"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."
"There is another thing about inter-planar travel that you ought to remember," said Bristol. "When a ship returns to our universe, it causes a wide area disturbance; you have probably heard it called space shiver or the bong wave. The beta universe is so much smaller than our own alpha that you can imagine a spaceship when shifted toward it as being several beta light-years long. Now, if you think of a ship, moving between the alpha and beta lines on this envelope, as getting tangled in the dotted lines that connect the points on the two lines, that would mean that it would affect an area smaller than its own size on beta—a vastly larger area on alpha.
"So when a ship returns to alpha, it 'twangs' those connecting lines, setting up a sort of shock in our universe covering a volume of space nearly a parsec in diameter. It makes a sort of 'bong' sound on your T.V. set. Naturally, this effect occurs simultaneously over the whole volume of space affected. As a result, when an invader arrives, using inter-planar ships, we know instantaneously he is in the vicinity. Unfortunately, his sudden appearance and the ease with which he can disappear makes it impossible, even with this knowledge, to make adequate preparations to receive him. Even if he is in serious trouble, he has gone again long before we can detect the bong."
"Well, dear," said Anne.