Seeking clarity on the physical definition of an observer

[QuarkyMeson]

Photodetectors are fermionic. Detection happens via electrons transitioning in silicon or being freed from a photocathode. A many-body bound state also has a well-defined aggregate fermion-number current giving collective 4-velocity.

Sure, although it started as "anything with its own Dirac current." Now it's "anything with an aggregate fermion current." That's every piece of ordinary matter in the universe. You also still have the problem with the free electrons for example, where they clearly have a Dirac current but don't always interact, and even if they do sometimes carry no which-state information. Did they still act as an observer? Was a measurement performed?

So the definition can't tell apart things that measure from things that don't. If everything is an observer, calling something an observer tells us nothing. The moment you try to fix that by adding a qualifier that it has to measure something, that condition is doing all the work and the current condition is doing none. That's just standard decoherence with an extra label slapped on.

 

[danieltanfh95]

Sure, although it started as "anything with its own Dirac current." Now it's "anything with an aggregate fermion current." That's every piece of ordinary matter in the universe.

The aggregate fermion-number current is the Dirac current for many-body fermionic systems. Single-particle Dirac current is the special case while aggregate is the general one in QFT, and I used the simple case as a demonstration. You can check out Peskin & Schroeder, An Introduction to Quantum Field Theory (1995) for the standard treatment.

You also still have the problem with the free electrons for example, where they clearly have a Dirac current but don't always interact, and even if they do sometimes carry no which-state information. Did they still act as an observer? Was a measurement performed?

There might be a misconception here. The framework distinguishes SR-observer (anything with a worldline, standard SR usage) from measurement-observer (SR-observer that forms records via decoherence). Free electrons are SR-observers but not usually measurement-observers. This is not a contradiction.

So the definition can't tell apart things that measure from things that don't. If everything is an observer, calling something an observer tells us nothing. The moment you try to fix that by adding a qualifier that it has to measure something, that condition is doing all the work and the current condition is doing none. That's just standard decoherence with an extra label slapped on.

"Just standard decoherence with a label" is wrong: standard decoherence produces branches but does not tell you which branch the observer sits in (Wallace, Zurek, Schlosshauer all acknowledge this gap). The gap is closed by identifying where the observer sits with where the physical matter current is. The current does the branch-localization work, decoherence does the record-formation work, and neither is redundant.

 

[renormalize]

The current does the branch-localization work, decoherence does the record-formation work, and neither is redundant.

Can you supply a specific reference that explicitly states this (in so many words) about the Dirac current?

 

[danieltanfh95]

Can you supply a specific reference that explicitly states this (in so many words) about the Dirac current?

I cannot provide a single-line quotation because the two halves sit in separate literatures. The decoherence programme (Zurek, Wallace, Schlosshauer) documents that decoherence alone does not select which branch the observer occupies, and the beable programme (Bell, Allori, Struyve) proposes fermion-number density as the physical anchor that closes this definite-outcomes gap. The composition is the natural completion within the beable programme, though I do note that it may not be the unique resolution (Bohmian particle beables, GRW collapse, and Everett-branch-realism are alternatives).

On the decoherence side:

That decoherence produces branch structure but does not by itself select which branch the observer occupies is standard. Wallace (2010) "Decoherence and Ontology" (in Saunders, Barrett, Kent, Wallace eds. Many Worlds? Everett, Quantum Theory, and Reality, Oxford University Press; arXiv:1111.2189, Section 4) makes the point directly:

"A pure-decoherence solution to the measurement problem turns out to be impossible on technical grounds: the decoherence criterion is both too strong, and too weak, to pick out an appropriate set of classical histories from the unitary quantum dynamics."

Schlosshauer (2005) arXiv:quant-ph/0312059 surveys additional admissions from Joos, Zeh, Adler.

On fermion-number density / Dirac current as physical beable:

Bell (1984) "Beables for Quantum Field Theory" (CERN preprint TH.4035/84, reprinted as Chapter 19 in Speakable and Unspeakable in Quantum Mechanics, 2nd ed., Cambridge University Press 2004) argues that fermion-number density is the beable in QFT. Struyve (2011) "Pilot-wave theory and quantum fields" (Rep. Prog. Phys. 74: 106001, Section 4.1, arXiv:1101.5819) summarizes:

"Bell was the first to present a pilot-wave approach to quantum field theory in terms of a particle ontology. He considered a spatial lattice and introduced an actual configuration given by the fermion numbers at the lattice sites."

Allori, Goldstein, Tumulka, Zanghì (2008) "On the Common Structure of Bohmian Mechanics and the Ghirardi-Rimini-Weber Theory" (Br. J. Phil. Sci. 59: 353-389, arXiv:quant-ph/0603027) formalize the primitive-ontology framing:

"They are ultimately not about wave functions but about 'matter' moving in space, represented by either particle trajectories, fields on space-time, or a discrete set of space-time points. The role of the wave function then is to govern the motion of the matter."

In conclusion, the two halves together give the standard beable-programme response: decoherence produces branch structure and record redundancy, and the beable (fermion-number density / aggregate Dirac current) selects which branch the observer's matter physically inhabits.

