The Science of Avatar

⁸ A SHIP TO SAIL

The Interstellar Vehicle Venture Star is a starbound freighter, one of a fleet of twelve sister ships regularly plying the route between Earth and Pandora. The frequency of the voyages is why Colonel Quaritch is able to offer Jake a ride home only a few weeks after his arrival. Typically the ship will carry two hundred sleepers like Jake out to Alpha Centauri, and hundreds of tonnes of unobtanium back to Earth. In addition to a fifteen-strong crew there are also ten medical personnel on board, woken at the end of the journey to supervise the waking of the cryosleepers.

If you have your Avatar DVD to hand, take a look at the early scenes featuring Venture Star, as we see it in orbit around Pandora. It certainly looks an impressive piece of engineering, and so it should. James Cameron wrote a “bible,” a ten-page briefing document, detailing how the ship works, drawing on our best understanding today of how to build a starship. (There have been more starship studies than you might think; I’m personally involved in a study called Project Icarus, about how to send an unmanned probe to a nearby star.)

At the rear of the ship is an engine stack. The ship’s main drive is a rocket powered by the annihilation of matter and antimatter—in fact hydrogen and antihydrogen, contained in those big spheres, cryogenically cooled and held safely in magnetic bottles. As we’ll see the rocket engine is only used at the Pandora end of the journey. The engine stack is at the end of a long strut that leads to the crew compartments, which include a rotating arm. These components in turn huddle behind an array of forward-facing shields.

That ship is big, no less that fifteen hundred metres long. One reason for its sheer size is the need to keep the crew separated from the hazardous radiations of the engine. We have no spacecraft of anything like that dimension. Our largest space artefact is the International Space Station (ISS), which is seventy-three metres long and a hundred metres wide, including the solar panels, and masses over three hundred tonnes—a total mass which is in fact less than Venture Star’s cargo capacity.

How does such a ship fly to the stars?

Venture Star undergoes short bursts of acceleration at the beginning and end of the voyage, and then spends most of its transit coasting, with the engines powered down. You can tell it must cruise because you can see that rotating arm turning around the ship’s spine, evidently a device to give the handful of alert crew artificial gravity. This makes engineering sense; Apollo cruised most of the way to the moon and back, with most of the fuel load of its huge Saturn V booster burned up in the first few minutes of the journey. During the boost phases, the fragile rotating arm is folded back against the ship’s spine.

When Venture Star is accelerating from the solar system, it doesn’t rely on its own engines at all. Instead it is pushed by beamed power from Earth, light from a tremendous laser that is caught by a huge sail, a bowl sixteen kilometres across held stable by rotation. The ship carries big mirror shields that in this phase of the voyage protect the habitable compartments from the laser beam’s intense glare. When the acceleration phase is over the sail is folded away. All this is done to minimise the mass of fuel Venture Star must carry.

The “lightsail” is in fact an interstellar propulsion technology whose underlying principles were, remarkably, defined and demonstrated by the end of the nineteenth century, with a theoretical prediction of the pressure exerted by light by the Scottish physicist James Robert Maxwell, followed by an experimental demonstration by a Russian scientist called Peter Lebedev in 1900. Later the American physicist and science fiction writer Robert L. Forward did much to develop the idea. In his novel Rocheworld Forward described a manned starship propelled by the collected light of a thousand laser stations in orbit around planet Mercury. But the basic idea is more elegant, even beautiful; if you didn’t mind a journey time of a few thousand years you could sail to the stars powered by sunlight alone, pushing a huge, filmy sail.

Pushed by the energy beam, Venture Star accelerates at an uncomfortable (for the awake crew) one and a half gravities for a hundred and sixty-eight days. Then it cruises. Jake Sully, having slept away the journey to Alpha Centauri in cryosleep, is told on waking that the journey took five years, nine months and twenty-two days. That’s certainly a long enough time to justify putting most of the passengers in the freezer rather than try to keep them fed, watered and occupied all that way; an active human consumes around two tonnes per year of oxygen, water and food.

But it’s still a pretty rapid crossing. According to my venerable Norton’s Star Atlas, Alpha Centauri is 4.39 light years from the sun. Despite a perhaps confusing name, a “light year” is a unit of distance, not time; it’s the distance a light beam travels in a year, around nine trillion kilometres, or about sixty thousand times the distance between Earth and sun. So it would take a beam of light four years, four months and around nineteen days to reach Alpha Centauri. Any estimate of the ship’s precise speed depends on whether Jake’s five years, nine months and twenty-two days is measured on Earth or aboard the ship—as we’ll see, there is a difference! But you can see immediately that to cross more than four light years in less than six years Venture Star must have been travelling at a respectable fraction of the speed of light. In fact, the cruise speed is seventy per cent of lightspeed, and the ship travels at this speed for five years ten months.

Such a high velocity immediately raises another hazard: interstellar debris. Space isn’t empty, not even between the stars. Out there between the sun and Alpha Centauri the average density of matter is around one hydrogen atom per cubic centimetre. That may not sound a lot, and in the four-light-year-long tunnel bored by Venture Star the ship will only encounter a gram or so of material. But a gram hitting you at seventy per cent lightspeed would be equivalent to the three hundred tonnes of the International Space Station hitting you at Earth-orbital speeds—whammo! So after launch the ship is turned head over heels, so that during the cruise those mirror shields that protected the crew from the laser beam are held ahead of the craft, to act as multi-layer protection against the debris.

