Friday, October 25, 2019

Quantum measurement problem, explained

Dr. Bee explains the quantum measurement problem:
The problem with the quantum measurement is now that the update of the wave-function is incompatible with the Schrödinger equation. The Schrödinger equation, as I already said, is linear. That means if you have two different states of a system, both of which are allowed according to the Schrödinger equation, then the sum of the two states is also an allowed solution. The best known example of this is Schrödinger’s cat, which is a state that is a sum of both dead and alive. Such a sum is what physicists call a superposition.

We do, however, only observe cats that are either dead or alive. This is why we need the measurement postulate. Without it, quantum mechanics would not be compatible with observation. ...

Why is the measurement postulate problematic? The trouble with the measurement postulate is that the behavior of a large thing, like a detector, should follow from the behavior of the small things that it is made up of. But that is not the case. So that’s the issue. The measurement postulate is incompatible with reductionism. ...

I just explained why quantum mechanics is inconsistent. This is not a 'vague philosophical concern'.
She also says QM is incomplete.

This so-called measurement problem is a 'vague philosophical concern' in the sense that it does not present any practical difficulties.

When you say a theory is inconsistent, that usually means that it allows computing two different outcomes for some proposed experiment. That never happens with QM.

To see that there is an inconsistency, you have to wade thru a discussion of not seeing cats that are alive and dead at the same time.

It is not clear that this problem is a problem.

If anything good comes out of quantum computing research, it could be a better reductionist understanding of quantum measurement. Quantum computers seek to string together qubits as much as possible without measuring them. Because the computation depends on this lack of measurement, maybe the experiments could tell us more precisely just what a measurement is.

But the quantum computing research has told us nothing of the kind. Good old QM/Copenhagen is the underlying theory for all these experiments, and we have no clue that the 1930 theory is not good enuf.

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