Quantum mechanics developed in the early part of the twentieth century in response to the discovery that energy is quantized, that is, comes in discrete units. At the microscopic level this leads to odd phenomena: light displays particle-like characteristics and particles such as electrons produce wave-like interference patterns. At the level of ordinary objects such effects are usually not evident, but this generalization is subject to striking exceptions and puzzling ambiguities.
The fundamental quantum mechanical puzzle is ’superposition of states’. Quantum states can be added together in a manner that recalls the superposition of waves, but the effects of quantum superposition show up only probabilistically in the statistics of many measurements. The details suggest that the world is indefinite in odd ways; for example, that things may not always have well-defined positions or momenta or energies. However, if we accept this conclusion, we have difficulty making sense of such straightforward facts as that measurements have definite results.
Interpretations of quantum mechanics are, in one way or another, attempts to understand the superposition of quantum states. The range of interpretations stretches from the metaphysically daring to the seemingly innocuous. But, so far, no single interpretation has commanded anything like universal agreement.
Validity of local thermal equilibrium in anomalous heat diffusion
