Like all wave phenomena, light has mechanical properties. Johannes Kepler suggested that comet tails always point away from the Sun because light carries linear momentum. In 1905, John Poynting developed the theory of electromagnetic radiation pressure and momentum density and, in 1921, Albert Einstein showed that Planck's blackbody law and the motion of molecules in a radiation field could be explained if the linear momentum of a photon isℏk.(The wave numberk = 2&pgr;/&lgr;andℏ = h/2&pgr;,where &lgr; is the wavelength andhis Planck's constant.) In modern times, light's linear momentum has been directly exploited for trapping and cooling atoms and molecules.