Tuesday, December 22, 2009

The science of Avatar

A very nice post


Copernicus’ Law of Science Fiction: Bending the laws of
physics out of service to the story is fine, doing it out of ignorance
is unconscionable.

I don’t mind if the ships in Star Trek can go faster than the speed of
light – otherwise the story would be pretty boring. And I know
there’s no sound in space, but I want Star Destroyers to
rumble, and the Millennium Falcon to have that iconic whine. But if a
director casually gets science wrong for no real reason other than
that he is stupid or lazy (see ARMAGEDDON, THE CORE, and THE DAY AFTER
TOMORROW, to name a few), then to hell with him. If the filmmakers
don’t respect the intelligence of the audience, I’m not going to
respect the movie.

Fortunately, James Cameron has a knack for science that rivals his
moviemaking skills.


Historically, movie directors have had their asses kicked by
astronomers as far as taking us to exotic worlds. For the most part,
movie planets look like an extreme form of Earth -- they almost always
have an oxygen atmosphere at an Earthlike pressure and gravity. Movie
planets don’t even come close to matching the diversity of worlds in
our solar system: the surface of Io is a mottled, sulfurous
orange-yellow, constantly being repaved by volcanoes shooting hundreds
of miles into the sky. Titan has a thick smog atmosphere that blots
out the sun and rains hydrocarbons. Mars has planet-wide dust storms
and a 17-mile-high volcano that nearly reaches above the atmosphere.
Venus has a crushing, choking sulfur dioxide atmosphere with a
pressure 92 times that of earth, and a temperature that can melt lead.
Enceladus shoots ice geysers into space. And the real Pandora orbits
within the rings of Saturn. These are only a few of the hundreds of
planets, minor planets, and moons in our solar system: we’ve
discovered hundreds elsewhere in the galaxy, some of which seem even
crazier: super-Earths, nearly boiling puffed-up Jupiters, and objects
that may be free-floating rogue planets without a star.
So I can’t think of a better use for 3d and a few hundred million
dollars of effects than filmmakers starting to raise the bar to
finally approach the awesome reality of nature. Due to the limits of
budgets, finances, and creativity, I can’t think of another film that
has attempted something near the scale of what Cameron has done here.

I’ll address the different aspects of the science in sections.


From a visual perspective, Avatar’s Pandora is breathtaking. While
most movies have only hinted at the exotic nature of their worlds with
an establishing matte painting or two, here Cameron takes us on an
elaborate three-dimensional tour though various habitats, from the
treetops to the forest floor. He’s created a whole ecosystem, from
semi-intelligent trees to giant land and air creatures. Most seem
inter-related via symbiotic relationships. In fact, Cameron has taken
the Gaia hypothesis, that the biosphere of the Earth is itself a kind
of living entity, and sexed it up – the biosphere of Pandora is
essentially a god, and it’s networked! Creatures can plug into each
other via what amounts to USB hair and fiber optic roots. While some
of these ideas are not without their faults (see below), Cameron gets
points for creativity – this is true science fiction, not space opera.
I do have one minor complaint, that given their networking abilities,
the Na’vi should not be so technologically inferior to the humans. On
Earth, the largest barrier to technological progression was that
information that existed in the brains of primitive humans could not
be easily shared or preserved. As soon as writing was developed,
suddenly it was possible to store information outside of the brain,
and record and build upon knowledge. The knowledge available to a
human or tribe went from one brain’s worth (and a minimal amount of
oral tradition), to thousands, and ultimately billions of brains’
worth. The result was a technological and social explosion. Hominids
have had technology like spears for about half a million years, but
only 7,000 years after the development of writing we had left the
planet. And the sharing of knowledge is still undergoing a revolution
with the development of the internet. Now we have instantaneous
access to the combined knowledge of the entire history of humanity.

Since the Na’vi have had the ability to download information and share
it in a massive network for long periods of time (evolutionary
timescales), they should be way ahead of us in terms of technological
development. Still, I have to give Cameron a pass here. It is
thematically necessary that the Na’vi are technologically primitive,
and their root-network is necessary to the plot. Maybe you could say
that they could have evolved more technology, but they don’t need it
or want it. Still, that reeks of the “Noble savage” idea, and I have
to agree with Stephen Pinker that that is a bunch of hoo-ha.

But my major complaint from an evolutionary standpoint is that there
is no way in hell that life on Pandora would evolve to look so similar
to Earth life: there are humanoids, space horseys, hammerhead
rhinoceri, and pseudo-pterodactyl beasties. And to make it worse,
they have DNA, and the DNA is close enough to our own that Na’vi and
human DNA can be combined! Again, I have to give Cameron a pass.
First, it is easier for the audience to relate to familiar things.
And more than that there is a significant plot point that I won’t
spoil towards the end of the film that hinges on humans and Na’vi
having similar DNA.

