I see quantum measurement theory as the greatest challenge of quantum physics. The challenge is to bring observer part of the physical system: nothing less than an extension of physics to a theory of consciousness is required.  

This requires answer to many questions. What is "self"? Is it possible to have consistency with the causality of field equations and that of free will? What is the relationship between the geometric time of physicist and experienced time which we know to have different properties?   Do we really understand the emergence of arrow of time and is it always the same? Is the existing quantum theory able to describe living systems, which might be macroscopically quantum coherent?  Etc..

Marvellous expansion of world view would be  waiting us if we only would be ready to challenge the materialistic worldview still regarded almost as a  definition of what it is to be  scientific.

Dear M. Pitkanen:

   Yes, in moments of depression I have also thought that physics should go into psychological problems. Let us hope that it will prove unnecessary.

Dear Raul and Pitkanen,

May I request you to go through my Psychophysical interpretation of quantum theory available on my RG page to see how consciousness has been there all along and how our fears that the inclusion of consciousness would complicate matters are unfounded. It is so straightforward and so simple to admit the straight fact that we are all conscious beings making conscious observations and taking conscious decisions all the while. Trying to deny the obvious really lands anyone in problems, not just QM.

Regards,

Rajat

Dear Amin, Manuel, Matti, Raul, Rajat: thank you for your answers. What do you think about Bayesian interpretation of Quantum mechanics? Or vaxjo interpretation? 

@Prof. M. Pitkanen and Raul: yes measurement problem seems interesting. I just found a paper in this regard. Perhaps you would like to make comment. Best wishes

http://arxiv.org/pdf/quant-ph/0509042.pdf

The "interpretation of quantum mechanics" and the theory of quantum-mechanical measurements and observations were definitely not settled in 1927 and still give rise to truly enormous confusion - as some of the entries in this blog show. Personally, I think that things have become much clearer, in recent years, and it is obviously time to clarify these matters in as definitive a way as possible! (I regard it as an intellectual scandal that the meaning of the most successful theory of physics has remained so mysterious and confusing during almost ninety years.)

However, I would like to express a word of caution: Don't think that these problems are easy and that you will find the correct views nicely explained in the literature! And don't mix these problems with problems in psychology and the like! Quantum mechanics - as we know it - is a physical theory describing a vast domain of Nature; but it has not been created to solve the enigmas of the mind. And, incidentally, the "observer" and his consciousness (nor, for that matter, gravity) do not play any role in a reasonable and convincing interpretation of quantum mechanics!

Dear Prof. Jurg Martin Frohlich: thank you for sharing your opinion. Yes, i also think that one of measurement problm is to clarify whether there is objective reality at microscale, or it is observer-dependent reality. After ninety years, perhaps now is the time to find a resolution on this problem. Thanks

Quantum Physics is a composite patchwork of :

- Quantum Mechanics proper, as a set of axioms with Hilbert spaces and observables, set up originally by von Neumann, about the relationship between data observed and physical quantities represented by mathematical objects

- Wave Mechanics, and the basic idea that particles can behave like waves and the converse, mostly tested and used in electrodynamics

-Particles Physics and its embodiement in the Standard Model, which does not consider gravity or General Relativity

The first is quite different from the others, as a look at the axioms show : this is not a theory in Physics at all. Just a sophisticated theory on information (if one wants it) and perfectly understandable in mathematical terms. All the axioms can be proven for models (in any fields) which meet some general criteria. This discrepancy between the theories - on ine hand a mathematical theory, and on the other hand physical theories based upon assumptions on the physical world -  is at the core of many, if not all, interpretations (or rather misinterpretations) of Quantum Physics. It is futile to look for an explanation of mathematical theorems in the physical world. It should lead physicists to focus more on the concepts of their trade, but this does not give the best return in the media : dark matter or parallel universe sounds better.

I think it's true. The measurement problem is not solved. 

