PUTTING a jar full of metal beads on a subwoofer makes more than
just a rattle. Add light and it can become a laser that changes
frequency when the beads are shaken harder. Such a tunable laser
could lead to crisper projected images and help unlock the
mysterious behaviour of granular materials, seen in sand
castles, dunes and avalanches. Ordinary lasers work by bouncing
photons back and forth between two semi-transparent mirrors
separated by a medium such as a crystal or a gas. The medium is
chosen so that as the photons travel through it, they stimulate
atoms to emit new photons - all with exactly the same
wavelength, phase and direction - which build the beam. In the
past decade, physicists have set about building so-called random
lasers, which turn that picture inside out by embedding the
mirrors - often a suite of tiny grains - inside the medium.
Random lasers have been shown to remove the speckling from
images created by laser projectors, but the grains used were
small, making them hard to control and s