The Science of Avatar

by Stephen Baxter

  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.

  PART FOUR

  PANDORA

  “You are not in Kansas any more…”

  —Colonel Miles Quaritch

  12

  FIRST PORT OF CALL

  The very first interstellar journey we make is likely to be, just as in Avatar, to our sun’s nearest neighbour.

  Alpha Centauri is a triple star system. The two principal stars, known as A and B, are bound close together by gravity. The twins don’t orbit each other, but both circle a common centre of mass, just a point in space, following looping elliptical trajectories. Each of the two central suns is similar to our sun, A in particular, but these near-twin stars are no further apart than the planets in our solar system. Alpha B comes about as close to A as the planet Saturn does to the sun, though it loops out to Pluto’s distance.

  Imagine standing on a planet orbiting A, the brighter star (as Polyphemus does). From here A looks like our sun in the sky. The companion, B, is a brilliant, orange-ish star. Even at its furthest distance from A, B is about two hundred times brighter than the full moon; at its closest it is over two thousand times as bright as the moon. In fact it shows a disc to a sharp enough naked eye.

  And somewhere in the complex sky around you is Proxima, the third star in the system, orb
iting the main binary pair four hundred times further away from those twins than they are from each other, trundling around an orbit that takes half a million years to complete. (Proxima is so far out that there’s some controversy about whether it’s really part of the Alpha system at all.) Proxima is actually the closest star of all to the sun, which is why it’s so named: like “approximate,” the name “Proxima” comes from a Latin root meaning “near.” Proxima is an unspectacular red dwarf, a minor component of this system—but of great interest to astronomers, for it is actually more representative of the Galaxy’s stars than either Alpha A or B, or indeed the sun; seventy per cent of stars are like Proxima.

  You are here! Alpha Centauri: the first port of call beyond the sun’s realm.

  As the closest star system, Alpha Centauri has, not surprisingly, featured in many starship studies, and fictional depictions of interstellar travel. For example there’s Leigh Brackett’s thrilling Alpha Centauri—Or Die! (1963), Encounter with Tiber (1996) co-written by moonwalker Buzz Aldrin with John Barnes, my own Space (2000)—and Footfall by Larry Niven and Jerry Pournelle (1985), about an invasion of Earth from Alpha Centauri, rather than the other way around as in Avatar. Avatar in fact seems to be the first depiction of the system in the movies, although it was the target for the hapless star travellers of the TV series Lost in Space (1965–8).

  We’ve known Alpha Centauri is the closest star system for nearly two centuries now. This was established in 1832 by a Scottish astronomer called Thomas Henderson, working at an observatory in South Africa (Alpha Centauri is invisible from the northern hemisphere). He used a method called parallax. If you hold a finger up closely before your nose, and then inspect it through first one eye and then the other, you’ll see it apparently shift against the more distant background. If you know how far apart your eyes are, and you measure the apparent shift, you can do a bit of geometry to work out how far your finger is from your nose. This is the method Henderson used, scaled up a mere hundred thousand trillion times. He knew the diameter of Earth’s orbit around the sun, and by studying the way Alpha Centauri apparently shifted across the background of more distant stars as Earth crossed from one side of its orbit to the other in the course of a year, he was able to establish Alpha’s distance. Parallax was a well-established method at the time, having been used to measure the distances between the sun’s planets. But the interstellar distance Henderson worked out was so large it made him hesitate to publish his result; suddenly the universe was bigger than everybody had thought.

  Even so, a starry night seen from Alpha Centauri might seem nostalgically familiar.

  Of course if you stand on a world of Alpha A, because of the glare of B, you won’t get many dark starry nights. And if your world is a Pandora, a close-in moon of a giant planet, the glare of that primary world will crowd the sky even more—although you will get a spectacular show as the giant goes through its phases, and eclipses one or other of the suns.

  With time however you’ll see B track slowly around the sky, like an outer planet in our solar system. Sometimes B will be in the “night sky” of A, and will banish the darkness. But when B is in the daytime sky, and especially when the suns are close together, they will act as if they are a single point of light, like our own solitary sun, and the day–night cycle will seem normal to a terrestrial like you. You may even see a very strange solar eclipse indeed—the eclipse of one sun by another, as B passes behind A.

  And there will be a few nights, when the suns are close together and both below the horizon—and when your local Polyphemus has set too—when the distant stars will at last be visible.

  You’re a mere four light years from home. If you look around the sky, just as you saw from Venture Star, the constellations are little changed, because most of the stars are much further away than that. But if you look back the way Venture Star came, you will see a compact constellation familiar to any amateur astronomer. That W shape is surely Cassiopeia, one of the most easily recognisable of our star figures. But there is an extra star to the left of the pattern, turning the constellation into a crude zigzag. That star is our sun: just a point of pale yellow light, bright, but not exceptionally so. And from where you stand, the sun, the Earth and all the planets, and all of human history before the first colonists left for Alpha Centauri, could be eclipsed by a grain of sand.

