Uncertainty relation
The quantum relation that a particle's position and momentum cannot both be pinned down as precisely as you like at the same time. Set out by Werner Heisenberg in 1927.
In March 1927 Heisenberg offered a thought experiment: to find where an electron is, you shine light of short wavelength on it, and the kick from the light disturbs its momentum. He argued that the blur in position multiplied by the blur in momentum cannot be made smaller than roughly Planck's constant h[1].
The same year the American physicist Earle Kennard proved the relation as an exact inequality[2]. Today it is understood less as disturbance by measurement than as a property of quantum states themselves, which do not hold a sharp position and a sharp momentum at once.
Because of this relation, the old quantum theory's picture of an electron circling the nucleus along a fixed line could no longer stand. In the same paper Heisenberg wrote that "the 'path' comes into being only because we observe it."
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.
- Planck constantThe fundamental constant, written h, that sets the step size of energy for light and matter. Frequency times h is the smallest unit of energy for that light.
- 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.