Consistent histories -- particle positions prior to measurement

[nickv2423]

I’m having a tough time understanding the CH interpretation. Only one of the histories happens when a measurement is made, but it seems to me it is saying that even prior to measurement a particle (ie electron) has a position and trajectory that we just don’t know about unlike standard QM. Is this what CH posits?

 

[PeterDonis]

Moderator's note: Thread moved to the QM interpretations subforum.

 

[gentzen]

but it seems to me it is saying that even prior to measurement a particle (ie electron) has a position and trajectory that we just don’t know about unlike standard QM. Is this what CH posits?

At least CH does not reject this statement, if you limit the meaning of „trajectory“ to suitably coarse grained descriptions.

 

[gentzen]

I’m having a tough time understanding the CH interpretation.

Are you aware that CH also has a formalism? Is your tough time related to that formalism? Or more to boiling down CH to „some essence“ that can be easily compared with other interpretations?

 

[nickv2423]

Are you aware that CH also has a formalism? Is your tough time related to that formalism? Or more to boiling down CH to „some essence“ that can be easily compared with other interpretations?

yea I’m having a tough time trying to comprehend what the formalism is saying. Are the coarse grained descriptions supposed to be the path they actually took prior to measurement or just a probability distribution

 

[gentzen]

yea I’m having a tough time trying to comprehend what the formalism is saying.

The formalism offers a way to have coarse grained descriptions and their compatible refinements.

Are the coarse grained descriptions supposed to be the path they actually took prior to measurement or just a probability distribution

You are supposed to use those coarse grained description in a suitable way to allow reasonable stories for the experimental setup under discussion. In this sense, the path is one the system might have taken (in a reasonable story). Measurements provide information about the path taken "in a specific instance" of the experiment. If the experiment is repeated many times, the frequencies of the measured results are hopefully consistent with the predicted coarse grained probabilities.

If you use a coarse grained description that is unsuited for the experimental setup under discussion, you still won't get any contradictions, but all computations done with that description will be useless nevertheless. CH claims that this is not a problem of the formalism. Opponents of CH claim that it is a problem, because CH does not offer sufficiently good criteria to distinguish suitable from unsuitable descriptions.

 

[nickv2423]

The formalism offers a way to have coarse grained descriptions and their compatible refinements.


You are supposed to use those coarse grained description in a suitable way to allow reasonable stories for the experimental setup under discussion. In this sense, the path is one the system might have taken (in a reasonable story). Measurements provide information about the path taken "in a specific instance" of the experiment. If the experiment is repeated many times, the frequencies of the measured results are hopefully consistent with the predicted coarse grained probabilities.

If you use a coarse grained description that is unsuited for the experimental setup under discussion, you still won't get any contradictions, but all computations done with that description will be useless nevertheless. CH claims that this is not a problem of the formalism. Opponents of CH claim that it is a problem, because CH does not offer sufficiently good criteria to distinguish suitable from unsuitable descriptions.

Ok I think I understand. So is this path the system may have taken physically real or is it just a probability function like in regular QM?

 

[gentzen]

Ok I think I understand. So is this path the system may have taken physically real or is it just a probability function like in regular QM?

The path is a possible consistent story. But it is not the only possible story.
In this sense, it is not physically real. But it is more than just a probability function.

 

[nickv2423]

The path is a possible consistent story. But it is not the only possible story.
In this sense, it is not physically real. But it is more than just a probability function.

So just to clarify is this saying that the system has a single position/path prior to measurement that we just don’t know about? Like in bohemian mechanics

Source: https://www.physicsforums.com/threa...ticle-positions-prior-to-measurement.1085187/

 

[gentzen]

So just to clarify is this saying that the system has a single position/path prior to measurement that we just don’t know about? Like in bohemian mechanics

Well, you can tell a story where the system has ...
You can also tell another story, consistent with itself and measurement results, but inconsistent with the previous story.

That is a bit different from Bohmian mechanics, where there is only one story, which is unique. But the independence from measurement (that "we just don't know" part) and the absence of wavefunction collapse is just like in Bohmian mechanics.

 

[bhobba]

I have posted this many times, but I will do it again:



Basically, decoherent histories (also called consistent histories) is many worlds, except that all possible outcomes are treated as on equal footing rather than real. Personally, I think it's just a semantic distinction, but you can make up your own mind on that. For a more detailed discussion showing how nuanced it is see:
https://www.mdpi.com/2624-960X/5/1/12/pdf

The best book I have read on decoherent histories is the Emergent Multiverse, which is about many worlds:

https://www.amazon.com.au/Emergent-Multiverse-Quantum-According-Interpretation/dp/0199546967

Watch it - it is mathematically sophisticated.

When I read it, I simply replaced 'real' world (other than the one we experience) with potentially real, and it was much more palatable to my sensibilities. Just a personal preference, of course, with no scientific value.

