[alpineidyll3]

Collapse is only mysterious to people who never learned thermofield theory, or non-equilibrium quantum dynamics. Unfortunately this is also a sizable fraction of all physicists. Thus we have articles like this :/

Basically all the work in getting large scale quantum computers to work lies in these fields. They will continue to grow.

[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.

[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.

[macspoofing]

>Collapse is only mysterious to people who never learned thermofield theory, or non-equilibrium quantum dynamics.

Oh yeah? Then what is the physical explanation for collapse?

[btilly]

What is the physical evidence that collapse actually happens?

Flash news. Nobody has ever produced any.

To the contrast we have lots of lines of evidence that an observer described by quantum mechanics should, upon observing a quantum experiment, be thrown into a superposition of observers. Each of which appears to have observed collapse. The notion is utterly repugnant to our biases so many reject the idea out of hand.

But as we create ever more complex but controlled systems, we can perform ever more elaborate experiments verifying that quantum mechanics works exactly as predicted. At some point if we take seriously the idea that the most successful scientific theory of all time is an accurate description of ourselves, then we have to accept that perhaps there is no collapse after all.

[gmkiv]

I happen to agree with you, the observer is a quantum system must get entangled with the quantum system, but that still doesn't explain probabilities. If you prepare a system - say sqrt(1/3) spin-down + sqrt(2/3) spin-up, and then observe it, repeatedly, your subjective experience is that you saw spin-down 1/3 of the time, and spin-up 2/3 of the time. I don't understand how purely unitary evolution can explain this. Does it?

[lmm]

> I don't understand how purely unitary evolution can explain this. Does it?

What's the alternative? Assuming unitary evolution and some fairly common-sense axioms about how we'd expect subjective experience to behave (things like: we never experience being in a branch that has amplitude zero; if we experience being in a given branch then we continue to be in that branch), the Born probabilities are the only model anyone's ever come up with for how our subjective experience should go. So what's there to explain?

[millstone]

The alternative is non-MWI theories, which typically introduce the Born rule via new axioms.

Regarding what's to explain, it's quantum randomness (which distills the Born rule objection). Our subjective experience is that we see spin-down 1/3rd of the time, and our theories say the result is otherwise impossible to predict, even in principle. But a deterministic theory cannot produce a random outcome, even a subjective one.

[lmm]

> Our subjective experience is that we see spin-down 1/3rd of the time, and our theories say the result is otherwise impossible to predict, even in principle. But a deterministic theory cannot produce a random outcome, even a subjective one.

Whyever not? What else would you expect the subjective experience of being in a state like 1/sqrt(3)|x> + 2/sqrt(3)|y> to be like?

[Orlanthai]

Derivations like that don't work, you've just declared it by fiat but there's no such proof that is known to work.

[lmm]

There's no proof that it's the only possible way - but no-one's ever been able to come up with a concrete alternative.

[im3w1l]

To paraphrase you, how do we get from probability amplitude to observed frequencies if there is no collapse?

This is were we have to invoke philosophy. Specifically how does consciousness interact with time? The common-sense thinking is that our soul is tied to our body and is traveling forward through time with it. Another way of thinking is that the soul is tied to a given position of the space-time-probability. It does not travel. You today is not the same as you tomorrow or yesterday. The you that observes spin up is not the same you as the one that observes spin down. Your soul is perceiving reality from a randomly chosen vantage point among all the possibilities with which have a compatible body. If we condition on those bodies belonging to experimenters who have observed frequencies, then we get the distribution.

This is one possibility anyway.

[LittleTester]

No it can't. There have been many attempts and they don't work. The Born rule is independent of unitary evolution. The closest one can get is to declare that the quantum state is fundamentally a statistical object (i.e. the only information in it is observation probabilities) and then with certain assumptions about the size of the state space you can show that the Born rule is the only possible rule for connecting the state to statistics consistent with the unitary dynamics.

So under the assumption that the state encodes probabilities, state space assumptions and consistency with unitary evolution you get the Born rule. However this is not the same as the Born rule arising dynamically from unitary evolution alone.

[Enginerrrd]

Isn't your subjective experience just one probabilistic eigenvalue of a particular combination of operators corresponding to your observation? How does unitary evolution break down here?

[LittleTester]

It's not unitary evolution breaking down, just that the Born rule isn't a consequence of unitary evolution. They're separate independent hypotheses. In most derivations of QM from an axiomatic basis they're consequences of separate combinations of axioms.

