In the long tradition of bold ideas meeting the immovable wall of physical law, a proposal for a neutrino laser — conceived a year ago by physicists at MIT and the University of Texas at Arlington — has been formally and mathematically ruled impossible. Nobel laureate Wolfgang Ketterle and colleagues published two papers in Physical Review Letters demonstrating that neither engineering ingenuity nor future technology can overcome the twin barriers of catastrophic atomic recoil and the quantum nature of fermions. The episode is less a story of failure than a reminder that science's greatest dis