The science in Avatar
10/11/2020 ⋅ No comments
Avatar is a 2009 movie where the story takes place in 2154 on an exo-moon (Pandora) orbiting a gas-giant exo-planet (Polyphemus), both orbiting the star Alpha Centauri A, thus at 4,36 light-years from Earth.
Humans went there boarding space-ships in order to mine a very rare, useful — and fictional — element: unobtainium. The exo-moon itself is inhabited amoung others creatures by the Na’vi: a population of intelligent humanoid bipeds living in harmony with Pandora and its nature, and who do not welcome the humans very much.
The story relates the adventures of Jake Sully, a paralyzed man who’s conscience is transposed into a clone of a captured Na’vi. In his new body, he will infiltrate the Na’vi. In the end, he will have befriended them and he’ll stay on Pandora, keeping his new and functioning body, since his own is paralyzed.
As a whole, the movie does not really have many scientific flaws (except for the “USB-sex” part, I guess), and we can discuss some interesting details: as you’ll read, some of those phenomena might actually be realistic, at least conceptually.
Pandora, moon of it’s planet Polyphemus
Pandora orbits its host planet Polyphemus, it self orbiting its star Alpha-Centauri-A, which is, by the way, a real star that happens to be our own nearest neighboring star in the galaxy, and also one of the brightest (but it’s not always visible from the northern hemisphere).
I’m not convinced that we will be capable to go to Alpha-Centauri before the year 2154: in the movie, the passengers on the space-ship are maintained in a cryogenic sleep during a 5 years, 9 months and 22 days trip.
This allows us to deduce that they are traveling at about 75% the speed of light, which is very, very fast, and much faster than anything we are able to even dream of doing right now.
But after all, why not, I guess? Who in it’s right mind would have thought in 1850 that we would actually walk the Moon about a century later (except Jules Verne, maybe)?
One strange point about the gas-giant, Polyphemus, is that it’s resembling Jupiter very much, whereas the name “Pandora” is in reality the real name of a real moon orbiting… Saturn!
Maybe this is done on purpose, but it’s funny to notice.
This being said, let’s talk about Pandora itself. It resembles strangely to… Earth !
In itself this isn’t a concern, but we must remember that a gas giants like Jupiter or Polyphemus generates a tremendous gravitational pull. At such point that its closest moons would be knead and distorted by the tidal forces: the side of the moon facing the planet will always be closer to the planet than the opposite side, an therefore it will be tugged towards it, a bit like the oceans on Earth being pulled by the Moon, responsible for the tides, but an order of magnitude more strongly.
To see this, lets compare with an actual moon of Jupiter, Io, located about as close to Jupiter as the Moon is from Earth. Io is so knead and heated by those forces that it has more than 400 active volcanoes and therefore constitutes the most active celestial body of our entire solar system.
If I check in Celestia, an open-source stellarium software; positioning Io in a way it scales like Pandora in the movie, you can see that Jupiter is quite far away. This is to say that, in Avatar, Pandora would be even closer to Polyphemus than Io is to Jupiter.
Actually, there is a limit for how close a moon can be to its planet : the Roche Limit (named after Édouard Roche).
To illustrate this, imagine standing on the Moon, facing the Earth. The Moon has lower gravitation than the Earth, but since you are so much farther away from Earth, you remain on the Moon. Now bring the Moon closer to Earth: the pull of the moon stays the same, but the pull from Earth gets stronger and stronger as you get closer and closer.
There will be a tipping point at which, even having your feet on the Moon, the pull of Earth will begin the exceed the pull of the Moon. At that point you will begin to quit the Moon surface and fall towards Earth!
Applying this to a Moon closing in to its planet, there is a point — the Roche Limit — at which the cohesive forces of the moon's ground is no longer strong enough to counteract the attraction of the large body: the moon gets torn apart and splits itself into pieces gradually falling on the planet.
That is exactly why meteorites usually break into pieces right after entering the Earth’s atmosphere.
Now, if we say that Pandora has a density of Earth and Polyphemus that of Saturn (a little lower than Jupiter's density), then the Roche Limit here is 73'509 km (45'676 miles).
Using Celestia, and if I scale Jupiter to look as big as Polyphemus in the movie, the distance at which Pandora orbits Polyphemus is… 56'000 km (34'804 miles).
In other words, Pandora could not exist: she would already be torn apart by the tidal forces of its host planet.
One could argue that the planet only looks close the Pandora due to the optics used to film the movie. If that is the case, the tidal forces would still be more than enough to induce intense volcanic activity in Pandora. After all, there is an order of 5 between my calculated distance between Polyphemus and Pandora, and the real distance between Jupiter and Io.
To conclude on this part: Pandora should look more like a volcanic hell covered in lava than to a green heaven.
The flying mountains “Hallelujah”
It is said that those mountains are levitating thanks to the unobtainium that compose them, and acting as a superconductor.
This isn’t unrealistic, except for the part where all known superconductors on Earth right now have to be cooled very much in order to keep displaying their superconducting properties.
Superconductors are substances that have no electrical resistance: current can flow indefinitely without any looses. That said, current produces a magnetic field, than can oppose an existing magnetic field and thus float above a magnet.
The trick of superconductors is that they sort of “freeze” onto any existing magnetic fields. This is due to some of the magnetic lines to slip into the superconductor, inducing current vortices. By remaining in place, due to the absence of resistivity, those vortices trap the magnetic lines, like a series of threads or branches frozen in a lump of ice.
When a superconductor encounters a magnet, they become locked one onto another and at a distance: the magnet can levitate in a stable state above a superconductor.