  Alpha Centauri, then, is a spectacular place. But the key question is: are there planets? Could Pandora actually exist?

  13

  FINDING NEW WORLDS

  In the Avatar universe the geography of the Alpha Centauri system has been worked out in some detail.

  All the three stars, Alpha Centauri A, B and C, have planets. Even C, the red dwarf, has a close-in gas giant and two rocky worlds. B has one gas giant and five rocky worlds, and an asteroid belt; B’s subsystem is perhaps most similar to our own solar system.

  A, the largest star, has three gas giants and three rocky worlds. Polyphemus is one of the gas giants, with similar size and mass to Saturn in our system, though without the rings. It orbits at about the same radius from Alpha A as Earth does from the sun—unlike Saturn, which is about nine times further out from the sun than Earth. Interestingly, rather like the Trojan asteroids in our solar system (see Chapter 6), two rocky bodies share Polyphemus’ orbit, at points of gravitational stability sixty degrees ahead of and behind the planet: one significant rocky world and one planetoid. Polyphemus has fourteen moons (compared to Saturn’s astounding sixty-two, at the latest count, of which seven are spherical). All these (fictional) bodies have names, by the way. All of them await explorations of the imagination, in movies, books and comics…

  The world we care most about is, of course, Pandora, fifth moon of Polyphemus.

  The larger moons, like Pandora, probably formed from the same swirl of debris that formed Polyphemus itself; the smaller ones may be captured asteroids. There are limits on where big moons might be found in relation to the primary world. Sensible spherical moons need to be outside the primary’s “Roche limit,” within which tidal effects are so strong they pull the moon apart; inside the Roche limit you may get shapeless asteroid-like lumps of rock, but not round worlds. The precise distance depends on the mass and rotation of the primary, and on the composition of the moon, but as a rule of thumb the Roche limit is around two and a half times the primary’s radius, measured from the planet’s centre. Thus Saturn’s innermost spherical moon Mimas is three Saturn radiuses out. You can see from the onscreen size of Polyphemus in Pandora’s sky that Pandora is safely out beyond the Roche limit. Some close-in moons of gas giants are “tidally locked,” so that they keep one face permanently set towards the primary, as the moon does to the Earth. This isn’t the case with Pandora; during its twenty-six-hour day Polyphemus rises and sets.

  In real life we’ve yet to detect any worlds of Alpha Centauri. But we have found an awful lot of worlds orbiting other stars.

  One of the true scientific miracles of my lifetime has been the discovery of “exoplanets,” indeed in some cases whole other solar systems. When I was a boy not a single planet beyond the sun’s family was known. Some scientists maintained there were no other worlds—that the solar system was a freak, a matter of chance. Now, at the time of writing, we know of more than four hundred other worlds. We’re starting to learn a good deal about the distribution of planets and planetary systems, and are coming up with new theories of planetary formation. And we have new ideas of how planets may be habitable, suitable for life, even if in some cases they are dramatically different from our own Earth. It’s certainly timely for Avatar, a movie of travel to alien worlds, to appear just now. Suddenly we see a sky full of Polyphemuses—and, maybe, Pandoras.

  The challenge of detecting worlds beyond our own is formidable, because planets are small and faint compared to their parent suns.

  Suppose we were studying the solar system from a planet of the star Altair, in the constellation of the eagle (Aquila), about seventeen light years awa
y. Even mighty Jupiter, the largest of the sun’s planets, would be lost in the sun’s glare. Jupiter’s apparent distance from the sun, from the point of view of an Altairean, would be only one-thousandth the width of a full moon seen from Earth, and its light, which is just reflected sunlight, only a billionth of the sun’s. It was once believed that you would need truly ginormous telescopes flying in space to resolve worlds like Jupiter out of the glare, let alone Earths, smaller, closer to the sun, even fainter. Not so.

  While there had been tentative observations of planets orbiting pulsars (small supernova remnants) since the 1980s, in 1995 the scientific world was startled by the first observation of a planet orbiting a star called 51 Pegasi, a “main sequence” star (that is, a star in the middle of its normal lifetime, like our sun). The discovery was made not with giant telescopes but with improved instruments, careful observation and a dash of ingenuity.

  An exoplanet is generally detected indirectly: not by observations of the planet itself, but by studying its effects on its parent star. The most productive technique to date has been the “radial velocity” method. As the planet orbits its star, the star itself is pulled out of position, just a little, and if some of this motion is towards or away from Earth you can detect it with a subtle shifting of the lines of the star’s light spectrum. This is the Doppler effect, the same phenomenon that causes the blue shift and red shift so familiar to hardened interstellar travellers like us. Alternatively there is the “transit” method. If the planet happens to pass across the face of its sun as seen from Earth—just like transits of Venus and Mercury, planets inside Earth’s orbit crossing the face of our sun—the dip in the star’s apparent brightness can be detected. Other techniques include using stars in the line of sight as gravitational “lenses.”