Amusement parks around the world are locked in a race to build the fastest, tallest and most terrifying roller coasters imaginable. Engineers are pushing the limits of speed, height and G-forces, designing rides that twist, drop and launch like never before.  But as coasters reach new extremes, the thrill comes with genuine risk, raising the question: how far can human tolerance — and physics itself — allow the quest for ultimate adrenaline to go? Recently, at the Six Flags Over Texas park in