Why do aliens who visit us have higher tech than us? Couldn't they wait for us to have it too?

Question

It is a well known fact that if aliens invaded, it would be our puny kill-one-person-then-reload-for-half-a-second-or-more-firearms against their super-cool-ultra-mega-thingy-laser-mumble-quantum-force-unobtanium-whatever-ray-guns. We might also have some inferior reverse-engineered-tech-that-the-government-has-been-working-on-since-that-flying-saucer-crashed-and-nobody-noticed-and-it-was-all-hushed-up.

Why are we always so inferior (other than the obvious need for there to be no chance for us to win) to them, or to put it another way, why are they superior to us?

According to some theories, life is so easy to create, that all of it should have appeared at roughly the same time. So, we have had as much time as them to develop our war machines.

Yet they are 2-3 times further ahead than us (we haven't even started at FTL, yet they cross the galaxy regularly to expand their empire). Why?

Answer 1

TL;DR: In order to interstellar travel you have to have the best technology possible

Space is hard and evil

Not necessarily evil towards you but just ... simply evil to anything and anyone. Even the smallest imperfection on a space ship traveling for a decade can cause huge problems. You must have technology good enough to build a ship that can carry life over interstellar distances

Space travel is hard (and boring)

Take for example the Mars mission. We are seriously discussing how to visit another planet in our solar system and according to some on-Earth and outside-Earth experiments, we know that it is not going to be easy.

One of ways to perform interstellar travel would be to increase speed. But that also needs advanced technology.

So, this planet can bear life. But is it safe?

Imagine that the best alien telescopes discovered our planet and this is the best that they can see:

Image source: Wikipedia

Imagine you have the technology for interstellar travel. You have engines and ships. You solved all the tiny-winey problems about how to get there. However, for whatever reason you simply don`t know what is going to await you there. Even if you are the most peaceful species in universe, you will most probably also pack your lasers and ion cannons, simply because you do not want to get destroyed by rocks and sticks.

We are not packing any weapons to Mars, because we know it is empty. But I doubt that any travel to another star would be without weapons.

Answer 2

We think we are advanced but still barely able to send devices within the gravitational influence of our star. We managed to send living samples of our species on our satellite, but just for few days and then we discarded the blueprints.

Now an alien race is visiting us, showing that they are able to mass travel trhough space. I would say that the mere fact that they show up on our door is a consequence of their more advanced technology.

You don't ride your bike until you are able to do so.

Answer 3

By definition they have to have higher level tech just to be able to reach us in the first place! We can barely get unmanned probes to the outer solar system and that takes years. We've had a bare handful of humans walking on our own moon and even that was decades ago. None of us has ever walked on another planet.

Any alien species able to reach us here has to be more advanced, in propulsion and life support if nothing else.

For something set further in the future this doesn't always have to be the case, but certainly for any sort of "alien invasion" scenario it does.

So to answer your original question - different life will always evolve and develop at different speeds. Look at how much we advanced in the past century compared to the millennia before. Imagine if the dinosaurs had developed tool using intelligence, they could have been flying to the stars millions of years ago.

The more advanced and aggressive species will naturally spread further combined by the fact that a species less advanced than us cannot even reach us and the conclusion is pretty much pre-ordained.

Answer 4

I think that others have covered the simple truth - aliens must be more advanced to have come into contact with us - pretty well.

As for the reason for how they had the chance to get advanced, just consider humanity.

• Life has been around on Earth for billions of years.
• Vertebrates have been around for hundreds of millions of years
• Humanity has existed for hundreds of thousands of years
• In the modern world, 100 years is long enough for technology to be utterly obsolete in most cases

Now assume that there's a 0.01% difference in the progression of vertebrate life-evolution-civilisation-technology on another planet. 0.01% equates to tens of thousands of years - it's bronze- or stone-age technology vs ICBMs, satellites and fifth-generation fighters.

In fact, much better to look at it this way - the chances of there being another civilisation at roughly the same technology level as ours are incredibly tiny. We'd easily expect them to be thousands of years different, but they could well be millions of years ahead of us (or behind, of course - but then we'd have to go to them).

Humanity hasn't really evolved along a nice, linear timeline from single-cell to high-functioning tool use; there's no particular reason that we couldn't have evolved hundreds of millions of years ago instead, and been millions of years earlier on the timeline.

Answer 5

Summarizing, mostly: we are an extremely young technological civilization, so anything that finds us should statistically be a lot more advanced.

