Quantum measurement problem (ie double slit experiment) question

[NotYourGrandmasJam]

TL;DR

Looking for insight and explanation regarding the quantum measurement problem, the current gaps in understanding and whether an ontologically new approach is warranted or even viable.

Retired engineer and physics hobbyist/enthusiast here, first post, humbly asking for insight and explanation on this topic. I think I understand the current landscape of the quantum measurement (electron slit experiment et al) problem. Copenhagen treats collapse as fundamental, decoherence explains the loss of interference through environmental entanglement, and many worlds avoids collapse by retaining all branches. All of these seem to be experimentally compatible and internally consistent but have gaps. Seems like the gaps are not necessarily regarding the how but in the why. Like decoherence explains why branches stop interfering, but not why a single outcome ever becomes physically realized.

I’ve been wondering whether the solution to close the gaps points to a missing physical ingredient rather than a missing interpretation. In particular, whether it has been considered that an outcome selection during active "measurement" could reflect a universal constraint associated with irreversible coupling to a macroscopic, informationally dense environment. If plausible, wouldn't introducing a "which path" detector not “observe” the particle in an epistemic sense, but alter the local physical conditions so that the distributed quantum configurations can no longer be sustained? Once coupling exceeds a critical threshold, could one trajectory become physically anchored while alternatives are dynamically suppressed? If that could be the case, could it be that the interference disappears not because information is known, but because the system can no longer support superposition under the new constraints? Would that leave unitary evolution and decoherence intact, remove any special role for observers, and treat collapse not as a probabilistic add on but as a physical convergence process?

I guess the totality of my question is whether something like a universal anchoring or a saturation principle operating where current formalisms fall silent is already postulated and ruled out, or whether it could plausibly complement the reults that are accepted and yielding what we already know works?

Seems to me if that could be the case, there are mathematical and experimental questions that can be answered there without deviating from confirmed observations/experimental results and knowns and possibly get into the why. I understand that the why doesn't always matter but my instinct is the why may be what's missing and the way ahead everywhere. Building from the top down maybe? I have been thinking about this broadly regarding phenomena in QM and GR with some potentially novel outcomes while trying as best as I can to be focusing on the math and falsifiability. Would appreciate educated opinions on whether this is crackpottery, already thought about and understood/dismissed or something worth discussing. I sincerely appreciate any feedback. Thanks very much.

 

[PeterDonis]

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

 

[PeterDonis]

Would that leave unitary evolution and decoherence intact

Short answer: no. The things you are vaguely describing all involve a change to the dynamics, making it no longer unitary. Indeed, anything that makes collapse, in the sense of selecting a single outcome to a measurement, an actual physical process must involve such a change to the dynamics. There are various speculations in the literature about such things, and even some explicit models, such as the GRW stochastic collapse model. No such things have worked out so far (the GRW model makes predictions that have already been falsified by experiment, for example).

 

[NotYourGrandmasJam]

Short answer: no. The things you are vaguely describing all involve a change to the dynamics, making it no longer unitary. Indeed, anything that makes collapse, in the sense of selecting a single outcome to a measurement, an actual physical process must involve such a change to the dynamics. There are various speculations in the literature about such things, and even some explicit models, such as the GRW stochastic collapse model. No such things have worked out so far (the GRW model makes predictions that have already been falsified by experiment, for example).

Thanks for the response and appreciate the insight. Not trying to be vague, I can be very specific if required and apologies, not sure what length and depth of post is appropriate here. I am happy to discuss the finer points and math as I understand it, I was just throwing this out there after some reflection and wanting to share a question. There aren't many places for a physics hobbyist to engage with ideas.

That said, I think there may be a slight category mismatch here in interpretation, so let me clarify what I am not proposing.

I’m not suggesting a modification to the Schrödinger equation, or a non unitary dynamical collapse mechanism in the GRW sense. I think it's obvious unitary evolution remains intact throughout. The question I’m raising is about the conditions under which a unitary description ceases to be physically adequate and not about altering the dynamics themselves

Decoherence already accepts this distinction implicitly from what I understand. The global evolution remains unitary but local descriptions become effectively classical once environmental entanglement and irreversibility grow beyond recovery. What I’m asking is whether outcome definiteness could similarly reflect a physical saturation or anchoring condition. Like a constraint on what configurations can remain viable. Not an additional stochastic term or explicit collapse law.

I don't think collapse here would be a new dynamical process competing with unitary evolution, I think it might be a statement about when distributed alternatives stop being supportable under irreversible coupling to a macroscopic environment. I think that is categorically different from GRW style models which introduce anything explicit and non unitarity and therefore make falsifiable dynamical predictions.

If the claim is that any account of outcome selection must necessarily alter the dynamics, I’d be interested in where that necessity is proven rather than assumed. As far as I can tell, the formalism itself is silent on why one decohered branch becomes physically realized and interpretations differ precisely because this step is not fixed by unitary evolution alone.

Totally happy to be corrected if I’m missing something essential, Just looking for insight and trying to educate myself. My intent here is to ask whether the “why” of definiteness must live in the dynamics or whether it could live in a physical constraint on description that all interpretations already rely on tacitly. Thanks again.

 

[PeterDonis]

I can be very specific if required

Not about any personal theory you might have; that's off limits here.

