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Old Aug 28th 2017, 09:23 AM   #1
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What is a measurement in quantum mechanics?

I'm sure that all of you have heard of Schrodinger's cat thought experiment, right? How many of you know the purpose of it? I ask because I thought it was clear but this journalist doesn't appear to get it. See:
Schrödinger?s Cat explained - Telegraph
The experiment was designed to illustrate the flaws of the ‘Copenhagen interpretation’ of quantum mechanics, which states that a particle exists in all states at once until observed.


If the Copenhagen interpretation suggests the radioactive material can have simultaneously decayed and not decayed in the sealed environment, then it follows the cat too is both alive and dead until the box is opened.
If you read the entire page you'll see that he doesn't get it.

The purpose of Schrodinger's cat in the box experiment was not to "illustrate the flaws of the ‘Copenhagen interpretation’ of quantum mechanics". It was designed to demonstrate that what holds on the subatomic level does not hold on the macroscopic level. The most widely accepted interpretation of this scenario among experts is that the triggering of the Geiger counter constitutes a "measurement," in the statistical sense, not the intervention of a human observer.

A measurement is said to take place the moment a macroscopic system interacts with a microscopic system.
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Old Aug 28th 2017, 04:26 PM   #2
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I'm not sure

Are you asking a question, or offering discussion about a phenomenon, or making a statement?
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Old Aug 28th 2017, 04:52 PM   #3
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Originally Posted by studiot View Post
I'm not sure

Are you asking a question, or offering discussion about a phenomenon, or making a statement?
Making a statement so I'll have something to reference when this subject comes up again. Those who are experts in quantum mechanics knows this fact as do those who write the textbooks. It's just never explained that well and gives misleading ideas such as that in the URL I gave. I.e. the guy who wrote that made the mistake of assuming that it shows flaws in the Copenhagen interpretation, which it doesn't.

Are you familiar with Griffiths text on QM. If so then what I described is in there.

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Old Aug 28th 2017, 06:22 PM   #4
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Occum's Razor

I agree with Pmb,
The interaction of the particle with the macroscopic world (as represented by the Geiger counter) should be considered the point of collapse of superposition.

The absurdity of the observer could otherwise be taken further.

If the Geiger counter is not observer enough and the cat is not observer enough, then why should "you" (as the opener of the box) be sufficient as an observer?
There would then be a superposition of you's, one that saw a dead cat one that saw a live cat.
etc. ad infinitum.

I have seen that as a many-worlds interpretation, and I can't say I like it...
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Old Aug 29th 2017, 02:29 AM   #5
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Originally Posted by Woody View Post
I agree with Pmb,
The interaction of the particle with the macroscopic world (as represented by the Geiger counter) should be considered the point of collapse of superposition.

The absurdity of the observer could otherwise be taken further.

If the Geiger counter is not observer enough and the cat is not observer enough, then why should "you" (as the opener of the box) be sufficient as an observer?
There would then be a superposition of you's, one that saw a dead cat one that saw a live cat.
etc. ad infinitum.

I have seen that as a many-worlds interpretation, and I can't say I like it...
Have you read the article in Physics Today called Quantum Mechanics and Reality by Bryce DeWitt, Physics Today, Vol. 23, No. 9 (September 190)? It's an article on what you just mentioned, i.e. many worlds. However I haven't read it in many years. I'm browsing through it this morning.

I've attached a copy of it. Let me know what you think. I'd appreciate hearing your views on it.
Attached Files
File Type: pdf DeWitt, B.S., Physics Today, 23(9), Sep (1970).pdf (1.62 MB, 3 views)

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Old Aug 29th 2017, 07:28 AM   #6
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You need to understand the mathematics of Hilbert spaces to understand this issue.

The linear combination may be finite or infinite, there is not restriction.

The observables correspond one-to-one to the self adjoint operators in a separable Hilbert space of infinite dimension, H. The pure states correspond one-to-one to the one dimensional subspaces of H. Every state is a (possibly infinite) convex combination of pure states. (This is a mathematical statement of waveform generation and collapse)

'Collapse of the waveform' is linked to the so called 'measurement problem' or measurement paradox and is very difficult to discuss without some higher maths.

Try reading here

The Measurement Problem

Essentially the measurement problem, waveform collapse and 'paradoxes' like Schroedinger's cat are about the crossover point from deterministic classical mechanics to probabilistic quantum mechanics and how this is handled.
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Old Aug 31st 2017, 05:50 PM   #7
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crackpot speculation

I have skim read the articles attached, and must admit much of it was beyond me.
However, I note that the conclusion of the DeWitt article admits that there is no observable difference between the two interpretations.
It thus becomes largely moot issue, one might even say even a matter of personal preference.

One thing I might venture though is that the collapse of the waveform seems to indicate information travelling back in time.
The probabilistic emission of the radio-active particle creates a superposition of states, but the detection or non detection presents just a a single state.
So was there a superposition of states that instantaneously collapses at the point of detection, or has the detection precluded the superposition of states in the original radioactive decay?

If one were to postulate that the probabilistic waveform is a four dimensional function, then the starting and ending conditions would be inherent features of its form.
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