Suppose life appeared in many planets at roughly the same time and also evolved toward intelligence and interstellar travel capability at roughly the same rate. There have been primates for about 50 million years. Suppose in an almost-exact copy of Earth a species had evolved from the first primate at a rate 0.01% faster than ours. That would mean a 5000-year head start by now. These alternative Homo sapiens would have been developing nuclear weapons and landing on their Moon at aroud the same time our ancestors were building Stonehenge. And the whole scenario is of course wildly unrealistic.

The idea that "life is easy" is misleading. Life might appear easily, but there's no guarantee that it will endure, and in any case most experts think that the predominant kind of life in the universe must be microscopic. If intelligent life were so common, we'd be finding signs of it all over (see Fermi paradox) unless all intelligent life, by an incredible coincidence, was just a bit less advanced than us or had a bad case of paranoia that led it to avoid all contact with the rest of the universe and to erase all traces of its presence.

Also, neither biological evolution nor technological progress work linearly or predictably. It makes no sense speaking of an alien species being n times more advanced. How do you measure that? There are things like the Kardashev scale, for sure, that attempt to measure technological advancement by isolating one variable (in this case, the amount of energy employed for communication), but that's it.

There's no guarantee that, if FTL is possible (to name one of the key technologies needed to go out and conquer SPACE!!!), then all technological species will be able to discover it. It may be that FTL depends on mastering states of matter only readily found near the cores of giant planets. It may be that you can only construct an FTL engine near a neutron star or black hole, and only civilizations living close to one of them will ever make the jump.

In many commercial science-fiction scenarios you have advanced aliens laying waste to Earth (or to human colonies) thanks to superior technology, and Earthlings doing their best to resist and ultimately fighting back. In more realistic scenarios humanity would not only not have a chance, but probably wouldn't even know what hit it. If you can master FTL travel for big ships, or even NAFAL interstellar travel, then you can produce, control and direct huge amounts of energy: more than enough to reduce Earth's biosphere to ash from a good distance.

Answer 6

As other have pointed out, aliens must have MUCH higher tech just to get here.

Then, someone said that doesn't mean they brought weaponry, or have more advanced weapons.

I don't think that's accurate. When you can manipulate energy the way a star-faring race can, your everyday items are weaponry to us.

As an example, take a common Caterpillar bulldozer. They're everywhere. Say your construction brigade landed on a planet of medieval human types. A common bulldozer could pull down any castle. It could apply the muscle of 1,000 men, easily. The point is that a common appliance for us would seem like a magical mechanical dragon to them. A monstrous weapon indeed.

Similarly, an alien cigar lighter could probably melt the Pentagon.

Answer 7

This isn't always true. Star Wars and Star Trek are both extremely famous SF worlds where Humans often meet civilizations far behind them in technology (and sometimes ahead). There are many other examples.

In fact, the idea that aliens marginally more advanced than us would show up in our civilization is exceedingly unlikely.

Interstellar civilization is a relativistic phenomina. If you can colonize other stars, the amount of time it takes to colonize a galaxy a blink of cosmic eye, even with slower than light ships.

Our primitive Voyager travels at 62000 km/h. At that rate it takes a mere 1.8 billion years to cross the Galaxy.

If we had a civilization that sent "civilization seeds" (generation ships, star wisps, whatever) at Voyager speeds, and the new civilization took 100,000 years to mature to the point where they could send out a similar seed to two other stars within 10 light years, the net velocity of the probe would drop a mere 1/3.

After even a handful of doublings, you'd run short of stars within 10 light years. If you can send the ships further, you do.

There are 100 billion stars in the Milky Way. This is a mere 36 doublings. At 300,000 years per generation, that is 11 million years.

The limit becomes the speed of the colony ships, not producing them, very quickly.

It takes 10% of the lifetime of the galaxy to cross the galaxy at voyager speeds. Getting a star wisp up to faster speeds is not that hard with todays technology.

The hard part becomes surviving the cold eons between stars, stopping at the other end, and sending enough resources to start a new civilization (even a machine one) at the other side.

If the star wisps move 10x faster, at 0.0005c, they cross the galaxy in 200 million years.

If we go science fantasy and go FTL, the effect is even larger.

Now, why does this imply there aren't more advanced civilization? Because the speed at which they can travel means that the time between them reaching the stars, and them being here, is short compared to the history of life on Earth.

