Measurements, errors, and negative kinetic energy

Abstract

An analaysis of errors in measurement yields insight into the penetration of quantum particles into classically forbidden regions. In addition to physical values, realistic measurements yield ‘‘unphysical’’ values which, we show, can form a consistent pattern. An experiment to isolate a particle in a classically forbidden region obtains negative values for its kinetic energy. These values realize the concept of a weak value, discussed in previous works.