But of course these particles weren't passive.
Watching the cloud swirl and undulate as it came closer, I saw that parts of it occasionally caught the sunlight in a way that turned it a shimmering, iridescent silver. Then the color faded, and the swarm became black again. That had to be the piezo panels catching the sun. But it clearly demonstrated that the individual microunits were highly mobile, since the entire cloud never turned silver at the same time, but only portions, or bands. "I thought you said the Pentagon was giving up on you, because you couldn't control this swarm in wind."
"Right. We couldn't."
"But you must have had strong wind in the last few days."
"Of course. Usually comes up in late afternoon. We had ten knots yesterday."
"Why wasn't the swarm blown away?"
"Because it's figured that one out," Ricky said gloomily. "It's adapted to it."
"How?"
"Keep watching, you'll probably see it. Whenever the wind gusts, the swarm sinks, hangs near the ground. Then it rises up again once the wind dies down."
"This is emergent behavior?"
"Right. Nobody programmed it." He bit his lip. Was he lying again?
"So you're telling me it's learned ..."
"Right, right."
"How can it learn? The agents have no memory."
"Uh ... well, that's a long story," Ricky said.
"They have memory?"
"Yes, they have memory. Limited. We built it in." Ricky pressed the button on his radio. "Anybody hear anything?"
The answers came back, crackling in his handset.
"Not yet."
"Nothing."
"No sounds?"
"Not yet."
I said to Ricky, "It makes sounds?"
"We're not sure. Sometimes it seems like it. We've been trying to record it ..." He flicked keys on the workstation, quickly shifting the monitor images, making them larger, one after another. He shook his head. "I don't like this. That thing can't be alone," he said. "I want to know where the others are."
"How do you know there are others?"
"Because there always are." He chewed his lip tensely as he looked at the monitor. "I wonder what it's up to now ..."
We didn't have long to wait. In a few moments, the black swarm had come within a few yards of the building. Abruptly, it divided in two, and then divided again. Now there were three swarms, swirling side by side.
"Son of a bitch," Ricky said. "It was hiding the others inside itself." He pushed his button again. "Guys, we got all three. And they're close."
They were, in fact, too close to be seen by the ground-view camera. Ricky switched to the overhead views. I saw three black clouds, all moving laterally along the side of the building. The behavior seemed distinctly purposeful.
"What're they trying to do?" I said.
"Get inside," Ricky said.
"Why?"
"You'd have to ask them. But yesterday one of them-"
Suddenly, from a clump of cactus near the building, a cottontail rabbit sprinted away across the desert floor. Immediately, the three swarms turned and pursued it. Ricky switched the monitor view. We now watched at ground level. The three clouds converged on the terrified bunny, which was moving fast, a whitish blur on the screen. The clouds swirled after it with surprising speed. The behavior was clear: they were hunting. I felt a moment of irrational pride. PREDPREY was working perfectly! Those swarms might as well be lionesses chasing a gazelle, so purposeful was their behavior. The swarms turned sharply, then split up, cutting off the rabbit's escape to the left and right. The behavior of the three clouds clearly appeared coordinated. Now they were closing in. And suddenly one of the swarms sank down, engulfing the rabbit. The other two swarms fell on it moments later. The resulting particle cloud was so dense, it was hard to see the rabbit anymore. Apparently it had flipped onto its back, because I saw its hind legs kicking spasmodically in the air, above the cloud itself.
I said, "They're killing it ..."
"Yeah," Ricky said, nodding. "That's right."
"I thought this was a camera swarm."
"Yeah, well."
"How are they killing it?"
"We don't know, Jack. But it's fast."
I frowned. "So you've seen this before?"
Ricky hesitated, bit his lip. Didn't answer me, just stared at the screen.
I said, "Ricky, you've seen this before?"
He gave a long sigh. "Yeah. Well, the first time was yesterday. They killed a rattlesnake yesterday."
I thought, they killed a rattlesnake yesterday. I said, "Jesus, Ricky." I thought of the men in the helicopter, talking about all the dead animals. I wondered if Ricky was telling me all he knew.
"Yeah."
The rabbit no longer kicked. A single protruding foot trembled with small convulsions, and then was still. The cloud swirled low to the ground around the animal, rising and falling slightly. This continued for almost a minute.
I said, "What're they doing now?"
Ricky shook his head. "I'm not sure. But they did this before, too."
"It almost looks like they're eating it."
"I know," Ricky said.
Of course that was absurd. PREDPREY was just a biological analogy. As I watched the pulsing cloud, it occurred to me that this behavior might actually represent a program hang. I couldn't remember exactly what rules we had written for individual units after the goal was attained. Real predators, of course, would eat their prey, but there was no analogous behavior for these micro-robots. So perhaps the cloud was just swirling in confusion. If so, it should start moving again soon.