Planck, Max Karl Ernst Ludwig

German physicist who was awarded the Nobel Prize for Physics in 1918 for his formulation of the quantum theory in 1900. His research into the manner in which heated bodies radiate energy led him to report that energy is emitted only in indivisible amounts, called ‘quanta’, the magnitudes of which are proportional to the frequency of the radiation. His discovery ran counter to classical physics and is held to have marked the commencement of modern science.

Measurements of the frequency distribution of black-body radiation by Wilhelm Wien in 1893 showed the peak value of energy occurring at a higher frequency with greater temperature. This may be observed in the varying colour produced by a glowing object. At low temperatures, it glows red but as the temperature rises the peak energy is emitted at a greater frequency, and the colour become yellow and then white.

Wien attempted to derive a radiation law that would relate the energy to frequency and temperature but discovered a radiation law in 1896 that was valid only at high frequencies. Lord Rayleigh later found a similar equation that held for radiation emitted at low frequencies. Planck was able to combine these two radiation laws, arriving at a formula for the observed energy of the radiation at any given frequency and temperature. This entailed making the assumption that the energy consists of the sum of a finite number of discrete units of energy that he called quanta, and that the energy ε of each quantum is given by the equation: ε = hv, where v is the frequency of the radiation and h is a constant now recognized to be a fundamental constant of nature, called Planck's constant. By directly relating the energy of a radiation to its frequency, an explanation was found for the observation that radiation of greater energy has a higher frequency distribution.

Planck's idea that energy must consist of indivisible particles, not waves, was revolutionary because it totally contravened the accepted belief that radiation consisted of waves. It soon found rapid acceptance: Albert Einstein in 1905 used Planck's quantum theory as an explanation for photoelectricity and in 1913 Danish physicist Niels Bohr successfully applied the quantum theory to the atom. This was later developed into a full system of quantum mechanics in the 1920s, when it also became clear that energy and matter have both a particle and a wave nature.

Planck's constant, a fundamental constant (symbol h), is the energy of one quantum of electromagnetic radiation divided by the frequency of its radiation.