Morbert said:
Could you expand on this? I.e. What do you think is the significance of the Diosi paper re/ incompleteness of CH?
gentzen said:
The Diosi paper is a strong indication that a closer investigation of (tensor) products should have been done, or rather that issues which occur in the context of (tensor) products had been neglected before. Which issues exactly I cannot say, because I have not done that investigation either. Maybe this is related to what Griffiths writes in the section "8.4 Open issues" (or maybe not):
I now studied more in detail what people do with CH and what they
wrote about it. Basically, they do what is convenient, and this includes working with inhomogeneous histories by just algebraically adding them. Section "
8.4.1 Entangled histories" is about something less convenient, even so it also happens when you use such inhomogeneous histories for coarse graining. In conclusion, I would say that this section is unrelated to a closer investigation of (tensor) products.
Getting rid of the real part in the consistency condition is desirable, because it simplifies the structure of the theory, and is also the right thing to do in many ways. But improving the analysis of (tensor) products and independence would probably just indicate that those don't force giving up that real part. In a certain sense, CH is constructed in a simple but non-relativistic way. And classical logic (and probability theory) too are enforced in CH in a rather simple way. That is a strength, not a weakness. But it makes it hard to upgrade CH to QFT (without sacrificing that simplicity), and also hard to "properly" analyse locality with it (without slightly cheating, by going outside of the provided logical language, and by "proving too little").
Morbert said:
@gentzen I think it's ok to interrogate assumed distinctions in interpretations. Lubos Motl once called consistent histories a homework problem for Copenhagen. There's no problem with Saunders arguing that Hartle agrees with him if his argument is interesting (I don't know if it is). And if consistent histories as a project ends up being useful to people doing work on many worlds, great!
Saunders claims
here that people like Halliwell or Hartle would actually support MWI. I always wondered where the claim that CH and MWI were nearly identical came from. This video made me look into Saunders writtings, and indeed I could find that claim there, written in way that I did not like. Wallace later demonstrated how such "content" can be written in an appropriate way, by inserting footnotes like "I do not want to make any historical claim here as to the influence or otherwise of Dowker and Kent’s work on proponents of consistent-histories approaches: my account is intended to capture the logic of the situation, rather than its chronology." or by parenthetical clarifications like "(And indeed, this is how Hartle, on my reading, does seem to understand it; see Hartle 2010.)"
For Saunders, I didn't yet read stuff that felt like being influenced by CH, Hartle, or Gell-Mann. For Wallace, it was rather the opposite, that I first heard stuff from him, which I thought made sense, and later found out that Hartle had said similar things (I don't care who said them first, they could be wrong anyway)
What is an Observer? A Panel with James Hartle, Susanne Still, David Wallace, and Alan Guth
The Return of the Observer by James Hartle
For Wallace, what confuses me is how he can base arguments for MWI on CH, and then claim that MWI would just work for QFT, while CH seems non-relativistic to me.
Morbert said:
Also, Weinberg's objections here seem somewhat subjective. Just as a the freedom of a physicist to choose a coarse-graining is a feature not a bug, perhaps the ability to choose from different fine-grainings is also a feature.
I find his objections fair, because both Griffiths and Omnès are quite clear that they like Copenhagen in general. So it seems fair to use the general objections against instrumentalism against them.