Is there any experimental proof that an electromagnetic wave has momentum?
Not only do electromagnetic waves have linear momentum, they also have angular momentum.
In a recent paper in Physical Review Letters, experimentalists at Cornell took advantage of the angular momentum of light to apply precise amounts of torque to a very small quartz bead.. See La Porta and Wang, "Optical Torque Wrench: Angular Trapping, Rotation, and Torque Detection of Quartz Microparticles".
Light is an electromagnetic wave that cany carry angular momentum by way of circular polarization. Transverse waves such as light, can have polarization in two directions, "left-right", "up-down" or any combination thereof. As an anology, you can shake the end of a slinky up and down, left to right, or at some arbitrary angle. Linear polarization occurs when these two components occur in phase, e.g. you shake the end of your slinky back and forth along the same path. When the two components are of equal amplitude but 90 degrees out of phase, however, circular polarization is created. This is what happens when you shake the end of your slinky in a circular motion.
In their paper, La Porta and Wang use the angular momentum created by circular polarized light to torque a quartz bead. Not only can they apply torque to the bead, but, by conservation of angular momentum, the bead can apply a torque to the light, which is detected by a change in the circular polarization.
La Porta and Wang provide soilid experimental evidence that electromagnetic waves do indeed have angular momentum.
Andrew Leifer, None, Physics Undergrad, Stanford Univerisity & JILA-CU