A recap: in the original 1951 version of the film, mankind is beginning to tinker with space rockets and H-bombs. Humanity had been under observation by a galaxy-spanning civilization prior to this; they had been satisfied to leave us to our own devices prior to this, but now that we might pose a threat, an emissary was sent to give a message to Earth. The message was simple:
The aliens don’t care what we do, so long as we don’t spread out troubles off-world. If we grow up, we can join the larger civilization; if we don’t, they’re perfectly capable and willing to simply erase us. To display their power, alien technomagic is used to shut down all human electrical systems across the planet, except for things like hospitals and airplanes. This is a temporary blackout to bop humanity upside the head. Power comes back on, alien flies off with a “don’t be a dumbass” message, credits roll.
In the 1951 original, the aliens are kinda dicks (lets face it: humanity armed with gigaton-yield H-bombs and 10,000-ton Orion battleships poses precisely no threat to the aliens given their vast technological superiority), but their motives are understandable and they demonstrate an unwillingness to kill unnecessarily.
But then there’s the 2008 remake.
In the remake, the aliens aren’t here to deliver a message. That task was left up to the film makers: the remake isn’t a warning about militarism and nuclear power, but rather about how evil and stupid humanity is for trashing the environment. The alien plan is to go straight to wiping out humanity. The reason? There are extremely few life-bearing planets in the universe, humans have messed up this one, so we are to be wiped out to let terrestrial life get on without us. The mechanism of the apocalypse is rather confounding, however. The aliens have a form of very small self-replicating robots that eat anything and make more of themselves. Not quite nanotechnology (the robots seem to be about the size of gnats), but the result is still a “gray goo” where cars and buildings and people and trees are converted into more little robots. Given the abilities of the aliens, you’d expect that the robots would eat all the people an all our stuff, but no: the robots are going to eat *everything.* The plan is to shave the entire biosphere down to bedrock, stripping the world of all life. The aliens would then re-seed the Earth with selected species that they have spirited away to safety in a few little “arks.”
Yeah. It really is that dumb. In order to save the Earth, they are going to wipe it out and start again, and just sorta hope that the limited lifeforms will be able to successfully take root and restore the planet.
So in 1951, the aliens are a bit dickish. In 2008… they are monsters. Worse, they’re *dumb* monsters. Given their power, you’d think they’d ignore Earth and use their gray goo to terraform Mars and Venus… turn the Venusian atmosphere into a vast collection of carbon based fusion engines that can not only speed up the planets rotation to give it a reasonable day, but also move it away from the sun a bit. Shoot a whole lot of stuff from Venus to Mars. Build similar fusion engines on Europa and move it out of Jovian orbit and shove it in-system. Wander past Mars while chucking a good fraction of the water ice onto Mars to give it a real hydrosphere. Drop Europa into Venusian orbit, transfer more water to the planet. In the end, turn three dead worlds into three living worlds.
But… no. Because life is so rare and precious, the aliens are going to kill all life on Earth because they’re annoyed with one species.
Anyway, what actually happens in the movie? Well, the roboswarm is unleashed on the East coast (Virgina, I believe) and begins to eat its way to New York City. It’s shown converting a football stadium, trucks on the highway and other human-occupied vehicles and structures into more little robots. The swarm gets to Central Park and starts eating everything when the aliens change their minds, shut the swarm off, and take off. As they go, more technomagic used to shut down human machinery across the entire planet. Roll credits.
So, what happens next? Well, it can go two ways, based on your assumptions about that shutdown magic: is it a short-term phenomenon, lasting maybe a few hours, or is it permanent? As memory serves, some mention is given that this might be a permanent issue. So, let’s go with that.
What all shuts down? Clearly, all electrical and electronic systems shut down. But it also appears that non-electrical systems also stop. What looks like an old-school mechanical wristwatch is shown to have stopped. An oil refinery is shown shutting down… including a “flare” that simply stops burning. It would seem that the alien technomagic is really quite powerful.
So, mankind loses all out stuff. Given how bloodthirsty these aliens are, you can assume that the hospitals also shut down. Everyone currently opened up on operating tables can be assumed to be dead in a few moments. Airliners soon begin dropping from the sky. Crews of submarines suffocate. Crew and passengers on ships at sea start dying within a few days.
There is no food transport, so cities quickly devolve into horror zones. Of course, on the East Coast of the US, entire counties have simply ceased to exist, replaced with bedrock covered with a layer several yards thick of dead micro-robots. On the edge of that field of destruction would be horrors straight out of Lovecraft: people and animals consumed to greater or lesser degrees stuck in buildings converted to Swiss cheese. Survivors have to deal not only with the ragged edges of eaten-off bits, but also their bodies being full of multitudes of sharp-edged dead microbots.
Blood and screams everywhere.
