What Is Surprising About Wave Function Collapse?

[Jimster41]

QM founders highlighted that there is no QM without classical Mechanics. Classical Mechanics is about a word where we neglect microscopic details. You have heat pressure, mean values and probabilities. The is no Schrodinger cat in this word. But you have to use CM to describe the apparatus in a laboratory, its environment and so on.
So we need a frontier. The problem is not to find where this frontier is: You put it where you want! It may include an observer who looks at the apparatus.
This frontier has to be seen as a boudary in space time. You can choose it to wrap only the particle between two moments or the whole laboratory between 2014 and 2015.
Once you have this frontier QM tells you that this boudary is a black box. Not a black hole but not so far. Inside the box you have amplitudes of probabilities that you have to sum Outside you have probabilities. Inside you have Schrodinger equation outside you have
https://en.wikipedia.org/wiki/Diffusion_equation
The is no collapse in QM. Seen from the outside QM has given a probability to the boundary that YOU hav chosen. I think that the question of when did the collapse occurred has no sense.
Collapse is an interpretation of QM for observers who live in a classical word where there is no amplitudes to collapse.

Interesting, and surprising. So did the founding fathers of QM think that there could be a classical world without QM, once they had discovered QM?

 

[naima]

Heisenberg's point of view was interesting. He did not say: we have an electron with an energy E1 and it jumps to a random energy E2 < E1 . He was only interested by what he could measure: the spectrum of the emitted photons. His matrix gives a probability to each couple of (E1, E2) .
I think that the good point of view would be to consider only couples (preparation, output) as the objects of our studies. And to calculate their probabilities. This is another way to say what i said in post 24. Where dou you need collapse?

 

[naima]

Interesting, and surprising. So did the founding fathers of QM think that there could be a classical world without QM, once they had discovered QM?

Yes there is is a frontier between QM and CM but you put it where you want.
You need a classical word in which information escape to explain decoherence.

 

[bhobba]

You need a classical word in which information escape to explain decoherence.

You don't. Decoherence follows directly from tracing over the environment which has nothing to do with a classical world.

Thanks
Bill

 

[Jimster41]

So is it accurate to say there are non probabalistic candidates systems that may account for the existence of the classical world.

I'm just struck by the image of them digging up a (causal) tree to find the roots (what is stuff made of?) then when they find the roots they say the roots wouldn't exist without the tree?

Or the causal flower, if you prefer.

 

[bhobba]

So is it accurate to say there are non probabalistic candidates systems that may account for the existence of the classical world.

Of course. BM is classical and deterministic and accounts for the classical world.

Thanks
Bill

 

[Jimster41]

Doesn't BM also say that world has non-local hidden variables? Or a universal pilot wave of some kind? I can imagine that those terms are somehow considered canonically technically consistent with "classical", but uh...

 

[bhobba]

Doesn't BM also say that world has non-local hidden variables? Or a universal pilot wave of some kind? I can imagine that those terms are somehow considered canonically technically consistent with "classical", but uh...

I think what classical means isn't pinned down exactly but comes from context. Lots of things are like that. But if by classical you mean local and realistic - well there is this theorem by Bell that says you can't have that.

Thanks
Bill

 

[naima]

Do not forget that in classical physics you have statistical mechanics. You use it when you speak of pressure and temperatures. When you describe an apparatus that measure a quantum property you need the language of classical physics.
Can you imagine that the CERN announced they discovered a new particle like that:
We used the tensor product of hilbert spaces (see Appendix A for the complete details of the apparatus) and we (see App B for our hilbert spaces) found that ...
This is the ficticious world the Founders of QM refused.
It seems that bhobba thinks the environment has nothing to do with the classical world. It is his own point of view, it is not the initial Copenhague interpretation. Tracing out is accepting classical world.

 

[bhobba]

When you describe an apparatus that measure a quantum property you need the language of classical physics.

You do not need the language of classical physics to define an observation eg it can be defined as the improper state just after decoherence.

Thanks
Bill

 

[naima]

Do you refuse the point of view of those who discovered QM?

 

[bhobba]

Do you refuse the point of view of those who discovered QM?

Yes. A LOT has been discovered since then.

Thanks
Bill

 

[atyy]

Yes. A LOT has been discovered since then.

Thanks
Bill

But so far it has only been shown that decoherence allows the classical world to rmerge if additional assumptions (eg. Bohmian Mechanics) are introduced to define the environment, choose the preferred basis and say when an observation occurs. No one has shown that this can be done in a minimal interpretation.

For a minimal interpretation, the founders of quantum mechanics were essentially right.

 

[bhobba]

For a minimal interpretation, the founders of quantum mechanics were essentially right.

Add any assumptions you like - the fact remains an observation can be defined independent of the existence of a classical world. If you want to see a rigorous exposition of it you can see Wallace's book on MW.

Thanks
Bill

 

[atyy]

Add any assumptions you like - the fact remains an observation can be defined independent of the existence of a classical world. If you want to see a rigorous exposition of it you can see Wallace's book on MW.

