Divine Neutrality, Blog. Science, Philosophy

What is Measurement?

January 17th, 2008

The Law of Happen

I am reading some 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 state – encounters a macroscopic measuring device. In doing so the Hilbert Spaces of the two become entangled. A pointer state of the device signals the system’s state. This is the scheme set down by von Neumann in 1932 and explained and expanded upon by Schlosshauer in a 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 free electrons moves pointers.) The reaction

(photon + bound electron) yields (free 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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