“The uncertainty principle. The uncertainty principle, introduced by Heisenberg in 1927, states that the position of a particle cannot be determined, but provides a mathematical formula relating its uncertainty to the uncertainty of the particle's momentum. Now in QFT what we call particles are really fields, and since fields spread out, there is no precise "position". However there is a property of fields in general, known as Fourier's theorem, that relates the spatial spread of any field to the spread of its wavelengths. In QFT, the wavelength of a quantum is related to its momentum, and I still remember my moment of insight when I realized that Heisenberg's relation between the uncertainties of position and momentum is simply Fourier's theorem.”