Roger Strang sat up, suddenly alert. "That boy," he said, his voice sharp. "You nearly made a sieve of him, trying to shoot him. Why didn't he die?"
"Because he was on a high-order variable. Picture it this way: From any point in time, the possible future occurrences could be seen as vectors, an infinite number of possible vectors. Every activity that makes an alteration, or has any broad effect on the future is a high-order variable, but many activities have no grave implications for future time, and could be considered unimportant, or low-order variables. If a man turns a corner and sees something that stimulates him into writing a world-shaking manifesto, the high-order variable would have started when he decided to turn the corner instead of going the other way. But if he took one way home instead of another, and nothing of importance occurred as a result of the decision, a low-order variable would be set up.
"We found that the theory of alterations held quite well, for low-order variables. Wherever we appeared, whatever we did, we set up a definite friction in the normal time-stream, a distortion, like pulling a taut rubber band out. And we could produce changes—on low-order variables. But the elasticity of the distortion was so great as to warp the change back into the time-stream without causing any lasting alteration. When it came to high-order changes, we simply couldn't make any. We tried putting wrong data into the machines that were calculating specifications for the Barrier, and the false data went in, but the answers that came out were answers that should have appeared with the right data. We tried to commit a murder, to kill David Strang, and try as we would we couldn't do it. Because it would have altered a high-order variable, and they simply wouldn't be altered!"
"But you, Morrel," Roger exclaimed. "How about you? You were top man in the Barrier Base Security office. You must have made an impression."
Morrel smiled tiredly. "I really thought I had, time after time. I would start off a series of circumstances that should have had a grave alterative effect, and it would look for awhile as if a long-range change was going to be affected—and then it would straighten itself out again, with no important change occurring. It was maddening. We worked for five years trying to make even a small alteration—and brought back our data—" He pointed to the papers on the floor. "There are the calculations, applied on the Equation. Meaningless. We accomplished nothing. And the Dictator is still there."
Drengo slumped in his chair. "And he's started the war. The real attack. This bombardment outside is nothing. There are fifteen squadrons of space-destroyers already unloading atomic bombs on the surface of Mars, and that's the end, for us. Farrel Strang has started a war he can never finish—"
Roger Strang turned sharply to Drengo. "This Dictator," he said. "Where is he? Why can't he be reached now, and destroyed?"
"The Barrier. He can't be touched in the Palace. He has all his offices there, all his controls, and he won't let anyone in since the attempted assassination three months ago. He's safe there, and we can't touch him."
Roger scowled at the control panel on the wall. "How does this time-portal work?" he asked. "You say it can take us back—why not forward?"
"No good. The nature of Time itself makes that impossible. At the present instant of Time, everything that has happened has happened. The three-dimensional world in which we live has passed through the fourth temporal dimension, and nothing can alter it. But at this instant there are an infinite number of things that could happen next. The future is an infinite series of variables, and there's no conceivable way to predict which variable will actually be true."