Polarization of a photon how do we know?

[bhobba]

Everything should be made as simple as possible, but not simpler

Yea - trouble is what is simpler is not universally agreed.

If you want to delve into interpretations best, like I said, to start with Ballentine - then branch out.

He not only carefully explains the Ensemble interpretation, but you will learn the correct axiomatic treatment and understand where things like Schroedinger's equation etc really comes from (symmetry) and the QM is contained in just two axioms. They are the axioms that need interpreting which greatly simplifies the issue.

Thanks
Bill

 

[vanhees71]

However, the cut is also necessary, as I will explain below. The reason I don't like Ballentine is he fails to mention the cut, and it seems that he is lacking collapse or an equivalent postulate that is necessary to describe filtering experiments.

But there is no such cut in quantum mechanics. The macroscopic behavior of macroscopic systems is an emergent phenomenon which is, to a certain extent, understandable from quantum many-body theory. Of course, Bohr is right in saying that we can learn about quantum systems only by making measurements with macroscopic measurement devices which allow to fix (practically) irreversibly the outcome of this measurement. Nevertheless, the measurement apparatus or any other macroscopic system that behaves according to classical physics, itself is not something contradicting quantum theory, but the relevant macroscopic observables are coarse grained (averaged) quantities over very many microscopic degrees of freedom.

So I also think, the idea of a cut, where quantum theory becomes invalid and the classical laws become valid, is flawed. Of course, you can argue about whether you find the arguments of quantum statistics, deriving the classical behavior of macroscopic many-body systems, convincing.

 

[bhobba]

So I also think, the idea of a cut, where quantum theory becomes invalid and the classical laws become valid, is flawed. Of course, you can argue about whether you find the arguments of quantum statistics, deriving the classical behavior of macroscopic many-body systems, convincing.

These days now we understand decoherence a lot better there is no need for a cut.

We simply say an effective observation has occurred after decoherence.

Thanks
Bill

 

[atyy]

But there is no such cut in quantum mechanics. The macroscopic behavior of macroscopic systems is an emergent phenomenon which is, to a certain extent, understandable from quantum many-body theory. Of course, Bohr is right in saying that we can learn about quantum systems only by making measurements with macroscopic measurement devices which allow to fix (practically) irreversibly the outcome of this measurement. Nevertheless, the measurement apparatus or any other macroscopic system that behaves according to classical physics, itself is not something contradicting quantum theory, but the relevant macroscopic observables are coarse grained (averaged) quantities over very many microscopic degrees of freedom.

So I also think, the idea of a cut, where quantum theory becomes invalid and the classical laws become valid, is flawed. Of course, you can argue about whether you find the arguments of quantum statistics, deriving the classical behavior of macroscopic many-body systems, convincing.

The cut doesn't mean that things on the classical side cannot also be included in a quantum description. What the cut means is that although we can shift it and place it in many different places, we cannot get it of it completely, because within Copenhagen and Ensemble interpretations, we don't know what physical meaning the "wave function of the universe" can have. The coarse graining doesn't solve the problem, because the underlying fine grained theory must also make sense, but it doesn't appear that an underlying fine grained quantum theory with only unitary evolution of the wave function can be extended to the whole universe, unless one takes something like a Many-Worlds approach.