Quantum

Origin

The modern concept of the quantum in physics originates from December 14, 1900, when Max Planck reported his findings to the German Physical Society. He showed that modelling harmonic oscillators with discrete energy levels resolved a longstanding problem in the theory of blackbody radiation. In his report, Planck did not use the term quantum in the modern sense. Instead, he used the term Elementarquantum to refer to the "quantum of electricity", now known as the elementary charge. For the smallest unit of energy, he employed the term Energieelement, "energy element", rather than calling it a quantum.

Shortly afterwards, in a paper published in Annalen der Physik, Planck introduced the constant h, which he termed the "quantum of action" (elementares Wirkungsquantum) in 1906. In this paper, Planck also reported more precise values for the elementary charge and the Avogadro–Loschmidt number, the number of molecules in one mole of substance. The constant h is now known as the Planck constant. After his theory was validated, Planck was awarded the Nobel Prize in Physics for his discovery in 1918.

In 1905 Albert Einstein suggested that electromagnetic radiation exists in spatially localized packets which he called "quanta of light" (Lichtquanta). Einstein was able to use this hypothesis to recast Planck's treatment of the blackbody problem in a form that also explained the voltages observed in Philipp Lenard's experiments on the photoelectric effect. Shortly thereafter, the term "energy quantum" was introduced for the quantity hν.