 

[renormalize]

Thanks for the references.

I cannot provide a single-line quotation because the two halves sit in separate literatures.

Given that admission, can we readers conclude that your take on the combination of decoherence and the fermion current constitutes an unpublished personal theory?

 

[danieltanfh95]

The statement above embeds three mischaracterizations worth addressing individually.

Given that admission.

"Admission" mischaracterizes my statement. That no single paper states the exact combined phrasing is a structural fact about the beable and decoherence programmes sitting in separate literatures, not an admission that the composition lies outside published work.

your take on the combination of decoherence and the fermion current

"Your take" mischaracterizes authorship. The beable programme (Bell, Allori-GTZ, Struyve) is published, and applying its logic to composite fermionic observers is the natural application, not my personal construction. Allori, Goldstein, Tumulka, Zanghì (2011) "Many-Worlds and Schrödinger's First Quantum Theory" (Br. J. Phil. Sci. 62: 1-27, arXiv:0903.2211) applies primitive ontology explicitly to the branch-selection question, using mass-density primitive ontology to resolve which world is actual. The composition sits in the programme's own tradition.

constitutes an unpublished personal theory?

This is not a personal theory. It adds no physics content beyond what is published, meaning new dynamical laws, new fields, new ontological commitments, or modifications to standard QM. Fermion-number density is Bell's beable, decoherence is standard, and the composition applies the programme's logic without added content.

 

[renormalize]

That no single paper states the exact combined phrasing is a structural fact about the beable and decoherence programmes sitting in separate literatures, not an admission that the composition lies outside published work.

Then it should be possible for you to cite an exposition of that composition that lies within "published work". Please do so.

 

[danieltanfh95]

Then it should be possible for you to cite an exposition of that composition that lies within "published work". Please do so.

My reading had been that this is derivation from cited premises, in the same category as applying Noether's theorem to a specific Lagrangian or Bell's inequality to a specific experimental setup.

On second thought, perhaps you are right. Applying AGTZ 2011's general primitive-ontology → branch-selection framing to Bell 1984's specific fermion-number-density beable may be novel enough to warrant a paper of my own, rather than a forum comment.

Could you point me to a precedent where the specific application of an interpretive framework to a particular beable was treated as novel enough for standalone publication? It would help to see how the literature handles this.

 

[QuarkyMeson]

The aggregate fermion-number current is the Dirac current for many-body fermionic systems. Single-particle Dirac current is the special case while aggregate is the general one in QFT, and I used the simple case as a demonstration. You can check out Peskin & Schroeder, An Introduction to Quantum Field Theory (1995) for the standard treatment.


There might be a misconception here. The framework distinguishes SR-observer (anything with a worldline, standard SR usage) from measurement-observer (SR-observer that forms records via decoherence). Free electrons are SR-observers but not usually measurement-observers. This is not a contradiction.

I agree more or less, but that was essentially my point. The proposed definition of observer is excessively broad. If an SR-observer is simply anything with a worldline, then the category includes nearly every ordinary material system and contributes nothing specifically to the measurement problem.

If the claim is only that measurement records are carried by physical systems along worldlines, that is true, but again, trivial. It does not explain what distinguishes a measurement interaction from an ordinary interaction, and it does not by itself select one component of a decohered state as the branch in which the observer is located.

So what does this definition actually add? What problem does it solve that is not already handled by the dynamics of interaction, decoherence, and record formation? I guess this is the payoff:

"Just standard decoherence with a label" is wrong: standard decoherence produces branches but does not tell you which branch the observer sits in (Wallace, Zurek, Schlosshauer all acknowledge this gap). The gap is closed by identifying where the observer sits with where the physical matter current is. The current does the branch-localization work, decoherence does the record-formation work, and neither is redundant.

But I just don't see how to get there. I'll be the first to admit that I don't fully understand the argument being made and it is perhaps my own lack of understanding.

The beable programme (Bell, Allori-GTZ, Struyve) is published, and applying its logic to composite fermionic observers is the natural application, not my personal construction. Allori, Goldstein, Tumulka, Zanghì (2011) "Many-Worlds and Schrödinger's First Quantum Theory" (Br. J. Phil. Sci. 62: 1-27, arXiv:0903.2211) applies primitive ontology explicitly to the branch-selection question, using mass-density primitive ontology to resolve which world is actual. The composition sits in the programme's own tradition.

A cursory reading of this paper seems to suggest the opposite:

It is worth noting that this objection does not apply to Sm, as there is no easy, clean, and precise way of getting rid of all but one world in Sm. Bell, however, can remove most worlds from his picture, and thus proposes the following for the one remaining world:

instantaneous classical configurations [Q] are supposed to exist, and to be distributed ... with probability |ψ|2. But no pairing of configurations at different times, as would be effected by the existence of trajectories, is supposed. It is not clear what is meant by the last sentence, given that for every time t a configuration Q(t) is supposed to exist.

