Ark

by Stephen Baxter

  ′I won′t let it happen again.′

  He nodded. ′Good answer.′

  ′I′ve got my assignment.′ She dug her handheld out of her bag, and tried to show him her study of the ecological disaster unfolding up in the Rockies, of how the tree line had already descended so far that the old regions of montane forest and shrubland, with their ponderosa pines and cactuses, were withering, whole ecozones disappearing.

  But Harry waved that away. ′You′ve both made yourselves late for Dr Zheng′s class, haven′t you? Pop quiz.′

  Zane fretted, and shuffled from one foot to another. ′Can′t we just go to class? A quiz will make us even later.′

  ′Then you′ll just have to make up that much more work, won′t you? OK. Overnight the Ark executive announced they′ve finally made their decision on where to locate the space launch centre - at Gunnison, Colorado. Why there?′

  Holle glanced at Zane. ′I didn′t know about Gunnison. I listened to the news. But it wasn′t in the bulletin I saw—′

  Harry said, ′Of course not. You know as well as I do about the secrecy around the project. You can′t keep a space centre under wraps, and there will be an official announcement later today. But both your fathers are at the centre of the project. You both are. You should know everything they know.′ He dug into the pile of books on the trolley, found an atlas, and threw it at Holle; it was a big, heavy, pre-flood volume, and she had trouble catching hold of it. ′Why Gunnison? Work it out. I′ll give you five minutes. Otherwise, another question.′ And he walked away, towing his trolley.

  The two of them knelt on the floor and spread out the atlas, looking for the right map. ′What an asshole,′ Holle murmured.

  ′He′s our pastoral tutor,′ Zane said. ′Looking after our overall personal development, while the specialist teachers - hey, look, here it is. Colorado.′

  They peered at the map, a splash of yellow and green laced across with roads marked in orange and blue. Denver showed up as a knot of development where major highways intersected. The map was pre-flood, but the shoreline of the great inland sea that had washed across the eastern United States, now reaching as far as a line from the Dakotas down to the Gulf, was still too far east to have shown up on this map.

  Zane looked at her doubtfully. ′So why would you build a space centre in Colorado at all?′

  ′The government would need to keep it close to Denver to make sure it was safe.′ Her father talked this sort of thing through with her. As the flood bit away at the remaining land area, more roads and rail routes were cut, more people joined the homeless throngs that washed back and forth across the high ground, and the government′s political control was weakening. The news bulletins were full of growing tension over a would-be separatist Mormon state in Utah; there was even talk of war. ′Somewhere in Colorado. But where?′

  ′High up enough that it won′t flood before 2040.′

  ′But that still leaves a lot of choice.′ She thought about where Cape Canaveral had been situated - on the Atlantic shore, the eastern coastline of America. Why there? For safety reasons, she remembered. You always launched rockets eastward, to get a boost from Earth′s own rotation. Launching from Canaveral had meant that any failure would result in a rocket, flying east, falling harmlessly into the sea. Now the same principle surely applied. ′Look,′ she said, stabbing a finger at the map. ′Gunnison. Twenty-three hundred metres above the old sea level. In 2040 it will be close to the eastern coast of the surviving land. A safe place to launch east.′ What else? She dug her handheld out of her bag and quickly interrogated it. ′The town′s on a valley bottom, so plenty of flat land. There′s an airport nearby, so you have transport links, and this reservoir, the Blue Mesa, can provide water. And it′s a college town, so there′s a population of workers already in place—′

  Harry Smith approached them. ′Actually that took you only four minutes. Yes, that′s why Gunnison, Colorado, is going to host the world′s latest, and maybe last, space launch facility. Twenty years ago you′d never have believed it. Good bit of deduction, Ms Groundwater. OK, you′re free to go.′

  They got to their feet, handed back the atlas, and ran for the stairs.

