At present, antimatter costs about $60 billion per microgram with an available supply measured in nanograms and a correspondingly short shelf life.  So, assuming antimatter / fusion technology exists, it's still a little iffy whether such fuel could actually beat the cost of gasoline.

Just storing antimatter would be a huge challenge. If a particle of antimatter bumps into a particle of matter, the two annihilate each other and turn into pure energy. In theory, antimatter could be stored in a magnetic field but that still leaves the problem of keeping all particles of ordinarily matter completely separated from it. Most likely, some amount of storage deterioration would have to be tolerated. However, the product of deterioration would be gamma radiation and a considerable amount of heat, yet another sticky problem to solve.

Keep in mind, that the spacecraft depicted in the movie is around a mile long, hence, the amount of antimatter required for making the trip would be on the order of thousands of tons. Even if the antimatter fusion hybrid engine beat the cost of gasoline by a factor of 1000, the cost of transporting a kilogram of cargo one way between Earth and Pandora would still be millions of dollars. The cost of a one-way ticket for a human could easily top a quarter of a billion dollars.

As for the local human security force or militia, most of their armament, especially the heavier stuff, would have to be manufactured on Pandora. Otherwise, the costs would be prohibitive again due to shipping expense. A commonplace assault rifle shipped from Earth could easily rival the cost of a battle tank on Earth. Ammunition would also need to be locally produced or the 3 seconds required to empty a typical assault rifle on fully automatic fire could easily cost a million dollars.

Aside from lower cost, the big advantage of locally produced ammunition would be optimization for local conditions. Since game laws or Geneva convention restrictions would not apply, rifle bullets could contain neurotoxins, explosives, or any other feature needed to deal with either the local blue guys or big beasts.

To an elephant hunter in the early 1900s an AK-47 assault rifle cartridge (2,000 joules of muzzle energy) would seem dangerously inadequate compared to the premier elephant rifle cartridge of the era, the .600 nitro express (10,300 joules of muzzle energy). Yet in modern times, the AK-47, most likely fired in fully automatic mode, seems to be the firearm of choice for elephant poachers. Poachers have used such weapons to kill entire families of elephants at the same time. It's hard to believe that the miniguns, automatic cannons, and assault rifles of even 2009 would be incapable of taking out a dinosaur-like beast the size of an elephant.

Strangely, the Avatar militia doesn't seem all that better equipped than conventional 2009 military. Where are the super-capable drone aircraft and robots? Why are the infantrymen not wearing invisible camouflage? Why don't they seem to have any mature versions of the smart bullets or extreme accuracy guided bullets currently under development? In the future, accuracy and kill rates for individual bullets will likely be much higher. Hence, it may not be necessary to spray the jungle with dozens of bullets for each attacker killed. With nanobot technology and magnetorheological fluid body armor along with fibers such as m5 polymer that's superior to the Kevlar currently used in body armor, it seems like infantry would be essentially immune to arrows, even Navi ones.

On Pandora, we once again see military guys tromping around in Amplified Mobility Platform suits (AMP) apparently borrowed from Matrix movies. However, at least these suits have enclosed cockpits, offering some protection to their operators. Against modern weapons, AMPs with their high profile and lack of heavy armor plating would be easily detected and knocked out, but on Pandora, they make tactical sense. They could be designed to be arrow resistant and used to transport heavy weapons into places where normal vehicles would bog down.

While such suits might be conceivable, it's unlikely that they could move with the agility depicted in the movie just from a mass to strength ratio (see the chapter on scaling factors in our book). Make a device 2 times taller than a human out of the same material with the same proportions and it will take 8 times as much energy for the device to jump as high as a human can. Make the device 3 or more times wider with metal construction and the energy requirements of jumping get unmanageably high--the same reason sumo wrestlers don't play basketball. Such a device could greatly exceed human capacity for picking up heavy objects but would be ponderous in a fist fight or wrestling match.

Still, the main plot device in the movie is not the now stereotypical AMPs, it's the Avatars. These are remotely controlled 10 ft tall artificial blue guys grown using a combination of human and Navi DNA. The human controlling the Avatar resides in a special chamber located some distance away but has the illusion of actually occupying the Avatar's body. Aside from the complexities of mixing DNA and creating an Avatar, there's going to be a major wireless communication challenge not to mention bandwidth problem. The Avatar is going to have to transmit super high resolution video from two eyes, stereo audio, smell, taste, and balance data not to mention the touch, pain, and hot/cold sensations from thousands of sensors in the Avatar's skin. In return the Avatar is going to have to receive perfectly coordinated control signals for hundreds of different muscles along with audio output.

