"No problems," John said. "I've aced all the quizzes in physics and physics lab."

"Good, good." Wilson paused, still staring at the device. "Carry on."

After Wilson had disappeared, John packed up the device and exited as quickly as he could.

They built the prototype out of the wood plank. Henry found a dozen more ball bearings at a local industrial supply store. John carved flippers out of wood and placed them so that a player could flip them from underneath. It was more work than an electric flipper, but it got the idea across. As he used the rasp and the file to carve the flipper, John realized that the shape and length of the flipper could be varied. The prototype was done by the end of the week.

It was clunky and hard work to play, but it was fun. John's wrists ached after whacking at the flippers all afternoon. The crew-Henry, Grace, and John-spent the evening talking about how each component would work. Casey was busy with a project, but John kept glancing at the door, expecting her to show. After the first night, John added a spring launch mechanism, so they didn't have to drop the ball in at the top.

"This is a lot more fun than those video games," Grace said, referring to the ghastly Pong-like Electrux game in the Student Union.

"The prototype just proves that the ideas work," John said. "Now, we have to ramp it up, so to speak."

"That was bad," Grace said.

"Grace," John said. "I want you to build a flipper prototype. When the player presses a b.u.t.ton, the flipper will move, about thirty degrees from here to here." He showed her the angles with his hands. "Use a solenoid. It needs to be strong enough to launch a steel ball bearing two meters up an incline plane of ten degrees.

"Henry, I want you to build a prototype b.u.mper. When a ball hits a b.u.mper, it triggers a solenoid that bounces the ball back in the opposite direction." He drew a diagram on the chalkboard, a triangle. "The first one should be about this size, but we'll want to be able to make any shape of b.u.mper. I'm going to work on sound, operation, and scoring. We'll worry about flags and other c.r.a.p later."

"Flags?" Grace asked.

"More ways to score points."

"How can you know so much about these things when I've never even heard of them?" Grace said.

"I spent a lot of time playing pinball... in Vegas," John said.

Grace had a flipper done first, and John suspected that she had worked on it when she should have been working on her lab reports. They were all three working in the lab after dinner two days later when Grace called John over. She showed him a small red b.u.t.ton mechanism that she held in her hand. Wires pa.s.sed from it to a block of wood, where one of John's carved wooden flippers was mounted.

"Watch this," she said, and pressed the b.u.t.ton. The flipper spasmed, the block of wood jumping.

"Wow. Kinda powerful."

"Yeah," she said, smiling at the thing. She made it hop a half-dozen times.

"Let me try," John said. He took the b.u.t.ton from her and pressed it, holding it down. The flipper jumped and came back to the starting position. "Is there any way to keep the flipper up while the b.u.t.ton is depressed?"

Grace frowned. "That wasn't in your list of requirements, John."

"I'm sorry. I forgot that one. But I like this. It's exactly what I had in mind otherwise."

When John looked over Henry's shoulder to check his work, he pointedly stopped what he was doing.

"You may want to tighten that. ..."

"It's not ready yet."

John shrugged and let him have his peace. Managing a team was tough work, he decided, so he returned to tracking down a quarter box and designing an underlaying physical and electrical framework. He could have used Casey's help, but she had begged off again.

Henry was a mechanical engineer. He'd worked in his father's auto garage as a kid, and knew how to weld. He built a table in no time, one that opened up as John suggested so that they could easily work with the electronics underneath the table.

"Every solenoid that clicks may need to trigger a sound and a score," John explained.

They built a row of bells and buzzers into the base of the thing, and a backboard that housed a small a.n.a.log scoreboard. Within a week their prototype could rack up a score and emit sound when they touched various triggers on the play field.

Two weeks later, Henry put two of Grace's flippers into the play field and they played for an hour before the right flipper's solenoid burned out.

"c.r.a.ppy equipment," Grace said, pulling the mechanism out and poking at it with a soldering iron. "It worked for a while. Not bad, I say."

"Neat," Henry said. "I'll have a b.u.mper for tomorrow. We can put it here." He pointed to a spot next to the flipper. A wooden b.u.mper sat there now.

"This is coming along very nicely," John said. "We need a ball return mechanism." When the ball fell out the bottom of the play field, it landed in a cup. Players manually picked it up and put in the launch lane.

Henry nodded. "I've got some ideas on that."

Grace said, "Why don't we have two boards back-to-back for double play?"

"What?"

"Pinball is fun, for one person. Why not put two boards together, with a player at each end, with the goal of trying to get the ball in the other person's drop area?"

"Mechanical soccer," Henry said. "Neat."

"That's not traditional pinball," John said.

"So?" Grace said. "We don't have to build a traditional pinball machine, do we?"

John nodded. "I guess we don't. We're doing this for fun."

"All right."

The next night, they tore off the backboard and built another play field. They decreased the slope a bit.

"You know," Grace said, "it would be better if there were several sets of flippers; then you could pa.s.s the ball back and forth."

John shook his head. "Let me tell you about foosball," he said with a laugh.

"Yeah?"

"Never mind."

The next time he came to the lab, John did so after midnight. He wanted no one to interrupt him, especially Wilson. The lab was dark and empty. John opened his lab book on his desk and scattered some experimental data around to act as cover if anyone came in. Then he turned on the spectrometer. He'd seen it on one of his casual tours of the lab: a brand-new gamma ray spectrometer from Aggison-Hewlett.