  ′And, Ms Groundwater - don′t be late again. Next time you might find somebody else sitting in your seat …′

  12

  When they got to the classroom, in the back of a large, emptied-out chamber labelled ′Edge of the Wild′ on the museum′s second floor, Liu Zheng was in full flow. He stood before an interactive whiteboard, rapidly assembling and erasing graphics, and allowing annotated equations to scroll by. ′The essence of an Alcubierre warp bubble is simple,′ he said. ′Conceptually at any rate. You have an isolated region of spacetime.′ This was marked as a circle on his two-dimensional diagrams, but he mimed a sphere with closed hands. ′Your spaceship is in this zone here …′

  As he talked, a dozen kids all about Holle′s age sat at tables before him, and worked at handhelds and laptops, muttering and murmuring in pairs and threes. Zane led Holle to an empty table. As she passed, the students glanced at her indifferently and looked away.

  Holle recognised a few of the kids in here, including Kelly Kenzie, a friend or maybe rival since they were little. Kelly was locked in intense conversation with a red-haired boy who looked a bit older. There were Cora Robles and Susan Frasier working in a huddle, two bright, pretty girls together. And Thomas Windrup and Elle Strekalov, sitting so close they might have been conjoined twins, as they had been all the way through grade school. Elle was a lot better-looking than Thomas, and the class gossips didn′t know why they stayed together. There was a lot of noise in here, and among the noisiest were Joe Antoniadi and Mike Wetherbee. Joe, an Italian-American whose family had fled New York, was likeable, friendly, easy to impress. Even as Zheng talked, Mike was cracking jokes in his broad Australian accent and making Joe laugh. Mike′s family were refugees from an almost entirely abandoned continent.

  They reached their table. Zane had a laptop, and Holle dug her handheld out of her bag.

  If the students had been indifferent to Holle, Liu Zheng didn′t so much as register her presence. He just carried on with what he was saying. ′So how do you fly to the stars? Well, you engineer the spacetime metric. You arrange it for spacetime to expand behind you, mimicking the inflationary conditions of the early universe. And you make spacetime collapse ahead of you, mimicking a black hole, say. Thus your spacetime bubble is pulled and pushed, driven ahead across the manifold. You are riding a propagating wave in spacetime.′

  ′Like surfing!′

  ′Yes, Mister Meisel. Though I myself have never surfed.′

  Holle thought she understood. The spacecraft would be embedded in spacetime like a toy insect in a block of glass. You didn′t transport the ship itself, but a whole chunk of the spacetime around it.

  ′This is the essence of the warp bubble. The transported spacetime must be large enough to keep you away from the regions of heavy curvature associated with the warp bubble itself - which would manifest, of course, like strong gravity fields. But what of travel faster than light? Einstein tells us that it is impossible to move faster than lightspeed as measured against local landmarks.′ He emphasised the words heavily. ′The trick is to carry those landmarks with you. The ship itself is not travelling at all relative to the spacetime bubble around it. It is the bubble itself that propagates at multiples of lightspeed, as desired. You are not travelling faster than light, because you are carrying the light with you …′

  Zane was already working, paging through notes on his laptop. Holle arranged for the board′s contents to be downloaded to her handheld, and she made notes alongside Liu′s diagrams and equations. All around her the students chatted, argued, joked, and scrolled through what looked like entirely disparate bits of work. This was not like the mostly calm, mostly studious atmosphere she had got used to at the grade schools in Denver.

  ′The warp bubble as a method of transportation has some paradox
ical properties. Because the ship is stationary relative to local landmarks, there are none of the effects we associate with special relativity: no time dilation, no Lorentz-Fitzgerald contraction. Clocks aboard the ship stay synchronised with those at the starting point, and indeed at the destination. And there will be no inertial effects.′

  ′What does that mean?′ Holle whispered.

  ′You wouldn′t feel any acceleration,′ Zane said. ′The ship′s not moving compared to the spacetime it′s embedded in. So you don′t get squished against the back wall of your cockpit when you turn on the warp drive.′

  ′However, there are issues of control, for you run the risk of outrunning any signal sent forward to control your own bubble. Therefore, we think it likely that any piloted mission will have the parameters of bubble formation, propagation, dissipation and so on loaded by a remote station before launch; the crew of the starship inside the bubble will essentially be passengers.′

  Joe and Mike burst into gales of laughter over some private joke.

