Wavefunction collapse

3nTr0pY

Would it be fair to say that before an observation, a wave-particle is in a superposition of many possible states but that after the observation, the wave-particle is found only in one state?

Would that be analogous to saying that it goes from behaving in a very wave-like manner to behaving in a very particle-like manner because the wavefunction has collapsed?

I just want to be clear on what's going on.

Dmitry67

Wavefunction collapse is an artifact of a (currently abandoned) Copenhagen Interpretation. It is replaced with Quantum Decoherence.

3nTr0pY

Sorry, I'm referring to the Copenhagen interpretation here. Would my description of the Copenhagen interpretation be correct?

Matterwave

Quote by Dmitry67

Wavefunction collapse is an artifact of a (currently abandoned) Copenhagen Interpretation. It is replaced with Quantum Decoherence.

Currently abandoned? I don't think so...

Bill_K

Would it be fair to say that before an observation, a wave-particle is in a superposition of many possible states but that after the observation, the wave-particle is found only in one state? Would that be analogous to saying that it goes from behaving in a very wave-like manner to behaving in a very particle-like manner because the wavefunction has collapsed?

Not really. Say the system is initially an eigenstate of J_z and you measure J_x. The state will collapse to an eigenstate of J_x. Now if you measure J_z it will collapse to an eigenstate of J_z (maybe not the initial one.) There is really no sweeping distinction between states that exist before an observation and those that exist after.

Matterwave

Yes, also the thought of a collapsed wave function acting as a particle, could only make sense for position measurements of the particle. If you measured the particle's momentum, though, it would become "less" like a particle in that it's position-space wave function would be spread out.

3nTr0pY

Quote by Bill_K

Not really. Say the system is initially an eigenstate of J_z and you measure J_x. The state will collapse to an eigenstate of J_x. Now if you measure J_z it will collapse to an eigenstate of J_z (maybe not the initial one.) There is really no sweeping distinction between states that exist before an observation and those that exist after.

But how would you know that it is initially in eigenstate J_z? Surely in the general case, you have no knowledge of the initial eigenstate and so it acts as a superposition of all possible eigenstates? And then once an initial measurement is made such a superposition no longer exists.

I've not been studying quantum for long so bear with me.

Yes, also the thought of a collapsed wave function acting as a particle, could only make sense for position measurements of the particle. If you measured the particle's momentum, though, it would become "less" like a particle in that it's position-space wave function would be spread out.

Good point. Fourier transforms and that.

Thing is, I'm trying to explain some basic quantum within a limited time frame to an audience that doesn't know physics. So perhaps its alright if I gloss over the complexities. I just don't want to say anything that is plain wrong.

*yoda jedi*

Quote by 3nTr0pY

But how would you know that it is initially in eigenstate J_z? Surely in the general case, you have no knowledge of the initial eigenstate and so it acts as a superposition of all possible eigenstates? And then once an initial measurement is made such a superposition no longer exists.

I've not been studying quantum for long so bear with me.

Good point. Fourier transforms and that.

Thing is, I'm trying to explain some basic quantum within a limited time frame to an audience that doesn't know physics. So perhaps its alright if I gloss over the complexities. I just don't want to say anything that is plain wrong.

http://plato.stanford.edu/entries/qm-collapse/

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Dmitry67	Wavefunction collapse is an artifact of a (currently abandoned) Copenhagen Interpretation. It is replaced with Quantum Decoherence.

3nTr0pY	Sorry, I'm referring to the Copenhagen interpretation here. Would my description of the Copenhagen interpretation be correct?

Matterwave	Yes, also the thought of a collapsed wave function acting as a particle, could only make sense for position measurements of the particle. If you measured the particle's momentum, though, it would become "less" like a particle in that it's position-space wave function would be spread out.