Quantum mechanics
The physical theory describing the behaviour of the very small, such as atoms and electrons. It took shape in 1925–26 and describes the state of a particle in terms of probability.
In 1925 Werner Heisenberg created a theory, matrix mechanics, that treats the electrons in an atom using only observable quantities[1]. The following year Erwin Schrödinger gave a different form based on a wave equation, and the two turned out to be the same theory. That same year Max Born proposed reading the size of the wave as the probability of finding the particle there[2].
In earlier physics, knowing the present state fixed the future uniquely. In quantum mechanics only probabilities can be known. Its predictions are nonetheless extraordinarily accurate, and much of modern science and technology, including the chemical bond, semiconductors, lasers and nuclear fusion, rests on it. What probability and superposition actually mean is still debated.
Related terms
- Matrix mechanicsThe first form of quantum mechanics, born in 1925: it describes electrons in atoms using only observable quantities, calculated as tables of numbers called matrices.
- Schrödinger equationThe basic equation of quantum mechanics, which governs how the wave of an electron or other particle varies in space and time. Written down by Erwin Schrödinger in 1926.
- Old quantum theoryThe theories of roughly 1900 to 1925, from Planck onwards, that explained phenomena by adding quantum conditions on top of classical physics.