What are some theories on why we haven't been visited by aliens yet?

 

What are some theories on why we haven't been visited by aliens yet?

Originally Answered: What are some theories on why we aren't visited by aliens yet?

The classic Fermi paradox makes for very interesting analysis. There are some very compelling theories on why we haven't been visited by or been made aware of other intelligent life forms out there if there are potentially over a 100 billion Earth-like planets out there.

To understand these theories, there are certain concepts and theories one needs to be familiar with: Kardashev Scale, Drake's Equation, Great Filter, and the Wow! Signal. This is a longish answer, but bear with me while I get to it. I took a fair bit of help from this Wait But Why post to give my answer a skeleton. Tim's done a great job of it and I'd encourage everyone to read it. I've added a lot of extra information, I hope you find it relevant.

First of all, how do you define alien life? Hardcore weirdos like in our famous movies or ant like creatures we can hardly see even if we zoom in on a body's surface, or better still, microbial organisms? For the purposes of this answer and for aliens to contact us -- for them to contact us -- let us assume they are "intelligent". Which brings us to, how do we define intelligent life and what if there's life more intelligent than us?

Types of Intelligent Civlizations
The first thing we need to understand is the Kardashev Scale. The Kardashev scale measures the technological advancement of a civilization based on the amount of energy it has at its disposal. There are three types of civilizations: Type 1, where a civilization is technologically capable of using all the energy sources on its planet (Carl Sagan puts us at 0.7, Michio Kaku says we might become Type 1 in about 200 years); Type 2, a civilization with enough technology to harness energy from its system's host star; and Type 3, a civilization capable of harnessing the power of its galaxy and thereby colonizing the galaxy. All these Types are used broadly while talking about colonization in space and the ability to terraform and mine other planets.

(Image from October | 2012 | AstroWright)

Sorry.

Why do we talk about these civilization types? Are they some form of measurement for future technological advancement? Not necessarily. Our Universe is 13.77 billion years old. Our Solar System is 4.6 billion years old. The Earth is about 4.54 billion years old. There are planets outside our Solar System that are more than twice as old as Earth. Even if out of the 100 billion Earth-like planets, only 1% have the right conditions to harbor life, we still have about 1000000000 planets.

(Image from theskyisbeautifultonight)

Drake's Equation, while isn't aimed at estimating the number of civilizations out there but only at the probability of communication, is a trippy thought experiment that also makes us think along the same lines:

N = the number of civilizations in our galaxy with which radio-communication might be possible
R* = the average rate of star formation in our galaxy
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support life per star that has planets
fl = the fraction of planets that could support life that actually develop life at some point
fi = the fraction of planets with life that actually go on to develop intelligent life
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time for which such civilizations release detectable signals into space

Currently, we estimate that R* and fp are quite high, with ne being moderately high. We do not have any proof of fl, fi, or fc yet.

On paper, some of these 1000000000 planets could easily be sustaining Type 2 or even Type 3 civilizations. But in reality, are they? If these civilizations indeed do exist, why have they not contacted us? Or have they contacted us? In fact, can they theoretically even exist at all? Is it even possible, there might not be any other civilizations at all?

Let's simplify these questions and break them down into three: 1) that life either doesn't exist or is of lower intelligence than us, making it hard for them to contact us, 2) there are Type2 and Type3 civilizations, but they choose not to contact us or are contacting us and we can't recognize them, and 3) we have been contacted and we know it. There are ample theories to explain the former two scenarios, and one piece of evidence for the third:

I. Life doesn't exist elsewhere, or if it does, is of lower intelligence than us

To understand this further, we need to know about how life came by and the Great Filter.

Origin of Life on Earth
Life arose on Earth from non-living organic compounds about 3.8 billion years ago: 700 million years after the Earth formed. We have proof of this in the form of sedimentary rocks in Isua, Greenland (the oldest known sedimentary rocks) that show evidence of life; and from the famous Miller–Urey experiment. This leads us to believe that planets that are older than us have had ample time to a) create conditions enough to sustain life, and b) have and contain civilizations more advanced than us. But there's more to it than just that.

The Great Filter
The Great Filter is a filtering event that exists somewhere in the journey from abiogenesis to Type 3 civilization which prevents civilizations as a group -- or lifeforms, really -- to reach Type 3. Simply put, it's a filter that is so catastrophic to civilizations, it simply wipes them out. However, by an extremely rare occurrence, one or some civilizations might cross it to advance further.

(Image source: Wait But Why)

Since we are not entirely sure what the Great Filter is, we are also not sure if we've crossed it or not. The GF could be in our past, could be in our future, or could even be the present.

• If it's in the past, we've crossed it, which gives us no hope that some other life might have crossed it or cross it in the future. The GF could be the origin of life itself (considering how much bombardment planets go through), or the first endosymbiontic process which gave rise to the first multi-cellular organism, or the first cyanobacteria that produced oxygen to sustain life, or the Cambrian explosion that lead to diversity in life, or the first appearance of Consciousness, or anything at all that is exceptionally rare in occurrence. "Rare" is the keyword here. Therefore, if the GF is in the past, it most likely means that we are rare ourselves and not many attempts at life have made it this far. However, if we identify another species that has undergone any of these changes outside our planet, we can safely rule all of these events out as the GF because these events aren't rare enough. This also supports the theory popularly known as the Rare Earth Hypothesis, which simply states that we are all alone because we, and only we, have the right conditions to harbor an intelligent civilization. There is no other life anywhere.
• The GF could be our present or in the very recent past. It could be quite possible that all of these millennia, the Universe was so hostile to life that no conditions existed for life to survive beyond infancy, except for now. We could be in the process of a massive biological transition and the first planet to do so. There could be billions of Earth-like planets, but nothing that is capable of harboring life. We could be the first or there could be other lesser developed lifeforms in the same transitionary period as us and do not have the technology to contact us yet. We ourselves have only started looking for signals have only very recently broadcast our own existence.
  