  Holle leaned over to Zane. ′Is it always like this?′

  ′Like what?′

  ′Noisy. Everybody messing around.′

  He shrugged. ′There are no rules. They just put the material in front of you and expect you to make what you can of it.′

  ′And if you can′t cope with that,′ said the boy next to Kelly, twisting back, ′you can go back to the kiddie schools and play with the plastic bricks. There′s always somebody ready to take your place.′ He grinned. ′Don Meisel. Who the hell are you?′

  Holle found herself blushing as she told him her name. A year older than Holle, Kelly was growing into a tall, confident blonde, no great beauty but a leader. And this boy Don, who Holle hadn′t met before, looked that bit older again. His eyes blue, his hair red, a stronger colour than Holle′s own pale strawberry, he looked relaxed in his own skin, lively, fearless.

  ′Your first day?′ Don asked.

  ′Yes,′ said Holle. ′It′s great.′

  ′Sure it is. That a Scotch accent?′

  ′Scottish, I—′

  ′You up to scratch on relativity?′

  Her dad had gone through it with her. ′Sure.′

  ′Special relativity is trivial,′ Kelly said. ′Nothing more than Pythagoras′s theorem. So how are you on your Christoffel symbols?′

  ′On my what?′

  Kelly and Don just laughed, and turned away.

  Zane said, ′They′re teasing. They′re talking about tensor calculus. The mathematics of general relativity. Which is what you need to describe how spacetime curves around a warp bubble …′ He showed her some of Liu′s equations. She recognised derivatives, but some of the symbols were strewn with superscripts and subscripts. ′That is a tensor,′ Zane said. ′A kind of multidimensional generalisation of a vector, which is a quantity with both magnitude and direction—′

  ′I′m eleven years old,′ she said. ′My dad has been cramming me since I was six, when he got attached to Ark One. But how am I supposed to know about tense—′

  ′Tensors?′ He shrugged. ′Liu′s actually a good teacher, even if he never looks you in the eye. And if you don′t learn—′

  ′I′ll be out. I know.′

  ′I′ll help you.′

  ′Thanks,′ she said sincerely. ′So what′s with those two? Kelly and Don. Are they going out?′

  Zane just blinked. He didn′t reply. That wasn′t the sort of thing Zane ever took any notice of.

  Holle said, ′Kelly was always the boss at grade school. Maybe they′ll drive each other on.′

  ′Or they′ll crash and burn together.′

  The lesson didn′t get any easier.

  ′After five years of intensive study, we do now have a handle on how a warp bubble might be created,′ Liu Zheng was saying. He filled up his board with new kinds of diagrams, showing sheets and cylinders. ′The expansion or contraction of spacetime locally reflects a change in Einstein′s cosmological constant omega, which, as you know, describes vacuum energy, which is like an antigravity field that permeates spacetime - the engine of universal expansion.

  ′Now, we believe that our universe has a small extension in higher dimensions - higher, that is, than the three of space and one of time we experience. But those extra dimensions are small. Our universe is like a hosepipe, rolled up around the extra dimensions. The cosmological constant is inversely proportional to the fourth power of the characteristic radius of that hosepipe. Inversely. So the smaller that hosepipe radius, the higher the constant and the greater the expansive effect. Therefore, if you can change that radius locally, you can adjust the cosmological constant, and thus control the expansion of spacetime as you desire. To make a spacetime bubble you pinch the hosepipe.

  ′But how to pinch that hosepipe? On face value that would seem to require reaching out of the local three-dimensional plane of the universe itself …′

  Now he spun off again, into ′string theory′, which described space as filled not with point particles like electrons and quarks and neutrinos, but with strings, tiny filaments whose characteristic vibrations determined the properties of the ′particles′ they defined, such as charge and mass. Holle had heard of these ideas. It was as if the whole universe was a symphony played on tiny violins.