Even a small difference in how they develop technologically would make a huge difference in the scale of their civilization. If they developed 1% earlier, they'd already be here. If they developed 1% later, we'd reach their planet in the equivalent of the dinosaur era. The ridiculous level of precision for them to be anywhere near us in civilization scale is not practical without some quite recent galaxy-wide synchornizing event.

These don't make great stories.

Now, if you want a story with humans that are "like us" and you still want interstellar civilization, the easy way is to give the aliens that civilization. This naturally places them far away from us.

Answer 8

Even accepting your premise of equal starting times (stars are constantly being formed, ours is supposed to be 3rd a generation) there is plenty of room for optimization in our route to the stars. What if we didn't waste the first couple billion years of life being single celled? We appear to be the descendants of survivors of four major mass extinctions, were all of those really necessary?

It seems the answer to those questions are tied up with the speed of evolution and I know far too little about that to guess if it could be other than it is for us. But how much time do you really need to cut out to leave us on the knife end of a gun fight?

Think about if the United States made war on some tribe in the Amazon. We can't be much more than fifty thousand years ahead of the most isolated tribes since we both walked out of Africa around then. Less than 1% of 1% since cells decided not to go it alone.

Answer 9

It is a well known fact that if aliens invaded, it would be our puny kill-one-person-then-reload-for-half-a-second-or-more-firearms against their super-cool-ultra-mega-thingy-laser-mumble-quantum-force-unobtanium-whatever-ray-guns.

It's not a fact. We have 1-kill/1-reload weapons because they're what we need on Earth right now, but when the aliens turn up, we upgrade to chainguns mounted on exosuits with supplementary rocket launchers.

When they fly in with their interstellar spaceships, we fight back with ours and destroy them before they get to Earth.

Asimov's Green Patches has humans flying to an alien planet with an alien life form capable of mind control; reading the pilot's mind about Earth, it sneaks onto the ship and disguises itself as a section of wire in the control board. By the time the ship returns to Earth, the alien is poised to invade ... 'If the stowaway manages to reach Earth, it will eventually convert all life there into a single organism with a unified consciousness — and green patches of fur instead of eyes', but the section of control board it chose is related to landing and when that engages the creature is electrocuted - the sneak invasion attempt happened without the aliens developing space travel and is countered because of our superior technology. (Thanks to Henning Makholm for the story identification I couldn't remember)

When the Vogon constructor fleet flew in we had no way to fight back at all - they didn't quite invade, but they did wipe us out, guns weren't involved, the power difference was too great.

When the Independence Day aliens flew in, there was gunfighting but we properly fought back with superior computer technology:

When the aliens invaded in the Midwich Cuckoos:- 'The Russian town was recently "accidentally" destroyed by the Soviet government, using an "atomic cannon" from a range of 50–60 miles.' - they got nuked.

When MorningLightMountain comes for the Commonwealth:- 'the small human resistance that exists on what remains of the Commonwealth worlds attacked by the Primes. Human resistance forces have found two ways to fight back: using the Prime weapons (primarily directed-energy weapons) against the invaders, and disrupting communication between the slave caste (motiles) and the commanding caste (immotiles) of the Primes. Meanwhile, the humans in the remaining Commonwealth pursue other plans: to develop a set of weapons and warships to defend against the next Prime invasion and force the conflict back into Prime space;' - humans aren't fighting with our weapons of the time, we're fighting with stolen weapons of theirs to keep them away while we build better weapons.

Andromeda Strain - "An accidental invasion by an extraterrestrial microbe that almost instantly clots human blood or causes insanity."

Day of the Triffids is a biological invasion where a) they have no tech weapons, guns work fine against them, and b) so do flamethrowers.

WorldWar appears to have aliens arriving with similar level of technology due to a mistake in information collection, and humans fighting back with equivalent guns and also nukes.

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In summary, as other people have said, if you want to tell a story where the aliens invade Earth, they necessarily must have the technology to get here. Yet we don't want Earth invaded so if we possibly can, we will try to intercept in space far away to neutralize it before it gets here, so that puts an upper-bound on our technology level as well. Not only must they be able to get here, we must not be able to get usefully off the ground.

And if you want the invasion to look like humanoid soldiers which current day humans can fight against, the aliens must be similar to us - if they are invincible robot humanoids, no appreciable fight can happen and humans will lose. If the invasion is a spec of nanomatter which turns the Earth into grey goo, humans will lose. Yet if they are humanoid, they need portable, powerful weapons allowing a small attack force to take on a planet - so they must be more powerful weapons.

