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
Symmetry in quantum mechanics There are rotational matrices for each spin quantum number. Evaluating the exponential for a given "z"-projection spin quantum number "s" gives a (2"s" + 1)-dimensional spin matrix. This can be used to define a spinor as a column vector of 2"s" + 1 components which transforms to a rotated coordinate system according to the spin matrix at a fixed point in space. For the simplest non-trivial case of "s" = 1/2, the spin operator is given by where the Pauli matrices in the standard representation are: The total angular momentum operator is the sum of the orbital and spin and