@Jean Claude and Federico: thank you, yes measurement problem and also collapsing wavefunction are not resolved issues yet. Best wishes

To Victor,

What is a wave function ? Can you give any definition, mathematical or physical, other  than what is said in the books written by the great priests ? Or have we to choose between some hundred of interpretations  (the Boehmian one, the Everett one,...) ? After nearly one century is still Physics a game of riddels between ermarked players ?

Dear Jean Claude: according to a definition, wavefunction is the quantum state of an isolated system of one or more particles. Another definition (which i object) is that wave function reflects a probability to find electron at certain orbit. I prefer to another way to define: wave function is tendency to make structures. In my view, a lot of interpretation problems come from two sources: a. rejection that there is exact corespondence between electromagnetic and quantum character of particles, just like the exact correspondence between Poisson bracket and commutator bracket, b. denial the electromagnetic meaning of quantum wavefunction. Unless these two problems are resolved then i doubt we can find satisfactorily answer to ninety years old problem of interpretation of QM. What do you think?

http://en.m.wikipedia.org/wiki/Wave_function

Dear Victor,

"a quantum state of an isolated system of one or more particles". So we need to define a quantum state, a particle, and a system...

However, yes, I have an answer, in the same framework as the QM. It is quite easy to understand : when you have variables defined as maps on the same set E, and with very general conditions, we see that the probability to measure a value of an observable is given by a map on this set. This is exactly a wave function. But we do not need any particle or quantum state. Actually, as a quantum state (meaning the value of an observable for a given value of a state) is a vectorial or tensorial object, the starting point is a tensorial quantity (the value of the observable at a given point of E), and we need a scalar product (provided for by the Hilbert structure) to define the wave function, or more exactly its square value. It is obvious that we cannot have a function (meaning scalar valued)  defining quantum states.

Just good old mathematics, without the need of any physics. I will detail all this in a book to be ready (I hope !) in some months (not years).

Dear Jean Claude: thank you for your answer, surely you can send your paper privately if you wish. I will read it. Thanks

``These authors say that interpretation problems were not solved at 1927. What do you think?''

It is certainly true that interpretation problems were not solved at 1927. They are not completely solved even today. Whether the Solvay Conference of 1927 can be very helpful in this respect is questionable because at that time it was a matter of groping in the dark, only few experimental evidence being available, and the theory still having to be developed.

Moreover, what perhaps is equally important, is that there was only a vague idea that the methodology of classical physics would no longer be applicable to microscopic physics. At that time logical positivism/empiricism was becoming the predominant philosophy, having had considerable influence in establishing the predominance

of the Copenhagen interpretation. However, in the second half of the 20th century this philosophy has convincingly been demonstrated to fail being able to back a methodology able to deal with the quantum world.

For this reason I think that in order to develop such a methodology, as well as an interpretation going with it, we will have to take into account the experimental evidence available today. In particular, measurements that cannot be dealt with by

standard textbook quantum mechanics but need a generalization of the quantum mechanical formalism are abundantly available now, thus making it possible to obtain a better idea of the meaning of their  outcomes. In my view these measurements point into the direction of a (considerably amended) version of the Copenhagen interpretation. Details can be found in my book Foundations of quantum mechanics,

an empiricist view, a pre-publication version of which can now be downloaded as Publ. 52 from my website http://www.phys.tue.nl/ktn/Wim/publications.htm#main .

Willem de Muynck

Wednesday, February 4, 2015  12:22

Sorry; but how do you (Willem de Muynck) know that:

""They are not completely solved even today." ? It might be more appropriate to say: "A complete solution is not known to me."

I am more optimistic! I think we understand these problems much better than we did even a few years ago! Let's be optimistic that, most of the time, scientists will end up solving important problems (assuming they are reasonably precisely stated, which, in the example of the measurement problem in QM is the case).

Dear Jürg Martin Fröhlich,

Thanks for your correction. Maybe I expressed myself in too pessimistic a way. I took it for granted that the most promising (and most cautious) interpretation I know of (which is discussed in my book) may be not acceptable to everyone.

Willem de Muynck

Dear Manuel, Willem and Dr. Jurg Frohlich: thank you for all your comments. Best wishes