Another factor is if you want to make it a relatable war story, it's basically going to look like World War II. No easily relatable and exciting war story is about how NSA cryptographers analyze alien radio signals with a deep-learning neural network to find a padding-oracle attack on the encryption used to authenticate the soldier with the directed energy weapon and then asking the populace to download an Android app which reuses smartphone wifi radios to interfere with it and the alien guns stop working. How dull would that be?

And again, right now humans use 2017 level technology to fight - we don't nuke each other because Earth is too small and life is too fragile for nuke-levels of energy and radiation. We're not developing more powerful weapons, because we don't need them. So when the aliens arrive, we're unprepared for bigger and more powerful weapons. we're developing more precise weapons which move humans away from the battlefield instead - a future fighting force won't have phased plasma rifles in the 40W range, and wasteful suppression fire, they will have 1-shot/1-kill personal self-targetting sniper/cruise missile launchers. Probably with the humans far away.

But other stories do exist - where the invasion is sneaky rather than ships full of soldiers, where the humans have higher technology but it's not a technological invasion, where the humans fight back with more powerful weapons than guns, where the humans fight back in space first, and more. I suspect stories where aliens invade by nudging an asteroid to destroy the native Earth dinosaur species and seed alien biped species instead, exist as well.

You get the story you want to tell.

PS. when was the last time you read a gripping story about the termite mound that spent years gearing up for war and then an anteater tore the side off and ate 20,000 of them and all their preparations for war were useless?

• The humans are going to win, fact.
• Both sides need to be roughly evenly matched or the stronger side will easily win and there's no story.
• Rooting for the underdog is more fun, stories are better with tension and imminent disaster, so the humans need to be the weaker side, therefore the aliens get to be stronger.

Answer 10

basically, why are the stereotypical alien invaders 4-5 times more advanced than us?

Because low tech aliens aren't interesting enough to become stereotypical. Or if they are interesting, they are unique and therefore not a sterotype.

For instance, the Honorverse by David Weber; there are about six known alien sophont species but since humanity is an interstellar civilisation everyone else is uninteresting. The first book was set in the same solarsystem as the latest discovered aliens and they had exactly zero impact on the story because what could a neolithic race do in a space battle?

The sophont race humanity meets during the series is too unique to become a stereotype.

Alien is also an example of a non-technological race that are just too unique to become a stereotype.

Compare this to the Independence Day aliens; the came, they saw they stomped. Could describe any number of alien conquerors: Mars Attacks, Signs.

They could be completely different, hive mind, facist, democracy, a non sentient cloud of locusts, a byzantine buerocracy based entierly on poetry... There isn't much difference from the point of view of humanity, all we see is the sole of a boot as it stamps down.

One attribute, overwhelming power, and no other discernible characteristics: instant stereotype.

Answer 11

Consider what would happen if an alien ship appeared, alien soldiers jumped out, and all they had were Earth-style weapons.

All one thousand of them. Against seven billion humans. They would be beaten into the ground before most of the world even knew they had landed!

To compensate for their inferior numbers, they need superweapons to stand the slightest chance.

Answer 12

Life and technology should happen earlier in older stars

The sun is young relative to the universe

Figure 1 shows a planetary nebula that is result of a sun-like-star that already died (source: wikipedia).

The sun is a middle aged 3rd generation star. To put another way the mainstream theory is the universe was around for 9 billion years before the sun was born. .

Life and technology stalled many times on earth

It is not difficult to believe that a solar system similar to ours can have developed life and technology faster. Even a percent faster means 4.5 million years before.

Alternative solar systems may be more suited to space fairing civilizations

Figure 2: Real planets around habitable zone of other stars (source: NASA). Keppler Telescope found in at least 8 earth-sized planets on the habitable zones of G and K stars on a very tiny corner of our galaxy using a detection method that require a rare alignment between the planet, its star and the earth to be found. This means our galaxy alone should have several hundred thousands of planets such as this.

In a superearth the cost of chemical rockets may be too high. Then, the aliens get right to advanced propulsion and discover new physics that is key to FTL, advanced computers and weapons.

