Rogue II by Eric J. Heller.

As far as unadulterated, standard quantum mechanics is concerned — no surreal particle trajectories à la Bohm, no nonlinear modifications of the Schrödinger equation à la Ghirardi, Rimini and Weber or Pearle, no extraneous axioms like the traditional eigenstate-eigenvalue link or the various "beable" conditions introduced by modal interpretations — the only sufficient condition available is to be measured.

To be is to be measured.

The properties of things (or the values of observables) are extrinsic in this particular sense: they exist only if, only when, and only to the extent that they are measured. In other words, they supervene (in this particular sense) on property-indicating events or states of affairs.

Supervenience is a technical term of philosophy for a non-specific relation of determination between two types of properties. Properties of type B are said to supervene on properties of type A if objects cannot differ in their B-properties without differing in their A-properties. Precisely such a relation seems to exist between the properties of the quantum world and the events and states of affairs from which they can (in principle) be inferred.

The invariable reference to "measurement" in standard formulations of quantum mechanics was famously criticised by John Bell:

To restrict quantum mechanics to be exclusively about piddling laboratory operations is to betray the great enterprise.

Indeed. But to restrict quantum mechanics to be exclusively about piddling laboratory operations is to misunderstand the reference to "measurement" in every standard formulation of quantum mechanics. The measurements presupposed by quantum mechanics include but are not limited to piddling laboratory operations:

• Any event or state of affairs from which the truth value of a proposition of the form "system S has the property P" (or "observable O has the value V") can be inferred, qualifies as a measurement.

In A scattering event we learned that under the conditions stipulated by Rule B, the question "Which outgoing particle is identical with which incoming particle?" lacks an answer. In other words, the proposition "this outgoing particle (say, W) is identical with this incoming particle (say, N)" lacks a truth value. It is neither true nor false but meaningless.

In Meaning of "both" we learned that under the conditions stipulated by Rule B, the propositions "the electron goes through L" and "the electron goes through R" lack truth values; they are neither true nor false but meaningless.

The supervenience of the properties of the quantum world on property-indicating events explains why this is so — and why, in the first place, there are two fundamental rules rather than one.

• If nothing indicates which alternative took place, Rule B applies, and if Rule B applies, the distinctions we make between alternatives are distinctions that Nature does not make. They correspond to nothing in the real world. They exist solely in our minds.