Whereas a magnet always ends up flipping itself when levitating above another magnet:
On Pandora, if those mountains are superconductors, we can guesstimate the intensity of the magnetic field in the mountains, based on the fact that the magnetic force has to counteract the local gravity.
If one of those mountains measures something like 100m × 50m × 50m, with an earth-like density, and considering the gravity on Pandora like 80% of Earths, we find that the magnetic fields is around 2,33 teslas in that region.
This doesn’t sound like a lot, but 2,33 tesla is actually enormous: it’s about 52'000 times the earths magnetic field.
Needless to say that in those areas, any communication device would be jammed, that any metallic planes would instantly crash on the ground, and even Sully's wheelchair would sink into the ground. Even guns and knives would irremediably be crushed to the ground with a force of several tons.
Where does that magnetic field come from?
Our Earth's magnetic field is produced in the Earth's inner-body: a solid iron core suspended in a liquid molted iron mantle. The rotation of the solid core in the liquid mantle generates and sustains a magnetic field thanks to what is called the dynamo effect (and — just so you can show off — the study of magnetic fields in liquids and gaseous phases is called magneto-hydrodynamic).
It is unlikely for a small rocky planet to create a magnetic field 52'000 times stronger that Earths on its own.
In the universe, the strongest magnetic fields are produced by magnetars: rapidly rotating neutron stars. Their magnetic field is estimated to be around several billion teslas, and they emit X-rays and high energy gamma-rays. Life near those stars would be impossible due to the magnetic field on its own but also those X and gamma radiation. Anything put near one of those stars would be vaporized in a few seconds.
Let us get back to Pandora: where could such a strong magnetic field come from, if it’s so unlikely for its inner-core to produce it?
One can suppose Polyphemus to be the culprit for that. In out solar system, Jupiter produces a very strong magnetic field.
This, by the way, is responsible for the formation of plasmic currents between one of its moon, Io, and Jupiter’s poles.
Jupiter and Io creates what is knows as Io’s plasmic torus. Jupiter's rotation is four times faster than the Io's rotation around Jupiter. As such, Jupiter's rotating magnetic field scraps about 1'000 kilos (~2'000 pounds) of sulfur and oxygen ions from Io's atmosphere every single second. Those ions are accelerated to match Jupiter's rotation speed, and they end up forming a giant donut shaped (a torus that is) ion rich region around Jupiter.
The interaction of this torus and Io now forms an enormous 400'000 volt potential differential with Jupiter itself, inducing a gigantic ionic current of several million amperes following Jupiter's magnetic fields from Io to the gas giant's poles.
It is estimated that those phenomena have a power of around 2,5 terawatt.
(image — The plasma torus (in red) and the ionic current between Io and Jupiter (in green))
If Pandora develops something similar, it might be that this ionic current induces a very strong magnetic field in return.
I seriously doubt that this hits our 2,33 tesla estimate, and Pandora's sky would be incredible colorful due to the auroras, but it would be one plausible explanation.
It is also possible that the moon itself produces an atmospheric layer protecting all life on Pandora from those ionic currents and magnetic fields.
This is actually what happens on Earth, where our planet protects us from the deadly cosmic rays and the Sun's ultraviolet radiation.
We can imagine a similar process taking place on Pandora, protecting all life forms and making mountains levitate.
… this doesn’t explain why the planes are not crashing, though.
Bio- and Tribo-luminescent plants
One of the specificity on Pandora I particularly liked, were the luminescent plants. The jungles should be amazing to see…
Here on Earth, we also have light emitting plants and animals. The glow-worm is one very common example.
Above that, phosphorescence (storing the incoming light during the day and continuing to emit it slowly all night long) is also something we have here on earth, especially in some rocks and minerals.
Components that include some form of phosphor often display this behavior (hence the name “phosphorescence”). Some more exotic and artificial compounds (like the so called Kryptonite), that include rare earths like strontium, dysprosium or europium exhibit this phenomenon much more strongly, glowing up to twelve hours after it has been exposed to natural light.
This behavior solely depends on the chemical compounds. Some projects aim to make artificially phosphorescent indoor plants by making the plant synthesize those compounds itself. Some phosphorescent bunnies have also been made using GMO's: we successfully transplanted genes of luminous algae to a bunny's fur, making a “glow in the dark” bunny.
On Pandora, some plants only become luminous when we are walking on them.
One can see this as light being emitted when a compound is mechanically distorted. This is known as triboluminescence. Here, deforming the compounds actually break some high energy crystalline bonds, and the released energy is then emitted in the form of visible light.
It is not absurd for a plant to be able to evolve such mechanism, and this is also something we encounter here on Earth. Some algae do emit light when the waves wash them on the shore.
This movie, apart from some scientific freedom that the creators granted themselves to make the story possible (like the metallic planes that do not crash when entering a strong magnetic field), is a concentrate of small amounts of real science: telluric magnetic fields, superconductivity, bioluminescence…
I haven’t spoken about transporting one’s consciousness into another body: I consider this to be the “fiction” part of “science-fiction”, and have no problem with this. This blog is not about judging a movie about it’s scientific accuracy: it’s merely about telling how some parts of the movie could actually be very so real!
There is some research done about artificial intelligence and supposedly transporting some existing conscience into a machine, but it is still very, primitive, if we can say it is at all. In fact, to this day, we still struggle to find a real definition of intelligence and consciousness…
Sciences are amazing, but nowadays, all the different research fields (biology, technology, physics, astronomy…) have merged and we find this back in arts, as in this sort of movies. This is most interesting, but it can also be quite frightening (just watch Minority Report, or even the Matrix).