Divine Neutrality

I am reading some wonderful papers on the Measurement Problem.

What strikes me is how measurement is visualized. It is visualized as taking place in a laboratory. The system - an isolated entangled state - encounters a macroscopic measuring device. In doing so the Hilbert Spaces of the two become entangled. Somehow a pointer state of the device must result. This is the scheme set down by von Neumann in 1932 and lucidly explained and expanded upon by Schlosshauer in a well written review article. Another gripping article is by Geoffrey Sewell, who says, effectively, that there is no measurement problem.

But is measurement about laboratories?

In the laboratory a photodetector signals the arrival of each photon and a counter accumulates the counts. It works because the photon is absorbed, ejecting an electron. (A current of electrons moves pointers.) The reaction

(photon + electron) yields (electron*)

is what marks the measurement.

But is not any green leaf a photodector? The photon gets absorbed via photosynthesis. The leaf’s vitality is a photon count accumulator. The reaction

(photon + water + carbon dioxide) yields (sugar + oxygen)

marks the ‘measurement’.

Surely every chemical reaction that goes to completion is a measurement event; the reactants disappear and the products appear. Isn’t every inelastic scattering a measurement event? In every such event the original quantum system is destroyed and something new emerges. It is just the property of any chemical reaction.

What, then, constitutes a measurement?

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A commonplace computational practice in quantum mechanics generates the most profound conceptual challenge to the theory. The challenge is called the measurement problem. Here are some quotes summarizing the problem.

“The quantum measurement parodox.. stated succinctly… In quantum mechanics all possibilities… are left open whereas in … experience a definite outcome always (occurs).”
A. J. Leggett in Foundations of Physics. 18, 939 (1988)

“How is the measuring instrument proded into making up its mind which value it has observed?”
Bryce S. Dewitt, Physics Today 23, 30 (1970)

“Some explanation must be provided for the fact that the Hilbert—space vector… collapses onto a certain eigenvector during a measurement process…”
J. Bub, Nuovo Cimento v. 57, Nr.2, 503 (1968)

The probability amplitudes evolve deterministically until a measurement is made: the measurement stops the evolution. What is the essential element that changes the evolution of the system from being in a state
|S> = (superposition sum of many states |n>),
into being in a state, say, |n=3>, one from among the many in the superposition?
Marvin Chester, never published

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