As Alpha Centauri approaches, the ship is flipped over again and the great antimatter engine is at last fired up, burning to give a deceleration of one and a half gravities for another hundred and sixty-eight days. On return to Earth, the sequence is reversed, with the antimatter engine pushing Venture Star on its way, and the beamed-energy laser bank slowing it down at the solar system. (Incidentally many candidate designs for starships use hybrid designs like Venture Star, with more than one propulsion system; the huge distances involved push our technologies to the limit.)

How long does the journey take? Well, if you add up all the times I quoted above you’ll find the total one-way mission duration, including acceleration, cruise and deceleration, is about six years and nine months—

Wait. Jake Sully was told he’d been sleeping for five years, nine months and twenty-two days. That’s a discrepancy of a year. What’s gone wrong?

The reason these numbers are different is because of another aspect of that tremendous velocity, those vast distances, that no amount of ingenuity will let you engineer away: relativity.

⁹ TWINS AND TIME

The timestamp on Jake Sully’s video diary tells us that his adventure among the Na’vi begins in May of the year 2154. But if Jake’s image were beamed directly to Earth—at lightspeed, the fastest possible—it wouldn’t arrive for another four years, four months; his May 2154 journal entry couldn’t be read until September 2158.

Even our nearest neighbour the moon is a bit more than a light-second from Earth. The two-second round-trip delay was noticeable during communications between Houston and the Apollo astronauts. This made little practical difference for Apollo, but it would for RDA on Pandora. It would take a whole four years for a plea for orders from Hell’s Gate administrator Parker Selfridge to reach his superiors back on Earth, and four more years for any reply to come back. Until an effective faster-than-light communicator is invented (see Chapter 11), interstellar colonialism will be much like empires on Earth before the advent of the telegraph and radio, when messages from London, carried overland or by ship, could take weeks or months to reach British outposts in India or Australia. (In fact RDA is rather like the East India Company, used by the imperial British to subdue that subcontinent and open up its resources; the London government profited through taxes while letting private enterprise take the strain.) And, just as in imperial days, because of lightspeed delays interstellar colonial administrators will enjoy a great deal of autonomy—a situation that can go horribly wrong.

But there’s more to lightspeed and time than this kind of administrative challenge. If it’s 19 May 2154 at Alpha Centauri, what’s the date on Earth?—which is, after all, more than four light years away. Come to that, what time is it on the moon right now? If Charlie Duke, who worked as ground communicator at Houston during the landing of Apollo 11, had tried to synchronise his watch with Neil Armstrong’s on the moon, there would have been a comedy of errors as Duke’s “Mark!” reached Armstrong’s ears a whole second later, with Armstrong’s own “Mark!” coming back to Earth a second after that.

I suppose Duke and Armstrong could have agreed between themselves that Houston time is the “master,” and any records made by Armstrong and Aldrin on the moon could reflect this. Perhaps in the Avatar universe some such administrative arrangement will be made, so that on Jake’s video-record time stamps a date of 19 May 2154 as recorded on Pandora will have an unambiguous meaning, whoever’s viewing it, on Earth or Pandora. Similar reconciliations have been made in the past. In the nineteenth century the coming of the railways, and the need to draw up timetables everybody could agree on, inspired the first moves to set up national time frames. In Britain, for a long time, the official time was called “railway time.”

But that’s just bureaucracy. There’s something more fundamental here. When is it “really” 19 May—when it comes up on the calendars on Earth, or on Pandora? Is there some universal time frame we can all appeal to? Can we synchronise our local times by the ticking of some cosmic clock?

Unfortunately (or wonderfully, depending on your point of view) the universe we live in is stranger than that. Albert Einstein proved that not only is there no universal time frame, there isn’t even a universal rate at which time passes. This is all because of relativity.

Relativity is a conceptual challenge.

The trouble is, relativistic effects depend on lightspeed, which is a very high speed (three hundred thousand kilometres a second in a vacuum). So on the scales of the short distances and low speeds on which we generally live our lives, relativistic effects are unimportant, too small to be noticed (but not too small to be measured by fine enough instruments, which is how we know Einstein’s ideas are correct). Relativity isn’t part of our everyday “common sense” universe; we didn’t evolve with it, and so it’s hard for us to grasp.

But special relativity, at least, isn’t terribly hard mathematically. (“Special” relativity deals with the mechanics of motion; Einstein’s later theory of “general” relativity deals with gravity: curved spaces, black holes and wormholes.) After all the theory was dreamed up, at the beginning of the twentieth century, by a young patent clerk in Switzerland with too little to do and too much imagination, who wondered how the universe would look if you could travel with a light beam…

And Jake aboard Venture Star, barrelling through space at over half lightspeed, certainly can’t ignore relativity’s effects.

The basic principle of special relativity is to do with lightspeed itself. Suppose you’re travelling on a train coasting at a uniform hundred kilometres an hour. I choose a more modern carrier, a French TGV perhaps, and on a parallel track I overtake you at a hundred and twenty kilometres an hour. From your point of view, if you measure my motion, you’ll see me pass at the difference of our velocities—my hundred and twenty less your hundred means you see me pass at twenty kph.

