local interpretations (including Bohm's and Copenhagen) be unphysical?

 

[lalbatros]

kvanti,

Wouldn't the wavefunction of a photon collapse when interacting with a beamsplitter?

The beamsplitter in an interferometer experiment plays exactly the same role as the two-slit-screen in the Young's two slit experiment. For the latest, I guess, you would not say the (interaction with) screen makes the collapse happen. It is rather a photon detector -of any kind- (including an obstacle on one slit) that performs the measurement.

Similarly, the quantum computer never collapses any wave function since it would then behave -at best- as a classical computer. A measurement would occur only when some result is read in some way, with a printer or -more realistically- with photon detector. And you certainly know all the funny things that follow, like to possibility to detect spying for example.

Michel

Postscriptum
Of course, the comparison with a screen is not totally perfect. With a screen you could always add more slits by removing some part of the screen (a part that is truly performing a measurement and really constrains the experiment).

But of course this is irrelevant since we are discussing what happens behind the screen.

 

[selfAdjoint]

The beamsplitter in an interferometer experiment plays exactly the same role as the two-slit-screen in the Young's two slit experiment. For the latest, I guess, you would not say the (interaction with) screen makes the collapse happen. It is rather a photon detector -of any kind- (including an obstacle on one slit) that performs the measurement.

It's really hard to see in an intersction interpretation how some interactions "collapse the wave function" and others don't. Why does passing through a beam splitter, or a slit, not produce a collapse, while passing through a counter does? In path integral form, the paths aren't "added up" until the "measurement" either. What happens where there is no measurement? Inside the Sun, as I never tire of asking?

 

[kvantti]

It's really hard to see in an intersction interpretation how some interactions "collapse the wave function" and others don't. Why does passing through a beam splitter, or a slit, not produce a collapse, while passing through a counter does? In path integral form, the paths aren't "added up" until the "measurement" either. What happens where there is no measurement? Inside the Sun, as I never tire of asking?

This is exactly my point and I find it strange that Copenhagenists don't see a problem in it. How does a measurement differ from any interaction the photon encounters? If you answer "we gain information", then what is this information physically? Knowledge? And what is knowledge then? And why does knowledge affect reality? And wouldn't the wavefunction have to interact with the beamsplitter in order to be reflected from it?

To me, the CoI is just another way to say "shut up and calculate".

 

[Hurkyl]

What is a "measurement" then?

That, of course, is the hard question. It's something that collapses the wavefunction, but it's not clear exactly what sorts of things would do that.

Wouldn't the wavefunction of a photon collapse when interacting with a beamsplitter?

(If we assume there are no hidden variables) we can prove, by experiment, that it does not. Thus, beamsplitting doesn't perform any sort of measurement.

And what do Copenhagenists have to say about this paper?

Based on its abstract and your description, it sounds like it's proving that unitary evolution is local. Even if I put my Copenhagen hat on, I would still agree with that... although I could manage some objections if I really wanted to.

if we chop something with the Occam's razor

Occam's razor is fun, because everybody has their own idea what should be chopped. There are two conflicting chops here:

(1) We can still explain observations if we give up the assumption that measurements have absolute outcomes. So, we should lop off that assumption.
(2) It is impossible to remember a particular outcome and see any effect of the branches where the other outcome happened. So, we should chop them off.

Actually, relational QM essentially makes both chops. So it's the superior interpretation, from that point of view.

 

[lalbatros]

selfAdjoint,
kvantti,

It's really hard to see in an intersction interpretation how some interactions "collapse the wave function" and others don't.

Remember the title of this thread.

My question was precisely about the collapse of the wave function and specifically about how the ad-hoc postulate has (mis-) led people to unrealistic discussions (by unrealistic I mean useless interpretations ...).

You will find as many people as you want to discuss about interpretations. But all will agree that the Schrödinger equation applies to any system including a measurement instrument, and at any time. And I agree that any interaction, was it with a beam splitter or with a photomultiplier, should be included in the relevant simulation of a complete experiment (or phenomenon).

