Speakable and Unspeakable in Quantum Measurements

Abstract

Quantum mechanics, in its orthodox version, imposes severe limits on what can be known, or even said, about the condition of a quantum system between two observations. A relatively new approach, based on so‐called “weak measurements”, suggests that such forbidden knowledge can be gained by studying the system's response to an inaccurate weakly perturbing measuring device. It goes further to propose revising the whole concept of physics variables, and offers various examples of counterintuitive quantum behavior. Both views go to the very heart of quantum theory, and yet are rarely compared directly. A new technique must either transcend the orthodox limits, or just prove that these limits are indeed necessary. Both possibilities are studied and orthodoxy is vindicated.

On Weak Values and Feynman's Blind Alley

“…It can also expose the researher's position inside the proverbial blind alley, reserved for the heretic (whatever this may mean, not so good). A comparison of this kind is long overdue in the case of weak measurements, and we will try to make it next (see also [13]).…”