The principle in quantum theory, formulated by Werner Heisenberg in 1926, which holds that the values of certain pairs of variables cannot BOTH be known exactly, so that the more precisely one variable is known, the less precisely the other can be known. For example, if the speed or momentum of a particle is known exactly, then its location must remain uncertain; if its location is known with certainty, then the particle’s speed or momentum cannot be known. Formulated another way, relating the unvertainties of energy and time, the uncertainty principle permits the existence of ultra-short-lived microscopic particles (virtual particles) in apparently empty space, which briefly blink into existence and blink out again.