Hi Lubos,
I was intrigued by this comment "If someone analyzes why and how a particular measurement apparatus works, it would be useful". I was interested in this question myself, but with little luck. I was hoping you have something interesting to say about this.

Prathyush Manchala

Hi PM, what are you exactly interested in? Did you understand what I actually wrote? I wrote that you have to say which particular apparatus you want to understand.

So again, if you're asking such a question, then I must repeat my question about your interest: What apparatus do you want to be clarified?

If you don't complete this question by adding the name of the apparatus, the question makes no physical sense and it's meaningless to study "how the apparatus or its measurement works". By definition, it just works - it transforms some information about the external objects or Nature that is hard to access by human senses to some information (like the position of a pointer or a digit on a display) that is easy to access. This "conversion" is done in many ways, by different apparatuses.

Hi,

Any apparatus of your choice actually. But I would like to see a formal treatment of the situation. Photographic plates, bubble chambers, Photomultipliers, calorie meters or a voltmeter as you suggested. I understand verbally what happens in each of these situations (except the voltmeter which I did not think about yet). But I have trouble understanding how to treat them formally.

It's a lot of textbooks you want to be written. ;-) What are you exactly dissatisfied about when it comes to the existing explanations how the gadgets work, e.g. those at Wikipedia?

Weinberg says;

"...and if we don’t use the things that we have learned, then the story we tell has no point.”
___________________________________

Thats found complete agreement, and the punch line was the title of this blog post.

in the final analysis, so "last part" in everything which was related about that.

Wow

"striking" right? Not in the references...

You have confused the two topics and blog posts. This thread is about QM, the quote you mentioned is from the blog post about the history.

III. COLLAPSE OF THE DENSITY MATRIX

"It is striking that although the detailed time-dependence of the density
matrix depends on the coefficients ℓnα and hα appearing in the matrices in
the Lindblad equation, the asymptotic limit for t → ∞ for both complete
and incomplete measurements does not depend on these details, depending
only on the initial condition ρ(0) and on what it is that is being measured.
This, of course, is just what we require of a measurement."

(Last part of the Weinberg's recent paper)
_______________________________________

My comment;

He said that, what we measure things like how its expected toward us (Or parallel to our expectetion).

İf these not hidden variables, what kind of influence create this "striking" effect on him?

#Then, initial conditions are seperable to historical blog post as you mention it, or just a coincidence (my confusion).

İsn't that informational reference on air?
(İn his mind)

İn example;
_______________________________________
...
Such unjustified idealizations long ago germinated the well-known
paradoxes concerning the alleged collapse of the wave function, and
voluminous quasimystical discussions of the role of consciousness in determining the quantum state.
...
On the formal side, in lieu of any obscurantism about duality or matter waves, we prefer the following three introductory axioms;

I. For each physical system there is a Hilbert space.

II. Each Hermitian operator A on that space represents an
observable; i.e., each such operator is associated with a class of measurement procedures.

III. For each preparation sheme there is a statistical operator p such that the arithmetic mean of A-data gathered from an ensemble
generated in the manner represented by p is given by Tr(pA).

[Preparation and Measurement in Quantum Physics /James L. Park and William Band ~ Received October 18, 1991]
________________________________________

Ps; formal only and 'our' hidden variables rather than particles...

Dear Lubos, This discussion is of course way over my head, but doesn't the word "collapse" connote a process that takes place over some finite if infinitesimal duration in time? I wonder if the use of that word itself might be an unconscious source of misunderstanding? In which case wouldn't maybe "realization of the wave function" be a better ordinary language description, understood to mean that only the measurement itself is real? Strictly from the peanut gallery.

Excellent response by Lubos as usual. I have to say though that he's written pretty much the exact same thing on this blog 50 or so times. You know, I'm sitting here thinking to myself about why Einstein just couldn't accept quantum mechanics. My thoughts have ranged from purely moral explanations of his character to his belief that his classical field theories should account for all known physical phenomena -- he thought his generalized theory of gravitation may explain quantum phenomena in a classical way. Tonight, I had another thought. Einstein insisted that some observables should have simultaneous physical reality. In proper quantum mechanics, there may be no subspace in the Hilbert space of a spin 1/2 particle corresponding to simultaneous physical reality: S_x = 1/2 & S_y = 1/2. This is a simple mathematical consequence of Hilbert spaces. I wonder if Einstein simply had a misunderstanding of Hilbert spaces. Anybody have a thought on this?

Thanks, Justin - I am getting bored by posting the very similar stuff often, too, and the frequency is decreasing, I think.

