That much had been understood by the earliest theorists. What they hadn’t quite grasped was that hyper space, formed by the combined gravitational distortion of an entire universe’s mass, was itself crossed and crisscrossed by permanent waves or currents of focused gravity. They were widely separated, of course, but they also might be dozens of light-years wide and deep, and they were deadly to any ship which collided with one. The gravitational shear they exerted on a starship’s hull would rip the hapless vessel apart long before any evasive action could even be contemplated, unless the ship happened to impact at precisely the right angle on exactly the right vector, and its bridge crew had both the reflexes and the reaction mass to wrench clear in time.

As time passed, the survey ships that survived had mapped out reasonably safe routes through the more heavily traveled regions of hyper-space. They couldn’t be entirely relied upon, for the grav waves shifted position from time to time, and sticking to the safe lanes between waves often required vector changes reaction-drive ships simply could not make. That meant hyper voyages had tended to be both indirect and lengthy, but the survival rate had gone up. And as it climbed, and as physicists went out to probe the grav waves they now knew existed with ever more sophisticated instruments, observational data increased and ever more refined theories of gravity were proposed.

It had taken just over five hundred years, but finally, in 1246 P.D., the scientists had learned enough for the planet Beowulf to perfect the impeller drive, which used what were for all intents and purposes “tame” grav waves in normal-space. Yet useful as the impeller was in normal-space, it was extraordinarily dangerous in hyper. If it encountered one of the enormously more powerful naturally occurring grav waves, it could vaporize an entire starship, much as Honor herself had blown the Havenite courier boat’s impeller nodes with Fearless’s impeller wedge.

More than thirty years had passed before Dr. Adrienne Warshawski of Old Earth found a way around that danger. It was Warshawski who finally perfected a gravity detector which could give as much as five light-seconds’ warning before a grav wave was encountered. That had been a priceless boon, permitting impeller drive to be used with far greater safety between grav waves, and even today all grav detectors were called “Warshawskis” in her honor, yet she hadn’t stopped there. In the course of her research, she had penetrated far deeper into the entire grav wave phenomenon than anyone before her, and she had suddenly realized that there was a way to use the grav wave itself. An impeller drive modified so that it projected not an inclined stress band above and below a ship but two slightly curved plates at right angles to its hull could use those plates as giant, immaterial “sails” to trap the focused radiation hurtling along a grav wave. More than that, the interface between a Warshawski sail and a grav wave produced an eddy of preposterously high energy levels which could be siphoned off to power a starship. Once a ship had “set sail” down a grav wave, it could actually shut down its onboard power plants entirely.