What is the Measurement Problem in Quantum Mechanics?

[sciboudy]

I'm not good in quantum but i heard about this cause (cat) what happen to this cat
and why he use cat although quantum used for microscopy system ? :yuck:

 

[Goldstone1]

The cat is in a box with a gieger counter. If normal quantum effects (such as a wave function) governs macroscopic systems, then if the geiger counter releases a poisenous gas, is the cat dead or alive? Quantum theory (old quantum theory) would say that the cat is both dead and alive becuase no one has made a weak measurement (observation) on the cat. But decoherence is relatively new concept which was recenently proven in 1996 by Alain aspect that particles in large enough groups begin to decohere, and quantum eigenstates appear. The cat will be dead if the gas goes off, no denying it - macroscopic systems like cats are not in superpositions... these are quantum effects being badly applied to the world at large that we deal with every day.

 

[Drakkith]

Just in case you didn't know, Schrodingers Cat is simply a thought experiment. It didn't actually happen.

 

[sciboudy]

thank you very much

 

[rgmcc]

Correct me if I'm wrong, but this thought experiment is still a good illustration of the unsolved 'Measurement Problem' in QM--
in principle, if you work out all the math using QM to describe the cat's fate, you will get an 'answer' consisting of two components, 'Alive Cat and Dead Cat' (even considering decoherence). Various interpretations of QM then 'trim down' this dual result into only one of its components, for instance by saying that they are actually random probabilities, or that both outcomes occur in parallel realities.

 

[Drakkith]

I think that is correct rgmcc.

 

[harrylin]

Correct me if I'm wrong, but this thought experiment is still a good illustration of the unsolved 'Measurement Problem' in QM--
in principle, if you work out all the math using QM to describe the cat's fate, you will get an 'answer' consisting of two components, 'Alive Cat and Dead Cat' (even considering decoherence). Various interpretations of QM then 'trim down' this dual result into only one of its components, for instance by saying that they are actually random probabilities, or that both outcomes occur in parallel realities.

I guess that it depends on one's mind if one deems it "solved" or "unsolved"... wasn't it the very purpose of Schrodinger's cat example to emphasize that QM only tells us about probabilities of what we will detect, and not about what goes on in the box?

Thus Schrodinger explained:

That prevents us from so naively accepting as valid a "blurred model" for representing reality. In itself, it would not embody anything unclear or contradictory. There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks.

See also the discussion here:
http://en.wikipedia.org/wiki/Schrödinger's_cat

 

[BruceW]

Yeah, whether state reduction happens at the cat depends on your interpretation of quantum mechanics.
I personally think that state reduction happens at any classical object, so the cat wouldn't be a superposition of dead and alive.
But there are other interpretations which would say the cat was a superposition of alive and dead (for example many-worlds interpretation).

 

[Goldstone1]

Yeah, whether state reduction happens at the cat depends on your interpretation of quantum mechanics.
I personally think that state reduction happens at any classical object, so the cat wouldn't be a superposition of dead and alive.
But there are other interpretations which would say the cat was a superposition of alive and dead (for example many-worlds interpretation).

I must say your ''hunch'' if I can call it that, concerning that ''I personally think that state reduction happens at any classical object, so the cat wouldn't be a superposition of dead and alive,'' is quite an accurate assumption. Whilst we cannot know the previous state without disturbing a quantum wave, there seems to be a cut-off for energies when particles are bundled into enough entangled states.

 

[Goldstone1]

Correct me if I'm wrong, but this thought experiment is still a good illustration of the unsolved 'Measurement Problem' in QM--
in principle, if you work out all the math using QM to describe the cat's fate, you will get an 'answer' consisting of two components, 'Alive Cat and Dead Cat' (even considering decoherence). Various interpretations of QM then 'trim down' this dual result into only one of its components, for instance by saying that they are actually random probabilities, or that both outcomes occur in parallel realities.

Who says it is unsolved? The measurement problem has been solved by quantum mechanics describing it as a weak measurement on a system. It's no if's or but's... the state of the system is reduced to it's own collapse at a certain measurement: These are of course macroscopic objects, and the cut-off frequency of these information packets has been observed at a molecular level, so it must appear at that level, the level from which an object is real or not.

 

[rgmcc]

Who says it is unsolved? The measurement problem has been solved by quantum mechanics describing it as a weak measurement on a system. It's no if's or but's... the state of the system is reduced to it's own collapse at a certain measurement: These are of course macroscopic objects, and the cut-off frequency of these information packets has been observed at a molecular level, so it must appear at that level, the level from which an object is real or not.

I think that your view is not the mainstream consensus.

For example, from Wikipedia:

"The measurement problem in quantum mechanics is the unresolved problem of how (or if) wavefunction collapse occurs. The inability to observe this process directly has given rise to different interpretations of quantum mechanics, and poses a key set of questions that each interpretation must answer."

from
http://en.wikipedia.org/wiki/Measurement_problem

Brian Greene also says something similar in Fabric of the Cosmos if I recall correctly; decoherence alone does not even really address the fundamental measurement problem.

 

[Goldstone1]

I think that your view is not the mainstream consensus.

For example, from Wikipedia:

"The measurement problem in quantum mechanics is the unresolved problem of how (or if) wavefunction collapse occurs. The inability to observe this process directly has given rise to different interpretations of quantum mechanics, and poses a key set of questions that each interpretation must answer."

from
http://en.wikipedia.org/wiki/Measurement_problem

Brian Greene also says something similar in Fabric of the Cosmos if I recall correctly; decoherence alone does not even really address the fundamental measurement problem.

Surely the classical and the quantum mechanical have been defined quite well at the level of decoherent systems? The issue has been addressed, we have confirmation that the wave function is small for a large configuration of particles, rather than a lone one singing in the rain.