Measurement theory in quantum mechanics

Karthik S. Joshi, R. Srikanth, Urbasi Sinha

More general probability sum-rules for describing interference found in quantum mechanics (QM) were formulated by Sorkin in a hierarchy of such rules. The additivity of classical measure theory corresponds to the second sum-rule. QM violates this rule, but satisfies the third and higher sum-rules. This evokes the question of whether there are physical principles that forbid their violation. We show that in a theory that is indistinguishable from quantum mechanics in first and second-order interferences, the violation of higher sum-rules allows for superluminal signaling, essentially because probability measures can be contextual in such theories.

Cortland M. Setlow

Günther Ludwig

No measurement is possible without any theory. With respect to a theory under consideration one has to distinguish between direct and indirect measurements. Direct measurements are defined by pre-theories, i.e. without the theory under consideration. The direct measurements for quantum theory are given by measurable processes on macroscopic devices. It is demonstrated how indirect measurements in quantum mechanics can be reduced to direct measurements. A theory of the measuring device is in this context not possible by quantum mechanics alone, but by a more comprehensive theory of macrosystems.

Gordon Reece

Jan Peřina, Z. Hradil, Branislav Jurčo

Paul K. Feyerabend

THE PROBLEM Within classical physics the relation between physical theory and ordinary experience was conceived in the following way: ordinary experience is something that can be described and understood in terms of physics. Such a description involves, apart from physical theory, certain approximations. But the conditons under which those approximations apply (they correspond to the initial conditions of, say, celestial mechanics), taken together with the theory, are supposed to be sufficient for giving a full account of ordinary experience. Consequently, classical theory of measurement (which, like any other theory of measurement, links together terms of ordinary experience and theoretical terms), is a piece of applied physics and all processes which happen during measurement can be analysed on the basis of the equations of motion only. When we enter quantum mechanics (QM), we are apparently presented with a completely different picture. For according to the current interpretation of elementary quantum mechanics, ordinary experience – and this now means classical physics – and physical theory (and this now means QM), belong to two completely different levels and it is impossible to give an account of the first in terms of the second. Any transition from the quantum level to the level of classical physics must be taken, not as a transition within QM from the general to the particular, but as an essentially new element which is incapable of further analysis.