The principle that it is not possible to know with unlimited accuracy both the position and momentum of a particle. This principle, discovered in 1927 by Werner Heisenberg, is usually stated in the form: Δ‎xΔ‎p_x ≥‎ h/4π‎, where Δ‎x is the uncertainty in the x-coordinate of the particle, Δ‎p_x is the uncertainty in the x-component of the particle’s momentum, and h is the Planck constant. An explanation of the uncertainty is that in order to locate a particle exactly, an observer must be able to bounce off it a photon of radiation; this act of location itself alters the position of the particle in an unpredictable way. To locate the position accurately, photons of short wavelength would have to be used. These would have associated large momenta and cause a large effect on the position. On the other hand, using long-wavelength photons would have less effect on the particle’s position, but would be less accurate because of the longer wavelength. The principle has had a profound effect on scientific thought as it appears to upset the classical relationship between cause and effect at the atomic level. An important consequence of the uncertainty principle is that it is not possible to describe the motions of electrons in atoms and molecules by exact orbits.