
Robert Griffiths replied to the topic Three arguments for the reality of wavefunction collapse in the forum 2017 International Workshop: Collapse of the Wave Function 3 months, 3 weeks ago
5 June 2017
Dear Shan,
First me suggest you take a look at my contribution to this workshop under the heading of “Wavefunction Collapse Not Needed”, as then some of the following comments will make more sense. In particular, note the representation of physical properties by means of Hilbert subspaces or the corresponding projectors. This idea…[Read more]

Robert Griffiths started the topic Wavefunction Collapse Not Needed in the forum 2017 International Workshop: Collapse of the Wave Function 3 months, 3 weeks ago
There is no need for wavefunction collapse in standard quantum mechanics. Probabilities assigned by the Born rule are sufficient. In the case of measurements of a simple sort, Kraus operators can be used to generate a “collapsed state” of the measured system, but this is fully justified using the Born rule and conditional probabilities; no…[Read more]

Robert Griffiths replied to the topic Collapse. What else? in the forum 2017 International Workshop: Collapse of the Wave Function 3 months, 3 weeks ago
Dear Nicolas,
I appreciate your including some references [19] to older work on the consistent histories (CH) approach, at the beginning of your discussion of Bohmian mechanics (BM) in Sec. III. That could, unfortunately, give the reader the mistaken impression that CH has something to do with BM, whereas the two approaches are quite different.…[Read more]

Robert Griffiths replied to the topic Why Bohmian theory? in the forum Bohm’s theory 2 years, 2 months ago
Dear Aurélien,
Let me reply to your opening paragraph by saying that the ontology of CH and of BM are very different; in fact, there is little if anything in common. In CH the ontology begins with Hilbert subspaces, whereas in BM the central idea is a collection of classical particle positions. Certainly one can find Hilbert subspaces which…[Read more]

Robert Griffiths replied to the topic Why Bohmian theory? in the forum Bohm’s theory 2 years, 2 months ago
Dear Arélien,
You raise a couple of points. For convenience in replying let me number them, as I find this helps me keep track of things.
1. Approximate paths. In the CH there is nothing except Hilbert space and Hilbert subspaces at the fundamental level, and in Hilbert space there are no mathematically precise positions and thus no…[Read more]

Robert Griffiths replied to the topic Why Bohmian theory? in the forum Bohm’s theory 2 years, 2 months ago
Dear Travis,
The standard simple example is found in Bell’s SPEAKABLE … on p. 111 (I think the same in both editions) and given in a figure on the bottom of that page. Bell seems to ascribe it to Wheeler, and maybe that is where it originated, but Bell’s version is very clearly worded. The basic issue is that two wave packets cross in empty…[Read more]

Robert Griffiths replied to the topic Why Bohmian theory? in the forum Bohm’s theory 2 years, 2 months ago
Dear Aurélien,
The consistent histories (CH) approach is best thought of as an interpretation of QM in terms of ‘events’, not just measurement outcomes, inside a closed quantum system, without making any reference to things outside this system. Measurement apparatus, if any, is to be included as part of the closed system, and described in…[Read more]

Robert Griffiths replied to the topic Why Bohmian theory? in the forum Bohm’s theory 2 years, 2 months ago
Dear Dustin,
I appreciate your taking a look at [1] in my previous post, and I hope you will look at some of the work referenced there. Regarding the connection of CH and BM the following additional comments may be helpful.
In BM the psi of (Q,psi) in your notation is what I call the uniwave: unitarily developing wave function of the…[Read more]

Robert Griffiths replied to the topic What are the most pressing problems? and how to solve them? in the forum Panel Discussion 2 years, 2 months ago
Dear Jiri,
My use of probability theory is standard (Kolmogorov), but the sample space is quantum mechanical (projective decomposition of the Hilbert space identity), and the single framework rule is strict. It may be that your ‘context’ is similar to my ‘framework’. But my approach is time symmetric, so there must still be a difference…[Read more]

Robert Griffiths replied to the topic Retrocausation vs Retrodiction in the forum Timesymmetric theories 2 years, 2 months ago
Dear Ken,
I myself do not see where anything but confusion results from identifying framework choices with hidden variables. If you call a choice of coarse graining of the classical phase space a ‘hidden variable’ you are certainly not using the latter term in the way it is employed in Bohmian mechanics, which was one of Bell’s motivations:…[Read more]

Robert Griffiths replied to the topic Retrocausation vs Retrodiction in the forum Timesymmetric theories 2 years, 2 months ago
Dear Mark,
The ontology of CH is explained in some detail in [1]. Fundamentally it is based on the assumption that physical properties can be represented by subspaces of a Hilbert space. This last is, indeed, a mathematical formalism, but it enables one to reason rationally about the physical world. If you want to address the question of…[Read more]

Robert Griffiths replied to the topic The Merits of the de BroglieBohm Theory in the forum Bohm’s theory 2 years, 2 months ago
Dear Jean,
My critique of Bohmian mechanics (BM) was posted in the Why Bohmian Tbeory
thread begun by Shelly, and you might want to take a look at it. A key problem
is that Bohmian trajectories can give the wrong answer, at least from the
perspective of an experimentalist who doesn’t like to be told that his detector
was triggered by a particle…[Read more] 
Robert Griffiths replied to the topic Consistent Histories Essentials in the forum Consistent histories 2 years, 2 months ago
Dear Miroljub,
In reply to your first remark, replacing projectors with density operators does not make much sense to me, but maybe there is a way to do it that is interesting. One might, for example, purify the density operator by using a reference system, still regarded as part of the closed system whose overall history one is trying to…[Read more]

Robert Griffiths replied to the topic Why Bohmian theory? in the forum Bohm’s theory 2 years, 2 months ago
A good understanding of quantum theory using a precise formulation free of conceptual problems, such as measurements and observables, no superluminal influences, no tension with relativity, a decent ontology, a Schrodinger cat taught not to bite, etc., should make it possible to provide a detailed critique of Bohmian mechanics: what it gets…[Read more]

Robert Griffiths replied to the topic Why Bohmian theory? in the forum Bohm’s theory 2 years, 2 months ago
Dear Richard and Travis,
Due to some health problems I only just got around to reading your exchange, which I found quite interesting; this is the sort of thing which a workshop of this type should facilitate. I hope to add a second comment, but let me start off with the first, addressed, Richard, to you. In your most recent #2815 you say:

Robert Griffiths replied to the topic God knows where all the particles are! in the forum Bohm’s theory 2 years, 2 months ago
Dear Travis,
Early in this thread you made reference to your contribution to Healey’s “Comments on Bohmian mechanics”. I took a look at it, and maybe these remarks should go there, but they also seem to fit very well in this discussion.
I once heard the story, no doubt apocryphal, that when one of Wigner’s students would go to him claiming…[Read more]

Robert Griffiths replied to the topic Consistent Histories Essentials in the forum Consistent histories 2 years, 2 months ago
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 preprobabilities, i.e., they generate probability distributions on properly defined sample spaces. But one should not confuse…[Read more]

Robert Griffiths replied to the topic Retrocausation vs Retrodiction in the forum Timesymmetric theories 2 years, 2 months ago
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]
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