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  "Computer, oscillate the gravity until all the objects are free from the floor and ceiling, then shift to zero-g, with the objects floating in the chamber."

  "Doing so."

  "My word, that's incredible," said Rissa. "The stuff can pass right through other matter."

  Jag grunted. "The original samples we tried to collect must have leaked through the probes' walls, pushed out by the force of their acceleration toward Starplex."

  By bouncing the apparent source of gravity inside the chamber between the top and the bottom, PHANTOM eventually got all the gravel pieces to float freely. But Jag's fur danced when he saw the results of two pieces moving together. He'd expected to see them hit, then bounce off.

  Instead, when they got to just a few millimeters apart they deflected away from each other.

  "Magnetic," said Rissa.

  Jag moved his lower shoulders. "No, there's no magnetism at work here

  --there are no charges present."

  There were four articulated arms ending in tractor-beam emitters inside the chamber, and Jag operated all of them in unison, controlling one with each hand. He used one beam to lock onto a piece of translucent gravel a centimeter in diameter, and used a second beam to grab another piece of equal size. He then operated the controls to move the two pieces together. Everything went fine until the chunks were within a very short distance of each other, but then no matter how much power he fed into the tractor beams, he was unable to bring them any closer.

  "Amazing," said Jag.

  "There's some sort of force repelling them--a nonmagnetic repulsive force. I've never seen anything like it."

  "That must be what keeps the haze of gravel from coalescing," said Rissa.

  Jag lifted his upper shoulders. "I suppose. The net effect is that the material in the haze between the spheres is bound together gravitationally, but it won't ever coalesce more than it already has."

  "But then what keeps these pebbles together? Why doesn't that repulsive force blast them apart?"

  "They must be locked chemically. I suspect they were originally formed under great pressure--pressure that defeated the repulsion we're observing. Now that their constituent atoms are bonded, they stay together, but it would take great effort to combine the pebbles into bigger groupings."

  "Oh, hell," said Rissa. "You know what I'm thinking . . ."

  Jag's four eyes went wide. "The Slammers! We've only ever seen what their weapon did to one of our probes.

  Perhaps if they turned it on a world, this might be the result.

  Quite the doomsday device: not only does it destroy the planet, but it also imparts a force to the rubble to prevent it from ever collecting back together to form another world."

  "And now there's an open shortcut leading from here to the Commonwealth worlds. If they were to come through--"

  At that moment, Jag's wall beeped, and the elderly face of Cynthia Delacorte appeared on it. "Jag, it's--oh, hi, Rissa..

  Listen, thanks for sending up those samples. Do you know that this stuff sinks into normal matter?"

  Jag lifted his upper shoulders. "Incredible, isn't it?"

  Delacorte nodded. "I'll say. It's not normal baryonic matter. It's not antimatter, of course. We'd have been blown out of the skies if it were. But where normal protons and neutrons consist of combinations of down quarks and up quarks, this stuff is made of matte quarks and glossy quarks."

  Jag's fur danced excitedly. "Really?"

  "I've never heard of those kinds of quarks," said Rissa.

  Jag made a sound like she was a fool, but Delacorte nodded. "Since the twentieth century, humans have known of six flavors of quarks--up, down, top, bottom, strange, and charmed. In fact, six was the maximum number allowed for under the old Standard Model of physics, so we'd pretty much given up looking for more, which turned out to be a big mistake." She looked pointedly at Jag. "The Waldahudin had only found the same six flavors, too. But when we met the Ibs, they were aware of two more, which we refer to by opposing lusters, glossy and matte.

  There's no way you can get them by breaking down normal matter,' but the Ibs had 'done unique work pulling matter out of quantum fluctuations.

  In their experiments, luster quarks were sometimes produced, but only at very, very high temperatures. What we've got here are the first-known naturally occurring luster quarks."

  "Incredible," said Jag. "You've noticed thefardint things carry no charge? What explains that?"

