Back action (quantum)

In quantum mechanics, back action is the phenomenon in which the act of measuring a property of a particle directly influences the state of the particle. In all scientific measurement, there exists a degree of error due to various factors, such as unaccounted-for variables, imperfect procedures, or imperfect measurement devices.

In classical mechanics, it is assumed that the error of any experiment could theoretically be zero if all relevant aspects of the configuration are known and the measurement devices are perfect. However, quantum mechanical theory posits that the act of measuring a quantity, regardless of the degree of precision, carries inherent uncertainty because the measurement itself influences the quantity being measured. This behavior is known as back action, and it occurs because quantum uncertainty carries minimum fluctuations as a probability. For example, even objects at absolute zero still exhibit "motion" due to such quantum fluctuations.