Just imagine what would happen if service were interrupted even briefly during a fight or when an avatar was leaping from vine to vine across a chasm. Imagine what it would be like to converse with an avatar while having even slight transmission delays or breakups. The senor data and control signals would need to be transmitted at a real time rates with enough redundancy and error correction to insure almost 100% reliability.

In an environment largely free of other electromagnetic (EM) noise and transmissions, the communications might be possible if facilitated by the equivalent of a cell-phone-like network of hidden towers and orbiting satellites to relay the signals and prevent them from being blocked by things like mountains. If an Avatar went in a cave he would need to spread smart dust along the way to form a bread crumb-like ad hock wireless network capable of relaying the Avatar's required control and sensor information. In fact, marking territory with smart dust would likely become a regular Avatar ritual. But, what about communication in the floating mountains?

Although they were visually stunning the floating mountains were arguably the most ridiculous features in the entire movie. They were gigantic chunks of real-estate floating in the sky apparently suspended  by some form of magnetic field. We surmised this from the fact that electronic instruments on all aircraft in the vicinity are driven crazy by the area's "high flux level", presumably a high magnetic flux level. Magnetic fields are also about the only available explanation. Other possibilities such as buoyancy, wind, and electrostatic forces are even more easily ruled out.

Unfortunately, the magnetic field strength needed to levitate mountains would have numerous disastrous side effects on the movie's story line, that is if the movie stuck to real physics. First such a field would rip magnetic materials out of aircraft if they flew into it, but that's just the beginning. Even if the aircraft had no magnetic materials in them, moving a conductive material of any type--magnetic or otherwise--through the field would create a large voltage difference across it. All electronic systems or controls, not just the navigation systems would be seriously disrupted. It's doubtful that an aircraft could fly in such conditions.

Since both the human brain and heart are conductive, both would have voltage differences generated across them by the motion of riding in an aircraft. Any variation in direction or magnitude of the magnetic field, even subtle changes from vibrations or body movements would induce random currents in the brain with an endless list of possible results including memory loss, hallucinations, psychotic events, and seizures. Randomly induced currents in the heart could result in fibrillation and death.

Strong carefully controlled magnetic fields are used for inducing currents in the brain as a treatment for depression. By comparison, the currents induce by flying around in a magnetic field strong enough to levitate mountains would be orders of magnitude larger and more random in nature. However, the floating mountains aren't the only questionable features on Pandora.

To its credit, the movie lets us know that gravity conditions are lower on Pandora than on Earth. This helps make the 10 ft height of the blue guys a little more feasible. Otherwise their ability to jump around would be reduced. Their hearts and circulatory systems would also need to be much more robust just to overcome the blood pressure differences between their head and feet, a problem that might otherwise limit their life spans. However, the effects of reduced gravity apparently don't apply to humans. We don't see them gaining any increased mobility.

The blue guy's slender build helps reduce their inertia, again a critical factor in the ability to make quick movements. Inertia comes in two forms: linear inertia, directly proportional to mass and rotational inertia, directly proportional to mass and the square of the distance of the mass from the center of rotation. Creatures with high inertia will have difficulty moving fast. Since inertia is independent of gravity, reducing gravity does not help reduce the effects of high inertia.

An arm twice as long as normal will have 8 times as much linear inertia but 32 times as much rotational inertia assuming it has the same proportions and is made from the same materials. Making the longer arm thinner would definitely help. Slender 10 ft high blue guys would be able to move faster than heavy-set blue guys.

Of course the blue guys have oversized eyes with large pupils because the moviemakers know that audiences respond favorably to large eyes with dilated pupils. Such large eyes and pupil dilatations would work quite well in a dimly lit nighttime environment where practically all the vegetation phosphoresces but would be over-designed in bright daylight. Blue guys would probably not want to look directly at the sun for risk of frying their retina. On the other hand, maybe the daytime star light is not so intense on Pandora.

The prevalence of six-legged beasts on Pandora is hard to explain from a natural selection standpoint. How would an extra set of legs impart a survival advantage? How would multiple nostrils at the base of their necks be helpful? It seems like rain water would run down their necks and collect in the nostrils.

Why would the blue guys be bipedal, have only four limbs, and human type nostrils if evolved from six-legged neck-nostrilled ancestors? Furthermore, why would a diversity of Pandorian life-forms including the Navi have a tentacle-like appendage designed for directly connecting to and sharing information with the brains of other life-forms? How could all these neurological connector organs be compatible, unless the diversity of life-forms had the remarkably similar DNA structures required to produce such organs and the brain structure needed to use them? Again, such connectivity implies common ancestry that doesn't otherwise seem to be reflected in the Navi.

We also find it puzzling that a race of sentient beings like the Navi, who supposedly live in perfect harmony with nature would have warriors. In fact, if they were so harmonious and interconnected, why would they even know about warfare?