Chemical factories and storage facilities on the edge of the swarm are toxic nightmares since the pipes and tanks have been partially consumed, letting fun stuff like hydrofluoric acid and the like spill out. Oil refineries and storage tanks are apocalyptic nightmares. Any nuclear powerplants that have been partially consumed can be expected to melt down in short order.
Longer term: a massive crash in human population. If the alien technomagic continues long-term, anything more advanced than steam power – and perhaps not even that – won’t work. This means tractors. No modern fertilizers. No food processing facilities. So never mind the trouble in transporting food… there won’t *be* any food.
The aliens ill-conceived mission was to save terrestrial life from the ravages of mankind. But after the power goes out… a lot of species would find themselves on the verge of extinction. Take a major city like Tokyo. Even though they will have no news from the outside world – (the last anyone heard from the TV, the aliens had unleashed some sort of nightmare in the US and now the power is down and even flashlights don’t work) – the people of Tokyo will, after a few days, largely come to understand that rescue ain’t coming. Tens of millions of people wholly dependent upon technology that no longer works would strip the city bare of processed food in a few days. And once all the canned veggies and packaged ramen and dehydrated kaiju has been consumed, there will still be millions of people who need to eat. People will start flooding out into the countryside seeking food. And what will they find? Farms. Unless the farmers are able to defend their farms – an unlikely prospect – the starving hordes will consume all the critters. Farms and ranches are able to support humanity because the critters are harvested in a rational, sustainable fashion, but in a world without power, people will slaughter what the find, when they find it.
Cows. Sheep. Pigs. Dogs. Cats. Horses. Deer. Chickens. All will soon be on the edge of extinction, and several will very likely go over the edge.
And then winter sets in.
You want to stay warm? Guess what you won’t be using: heating oil, electricity, even coal. All gone. So if you want warmth, that means burning stuff. What will there be to burn? Well, lumber and paper and building supplies and building, at least for a while. But how many home are equipped with proper fireplaces or wood burning stoves these days? If the cities haven’t burned to the ground, you can bet that a whole bunch of them will explode come winter. Forests will be chopped down just as fast as human labor can handle it.
And then summer comes… with no air conditioning or electric fans or water. Three cheers for heat stroke and cholera!
And all those nuclear reactors, sitting idle with no power to run their cooling systems? Meltdowns. Lots of ’em. With absolutely no way to deal with them, they will explode, burst into flame and remain glowing radioactive sores for the duration.
So, in short: the population crashes 90, 95% in the first year or so. What about long term?
Human society will be forever mutilated. Many regions will be in the dark about what happened, with perhaps not even a rumor about aliens. All they’ll know is that the power went out and they can’t get it back on. Most of the planet will know something about the aliens, and will be able to put two and two together. Much of the industrialized world will know a fair deal about the aliens, including the fact that an alien weapon was in the process of consuming the eastern seaboard when the lights went out. Their descendants will tell tales of the demons from the sky that came down and ended the Golden Age. Humans will *hate* and fear the aliens and incorporate that into new myths and religions.
An important point was that Klaatu the alien told a human woman and child what it was all about, about how humanity needed to get its act together. But this message is almost certain to not spread. The two are left alone in the meddle of Central Park, surrounded by a New York City that has been largely devastated by the roboswarm. It’s by no means certain that these two will even live to meet another human. Manhattan has been trashed; there’s likely nobody else left on the island. the bridges are probably eaten away, the tunnels flooded. Unless there are boats, they’re probably stuck there, on an island with no buildings that are not structurally compromised, with a good chance of no access to food and clean water. Additionally, they are surrounded by many square miles of the dead microbots. These little beasts are built like insects… insects composed of sharp edges and blades. When the wind blows, the air will be filled with microscopic razor blades. These two survivors will probably drown in their own blood by the end of the day as every breath brings in more little razors to chop up their lungs. So the aliens message will be lost on humanity: aliens showed up, butchered tens of millions of Americans, and then vanished.
Humans are reasonably clever. It’s possible that technology will survive here and there… perhaps the technomagic doesn’t work underwater or in deeply buried facilities. Perhaps the technomagic can be countered: one of the first things people will try is surrounding simple electrical devices with Faraday cages. Perhaps that will work, or some other countermeasure will function. If so, human society might survive in some recognizable, albeit limited, form. And if so, preparing mankind for the return of the aliens will be the main concern after simple survival. So if the aliens show up again a century or two later to see how things are going, to see if humanity has turned into nature freaks living at one with Mother Gaia, they might find a largely depopulated world stripped of much of it’s forests, with vast swathes of radioactive ruin, bristling with weapons – nukes, lasers, missiles, etc. – shielded against the alien technomagic. If the aliens unleash another roboswarm, humanity may well have discovered a way to simply shut it off. But even if so, the aliens are sure to still be far in advance of mankind; the fight will still be decidedly one-sided. But int he process, the planet will be wholly trashed.
In short: Aliens come to save life on Earth, because life is so rare and precious. In the end, they ruin the surface of the planet because they (in actuality, the writers) are monumentally dimwitted.