I must mention that this is decoherent histories, as applied to Quantum Field Theory (and examined in the Emergent Multiverse as part of the many-worlds interpretation). Ordinary QM is simply wrong and requires no interpretation. The non-relativistic limit of QFT is actually two Schrodinger equations, one for the particle and one for the antiparticle, in the Heisenberg picture, and acting not on a state but on the quantum field (that does not go away in the non-relativistic limit). I don't know the answer to why ordinary QM is a good approximation to QFT in the low-energy limit. I only have a model suggested by QFT - but not derivable from it. Maybe someone reading this can resolve it.

However, before delving into interpretations of QFT, it is probably a good idea to see its application to ordinary QM.

Thanks
Bill

 

[bhobba]

So just to clarify is this saying that the system has a single position/path prior to measurement that we just don’t know about?

I think QFT sheds light on this. The system is a disturbance in a quantum field or fields; there are no paths or positions (they are disturbances we can sometimes interpret as such). A measurement is an interaction between the fields of two quantum systems that yields a result (number on a digital readout, etc.) that we as human beings interpret as a measurement.

Thanks
Bill

 

[Demystifier]

CH is not about what the reality is, but about what we can say about reality. It is a development and formalization of the Bohr's complementarity principle. According to CH, the set of claims we can say is not unique, but depends on the framework in which we are talking. That is very Wittgenstein-ish in spirit. For instance, Wittgenstein said “The limits of my language mean the limits of my world.”, while CH says something like "The limits of a framework define the limits of describable quantum reality."

Speaking of Wittgenstein, see also https://arxiv.org/abs/2512.06034 .

 

[Morbert]

I’m having a tough time understanding the CH interpretation.

The formalism of CH aligns with a few different interpretations. David Wallace uses it heavily when presenting the Many-Worlds interpretation. Roland Omnes (one of the primary developers of the formalism) uses it to flesh out a "neo-Copenhagen" interpretation. I could see it readily applied to QBism.

I'll present the position of Harlte, Gell-Mann, and Griffiths. They are the ones who come closest to treating CH as a standalone interpretation.

Are the coarse grained descriptions supposed to be the path they actually took prior to measurement or just a probability distribution

The probability distribution is over alternative coarse-grained descriptions, one of which occurs. Competent measurement reveals which one occurs

So just to clarify is this saying that the system has a single position/path prior to measurement that we just don’t know about? Like in bohemian mechanics

Not quite. Alternative fine-grained paths won't typically be consistent. The alternative histories (one of which occurs) are made up of coarse-grained propositions. The closest you will get to Bohmian mechanics is Extended Probability Ensemble Decoherent Histories.

I think QFT sheds light on this. The system is a disturbance in a quantum field or fields; there are no paths or positions (they are disturbances we can sometimes interpret as such). A measurement is an interaction between the fields of two quantum systems that yields a result (number on a digital readout, etc.) that we as human beings interpret as a measurement.

In CH-QFT, the histories would be four-dimensional field configurations.

 

[bhobba]

The formalism of CH aligns with a few different interpretations. David Wallace uses it heavily when presenting the Many-Worlds interpretation.

Indeed, he does, although he does not give a detailed discussion of the connection, but does examine it (see page 91 - The Decoherent Histories Framework). But as I said, if you read the Emergent Multiverse and, instead of thinking in terms of ontologically real worlds, you think in terms of treating all the possible worlds on equal footing, you get close to CH. I added a link to a paper that examines this in more detail, even one in which the histories are ontologically real (which is far too weird for me). Still an interesting intellectual exercise.

Thanks
Bill

 

[gentzen]

I'll present the position of Harlte, Gell-Mann, and Griffiths. They are the ones who come closest to treating CH as a standalone interpretation.

The probability distribution is over alternative coarse-grained descriptions, one of which occurs. Competent measurement reveals which one occurs

Not quite. Alternative fine-grained paths won't typically be consistent. The alternative histories (one of which occurs) are made up of coarse-grained propositions.

Honestly, I find this description unnecessarily ambiguous. And assuming you had used clearer language, would different CH proponents really disagree on this technical level?

And I disagree with your claim that Griffiths would share the position of Hartle and Gell-Mann. I am certainly not alone with that opinion, see for example "A Brief Historical Perspective on the Consistent Histories Interpretation of Quantum Mechanics" (https://arxiv.org/pdf/2103.05280):

The formalism of the consistent/decoherent histories approach will be presented in §2, reviewing it in historical context in §3. Robert Griffiths and Roland Omnès, whose seminal works will be explored in §3 and §4, considered their contributions to be “a clarification of what is, by now, a standard approach to quantum probabilities” (Freire, 2015, 321), namely, the Copenhagen perspective.
Gell-Mann and Hartle, on the other hand, who first applied the expression “decoherence” to histories (Gell-Mann and Hartle, 1990a, 1994a,b), following in the footsteps of Zeh (1970) Zeh (1970), Zurek (1981), and Joos (see for instance Joos and Zeh (1985)), considered themselves to be post-Everettians, ascribing to Everett’s 1957 work the merit of first suggesting “how to generalize the Copenhagen framework so as to apply quantum mechanics to cosmology” (Gell-Mann and Hartle, 1990b, 323).

