If Nick Hawker’s computer models are right, the snap of a pistol shrimp’s claw holds the secret to boundless clean energy. Mr. Hawker’s company, First Light Fusion , is one of around two dozen startups chasing the dream of generating electricity by squeezing atoms together. Fusion, first theorized a century ago and shown to be possible decades later, is the same power that lights the sun and every other star, as well as hydrogen bombs. All it requires is the force to press small atoms together to build bigger ones—a process that releases huge amounts of energy, no greenhouse emissions and limited radioactivity.