This works fine at our everyday scales, but not when lightspeed is involved.

Suppose instead of riding my TGV I fire a laser beam along the track beside you. I would measure the speed of that beam at the standard three hundred thousand kilometres per second (ignoring for now the complication that lightspeed varies in different media, such as air). You, in your carriage, by analogy with the overtaking trains, ought to measure the speed of the same beam at three hundred thousand kps, less your hundred kph. Correct? Wrong—you would derive the same speed as I would for the light beam, even though we are moving at different velocities. I know this defies common sense, but, remember, we are tiptoeing into realms outside our everyday experience.

The explanation derives from physics older than Einstein’s, the theory of electromagnetism developed in the 1860s by Edinburgh physicist James Clerk Maxwell—the same man who predicted that a beam of light can exert a pressure.

Light can be regarded as an electromagnetic wave, and so all its properties, including its speed, are predicted by Maxwell’s theory. But that’s a puzzle. Einstein clung to a basic principle of physics: in a lab moving at a uniform speed, so suffering no acceleration—a lab such as you could set up on your hundred-kph train—you ought to find reality obeying the same physical laws as in any other lab moving at a uniform speed, even if the speed is different. You couldn’t feel the motion, so, said Einstein, it should make no difference to the physics.

And that’s why the speed of light is such an oddity, unlike other speeds such as the speed of a moving train, or even the speed of sound. Lightspeed is a physical constant, a fundamental part of the fabric of the universe, like the charge on the electron. You can measure it as a ratio of other physical quantities that you can determine in the lab. And if two observers measure the speed of the same light beam, even if they are moving at different speeds themselves, they have to get the same answer.

So what happens when you do try to measure lightspeed aboard a moving train? If the speed always comes out at the same answer, your measurements of distances and time must vary, depending on how fast you travel. This is where the famous contraction of space and dilation of time at high speeds comes from. Specifically, as seen from back on Earth, Jake’s rulers shrink in the direction of Venture Star’s motion, and his clocks slow down; “dilation” means stretching. But Jake doesn’t notice, because his body “shrinks” in proportion, and the internal clocks of his body “slow down” too.

I know—it’s extraordinary. But if you hang on to the basic idea that distances and times adjust themselves so that lightspeed always comes out to the same number whatever your own speed, then you have the essence of special relativity.

So what does this mean for Jake on his starship?

Suppose Jake sails off on Venture Star, waved away by his twin brother Tommy. (Yes, I know, if Tommy had been alive Jake wouldn’t be going to Pandora as his replacement at all—but bear with me.)

When he’s woken, Jake is told that he’s been in cryosleep for five years, nine months and twenty-two days. But as Tommy sees it from Earth, Jake’s time slows down and his distance is compressed, and by Tommy’s clocks Jake has taken six years nine months to get to Alpha Centauri. If Jake comes straight home he will arrive back several years younger than his twin, for less time will have passed for him on both legs of the journey. An extraordinary thing: when the twins are reunited, Jake is suddenly younger than his twin. Isn’t he?

You might be troubled by something fishy here. That was Tommy’s point of view. What does Jake see? For him, Earth with Tommy aboard apparently sails off at seventy per cent lightspeed. From Jake’s point of view, isn’t it Tommy whose clocks should be slowed down? And if Jake were to return, shouldn’t it be Tommy who’s younger? This is known as relativity’s “twin paradox”—and the name is why I was so keen to bring Tommy back from the dead, briefly.

The resolution is that the twins’ situation isn’t symmetrical, because of those acceleration phases. Jake undergoes accelerations that Tommy doesn’t; Jake feels the boost phases (or would if he was awake), which Tommy doesn’t. When he got back to Earth Jake would find he was the younger twin—and Tommy would agree.

Because of calculations like this, Einstein, dreaming in his patent office, realised there could be no universal clock, no universal time. Every clock in the universe is in motion, and most of those motions will be different: Venture Star’s chronometers sailing the gulf between the stars, Tommy’s clock on Earth sailing around the sun, Quaritch’s clock in his Dragon gunship as he flies over Pandora, as it orbits its parent world Polyphemus, as it orbits Alpha Centauri A. Each of those clocks is measuring only its own “local” time, which runs at a different rate from the time measured by any other clock, a time with meaning only for those travelling with that clock. In Einstein’s universe there is no grand frame, no tremendous cosmic graph with universally agreed axes. All that exists are events: points in space and time. But there is good mathematics that lets us handle all of this. And relativity helps us understand causality. Because the scattered events can only be connected by effects that travel at lightspeed or less, lightspeed is what makes sure cause and effect occur in the right order.

Whether you think this kind of universe is entrancing or appalling depends on your point of view. But, according to our very best measurements, this is the universe we’re stuck with, and we’ll have to face relativistic consequences if we ever build a working starship.

But that is a big if.

¹⁰ THE ULTIMATE ROCKET

Humans have already sent off four interstellar craft, of a sort: the unmanned Pioneer and Voyager probes of the 1970s. Having been launched by chemical-propulsion rocket boosters, they escaped from the solar system after slingshotting off the gravity wells of the giant planets. None of them is heading for Alpha Centauri, but Voyager 2 will pass within a light year of the nine-light-year-distant star Sirius—after a cruise, not of five or six years, but nearly four hundred thousand years.

Our modern chemical-engine rocket technology is clearly far too feeble to challenge the huge distances to the stars. How can we do better?

We’ve already looked at lightsails, interstellar sailing ships pushed by light itself. But what about rockets? All our spacecraft so far have been powered by rockets. Can we get to the stars that way?

Isaac Newton understood that to make any rocket work, you have to throw something out the back, the faster the better, and that applies whether you’re talking about a Chinese firecracker or Venture Star. (This is Newton’s Third Law of Motion.) The velocity increase you get per kilogram of fuel depends on the velocity of your exhaust products. The space shuttle’s oxygen-hydrogen fuel, which is the best possible chemical system, has an exhaust velocity of a weedy few kilometres a second.

We could do better with a starship we could probably build tomorrow, if we really had to. Perhaps mankind’s earliest practical dream of a starship came out of our worst nightmare: a ship driven by nuclear bombs, a whole stream of them, thrown behind a huge spring-loaded pusher plate and detonated. Project Orion was run from 1957 to 1965 by General Atomic, a division of a company that also built nuclear submarines and intercontinental ballistic missiles. It was a time of extravagant dreams inspired by the new technology of thermonuclear detonations, the energies of the sun brought down to Earth. Orion was like putting a firecracker under a tin can to fire it into the air: not pretty, but effective. One analysis predicted that it would be possible to have sent humans as far as Saturn by 1970. However, growing opposition to nuclear weapons through the 1960s caused the Orion concept to be viewed with suspicion. The final straw was an unwise presentation to President Kennedy of a model of a spaceborne Orion-technology battleship, bristling with nuclear missiles. Kennedy was disgusted, and the project was canned.

A more refined version of the Orion idea is a technology called nuclear pulse propulsion. This was used in a conceptual study called Project Daedalus by the British Interplanetary Society in the 1970s. The ship would be driven forward by a series of micro-explosions, pellets of deuterium and helium-3 blasted by lasers and blowing up behind a pusher plate. This kind of drive could throw out its exhaust at something like a thousand kilometres a second, maybe hundreds of times better than chemical technology.

The ultimate rocket exhaust speed, however, is the speed of light—the universe’s speed limit, around a hundred thousand times the shuttle’s exhaust velocity. With such an exhaust velocity you’d have the best rocket you can possibly build.

And this is what must have drawn the attention of RDA’s rocket engineers to antimatter.

Antimatter’s existence was predicted theoretically as long ago as 1928, by a young physicist called Paul Dirac. Dirac was seeking a way to unite Einstein’s special relativity with another theory: quantum mechanics, the theory of matter, energy and motion on very small scales—brand new in 1928, still brain-bending today, and happily we don’t need to look at it too closely in this book. Dirac found that his resulting theory contained a prediction that for every type of subatomic particle there must exist an anti-particle: that is, a particle with the “quantum numbers” that define it all having opposite signs. Thus to the negatively charged electron there is an “anti-electron,” also called a positron, with positive electric charge, and other less familiar properties similarly mirror-image reversed.

Rocket engineers, and other fans of big explosions, soon realised that this “antimatter” had the beguiling property that if a chunk of it came into contact with an equal-sized chunk of normal matter, both chunks would be annihilated completely, in a flash of radiation. This made it stupendously efficient as an energy source, with all the propellant mass turned to energy; even the nuclear fusion processes that power the sun and thermonuclear bombs only turn a few per cent of the fuel mass to energy. A single gram of antimatter could deliver more energy than is contained in a thousand space shuttle external tanks full of fuel.

And because the result of the annihilation is pure radiation, you could stick the stuff in a rocket and immediately get that ideal lightspeed exhaust.

The first conceptual antimatter rocket design was by a German engineer called Eugene Sanger (who, in the 1930s, had sketched a rocket bomber-plane that could have struck New York; happily it was never built). In the 1950s Sanger produced a rocket design based on the annihilation of positrons with electrons; the gamma-radiation “exhaust” would fly out at lightspeed. The problem, however, was directing that exhaust. The gamma-ray photons fly out of annihilation events in all directions. If they were charged particles you could use a magnetic field to point them in the right direction—namely, out the back of the rocket. But as photons have no electrical charge there was no way of controlling them.

In the 1980s Robert L. Forward, of lightsail fame, came up with a workable design based on protons, massive fundamental particles. These annihilate with their antimatter twins, antiprotons, in two stages. First they produce particles called pions, some of which are charged. The pions soon decay to gamma rays—but not before you can use a magnetic field to hurl these charged particles out the back of your rocket as your exhaust. So this is a “pion” rocket rather than a true “photon” rocket, but it’s the closest anybody has come so far to the ideal.

Venture Star’s engine is an advancement on these lines, depending on a hybrid system, using a deuterium fusion process along with the antimatter annihilation.

When dealing with antimatter there are always practical problems of containment. You have to keep your antimatter from any contact with matter, even the walls of any fuel tank, if you want to live through the trip. The only way we know to do this is with magnetic fields, perhaps with the antimatter in the form of plasma, a charged gas. The most famous fictional use of antimatter as a fuel has probably been in Star Trek, in which it provides the energy for the faster-than-light warp field. The antimatter is contained in “pods”; starships are regularly wrecked when the containment fails. In the modern world the “Penning traps” used for such purposes, to contain the tiny amounts of antimatter produced in particle accelerators, are minuscule by comparison to what you’d need for Venture Star. And they’re short-lived. The longest anybody has trapped a handful of antihydrogen atoms so far is a mere thousand seconds, about sixteen minutes.

Aboard Venture Star, this problem has been solved by using Pandoran unobtanium, a room-temperature superconductor, to generate the intense magnetic fields necessary for successful containment (see Chapter 15).

More fundamental than containment, however, is the problem of where the antimatter is going to come from in the first place.

How much antimatter would Venture Star need?

Robert Forward came up with some numbers for his piondrive propulsion system. He figured that to get a small unmanned one-tonne probe to Alpha Centauri at a tenth lightspeed would require around a third of a tonne of antimatter. Venture Star is a lot bigger than that, and goes a lot faster, and, as you can imagine, the fuel load thereby increases; probably the antimatter required is going to be the same order of size as the mass of the ship itself—hundreds of tonnes, perhaps, or thousands. That’s why those spherical fuel pods in Venture Star’s engine stack are so large. Where is RDA going to find that much antimatter?

The trouble is, antimatter doesn’t seem to be easy to find in nature. Here we’re getting into questions of physics and cosmology. Dirac’s equations were symmetrical—they predicted that equal amounts of antimatter and matter should have come spilling out of the Big Bang in the first place. If so, where is the antimatter? How come we don’t see matter-antimatter annihilation events all around us? As far as we can tell the observable universe is basically just matter, aside from traces of antimatter emerging from natural high-energy events like supernova explosions, which leave signatures in cosmic rays.

The answer seems to lie in the subtleties of high-energy physics and the details of creation after the Big Bang. The laws of physics may not be quite symmetrical after all. A bit more matter than antimatter came spewing out of the Big Bang. A carnival of annihilation followed, filling the universe with a bath of radiation, and eliminating all the antimatter, and all but a trace of the matter. The excess of matter over antimatter had only been one part in ten billion, but that was enough to provide all the matter that makes up the galaxies, stars, planets, and you. This is a nice bit of physics, though the details are far from settled. But for a would-be antimatter rocket engineer all that’s important is that nature seems stingy when it comes to coughing up the juice.

Could we manufacture it? At the moment our only antimatter “factories” are particle accelerators, like Fermilab in Chicago. The antimatter produced by slamming fundamental particles into each other at near-lightspeed inside such machines amounts to around one ten millionth of a gram per year. (And that’s at a cost of around a hundred thousand trillion dollars per kilo! This, you will note, is somewhat higher than unobtanium’s twenty million per kilo, as Selfridge quotes to Grace Augustine.) To make a few hundred tonnes at that rate would take millions of billions of years, a time which exceeds the age of the universe by a factor of… oh, let’s not even go there.

There will clearly have to be a revolution in antimatter procurement to make all this work, and maybe that will come. Antimatter does have some practical applications today, such as in the PET (positron emission tomography) imaging system used in medicine. Maybe that will promote advances in its manufacture and storage. And Robert Forward pointed out that a factory dedicated to producing antimatter could be a lot more efficient than high-energy physics experiments producing it as a by-product.

This is what has been achieved in the universe of Avatar, in which a tremendous particle accelerator on the far side of the moon churns out antimatter in the quantities needed to send Venture Star and its sisters to Pandora—and the reason this giant engine is on the lunar far side is to keep the Earth safe from the huge energies it handles.

Interstellar travel is hugely challenging. For now we can say that we know Venture Star could work in principle, but we don’t yet know how to build it, and couldn’t yet manufacture the antimatter needed to run it. But we do believe it could one day exist, and could take us to Pandora.

And, even though Jake Sully sleeps through the whole thing, the journey itself would be a tremendous adventure.

¹¹ STARS TO SAIL BY

Interstellar distances are appalling. To scale, the stars are like grains of sand separated by kilometres.

Thomas Henderson, the first man to measure the true distance to Alpha Centauri in the nineteenth century (see Chapter 12), was so shocked by his result that he hesitated to publish it. It will be daunting even for an interplanetary civilisation; the distances between the stars are hundreds of thousands of times the distances between the planets of the solar system.

That’s why the cruise of Venture Star, even to the nearest star system and even moving at a respectable fraction of lightspeed, will take years. And why the journey itself is a significant challenge.

To begin with, Jake Sully’s five years, nine months and twenty-two days is a long spaceflight. The longest human spaceflight to date was by Valeri Polyakov, a Russian cosmonaut who stayed on the Mir space station from January 1994 to March 1995, during which time, endlessly circling the Earth, he travelled some three hundred million kilometres, or around seventeen light-minutes. That’s why the fifteen-strong crew of Venture Star is rotated in three waking shifts of five each, so nobody has to endure the whole journey.

What about life support? Whether you’re on the moon or on Mars or suspended between the stars, there are common technological challenges in maintaining small habitable volumes for long periods, with closed loops of air, water and other essentials. We don’t know how to do this yet; small systems tend to be unstable, as discovered from the “Biosphere II” experiment in Arizona in the 1990s. Today we are running simulated long-duration “missions” on Earth, such as the Russian Mars500 project, in which six Russians, Chinese and Europeans were locked away in steel tanks without resupply from outside for the length of a near-future Mars mission. The “mission” had such real-life features as communications time delays, and a “landing” in which the crew were separated into “surface” and “orbit” teams. Perhaps soon, according to President Obama’s new vision (see Chapter 6), we will be running real space missions to near-Earth objects that could last hundreds of days away from the Earth.

By the time we launch Venture Star we’ll surely have solved these problems. Even so, to survive more than five years, even with their passengers stored in cryosleep, the waking crew of Venture Star will have to manage their resources with almost one hundred per cent efficiency. It is a supreme irony that to reach their interstellar goal the crew, citizens of an evidently supremely wasteful civilisation, will have to become experts at recycling.

They will also have their own health to think of.

The design of Venture Star and its mission must be constrained by human factors. The higher the acceleration during the boost phase, and the longer it can be sustained, the better, as the overall mission time is reduced. But how much acceleration can a human body stand?

Since the arrival of high-performance aircraft and the space age our tolerance of G-forces has been studied by organisations like NASA and the military. Most of us can withstand a couple of G (Earth standard gravities) for short periods. That’s what you would experience on a mild roller-coaster, though some can pull you through as much as five G, briefly. We are most vulnerable to accelerations when we’re standing, because that drains the blood away from the brain; ten seconds at five G leads to tunnel vision and then blackouts. Fighter jets can impose up to nine G vertically, and pilots trying to stay conscious wear stretchy “G-suits” to force the blood up to the brain. Pilots with the highest tolerance are known in the trade as “G-monsters.” You can improve your G-tolerance with training in centrifuges, like the spinning rotor arm on Venture Star, though turning a lot faster. The secret is to tense your leg and abdominal muscles to force the blood to the upper body; you strain, as if you were suffering a particularly difficult bowel movement.

It seems unlikely however that without major re-engineering the human body is ever going to be able to function effectively in gravity fields of more than a few G. You could move around in an exoskeleton like Colonel Quaritch’s AMP suit if you had to, but your cognitive functions would likely be impaired. The RDA designers probably pushed the G load in the boost phases as high as they could. But even to withstand months at Venture Star’s one and a half gravities, the crew must have been hardened by some serious time in the centrifuge.

Meanwhile the long cruise phase holds its own hazards as well: not too much gravity, but too little.

On Pandora we see Colonel Quaritch ferociously exercising, because, he says, low gravity makes you “soft.” He’s probably right. Without gravity pulling on your body, you would suffer what’s become known as “space adaptation syndrome.” You’d suffer immediate effects such as a redistribution of the fluids in your body, and in the longer term a wasting of your relatively unused muscles, in your legs, for example. There are other effects which appear to be permanent, such as a decrease of bone density.

To compensate, astronauts on the space stations have always tried to exercise, to put their bones and muscles under regular stress. One good recreational way to do this, incidentally, might be through contact sports like wrestling or sumo, where you stress your body against somebody else’s—I can see Quaritch putting his rookies through that, en route to Pandora.

On board Venture Star there is a higher-tech solution. The awake crew have been given artificial gravity during the cruise by that rotating “arm” turning around the ship’s spine.

It certainly would feel like gravity if you stood inside one of the compartments at either end of the arm. On Earth, the planet’s gravity is constantly pulling you down towards the centre of the world; you’re stopped from falling by the reaction of the ground beneath your feet, pushing back at you. Inside the ship’s rotating compartment the floor is similarly pushing at your feet, so it feels like a reaction against gravity. But in fact the floor is pushing to keep you moving in a circle. If the compartment suddenly dissolved and you were released, you’d go flying off in a straight line at a tangent to the circular motion—just like a bolas whirled and released by a Na’vi hunter. The artificial gravity you feel is what the engineers call a “fictitious force”; it is a “centripetal force,” which means “centre-seeking.”

As you can imagine, the faster you are whirled around by the ship’s arm the greater the apparent gravity. And also the longer the arm is, the more “gravity” you would experience—but the greater the engineering challenge, for all that spinning mass would have to be compensated for, if the ship itself wasn’t to start spinning the other way in reaction.

How much gravity is “enough” for the human body—a sixth of Earth’s like the moon, a third like Mars? We know something of the effect of extended periods of zero gravity on human physiology, but we know nothing at all about extended periods of partial gravity, as you might experience in Venture Star’s spin module, or on low-gravity worlds like the moon, Mars and Pandora. We’ll have to find this out before we can design a ship like Venture Star.

And there’s another “fictitious” force to contend with in a spinning environment, called the Coriolis force. This acts on a moving body to curve its motion in the opposite sense to the spin. This has real consequences for us here on the turning Earth, such as the deflection of moving masses of air into weather systems. In a spinning habitat Coriolis effects will interfere with the inner ear, causing dizziness, nausea and disorientation. Experiments have indicated that at two rpm (revolutions per minute) or below, most people will suffer no adverse effects from Coriolis forces; at seven rpm or above, most people will suffer. Venture Star’s arm turns at around three rpm—you can see this in the movie and time it—which looks a sensible compromise.

Maybe the human body is going to prove more adaptable to long-term spaceflight than we think. I once met Sergei Krikalev, the cosmonaut who holds the record for the most time in space accumulated on separate missions, an astounding eight hundred and three days. And I have to say he looked pretty healthy to me.

There would be plenty of work for the crew to do through the long cruise. There would be basic systems maintenance; in a system as complex as a starship, over such a long journey, you can bet that a lot of glitches, and even multiple failure modes where one fault compounds another, are going to crop up. This is one reason an awake human crew will be required, for their flexible problem-solving capability—evidently beating out the capabilities of even the super-advanced artificial intelligences of the twenty-second century.

And, outbound, the most essential work the alert crew will have to undertake is to care for their precious live cargo: the avatar bodies being grown in their tanks, and Jake Sully and the other (human) passengers undergoing “cryosleep,” suspended animation.

The idea of using cold to induce suspended animation—to halt, temporarily, all the body’s functions—has a long history. There have always been cases of humans being saved for example from near-drowning accidents by hypothermia, the deep chilling of the body, which induces a kind of natural cryosleep. In antiquity the pioneering doctor Hippocrates advocated packing wounded soldiers with snow to keep them alive. There is good science behind this. For every six degrees’ drop in your core body temperature your metabolic rate drops by fifty per cent.

Deep cold is already used routinely in medicine. Some tricky heart operations require that the body’s blood flow be cut off entirely, while the surgeons get on with their repair work. But at normal body temperature, brain cells can survive only five minutes or so without oxygen from the blood. After that you get brain damage, and, ultimately, death. This survival interval can be greatly extended if the patient is cooled down, to give the surgeons a chance to do their work. The technique is known as Deep Hypothermic Circulatory Arrest. Suspended animation has other potential applications, for instance for patients waiting for organ donation—or, to go back to Hippocrates, to stabilise soldiers critically wounded on the battlefield.

But there are complications. Cells can be damaged by the cold itself; Jake wouldn’t have been helped to wake up with frostbite. In the Avatar universe RDA scientists have found a way to use microwaves to “jostle” water molecules in cells, and so prevent the formation of damaging ice crystals. But even without actual damage the effects of cooling on the body are complex; humans after all are not animals that naturally hibernate. For example, immunity reactions are slowed.

For now, NASA and ESA are not funding any research into suspended animation, though both appear to be keeping an eye on developments elsewhere.

One last job for Venture Star caretaker crew, and perhaps the most glamorous, is interstellar navigation.

Navigation is the science of figuring out precisely where you are and where you’re heading. And, given the vast distances involved and the relative smallness of the target, you might imagine that navigation and some kind of mid-course corrections will be necessary during Venture Star’s cruise.

To some extent interstellar navigation will be based on principles developed over millennia on Earth, principles we’ve already adapted as we’ve sent probes out beyond the planets, and have landed humans on the moon at target destinations with an accuracy of metres. We’ve all become used routinely to locating our positions with enormous precision thanks to the GPS system of satellites, a system consulted by smart phones and satnav systems. Conceivably, by the time Venture Star carries Jake Sully to Pandora, some chain of interstellar location beacons could be established to help a passing starship figure out its position. The receipt of pulses from beacons on Earth and at Alpha Centauri could also be useful.

Alternatively, many vehicles (and indeed modern mobile phones) carry accelerometers which can sense movement; keeping track of this allows “inertial navigation,” with which a ship computes where it must be in space simply from its internal sensing of motion. But inertial navigation systems tend to accumulate errors.

Venture Star’s principle system of navigation is in fact the very oldest: by the stars. Many unmanned spacecraft have carried star sensors for just this reason; out in space, surrounded by a shell of brilliant stars, it’s easy to pick out target stars by their characteristic light, and so to fix your position in three dimensions.

Before any star mission becomes practical, a vast exercise will be needed in nailing down interstellar distances, star positions and velocities precisely. Work has already begun on such a catalogue of stars, with the first space probes dedicated to “astrometry.” ESA’s Hipparcos space mission (High Precision Parallax Collecting Satellite), which ran from 1989 to 1993, produced a high-precision mapping of a hundred thousand stars. The upcoming Gaia, ESA’s successor to Hipparcos to be launched in 2012, is set to catalogue a billion stars.

For Venture Star it won’t be sufficient to treat the stars as fixed markers, as navigators can on Earth. From the ship, moving among those very stars, the crew will see the stars themselves shift across the sky—though not by much; the distance to Alpha Centauri, four light years, is still relatively short compared to the distances to the furthest visible stars. The stars of the Orion constellation, for example, are scattered through a volume of space a thousand light years deep, and the nearest of them is no closer than five hundred light years from the sun. Perhaps the interstellar navigators will actually measure the shifting of nearby stars against the background to fix their position (this is effectively how Thomas Henderson calculated the distance to Alpha Centauri in the first place (see Chapter 12)).

And, as you might expect, as Venture Star is travelling at a respectable fraction of the speed of light, there will be relativistic effects to take account of.

