Why is no-one ‘knocking on the door’?


OK; I admit that this is one of my more wacky posts but there is a human appetite for unsolved mystery and this subject is probably the biggest one of all and fun to speculate on. It has implications for the directions science and technology should take and ultimately we will probably find a definitive answer one way or another. If you have never heard of the ‘great filter’, Fermi or the Drake equation I hope this goes a little way to stimulate an interest.

The Great Filter

We are missing something. The longer this goes on the more certain we can be that we are missing something! I am referring to what has become  the biggest mystery humanity still has to solve. It appears increasingly possible we may ‘be alone’ after all. Along with so many others I have had a lifelong fascination with  SETI; the Search for Extra Terrestrial Intelligence. It has been one of my biggest expectations that SETI would be successful ‘very soon’. Each year the search has become ever more advanced, thorough and imaginative and yet with over 50 years of searching still ‘not a peep’. Success would shake our civilization to its core by challenging our self-identity, discrediting the world’s religions in an instant and give us a much needed collective kick up the backside and outward focus. As Arthur C Clarke summed it all up well: “Sometimes I think we’re alone in the universe, and sometimes I think we’re not. In either case the idea is quite staggering”.

There is so much we do know already. The scale of the universe is simply mind-numbing. Our own galaxy, the Milky Way, has an estimated 100 billion stars and is comparable to another 100 billion galaxies! In recent years we have proven the existence of over 1000 exoplanets orbiting other stars and some of these are known to be in ‘goldilocks zones’. Recent data from the Keplar mission also suggests there is as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way, 11 billion of which may be orbiting Sun-like stars.

The Drake equation is a famous attempt to calculate how many civilizations we should be able to contact in the Milky Way and well worth appreciating.


The problem is if you insert pessimistic values you end up with an answer of 1 (us) and if you use optimistic values you end up with a value in the multi-millions! We can now add realistic values to the first 3 variables. The problems remain with the remaining four given we only have one real case-study to apply values for life-related variables; our own planet. How often does life start? How often does it become multi-cellular? How often does multi-cellular life become intelligent? How long does an intelligent civilization tend to last? With a sample of 1 we just don’t know. And this leads us to an uncomfortable, unavoidable choice we need to face concerning an idea called “the Great Filter”. As time goes on and if SETI continues to fail we have one of three possibilities to grudgingly accept; We are first, we are rare or we are fucked! An excellent resource to briefly explain the Great Filter idea is as follows:


I want to explore these ideas a little more to present a few of my own thoughts.

There are things we do know. Life seems exceptionally resilient and resourceful. Fossil records show life started extraordinarily early and thus easily on the young Earth (The Earth is 4.5 billion years old but life has been active for 3.8 billion years) and has stubbornly stuck around ever since surviving nearly supernova, asteroid impacts, chaotic climate swings, reversal of the magnetic poles and numerous other challenges as well as continuous background radiation. It seems plausible given the timespans and locations available that the spark of life should be being endlessly repeated. However in our galaxy alone many generations of stars have lived and died before our own sun came into existence so it seems improbable that we are the early birds on the galactic (and cosmic) scene in terms of life emergence. When we start digging around under the ice on Europa and in the Martian permafrost if we do find evidence of life the idea of us being first will seem much less likely. And even without this evidence it is already believed life may have started independently on Earth several times;


It’s important to remind ourselves at this point (without being too big-headed about our own species) that there is a big distinction between life and intelligent life. I am making an assumption that if you stick a suitably sized rocky planet in the habitable zone of a yellow dwarf star and simmer gently for a few billion years life will start by natural processes and that multi-cellular development is inevitable as the laws of evolution take over. If you want evidence for how tough Earth life can be look up ‘Tardigrades’ and their abilities. The big unknown remains, however, how often does this ‘life’ become intelligent, contactable and communicative as attempted by the Drake equation. I am confident that the sheer numbers involved in our stellar surroundings mean that the idea of us being first is basically ridiculous.

The second option for the Great Filter is that we are ‘rare’. At first sight our planet seems fairly normal and orbits an ordinary star in an ordinary galaxy. To my mind there are a few potential themes which make our home and personal circumstances unusual to our obvious benefits:

  1. Our Earth/Moon Relationship. Our moon is big relative to our planet and formed early in the Earth’s history as a result of a head-on collision between the young Earth and a mars-sized body named Theia. This moon gives us unusually strong tides and the ancient impact massively changed the geological and tectonic make-up of the planet. Would our mineral-rich mines and resources have been so close to the surface of our planet without this ancient cataclysm. How important is this moon and the timing of its formation to our success?


  1. The Earth is large enough to have held onto its atmosphere. A smaller planet with a lower gravity would have leaked away its atmosphere like Mars has done. I never understand why people entertain the idea of terraforming Mars; it will simply leak away its new atmosphere as fast as we create it!


  1. The period of “heavy bombardment” early in the Earth’s history provided large quantities of water (probably via comet impacts). Our large proportion of oceans act as an important planetary regulator for our atmosphere and makes the Earth much less sensitive to our civilization’s recent explosive high population growth. Perhaps other galactic civilizations are less lucky than us in their land/ocean rations and have runaway greenhouse effects.


