Werner Heisenberg

 was known to the First Doctor as one of the notable scientists in Germany in January 1933. (AUDIO: The Alchemists)

Heisenberg's uncertainty principle stated that the very act of measuring something changed its nature. (AUDIO: The Time of the Daleks)

Alistair Gryffen used the exclamation "what the Heisenberg?" to express shock. (TV: Alien Avatar)

Behind the scenes
The Time of the Daleks is confusing Heisenberg's uncertainty principle with the observer effect. The two are frequently confused. In the real world, the uncertainty principle instead refers to a fundamental limit to accurate knowledge when attempting to precisely measure both position and momentum of a particle at the same time. Rather than commenting on the impact of in quantum mechanics, the uncertainty principle holds "we cannot know the present state of the world in full detail". Both theories, for different reasons, suggest that predicting the future with complete precision should be impossible.