| Machine Causes Light to Behave in 'Impossible' Ways
Physicists have built a ring in which pulses of light whip circles around each other and the normal rules that govern light's behavior no longer apply. Under normal circumstances, light displays certain kids of physical symmetry. First, if you were to play a tape of light's behavior forward and then backward, you would see it behave in the same way moving in both directions in time. This is called time-reversal symmetry. And second, light, which can move through the world as a wave, has what is called polarization: how it oscillates relative to the motion of the wave. That polarization usually stays the same, providing another type of symmetry. But inside this ring-shaped device, light both loses its time-reversal symmetry and changes its polarization. Inside the ring, light waves turn circles and resonate with one another, producing effects that don't normally exist in the outside world. Researchers already knew that, under certain circumstances, when light is bouncing around inside