"At any rate, distance traveled depends on mass--the bigger the ship, the shorter the distance traveled on each return to our own universe--and not on velocity in 'alpha.' Other parameters, entirely under the control of the traveler, also affect the time that a ship remains in the inter-planar region. "There are refinements, of course. Recently, for example, we have discovered a method of multi-transfer. Several of the transmitters that accomplish the transfer are used together. When they all operate exactly simultaneously, all the matter within a large volume of space is transferred as a unit. With three or four transmitters keyed together, you could transfer a comet and its tail intact. And that's how inter-planar traveling works. Clear now?" "And that's why they call it 'stitching,'" said Anne with seeming delight. "You just think of the ship as a needle stitching its way back and forth into and out of our universe. Why didn't you just say so?" * * * * * "I have. Many times. But there's another interesting point about stitching. Subjectively, the man in the ship seems to spend about one day in each universe alternately. Actually, according to the time scale of an observer in the 'alpha' plane, his ship disappears for about a day, then reappears for a minute fraction of a second and is gone again. Of course, one observer couldn't watch both the disappearance and reappearance of the same ship, and I assume the observers have the same velocity in 'alpha' as does the stitching ship. Anyway, after a ship completes its last stitch, near its destination, there's a day of subjective time in which to make calculations for the landing--to compute trajectories and so forth--before it actually fully rejoins this universe. And while in the inter-planar region it cannot be detected, even by someone else stitching in the same region of 'alpha' space. "That's one of the things that makes interruption of the enemy ships entirely impossible. If a ship is in an unfavorable position, it just takes one more quick stitch out of range, then returns to a more favorable location. In other words, if it finds itself in trouble, it can be gone from our plane again even before it entirely rejoins it. Even if it landed by accident in the heart of a blue-white star, it would be unharmed for that tiny fraction of a second which, to the people in the ship, would seem like an entire day. "If this time anomaly didn't exist, it might be possible to set up defenses that would operate after a ship's arrival in the solar system but before it could do any damage; but as it is, they can dodge any defense we can devise. Is all that clear?" Anne nodded. "Uh-hunh, I understood every word."