  Delacorte nodded, then looked at Rissa. "Electrons have a charge of negative one unit, up quarks have a positive two-thirds charge, and down quarks have a negative one-third charge. Each neutron is made of two downs quarks and an up, which means the net charge is zip.

  Meanwhile, each proton consists of one down and two ups, which gives a charge of positive one. Since atoms have equal numbers of protons and electrons, they have an overall neutral charge."

  Rissa understood that the explanation had been for her benefit. She nodded at the wall monitor for Delacorte to go on.

  "Well, this luster-quark matter consists of what I'm calling para-neutrons and para-protons. Para-neutrons consist of two glossy quarks and one matte, and para-protons consist of a pair of mattes plus a glossy. But neither glossies nor mattes carry any charge whatsoever--so regardless of how you combine them, there's no charge on the nucleus.

  And without a positive nucleus, there's nothing to attract negatively charged electrons, so a luster-quark atom is solely a nucleus; it has no electron orbital shells. The bottom line is that luster matter isn't just electrically neutral. Rather, it's nonelectrical; it's immune to electromagnetic interactions."

  "Gods," said Jag. 'q'hat would explain why it can sink into solid objects. It would probably pass through completely unhindered if it weren't for drag caused by the regular-matter carbon grains and hydrogen polluting it, and--of course! That explains why we can see it, too. If it were purely luster quarks, it would be invisible, since the reflection and absorption of light depend on vibrating charges.

  We're just seeing the interstellar dust that's caught gravitationally inside the luster matter, like sand in jelly."

  He looked at the wall screen. "All right--it doesn't interact electromagnetically. What about the nuclear forces?"

  "It is affected by both the strong and the weak nuclear force," said Delacorte. "But those forces are so short-range, I doubt we'd get any interaction through them with regular matter except at incredibly high pressures and temperatures."

  Jag was quiet for a moment, considering. When he next spoke, his barking was subdued. "It's incredible," he said.

  "We knew the Slammer weapon could break chemical bonds, but changing regular matter into luster matter is --"

  **DELTA DRACONIS**

  "What was Saul Ben-Abraham like ?" asked Glass.

  Keith looked around the forest simulation, thinking of all the ways he could describe the man who had been his best friend. Tall.

  Boisterous. A guffaw that could be heard a kilometer away. A guy who could identify any song in three notes. A man who could drink more beer than anyone Keith had ever met--he must have had a bladder the size of Iceland. Finally, Keith settled on, "Hairy."

  "I beg your pardon?" said Glass.

  "Saul had a great beard," said Keith. "Covered most of his face. And he had this one giant eyebrow, like a chimp had laid its forearm across his head. The first time I ever saw him in shorts, I was amazed. The guy looked like sasquatch."

  "Sasquatch?"

  "A mythical primate from my part of Earth. I still remember seeing him in shorts for the first time and saying, gee, Saul, you've got hairy legs. He let out that great laugh of his and said, 'Yes--like a man.'"

  I said it was more like ten men." Keith paused. "God, how I miss him Friends like that, who mean that much to you, come along perhaps once in a lifetime."

  Glass was quiet for several seconds. "Yes," he said at last. "I suppose that's true."

  "Of course," said Keith, "there was mor
e to Saul than just a thick coat of fur. He was brilliant. The only person I've ever met who I thought might be brighter than him is Rissa. Saul was an astronomer. He's the person who discovered the Tau Ceti shortcut, from its footprint in hyperspace. The guy should have won a Nobel prize for that . . . but they don't like to award them posthumously."

  "I appreciate your loss," said Glass. "It's as if--oh, excuse me. My reckoner says I've got an incoming thought package. Will you excuse me for a little while?"

  Keith nodded, and Glass took an odd step, sort of sideways, and disappeared. Doubtless he'd gone through a door hidden by the forest simulation filling the docking hay--the only direct visual evidence Keith had had that he wasn't actually back on Earth. Well, if there was a door, Keith wanted to find it. He patted the air in the spot that Glass had disappeared from, but there was nothing.

  There had to be a wall somewhere around, though. The bay wasn't that big. Keith began to walk, figuring he was bound to hit a wall eventually. He continued on for perhaps five hundred meters without encountering any obstruction.