More precisely, I think that the "collapse of the wave function postulate" is not to be interpreted as a special interaction that supersedes the Schrödinger equation. But it is simply the physical meaning of the wavefunction as a "probability amplitude"! Interpreting further will not explain anything or predict anything in a lab. Still you need the rule for probabilities, since the probabilities are the real experimental data. But having introduced the unphysical and unneeded "collapse" in the postulates was -I think- a bad idea. The wavefunction is a probability amplitude before, during and after an experience is carried out, and it evolves according to the same laws.

That being said, we must still recognise however that, in an interferometer experience, a beam splitter or a slit diaphragm or a photomultiplier do not precisely play the same role, even if each does interact somehow during the experiment. It is clear that the statistical evaluation of the wavefunction is performed by the photomultiplier. In this sense, the photomultiplier performs the measurement (or the counting) while the beamsplitter and eventually the diaphragm is the system under test.

Further, we should recognize that these interactions may have very different effects on the system. Clearly, the interaction with the photomultiplier (or with the diaphragm) has a much more "important" effect on a photon that the beamsplitter! In a sense the beamsplitter keeps a photon alive, while a photomultiplier kills it ! The interaction with the photomultiplier is clearly irrevesible, not so for the beamsplitter.

Maybe I could change my mind about the postulate, if I was able to understand further the irreversibility of some interactions.

Michel


Postscriptum
The irreversiblity of the measurement (yes!) has been interpreted as due to an interaction with a classical system. By "classical" system, I understand at least a system with a practically continuous spectrum.

Continuous spectra are known to give rise to irreversible behaviour, like atomic emission or nuclear decay. Can give rise to philosophical discussions too!

 

[Hurkyl]

But all will agree that the Schrödinger equation applies to any system including a measurement instrument, and at any time.

Not true. Some will argue that macroscopic scales are outside of QM's domain of applicability.

More precisely, I think that the "collapse of the wave function postulate" is not to be interpreted as a special interaction that supersedes the Schrödinger equation. But it is simply the physical meaning of the wavefunction as a "probability amplitude"!

That last sentence is an interpretation.

But this one's not a matter of interpretation; it's a matter of the mathematics. Time evolution in QM is unitary. Wavefunction collapse is not unitary. Therefore, a wavefunction collapse cannot occur through ordinary time evolution. The different interpretations are all different approaches to this problem.


Copenhagen says that measurements have outcomes, and thus collapses happen.

MWI says that measurements don't have outcomes, and thus collapses don't happen.

Bohm says that collapses don't happen, but there's a pilot wave choosing outcomes.

Relational QM says that it's all a matter of perspective.

 

[lalbatros]

Hurkyl,

Some will argue that macroscopic scales are outside of QM's domain of applicability.

Maybe. But anyway the scientific truth is not based on a majority, most people agree with that!

Quote:
... I think that the "collapse of the wave function postulate" is not to be interpreted ... it is simply the physical meaning of the wavefunction ...!

That last sentence is an interpretation.

I think you are joking!
That the SE governs quantum systems is not an interpretation.
The probability amplitude is not either, it is the root definition in QM.

Time evolution in QM is unitary.

Again, here I think you try to start a debate for the fun.
Classical mecanics is microscopically reversible too, but nobody thinks it contradics thermodynamics.
On the contrary, the microscopic reversible laws are sufficient to explain the macroscopic irreversibilities.

In addition, we can go back to the physics of atomic emission.
All laws (interactions) are indeed reversible, still -with a continuous spectrum- an irreversible decay is easily predicted. This is just decoherence occurring by summing up modes from a continuous spectrum. Why should we believe something special is needed to understand a "quantum measurement irreversibility".

Michel

 

[selfAdjoint]

But anyway the scientific truth is not based on a majority, most people agree with that!

Classical mecanics is microscopically reversible too, but nobody thinks it contradics thermodynamics

Consistency is the hobgoblin of small minds.

