Stephen Baxter - The Science of Avatar

There are plenty of subtleties beyond this simple argument. Even given their height the Na’vi look remarkably slender—narrower bones mean higher pressure—but, as we’ll see in Chapter 25, their bones are strengthened by a naturally occurring carbon fibre.

And we should remember that the Na’vi didn’t have to be as tall as they are. No animal has to grow as large as the laws of physics allow it to. The Na’vi’s apparent close relative, the prolemuris, is no more than a metre and a half tall, just as on Earth our hominid ancestors were all chimp-sized until the emergence of Homo erectus , about as tall as us, a couple of million years ago. The Na’vi are as tall as they are because something in their evolutionary history made it right for them to be so. However their height does illustrate that a human body form that would be impossibly tall and slender on Earth can work on Pandora.

What are the limits? How big could a land animal grow on Pandora?

Pandoran beasts are big. Even the direhorse is larger than any horse on Earth. The heaviest living land animal on Earth is the African elephant; a bull can stand some four metres tall at the shoulder. The heaviest animal of all was the brachiosaurus which died out some hundred and thirty million years ago, and stood around seven metres tall at the shoulder. The largest land animal we see on Pandora in Avatar is probably the hammer-head titanothere at maybe six metres tall—like an elephant scaled up in Pandora’s gravity field. Perhaps greater beasts roam in parts of Pandora yet unexplored.

Pandora’s low gravity would help you fly, especially with the aid of that thick air. On Titan, the air is so thick and the gravity so low that a human could fly by the power of her own muscles, flapping artificial wings. So we could have predicted big flying animals on Pandora.

Earth’s largest flying creature was the winged reptile Pteranodon ingens , which flew over Kansas some eighty million years ago, with a wingspan of around nine metres. On Pandora a mountain banshee exceeds that at around twelve metres wingspan, and a leonopteryx would dwarf it, with a wingspan of thirty metres. The size a flying creature could reach depends on other factors than gravity, such as the density of the air and the oxygen content—the more oxygen, the more energy you have available to keep you aloft.

On Earth you have to look to the sea for the real monsters in size. The blue whale is thought to be the heaviest animal ever to have existed, weighing in at some hundred and ninety tonnes (compared to around five tonnes for an African elephant). If we visit Pandora’s oceans in the future, there will be monsters, I have no doubt.

And what of the tremendous trees of Pandora?

On Earth, the basic physical constraint on tree height is the need for the tree to be able to lift water to its uppermost leaves. The tallest known tree on Earth is a sequoia in northern California, at a hundred and sixteen metres tall. The theory says that a tree could possibly reach as much as a hundred and thirty metres—and there have been historical accounts of trees a hundred and twenty metres tall. By comparison Hometree on Pandora is some three hundred metres tall, nearly three times the size of that big old sequoia. This is more than the simple gravity scaling might suggest, but Hometree evidently has a different architecture from a sequoia, with pillar-like multiple trunks, themselves as sturdy as sequoias, enclosing a large internal hollow.

Pandora’s low gravity would enable some wistful architectural designs: impossibly long arches, impossibly slender columns. We don’t see any native architecture on Pandora; with the hometrees available for habitation I suppose building is unnecessary. And the humans at Hell’s Gate show no imagination in their own functional building schemes. Maybe the Stone Arches are a glimpse of what would be possible.

But in fact the Stone Arches seem to be a product of the single most remarkable physical phenomenon on Pandora: its unobtanium, and the magnetic fields with which it is associated. And if you followed Jake Sully to Pandora you would very quickly learn that unobtanium is the reason you, and RDA, are here.

What is it about unobtanium that makes it so valuable?

Unobtanium is a room temperature superconductor—we’ll find out later what that means. On Parker Selfridge’s desk we see demonstrated one of its apparently magical properties, that a chunk of it can float in the air, defying gravity, over what looks like a magnet. Unobtanium has shaped Pandora’s geology. It is unobtanium’s gravity-defying properties that hold up the floating Hallelujah Mountains. When Jake climbs the “stairway to heaven” on his way to Iknimaya , his mountain-banshee challenge, you can see what look like lumps of rock embedded in the roots and tendrils, straining to rise like trapped balloons, boulders presumably laced with unobtanium.

But the real value of unobtanium lies in its superconducting properties, which have led to a new industrial revolution on Earth, including the building of Venture Star- class starships—and generating vast profits in the process.

Is all this fanciful?

The very name “unobtanium” suggests that we’re dealing with impossible physics. According to science-fiction archivist David Langford, the word is an engineer’s in-joke dating from the middle of the twentieth century, applied to any ideal substance you need to achieve the impossible—frictionless bearings, for example. The word “unobtanium” was actually formally defined in the U.S. Air Force University’s Interim Glossary of 1958 as “a substance having the exact high test properties required for a piece of hardware or other item of use, but not obtainable whether because it theoretically cannot exist or because technology is insufficiently advanced to produce it.” The word has been used in science fiction before, for instance in David Brin’s 1983 novel Startide Rising . Cameron has suggested that maybe the discoverers of unobtanium on Pandora adapted the old tongue-in-cheek name as a joke for this magical stuff, and it stuck.

But in fact there may be nothing unobtainable about unobtanium. Superconductivity is a real property. And a superconductor really can defy gravity, at least in the presence of a magnetic field.

As the name suggests, a superconductor is a material that is a “super” conductor of electricity—so super, in fact, that unlike common conductors like copper wire, it conducts with virtually no resistance at all . This means that no electrical energy is wasted in heating up the conductor, and the current could apparently run for ever, without losses.

This seemingly impossible property was first discovered by accident, as a consequence of research into low temperature physics.

In 1908 the Dutch scientist Kamerlingh Onnes was the first experimenter to turn the gas helium into a liquid. Whereas water liquefies from steam at a hundred degrees centigrade, to liquefy helium you need to reach the astoundingly low temperature of just four degrees above absolute zero—around two hundred and seventy degrees below zero centigrade. Having achieved his liquid helium Onnes tried dunking familiar materials in it, just to see what happened. (Well, you would, wouldn’t you?) And he discovered that in certain pure metals, as they cooled down, electrical resistivity suddenly switched off—or at least, it dropped to values too low to measure.

The industrial applications of such a substance are startling. You could run extremely high currents, for instance to power the very strong electromagnets needed by fusion reactors and starship antimatter traps, without the fear of heat damaging your apparatus. Low-loss power transmission lines are another possibility. Heat produced by electrical resistance is a problem in computers, forcing a limit to how much connectivity you can jam into a finite space—the smaller your computer is physically, the faster it can operate. With superconductivity there would be no heat limitations, in principle.