Observer modified mechanics or something

[Bob_for_short]

According to Severian, "Quantum mechanical wavefunctions are only quantized if you set boundary conditions. And this is not really a 'quantum' phenomena, as ordinary waves are similarly quantized (pluck a guitar string and you get one note).

 

The unique thing about QM is that observations change the state of the system, so it would have been better to incorporate that into a name (Observer modified mechanics or something)."

 

I would like to clarify this question: a single measurement do not modify the system state ψ but supplies a bit of information about the state. Only an infinite number of measurements cover every possibility contained in the QM state, for example, the ψ(x) profile in a double slit experiment.

 

The same is valid in Classical Mechanics: we exchange with many-many photons with a body to finally obtain an average - the "deterministic" center of inertia coordinates (think of one-slit QM experiment). Without sufficient exchange the state infirmation is not complete in both CM and QM. Let us note that in both cases the exchange means elementary interaction with a system so observing includes and only possible due to interaction (or "change of the state" in a narrow sense).

Edited September 23, 2009 by Bob_for_short

[Severian]

I would like to clarify this question: a single measurement do not modify the system state ψ but supplies a bit of information about the state. Only an infinite number of measurements cover every possibility contained in the QM state, for example, the ψ(x) profile in a double slit experiment.

 

I disagree. Any 'measurement' does change the system state (unless the system is already in an eigenstate of the variable you are measuring). So if you measure a particle's momentum, for example, after your measurement, the state will be a momentum eigenstate, even if it wasn't before.

[Bob_for_short]

Do you measure the electron or photon momentum (or coordinate) in a double or one-slit experiment?

[Severian]

It is a position measurement. Once the electron (photon experiments are a bit unimpressive since we always knew light was a wave!) hits the screen, you collapse its (position space) wavefunction to a point, and get a data point. When you add them all up you see the interference pattern (in a double slit experiment).

[Bob_for_short]

It is a position measurement. Once the electron hits the screen, you collapse its (position space) wavefunction to a point, and get a data point. When you add them all up you see the interference pattern (in a double slit experiment).

 

If it is a position measurement, then one point suffices. Why then to talk about the space wave function?

 

In my opinion, the system state information is obtained in a series of measurements, so each measurement is not a collapse but retrieval of a piece of the wave function. The state wave function is thus built from pieces since one measurement contains too little information about it.

Edited September 23, 2009 by Bob_for_short