Almost unwillingly, Davidson reached out to touch what was left of the cigarette. "Why you?" he asked. "If your time machine is built, then everything in the world ought to be equally fair game. So why don't things disintegrate in my hands, too?"
"Again, I don't know for sure. I suspect the probability shifts cluster around me because I'm the only one who knows how to make the machine." Garwood seemed to brace himself. "But you're right. If the machine is actually made, then it's all out of my hands... and I can't see any reason why the effect wouldn't then mushroom into something worldwide."
A brief mental image flashed through Davidson's mind: a black vision of the whole of advanced technology falling to pieces, rapidly followed by society itself. If a superpower war of suspicion didn't end things even quicker... "My God," he murmured. "You can't let that happen, Doctor."
Garwood locked eyes with him. "I agree. At the moment, though, you have more power over that than I do."
For a long minute Davidson returned the other's gaze, torn by indecision. He could do it—he could simply let Garwood walk. It would mean his career, possibly, but the stakes here made such considerations trivial. Another possibility occurred briefly to him... "Why did you need the computer?" he asked Garwood. "What were you trying to do?"
"Find a solution to my equations that would allow for a safer form of time travel," Garwood said. "Something that would allow us to observe events, perhaps, without interacting with them."
"Did you have any luck?"
"No. But I'm not ready to give up the search, either. If you let me go, I'll keep at it."
Davidson clenched his jaw tightly enough to hurt. "I know that, Doctor," he said quietly. "But you'll have to continue your search at Backdrop."
Garwood sighed. "I should have known you wouldn't buck your orders," he said bitterly.
"And leave you out here, threatening a community of innocent bystanders?" Davidson retorted, feeling oddly stung by the accusation. "I have a working conscience, Doctor, but I also have a working brain. Backdrop is still the safest place for you to be, and you're going back there. End of argument." Abruptly, he got to his feet. "Come on. I'll have some of my people pack up your stuff and bring it to Backdrop behind us."
Reluctantly, Garwood also stood up. "Can I at least ask a favor?"
"Shoot."
"Can we drive instead of flying? I'm still afraid of what influence I might have on a plane's engines."
"If you can sit this close to that terminal without killing it, the engines should be perfectly safe," Davidson told him.
"Under the circumstances, 'should' is hardly adequate—"
"You're arguing in circles," Davidson pointed out. "If you get killed in a plane crash, how is anyone going to use your equations to build a time machine?"
Garwood blinked, then frowned. "Well... maybe I wouldn't actually die in the wreck."
"All right, fine," Davidson snapped, suddenly tired of the whole debate. "We'll put an impact bomb under your seat to make sure you'll die if we crash. Okay?"
Garwood's face reddened, and for a second Davidson thought he would explode with anger of his own. But he didn't. "I see," he said stiffly. "Very well, then, let's find a phone booth and see what Saunders says. You will accept suggestions from Saunders, won't you?"
Davidson gritted his teeth. "Never mind. You want to sit in a car for fourteen hours, fine. Let's go; we'll radio Chanute from the car and have them call in the change of schedule to Backdrop. And arrange for a quiet escort."
V
"I hope you realize," Garwood said heavily, "that by bringing me back you're putting everyone in Backdrop at risk."
Saunders raised polite eyebrows. Polite, stupidly unconcerned eyebrows. "Perhaps," he said. "But at least here we understand what's going on and can take the appropriate precautions. Unlike the nation at large, I may add, which you've just spent nearly four months putting at similar risk. Under the circumstances, I'm sure you'd agree that one of our concerns now has to be to keep you as isolated from the rest of the country as possible." He shrugged. "And as long as you have to be here anyway, you might as well keep busy."
"Oh, of course," Garwood snorted. "I might as well help Backdrop to fall apart that much soo—"
He broke off as a muffled cracking sound drifted into the room. "More of the plaster going," Saunders identified it off-handedly. "Nice to hear again after so long."
Garwood felt like hitting the man. "Damn it all, Saunders," he snarled. "Why won't you listen to reason? A working time machine cannot be made. The very fact that Backdrop is falling apart around me—"
"Proves that the machine can be made," Saunders cut him off. "If you'd stop thinking emotionally for a minute and track through the logic you'd realize that." Abruptly, all the vaguely amused patience vanished from his face, and his eyes hardened as they bored into Garwood's with an unexpected intensity. "Don't you understand?" he continued quietly. "When you left, the probability-shift damage to Backdrop dropped off to near zero. Now that you're back, the destruction is on the increase again."
"Which is my point—"
"No; which is my point," Saunders snapped. "The probability-shift effect cannot exist if a working time machine isn't possible."
"And yet that same effect precludes the manufacture of any such machine," Garwood pointed out. "As I've explained to you at least a hundred times."
"Perhaps. But perhaps not. Even given that the concept of time-travel generates circular arguments in the first place, has it occurred to you that a working time machine might actually prove to be a stabilizing factor?"
Garwood frowned. "You mean that if we have the theoretical capability of going back and correcting all these alterations of history then the wild fluctuations will subside of their own accord?"
"Something like that," Saunders nodded. "I did some preliminary mathematics on that question while you were gone and it looks promising. Of course, we won't know for sure until I have all the equations to work with."
"And what if you're wrong?" Garwood countered. "What if a working time machine would simply destabilize things further?"
A flicker of Saunders's old innocent expression crossed the man's face. "Why, then, we won't be able to make one, will we? The components will fall apart faster than we can replace them."
"In which event, we're back to the probability-shift effect being a circular paradox," Garwood sighed. "If it prevents us from building a time machine, there's no time travel. If there's no time travel, there's no change in probabilities and hence no probability-shift effect."
"As I said, time travel tends to generate paradoxes like that." Saunders pursed his lips. "There's one other possibility that's occurred to me, though. The man who brought you back from Champaign—Major Davidson—said in his report that you'd been trying to find an alternative solution to the time travel equations. Any luck?"
Garwood shook his head. "All I found was blind alleys."
"Maybe you just didn't get to look long enough."
Garwood eyed him. "Meaning...?"
"Meaning that one other possible explanation of the probability-shift effect is that there is indeed another set of solutions. A set that will let us build the machine and still be able to go back and change things."
Garwood sighed. "Saunders... don't you see that all you're doing is just making things worse? Isn't it bad enough that things fall apart around me?—do you want to see it happening on a global scale? Stabilization be damned: a time machine—a real, functional time machine—would be the worst instrument of destruction ever created. Ever created."