One way out of both my evolutionary nitpicks is the panspermia
hypothesis -- that life in the galaxy was seeded in multiple places by
an advanced civilization. But even then the odds against evolution
producing such similar animals on different planets is astronomical.
Since we have a clear record of evolution on Earth, some civilization
would have had to keep taking specimens from earth, first
pterodactyls, and ultimately humans (after they evolved), and then
would have had to deliver them to Pandora, possibly modified via
genetic engineering. That would be an interesting sequel: humans and
Na’vi come together to confront their godlike humanoid ancestors!

Grade on astrobiology: A for the scale of the ecosystem, C for being
too much like Earth – call it a B overall.


Pandora is a moon of Polyphemus, a fictional gas giant orbiting Alpha
Centauri A. I’ve always wanted to know what the view would be from
the moon of a gas giant. Can you imagine a quarter of the sky being
taken up by a massive cloud-covered planet visible night or day? We
get to see it in Avatar, and since Jupiter is the king of the gods,
maybe majestic is an appropriate word to describe it. I wonder if
Cameron’s choice to set this on the moon of a gas giant wasn’t a slap
in the face to Lucas, as if to say “this is RETURN OF THE JEDI done
right.” (I know it is ambiguous in the Star Wars universe whether or
not Endor orbits a gas giant.)
But what had me really geeking out is the choice of the star system.
Alpha Centauri A is perfect. First, as the closest star system to the
sun (4.37 light years), it may well be the first star we travel to.
Second, it is familiar in that you can see it with the naked eye if
you live in the southern hemisphere – it is the brightest star in
Centaurus. Actually, what appears to be a single star can be resolved
as a binary system if you use a telescope. It is Alpha Centauri A, a
bit more massive than the sun (1.1 solar masses), and Alpha Centauri
B, a bit less massive than the sun (0.9 solar masses). The choice of
G-type stars near the mass of the sun is great – they last for
billions of years – plenty of time for life to evolve. They are in an
elliptical orbit around a common center of mass, which means they come
together and drift apart over the course of one 80 year orbit. The
two stars get as close as 11 astronomical units (an AU is the average
Earth-Sun distance; 11 AU is about the distance to Saturn), and get as
far apart as 36 AU (about the distance to Pluto).

Would you see the companion star (Alpha Cen B) in the sky from
Pandora? That depends on where it is in its orbit. At the farthest
distance it would be a few hundred times the brightness of the full
Moon as seen from Earth. But your eyes are logarithmic detectors, so
it would actually only seem a few times brighter than we perceive the
Moon. At its closest approach, Alpha Cen B would be a few thousand
times as bright as we see our Moon. This is not all that bright – in
comparison, on Earth the Sun is about half a million times brighter
than the Moon. So on Pandora, if Alpha Cen B is up in the daytime
then you might not even notice it, depending on how far away it is in
the sky from Alpha Cen A. But if it is up at night (as it would be
for half the year), it would never get completely dark – the sky would
just be kind of dark blue.

Technically, there is a third star in the system, Proxima Centauri,
but it is a tiny red dwarf a huge distance, about 12,000 AU, away – it
is not even clear it is bound to the system. At any rate, it would
not be prominent in the sky as seen from Pandora. Incidentally, my
first job as a graduate student was to help calibrate the fine
guidance sensors on the Hubble Space Telescope to help my advisor look
for planets around Proxima Centauri. Sadly, we didn’t find any.

It is an interesting question as to whether planets around either
Alpha Cen A or B could exist in stable orbits that would last for
billions of years. You might think they couldn’t because the gravity
of the other star would perturb any forming planet.
However, simulations show that at least at Earth-like distances,
stable planets can form in that system.

Grade for astronomy: for the choice of star system, setting in on a
moon, and around a gas giant, Cameron gets an A+.


Electromagnetic radiation comes in many forms, gamma rays, x-rays,
ultraviolet, visual, infrared, and radio. Our eyes evolved to see in
the narrow range that the sun has its peak output -- the visual band
-- and the flora and fauna of Earth evolved pigments and colors that
work at these wavelengths. But this isn’t universal -- some animals
can see a narrower region of the spectrum than us, and others see
farther into the ultraviolet or infrared. Our cornea blocks most UV
light, but bees, for example, don’t have one and can see farther into
the UV. They can see patterns in flowers that we can’t.
In fact, colors are really something manufactured in our brain –
physically colors are just different wavelengths of light ranging
uniformly from short wavelengths (violet) to long (red). What we see
as blue or green or red helps us differentiate sky from grass from
blood, but to a creature from another world, all these things might
appear as the same color. In fact, you could imagine that bats might
use echolocation to “see” rough surfaces as one color and smooth
surfaces as another. So since colors are something created by our
brains and not intrinsic to the universe (only wavelengths of light
are), it is virtually certain Pandorans would see color differently
than we do.