What Bell presumably had in mind is that for every time t the configuration Q(t) is chosen independently with distribution |ψt|2. Let us call this theory Sip (S for the Schr¨odinger equation, i for independent, and p for particle ontology); in [3] it was called BMW for “Bell’s version of many-worlds.” So in Sip, the PO consists of particles, as in Bohmian or classical mechanics, but their positions vary with time in an utterly wild and discontinuous way. (Indeed, the path t 7→ Q(t) will typically not even be a measurable function.)

I'll have to go through the rest of the papers later.

 

[renormalize]

Could you point me to a precedent where the specific application of an interpretive framework to a particular beable was treated as novel enough for standalone publication? It would help to see how the literature handles this.

Even though I have read the Bell paper you cited, I am, alas, not knowledgeable enough about "beables" to intelligently advise you. That said, even Bell expressed uncertainty about the significance of the fermion current as a beable. Quoting him from that paper:
"... For one thing there is nothing unique about the choice of fermion number density as basic local beable. We could have others instead, or in addition. For example the Higg's fields of contemporary gauge theories could serve very well to define 'the positions of things'. ..."
The Higgs boson is certainly not fermionic!
That's why I believe you overreach in the claims you make regarding the Dirac current and decoherence. The best I can suggest is that you write-up an exposition and submit it for publication to a credible journal. In other words, run your claim up the flagpole and see who, if anyone, salutes. After publication, the topic would certainly be suitable for discussion on Physics Forums.
Perhaps someone more steeped than me in quantum interpretations will read and comment on our discussion in this thread (paging @PeterDonis).

 

[gentzen]

Could you point me to a precedent where the specific application of an interpretive framework to a particular beable was treated as novel enough for standalone publication? It would help to see how the literature handles this.

If you want, you can contact Arnold Neumaier. He recently embraced a research program (basically because TomS triggered discussions which triggered him to explain some related stuff in more detail) with similar "beables" and similar goals:
https://www.astronews.com/community/threads/a-qft-based-quantum-universe-without-many-worlds.12619/

Graduate Thread 'Universal quantum physics'

Nov 20, 2025

Based on the thermal interpretation, I developed a quantum version of the classical, mechanical universe suggested by Laplace over 200 years ago.

Abstract. The purpose of this paper is to propose a quantum version of the classical, mechanical universe suggested by Laplace over 200 years ago. The proposed theory operates fully within the established mathematical formalism of quantum field theory.
The proposed theory unifies the classical and quantum intuition about the macroscopic and microscopic, deterministic and stochastic, local and nonlocal aspects of our world. It entails...

• A. Neumaier
• Replies: 47
• Forum: Quantum Interpretations and Foundations

• 

https://www.astronews.com/community/threads/universelle-quantenphysik.12458/
Not yet accepted for publication, but not rejected either. It is not easy to publish, because ... (In my opinion: you don't have that single subject which is new and can guide the story of a readable paper.)

 

[PeterDonis]

"Branches always present" only requires unitary evolution

If by "unitary evolution" you mean "exactly unitary evolution all the time, even during a measurement", then yes. But that qualifier is crucial.

Also, you're equivocating on the meaning of the term "branches". See below.

plus that the Dirac matter field is physical.

As it stands, this is too vague to even respond to. See below.

That's shared by MWI, Bohmian mechanics, and consistent histories, not MWI-specific.

Only if you're willing to adopt a different meaning for the term "branches" for each interpretation.

The MWI says that the wave function is the physical reality. So "branches" in the MWi means physically real everything--including us humans--in every branch. And saying "all branches are present" is a direct statement of that physical reality. And note that what you are calling "the Dirac matter field" in the MWI is just the wave function, so in the MWI, saying "the Dirac matter field is physical" just means what I said just now--the wave function is the physical reality.

Bohmian mechanics says that the physical reality of objects like particles, baseballs, and us humans is in the particle positions. There are no "branches" in the particle positions: every particle has a single well-defined position at all times. The wave function (or quantum potential) in Bohmian mechanics is just part of what tells the particles how to move. So "branches" in Bohmian mechanics are really irrelevant, since the only portion of the wave function that matters is the one corresponding to where the particle positions are.

Also, in Bohmian mechanics, "the Dirac matter field" doesn't even make sense. "Matter" is the particle positions, not a field. The wave function is the wave function, not "the Dirac matter field".

I'm not as knowledgeable about consistent histories, but as I understand it, it does not assign the same meaning to "branches" as either of the other two.

it is incompatible with literal-collapse Copenhagen and objective collapse (GRW / Penrose).

Yes, I agree with this.

 

[PeterDonis]

Free electrons are SR-observers

Not if they aren't being measured. If they aren't being measured they do not have well-defined worldlines.

 

[PeterDonis]

It adds no physics content beyond what is published

It does if nobody has published the composition of the two separate lines of literature that you describe.

I am closing this thread for moderation for now. @danieltanfh95 if there is published literature on the composition you describe, please PM me links so the moderators can review it. Unless and until we have a valid reference to back up the viewpoint you are expounding, this thread will remain closed.