 

[lalbatros]

selfAdjoint,

I used a few emoticons to help with reading.

Michel

Trust thyself: every heart vibrates to that iron string.
...
The nonchalance of boys who are sure of a dinner, and would disdain as much as a lord to do or say aught to conciliate one, is the healthy attitude of human nature. A boy is in the parlour what the pit is in the playhouse; independent, irresponsible, looking out from his corner on such people and facts as pass by, he tries and sentences them on their merits, in the swift, summary way of boys, as good, bad, interesting, silly, eloquent, troublesome. He cumbers himself never about consequences, about interests: he gives an independent, genuine verdict. You must court him: he does not court you.

 

[selfAdjoint]

selfAdjoint,

I used a few emoticons to help with reading.

Yeah I got the point with the first quote, but you were so THERE with it I just couldn't resist!

Science history suggests that in the long run, the firm opinion of the most prominent and best trained scientists is dead wrong. This fact is what set Kuhn off, but he's no better,

 

[Hurkyl]

That the SE governs quantum systems is not an interpretation.

Right -- but the point is that there's disagreement over the question of whether macroscopic things like humans, measuring devices, and the universe are quantum systems, or not.

It could very well be that each little microscopic portion of a measuring device is (approximately) a quantum system, but when the device is considered as a whole, QM is inaccurate.

The probability amplitude is not either, it is the root definition in QM.

You're assigning physical meaning to a mathematical object. By definition that's an interpretation. But the point I wanted to make by this is that it's not universal -- for example, it makes no sense in MWI.

Again, here I think you try to start a debate for the fun.

I've never argued this side of the coin before. It's fun, and a lot easier! I'm not really trying to convince you; I just get the feeling that you don't really see the reasons people adopt the opposing position.

Classical mecanics is microscopically reversible too, but nobody thinks it contradics thermodynamics.
On the contrary, the microscopic reversible laws are sufficient to explain the macroscopic irreversibilities.

But that's still not a wavefunction collapse. Quantum thermodynamics is still a quantum theory -- it cannot magically select one outcome amongst many to "actually happen" (more or less). After decoherence, you're still stuck with the universe being in a superposition of you seeing a spin-up electron and you seeing a spin-down electron... and if you want to posit that your measurement had a definite outcomes, you need something more.

 

[lalbatros]

Hurkyl,

... but when the device is considered as a whole, QM is inaccurate ...

No, I cannot agree. QM is more general than CM, and all classical behaviour can be obtained from the pertinent limit in QM. This is how QM has been built, anyway, through the correspondance principle.

Quote:
The probability amplitude is not either, it is the root definition in QM.

... By definition that's an interpretation ...

No, this is the main bridge between theory and experience. It is absolutely necessary for checking experimental data against theoretical predictions and for translating theory in experimental expectations. A correspondance bridge between theory and experience could be called an interpretation, of course. But it is the kind of interpretation that matters, not a simple dictionary from a language to another. It is more like a correspondance between language and reality, the reality in the lab.

If you think twice, you will realize that the widespread use of the density operator (rho) is precisely justified by this useful "interpretation". Generally, experimental prediction by a theoretical model are expressed by an average of "detection" operator with the famous formula: <D> = tr[D rho] . This could be for example the intensity of an atomic emission. You will also agree, I think, that quantum mechanics can be fully constructed on the density operator. And this would smooth out much of the debate.

Quote:
Classical mecanics is microscopically reversible too, but nobody thinks it contradics thermodynamics.

But that's still not a wavefunction collapse.

This is my main point of interrest. I think this should not explain the |phi|² rule for quantum probabilities. But I do think it brings an irreversibility.

When dealing with a macroscopic system the outcome is simply quantum thermodynamics or statistics, for example the black body radiation theory!

