Schrödinger's cat Schrödinger's cat Schrödinger's cat is a thought experiment, sometimes described as a paradox, devised by Austrian physicist Erwin Schrödinger in 1935. It illustrates what he saw as the problem of the Copenhagen interpretation of quantum mechanics applied to everyday objects. The scenario presents a cat that may be simultaneously both alive and dead, a state known as a quantum superposition, as a result of being linked to a random subatomic event that may or may not occur. The thought experiment is also often featured in theoretical discussions of the interpretations of quantum mechanics. Schrödinger coined the term "Verschränkung" (entanglement) in
Sum rule in quantum mechanics to construct operator equalities, which are then applied to the particles or energy levels of a system. Assume that the Hamiltonian formula_1 has a complete set of eigenfunctions formula_2 with eigenvalues formula_3: For the Hermitian operator formula_5 we define the repeated commutator formula_6 iteratively by: The operator formula_8 is Hermitian since formula_5 is defined to be Hermitian. The operator formula_10 is anti-Hermitian: By induction one finds: and also For a Hermitian operator we have Using this relation we derive: The result can be written as For formula_17 this gives: See oscillator strength. Sum rule in quantum mechanics In quantum mechanics,