Heisenberg, Werner (Karl)

German physicist who developed quantum theory and formulated the uncertainty principle, which places absolute limits on the achievable accuracy of measurement. He was awarded the Nobel Prize for Physics in 1932 for his creation of quantum mechanics, work he carried out when only 24.

Heisenberg was concerned not to try to picture what happens inside the atom but to find a mathematical system that explained it. His starting point was the spectral lines given by hydrogen, the simplest atom. Assisted by Max Born, Heisenberg presented his ideas in 1925 as a system called matrix mechanics. He obtained the frequencies of the lines in the hydrogen spectrum by mathematical treatment of values within matrices or arrays. His work was the first precise mathematical description of the workings of the atom and with it Heisenberg is regarded as founding quantum mechanics, which seeks to explain atomic structure in mathematical terms. Heisenberg also was able to predict from studies of the hydrogen spectrum that hydrogen exists in two allotropes – ortho-hydrogen and para-hydrogen – in which the two nuclei of the atoms in a hydrogen molecule spin in the same or opposite directions respectively. The allotropes were discovered in 1929. In 1927 Heisenberg made the discovery of the uncertainty principle, for which he is best known. The uncertainty principle states that there is a theoretical limit to the precision with which a particle's position and momentum can be measured. In other words, it is impossible to specify precisely both the position and the simultaneous momentum (mass multiplied by velocity) of a particle. There is always a degree of uncertainty in either, and as one is determined with greater precision, the other can only be found less exactly. Multiplying together the uncertainties of the position and momentum yields a value approximately equal to Planck's constant. The idea that the result of an action can be expressed only in terms of the probability of a certain effect was revolutionary, and it discomforted even Albert Einstein, but is generally accepted today. In 1927 Heisenberg used the Pauli exclusion principle, which states that no two electrons can have identical sets of quantum numbers the same, to show that ferromagnetism (the ability of some materials to acquire magnetism in the presence of an external magnetic field) is caused by electrostatic interaction between the electrons.