A laser beam bounces off the surface of a silicon crystal to simulate an Advanced Photon Source (APS) X-ray. The APS will produce X-ray beams 10,000 times brighter than those from existing sources. It will accelerate positrons to energies greater than 7 GeV (Giga-electronvolts). Moving at nearly the speed of light, the positron beam will be bent by magnets into a circular path about 400 metres in diameter. Arrays of powerful magnets called wigglers" and "undulators" will vibrate the beam, creating tightly focused X-rays which can be tuned to different energies and used to reveal atomic and molecular structure in great detail."