Thanks
Bill

But it is misleading to state it as if one is still using a minimal interpretation. One needs something like BM or MWI. There is no consensus that any interpretation except Copenhagen covers all of quantum mechanics.

Not stating these nontrivial assumptions is like stating Gleason's without highlighting the contextuality assumption.

 

[bhobba]

But it is misleading to state it as if one is still using a minimal interpretation.

I am not claiming that. I am simply claiming an observation can be defined without reference to a classical world. Have the most assumption laden interpretation you can imagine - its not relevant - the only relevant thing is it can be done.

Thanks
Bill

 

[atyy]

I am not claiming that. I am simply claiming an observation can be defined without reference to a classical world. Have the most assumption laden interpretation you can imagine - its not relevant - the only relevant thing is it can be done.

Also, even with additional assumptions, we don't know whether it can be done. The extension of Bohmian Mechanics to all relativistic quantum theories remains a matter of research, and even proponents of Many-Worlds like Wallace and Carroll agree that problems remain.

 

[bhobba]

Also, even with additional assumptions, we don't know whether it can be done

There is a myriad of interpretations. Are you sure everyone has issues? Here is a little known one:
http://arxiv.org/pdf/quant-ph/9508021.pdf

And, as you know, all those things you call issues are rather controversial.

Thanks
Bill

 

[atyy]

There is a myriad of interpretations. Are you sure everyone has issues? Here is a little known one:
http://arxiv.org/pdf/quant-ph/9508021.pdf

And, as you know, all those things you call issues are rather controversial.

But they are controversial the other way - ie. it is not generally agreed that they don't have issues. Even in less controversial realms, there can be errors in work that has not been widely examined. For example, https://www.physicsforums.com/threads/status-of-lattice-standard-model.823860/.

So it is not correct of you to ask me whether I am sure there are issues. One rather should ask you whether you are sure there are no issues.

 

[bhobba]

One rather should ask you whether you are sure there are no issues.

Actually I believe a number don't not just those mentioned here eg GRW and mine. Not wanting to derail this thread if you want to pursue it best to have a new thread. But you are making a very strong claim - every single interpretation that can give meaning to an improper state without referencing classical physics is problematical.

Thanks
Bill

 

[atyy]

Actually I believe my interpretation doesn't. Not wanting to derail the thread if you want to pursue it best to have a new thread.

Your interpretation has issues, because you just wave your hands and say "factorization" can be done objectively, "preferred basis can be done objectively", "decoherence threshold can be done objectively", and it is misleading because you present it as a minimal interpretation.

At the very least it is non-minimal because to define the preferred basis objectively, you need the predictability sieve, which is not part of standard quantum mechanics.

Furthyermore, you simply have no definition of factorization and threshold, so it is ill-defined. And you do not show that there is a way of defining factorization and decoherence threshold such that orthodox quantum mechanics is recovered. For a given threshold, the collapse is a nonlinearity, and the onus is on you to show that it does not show up as a failure of superposition, as it does in GRW and CSL.

 

[bhobba]

Your interpretation has issues, because you just wave your hands and say "factorization" can be done objectively,

And you just wave yours and say assuming you can factor a system into what's doing the observing and what's being observed invalidates it.

But that my last comment in this thread, Start another.

Thanks
Bill

 

[atyy]

And you just wave yours and say assuming you can factor a system into what's doing the observing and what's being observed invalidates it.

That is exactly what I don't do. Copenhagen does have the measurement problem, and this problem is stated immediately, eg. Landau and Lifshitz and Weinberg. This is why Copenhagen is an honest interpretation - it is honest about its problems.

 

[Nugatory]

I think this episode of The Bhobba and Atyy Show is a rerun.

 

[Jimster41]

Do not forget that in classical physics you have statistical mechanics. You use it when you speak of pressure and temperatures. When you describe an apparatus that measure a quantum property you need the language of classical physics.
Can you imagine that the CERN announced they discovered a new particle like that:
We used the tensor product of hilbert spaces (see Appendix A for the complete details of the apparatus) and we (see App B for our hilbert spaces) found that ...
This is the ficticious world the Founders of QM refused.
It seems that bhobba thinks the environment has nothing to do with the classical world. It is his own point of view, it is not the initial Copenhague interpretation. Tracing out is accepting classical world.

So the founders of QM considered the classical description of the world primary? Or did they, and should we, consider it dependent on or emerged from the QM world?

If the latter why isn't it necessary for the LHC to describe things they discover always only at the level of fundamental detail? Why isn't it desirable?

I thought the sentence "tracing out is accepting the classical world" was a good one. My question is, would any intelligent machine have to struggle with the same problem of classical QM boundary interpretation and the notion of wave collapse we do? Couldn't a sufficiently comprehensive machine percieve reality as one big non-stationary probabilistic (or non-local) QM fabric and navigate in the very "fictitious" world the founders refused, one free of classical objects?

