Tuesday, 26 May 2015

"Where is Everybody?" - The Fermi Paradox and Possibility of Extraterrestrial Life

1950, Los Alamos National Laboratory New Mexico.

A group of scientists, Enrico Fermi and Edward Teller among them, are sitting at a table, discussing the some of the deepest mysteries of physics. They are on a lunch break at the facility where the foundations of modern nuclear physics were formed. Suddenly during the conversation, in an apparent juxtaposition, Fermi asks "Where is Everybody?". Surprisingly, the rest of the scientists know exactly what he means, to quote Edward Teller "The result of his question was general laughter because of the strange fact that in spite of Fermi’s question coming from the clear blue, everybody around the table seemed to understand at once that he was talking about extraterrestrial life."

Enrico Fermi's career was filled with asking random, but relevant, questions and computing answers and approximations quickly that were not only profound and yet simple but also incredibly long lasting. His answers to some of the biggest problems in quantum and nuclear physics were also randomly churned out, sometimes solving massive problems on scraps of paper that happened to be around. 


            The back-of-the-envelope physics calculation was often the calling card of Enrico Fermi

His quick and straight to the point way of utilizing his skills made him the inventor of the first nuclear reactor capable of creating a self-sustaining nuclear fission reaction. Despite the safety problems involved, Fermi was eager to utilize the power of the atom but was troubled by the lack of restraint he feared society in the future would have in using it. Fermi mused on and off about the contradiction about the probability of life forming in the universe and the fact that there was no evidence of biological activity in space. This contraction is known as The Fermi Paradox. 

Fermi's colleague, Edward Teller,  further remembers these musings as, "a statement that the distances to the next location of living beings may be very great and that, indeed, as far as our galaxy is concerned, we are living somewhere in the sticks, far removed from the metropolitan area of the galactic center." Fermi then made a series of rapid calculations using estimated figures in his classic style of "back of the envelope" estimations from first principles using minimal data.
Enrico Fermi followed up with a series of calculations on the probability of earth-like planets, the probability of life given an earth, the probability of humans given life, the likely rise and duration of high technology, and so on. He concluded on the basis of such calculations that we ought to have been visited long ago and many times over. The fact that there has been no evidence of visitation is the Fermi Paradox.
In this exercise, Fermi essentially anticipated and pre-dated many of the elements that went into the Drake equation, named after the radio astronomer Frank Drake who used this equation as a scientific sentence to create a dialogue around the idea of The Search for Extraterrestrial Intelligence, S.E.T.I.

The Drake Equation is a mathematical sentence which states some of the factors involved in making a possible estimate for the number of civilizations in the galaxy transmitting radio signals. Fermi made similar arguments using his famous methods for making clear estimates with minimal data available.

To ask such a question is perfectly normal, given our everyday experiences. Humans evolved on a planet teeming with life, even in places deemed unfit for ourselves to live we know that some form of life thrives in it, be it the hottest deserts, tallest mountains, deepest trenches or coldest wastelands. Therefore it is hard to conceive of a world, as real a place as our own world that is completely empty of life. 

On The Possibility of Simple Extraterrestrial Life:

On earth, most of us will wash our hands immediately after we pick something up from the ground. 
Why? because we know that the ground is filled with various bacteria that will quickly put our internal systems on their menus. So what would these same people do if they picked something up from Martian soil? Would they wash their hands knowing that there was nothing to wash off but sand? Is the presence of life around us so common that it drives our instincts?
Perhaps is it that we may not know, even when people are actually living on Mars, whether or not we are the invaders on a planet ruled by bacteria.

For most of the history of science we have had to rely on only one petri dish, namely Earth, to test the idea of how favorable life is in the universe as a whole. This makes the pursuit of question of the origin of life, when confined solely to the Earth, impossible to answer because we are overrun with variables.

During the 1990 flyby of Earth of the NASA Galileo spacecraft, the famous astrophysicist Carl Sagan carried out a controlled experiment using the NIMS instrument to look for biosignatures on Earth based on a few chemical markers characteristic of life. The spectrometer found abundant molecular oxygen in atmosphere, a sharp absorption edge in the red part of the visible spectrum due to vegetation, and atmospheric methane in extreme thermodynamic disequilibrium. All of these biosignatures are highly suggestive of life on Earth. 

Sagan's experiment is important because it allows us to narrow down the number of variables to that of essentially spectroscopy of a few characteristic chemicals present on an earth like planet, and assuming earth-like life. The search for earth-like planets beyond our solar system severally limits our variables we can receive from such sources. Hence limiting the search to simple, but easily collectible and hence abundant, variables that can be tackled with a group search of matching conditions is the only solution to finding out if a planet at the other end of the galaxy is habitable or not.

Future space-based optical telescope interferometers equipped with imaging spectrometers will be able to obtain integrated spectra of the full disk of earth-like exoplanets to search for biosignatures in their atmospheres. 

