The observer effect is the fact that observing a situation or phenomenon necessarily changes it. Observer effects are especially prominent in physics where observation and uncertainty are fundamental aspects of modern quantum mechanics. Observer effects are well known in fields other than physics, such as sociology, psychology, linguistics and computer science, but none of these other fields have experienced the same level of publicity and controversy as physics. This may be responsible for the widely held implicit assumption that “real” observer effects are exhibited only by quantum objects and not by classical objects. This misunderstanding may be due, to some extent, to confusing the observer effect with the Heisenberg uncertainty principle and with other quantum uncertainty principles. In fact, observer effects occur in both classical and quantum systems. This article presents a number of examples of observer effects in purely classical processes. It also introduces a framework for understanding and analyzing many of such effects for classical systems. Ignoring observer effects can cause errors in experiments at a macroscopic level where no quantum effects would be discernible. Consequently, there are practical reasons for being careful to address observer effects.
The usual formulation of the quantum Zeno effect (QZE) uses a sequence of rapid observations to prevent change. The same effect can be achieved with continuous observation. For partial inhibition of change we provide a quantitative relation between pulsed (at intervals $\ensuremath{\delta}t$) and continuous (with response time ${\ensuremath{\tau}}_{O}$) observations: the slowdown is the same for $\ensuremath{\delta}t=4{\ensuremath{\tau}}_{O}.$ Including the continuous observer in the Hamiltonian gives an alternative view of the QZE, showing the appropriateness of the name ``dominated time evolution.'' Finally, the variation on the QZE in which a nonconstant sequence of projections can force change is also shown to be achievable by time-varying continuous observation.
An observable on a quantum structure is any
σ-homomorphism of quantum structures from the Borel
σ-algebra into the quantum structure. We show that our partial information on an observable known only for all intervals of the form
(−∞,t) is sufficient to determine uniquely the whole observable defined on quantum structures like
σ-MV-algebras,
σ-effect algebras, Boolean
σ-algebras, monotone
σ-complete effect algebras with the Riesz Decomposition Property, the effect algebra of effect operators of a Hilbert space, and a system of functions, and an effect-tribe.