Nevertheless, after a particularly egregious incident, the blue guys, whipped into a frenzy by ex-marine Jake Sully (Sam Worthington) in an Avatar's body, have had it with the humans and prepare to drive them off Pandora. There's no lengthy discussion or thought given to a workable strategy. It's just let's round up all the tribes and make a single decisive frontal attack. Considering his military training, Sully should know better. Occasionally, a force with a vast numerically superiority has wiped out a much smaller but better armed force.  Certainly, it happened to Custer at the Little Big Horn, but such rare defeats have usually required foolhardy behavior from the losing side's commander.

On Pandora the human commander Colonel Miles Quaritch (Stephen Lang) is no such fool. Realizing that the Navi have started massing for an attack, he decides to blow up their sacred tree of life in a preemptive airborne strike before they can reach overwhelming numbers. He correctly determines that blowing up the tree of life would demoralize his enemy and send them packing, permanently ending the human/Navi conflict. According to the Colonel, his plan is the only way to avoid being overrun and wiped out.

Maybe so, but we would point out that a dug in force armed with modern automatic weapons, rapid fire cannons, and air support against a mass attack of warriors armed bows and arrows is not the same thing as Custer's disastrous cavalry charge. Custer's men were primarily armed with single shot rifles and were caught in the open

Colonel Quaritch knows about the tree of life thanks to Jake Sully who in Avatar form has previously gained the confidence of the Navi only to act as a quisling who passes vital information to the humans. Sully eventually discovers that running around the forest as a blue guy is actually pretty cool. He finds romance, has a change of heart, and with a group of human co-conspirators steals his Avatar along with the equipment needed to drive it so that he can return to the Navi.

Here's the problem: Sully would also need to have control of the cell-phone-like network required for maintaining the Avatar's control signals. Even if the corporation didn't fully control the network, what's to prevent them from wire-tapping into it and reading Sully's thought's as well as seeing everything he sees.  Yes, the information could be encrypted, by why would an evil corporation spend billions on a system with encryption that it couldn't decrypt? Even if that were the case, why couldn't they jam the transmissions or simply destroy the network? In fact, why would they even need Sully to gain information. Even in 2009 the U. S. military is already developing insect sized crawling and flying robots capable of spying on enemies. Smart dust prototypes are already about 5 mm on a side and cost around a dollar to make. A corporation capable of creating an Avatar could easily use advanced versions of these technologies to spy at will on a primitive culture like the Navi.

For some reason, while preparing to blow up the tree of life, Colonel Quaritch also to decides mount a land assault. The Navi meet the ground force head-on with a cavalry charge, a well known form of suicide since at least 1914. And guess what? They get creamed. Meanwhile other blue guys mounted atop flying critters swoop down on the aircraft from above.

Aircraft designed for ground assaults could indeed be vulnerable from above. These aircraft used high speed counter-rotating propellers contained in large circular shaped pods. Shooting one of the Navi's spear-like arrows or dropping big rocks into the rotor blades should have severely damaged them and disabled the aircraft. In addition to performing such attacks from the flying critters, in at least some places, the Navi could have hidden archers atop the floating mountains and rained arrows down on the aircraft attempting to pass between them.

Instead, the Navi riding the flying critters mostly shot arrows at the pilots sitting behind transparent canopies. Fortunately, for the blue guys, the canopies proved to be strangely under-designed for arrow resistance. In addition, the flying critters were able in several instances to grasp and fling some of the smaller-sized aircraft into nearby objects. The problem was that there just weren't enough warriors on flying critters available to handle all the aircraft.

To the movie's credit, when the blue guys learned an attack was imminent, they didn't use the usual lets-build-a-cleverly-designed-fortress-in-twenty-minutes strategy so often used in similar movie situations. The movie realistically portrayed the situation: the blue guys didn't have a lot of options and would have been crushed were it not for the fact that at just the right moment, when all was about to be lost, the tree of life sent hoards of critters to the rescue.

After the battle, the blue guys have a big celebration and send the surviving remnant of evil humans back to Earth, but why? Do they really think these people will be grateful and not bad mouth Sully as a terrorist or the Navi as soulless assassins? Do they not think the survivors returning to Earth will whip up popular support for a return invasion? And if an invasion force does return would it again fool around with aircraft or ground assaults when it could remain in orbit and rain warheads or herbicides down on the tree of life while infecting the Navi with designer viruses? We'll have to wait for the inevitable sequel to see if the invasion is deadly effective or just another version of Custer's last Stand. Meanwhile, save your 3D glasses. Judging from early box office numbers on Avatar, you're going to be using them a lot in the future.