'Gravity waves,' Mace said. 'Like a tractor beam.' Suddenly fear broke to the surface of Mace's hard features; his Eyes seemed even more incongruous, metal islands in a sea of human emotion. He pointed through the viewport, picking out a palm-sized patch of darkness. 'From the direction of the Virgo supercluster; although that's probably coincidence...'
'I caught an echo of the beam.'
'Kapur, I think I know how they did it.'
'The Snowmen?'
'Mach's principle. I think they can manipulate Mach's principle.'
Kapur shook his head.
With a kind of irritated patience, Mace said, 'The Spline is embedded in a Universe of matter. That matter tugs at the Spline with gravity fields — but the fields surround the ship uniformly; they are equal in all directions, isotropic and timeless.'
Kapur frowned. 'And you think the Snowmen have a way of making the field — unequal?'
Mace laughed uneasily. 'I guess you learn a lot in fourteen billion years.'
-Vacuum Diagrams, "The Godel Sunflowers"