http://www.newscientist.com/article/dn17319-physicists-create-black-hole-for-sound.html" The team cooled 100,000 or so charged rubidium atoms to a few billionths of a degree above absolute zero and trapped them with a magnetic field. Using a laser, the researchers then created a well of electric potential that attracted the atoms and caused them to zip across the well faster than the speed of sound in the material. This setup created a supersonic flow that lasted for some 8 milliseconds, fleetingly forming an acoustic black hole capable of trapping sound. I did not understand how this supersonic flow was capable of trapping sound. Anyone?