The development of quantum physics is regarded by many as the finest intellectual edifice of the 20th century. This position was not reached in a single step but rather during three main periods:
1. Max Planck's work on the 'Black Body' problem started the quantum revolution in 1900. He showed that energy cannot take any value but is arranged in discrete lumps – later called photons by Einstein.
2. In 1913, Niels Bohr proposed a model of the atom with quantised electron orbits. Although a great step forward, quantum physics was still in its infancy and was not yet a consistent theory. It was more like a collection of classical theories with quantum ideas applied.
3. Starting in 1925 a true 'quantum mechanics' – a set of mathematically and conceptual 'tools' – was born. At first, three different incantations of the same theory were proposed independently and were then shown to be consistent. Quantum mechanics reached its final form (essentially unchanged from today) in 1928.
Max Planck's paper Zur Theorie des Gesetzes der Energieverteilung im Normalspektrum (On the Theory of the Law of Energy Distribution in Normal Spectrum)