Now considering the interaction between a macroscopic system and a small coherent quantum, I expect the irreversibility should pop up because of the continuous spectrum. In other words, I think it should be possible to prove the (practical) equivalence between two descriptions:

the entangled state description (macro-micro) (the full density matrix)
the statistical projection description (the diagonal part of the density matrix)​

I think so because of the (striking) analogy with the nuclear decay I mentioned earlier. It is clear that this decaying system could be described with an entangled state. And it is equally clear that the projected statistics works just as well, since it is the usual point of view. Is that not called decoherence?

Having said that, I am still left with my |phi|² question that I launched in another thread.
But I didn't see why i would need a MW reality to understand QM.
I also understand by the discussion that the fathers of QM did their best with the projection postulate, but that it is indeed not absolutely necessary.

Michel

 

[Hurkyl]

No, I cannot agree. QM is more general than CM, and all classical behaviour can be obtained from the pertinent limit in QM. This is how QM has been built, anyway, through the correspondance principle.

I'm not asking you to agree -- just to acknowledge that it's not a settled question! The two main lines of objections, I think, are:

(1) There is no experimental support that QM adequately describes everyday objects, like tennis balls at room temperature.

(2) There is no theoretical proof that QM reduces to CM in the many-particle limit.

It is absolutely necessary for checking experimental data against theoretical predictions and for translating theory in experimental expectations.

Again, I'll point out MWI as a counterexample, since it does not make that assumption. Probabilistic predictions are made through other means. (and, I think, how to get probabilities is one of the main topics of research in MWI. Maybe vanesch can clarify if he appears)

But I do think it brings an irreversibility.

Unitary evolution of the universal wavefunction yields "irreversible" evolution of individual systems, such as decoherence. But it cannot provide a mechanism that selects one of the possible outcomes for us to actually observe.


I think I may be missing the goal of your inquiry... it might help if you restated just what you're after. (Or, if not, I'll try rereading the thread again when I have time)

 

[Demystifier]

The paper is a beautiful exposition, but I don't see how it will find a difference with standard QM. What my intuition says is that you will end up wanting to write the measurment problem in field theory terms and then his interpretation will give the same results as usual.

Hi,

I have put my name in Google and get pleasantly surprized by finding this discussion.
So let me answer.
As shown in the paper, the Bohmian interpretation leads to the result that the particle cannot be found at some positions at which the wave function does NOT vanish. I think it is a significant difference with respect to standard QM.

 

[CarlB]

Demystifier, Sorry for not getting back sooner. I lost track.

My intuition says that those places where the standard QM predicts a particle and your analysis does not are very close together. And building a measurement apparatus which is suffiicently high energy to detect the difference will give probabilities of pair creation. I should admit that I think in terms of massive fermions.

Carl

 

[Demystifier]

CarlB, I agree with you that it is difficult to measure this effect in practice. But at least it is possible in principle, so the usual claim that the Bohmian interpretation is physically useless because it does not lead to new measurable predictions - is wrong.

 

[vanesch]

But this one's not a matter of interpretation; it's a matter of the mathematics. Time evolution in QM is unitary. Wavefunction collapse is not unitary. Therefore, a wavefunction collapse cannot occur through ordinary time evolution. The different interpretations are all different approaches to this problem.


Copenhagen says that measurements have outcomes, and thus collapses happen.

MWI says that measurements don't have outcomes, and thus collapses don't happen.

Bohm says that collapses don't happen, but there's a pilot wave choosing outcomes.

Indeed. I would like to point out to lalbatros that the fundamental problem of the measurement process doesn't reside in the randomness or anything, but rather in the linearity of the time evolution equation (as Hurkyl pointed out, the unitarity of the time evolution), which can never produce a projection. So no matter how complicated the physical interaction is, if it is described by the language of quantum theory as we know it, then it will not project the wavefunction.
Now, one could think that this linearity can then simply be seen as a good approximation (some do that! GRW for instance), and that the "true" quantum time evolution is non-linear. This is a possibility but the problem is that in doing so, one looses the locality of the interactions (which IS conserved in the case of strictly unitary interactions).