 

[Morbert]

I added a link to a paper that examines this in more detail, even one in which the histories are ontologically real (which is far too weird for me). Still an interesting intellectual exercise.

I'm not sure if what the author calls "Many-Worlds variant of CH" is distinct from Many-Worlds, where choice of "history space" is a choice of description of the fundamental state (https://arxiv.org/pdf/quant-ph/0103092).

Re/ The Emergent Multiverse. Two issues with reading it as a CH textbook are i) While it makes use of the CH formalism, it doesn't center it like e.g. Griffiths does in "Consistent Quantum Theory"*. ii) The issue of MW and probability is discussed, but this is not an issue in CH, as there is no commitment to an underlying determinism.

* @nickv2423 as an aside this would be my book recommendation. It's available for free online https://quantum.phys.cmu.edu/CQT/

 

[Morbert]

Honestly, I find this description unnecessarily ambiguous. And assuming you had used clearer language, would different CH proponents really disagree on this technical level?

My language was clear. If you take issue with it, be specific.

And I disagree with your claim that Griffiths would share the position of Hartle and Gell-Mann. I am certainly not alone with that opinion, see for example "A Brief Historical Perspective on the Consistent Histories Interpretation of Quantum Mechanics" (https://arxiv.org/pdf/2103.05280):

Griffith's account is not a Copenhagen perspective at all, as he describes measurement as revealing pre-existing properties of a system. While his focus might not be cosmology, his ontology is much more aligned with Gell-Mann and Hartle.

 

[gentzen]

My language was clear.

Not for me.

If you take issue with it, be specific.

The worst offender is

Competent measurement reveals which one occurs

Depending on how one interprets this, either all proponents of CH would trivially agree with it, or else even Robert Griffiths would strongly disagree. I guess you had the interpretation in mind were everybody would trivially agree.
But on the other hand, you say you would describe the specific position of Hartle et al, and I don't see where other CH proponents would disagree, if I always interpret your words in the way that they don't contradict my opinions.

 

[Morbert]

Not for me.

The worst offender is

Depending on how one interprets this, either all proponents of CH would trivially agree with it, or else even Robert Griffiths would strongly disagree. I guess you had the interpretation in mind were everybody would trivially agree.
But on the other hand, you say you would describe the specific position of Hartle et al, and I don't see where other CH proponents would disagree, if I always interpret your words in the way that they don't contradict my opinions.

Omnes does not give ontic significance to histories. So e.g. if you have two histories: one where a particle has x spin-up before it is measured, and one where a particle has x spin-down before it is measured, and the physicist performs an x spin measurement and records an outcome "up", then according to Omnes, this does not mean the history where the particle has spin-up before it was measured actually occurred.

 

[gentzen]

Omnes does not give ontic significance to histories. So e.g. if you have two histories: one where a particle has x spin-up before it is measured, and one where a particle has x spin-down before it is measured, and the physicist performs an x spin measurement and records an outcome "up", this does not mean the history where the particle has spin-up before it was measured actually occurred.

I didn't know that Omnes takes that position (or has taken that position at some point in the past). So when you group Griffiths with Hartle, you are basically just objecting to the common grouping of Omnes and Griffiths?

 

[bhobba]

In CH-QFT, the histories would be four-dimensional field configurations.

Agreed.

But Wallice in the Emergent Multiverse goes further on page 302, where he describes the interpretation I currently favour: that QFT is literally ontologically real. He calls it spacetime state realism (although he admits the name is a bit of a misnomer). This view is presented in Art Hobson's Field and Their Quanta, which, IMHO, is a very underrated and not that well-known interpretation.

I will admit that for people with advanced knowledge, he does make dubious claims, i.e., the ordinary QM state is the non-relativistic limit of QFT - it isn't. Indeed, Thanu Padmanabhan (evidently a famous physicist in India, whom I had never heard of before), whose paper explained this, thinks there is a hidden problem in going from ordinary QM to QFT - one that needs resolution. He has written some interesting books on general physics and QFT, which I ordered while feeling generous (I buy far too many books on physics and QM in particular).

Thanks
Bill

 

[Morbert]

I didn't know that Omnes takes that position (or has taken that position at some point in the past). So when you group Griffiths with Hartle, you are basically just objecting to the common grouping of Omnes and Griffiths?

There can be different groupings depending on criteria, and Omnes and Griffiths are often grouped together because of their focus on minimal decoherence criteria. But as far as interpretations go, I read Omnes's work as less a distinct interpretation and more a logical grounding of Copenhagen QM.

 

[Morbert]

But Wallice in the Emergent Multiverse goes further on page 302, where he describes the interpretation I currently favour: that QFT is literally ontologically real. He calls it spacetime state realism (although he admits the name is a bit of a misnomer). This view is presented in Art Hobson's Field and Their Quanta, which, IMHO, is a very underrated and not that well-known interpretation.

I've touched on SSR occasionally here, as it's useful for understanding MWI and locality. But the issues SSR addresses are not issues in CH, as CH associates properties/true propositions with projectors on Hilbert space, not quantum states.