Uncertainty principle

You may be looking for the CC audio story or The Uncertain-Tea Principle.

According to the Eighth Doctor, Heisenberg's uncertainty principle stated that the very act of measuring something changed its nature.

Professor Osric extrapolated from this that Heisenberg's revolutionary idea, that to observe something was also to change it, might also apply to time. He believed that using a clock to measure time, "merely by counting off the seconds, disturb[ed], change[d] time", such that time travel, or at least a view into other times, might be possible with sufficient clocks and mirrors. (AUDIO: The Time of the Daleks)

Behind the scenes

The Eighth Doctor, Osric and Learman are in fact referring to the observer effect, not the uncertainty principle as it exists in the real world. The two are frequently confused, but are not at all equivalent.

In the real world, Heisenberg's uncertainty principle instead refers to a fundamental limit to what can be known when attempting to precisely measure both position and momentum of a particle at the same time. When precision is increased in measuring one quantity, precision is lost in measuring the other.

Thus, though both theories suggest that predicting the future with complete precision should be impossible, they follow wildly different lines of thought before arriving there. Rather than commenting on the impact of measurement in quantum mechanics, as does the observer effect, the uncertainty principle holds that "we cannot know [even] the present state of the world in full detail".