Hell's Gate

his body, his hands, and think about how old he was… how really really old.
        Lynda thought of something. “If we stop these… these vacii and can live a normal life, will Vic live to be 310?”
        The computer seemed to take a moment for reflection. “He will be a fixture of the present, will not wink out of existence. He will live a healthy life, though it is not certain he will grow to be 310. He will not be living a preordained life, but a future of his own choosing. His mortality should be every bit as shaky as anyone's in this era.”
        “You've more or less convinced me the vacii must be stopped,” Salsbury said. “But why? What are they… and where are they from?”
        “They are an intelligent extra-galactic race. Not only have they conquered faster-than-light travel, but probability travel as well. Or at least one probability line of them has.”
        Salsbury looked properly perplexed, and the computer's sensors must have registered the expression.
        “Imagine,” the computer said, “that this is not the only Earth that exists. There are thousands, millions, billions, countless Earths with slightly different histories. There are an infinity of probabilities, all existing in the same space and time, but separated by quasi-dimensional spaces. Traveling from one to the other of these probabilities involves finding the weak spots in the quasi-dimensional spaces, the places where the probabilities almost touch. Once these are found, equipment is erected to weaken these places further until, finally, a bubble develops between the two probabilities, a bubble through which you can pass. At first, living tissue cannot move through the bubble and survive, for it is a vacuum filled with randomly bouncing electrons freed when the quasi-dimensional space is broken down to form the bubble. These electrons have a mass all out of proportion to their size. Tremendous density. They're like bullets that are of micro-micro size; they corrode the flesh, though they do not harm the plasti-steel alloy of the robots specially built to transverse the primitive bubble.
        “Once on the other side, the robots can bring through equipment to set up a beam generator from this side of the bubble. When the beams from both sides are locked, the bubble becomes a doorway that even flesh can pass through without difficulty. The vacii have sent robots through to destroy you but have not yet opened the bubble to animal transport. They will do that shortly, as soon as they have killed you, or before.
        “But to return to the origins of the vacii, the lizard-men. They landed on an Earth of one of the other probability lines and conquered it. From there, they spread out in both directions on the plane of probabilities, defeating one counter-Earth after another. We are the seventy-sixth to fall. We have not essentially been conquered from space, but from our own other probabilities. Here, at Harold Jacobi's house, in the summer of 1970, the vacii took over this probability. They established as experimental station, then proceeded to worlds beyond ours, into other probabilities.
        “Unknown to the vacii manning the station, on this world, our world-the future from which you and I have come-man discovered time travel. It was obvious, at once, to those in our future, that a time machine could be used as a weapon against the vacii rulers. If someone could be sent into the past to stop the vacii takeover of our worldline, the future would be entirely different. Man would be free. And, perhaps, the other vacii empires could fall like dominoes, backwards through the other probability lines they conquered; one Earth becoming free after another.”
        That was it. But it was too complicated to grasp all its significances in one sitting. Salsbury could only let it settle into his mind where he could later proceed to try to understand it. The lizards in the wall were aliens. But they were coming from a counter-Earth, not directly from the stars. He had been sent from the future of this Earth to stop their invasion before it began…
        “What do you mean by experimental stations?” Lynda asked. “And what is the future like under the vacii?”
        “The vacii,” the 810-40.04 said, “are nearly emotionless creatures. Perhaps they do experience love, pity, and hate among themselves, though to a small degree; but they have no feelings toward men. They look to man as an inferior animal to be