Tom Stanton has spent years chasing a number that gravity itself seemed to forbid. On a quiet field somewhere in the UK, a 40-kilogram mass dropped a short distance, spun a carbon-fiber arm past 2,300 revolutions per minute, and sent a 4-gram projectile into the air at 776 miles per hour. That is nine miles per hour past the speed of sound. For the first time, a purely gravity-powered trebuchet crossed the barrier. Medieval engineers created these machines to fling huge stones at castle walls. The basic idea is to hoist a large heavy weight, let it fall down, and