Decoherence does not collapse wavefunc.

[arkajad]

There is a distinct physical process of decoherence which one can express easily in the density operator language (which you resist accepting) which is not the same as collapse and indeed shows that classical and quantum collapse are indistinguishable. (Classical collapse being the baysian updating of probabilities given subsequent observations.)

There are distinct physical events which one can express easily in the stochastic processes' language.

I did not see one event in finite time for an individual quantum system derived from the decoherence formalism. But if you show me one - I may even change the team.

BTW. Collapse is NOT bayesian updating probabilities. It is a sudden change of the wave function. Probabilities is a related (in a not so simple way)- but not the same - business. Moreover, they are not bayesian. At least not those that I am talking about.

 

[jambaugh]

There are distinct physical events which one can express easily in the stochastic processes' language.

I did not see one event in finite time for an individual quantum system derived from the decoherence formalism. But if you show me one - I may even change the team.

BTW. Collapse is NOT bayesian updating probabilities. It is a sudden change of the wave function. Probabilities is a related (in a not so simple way)- but not the same - business. Moreover, they are not bayesian. At least not those that I am talking about.

A baysian updating of probabilities is a "sudden change in the probabilities". (If you'll please let me invoke density operators...)

You can express the collapse either before or after you express the decoherence aspect of the measurement process. (There is no need to pay attention to the decoherence if you simply want to incorporate the new information of the measured value). But in detail, when you measure say X you are coupling the system via its X observable to a measuring device which itself is coupled to an entropy dump.

In the process the X observable becomes correlated to the recording variable (meter) of the measuring device while the whole system + meter decoheres. One is amplifying the quantum variable and like any amplifier you must have energy (to move the meter) and a heat sink (to make the meter settle down into the recorded position).

In so far as the description of the measured system goes you have a density operator diagonal in the eigen-basis of X and correlated to the density operator describing the meter. This meter description is highly separated and you can treat it as a classical system at this point. You have a series of probabilities for each correlated X value of system and "x" record on the meter.

Now you "collapse" by looking at the meter and updating the now classical probability distribution over the eigen-basis of X. This step is simply the same Baysian updating of the classical probabilities of the outcomes of the X measurement from what it was to certainty that a specific x value was measured. It is qualitatively no different from updating the expectation value of a lotto ticket after you read the results of the Sunday drawing in the morning paper.

Now there is a great deal of variability in the decoherence stage, you can entangle then decohere, entangle then measure the entangled partner. The uncertainty principle reflects the necessary correlation of observables non-commuting with X to variables in the measuring device which necessarily get coupled into the entropy dump. E.g. to settle down a needle on a literal meter you need to dissipate its momentum into a heat sink (friction and coil resistance). But once the system in question interacts with the auxiliary meter system the original system when considered by itself has effectively decohered as reflected in its reduced density op.

If you've further interest in the matter I'll see if I can cook up a detailed description of a particular act of measurement. (It's something I need to do anyway.)

 

[yoda jedi]

You are missing the point. Everybody is calculating lot of things. And you too. There is nothing wrong with calculations. There is nothing wrong with solving differential equations - they are on paper or in the mind.

The point is whether at the end of your calculation you get something that you can compare with observations. In this respect there is no difference between solving differential equations and models with collapses. In each case at the end you get numbers or graphs that you can compare with experimental data.

So, your war is misdirected.

a lot of physics professionals with papers in respected and prestigious journals.

BTW. Collapse is NOT bayesian updating probabilities. It is a sudden change of the wave function. Probabilities is a related (in a not so simple way)- but not the same - business. Moreover, they are not bayesian. At least not those that I am talking about.

i agree.

 

[arkajad]

If you've further interest in the matter I'll see if I can cook up a detailed description of a particular act of measurement. (It's something I need to do anyway.)

It's not been done so far? So unimportant? Amazing!

 

[A. Neumaier]

So decoherence does not collapse the wavefunction?

The relation is more complex.

Decoherence explains the dynamical decay of off-diagonal entries in a density matrix rho, thus reducing a nondiagonal density matrix (e.g., one corresponding to a pure state psi via rho = psi psi^*) to a diagonal one, usually one with all diagonal elements occupied. In particular, this turns pure states into a mixture.

On the other hand, the collapse turns a pure state psi into another pure state, obtained by projecting psi to the eigenspace corresponding to a measurement result. In terms of density matrices, and assuming that the eigenspace is 1-dimensional, a collapse turns a density matrix rho into a diagonal matrix with a single diagonal entry. This is not explained at all by decoherence.

A thorough discussion is given in Schlosshauer's survey article
http://lanl.arxiv.org/abs/quant-ph/0312059

See also Chapters A4 and A5 in my theoretical physics FAQ at
http://arnold-neumaier.at/physfaq/physics-faq.html#decoherence
in particular the entry ''Does decoherence solve the measurement problem?''

