What is Measurement?

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?

It seems to me that the essence of measurement is captured by any elemental process that procedes irreversibly. When that happens probabilities become converted into certainties. The very word, ‘happens’, captures the essentials of a measurement event. If something ‘happens’ then chemical processes are proceeding to completion. Reactants are producing products. In every such process there is an identity change. Out of the ingoing identities there arise new outgoing identities. In this view a measurement event is the creation of new identities from old. What is the elemental such chemical process where identities materialize? It is an inelastic collision – say, where two particles merge? Can one not assert, then, that an inelastic collision is the archetypical ‘measurement event’?

It seems to me that the Measurement Problem is really the Riddle of Materialization. Or perhaps it is best caste as, “What is the Law of Happen?” But, whatever the solution is, it must be perceivable in an elementary inelastic scattering event. Sewell says that the solution is contained within quantum theory itself. I would like to understand how.

Should the mechanics of materialization not be visible in a careful analysis of an idealized, say 1-D, inelastic scattering? I am going to try this and would appreciate any direction cogniscenti may wish to give. Perhaps all this has already been said and done. All are invited to comment.

To be discussed:
What is a ‘system’?
What is a ‘state of the system”?





2 responses to “What is Measurement?”

  1. Kieran O’Rourke

    A very insightful set of ideas on what constitutes a measurement has been gathered by Ron Garret using basic Quantum Information Theory.

    See: http://www.flownet.com/ron/QM.pdf

    His Google Talk summarizes his ideas and may motivate the reading of the PDF.

    Video: http://www.youtube.com/watch?v=dEaecUuEqfc

    Don’t be put off by the flippancy of the talk’s title, he is being provocative.

    This is one of the more stimulating expositions on the “Measurement Problem” that I have come across.

  2. I’m sorry to say that Garret has a number of errors and misstatements in his offering. And these discount its validity. And it seems to me that the kind of language he uses does, indeed, reveal something of his mindset – self-involved rather than scholarly.

    Here’s a misstatement example:

    “… in order to make particles stop behaving like waves and start behaving like particles. We only have to introduce some change that makes it possible in principle to determine which slit a particular photon passed through on the way to the screen and we will destroy the interference pattern exactly as if we had actually measured the particle’s position.”

    The destruction of the interference pattern in this experiment may be understood entirely using Maxwell’s electro-magnetic wave theory. It does not show waves becoming particles; interference destruction occurs with waves.

    The piece is marked by the avoidance of diagrams that might clarify what is being said. These are sorely needed especially in his description of an EPR type experiment with each of the pair of photons passing through distant slit pairs. Except for the Venn diagrams there are no figures in the text. This is a mark of obfuscation.

    The rest of the piece amounts essentially to ‘proof by assertion’. Not the work of a scholar. I’m afraid I must discount it.

    To see an animated motion graphic of the two slit experiment and a which-way ‘eraser’ go to: