Caltech-led astronomers estimate that at least 100 billion planets populate the galaxy so this begs the question; “Where are all the Alien tourists?”
Respected luminaries such as Enrico Fermi (1950) and Michael H. Hart (1975) postulated what has become known as the Fermi Paradox, with the limited real evidence available to them they were puzzled by the lack of Aliens landing on the Whitehouse lawn.
Now with the Kepler spacecraft finding exo-planets daily the answer to the question may be supported by real scientific evidence.
So what could be the reasons for a lack of Alien contact?
Of course, many suspect that Aliens live among us and are protected by a huge world-wide cover-up, a conspiracy theory that has prompted many a great movie (and some not so great). The Men In Black films being perhaps the most recent, entertaining and comedic.
There are however some very good reasons why we see no evidence yet of Alien life, intelligent or otherwise.
It is still possible that Earth may be unique in the universe but highly improbable. Just on a statistical level the odds are against it. The Earth is nothing special cosmically, a rather boring little blue-green planet on the outer edge of a regular spiral galaxy amongst 100 billion other galaxy’s in the observable universe.
The Drake Equation N = R* • fp • ne • fl • fi • fc • L was formulated by Dr. Frank Drake in 1961 to try to estimate the number of intelligent civilisations we might discover in our Galaxy. The discovery was expected to be made by receiving radio waves from Aliens and spawned the SETI Institute which has searched without success to this day. They are now trying to look for other communications such as Lasers as well as increasing their radio receiving capability.
N = The number of civilizations in The Milky Way Galaxy whose electromagnetic emissions are detectable.
R* =The rate of formation of stars suitable for the development of intelligent life.
fp = The fraction of those stars with planetary systems.
ne = The number of planets, per solar system, with an environment suitable for life.
fl = The fraction of suitable planets on which life actually appears.
fi = The fraction of life-bearing planets on which intelligent life emerges.
fc = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
L = The length of time such civilizations release detectable signals into space.
Recent discoveries by the Kepler mission has dramatically increased fp and ne is also now thought to be higher due to better understanding of which star types could have a habitable zone (orbits where liquid water could exist). fl has also increased from earlier estimates because of discoveries on Earth of organisms that exist in very inhospitable places, such as deep dark caves and ocean floor volcanic vents known as smokers.
From this we might conclude that the Galaxy should be teaming with life just as our own planet is. But fi , fc and L are probably the main reasons why we see nothing out there. (yet)
It took a very long time for intelligent life to emerge on Earth, 4.54 billion years is estimated as the age of the Earth and the Universe is only 13.75 billion years old. In all that time and for all the time that humans have roamed the Earth we have only been emitting radio waves for the last 100 years. Even as I write this blog post the radio waves we have emitted have only travelled 100 light years into space. 100 light years would cover outer space in which only 15, 000 stars inhabit. Already we are converting from powerful analogue radio transmitters to narrow beam digital signals and bouncing them off of satellites back to Earth. Over-ground communications are increasingly using fibre-optics for voice and data so no radio signals there and WiFi and mobile phone signals are weak and short-range, these will not leave the Earth. Our space radio foot print is therefore getting smaller and weaker and less discernible from the cosmic radio interference. So perhaps L is just 200 years for us and possibly the same for any technological civilisation as well. That is a very short window to detect anything on cosmic timescales.
In the original equation L may have been meant to refer to the longevity of an intelligent technological civilisation but even if we were to be generous and say a million years the chance of detecting the signals is low. Technology seems to be driven most by wars and ultimately unleasing the full power of nuclear, chemical and biological weapons will ultimately lead to a civilisations demise. Possibly even our fate in the near future.
There are of course many hazards apart from wars that could wipe out an alien civilisation such as; asteroid or comet impact, huge volcanic eruptions, supernova, gama-ray bursts, natural plagues, climate change, loss of atmosphere and water, reproduction failure and alien invasion. Anything that could shorten L would reduce our chance of detecting an Alien signal.
fc should be a large part of the set of fi since reasonable intelligence comparable to ours should lead to the discovery of radio as it requires minimal technology to both detect and reproduce. However, fc might be limited by environmental and physical factors. A highly evolved intelligence might be limited by a lack of some sensory apparatus that we enjoy and/or by a physical limitation in their ability to manipulate and experiment and develop technology. Abilities to communicate effectively and transfer knowledge might also be a limitation on their planet. Some animals like Dolphins on Earth are considered intelligent but we would not expect them to ever develop radio technology living as they do in a water environment (impermeable to most radio waves). Also they have no limbs to construct technology with. Although one day we may be able to discus Plato and Nietzsche with them and appreciate their musical culture (assuming that is what all the whistleing and clicking is about).
fi is the big question still to be answered since, as yet, we have not detected any life (at all) outside of the Earth so it is a big step to try to estimate the number of life-bearing planets on which intelligent life may emerge. We only know of one planet, ours, but statistically it would be likely that there are many other planets would develop life, and some at least would be as intelligent as us and probably more so.
There are other factors that may affect our ability to detect Alien transmissions not covered by the Drake equation;
1. Distance to planets from Earth
The power of a radio signal drops off the further the signal travels. This obeys what is known as the inverse square rule 
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2. Our equipments sensitivity to receive a signal
The more sensitive the receiver is the more noise we also receive. Separating out the signal from the noise is a huge challenge and some radio frequencies are naturally noisy due to natural cosmic phenomena. On Earth we also have a massive amount of man-made radio noise that we have to filter out or shield from.
3. Our ability to discern an intelligent signal
We may see a radio signal emanating from space but how do we know it’s from an intelligent source? In 1967 the first radio pulsating regular signal from space was detected and initially we thought it might be an Alien intelligence but it turned out to be what we now call a Pulsar, a fast rotating neutron star. So even if w’e do pick up a signal we may have trouble telling if it’s natural or Alien made.
4. The power needed to transmit a signal to us
As well as needing many years to reach us, an Alien signal would also have to be transmitted with enormous power, perhaps even requiring the harnessed power of an entire sun. This is possible if a tight beam and perhaps the power source is a fusion reactor (a sun in a bottle) but then it would have to be pointed directly at our solar system and we would have to be listening at exactly the right time and have our receiving dishes pointed in the right direction.
5. The willingness of Aliens to be detected
Our own history shows us that we engage aggressively in conquering other lands and plundering them of their natural resources, often with little regard to indigenous people, plants and animals. We should perhaps expect Aliens to do the same if they discover us. Or perhaps Alien civilisations have learnt early on to keep quiet in the cosmos as a way of self-preservation from other predatory species.
6. We may be too primitive or too dangerous to contact
If you look in your garden you may see an Ant hill, a seemingly highly evolved, organised and busy society. How often have you tried to communicate with your Ants? Highly advanced Aliens may just see us as a lower form of life not worth bothering with at this time. Or they may see us as a potential threat that should be stamped on before we overrun their neck of the woods. Our planet may be seen as a bio-hazard to be avoided and/or our species as an aggressive war mongering selfish heavily armed volatile rampant and chaotic infestation, definately to be avoided. Cautious Aliens would avoid advertising their local presence and only visit us covertly for scientific reasons.
So, in my opinion, it is no surprise that we have yet to detect Alien intelligence in the Galaxy, they are most likely there in hordes but we just can’t hear them or they don’t want to be found. I expect that since we are a long way from interstellar travel we will just have to wait for Aliens to find us and contact us.
All the more reason to look out for UFO’s in your neighbourhood !
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