 

[Alfrez]

It's not been done so far? So unimportant? Amazing!

Can someone comment on these debates between jambaugh and arkajad?

Jambaugh is bonafide copenhagen who believes that the wave function doesn't actually represent the properties of the system, but is just a part of a mathematical formalism that can be used to calculate probabilities of possible results of experiments, as someone put it. While arkajad is the other side where something may be occurring physically and so make valid the Interpretations like Many Worlds, Bohmian Mechanics. Jambaugh (I think) believes that Interpretations are not even possible at all by ontology. The following illustrates the point.

In the double slit, an emitter send a buckyball composed of 60 carbon atoms in pure state. We can measure interference if the system is not perturbed by external noise. Now Jambaugh believes what occurs between the emission and detection is completely indetermine, in fact, nothing happens physically. While arkajad believes the particle can either still exist as in Many Worlds, Bohmian or even in Cramer's Transductional Interpretation.

The implication if Jambaugh were right that nothing physically happens in between emission and detection is that it is possible reality only consists of measurements and what happens before measurement is physicality doesn't even exist. This means it is possible we are living inside some kind of computer simulation? In such event, only the output (measurement) is important, what goes on between and behind it is complex programming codes and execution. So between emission and detection in the double slit. Reality is being processed in some kind of computer codes. This means even if we can detect billions of galaxies. It doesn't mean they were even there, our measurements detect the photons coming from them. And since nothing happens in between, the photons don't have to travel or even exist... we can say that in the program, there is a subroutine to make it appear that photons coming from the alleged cosmos is being detected. Imagine the subroutine in the program that says to surround and shower the Earth virtual mode with virtual cosmos data. This is one possibility if we have to take seriously in the Copenhagen view that only measurement is meaningful, what happens in between is completely indetermiate as what Jambaugh emphasized using superior mathematics which may be nothing more than features of the programming language used in modelling us. Now I mention all this to make someone refute this hypothesis. By refuting it means arjakad is right that something physically exists.

 

[A. Neumaier]

a mathematical formalism that can be used to calculate probabilities of possible results of experiments

Quantum mechanics does much more than predict probabilities of possible results of experiments.

For example, it is used to predict the color of molecules, their response to external electromagnetic fields, the behavior of material made of these molecules under changes of pressure or temperature, the production of energy from nuclear reactions, the behavior of transistors in the chips on which your computer runs, and a lot more. Most of these predictions have nothing at all to do with collapse.

It is a pity that public reception of quantum mechanics is so much biased towards the queer aspects of quantum systems. The real meaning and the power of quantum mechanics does not come from studying the foundations but from studying the way how QM is applied when put to actual use.

 

[Alfrez]

Quantum mechanics does much more than predict probabilities of possible results of experiments.

For example, it is used to predict the color of molecules, their response to external electromagnetic fields, the behavior of material made of these molecules under changes of pressure or temperature, the production of energy from nuclear reactions, the behavior of transistors in the chips on which your computer runs, and a lot more. Most of these predictions have nothing at all to do with collapse.

It is a pity that public reception of quantum mechanics is so much biased towards the queer aspects of quantum systems. The real meaning and the power of quantum mechanics does not come from studying the foundations but from studying the way how QM is applied when put to actual use.

I quoted it out of context. The complete sentence is "|u>+|v> doesn't actually represent the properties of the system, but is just a part of a mathematical formalism that can be used to calculate probabilities of possible results of experiments." as "he put it" (which I mentioned). I replaced "|u>+|v>" with simple words "wave function" which created the confusion. Of course I know QM is used in many daily electronics and applications.

You and Jambaugh are bonafide Copenhagen. While Fredrik and others are Many Worlders. So I guess the debates are still valid. Both produce the same experiment outputs. Is there any implication by knowing the correct interpretations. Yes. Unification with General Relativity or comprehending Quantum Spacetime by ontology.

 

[A. Neumaier]

You and Jambaugh are bonafide Copenhagen.

No; I am not.

I have my own interpretation. It is superior to any I found in the literature, since it
-- needs only one world,
-- applies both to single quantum objects (like the sun) and to statistical ensembles,
-- has no split between classical and quantum mechanics,
-- has no collapse (except approximately in non-isolated subsystems),
-- has no concepts beyond what is taught in every QM course.
I call it the the thermal interpretation since it agrees with how one does measurements in thermodynamics (the macroscopic part of QM (derived via statistical mechanics), and therefore explains naturally the classical properties of our quantum world. It is outlined in my slides
http://arnold-neumaier.at/ms/optslides.pdf
and described in detail in Chapter 7 of my book
Classical and Quantum Mechanics via Lie algebras
http://lanl.arxiv.org/abs/0810.1019

Is there any implication by knowing the correct interpretations.

The main advantage of having a good interpretation is clarity of thought, which results in saving a lot of time otherwise spent in the contemplation of meaningless or irrelevant aspects arising in poor interpretations.