  Light, being massless, casts no shadow; under ordinary circumstances, photons
pass right through each other unimpeded. Here, we demonstrate a laser beam
acting like an object - the beam casts a shadow upon a surface when the beam is
illuminated by another light source. We observe a regular shadow in the sense
it can be seen by the naked eye, it follows the contours of the surface it
falls on, and it follows the position and shape of the object (the laser beam).
Specifically, we use a nonlinear optical process involving four atomic levels
of ruby. We are able to control the intensity of a transmitted laser beam by
applying another perpendicular laser beam. We experimentally measure the
dependence of the contrast of the shadow on the power of the object laser beam,
finding a maximum of approximately of approximately 22 percent, similar to that
of a shadow of a tree on a sunny day. We provide a theoretical model that
predicts the contrast of the shadow. Making light itself cast a shadow opens
new possibilities for fabrication, imaging, and illumination.