  1. There is increasing evidence that the tectonic activity character of our planet has been pivotal to life’s development. Enhanced by our unusually large moon plate tectonics have a) been fundamental to underpin the planet’s carbon cycle (giving balance to the atmosphere) and b) given us a strong magnetosphere to shield us from Solar radiation as well as c) giving us a very fertile planet with lots of young terrain. How many planets start life on tectonically barren worlds?


  1. We are exceptionally lucky to have inherited a planet rich with large hydrocarbon resources. The lucky break whereby the perfectly sized Chicxulub asteroid wiped out all the dinosaurs but left the door open for mammal domination set the stage for our civilization to inherit vast energy reserves for the stepping stones of our development. If we had evolved 100 million years earlier on a planet with no oil or gas reserves how would we have fared and would we be stuck in caves for millions of years with the ideas but not the resources to develop?


  1. We have been lucky to have had no large asteroid impacts in the last 200,000 years of our development. An unlucky impact in the ‘cradle of humanity’ in Africa would have left us smeared into the fossil record! Has our jovian dominated solar system shielded us from more regular asteroid impacts compared to other candidates? Having said that without the dinosaur-killing impact 65 million years ago mammals would never have had their ‘lucky break’ to inherit the Earth in the first place. How many planets are ‘stuck’ with non-intelligent reptilian ecosystem dead-ends?


  1. How many civilizations evolve with intricate tool-forming ability? Maybe civilizations are mostly made up of intelligent cephalopods/Cetacea or crocodilian rather than freakish upright apes with opposable thumbs! Intelligence does not necessarily run alongside suitable appendages for technological development! There may be countless super intelligent philosopher species out there with no technological skills.


  1. Our planetary landscape, character and ecological diversity has made Earth life advance more rapidly through enhanced evolutionary processes than perhaps is the norm. Perhaps other worlds get stuck in evolutionary dead-end ruts without emergent intelligence? Crocodiles have basically remained unchanged for millions of years; T-Rex wouldn’t notice much little visual differences if a modern croc was substituted with an ancient one in his local swamp!


9.How unusual is the strength of our planet’s magnetic field and internal dynamo? We really don’t know but it is clear the magnetosphere is an excellent barrier to damaging solar radiation. Perhaps other ‘goldilocks Earths’ elsewhere with under-developed (smaller) iron cores leave their fledgling lifeforms hopelessly irradiated. Perhaps the Earth has a freakishly large Iron core following the impact which created the moon.

When all these things are considered together (and some we may have overlooked) maybe Earth really is a ‘rare’ galactic gem. We have to hope this is the answer to the mystery because the third option for the great filter is that it is not behind us but in front of us. This is much more unsettling if true and means we are most likely ‘fucked’; this is the one option deeply uncomfortable to consider and we may need to tread very carefully indeed.  Many of the possible scenarios for this ‘end’ are obvious and well discussed but worth listing a few anyway:

1. Civilizations cannot contain their own planetary dominance and civilizational arrogance and burn out ‘Easter-Island style’ from lack of resource or environmental damage.

2. Civilizations are fundamentally self-destructive (we can sadly all relate to this idea at the moment).

3. Civilizations unknowingly repeatedly trigger an unforeseeable, unrecoverable technological catastrophe from such innovations as self-developing AI, nano-weaponry, super bio-weaponary or unstable high-energy particle accelerators.

4. Predatory advanced local alien civilizations consistently monitor and eliminate any emerging threats to their own interstellar dominance.

5. Unforeseeable pollution effects accumulate in the environment and render emerging civilizations unavoidably infertile.

6. It is in the ‘nature’ of advanced civilizations that they are time-limited and prone to rapid self-collapse through lack of resilience and foresight and resource sharing.

7. Maybe those rare civilizations that ‘survive’ are those more fortunate than us in having several habitable planets in their star’s habitable zones giving an easier insured stepping-stone process for stellar colonisation to counter aforementioned risks. Maybe our Solar system isn’t as perfect as we think.

8. Maybe those civilizations that make it multi-stellar are those in globular clusters with stars bunched considerably closer together…SETI researchers take note!

9. Do civilizations all make the same mistake of developing particle accelerators to a threshold energy level that then triggers an extinction catastrophe? Perhaps this event is completely unpredictable and unavoidable. ‘Curiosity killed the species’?

10. Do most biological civilizations become overwhelmed and replaced by their own AI creations meaning artificial not evolutionary life is the natural end-point of nearly all civilizations.

There is one more really nasty hypothesis which has been used to solve the central mystery. It is one which Elon Musk has talked about. It has been suggested that our entire universe is no more than a sophisticated simulation. You can hear him talk about this idea here:


I find this idea quite upsetting but can understand the logic of the argument. Perhaps when mathematicians and physicists delve deep enough they can settle this one soon as the evidence for or against this unsettling idea probably lies observable but deep in microphysics laboratories.


If we in reality ‘unlucky’ and in the ‘rare’ category it is possible to watch out for the potential stumbling blocks to our continuation and try to plan accordingly to avoid stumbling blocks. Should we be a species capable of overcoming the ‘great filter’  (unfortunately in my view still ahead of us) we could move on to to becoming a type II or III civilization with a future virtually without limit;all recorded human history so far would represent a tiny percentage of 1% of the whole human story. This is our ultimate challenge; can we put aside our own personal selfish needs and jump this barrier to reach civilizational adulthood and see the universe through to its end?