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## The Avengers, and Some Ideas for Successfully Invading Earth

We’re the leaders of an alien race. We’re planning to invade and exterminate/subjugate humankind. Here is a good plan for invading Earth, as detailed in the movie “Independence Day”.

• Have our central mothership parked well away from Earth. Humans don’t really have weapons to attack targets in space. Have a large (in the millions) invasion force on standby in the mothership.
• Given Earth’s well-spread military and civilian targets, bring along 36 smaller (but still city-sized) ships to send to strategic locations. Arm these ships with one, massive, city-levelling weapon. Make sure ships can withstand nuclear weapons. Have the mothership power a defensive shield around each smaller ship.
• Disguise ships as strange atmospheric phenomena as they get into position. Don’t show aggression too early.
• Fire main weapon on all ships simultaneously.
• Should humans attempt to fight back with fighter jets, send our fighters to engage. We don’t really need to do this, but decimating their feeble air force will underscore our obvious technological superiority.
• Continue using smaller ships to destroy cities and military targets. Once human population and military largely decimated, send in invasion force from mothership.

That is a pretty good plan. In fact, it’s so good that the aliens would have to do something really stupid, or the humans something utterly miraculous, for the plan to fail. (Spoiler alert) That miraculous thing turns out to be Jeff Goldblum, who is able to plug his laptop into the alien spaceship and – without as much as a text editor in sight – program a computer virus able to disable the shields of the entire fleet. That, and the fact that the main weapon of the smaller ships is prone to backfire, and the Earth is saved.

Now let’s go over the plan devised by the aliens in “The Avengers”.

• Open a single portal from our planet to Earth, over New York City.
• First, send through about 100 soldiers on small flying bikes. No need for a fully-enclosed fighter. Equip these bikes with a weapon capable of destroying a small sedan. Instruct the soldiers to fire at civilians and civilian traffic.
• Give the soldiers a spear capable of firing a laser beam. Encourage soldiers to eschew said laser beam in favour of hand-to-hand combat. Laser beams are expensive.
• Follow up with a large flying creature. Give the creature a thick armour, but no weapon apart from its bulk and mouth. It will fly alongside high-rise buildings, breaking windows and disfiguring the architecture with its fins. It will also serve as troop transports. It holds around 100 soldiers. Configure creature to shoot soldiers onto the outside of the human skyscrapers, because we’re pretty sure that’s where human beings hang out.
• When supervillain Loki commands: “Send the rest!”, send through all the creatures we have. We seem to have about 10. Maybe 20. That’s an invasion force of about 2000 soldiers. Our soldiers can be taken down with a bow and arrow, so hopefully the (7 billion) humans don’t have any of those. Or handguns. Or know how to karate chop the back of our necks.
• The bikes and flying creatures can travel at about 100 km/h. Once we have captured New York, proceed to Beijing. Hopefully the other independent sovereign nations of Earth will not have coordinated a military response in the 4-5 days it will take us to get there.
• Park our mothership just on the other side of the portal. Don’t worry about anything coming back through the portal. The mothership shouldn’t need shields of any kind, or the ability to shoot down incoming missiles.
• We possess no weapons remotely comparable to a nuclear weapon. We have no shields capable of withstanding a nuclear blast. I confidently predict that there is no need to worry about this at all.
• Have all soldiers, bikes and creatures remotely powered by the mothership. If the mothership goes down, or if the portal is temporarily closed, our entire invasion army will drop like a sack of potatoes. All the more reason not to worry about defending our end of the portal or the mothership itself.
That, obviously, is a pretty feeble plan. The movie hints that the invading force has underestimated the humans. That’s a major lack of intelligence, especially from Loki, who has witnessed both US defence research facilities and the S.H.I.E.L.D. flying fortress.

The writers of alien invasion movies face a massive problem: almost certainly, an alien invasion would be a massacre, a relatively minor extermination program. Wars between human beings are only close because our technology has progressed in step. An alien invasion would be like the modern American military attacking ancient Egypt. There isn’t any dramatic tension. To make an interesting plot, the aliens must be rather stupid (like overlooking the problem of Earth’s microorganisms and diseases), or cocky, or drastically misinformed, or else the humans must be extremely lucky and/or well-equipped.

That said, I very much enjoyed The Avengers, and add my personal approval to the other 97% of critics. I quite liked Independence Day as well. I suppose the only alien invasion stories worth making into a movie are the ones where we, somehow, win.

## James Jeans and our finite universe(?)

“Leave only three wasps alive in the whole of Europe and the air of Europe will still be more crowded with wasps than space is with stars, at any rate in those parts of the universe with which we are acquainted.”

I love a good illustration.

For whatever reason, I’m drawn to old popular-level science books. I just finished reading “The Stars in Their Courses” by James Jeans, first published in 1931. Jeans is best known in my field for the “Jeans length”. Suppose a cloud of gas is trying to collapse under its own gravity, but is being held back by gas pressure. Jeans showed that there is a critical length scale, such that if the object is smaller than the Jeans length then pressure wins and the cloud is stable, but if it is larger then gravity wins and collapse ensues.

Jeans gives an overview of all of the astronomy of his day. It’s mostly familiar material, of course; the interesting bit is the glimpse inside the mind of the great scientist. Here’s a neat illustration:

“If we could take an ordinary shilling out of our pocket, and heat it up to the temperature of the sun’s centre [40 million kelvin], its heat would shrivel up every living thing within thousands of miles of it.”

Repeating this calculation, I think Jeans is reasoning as follows. A shilling is about 5 grams of copper (specific heat capacity 0.385 J/gram/kelvin), and so at 40,000,000 K we have about $8 \times 10^7$ J of energy. This is ‘only’ 20 kg of TNT – most bombs are at least a tonne of TNT equivalent, and they don’t do miles of damage. That much energy could raise the temperature of the surrounding air to boiling point for about a 10 metre radius. Not too promising. However, the coin will be emitting thermal radiation at x-ray wavelengths. A lethal dose of x-rays is about 5 J/kg, so our coin has enough energy to kill about 100,000 people. One must factor in the fraction of energy emitted horizontally, the fraction absorbed by biological material, the cooling of the coin, etc, but certainly it’s a very dangerous coin.

Jeans’ views on cosmology are very revealing. He is writing within 5 years of the discovery of the expansion of the universe by Lemaitre (first!) and Hubble. Jeans says: (more…)