 

[vanesch]

Again, I'll point out MWI as a counterexample, since it does not make that assumption. Probabilistic predictions are made through other means. (and, I think, how to get probabilities is one of the main topics of research in MWI. Maybe vanesch can clarify if he appears)

It is in fact the "holy grail" of MWI, as indeed, there doesn't seem to be an obvious way now to generate probabilities. There are different MWI flavors and they often diverge on the details on how to see probability.

However, there is one red line through all these views: that is: instead of QM generating probabilities for events, it generates probabilities for observers.

That is, for each thing we call a "measurement", it is a measurement wrt to an observer, that is, a psycho-physical link to a subjective experience (that is what is, ultimately, an observer). You have to make a distinction between the "physical degrees of freedom" of the material carrier of an observer, and the observer itself (which is a subjective experience that goes with it). For a human, for instance, the physical degrees of freedom correspond to the quantum mechanical description of his body. Now, if these physical degrees of freedom occur in different terms in the wavefunction, which are entangled with other systems and have a certain stability in time and all that (it is on these points that different views on MWI differ), then that means that to the same set of physical degrees of freedom, can be associated different observers ("copies of them"), and the wavefunction generates in one way or another (again, different views here correspond to different flavors of MWI) a statistical ensemble of "observers".

In other words, by simply applying the Schroedinger equation, you arrive at a state which can formally be represented as:

|my-body-sees-dead-cat> |dead-cat>|stuff> + |my-body-sees-live-cat>|live-cat>|otherstuff>

The physical degrees of freedom of my body occur in two terms, entangled with other stuff. This generates a statistical ensemble for subjective experiences ("observers") to be attached to my body. MY personal subjective experience being one of them, I draw my subjective experience from that ensemble, which gives me the probabilistic impression.

Now, it is mostly disturbing to have to talk about "subjective experience" in a physical theory. This is what turns off most people from the start in MWI. But we have also to realize that the only way of knowing that there is in fact a kind of measurement problem in QM, is through subjective experience ! If it weren't because we "know" that we cannot be "at the same time" walking in the park and shopping in the grocery store, by our experience, then this would not be contradictory! All things you could calculate to both of these states in superposition would still be correct (they would happen then "simultaneously"). So it seems that the only objection against superposition comes from our subjective experience - in that case one shouldn't be surprised that it is also part of the proposed solution.

EDIT: let me clarify a bit more the last bit.
We have a serious problem in considering "ourselves in superposition". Nevertheless, if we say that we are to have a quantum description (a vector in hilbert space), then it should be obvious that the superpositions of "ourselves here" and "ourselves there" is entirely part of the description. So the problem is already introduced from the very start: if our bodies are to have a genuine quantum description, there's no way to exclude these superpositions.
You might hope somehow that some dynamical instability will remove these states, and quickly bring them to the "me here" state, or the "me there" state, but unfortunately, with a unitary time evolution, that's impossible to achieve (THIS is the fundamental problem):
if some initial state leads to "me here" and another initial state will lead to "me there", then the superposition of these two initial states will lead to a superposition of "me here" or "me there".

So logically, there are only 3 ways out:
- quantum descriptions don't apply to things like human beings, or we don't have to take them literally. Ok, but then QM is not a universal theory, and the question is then: what is a universal theory ? This includes essentially all "shut up and calculate" views, who only see the quantum formalism as a technique that allows to calculate outcomes of observations, as well as alternatives to QM (local or nonlocal realist theories...).
- the unitary evolution is not strictly correct, and the corrected dynamics WILL include these kinds of instabilities. This is very well possible. GRW is one such approach ; Penrose thinks that gravity in one way or another plays this role.
- these superpositions really do occur. In that case we have to say why we don't experience life that way. That's essentially MWI.

EDIT II:

for a (very good) overview of the ideas behind MWI, look here:

http://plato.stanford.edu/entries/qm-manyworlds/

 

[vanesch]

If you think twice, you will realize that the widespread use of the density operator (rho) is precisely justified by this useful "interpretation". Generally, experimental prediction by a theoretical model are expressed by an average of "detection" operator with the famous formula: <D> = tr[D rho] . This could be for example the intensity of an atomic emission. You will also agree, I think, that quantum mechanics can be fully constructed on the density operator. And this would smooth out much of the debate.