In a system with a few Mars and moon sized planets on the habitable zone the species may evolve more resilient to space radiation and the effects of zero g. Chemical rockets on those planets will be cheaper, then they become a space fairing civilization sooner and space research may allow breakthroughs in advanced physics. Something like the "em drive" could have been tested in weeks in a scenario like that. In opposition to the decades will take in our world.

In a earth like planet tidal locked to a red dwarf, solar an wind power generation may be stable and the world will never need oil. Taking power politics aside early in the technological development may allow a faster technological developement.

There was more than enough time for a mult-galaxy empire to arrise

There is space voids that some people think like could darken entitled galaxies with solar panels. I think is a stretch but...

To travel to another planet with a sufficient large military force implies to be more technologically advanced than we are

But in the end boils down to the simpler argument. To invade another planet you need technology to build massive space ships propelled with something more powerful than chemical or ion drives. And be able to survive in those ships for the duration of the flight here.

We are far from this technology level and then anyone that reach our planet with a military force big enough to invade will be more technologically advanced than us.

The physics of efficient space travel may have the same basis than the physics of the advanced weapons

Actually the advanced weapons can be a side effect of the advanced propulsion. And vice versa:

• Orion drive are fueled by cheap nuclear bombs. Implies mastery or immunity of radiation.
• Interestelar ramjet or deadalus project are fusion drives. Fusion in that scale may power in atmosphere destroyers, tanks, microwave beams, asteroid catapults and more.
• Warp drives may make excellent and weird bombs, personal shields and more.

Answer 13

Selection Bias

Selection bias is the selection of individuals, groups or data for analysis in such a way that proper randomization is not achieved, thereby ensuring that the sample obtained is not representative of the population intended to be analyzed

Lets assume there are a large number of species across the universe. All of them are on the same sort of timeline in terms of 'age since big bang' - some will have a head start, because they didn't get wiped out in the 'dinosaur' era, like our dinosaurs did. Others has a couple more catastrophes. And there's a spread of 'how optimal is your planet' syndrome - ours is pretty good, comparatively, as our large moon gives us access to heavier elements with relatively lower effort.

But if there's a big spread, then it stands to reason - technologically there's some in advance of us, and some behind us. The critical difference though is - the Universe is pretty huge, and faster-than-light travel may well be impossible as we currently understand it.

Inventing FTL means more tech than we have.

And a generation ship is ... probably just about on the edge of our technical capabilities. But generation ships are SLOW - the hint's in the name - multiple generations.

Nearest exoplanet candidates

... but we're also not sure there's any alien life on any of those, which would imply they're further away. (Or somewhere we're not looking, that's possible too).

So - net result. For aliens to come here they would have to be more technologically advanced, because we haven't gone there yet. Because we can't.

Answer 14

Because lasers look more awesome on screen than bullets. See also laser swords over metal swords. This is all based on The Rule of Cool.

As others have stated in better detail, the aliens would, by rule, have to have more advanced technology to successfully complete an interstellar voyage. But that doesn't mean they have to have super-advanced sci fi weapons too, if standard projectiles are better, for some specific definition of "better." But the rule of cool overrules that in fiction.

As a related point, watch District 9, in which the aliens weren't really that much more advanced than us, at least in weapons technology and survival skills. They had more advanced ships and biological sciences, I guess.

Answer 15

If an Alien civilization reached our solar system they would most certainly be more advanced than us because, even if they launched an interstellar ship as soon as they could, because even if you use what many people consider my extremely high chance number of advanced technology civs, the closest one would be 1000 light years away. The highest speed they could achieve would be 10% the speed of light which means by the time they got to us they'd be ~10000 years beyond 1960s tech. Even at a slower pace with a small population they'd still be so much more advanced by the time they arrived due to having info beamed to them and them constructing stuff on the way.

Answer 16

The level of technology development is driven by two factors.

1. How long it has had to develop.

2. How fast the technology is developing.

1 is easy--older alien civilizations are more likely to have higher-level tech than us, because they've had longer to develop it. The universe is 15 billion years old we have done all of our tech development from fire to atomics in the last 10,000 years. In terms of the life of the universe or the possibility of life in the universe a lead of 10,000 years is nothing. What chance would cave men have against us?

2 is much more complicated. There are many factors driving technological development. A big one is competition--look at the huge technological advances made during WW2 or the "space race".

Necessity is similar: if an alien species is threatened with extinction, due, say, to the failure of the ecosystem on its home planet, it is going to be very driven to get out there.