[Enginerrrd]

Thanks. Do you by chance have a good source for a gentle introduction into axiomatic QM? Like undergrad level is fine, I've taken basic QM and worked through Griffith's intro book on my own, and I've had a lot of math.

I'd love to read more but my google results aren't turning up a good definitive introduction.

[LittleTester]

Well in the most common family of interpretations "collapse" isn't an actual physical process, just Bayesian updating. So you wouldn't expect to find physical evidence of it in that sense.

It's true that from the perspective of an external superobserver the quantum state evolves to contain terms for each observer observation state. However since all interference observables turn out to be non-physical for macroscopic systems we get a superselection rule and so the probabilities for different macrostates are classical probabilities and thus reflect simple ignorance of the observer's post measurement state.

There's very little motivation for reading the quantum state "ontically" in the way you are doing.

[tsimionescu]

But this doesn't answer the question. If you claim that all of these possible observers 'exist', how does this have a physical meaning?

This is what I never understood about MWI, in what physical sense can the many worlds be said to exist? Where are they in our universe? What direction would we have to travel to find them? Do they exert gravity on us? If not, then how can we claim that they exist in a physical sense?

[Enginerrrd]

No you're thinking of MWI all wrong. Your conception of the universe you exist in as being non-quantum is fundamentally flawed. The universe with the superposition of all the possible observers exists more purely in hilbert space. Sean Carroll has even started to put together a model for how spacetime could emerge from that hilbert space.

So the universes all exist in the same place, since they are the same universe. Your idea of what an observation is, is just an eigenvalue of that corresponding operator.

[AgentME]

An object that moves far enough away from us is said to leave the observable universe, because with the continual expansion of space, it or anything it interacts with would have to travel faster than light back toward us in order to have any effect on us. Should we say that objects that leave the observable universe continue to exist? Should we amend our theories to include a new fall-off effect separate from gravity that says things stop existing when they exit our observable universe?

[akvadrako]

Existence isn't based on something affecting our world, obviously - that's just absurdly self-centered.

But anyway, the other worlds do effect our world - that's why we get interference patterns in double slit experiments.

[ClumsyPilot]

> "Existence isn't based on something affecting our world, obviously - that's just absurdly self-centered."

This is very unfair. This is a niche field with contested interpretations, don't make people feel stupid for asking fair questions.

It's obvious what the other person meant: what does 'our world' and 'other worlds' mean, and how do you know it's not just a figment of your imagination, as a scientific theory must be falsifiable -> i.e. measurable and provable / disprovable somehow.

You should at least point people to reading material before making fun of them.

[akvadrako]

I wasn’t making fun of anyone but your “obvious“ reading seems wrong.

I’m pretty sure he suggested that something only exists physically if it has some measurable effect on us.

[tsimionescu]

In physical terms, we do generally define existence that way - for example, we say that time and space didn't exist 'before' the big bang, because there was nothing that could have a position or change. I was thinking of the same notion of existence and how it can be applied to MWI - essentially existence in the physical sense must mean that something is measurable, that it has some effect on the world (perhaps in the past or in the future).

[akvadrako]

I don't think we do define existence that way. Say you and your friend both go to opposite ends of the visible universe; due to inflation you'll never be able to communicate again.

I suspect most people would say their friend continues to exist. This is very analogous to the many worlds situation.

[ben_w]

Speaking as a barely informed enthusiast, we can say they exist in the Occam’s Razor sense that the maths is much less complicated when we assume they do.

I think there’s also an experimental setup, whose name I forget, but which is essentially nested Schrödinger's cat setups: Alice is in a box, Bob is in a box which contains Alice’s box, Carol is outside; Alice goes into superposition of |Alice+> and |Alice->, Bob opens the box and Carol can now demonstrate that Bob is in a superposition of |observing Alice+> and |observing Alice-> instead of the combination of 100%|observing> and a superposition of |Alice+> and |Alice->.

[tsimionescu]

The maths is the same whether we interpret it as many worlds, wave function collapse, and others.

[ben_w]

Well of course. It they were different — or at least if they gave different conclusions — we could rule some of them out.

[nabla9]

Different worlds don't exist in extra space or dimension. They are orthogonal quantum states of the whole universe.

[nabla9]

Wave function collapse is not experimentally verified or observed physical phenomenon (so far). It's postulated by some interpretations of QM.

Apparent wave function collapse can be explained using quantum decoherence.