Quantum superposition the eigenvector corresponding to the measured eigenvalue. It is natural to ask why ordinary everyday objects and events do not seem to display quantum mechanical features such as superposition. Indeed, this is sometimes regarded as "mysterious", for instance by Richard Feynman. In 1935, Erwin Schrödinger devised a well-known thought experiment, now known as Schrödinger's cat, which highlighted this dissonance between quantum mechanics and classical physics. One modern view is that this mystery is explained by quantum decoherence. A macroscopic system (such as a cat) may evolve over time into a superposition of classically distinct quantum states (such as "alive" and
Introduction to quantum mechanics by a small amount, where only one line was expected. Pauli formulated his "exclusion principle", stating that "There cannot exist an atom in such a quantum state that two electrons within [it] have the same set of quantum numbers." A year later, [[George Eugene Uhlenbeck|Uhlenbeck]] and [[Samuel Goudsmit|Goudsmit]] identified Pauli's new degree of freedom with the property called [[spin (physics)|spin]] whose effects were observed in the Stern–Gerlach experiment. Bohr's model of the atom was essentially a planetary one, with the electrons orbiting around the nuclear "sun." However, the uncertainty principle states that an electron cannot simultaneously have an exact location