Free Debt of Ages by Steve White

really sure that for just those few moments the future was teetering on such a knife-edge, as you said earlier, that a few words from me could have tipped the balance?"
    "Yes," Tylar stated shortly. "Those moments may have been unique in all the timestream. I devoutly hope so. But at any rate, the inarguable fact is that an alternate timeline was created, by application of precisely the right stimulus at precisely the right instant. Our subsequent researches leave no room for doubt on the matter."
    After a few heartbeats, Sarnac decided it was incumbent on him to break the silence. "So, Tylar, what do you propose to do? Take me back to that same place and time and make sure that I keep my big mouth shut?"
    "Oh, no. I'm afraid it doesn't work that way. We've had a chance to investigate these matters in some depth, and while there's much we don't understand, one thing is clear: once an alternate timeline branches off, it can't be unmade."
    Sarnac felt that sensation of groping for an understanding of the basic fundamentals that was all too familiar when dealing with Tylar. "Uh, I don't quite get it. If what happened can't be undone, then what's the use of all this?"
    "Andreas' timeline can't be obliterated, true. But it can be altered under the right circumstances, just as ours can. It's precisely what my people and I are constantly on guard against, in our own reality. Well, in the alternate reality I'll have to bring about that which I've dedicated my life to preventing, and change history so as to assure that by the twenty-sixth century the alternate Earth is prepared to defend itself against the Korvaasha. I fear I'll be hard put to manage with equanimity such a . . . reversal of orientation."
    "So you're saying we should enter the alternate reality and go back to a point in time just after the branching-off, and talk the alternate Artorius into changing his mind? Well, it shouldn't be too hard." He turned to Artorius. "The decision you made was entirely reasonable, given the information available to you at the time."
    "So I've frequently assured myself," the former High King said drily.
    "Ah, I'm afraid it's not quite that simple." Tylar sounded apologetic. "You see, after the crucial turning point the 'fabric' of the alternate reality becomes very strong for some time, as one decision leads with inexorable logic to the next. We've investigated the matter thoroughly, and the next area of 'weak fabric' when history can be changed occurs twenty-one years later, in the alternate year 491 A.D."
    " What? But that's a lot of time, Tylar—time enough for a lot of water to have flowed over the dam. Won't the changes that lead to Andreas' world have accumulated a lot of, uh, momentum by then?"
    "Indubitably. There will be certain difficulties, and I anticipate an extended stay in the alternate universe to assure ourselves that matters are proceeding as planned. But," he added brightly, "anything worth doing is worth doing properly, as someone once said."
    "Your mother," Sarnac supplied. If you had one, he didn't add.
    "Ahem! Well, to business! You'll need to be supplied, via implant, with updated knowledge that Artorius and Andreas already possess. Then these data will have to be supplemented with a conventional briefing concerning the precise state of affairs at the moment when we'll—"
    "Tylar." Sarnac's tone achieved the not-inconsiderable feat of stopping the time traveller in his verbal tracks. "Look, I think I understand all this, at least on a superficial level. And I'm not blind to the ethical implications—I'm willing to admit a degree of responsibility. But like all adult human beings, I have to balance my responsibilities. And, as you know, I'm in command of a naval force that is shortly going into battle against some very dangerous enemies of the human race— our human race." His glance at Andreas was rueful but not apologetic. "That has to be my first priority."
    "But, my dear fellow, there's no conflict! As I