• The GF could be in our future, in which case, we're screwed. This means that life has often survived up to the GF and then died out, as we might one day be destined to. This could be a natural cataclysmic event or an obvious turn of technology advancement where civilizations just wipe each other out in war. Ironically, any evidence of extra terrestrial life switches our present condition to this situation by default, making us a doomed civilization. There could be other civilizations, but they all routinely die, just like we would. The fi in Drake's equation approaches zero, making the whole equation zero. A very good reason for this might be that civilisations died out before they could become an interplanetary species. We are currently in the middle of a mass extinction and could face the same fate thanks to global warming and global wars if we do not colonise Mars and beyond.
  

II. There are civilizations more advanced than us, but they choose not to contact us or are already contacting us, but we don't know it yet

Here, we come to the crux of the question: why have aliens not contacted us? There are hundreds of theories out there attempting to answer this question.

1. We are too primitive: Aliens are transmitting signals to us, but we do not have the technology to receive or identify them. We can only receive signals when they are being transmitted directly in the direction we are placed in. Further advancement of technologies could help here.
   
2. Higher Dimensions: Higher life is all around us in different dimensions, but as lifeforms, we are too limited by our own sense of physical dimensions to perceive them.
3. The Zoo Hypothesis: Aliens are observing us before making contact. Maybe they think we aren't ready to be contacted yet.
4. Ancient Alien theory: Aliens have already made contact but before our time. They could have helped shape civilization, or merely observed to make a note of our existence, or examined our planet for the future, or were simply too bored by life very similar to theirs and moved on. Just watch the show.
   
5. Utopian Aliens: There are advanced civilizations that are so happy with themselves and content with their development and resources that they do not have the curiosity or the need to explore.
6. Berserkers: Powerful aliens are out there and are super predators that destroy all life in the quest for colonization. This is one of the reasons many scientists, including Stephen Hawking and Carl Sagan, believe that broadcasting signals out into space (like METI) is a stupid idea. There are a few variations of this theory. One is that there is only one Super Predator that has eliminated all civilizations it has come in contact with and might kill us. Therefore, other civilizations that are aware of this -- if they exist -- are carefully silent. Another variation is that there are multiple berserkers out there who are killing themselves to establish dominancy: a Great Filter.
7. We are a rural planet: This is a Foundation-esque theory that thinks that the galaxy has already been colonized with multiple civilizations in regular contact with each other, but we are in a desolate, rural system which either hasn't been colonized yet or has been abandoned.
8. We haven't had enough time: We just started looking for signals less than 100 years ago. Maybe whoever is transmitting isn't transmitting continuously or isn't visiting regularly. Give it another 100 years. The L in Drake's equation is too low.
   
9. We are interpreting wrong: Everything we see and observe in the Universe, like Gamma Ray bursts or asteroids, or even natural phenomenon on Earth, such as the weather, are triggered by higher intelligence and we just don't know it.
10. Aliens are among us: .. and we are not equipped to identify them. Think MIB, but less creepy. Or more creepy, who knows.
    
11. We are wrong in everything we think we know, and we know and understand nothing: Self explanatory.

Lastly, there is yet another option.

III. We have been contacted and we know it

On August 15, 1977, researchers at SETI detected what is now famously known as the Wow! signal. The signal was exactly what we on Earth expect extraterrestrial intelligence to sound like, causing Ehman, who heard the signal, to write "Wow!" on the side. The transmission lasted 72 seconds (because of the Earth's rotation) and came from the Sagittarius constellation.

We have never detected this signal again. We have sent many responses towards this location, even in the form of a tweet-compilation with a decoder, but to no avail. We can't conclusively say it's an alien signal until we detect it again.

At the end of the day, we really know nothing at all and can only speculate about where we stand as a species in the vast emptiness that is the Universe. That we are alone, or that there are other civilizations out there; both prospects are equally terrifying and exciting. All we can do right now is to keep developing, keep searching, and keep hoping, and remembering that in all probability, "no sign is a good sign."

Sources and further reading:
Prevalence of Earth-size planets orbiting Sun-like stars (a study on number of Earth-like planets)
Kardashev scale
Where Are The Aliens? How The ‘Great Filter’ Could Affect Tech Advances In Space
http://www-eve.ucdavis.edu/grosberg/Grosberg%20pdf%20papers/2007%20Grosberg%20%26%20Strathmann.AREES.pdf (Paper on whether prokaryotic to eukaryotic evolution was a minor step or a major step. It has happened 46 times before on Earth).
http://www.ncbi.nlm.nih.gov/pubmed/18855114 (Paper on how we are in the midst of an astrobiological transition)
The Fermi Paradox - Wait But Why (again, read this!)
Fermi Paradox | SETI Institute
Page on nature.com (Isua rock formation and how they present evidence of life)
Page on osu.edu (Interesting paper on the decoding signals problem at SETI)
University of California Scientists: 'Camouflaged' Aliens Live Among Us