Falling Free

working shift yesterday in Hydroponics. Yet another legged Somebody who got to go places and make things happen. He’d actually been born on old Earth, Mr. Van Atta said.

There came a muffled tap on the door of the soundproof bubble, and Silver keyed her remote control to open the door. Siggy, in the yellow shirt and shorts of Airsystems Maintenance, stuck his head through. “All clear, Silver.”

“All right, come on.”

Siggy slipped inside. She keyed the door shut again, and Siggy turned over, reached into the tool pouch on his belt, jimmied open a wall plate, and jammed the door’s mechanism. He left the wall plate open in case of urgent need for re-access, such as Dr. Yei knocking on the door to inquire brightly, What were they doing? Silver by this time had the back cover off the holovid. Siggy reached delicately past her to clip his home-made electronic scrambler across the power lead cable. Anyone monitoring their viewing through it would get static.

“This is a great idea,” said Siggy enthusiastically.

Claire looked more doubtful. “Are you sure we won’t get into a whole lot of trouble if we’re caught?” “I don’t see why,” said Silver. “Mr. Van Atta disconnects the smoke alarm in his quarters whenever he has a jubajoint.”

“I thought downsiders weren’t allowed to smoke on board,” said Siggy, startled.

“Mr. Van Atta says it’s a privilege of rank,” said Silver. I wish I had rank…

“Has he ever given you one of his jubas?” asked Claire in a tone of gruesome fascination. “Once,” said Silver.

“Wow,” said Siggy, grinning in admiration. “What was it like?”

Silver made a face. “Not much. It tasted kind of nasty. Made my eyes red. I really couldn’t see the point to it. Maybe downsiders have some biochemical reaction we don’t get. I asked Mr. Van Atta, but he just laughed at me.”

“Oh,” said Siggy, and switched his interest to the holovid display. All three quaddies settled around it. An anticipatory silence fell in the chamber as the music swelled and the bold red title letters rotated before their eyes—”The Prisoner of Zenda.”

The scene opened on an authentically-detailed street scene from the dawn of civilization, before space travel or even electricity. A quartet of glossy horses, harness jingling, drew an elaborate box on wheels across the ground.

“Can’t you get any more of the ‘Ninja of the Twin Stars’ series?” complained Siggy. “This is more of your darned dirtball stuff. I want something realistic, like that chase scene through the asteroid belt…” His hands pursued each other as he made nasal sound effects indicating machinery undergoing high acceleration.

“Shut up and look at all the animals,” said Silver. “So many—and it’s not even a zoo. The place is littered with them.”

“Littered is right,” giggled Claire. “They’re not wearing diapers, you know. Think about that.”

Siggy sniffed. “Earth must have been a really disgusting place to live, back in the old days. No wonder people grew legs. Anything, to prop them up in the air away from—”

Silver switched the vid off with a snap. “If you can’t think of anything else to talk about,” she said dangerously, “I’ll go back to my dorm. With my vid. And you all can go back to watching ‘Cleaning and Maintenance Techniques for Food Service Areas.’ “

“Sorry.” Siggy curled his four arms around himself in a submissive ball, and tried to look contrite. Claire refrained from further comment.

“Huh.” Silver switched the vid back on, and continued watching in rapt and uninterrupted silence. When the railway scenes began, even Siggy stopped squirming.

Leo was well launched into his first class lecture.

“Now, here is a typical length of electron beam weld…” he fiddled with the controls of his holovid display. A ghost image in bright blue light, the computer-generated x-ray inspection record of the original object, sprang into being in the center of the room. “Spread out, kids, so you can all get a good look at it.”

The quaddies arranged themselves around the display in a spherical shell of attentiveness, automatically extending helping hands to neighbors to absorb and trade momentum so that all achieved a tolerable hover. Dr. Yei, sitting in—if you could call it that—floated unobtrusively in the background. Monitoring him for his political purity, Leo supposed, not that it mattered. He did not propose to alter his lecture one jot for her presence.

Leo rotated the image so that each student could see it from every angle. “Now let’s magnify this part. You see the deep-V cross section from the high-energy-density beam, familiar from your basic welding courses, right? Note the small round porosities here…” the magnification jumped again. “Would you say this weld is defective or not?” He almost added, Raise your hand, before realizing what a particularly unintelligible directive that was here. Several of the red-clad students solved the dilemma for him by crossing their upper arms formally across their chests instead, looking properly hesitant. Leo nodded toward Tony.

“Those are gas bubbles, aren’t they sir? It must be defective.”

Leo smiled thanks for the desired straight line. “They are indeed gas porosities. Oddly enough, though, when we crunch the numbers through, they do not appear to be defects. Let us run the computer scan down this length, with an eye to the digital read-out. As you see,” the numbers flickered at a corner of the display as the cross-section moved dizzyingly, “at no point do more than two porosities appear in a cross-section, and at all points the voids occupy less than five percent of the section. Also, spherical cavities like these are the least damaging of all potential shapes of discontinuities, the least likely to propagate cracks in service. A non-critical defect is called a discontinuity.” Leo paused politely while two dozen heads bent in unison to highlight this pleasingly unambiguous fact on the autotranscription of their light boards, braced between lower hands for a portable recording surface. “When I add that this weld was in a fairly low-pressure liquid storage tank, and not, for example, in a thruster propulsion chamber with its massively greater stresses, the slipperiness of this definition becomes clearer. For in a thruster the particular degree of defect that shows up here would have been critical.”

“Now,” he switched the holovid display to one in red light. “This is a holovid of the same weld from data bits mapped by an ultrasonic pulse reflective scan. Looks quite different, doesn’t it? Can anyone identify this discontinuity?” He zoomed in on a bright area.

Several sets of arms crossed again. Leo nodded toward another student, a striking boy with aquiline nose, brilliant black eyes, wiry muscles, and dark mahogany skin contrasting elegantly with his red T-shirt and shorts. “Yes, Pramod?”

“It’s an unbonded lamination.”

“Right!” Leo tapped his holovid controls. “But check down this scan—where have all our little bubbles gone? Anybody think they magically closed between tests? Thank you,” he said to their knowing grins, “I’m glad you don’t think that. Now let’s put both maps together.” Red and blue melded to purple at overlapping points as the computer integrated the two displays.

“And now we see the little bugger,” said Leo, zooming in again. “These two porosities, plus this lamination, all in the same plane. You can see the fatal crack starting to propagate already, on this rotation—” The holovid turned, and Leo emphasized the crack with a bright pink light. “That, children, is a defect.”

They oohed in gratifying fascination. Leo grinned and plunged on. “Now, here’s the point. Both these test scans were valid pictures—as far as they went. But neither one was complete, neither alone sufficient. The maps were not the territories. You have to know that x-radiography is excellent for revealing voids and inclusions, but poor at finding cracks except at certain chance alignments, and ultrasound is optimum for just those laminar discontinuities x-rays are most likely to miss. Both maps, intelligently integrated, yielded a judgment.”

“Now,” Leo smiled a bit grimly, and replaced the gaudy image with another, monochrome green this time. “Look at this. What do you see?” He nodded at Tony again. “A laser weld, sir.”

“So it would appear. Your identification is quite understandable—and quite wrong. I want you all to memorize this piece of work. Look well. Because it may be the most evil object you ever encounter.”

They looked wildly impressed, but totally bewildered. He commanded their absolute silence and utmost attention.

“That,” he pointed for emphasis, his voice growing heavy with scorn, “is a falsified