The problem with the use of the density operator this way, is that you *already* made use of the projection postulate in order to write <D> = tr[D rho], when you apply it to a reduced density matrix of a subsystem.

I think it should be possible to prove the (practical) equivalence between two descriptions:

the entangled state description (macro-micro) (the full density matrix)
the statistical projection description (the diagonal part of the density matrix)​

Sure, that's the entire content of decoherence theory. But, it doesn't give a solution to the measurement problem as such. It only indicates that *if you have a solution to the measurement problem*, that in that case, you can place the Heisenberg cut anywhere, once there is entanglement with the environment, because this will not numerically change the outcomes.

If you are interested in these issues, I suggest you look at "Decoherence and the appearance of a classical world" by Zeh, Joos et al. (look on amazon for details).

But I didn't see why i would need a MW reality to understand QM.
I also understand by the discussion that the fathers of QM did their best with the projection postulate, but that it is indeed not absolutely necessary.

The density matrix formalism uses the projection postulate in a disguised way. If you have a pure state: |psi>, and you calculate rho = |psi><psi|, and write out the matrix in your preferred basis, then you have the corresponding "Born rule probabilities" on the diagonal. However, the matrix is not diagonal in all generality. "After the measurement", it has become a statistical mixture, and now we should put all the non-diagonal elements to zero. It is this operation which is mathematically impossible by a unitary time evolution. So no time evolution operator you can think off can do this.

However, what you can do is to "trace out the environment". That is, calculate for an entangled state, the reduced density matrix that corresponds only to a subsystem. What decoherence theory shows you, is that, with enough "irreversible entanglement in practice", that this reduced density matrix will take on a certain diagonal form in a "basis which is robust wrt to the environment" (which comes down to the so-called pointer basis).
If you now interpret this reduced density matrix as a genuine density matrix, then it looks like the density matrix of a mixture.
However, it isn't really a mixture! The genuine density matrix is the larger matrix, including the environment, and there exists in principle a unitary operator which can "undo" the entanglement - which wouldn't be the case if we had a genuine mixture. We can establish correlations with the overall density matrix, which are not described by the local reduced density matrix.
The local density matrix is in fact obtained by assuming that at some point, we can apply the Born rule on the larger system, and then sum over the probabilities of the "irrelevant" degrees of freedom of the environment. That's exactly what is done by taking the partial trace. If you don't assume that, then there is no meaning to "taking the partial trace".

 

[Anonym]

Lalbatros:”Remember the title of this thread.
My question was precisely about the collapse of the wave function and specifically about how the ad-hoc postulate has (mis-) led people to unrealistic discussions (by unrealistic I mean useless interpretations ...).”

The “collapse of the wave function” is not a postulate and it is not interpretation dependent. The collapse is universally valid firmly established experimental result connected with the transition from Quantum world to Classical world (E. Schrödinger cat). The collapse is a key problem of the measurement theory and therefore also is called “The Measurement Problem”. The theoretical description of the natural phenomena is groundless without consistent and adequate measurement theory. There are and always will be people that claim that a quantum world is a classical world or a classical world is a quantum world, that the measurement problem do not exist, AB phenomenon do not exist,quarks do not exist, etc. These are the people that have no problems in their worlds. God bless them. But it is not interesting world.

Lalbatros:
”That the SE governs quantum systems is not an interpretation.

The probability amplitude is not either, it is the root definition in QM.

And I agree that any interaction, was it with a beam splitter or with a photomultiplier, should be included in the relevant simulation of a complete experiment (or phenomenon).”

May be you are right. Then the interaction should be treated as the communication message. It should be encoded by transmitter and decoded by receiver. The transmitter code is the complex Hilbert space mathematical structure. The receiver code should be matched. And the classical physics are dispersion free. Therefore, the real Hilbert space will do a job.