This is called the uncertainty principle, also known as the Heisenberg principle after the person who first proposed it.

Trends are subject to their own Heisenberg principle.

The Heisenberg Principle implies that no peoplewatcher should allow himself to be caught at it.

But if precognition really has something to do with time- I cite the Heisenberg Principle.

The Heisenberg principle helps determine the size of electron clouds, and hence the size of atoms.

The Heisenberg Principle clearly demonstrates the calculability of the uncertainties to which you so glibly allude.

His presence was undoubtedly a factor in many of the intimate scenes he reports, yet he never acknowledges the potential effect of the journalistic Heisenberg principle.

I know the Heisenberg Principle.

Bless the Heisenberg Principle!

The original heuristic argument that such a limit should exist was given by Werner Heisenberg in 1927, after whom it is sometimes named the Heisenberg principle.