  Of course, if his--he started to think the word "captor," again, but fought it down and substituted "host" instead--if his host were being clever, he could have manipulated the images to make.Keith think he was walking in a straight line when he was really going in a circle.

  Keith decided to rest. As much as he tried to find time to work out in Starplex's Earth gym, which had gravity set to a full standard gee, he'd lost some muscle tone because of all the time he spent in the lighter Wald-standard gravity used in the ship 's common elements. He really should take Thor Magnor up on his offer of playing handball; Keith and Saul had played the game regularly, but he'd given it up when Saul had died.

  Keith lowered himself to the ground again, which, at this spot, was covered with clover. Keith found it quite comfortable to sit on. He ran his hand through the clover, enjoying the feel of it against his skin, and looked around. It was a remarkable Simulation, he thought.

  So relaxing, so beautiful.

  He watched some birds moving high overhead, but they were too far away for him to identify the species.

  Keith plucked a piece of clover and brought it up to look at. Maybe this was his lucky day; maybe he'd find a four-leaf clover . . .

  What luck. He did.

  He plucked a few more pieces, and his jaw dropped.

  He pressed his face to the ground, and examined plant after plant.

  They were all four-leaf clovers.

  He brought one up to his face, held between thumb and index finger, and scrutinized it. It seemed like normal clover in almost every way. It even bled a little green plant juice from its severed stem. But each of these clovers had four leaves. Keith remembered from undergraduate botany that the genus name for clover was Trifolium-- three leaves. By definition, clover had three leaves, except in the odd mutant individual. But these plants all had four distinct oval leaves.

  Keith looked at the white and pink flowers growing from some of the plants. Definitely clover--but four-leaf clover.

  He shook his head. How could Glass have gotten all the 'other details right, but have made a mistake such as this? It didn't make any sense.

  He looked around again, searching for any other discrepancies.

  Most of the deciduous trees did indeed seem to be maple--sugar maple, in fact, if he wasn't mistaken. And those conifers were jack pine, and the big one a little farther along was a blue spruce. And-- And what kind of bird was that? Sitting in that blue spruce? Surely not a cardinal or ajay. Oh, it had the tufted head crest, but it was emerald green, and its bill was flat and spatulate, unlike that of most songbirds.

  It was Earth; no doubt about it. That was Earth's moon, still sitting high in the daytime sky. And yet, it wasn't quite Earth--some of the details weren't right.

  Keith chewed at his lower lip, puzzled . . .

  Chapter VII

  Jag and Rissa took an elevator up to the bridge, and soon the Waldahud was standing in front of the two rows of workstations, telling his colleagues of the fantastic discovery.

  "There's a metaphor that's been carried by the current for years," he barked, "that visible matter is just froth on an inky ocean of dark matter. We knew the dark matter was there because of its gravitational effects, but we've never seen it until now. Those spheres out there, and the gravel fog between them, are made out of dark matter."

  Lianne let out a low whistle. Keith raised an eyebrow. He knew a bit about dark matter, of course. CalTech astronomer Fritz Zwicky had deduced its existence back in 1933, through observations of the galaxies in the Virgo Cluster.

  Those galaxies were rotating around each other so quickly that if the visible stars were the only major source of mass present, the whole thing should have flung apart long ago. Subsequent studies showed that almost every large structure in the universe--including our Milky Way galaxy--behave as if there were far more mass present than could be accounted for by the suns and any reasonable number of attendant planets. Some previously undetected matter, dubbed "dark matter"

  because it was apparently neither luminous nor highly reflective, accounted for over 90 percent of the gravity in the universe.

  As usual, Thorald Magnor had his large feet up on his console, and his thick fingers interlaced behind his head, buried in his red hair. "I thought we'd already discovered what dark matter was," he said.

  "Only part of it," said Jag, lifting two of his four hands.