There are also less urgent factors which will affect the alien species, e.g how much do they value education? or what is the social status of engineers and scientists?

Answer 17

They have to have superior technology in order to reach us in the first place. It's something of a self-fulfilling prophecy.

Answer 18

Not all planets are created equally.

"According to some theories, life is so easy to create, that all of it should have appeared at roughly the same time. So, we have had as much time as them to develop our war machines."

I'm going to go with the assuming all life supporting planets formed and cooled at the same time.

Lets take Earth and an Earth like planet,(lets call it Learth) for example. Earth and Learth both had life start at roughly the same time. Unfortunately for Earth it went through several extinction level events. Learth did not necessarily go through these same events. So one of Learth earlier species had time to evolved into a species capable of developing tool, societies, culture and eventual Space travel. Earth had to wait till after 5(?) extinction level events before that happened.

Answer 19

TLDR: The premise is false. Aliens are not always advanced. In fact Earth may have even been "invaded" by mere alien microbes.

You ask, "Why are we always so inferior ... to them, or to put it another way, why are they superior to us?"

If you're asking why we are always so inferior in books and movies, the simple answer is that it is because Earth getting "invaded" by a bunch of protoplasmic microbes doesn't make as exciting a story. Yet in contrast to many those who imagine advanced alien invaders, there are many scientists who believe earth has actually been "invaded" by microbes, as discussed in the articles below. A short except from the Telegraph:

"The team behind the Rosetta comet landing mission have announced they have found the amino acid glycine and the element phosphorous in the dust surrounding 67P/Churyumov-Gerasimenko." "Comets contain an awesome cocktail of organics material that, if provided with the right conditions, could then go on to form life......So, in the case for Earth, this supports the idea that comets impacting Earth could provide the right ingredients for life."

http://news.berkeley.edu/2013/03/05/research-news-briefs-did-comets-seed-life-on-earth/ http://www.telegraph.co.uk/science/2016/05/28/comets-could-have-sparked-life-on-earth-as-building-blocks-of-dn/

Answer 20

The alternative to more advanced invaders is invaders with a natural, not technological, ability. If some kind of space-warping is possible according to the laws of physics, perhaps a creature exists that has a natural means of exploiting that principle and traveling between worlds. That creature does not need to be advanced enough to understand how it travels, it just does it. If it arrived here, we might be more advanced than it.

Answer 21

According to some theories, mostly by the astrophysics, life would not appear at the same time.
One reason why - the universe is expanding after the Big Bang. So some galaxies closer to centre would be (could be) long dead and abandoned while some planets would just start to create primordial soup.

IF your universe started as it is all at once then you would not have the problem with aliens.
BUT if you want to stick to our universe you need to follow the rules.

Answer 22

We cannot assume that all solar systems have asteroids although it is likely. If a planet is bombarded with such material, even in a random fashion, over billions of years, it will cripple the dominant species on the planet, who rely on brain power rather than defense mechanisms to survive. So the development of a species can be heavily influenced by external forces at work, beyond the control of the dominant species.

Answer 23

We have traveled to our moon, and have sent probes throughout our solar system. We move in space by throwing stuff overboard (i.e. thrust gases.) At the bleeding edge of our technology, it will take 100 years for us to reach our nearest neighbor.

So, until we have the ability to span the unbelievably great distances of space with ease, anyone that visits us must have made massive leaps of scientific advancement over what we can do. Therefore, visitors must be more advanced.

Of course, we don't know what we don't know. Most speculation involves straight-line advancements. We could uncover a fundamental truth next year, that would allow us to open a traversable wormhole using few WI-Fi Routers coupled to an empty tuna can. Then, we could be the ones visiting the far corners of the galaxies in something akin to the space shuttle.

Answer 24

So tactically, waiting for a potential threat to be on the par is not a sound strategy. Any advantage an enemy has is a disadvantage to you. So, if there is a species of intelligent apes that are showing signs that they may be a threat to you in the near future, giving them time to close the gap is going to make eliminating the threat are the more difficult if it manifests. Of the few things I give to the sequeal to Independence Day is that it's probably the only Alien Invasion story I am aware of that touches on the difficulty of presecutiong an interstellar scale war... the time to recieve and send reinforcements is going to be much more difficult than a more local war, which means all its logicistical supplies (ammunition, food, medicine, communications equipements sleeping quarters, ect) are going to have to be shipped in with the invasion fleet. And sure, an initial showing might result in the destruction of large urban areas... but as has been observed, "War never changes". Consider Sun Tzu is still taught in an age where all weapons he would have available to him are hopelessly obsolete.