If so doesn't that imply the collapse may only be an artifact of our specifically evolved and limited (layered, heirarchical) system of comprehension.

 

[bhobba]

If the latter why isn't it necessary for the LHC to describe things they discover always only at the level of fundamental detail? Why isn't it desirable?

Just because one can define an observation independent the classical world does not imply that you must always do so - in fact unless speaking of matters of principle such as how the classical world emerges from the quantum, you would have to have rocks in your head to do it.

Thanks
Bill

 

[Jimster41]

Just because one can define an observation independent the classical world does not imply that you must always do so - in fact unless speaking of matters of principle such as how the classical world emerges from the quantum, you would have to have rocks in your head to do it.

Thanks
Bill

Do you mean it would be inefficient?
On the plus side It would be free of arbitrary quantum classical boundaries, and more accurate?

 

[bhobba]

Do you mean it would be inefficient? On the plus side It would be free of arbitrary quantum classical boundaries, and more accurate?

Simply not necessary. They are writing for professionals that know what's going on, if they are in fact interested in the issue. Despite the interest it garners around here most physicists couldn't care less.

Its in fact a minor blemish in Copenhagen which is the interpretation most have a rudimentary exposure to:
http://motls.blogspot.com.au/2011/05/copenhagen-interpretation-of-quantum.html

There are a number of more modern interpretations like MW, Decoherent Histories, Ignorance Ensemble, Quantum Darwinisn and probably a myriad of others that were cooked up to rectify it. To forestall getting into a long drawn out rehash of things that have been discussed ad-nausium on this forum some of it is known to be controversial eg the use of decision theory in MW. Note - controversial means just that - it does not mean disproved.

Thanks
Bill

 

[atyy]

Simply not necessary. They are writing for professionals that know what's going on, if they are in fact interested in the issue. Despite the interest it garners around here most physicists couldn't care less.

Unless they happen to be very good ones like Dirac or Witten?

 

[bhobba]

Unless they happen to be very good ones like Dirac or Witten?

Of course some are.

Thanks
Bill

 

[lightarrow]

A measurement has a definite outcome because you constructed your measurement apparatus to give you one.

This statement is interesting. The standard example is with linearly polarized photons which pass through a polarizer at an angle α with respect to their polarization plane. But how do "I construct my apparatus so that it makes the photon pass through or not, with a probability cos²α?" It's not, instead, that I construct an apparatus and I simply observe it works in that way but don't know why, in the sense that don't know why a single photon passes or not?

--
lightarrow

 

[bhobba]

It's not, instead, that I construct an apparatus and I simply observe it works in that way but don't know why, in the sense that don't know why a single photon passes or not?

Its I construct an apparatus and know from theory how it works. Part of that theory is it only predicts the probability of a photon passing.

Thanks
Bill

 

[vanhees71]

You use some polarization foil and demonstrate by experiment that it works the way expected. That's how all measurement apparati work. Another question is, whether you can theoretically understand, how it works. That's usually very complicated. In Wikipedia you can read about various ways to polarize light. Some are simple to theoretically analyze (e.g., using reflection under the Brewster's angle):

https://en.wikipedia.org/wiki/Polarizer

The problem with all these "interpretation debates" is that many theoretical physicists (including myself) tend to forget about what's really done in the lab and then try to solve very complicated theoretical problems which are not really there. Sometimes one should simply look at what's done in the lab when some quantum phenomenon is studied. That's very helpful in disginguishing what's necessary to understand the underlying theory in a physical sense from metaphysical (often pretty useless and empty) problems of philosophers.

 

[naima]

That is why i like the Rovelli's sushis!
He writes that every experiment (including the lab if you want) is a sushi of space time.
you can see one of its faces how things where prepared and on the other faces what are the ouputs. Only the boundary of the sushi can be seen. Inside is the domain of superposition and of path integrals. This reminds me what the founders of QM called the frontier between classical and quantic world. Read Rovelli's last book.

 

[Jimster41]

I was more trying to understand more if a Turing machine could built that could calculate an awareness of pure QM reality without "classical measurement or observation" or whether it too would have come up against the puzzle of the quantum classical boundary.

It was a babbling naive attempt attempt to take this thought experiment somewhere new - so I could learn something. Ai was has been on my mind and this conversation was highlighting some confusing questions I have about subjectivity and measurement in those terms. I was hoping some experts might illuminate current understanding and terms re this angle on the problem.

My hunch is that thermodynamics would dictate it would have to be a computer equivalent to the world, or it would end up on the edge of crazy just like human-experimenters, watching the oracle of the inaccessible future constantly decide the cat's fate. Surprise!

Bhobba seems to be saying a real system could be be built that could define a measurement or be aware of a pure QM reality somehow, without crossing or dealing with the Quantum classical boundary. I am skeptical and honestly confused by it. It seems very intersting that the proposal could be made, without the machine necessarily being construct-able. And it seems at least to me like it could illuminate the debate.