  But, Liu said, the strings could interact with those rolled-up extra dimensions of spacetime. In particular the strings could wrap around the extra dimensions, like spider-web wisps around the hosepipe. That was how the dimensions stayed compact in the first place. And that meant—

  ′You can squeeze the hosepipe,′ Holle burst out, her imagination racing away.

  Liu turned to her. ′And how would you do that, Ms Groundwater?′

  ′With a particle acce—′ She stumbled over the word.

  ′Accelerator?′

  ′Yes. With an accelerator you′re manipulating matter at its smallest levels. You can yank on the tiny strings.′

  Everybody was staring at her. Don and Kelly looked around, Don with amusement, Kelly with something more like resentment.

  ′I′m sorry,′ she said. ′I was thinking out loud.′

  ′Don′t be sorry,′ Liu said. ′You got it about right; that is what we′re planning. We′re setting up a hadron collider outside the city, based on scavenged components from accelerators in the US and overseas. Though we′re still years away from even a ground test of the concept; the energies required are ferocious …′ He gestured at the board. ′And can you see how that basic concept is expressed in my equations?′

  ′No,′ she said frankly.

  Kelly laughed. ′That figures.′

  But Liu was unperturbed. ′That′s not important. Intuition is the thing. But though we have a conceptual design for the creation of the warp bubble, we have a fundamental problem. The energy requirement is literally astronomical. A warp bubble is an artefact of curved spacetime analogous in some ways to a black hole. Now, suppose we built a bubble a hundred metres in radius. That should be big enough to house a respectably sized spacecraft, shouldn′t it? Give me an order of magnitude estimate of the mass-energy required.′

  The students huddled over their computers. Kelly muttered, ′The radius of a black hole is twice the mass times the gravitational constant divided by speed of light squared …′

  ′Ten to power twenty-nine kilograms,′ Venus Jenning called out. She was a black girl whose family had come from Utah, fleeing the gathering Mormon uprising. As far as Holle could tell she′d figured that number out in her head. Even while she worked, she was reading a yellowing paperback book under her desk, a gaudy science fiction title.

  ′Give me that in English,′ Liu snapped back. ′What does that number mean?′

  Kelly said, ′One-tenth the mass of the sun. You′d have to convert one-tenth of all the sun′s mass to energy to be able to build a warp bubble of that size.′

  ′Not exactly practical,′ Liu said. ′And that remains our fundamental problem, after years of studying
this concept. We just don′t have the energy resources to build a warp bubble of the size we need.′ He drew a big red cross through the equations and diagrams on the board.

  Again Holle found herself thinking out loud. ′If the answer′s not the one you want, maybe you′re asking the wrong question.′

  Liu turned to her again.

  ′I′m sorry,′ she said. ′It′s something my dad always says.′

  ′Then what is the right question?′

  Zane said quietly, ′Maybe, how big a warp bubble can we create?′

  Liu thought that over. ′OK. Let′s run with that. What′s the most energetic event humans can control?′

  ′Nuclear bombs,′ Thomas Windrup called. ′Thermonuclear actually.′

  ′Right,′ Liu said. ′And the biggest blast of all was?′

  That sent them scrambling to their computers, and whispering into search engines.

  It was Susan Frasier who came up with the answer. ′30th October 1961. A Russian test. Fifty-seven megatons, detonated at Novaya Zemlya.′ She smiled, always friendly, always eager to please.

  ′All right. And if that mass-energy was applied to creating a black hole?′

  It took them a minute to find out how to convert energy measured in an equivalent tonnage of TNT into joules. This time Kelly made sure she was the first to come up with the final answer. ′Its radius would be ten to minus twenty-seven metres.′

  Liu said, ′Give me that—′

  ′In English,′ Don said. ′Well, it′s eight orders of magnitude above the Planck length, the smallest possible. But it′s only one-thousandth of the radius of a one-mega-electron-volt neutrino! You couldn′t even fit a neutrino in there, let alone a spaceship!′

  There was a ripple of laughter, and Zane blushed.