Concerning the look inside Einstein's head, I don't quite see the difference between your two explanations, and I agree with both. He just believed in a classical relativistic field theory to ultimately explain everything.

Dear Luke, the word "collapse" indeed invites lots of confusions and misunderstandings but I think that your particular one is the minor one - probably especially because you're confused about the *important* aspects of the collapse.

The point is that the "collapse" is just a "collapse of a wave function" and the wave function is the complexified quantum generalization of the "probability distribution" i.e. the information about the subjective probabilities.

The collapse of it - the wave function etc. - is therefore a subjective matter, a change of the observer's knowledge caused by the measurement. This change occurs for exactly analogous or "same" reason as the change of probabilities in any Bayesian inference.

The only physical purpose of the knowledge (stored in the wave function) is to help one to predict future observations. So the collapse - a change of the data needed to make future predictions - must take place quickly enough,before you actually make the future predictions, otherwise they will be done incorrectly.

But you may imagine that this change - collapse - takes place "immediately" (at t=const in a given reference frame) or gradually or in any other way. The difference between all these choices is absolutely unphysical - so it just doesn't matter how quickly or abruptly the collapse takes place as long as it is done in time before the following observations.

It is absolutely wrong to imagine that the collapse is a physical process in which something "objectively real" is changing in a way that different observers could agree upon, like a collapse of a building attacked by terrorists. The collapse is a *subjective* matter. I feel that you misunderstand this fundamental point as well, and this is why you worry about those "other" issues (the speed) that are actually completely unimportant.

If the collapse were a "collapse of something objectively real", it would be a problem for it to occur quickly - in principle superluminal speed is sometimes needed - because it would violate relativity (no information propagates faster than light). But because the collapse is really a change in the "observer's mind" only, it can occur e.g. instantly without violating relativity.

Lubos, some people I know can't/won't accept that a measurement inevitably disturbs the system. They say that there must be some undiscovered way to measure without disturbing. I tell them that this point has been falsified for 90 years. What else can I do?

Suffer for a while, like I do, have a drink, and enjoy the Sun. ;-)

i guess weinberg forgot a reference in his paper to where this paper refered,
http://fqxi.org/community/f...

Isn't this very point about the 'mysterious wave function collapse' allied with measurement very clearly explained (why it's not so mysterious) by Sidney Coleman in his amazing lecture 'Quantum Mechanics in Your Face'? Or am I wrong?

It's surely *a* nice explanation,
http://motls.blogspot.com/2...

although I am sure that very far from the only one. Lots of things have been written about this and a significant portion of it has been right and, originally, deep.

Thanks for the reply. I agree that it's not the 'only' explanation - at least not the routine text book (Copenhagen interpretation), is that correct? Please lemme know if I'm muddling it. BTW, as off topic, is anyone else having problems with the comments section not appearing at all under the posts? Or maybe it's just me? Thanks for your time.

EDIT: The comments are working again, thanks a lot Lubos.

Dear Umesh, it was hard to find about the split of the comments to the national editions, to find the exact reason, and write 2 lines of JavaScript code to fix the problem, but I did it:

http://motls.blogspot.com/2...

I believe that what Sidney Coleman did, at least the essence, never went beyond the normal quantum mechanics as invented by the Copenhagen school.

He credited Everett for some realizations that everything in QM works for systems of any size and there's no contradiction but I don't believe that Everett actually understood why QM works to the same extent as Coleman did.

I agree 100% with the physics explanations Coleman gave but less than 100% with some of the history.

P.S.: I must also brag about an e-mail related to a reply about the "two lines of code" I left at the Blogger Forum: :-)

Blogger Help Forum Google Product Forums
Hi lumo,

A reply of yours was so helpful that it was marked a best answer in this forum: Blogger Help Forum
Thanks for being a part of our community and for all you're doing to help other users!

The Google Community Team
To change what emails Google Groups sends you, manage your email notifications.
Visit the help center

Yes, amazing job - thanks for solving the issue! Everything's as usual now. OK, now I see clearer. Thanks a lot for the explanation as well.

Talking of measurement, what do you think
of the latest attempt to justify the de Broglie-Bohm interpretation of QM? See
link: “Experimental Nonlocal and Surreal Bohmian Trajectories” : http://phys.org/news/2016-0...

Lubos - Nicely done! And quickly too.
I dropped by TRF immediately after reading Weinberg's paper only to find that you had already dealt with it rather thoroughly.
I should have come here first.
Cheers!!!