He'd borrowed the spectrometer notes from a guy who'd taken the nuclear physics lab cla.s.s the semester before. It had a simple procedure for calibrating the spectrometer, then taking and printing a spectrum.

John calibrated it with the cesium sample, then set the device under the detector. He started it and waited.

It took a while, but a peak began to grow. He let it sit for an hour, nervous that someone would disturb him. To occupy his mind, he started filing at a new flipper; he'd made a dozen styles for the pinball machine, and to swap them the player could simply lift off the old flipper and replace it with a new one with the same mounting.

The spectrometer beeped. John examined the screen; there was a single sharp peak. He printed the spectrum, and used a ruler to figure the center of the peak. He guessed it was about 510 keV. Just one peak meant just one isotope inside, usually.

He opened the nuclear physics book and started working through the list of elements and their gamma ray energies.

He worked his way through the list eliminating anything with a half-life less than a year and anything with a gamma ray not within 50keV of 510. He ended up with Kr-85, which had a half-life of 10.3 years and a gamma ray of 540 keV.

He wondered if he had calibrated the device wrong.

John started over, and calibrated this time with a Cobalt 60 isotope, which had two distinct peaks at 1330 and 1170 keV. Again he put the device under the detector. Again he saw the same peak, and he calculated it to be at 510 keV.

Frustrated, he put the two spectra in his backpack and walked home. Could it be that the device contained an isotope that no one here had discovered?

The next day he wandered over to the spectrometer when someone was using it.

"Excuse me, can you help me with something?"

"Sure," the guy said in a Slavic accent.

John showed the spectrum to him, and asked, "What isotope makes a peak at 510 keV?"

The student looked at the spectrum and said, "None. You have annihilation peak here."

"Annihilation peak?"

"Sure. Gammas interact by three mechanisms. ..."

"Photoelectric effect, Compton scattering, and pair production. Of course!" John laughed as he realized what he was seeing.

"I'm Alex Cheminov, by the way," the student said. "You know your stuff. We could make a decent nuclear physicist out of you easily enough."

"John Wilson," John said, shaking hands. "I may have a few more questions. Do you mind?"

"Not at all."

John realized that the peak at 510 keV , really 511 keV, was from the gammas produced when a positron hit an electron and disappeared in a burst of radiation: two equal energy gammas at 511 keV. He was seeing the tail end of the pair production interaction of gamma rays in matter.

It only happened when the gammas were highly energetic, the spontaneous breakdown of a gamma ray into an electron and positron, antimatter, as it neared a nucleus. The positron would then bounce around, slowing down until it found another electron to interact with and generate the annihilation gammas. And that was what he was seeing.

John stopped. But the annihilation radiation was the tail end of a reaction. It was seen in addition to other methods of interaction. He should have been seeing at least one higher energy peak. But he wasn't.

Unless the positron wasn't being created by pair production. Unless there was another source of the positrons. Unless there was antimatter inside the device, powering it.

He laughed. It made sense. To move between universes required a lot of energy. And what better form was there of compact energy than antimatter? The device was powered by antimatter. It was a sound hypothesis.

One more mystery of the device fell before the sword of science.

"Science!" he cried, and as he was in the lab, not a single person looked up in surprise.

John watched Casey smile, and his heart jolted. They were standing on the edge of a chasm in Old Shady Park. Water had etched a fifteen-meter jag into the bedrock, already sc.r.a.ped clean of topsoil by glaciers. Autumn leaves tumbled around them. Browns, reds, oranges, and yellows covered the ground.

Casey wore no makeup. Her hair was pulled back into a ponytail. She was the most beautiful woman he had ever seen, and he hated himself for coming to desire her so. He was leaving this universe one day and would never come back.

"Let's go down," she said. She caught his look. "What's wrong? You look... pensive."

"I'm okay."

There was a stair that led them to the bottom of the gorge. The iron rail was wet, and the damp pulled the heat from John's hand. The steps were carved into the rock but patched with cement in places. Still they were mossy, and the footing was slick.

Casey slipped, exhaled sharply, and grabbed John's arm. She tensed, then relaxed into him.

"Thanks," she said.

"Sure."

The park was empty this early in the morning on a weekday. She had skipped her abnormal psychology lecture, and he had no cla.s.ses on Wednesday morning. She had told him they needed to spend some quality time together.

Something rustled in the leaves on the other side of the rail. A chipmunk raised its head to look at them, then scampered away.

"Look!" Casey cried, before it disappeared into a hole somewhere.

From below, the U-shaped falls seemed to close in on them. The sprinkle of water splashed in a small basin of rust-colored rock. John looked up into the falling water, past the trees, and into the cloudy sky. The moisture tickled his nose.

"I feel claustrophobic," Casey said.

Her voice echoed around the carved-out cavern behind the falls. John leaped across the weakly flowing stream. Graffiti was scrawled across the rocks behind the falls. A pile of beer cans were tossed in the dry grotto. It was clearly a hangout for local kids.

Casey hopped across the stream and joined him, hanging on to his arm.

She looked at the garbage and said, "People are so stupid. Look at this."

"Yeah."

John walked around the cavern. The floor had been rubbed clean and smooth over the years. During heavy rains, the place would fill up.