Dear Miroljub,

Let me respond to each of your points in turn.

1. Mixed states are often regarded on an equal footing with pure states.

There is no problem as long as both mixed and pure states are considered pre-probabilities, i.e., they generate probability distributions on properly defined sample spaces. But one should not confuse…[Read more]

Dear Ken,

I appreciate your comments. However, I think only confusion and not clarity will result from trying to put the CH idea of a ‘framework choice’, specifying a quantum sample space, in the same bin with ‘hidden variables.’ Let me explain the difference.

A framework is chosen by a physicist constructing a stochastic quantum description;…[Read more]

Regarding your #2781. I do not defend the use of probability theory in quantum textbooks, which leaves much to be desired. But the consistent histories approach used a time symmetric formulation of probability in the very first paper I published on the subject in 1984, and that is still true, at least in the way I formulate it. Let me suggest…[Read more]

Dear Miroljub,

I am not sure how to answer your question, but let me make the following comments, so you can see where my thinking is going.

The most common usage of density operators in ordinary quantum mechanics is, in the CH perspective, that of assigning probabilities to a quantum sample space at a single time, that is, to a projective…[Read more]

Dear Mark,

I have discussed in considerable detail why the outcomes violating Bell’s inequality are perfectly consistent with the complete absence of superluminal influences. There is a very complete, but not all that readable, treatment in Ch. 23 of my book (chapters available at http://quantum.phys.cmu.edu/CQT/) where I show by explicit…[Read more]

Dear Ken,

Thank you for your comments, but I think you may have misunderstood my point. When Alice says, on the basis of the output of her apparatus, that the particle had the property S_x = -1/2 when it entered her detector, she is NOT referring to a “hidden variable” in the sense that term is commonly used these days, as something in addition…[Read more]

Dear Jiri,

In reply to your #2698 and #2705:

The most accessible work I have published on EPR-Bohm is in “EPR, Bell, and Quantum Locality”, Am. J. Phys. 79 (2011) 954. arXiv:1007.4281. An additional and more technical treatment of nonlocality is in “Quantum Locality,” Found. Phys. 41 (2011) 705; arXiv:0908.2914. In these papers, as in Chs.…[Read more]

I confess I don’t know what a block universe is supposed to be. I looked briefly at the attachments at #2659 (Ken) and #2664 (Mark), and did not find them very enlightening. So maybe I’m just a blockhead. Anyway, here is my vague idea of what is involved, which then motivates a couple of questions which may be equally vague. Comments would be…[Read more]

Dear David,

I took a careful look at your attachment (Miller & Farr). Here are my thoughts.

I think you have taken up an interesting but subtle topic; causality in general, not just in quantum mechanics, has given rise to a lot of discussion. As opposed to statistical correlation, causes are supposed to precede their effects, so seem to be…[Read more]

Regarding my exchange with Miroljub, see #2525, #2547, #2556. We have had some exchanges by email and now agree that choosing a decomposition of the identity for use in a family of histories, and choosing a tensor product structure, as when discussing a hydrogen atom, are separate issues, and one choice does not necessarily affect the other.

Bob…[Read more]

Dear Miroljub,

There are certainly many situations in which the data you have or are considering can be placed in or represented by different consistent families which are mutually incompatible, and this is one of the standard objections to consistent histories. My response is that there is no “right” family; this is something I take up under…[Read more]

Dear Jiri,

Three comments.

First, I disagree with your assertion that it is possible to derive Bell inequalities in the framework of quantum mechanics. This claim is widespread, but it ignores a key assumption made by Bell and his followers: the existence of classical hidden variables not described by the quantum Hilbert space. Once one…[Read more]

Speaking as a physicist, the problem with thinking of the wavefunction as representing a quantum system goes back to the days when Born proposed that Schrodinger’s wave be interpreted not as something physically real but as a device for assigning probabilities. Thus a spherical wave emerging from a scattering event can be reconciled with the fact…[Read more]

Two comments.

First. I think it only creates confusion to put consistent histories in the Everett/many worlds camp, since in one respect they are directly opposite, as I pointed out in my first paper in 1984. Quantum time development in consistent histories is stochastic; in the Everett approach it is deterministic. For a more recent…[Read more]

Dear Mark
Do you think you could prepare a brief summary of what you are doing, say 4 or 5 pages, relating it to issues in quantum foundations and the work that other people are doing, and saying why we should be interested in it? I confess to having gotten essentially nothing from the abstract and introduction of your 54 page article, and lack…[Read more]

+classical physics
Is the macroscopic ‘classical’ world also governed by the laws of quantum mechanics? If so, how is the classical situation to be explained starting from quantum mechanics? If not, what modifications of quantum mechanics are expected on the road from microscopic to macroscopic physics?

+decoherence
Does decoherence…[Read more]