That was a question every paratimer has to answer, every time he talks paratime to the laity. Verkan Vall had been expecting it; he answered patiently.
"The Ghaldron-Hesthor field-generator is like every other mechanism; it can operate only in the area of primary time in which it exists. It can transpose to any other time-line, and carry with it anything inside its field, but it can't go outside its own temporal area of existence, any more than a bullet from that rifle can hit the target a week before it's fired," Verkan Vall pointed out. "Anything inside the field is supposed to be unaffected by anything outside. Supposed to be is the way to put it; it doesn't always work. Once in a while, something pretty nasty gets picked up in transit." He thought, briefly, of the man in the black tunic. "That's why we have armed guards at terminals."
"Suppose you pick up a blast from a nucleonic bomb," the pilot asked, "or something red-hot, or radioactive?"
"We have a monument, at Paratime Police Headquarters, in Dhergabar, bearing the names of our own personnel who didn't make it back. It's a large monument; over the past ten thousand years, it's been inscribed with quite a few names."
"You can have it; I'll stick to rockets!" the pilot replied. "Tell me another thing, though: What's all this about levels, and sectors, and belts? What's the difference?"
"Purely arbitrary terms. There are five main probability levels, derived from the five possible outcomes of the attempt to colonize this planet, seventy-five thousand years ago. We're on the First Level—complete success, and colony fully established. The Fifth Level is the probability of complete failure—no human population established on this planet, and indigenous quasi-human life evolved indigenously. On the Fourth Level, the colonists evidently met with some disaster and lost all memory of their extraterrestrial origin, as well as all extraterrestrial culture. As far as they know, they are an indigenous race; they have a long pre-history of stone-age savagery.
"Sectors are areas of paratime on any level in which the prevalent culture has a common origin and common characteristics. They are divided more or less arbitrarily into sub-sectors. Belts are areas within sub-sectors where conditions are the result of recent alternate probabilities. For instance, I've just come from the Europo-American Sector of the Fourth Level, an area of about ten thousand parayears in depth, in which the dominant civilization developed on the North-West Continent of the Major Land Mass, and spread from there to the Minor Land Mass. The line on which I was operating is also part of a sub-sector of about three thousand parayears' depth, and a belt developing from one of several probable outcomes of a war concluded about three elapsed years ago. On that time-line, the field at the Hagraban Synthetics Works, where we took off, is part of an abandoned farm; on the site of Hagraban City is a little farming village. Those things are there, right now, both in primary time and in the plenum. They are about two hundred and fifty thousand parayears perpendicular to each other, and each is of the same general order of reality."
The red light overhead flashed on. The pilot looked into his visor and put his hands to the manual controls, in case of failure of the robot controls. The rocket landed smoothly, however; there was a slight jar as it was grappled by the crane and hoisted upright, the seats turning in their gimbals. Pilot and passenger unstrapped themselves and hurried through the refrigerated outlet and away from the glowing-hot rocket.
An air-taxi, emblazoned with the device of the Paratime Police, was waiting. Verkan Vall said good-by to the rocket-pilot and took his seat beside the pilot of the aircab; the latter lifted his vehicle above the building level and then set it down on the landing-stage of the Paratime Police Building in a long, side-swooping glide. An express elevator took Verkan Vall down to one of the middle stages, where he showed his sigil to the guard outside the door of Tortha Karf's office and was admitted at once.