I’m not suggesting a modification to the Schrödinger equation...The question I’m raising is about the conditions under which a unitary description ceases to be physically adequate

You're contradicting yourself. If "a unitary description ceases to be physically adequate", then you have to modify the Schrodinger equation, because that equation is unitary.

Decoherence already accepts this distinction implicitly from what I understand.

I don't know where your understanding is coming from (you have given no references to actual textbooks or peer-reviewed papers), but it's wrong. Standard decoherence theory treats decoherence as a unitary process. There is no suggestion whatever of a unitary description ceasing to be "physically adequate" just because decoherence occurs.

local descriptions become effectively classical once environmental entanglement and irreversibility grow beyond recovery.

This is wrong. Decoherence does not give us a classical description, because a classical description would be a description with a single outcome, and decoherence does not give single outcomes. All decoherence does is explain why the different branches of the wave function that are produced by a measurement do not interfere with each other. It does not explain why just one branch actually occurs and the others don't. (That's assuming you are using a QM interpretation which says that there are single outcomes, instead of one like the MWI, which says all outcomes occur.)

I don't think collapse here would be a new dynamical process competing with unitary evolution

Then you haven't understood the math. The math of unitary evolution is quite clear: you cannot get single outcomes from unitary evolution alone. This has been well known in the QM literature for decades.

the formalism itself is silent on why one decohered branch becomes physically realized and interpretations differ precisely because this step is not fixed by unitary evolution alone.

You're understating it. Interpretations differ because only one decohered branch being physically realized is impossible according to unitary evolution alone. We know perfectly well what unitary evolution alone gives you: it gives you the MWI. Again, this has been well known in the QM literature for decades.

Totally happy to be corrected if I’m missing something essential

You are. See above.

 

[NotYourGrandmasJam]

Not about any personal theory you might have; that's off limits here.

That is understood and appreciated. Not a personal theory, just trying to interpret reality and appreciate your insight.

You're contradicting yourself. If "a unitary description ceases to be physically adequate", then you have to modify the Schrodinger equation, because that equation is unitary.

I don’t mean global unitarity fails. The Schrödinger equation remains unitary for the closed system (system + detector + environment). What ceases to be adequate is treating the subsystem as if it admits a purely unitary, coherent description after irreversible coupling.
This distinction is standard: even when the total state evolves unitarily, the reduced state generically evolves non unitarily (master/Lindblad-type effective evolution) once you trace over environmental degrees of freedom. That does not require modifying the Schrödinger equation in my opinion. it’s an emergent/effective description due to coarse-graining.

My point is in the same category as decoherence. Should be global unitarity and local effective classicality. The open question is whether decoherence alone is sufficient for definiteness (outcome selection), or whether there could be an additional physical admissibility/anchoring criterion at the subsystem leve. That is still compatible with global unitarity and governing when “multiple unrealized alternatives” stop being a meaningful physical description.

I don't know where your understanding is coming from (you have given no references to actual textbooks or peer-reviewed papers), but it's wrong. Standard decoherence theory treats decoherence as a unitary process. There is no suggestion whatever of a unitary description ceasing to be "physically adequate" just because decoherence occurs.


This is wrong. Decoherence does not give us a classical description, because a classical description would be a description with a single outcome, and decoherence does not give single outcomes. All decoherence does is explain why the different branches of the wave function that are produced by a measurement do not interfere with each other. It does not explain why just one branch actually occurs and the others don't. (That's assuming you are using a QM interpretation which says that there are single outcomes, instead of one like the MWI, which says all outcomes occur.)


Then you haven't understood the math. The math of unitary evolution is quite clear: you cannot get single outcomes from unitary evolution alone. This has been well known in the QM literature for decades.


You're understating it. Interpretations differ because only one decohered branch being physically realized is impossible according to unitary evolution alone. We know perfectly well what unitary evolution alone gives you: it gives you the MWI. Again, this has been well known in the QM literature for decades.


You are. See above.

 

[PeterDonis]

I don’t mean global unitarity fails.

Then you can't get single outcomes.

even when the total state evolves unitarily, the reduced state generically evolves non unitarily

Yes, but using a "reduced state" at all already assumes that you have a single outcome--without even trying to explain how you got a single outcome in the first place when the total state evolves unitarily. In other words, using the "reduced state" is a pragmatic method of making predictions, nothing more. It is not a foundational argument for why single outcomes occur.

once you trace over environmental degrees of freedom.

But just tracing along isn't enough for the "reduced state" dynamics you refer to. Even the traced out density matrix still has multiple outcomes in it. To make predictions using the "reduced state" dynamics, you have to throw out all but one of those multiple outcomes (you keep the one corresponding to the measurement result that was actually observed).

The open question is whether decoherence alone is sufficient for definiteness (outcome selection)

That's not an open question. The answer is no, it's not. Again, this has been known in the QM literature for decades.

That is still compatible with global unitarity and governing when “multiple unrealized alternatives” stop being a meaningful physical description.

You stil don't get it: "global unitarity" means multiple alternatives never stops being a meaningful physical description. As long as the evolution of the total system is unitary, you can never get single outcomes.

Once more: all this has been known in the QM literature for decades. There are no open questions about it.