Talk:Consistent histories/Archive 1

Projectors issue
A lot more explanation is needed as to what these projectors are, the setup of the projectors, the choice of the times etc.. Also I'm a little confused by your choices of indices. CSTAR 06:12, 28 Nov 2004 (UTC)

star vs dagger
I tried to make the article consistent with other articles on Wikipedia on related matters. * is almost universally used to denote adoint in Wikipedia, although this is a minor point, and admittedly physicists use dagger much more frequently. But since there has to be a common convention between the math articles and the physics ones, particularly in regard to C*-algebras I would have hoped some minor change would be aceptable. After all, we don't use the term $$\mathrm{C}^\dagger$$-algebras. That would mean I would have to change my nom-de-wiki to CDAGGER.


 * Oh, I did not realize that your nickname was a misnomer, CDAGGER. I hope that you will be able to fix your username. --Lumidek 18:40, 28 Nov 2004 (UTC)

Also note there are numerous articles on related subjects; for example, yes-no questions for instance are discussed in quantum logic).CSTAR 17:09, 28 Nov 2004 (UTC)

Consistent histories
I think the article is still confusing:


 * Why the double subscript in the definition?


 * $$H_i = T \prod_{j=1}^{n_i} P_{i,j}(t_{i,j})$$


 * There should be only one subscript.


 * Not really. The index "i" labels the different histories, and for different, i-th history, there are many projectors that are labeled by the extra index "j". But for every history there are different projectors, so one must use two different indices if he wants to distinguish them. --Lumidek 18:40, 28 Nov 2004 (UTC)


 * These special histories are by definition consistent. Apply the main property of the trace!


 * $$ \operatorname{Tr}(A B) = \operatorname{Tr}(B A)$$


 * For histories made of 0 or 1 projector only, the consistency is automatic as long as the operators are complementary. For more complicated histories it's not true and you've done some error if you think that it is true. --Lumidek 18:40, 28 Nov 2004 (UTC)


 * Please see the discussion on pp 138-140 of the Omnes reference given in the article There are of course other references, such as the original papers, but this is the one I happen to have in my home library.


 * I've read an article by Omnes, and this is a significant part of my knowledge about the subject, thanks. --Lumidek 18:40, 28 Nov 2004 (UTC)


 * The general Griffiths conditions are given on pp 162-163 (ibid). CSTAR 18:32, 28 Nov 2004 (UTC)

OK, that's fine, but the explanation here is confusing, still. For instance, do you explain what the relation between the time t'i,j are? This may be very clear to yourself, who is quite clearly an expert on the subject. But it doesn't say much for the article if my first reaction is to reach for another reference to try to understand what's written here.

Notice that I only made minor changes to the article (latex formatting and the infamous dagger), because I am not knowledgeable in this area. And the point of this discussion isn't to see who's right or who knows more about this. You clearly know more about this. I just want to be able to understand what's written. I'm sorry I still don't.CSTAR 18:57, 28 Nov 2004 (UTC)

CH vs MWI
Hello. I'm just wondering if I can get some clarification on the sentence "Because of some initial contributions of Everett, this interpretation is also sometimes considered to be a modern version of the Many-worlds interpretation."(MWI) Isn't Consistent Histories meant to be a revised vision of the Copenhagen Interpretation, and thus a rival to MWI? The Interpretation of quantum mechanics page notes they are quite different - differing in both determinism, waveforms, and of course number of universes. It also refers to Consistent Histories as Copenhagen "done right".


 * Hi Cstar, all interpretations are in some sense competing, and in some sense similar. An important part of the Consistent Histories is that they rely on an actual physical effect called Decoherence, and Decoherence was originally pointed out by Everett as a part of his many-worlds interpretation, see e.g. ... Yes, CH is called "Copenhagen done right"... All these 3 pictures - Copenhagen, MW, CH may be viewed as mainstream interpretations of QM, and the choice among these is a matter of philosophical preference among the physicists. On the other hand, the Bohmian and other interpretations are not mainstream in any sense, and the require physics to be modified in a controversial way. --Lumidek 01:11, 13 Dec 2004 (UTC)


 * Hey Lumidek ...you talking to me man? :) I'm not sure sure whether you're answering Culix or responding to me response. I agree with what you said, but I also suggested to Culix that CH is really a refinement of the older quantum logic approach. There is a nice paper by Chris Fuchs arXiv:quant-ph/2005039 which tries to make this connection.  Actually I like that paper because it points out that in this view one doesn't need anything quite as sophisticated as Gleason's theorem to introduce density (besides which Gleason's theorem assumes dimension >= 3 which makes it innapplicable to spin quantum systems. And I agree with mainstream vs non-mainstream interpretations.


 * By the way just saw a wacky movie What the Bleep Do We Know?! which does the "consciousness and QM" thing which we all "love" so much.

I was under the impression that Copenhagen and MWI were rival theories, because while Copenhagen theorizes that waves "collapse" into particles when observed, MWI proposes the existence of "shadow photons" and the like (see David Deutsch FoR book) that interfere and cause the photons we observe to change course. Thus I figured that if Consistent Histories were Copenhagen "done right", it would be based on Copenhagen and thus remain a rival of MWI. If anyone could offer some insight on this, it would be greatly appreciated. --Culix 20:26, 12 Dec 2004 (UTC)


 * Hmm. Caveat: I am not an expert on consistent hitories. That said, I would rephrase your description of consistent histories as: it is quantum logic done right.  Consistent histories allows for more sophisticated operations on quantum systems (described by histories) than does quantum logic (which only allows projections) and more realistic measurements (descibed in CH by Positive operator valued measures) than quantum logic, which only allows for projective measurements.  Yes I would characterize them as rival interpretations, but they are both path space intepretations. And MWI if done carefully, really is quite general. CSTAR 22:52, 12 Dec 2004 (UTC)

I did a PhD on constistent histories with Adrian Kent and know most of the people who developed the theory. Certainly I do not think that consistent histories has anything to do with the MWI interpretation. If you believed an MWI idea everytime you used a projection operator you have a split into seperate universes. It then makes no sense at all to talk about consistency of histories which are a sequence of events in time. The CH approach can be regarded in two different way. In a minimal interpretation it is a set of rules of discussing sequences of events that are consistent with the ordinary rule sof classical propability. In that sense it is successful and there are no problems. The grand aim of the program however was to actually choose a set of consistent histories onw of which will occur. These histories would describe classical sized objects as being in classical like states, such as having reasonably well defined positions. Knowone has yet found a way of doing this and there appear to be insurmountable difficulties. I would liek to emove the comparison to MWI.


 * My comment was more to the relation between quantum logic and CH, as for instance in (one of) Griffths 1996 paper. The Hilbert space is the time-ordered Hilbert space tensor product.  The logic is commutative given by a time-ordered family projections defining histories. It has the advantage that measurement is already built in. Anyone this approach should be explained.CSTAR 15:28, 21 Jan 2005 (UTC)