The Very Act Of Observing Disturbs

This succinct formulation of the observer effect is attributed to Heisenberg's lectures on quantum mechanics and appears in collections of his sayings, with one citation pointing to Natural Law and the Structure of Matter (1970). It is closely related to — but distinct from — the uncertainty principle: the uncertainty principle concerns the mathematical limits on simultaneous knowledge of conjugate variables, while the observer effect concerns how the act of measurement physically disturbs the system being measured. Heisenberg's own thought experiment involving a gamma-ray microscope to "see" an electron illustrated both effects, though Niels Bohr later pointed out errors in his reasoning while confirming the principle itself.