Why does QM split into two evolution types

[Thor Jackson]

Quantum mechanics uses two very different kinds of evolution:

• smooth, unitary evolution under the Schrödinger equation
• non‑unitary state change during measurement

What I’m trying to understand is whether this split is just a feature of how we describe QM, or whether the theory itself enforces a deeper structural progression.

In many situations, the sequence seems to be:

1. unitary evolution builds constraints (entanglement, correlations)
2. coherence is lost through interaction with the environment
3. a non‑unitary transition produces a new, stable state
4. the system continues evolving from that new configuration

My question is:

Is this ordering—constraint buildup → coherence loss → transition → stabilization—fundamental to QM, or is it just a convenient way we organize the theory?

I’m not assuming any particular answer.I’m trying to understand whether QM has an underlying structural logic that makes this sequence unavoidable.

 

[PeterDonis]

What I’m trying to understand is whether this split is just a feature of how we describe QM, or whether the theory itself enforces a deeper structural progression.

I think the answer to this depends on which QM interpretation you adopt.

Your step 3 (which is usually called "collapse" or "state reduction" or something similar) is necessary in order to use QM to make accurate predictions. In that sense it's "enforced" by the theory.

But different QM interpretations tell very different (and mutually inconsistent) stories about why your step 3 is necessary in order to use QM to make accurate predictions. In that sense, what your step 3 actually means depends on how you choose to describe what QM is doing (which is one way of viewing what different QM interpretations are doing in different and mutually inconsistent ways).

 

[Thor Jackson]

I think the answer to this depends on which QM interpretation you adopt.

Your step 3 (which is usually called "collapse" or "state reduction" or something similar) is necessary in order to use QM to make accurate predictions. In that sense it's "enforced" by the theory.

But different QM interpretations tell very different (and mutually inconsistent) stories about why your step 3 is necessary in order to use QM to make accurate predictions. In that sense, what your step 3 actually means depends on how you choose to describe what QM is doing (which is one way of viewing what different QM interpretations are doing in different and mutually inconsistent ways).

Thanks — that helps clarify the interpretational landscape.Let me try to sharpen the question a bit, because I’m not really asking why collapse is needed for predictions, but why the sequence of processes in QM always seems to follow the same structural order, regardless of interpretation.

Even in interpretations that deny literal collapse, the pattern still appears:

• unitary evolution builds correlations and constraints
• coherence is lost through interaction with the environment
• the system transitions into a more stable, effectively classical state
• future evolution proceeds from that new configuration

Different interpretations give different stories about why this happens, but they all preserve the same ordering of events.

So the deeper thing I’m trying to understand is:

Why does quantum theory enforce this particular progression — constraint buildup → coherence loss → state transition → stabilization — even in interpretations that disagree about what “collapse” means?

I’m not assuming anything beyond standard QM.I’m trying to understand whether this structural ordering is just a feature of our descriptions, or whether it reflects something built into the mathematical architecture of the theory itself.

 

[PeterDonis]

why the sequence of processes in QM always seems to follow the same structural order, regardless of interpretation

Um, because that's how the theory works? And doing it that way makes correct predictions?

I’m trying to understand whether this structural ordering is just a feature of our descriptions, or whether it reflects something built into the mathematical architecture of the theory itself.

Um, the latter? Since that "mathematical architecture" is what makes correct predictions?

 

[PeterDonis]

the system transitions into a more stable, effectively classical state

No, that's not what "no collapse" interpretations (like the MWI) say. They say unitary evolution just continues, all the time. Yes, part of that unitary process involves decoherence, when everything starts to become entangled with an arbitrarily large number of untrackable degrees of freedom in the environment. But there is never a "transition" where unitary evolution temporarily stops happening.

The main task such interpretations take on is to explain why it looks to us like a "transition" such as you describe happens, even though no such thing ever actually happens.

 

[Thor Jackson]

Um, because that's how the theory works? And doing it that way makes correct predictions?


Um, the latter? Since that "mathematical architecture" is what makes correct predictions?

I get that the formalism works that way — that part isn’t in dispute.What I’m trying to get at is something slightly different.

If the ordering were just a matter of “that’s how we do the calculation,” then different interpretations might rearrange the sequence or treat the steps as interchangeable. But they don’t. Even when they disagree about what collapse means, they still preserve the same structural progression:

• unitary evolution builds correlations
• coherence is lost through interaction
• a transition to a stable, effectively classical state occurs
• future evolution proceeds from that new state

So the question I’m trying to understand is:

Why does every interpretation — even ones that deny literal collapse — still preserve that same structural ordering?

If the ordering is built into the mathematical architecture, then there should be a way to point to which part of the architecture enforces it.If it’s just descriptive, then in principle one could reorganize the sequence without changing predictions.

I’m trying to understand which of those is actually the case.

 

[Thor Jackson]

Thanks — that’s a good clarification about how MWI treats the dynamics.I agree that in Everettian QM the evolution is always unitary and there is no literal “transition” where the Schrödinger equation stops.

But even in MWI, the appearance of a transition still follows a very specific structural order:

1. unitary evolution builds correlations
2. decoherence spreads those correlations into the environment
3. branches become dynamically independent
4. observers inside a branch see a stable classical outcome

So while nothing non‑unitary happens globally, the effective structure seen by an observer still unfolds in that same sequence.

That’s the part I’m trying to understand more deeply.

Even when interpretations disagree about what is “really” happening, they still preserve the same ordering of:

• correlation buildup
• coherence loss
• emergence of stable classical records

So my question isn’t about whether collapse literally occurs.It’s about why every interpretation — including fully unitary ones — preserves that same structural progression in the phenomena we actually observe.

If the ordering is purely emergent, then there should be a clear explanation of why that emergence always takes this form.If the ordering is enforced by the mathematical structure, then it should be possible to point to the specific features of the formalism that make it unavoidable.

That’s the distinction I’m trying to get at.

 

[PeterDonis]

If the ordering were just a matter of “that’s how we do the calculation,”

It's not. It's a matter of "that's how we make accurate predictions". That's how any physical theory works: you figure out what operations you have to do, in what order, to make accurate predictions. Then you do that.

emergence of stable classical records

No, MWI doesn't even say this. There are never any "stable classical records" in the entire wave function. There only appear to be in particular branches of the wave function.

If the ordering is purely emergent

It isn't, unless you think that the requirement to make accurate predictions is somehow "emergent".

If the ordering is enforced by the mathematical structure

It's not. It's enforced by the requirement to make accurate predictions.

That’s the distinction I’m trying to get at.

I don't think either arm of your distinction captures what is actually going on. See above.