Schrödinger 's Cat

Nuclear Physicist and Professor Jim Al-Khalili explains Schrödinger's infamous Schrodinger's Cat gedankenexperiment in this excerpt from BBC Four's "Atom" documentary.

The gedankenexperiment (thought experiment) has proved to be very useful in quantum theory. Physicists often conduct thought experiments prior to an actual experiment or when a particular physical experiment is impossible to conduct. (It was Einsteins gedankenexperiment of chasing a light beam which resulted in Special Relativity theory.)

Published by Schrödinger in the mid-1930s to explain wave function collapse in relation to large objects, Schrodinger's Cat experiment
presents a paradox which demonstrates that observation itself affects an outcome - because an "outcome" does not exist until it is observed.