Alpha Cen A has almost the same temperature as the Sun, but it is just
a bit hotter. As a result, the star puts out most of its light at
visual wavelengths just like the Sun. But the star’s output is only
part of the story – the oxygen and ozone in our atmosphere block much
of the ultraviolet light from the Sun, and water vapor blocks some of
the infrared light. Pandora doesn’t have an oxygen atmosphere (if the
movie mentioned what gasses it contains, I didn’t catch it), so we
might expect more of the ultraviolet light to reach the surface. The
creatures there might be able to see farther into the ultraviolet.
There might be all kinds of patterns that the inhabitants of Pandora
can see that just look blue to us. Maybe that’s which there are so
many blue colors in the film. To take this a step farther, I would
have loved to see a scene where a character sees beautiful colors or
patterns as an Avatar, only to have this beauty evaporate into a
uniform sea of blue when he sees the same vista with human eyes.

Another feature of Pandora adding to the ubiquitous shades of blue is
that bioluminescence seems to be a staple of the ecosystem. As
Massawyrm points out, this makes sense for a world that may spend days at a time
shrouded in darkness. Remember that a day occurs when Pandora rotates
on its axis. But it might take a month or so to orbit its gas giant,
which we know looms large in the sky, and could blot out the sun for

Grade for the astrophysics: For the fact that this world doesn’t have
an oxygen atmosphere, and the plausible use of color, A.


Since Pandora is a moon and is presumably smaller than the Earth, the
gravity would be lower. This is alluded to in the film, and creatures
do grow larger and survive falls from greater heights than you could
on Earth. I wonder if Cameron dialed in a different gravity to the
physics engine rendering everything. To my eye, for at least the
human scenes, the gravity looked just like Earth gravity, but then
again if the gravity is close the differences can be subtle.
Virtually all science fiction movies feature planets with gravity at
1g, since, of course, until now, filming has always been done on
Earth. Since here so much of the world was created inside the
computer, I would have liked to see this aspect pushed a bit farther.
In one of my biggest pet peeves regarding the science of Avatar, there
is one scene where the gas giant, Polyphemus, can clearly be seen to
be rotating in the span of about a second or two. Let’s say it
rotates about a degree out of 360 degrees in those 2 seconds. That
means it makes one rotation in 720 seconds, or 12 minutes! Jupiter
takes about 10 hours to rotate. So the gas giant in Avatar rotates
about 50 times faster than Jupiter. Winds on Jupiter can exceed 100
meters per second, so the winds on Polyphemus would have to exceed
5000 m/s – this is supersonic and clearly implausible. Here’s one
case where Cameron opted for visual effect over realism, but to me the
bargain isn’t worth it. It looks unrealistic and takes me right out
of the movie. But I do like the look of the clouds on Polyphemus –
they look like a cross between Neptune and Jupiter. The highlight is
a giant storm resembling Jupiter’s Great Red Spot. That is
particularly appropriate for Polyphemus, named after a mythological

But my biggest beef in Cameron’s trading physics for visuals is those
goddamn floating mountains. Seriously, floating mountains? How the
hell do they stay up there? This is such an egregious flouting of the
laws of physics that surely there is some reasoning behind it.

Between the fact that Pandora seems to be sort-of at 1g, the
impossible rotation of Polyphemus, and the floating mountains, physics
is one one area AVATAR gets a marginal fail on Copernicus’ Law of
Science Fiction. But on all the other aspects of science, Cameron
gets either a pass or passes with flying colors.

The dream of interstellar travel will only become a reality far beyond
our lifetimes. But I love the fact that today I can be deeply
immersed in not just a plausible, but a compelling alien world just by
putting on a pair of 3D glasses and visiting my local theater. Even
if I have to drive 100 miles to see it in IMAX, that is nothing
compared to interstellar distances! And I love that there is a
filmmaker that plays more than lip service to the science in his
films, stimulating discussion and thought about distant worlds among
geeks everywhere. I was inspired to do astronomy after seeing STAR
WARS as a kid. I’m willing to bet that a fair fraction of tomorrow’s
astronomers will have decided to devote their life to the discovery of
new worlds because of AVATAR.

