One of the fundamental principles of quantum mechanics. According to this theory, an observer cannot ascertain a particle's position and momentum at the same time. To be more exact, the act of observing a particle's position in a given space causes its momentum to change, while trying to measure its momentum makes it impossible to ascertain its position. This is not to say that the position and momentum of the particle are completely unknown before measurement; instead, the particle is in a superposition of multiple potential states, and there is no way to predict which state will result from the measurement. It is one of physics' most important theories, and helped bring physics into the realm of philosophy. Even in the spacedate era, its importance has not faded in the slightest.