[GuB-42]

I am not a physicist but isn't it an instance of "if you think you understand quantum mechanics, you don't understand quantum mechanics"?

The measurement problem, which deals with collapse is still unsolved. It is also the reason why there are so many weird interpretations of quantum mechanics and that even top physicists can't agree on one. In fact the same physicists tend to sweep the problem under the rug and instead focus on the equations that, to be fair, did a lot more to science and technology than trying to solve the measurement problem.

[alpineidyll3]

There is no measurement problem. It so happens that most measurement apparatus are macroscopic and at a temperature much higher than the quantum gap of the system being probed. Measurements in qm imply a rotation between systems, and mixture with an incoherent thermal state implies a quantum superposition will lose coherence. Full stop. This is all understood in full, grab any text on non equilibrium quantum dynamics. One can even simulate the collapse process in full detail, and obtain superior agreement with experiments.

If the measurement apparatus is coherent, measurements can be performed without collapse. This was well thought out even within wigner and einstein's lifetimes. c.f. the vaidman bomb detector.

[zeofig]

This is certainly my view as a physicist, although I'm not the greatest physicist by any measure. QM can give us a perfectly clear picture of "collapse", although it's not a trivial matter and it's hard to explain to laypeople. Especially when physics fans have these ideas about ~conscious observers collapsing the wavefunction~, or something.

[Orlanthai]

It depends on what one calls the measurement problem.

This solves the "consistency/small problem", i.e. treating the macroscopic apparatus as boolean is justified.

It doesn't resolve the "outcome problem", i.e. which outcome is selected. Of course if you accept the world is not deterministic this isn't really a problem.

[alpineidyll3]

One could argue even classical mechanics isn't deterministic as we think of it because of chaos, which has fascinating connections with QM. W Hoover (of the Nose-Hoover thermostat fame) did some great work with reversible thermostats exploring the instability of Newtons equations of motion.

[Orlanthai]

I think that's different. Chaos still uses classical probability and the randomness is just ignorance of underlying initial conditions. This is very different from QM.

[alpineidyll3]

You'd be surprised. You should read about many-body localization and the eigenstate thermalization hypothesis.

[Orlanthai]

I have. They still don't make Quantum Theory and Chaos similar. Rather for some systems QM can motivate ergodicity as well as classical chaos can. However that doesn't mean Quantum Probability and Classical Probability are alike simply because they give similar behaviour for certain systems for one specific limit. Their representation theory is completely different.

Chaotic systems don't have a Kochen-Specker or PBR theorem.

[thegabriele]

What do i measure of a quantum system if nothing collapse?

Honest question.

[alpineidyll3]

Measurement is a transfer of a state information from a subsystem 'being measured' to a 'measurement device' caused by an interaction. (ie: a quantum bus is a measurement) Just like in a classical situation, further dynamics can depend on the final state in the measurement device. The only difference is that all measurement states will be involved in the quantum measurement device, and all possibilities of dynamics conditioned on the measurement are explored. At any point in the future the whole chain of events can be collapsed, if the measurement device interacts with a classical incoherent object.

[kanzenryu2]

The first quote dates to the 1960s before decoherence was understood.

[LittleTester]

Decoherence doesn't really solve these issues. It gives you an approximately diagonal density matrix for the macroscopic degrees of freedom, but: (a) Not exactly diagonal (b) There isn't a unique decomposition of the macroscopic density matrix even after decoherence thus it cannot be taken as simply ignorance of some set of macrostates.

You need something stronger, namely superselection or irreversibility.

[alpineidyll3]

That's correct, But there are numerous good methods to propogate irreversible dynamics in qm. Many of these are exact in the limit of a noninteracting bath of bilinearly coupled oscillators which is sufficient to describe a measurement collapse. There's no mystery or inexact proscription to such a simulation of a collapse process. It's just complicated.

[Xcelerate]

> There's no mystery or inexact proscription to such a simulation of a collapse process. It's just complicated.

Then there are a lot of Nobel prize winning physicists who would love to be enlightened about how simple the mystery actually is.

[akvadrako]

You are really missing the point - the details of how a measurement happens with specific instruments is not what the measurement problem is about.

The issue is linear evolution means the measurement of a superposition leads to a superposition of measurement devices. If the quantum state is real that gives you many worlds.

If you are suggesting there is nonlinear evolution, well a) it must be non-local and b) the theoretical research suggests it would be inconsistent. QM is a very rigid theory - “an island in theory space”. It isn’t easy to slightly modify.

[Joker_vD]

> The issue is linear evolution means the measurement of a superposition leads to a superposition of measurement devices. If the quantum state is real that gives you many worlds.

And that's problematic because? Because it explains away the whole measurement problem, there is nothing to explain, it's an artifact of a macroscopic observer's point of view?

It's like saying that SR/GR with their space-time continuum being real is problematic, so let's keep to the (post)-Newtonian point of view, but oh no, it now has all this weird amendments and additional terms, and when you try to extend it to the whole of the Universe, it breaks down/gives really weird stuff. Well, duh, of course it does, if one tries to pull an owl on a globe, it simply won't fit.

[Orlanthai]

Well you don't need to have nonlinear evolution to get what alpineidyll3 is saying. It's sufficient for the observable algebra of macroobservables to be commutative. This allows the evolution to be linear and have no interferences.

The QM is "an island in theoryspace" idea isn't strictly true either. QM is one among an entire family of probability theories. It's only rigid when considered purely from the point of view of Probability theories based around vectors in Hilbert space. However considered as part of OPTs in general there's nothing that makes it difficult to modify.

[gus_massa]

Is it confirmed that decoherence is the answer to collapse? I strongly like the idea of decoherence, but IIRC it still need a few decades to settle.