I Does the Act of Observing a Detector Influence Quantum Measurements?

[sayetsu]

What counts as an observer, and what does it mean to "observe"? I've read a detector (machine) is an "observer." But how do we know it performed an observation without observing it? Is it possible our observation of the detector retroactively caused the wave function to collapse?

 

[A. Neumaier]

I've read a detector (machine) is an "observer." But how do we know it performed an observation without observing it?

You can ask the same question about a human observer. Your knowing has nothing to do with someone else's observations.

 

[Aidyan]

One can also ask how do you know that a rock exists unless observed? It has nothing to do with quantum physics. It is a more general philosophical question with which philosophers struggled long before quantum physics was born and without any reference to a wf collapse.

 

[PeterDonis]

What counts as an observer, and what does it mean to "observe"?

There is no rigorous definition of an "observer" or a "measurement" in QM. That is one of the main open issues with QM. For practical purposes, physicists simply assume a "measurement" has occurred and apply the projection postulate accordingly wherever it works best for making predictions and interpreting experimental data. But the QM formalism itself does not tell physicists when to do this; it just says "do it wherever it works best".

 

[EPR]

It's more consistent to treat everything in terms of quantum fields instead of applying wave particle duality(and collapse).

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[Morbert]

As others have mentioned, observers are not explicit in the formalism. Instead we have physical variables and their correlations. We try to identify the variables with enough classicality to be experienced by us, and see how they correlate with more explicitly quantum variables. This correlation is the closest the formalism gets to identifying an observation or measurement.