Mail Copernicus
-- Copernicus

Thanks for the enthusiastic response to the article. Thanks for all
the emails, and it is nice to see some interesting discussion in the
talkbacks (who knew?). A few updates:

People have sent links to several sources that explain many of the
questions I had. One is the pandorapedia. Another is A
Confidential Report on the Biological and Social History of
. And people sent the script treatment. I have not read
the entirety of that last one yet, but the first two are interesting

Levitating mountains: As dozens of people have pointed out, the
mountains supposedly contain unobtainium, a room-temperature
superconductor. Superconductors expel magnetic field lines, and as a
result magnets can levitate above a superconductor. Here
superconducting mountains are apparently levitating over the strong
magnetic field of the moon or planet, or both. I had thought about
some kind of mechanism like that but dismissed it for two reasons:
(1) how could mountains form, stay in place, be weathered and shaped,
etc. (2) if there is unobtanium in the floating mountains, why not get
it there so as not disrupt the Na'vi. But I think I was just
short-sighted. In the case of (1), the intention is that the
mountains started out attached, but broke off and floated upwards at a
certain point, and now they sort of float around. I buy that, at
least enough for a cool movie scene. And for (2), maybe the
unobtanium in the mountains isn't the right kind, or isn't pure, or is
hard to mine. Interestingly, a geologist emailed me with another
sighting indicating the strong magnetic field of the planet: the stone
arches seen at the climax seem to be from mineral growth along
magnetic field lines. Awesome.

Plenty of people have asked where the water comes from for the
waterfalls in the floating mountains. To me, it is just like a normal
mountain, with the bottom missing. Where does the water come form in
normal mountains with waterfalls: rain and snow. Yes there was tons
of water, but have you ever been to Yosemite in the spring? When the
snow melts it all comes down at once, and it is an impressive sight.

And this is a little out of the purview of this article, but plenty of
people have also asked why the humans didn't nuke the planet from
orbit. (A) maybe they didn't bring any -- the proverbial "somebody's
gotta go back and get a shitload of dimes" problem, and (B) you people
scare me! Why doesn't the US just nuke countries we don't like? That
is not cool, man.

Planet rotation: Someone affiliated with the film who asked me not to
name them (but who ought to know) says the planet rotation scene was
intended to be time lapse. Brilliant. I'll have to see it again to
confirm that, but I'd buy it, because I think there were other shots
where the planet didn't seem to be rotating quickly.

Oxygen atmosphere: I said that Pandora doesn't have an oxygen
atmosphere, but I was wrong about that -- it does, but it has other
gasses that are poisonous to humans. Clearly Cameron, a fellow diving
and deep sea enthusiast, thought of this, because the people only need
simple gas masks to breathe, and not huge oxygen tanks.

DNA: The Pandorapedia says the Avatars don't have DNA, just something
analogous so that you can map to it. Great! Although, from my
memory, the movie implies they do have DNA. I'd need to see it again
to be sure. Maybe the character explaining it knows as much about DNA
as most people do and just got it wrong. If I had a nickel for every
time I was at a partly and someone told me that they heard that "they"
(meaning scientists) have broken the speed of light, teleported
something, etc.

Interstellar travel: From the pandorapedia: "Mission Profile: 0.46
year initial acceleration @ 1.5 g to reach 0.7 c; 5.83 years cruise @
0.7 c; 0.46 year deceleration; 1 year loiter in orbit around Pandora;
Mission Duration: 6.75 + 1.0 + 6.75 = 14.5 Earth years. However,
relativistic effects shorten the time onboard ship to slightly less
than 6 years each way."

Hmm, I don't think that calculation is quite right, but it is close
enough. To see, let's take the special relativistic part, the
cruising speed. If ET is Earth Time, ST is Ship Time, v is velocity,
and c is the speed of light, then ET=ST/SQRT(1-v^2/c^2). So
ST=5.83*SQRT(1-0.7^2)=4.1 years for the cruising. Even if you assume
there is no time dilation on the accelerating and decelerating parts,
then the trip is only 5 years, not 6. Maybe they are including the
hanging out on Pandora time. To do the calculation correctly I'd have
to drop some GR on you bitches, and I'm too lazy and you'd be bored.
70% of the speed of light is a good figure though -- it is almost
plausible! From what I've read of the ship technologies, they sound
very well thought out too.

Eyes: One of those sources mentioned that some of the creatures have
two pairs of eyes -- one visual, and one that sees in the IR for
nighttime hunting. Sweet! This is not without precedent. We have
two separately evolved "circuits" for vision in our brains -- one
primitive automatic one and another one for conscious sight. Look up
"blindsight," where people with damage to the latter circuit can't
consciously see, but can catch a ball. And of course we have two
types of cells for day / night vision in our eyes: cones that allow
you to see color when there is plenty of light, and rods that allow
you to see black and white only, but give you night vision. Try this:
put an eye patch on while you are inside for about 30 minutes, then go out where it is dark and blink between your dark and light adapted eyes. You can really see the color difference. It is awesome.

Wow, it appears that many of my nitpicks about the science were actually taken into account by the filmmakers and there are answers. I'm impressed! Hats off to Cameron and company for getting all this right. I can't wait to use this film in my introductory astronomy classes.

there's a related post here


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