  "We've long known that baryonic matter--matter made up of protons and neutrons--accounts for less than ten percent of the mass of the universe. In 2037, we discovered that the ubiquitous tau neutrino has a very slight mass--about seven electron volts' worth. And we found that the muon neutrino also has a trifling mass, about three one-thousandths of an electron volt. Since these two types of neutrinos are so abundant, in total they account for about three or four times more mass than all the baryons do. But that still left us with as much as two thirds of the universe's mass unaccounted for--until now."

  "What makes you think the stuff out there is dark ' matter?" Keith asked.

  "Well," said Jag, "it isn't normal matter; that much is certain."

  Although he was trying to hide it, Jag was holding on to the beveled edge of Thor's console with one hand so that he wouldn't drop down onto four legs. Starplex operated on a four-shift cycle as a concession to the Waldahudin, who came from a world with a short day, but Jag had been working overtime. "In early dark-matter studies, there were two candidates for the material composing it, named WIMPs and MACHOs by human astronomers-- all of whom should have to swim in a river of urine, by the way. WIMPs are 'weakly interacting massive particles' -you see the gibberish foisted upon us in search of these silly acronyms? Anyway, the tau and muon neutrinos turned out to be WIMPs."

  "And MACHOs?" asked Keith.

  "'Massive compact halo objects,'" said Jag. "The 'halo' is the sphere of dark matter that has a galaxy at its center.

  The 'massive compact objects' were thought to be billions of Jupiter-sized bodies not associated with any particular star--a fog of gaseous worlds through which the luminous material of the galaxy moves."

  Lianne was leaning forward, chin resting on her hand. "But if the universe really were permeated with--with MACHOs," she asked, "wouldn't we have detected them by now?"

  Jag turned to her. "Even Jupiter-sized objects are puny on the cosmic scale. And since they're nonluminous, the only way we would see them is if one wandered in front of a star we happened to be observing.

  Still, the effect would be minor: just a slight gravitational lensing of the star's light, causing a temporary brightening. Such events have occasionally been seen; the oldest recorded observation of one was made by human astronomers in 1993. But even if space were lousy with MACHOs--enough so that they made up two thirds of all the mass in the universe--only one out of every five million stars you could observe at any given moment would likely be undergoing gravitational lensing due to one passing by." He
gestured toward the twinkling part of the starfield. "We only see gross effects here because we're so close to the field of dark matter, and because the dark matter itself is transparent.

  We're actually just seeing regular space dust, sprinkled throughout the dark-matter objects."

  Keith looked at Rissa, his eyebrows raised. She made no objection.

  "Well," said the director, "this certainly seems to be a major discovery, worthy of further--"

  "Forgive the interruption," said Rhombus, "but I'm detecting a tachyon pulse." Rhombus rotated the starfield hologram surrounding the bridge to bring the shortcut front and center; the effect on Keith's stomach was similar to what he experienced in a planetarium when the operator was trying to demonstrate that learning could be fun. Jag quickly took his seat on Keith's left. The shortcut was a pinprick of green--the color of whatever was coming through it--surrounded by the usual ring of violet Soderstrom radiation.

  "Is it a Commonwealth ship?" Keith asked.

  "No," said Rhombus. "There's no transponder signal of any kind." The green spot continued to grow. "Incredulous: that is bright"--PHANTOM's stilted translation of the words that were flashing over Rhombus's mantle. But the Ib was right. The shortcut was the brightest object in the sky?

  exceeding even the A-class star Jag had spotted earlier.

  "Let's give it lots of room, whatever it is," said Keith.

  "Thor, start backing us away."

  "Doing so."

  Keith looked to his left. "Jag, spectral analysis."

  The Waldahud read from one of his monitors. "Scanning.

  Hydrogen, helium, carbon, nitrogen, oxygen, neon, magnesium, silicon, iron . . ."

  "It looks pure green," said Keith. "Could it be a laser?"

  Jag turned his two right eyes to look at the director, while keeping his other two focused on his instruments. "No.

  There's nothing coherent about that light."

  The fiery green pinprick was growing wider; it had become a fiercely bright circle several meters in diameter.