The longer a war is prosecuted, the more likely the invasion force is likely to fail. This is because the defending forces have the supplies they can more readily rely on, where as the invading force will have to rely on either finding supplies they can use or those they brought. The defenders also have better knoweldge of the terrain and better population of able bodied defenders (an invasion enteres with its commited forces, but the defenders can quickly draft their civvilians to assist in defense operations.)... Even if military chains of command of the defenders are wiped out, guerillas with simple idealogies of "They ain't from here, kill them" are sure to be a tolling force on the continued pressence for the alien force. This is all before realizing that humans are evolved to live on Earth, where as the aliens are not... sure... they might be similar, but we'd be the most efficient of the two in terrain movement, air, water, and food consumption, and our bodies are designed to work on this gravity. The invading aliens might have some similarities, but the the idea that they are superior to us in every way on earth is laughable... especially considering where one of the most populous species for our size AND have some of our own phsyical superiorty in our own animal kingdom. For example, humans are amazingly resilient agaisnt tramatic pain and we can avoid going into shock a lot better than most animals. We're also the most physically enduring animals on the planet. There may be a lot of animals that can out run us or out swim us, or out climb us, but few do all three and none can outlast... The only feat of physical competition humans have ever one over the animal kingdom is the Ultramarathon... humans are slow and steady, and will tire out long after any animal that can take a lead by speed and recover faster than that same animal can... Its one of the reasons there are 7 billion of us. We are the Elizabeth Warren of the Senate that is the Animal Kingdom... still we persist!

Even if techonogogically superiority persists, a weapon does not win a war... numbers of weapons win a war. Even World War II, which was ultimately concluded by the Atomic Bomb, the fact remains that America had more than Japan could recover from was the deciding factor, not that at time of surrender, they had just one left and had Japan called that bluff, America would have a siginficant amount of time before they could rain down more. On the otherside of the globe, the Germans allowed themselves to believe that killing two allied tanks for every one german panzer was certain to ensure victory, but the decision was made by the fact that Americans could produce from raw materials to battle field five tanks in the time it took Germany to produce one. Over the scale of the war, the ability to field tanks faster fliped the number. From raw material to death on the battle field, America tank forces would net one new tank for every two that the Germans lost in combat. Taking this to alien levels, sure, you may have wiped out 50% of the population in the initial invasion... but that still leaves 3.5 billion people who all want to kill one of your guys... with those numbers, it's a lot more you have to kill.

In the alien situation, the prolonged occupation of Earth would mean that the Aliens would be expected to feed troops with dwindling food supplies and limited items from the planet that didn't poison them and probably didn't provide nearly enough nutrition to serve as an efficient replacement. So your troops are expected to engage enemy forces while malnourished and unhealthy because of it. Lack of proper nutriotion and stressful conditions (such as war would produce) mean the immune system isn't working at its best opening threat from Earth's oldest method of fighting alien invaders, germ warfare.

And then you have to deal with the fact that people are trying to kill you... no war is 100% causialty free for the victor. You're going to have losses no matter how much you try to prevent it. And resistence cells are notoriously hard to erradicate. The United States was able to defeat British Forces in the Revolutionary War and War of 1812 largely on guerilla tactics and Admiral Yammoto famously reminded the Imperial Japanese government of this fact to successfully convince them they only wanted non-mainland territorial concessions from the outcome. In turn, Northern Vietnam was able to hold out against the United States superior war machine by pretty much doing the same thing. Afganistan, the infamous "Grave Yard of Empires" has relied on it's natual hell-hole like living conditions and combined with hit and run tactics to win against every invasion force in the history of civilization by just waiting for them to declare "fine, you keep this dumb land anyway". It could even be argued that the legitimate Afgan government is whoever is winning the tribal fighting at any given time.

Depending on how much time the alien species needs to gather enough forces to send re-enforcements and resupplies to facilitate planetary occupation, the prolonged war could last years and when it ends, it's because the aliens consider the blood and treasure they put into verses the expected reward. Every day the invasion cannot definatively passify the invaders is another addition to that charge and sooner or later, it becomes untenable to take from a purely logistical nightmare forming around the defense.

TL;DR: Aliens will not invade and conquer Earth because that's where Afganistan is. No one conquers that place and surivives.