There will certainly be a “Doppler effect,” the same phenomenon that causes the pitch of a speeding police car’s siren to rise as it approaches you and drop when it drives away; the sound waves are bunched up one way, then stretched out the other. The maths for light at relativistic speed is different from the acoustic case, but the principle is the same. On a starship the Doppler effect will cause the light of the stars you are heading towards to be shifted towards the blue end of the spectrum (the shortest wavelengths), and those you are leaving behind shifted towards the red (the longest wavelengths), phenomena known as blue shift and red shift. If you go fast enough the “visible” stars might become invisible altogether because of this effect, with sullen red stars ahead of you, usually not seen at all, blue-shifted to visibility.

And then there’s an effect called “stellar aberration.” Aboard Venture Star you’re hurtling through a storm of starlight, as if you were running through raindrops. Just as if it would feel as if the rain was beating into your face even if it was falling vertically, so the apparent angle of the starlight is adjusted by your motion. At seventy per cent lightspeed, a star that was along a line of sight at forty-five degrees to your direction of motion would apparently be shifted down to about twenty degrees. Essentially the stars ahead of you would all seem to be bunched together in your field of view.

From the point of view of interstellar navigation, all these effects can be accounted for. But imagine a starscape at interstellar speeds! You would see all the stars in the sky scrunched up into a disc ahead of you, and these are not the familiar stars of our constellations but the much vaster population of cooler stars blue-shifted to brilliance, tens of thousands of stars usually invisible to the human eye. Behind you is only darkness, a part-sphere from which all the light has been deflected by aberration, all save for a point directly to the rear…

I hope Venture Star has an observation dome; it would be quite a view—but one which you might get sick of after the first five years of the flight.

Must it take so long to reach the stars?

If we’re limited by lightspeed, then it will always take a significant chunk of a (non-frozen) crew member’s life even to reach the nearest stars, and much of the Galaxy might forever be beyond us. If we’re limited by lightspeed. But are we? Will a warp drive, like the Enterprise of Star Trek, ever be possible?

The way to break Einstein’s speed-of-light law is to look at the small print. You can’t travel faster than light going through space-time… so what you must do is to go around space-time… or take it with you.

The idea of the space-time wormhole, a short cut through space, has become familiar to us through science fiction shows such as Star Trek: Deep Space Nine. Einstein himself (in his general theory of relativity) taught us that space-time is malleable, shaped by the mass and energy it contains. The idea of a wormhole is to bend space-time so severely that two points which are far apart are drawn together, through a higher dimension, and connected by a wormhole, a short tunnel. It would then be possible to cover immense distances without violating light-speed, by popping through the wormhole short-cut. Surprisingly the idea has a (reasonably!) firm theoretical footing. The astronomer Carl Sagan, wanting to use the idea for his science fiction novel Contact, asked physicist Kip Thorne to put some theoretical flesh on the notion. (Starship dreams are definitely an area where science fiction and science overlap.) Thorne found, to his surprise, that the concept made sense.

Another intriguing possibility is space-time surfing. In 1994 a physicist called Miguel Alcubierre, working at the University of Wales, showed that it may be possible to create waves of space-time. Because these waves are made of space-time they do not travel through space-time, and so aren’t subject to the light-speed law. A spacecraft could “surf” such a wave, and be carried at arbitrarily high speeds. Alcubierre’s surfing would have the advantage that you could go anywhere you liked; wormholes, by comparison, connect two fixed points. Alcubierre himself said in his paper that this is as close to the classic “warp drive” of science fiction that we are likely to come up with—and since that paper a generation of workers have toiled to find ways to make this practical. (Incidentally, because a faster-than-light starship breaks out of lightspeed’s causality cage, all that stuff in Chapter 9 about clocks and simultaneity becomes a lot more complicated. Such a starship can even become a time machine.)

If we ever do build a warp engine it will probably be long after Avatar’s twenty-second century—but there is one small chink of light.

According to Albert Einstein, nothing, not even information, can travel faster than light. But RDA do have a “superluminal” (faster than light) communication channel, which works by “McKinney quantum entanglement encoding.” Not very well, however; the bit rate is very low.

Quantum theory is all about information, specifically the information needed to specify the state of a particle like an electron: its charge, its spin, its velocity and so on. Suppose you have two electrons coming out of some process so that they share a property—spin, say, or momentum. They are said to be “entangled,” the information sets that describe them forever linked. The entanglement still holds true no matter how far they are separated—even if one electron stays on Earth and the other is carried to Pandora. If you now make a measurement of the entangled property of the particle on Earth, the state of its twin is immediately affected, instantaneously, regardless of light-speed. Einstein himself didn’t like this, he was no great fan of quantum mechanics as a whole although he contributed greatly to its development, and he called it “spooky action at a distance.” He was probably comforted by the apparent fact that you couldn’t send any useful information by this channel.

But in the universe of Avatar, a physicist called Albert McKinney has found a way to do just that, by exploiting another quantum property called “tunneling.”

It may be that when we reach for the stars for real, we will have a better theory of physics than we have today. As Dirac and others have argued, relativity, the science of the very big and very fast, and quantum mechanics, the science of the very small, must one day be united in a “quantum gravity” theory, out of which may naturally fall faster-than-light communications, and indeed something like a warp drive.

But this is for a more distant future.

So we’ve come to the end of Venture Star’s interstellar journey. The great engine has fired to slow us. The universe as seen from the observation dome has opened up like a flower in spring.

And laid out before us is a majestic spectacle: Alpha Centauri.