For this they would need to have a group search for potential biosignature gases at their characteristic spectra of light reflectance, including:

  • Water 2.7 microns, 6.3 microns, 19.51 microns
  • Nitrous Oxide 3.8 microns, 4.5 microns, 7.78 microns, 17 microns 
  • Methane 3.3 microns, 7.7 microns 
  • Ozone 9.65 microns 
  • Oxygen 0.69 microns, 0.76 microns, 1.26 microns 
  • Carbon Dioxide 2.7 microns, 4.3 microns, 15 microns 
  • Carbon Monoxide 4.7 microns 
  • Nitric Acid 11.5 microns
  • Chlorophyll a 6.76 microns 
  • Other potential biosignatures include Ammonia, Sulfur Dioxide, H2S, CH3Cl, and DMS

Some Solid Signatures on Rocky Planets include:

  • H2O Ice 1.25, 1.5, 2.0 microns
  • Silicates 1.0, 2.0 microns (broad)
  • Ferric Oxides 1.0 microns
  • Carbonates 2.35, 2.5 microns
  • Hydrated Silicates: 3.0-3.5 microns (broad)

Pigments in Earth-sized planets orbiting stars somewhat brighter than the Sun could absorb blue (450 nm) and reflect yellow, orange, red, or a combination of these colors.For stars cooler than the Sun (M Type), evolution might favor photosynthetic pigments to pick up the full range of visible and IR light. With little light reflected, plants might look dark to human eyes. The red edge spectral position could be shifted for other Earth-like planets with a different parent star.

Photosynthesis on Earth produces the most detectable signs of life at the global scale. The presence of oxygen or ozone in an atmosphere simultaneously with reduced gases like methane is considered a robust biosignature (Des Marais et al., 2002). A challenging, complementary observation to atmospheric oxygen would be detection of the vegetation red edge - the strong contrast in red absorbance and near-infrared reflectance of plant leaves due to green chlorophyll. Although the reason for the placement of the Earth’s rededge at 0.7 microns is still not fully explained, scientists have proposed it is due to the function of chlorophyll a (Björn et al. 2009). On an extrasolar planet however, the red edge may shift due to different types of plant life and/or the spectral class of the host star. 

Terrestrial biosignatures resulting from biological species include a disequilibrium in atmospheric gas species, the red-edge of plant life due to the enhanced reflectivity in the near-IR and strong absorption in the red, and biosignatures that vary with time, such as seasonal variations in atmospheric composition and/or surface albedo.

Any small amount of molecular oxygen in an earth-like planet's atmosphere produced by photolysis of water vapor is consumed through oxidation of surface rocks and volcanic gases. Thus, if oxygen and liquid water are simultaneously observed in a spectrum, there must be some additional source producing the oxygen. The most likely source would be oxygenic photosynthesis. If ozone and liquid water are seen in a spectrum, it would be a very strong biosignature. The formation of ozone (O3) requires the presence of oxygen in the planet's atmosphere, since UV radiation dissociates molecular oxygen, which then recombines to form ozone. Ozone has a spectral signature in the infrared part of the spectrum, making it easier to detect than oxygen (which is detected at visible wavelengths).

If both oxygen and methane are detected together, it is a strong indication that photosynthesis is occurring. Also, if imaging spectroscopy detects a seasonal trend (variation) of methane abundances, it is an indication of life because methane levels will eventually begin to decrease due to dissociation from stellar radiation. Methyl chloride might be an indicator of burning plant life due to fires. It is also due to an interaction between sunlight and ocean plankton and chlorine in seawater on Earth. However, oxidation acts as a sink and its signature may be too weak to detect. Nitrous oxide is released as vegetation decays on Earth. Nitrogen is released in the form of nitrous oxide. Since abiotic sources of this gas (lightning, etc.) are negligible, it could be used as a possible biosignature.

Photosynthesis from plant life on earth produces molecular oxygen. However, the dissociation of H2O by UV photons, which also produces O2,  is an inorganic process. On Earth, oxygen is stored in the atmosphere, and part of it is destroyed by oxidation of anoxidized rocks, freshly delivered by tectonic activity. However, the oxygen content of Earth's atmosphere is significant (about 20%) because of the dominance of oxygen production from biogenic sources. The ratio of O3/O2 is an indicator of the degree of evolution of biological activities on an earth-like planet. Ozone is produced by the photolysis of oxygen. Ozone is a good tracer of oxygen in the atmosphere of an Earth-like world. The spectroscopic detection of molecular oxygen and a reduced gas (methane or nitrous oxide) provides very strong evidence for the presence of life on an Earth-like planet.

The three spectra shown below for Venus, Earth, and Mars illustrate the effect of life on Earth. All three terrestrial planets shown strong absorption in their atmospheres due to carbon dioxide. Only the Earth's atmosphere shows two biosignatures due to life, water and ozone.

In our own solar system, the exploration of worlds like Mars, Europa and Titan open up the possibility to answer how life arose on Earth and how it can arise beyond it. If the conditions which allow for the origin of the molecules of life and the sustainability of biological functions of these molecules are favorable on some of these worlds, if it then turns out that the molecules of life and life itself did not evolve on them at all then it gives us the perfect example of an experiment and a control where life can emerge on one world and not the other even when conditions allow it. This would allow us to narrow down to the root cause of the origin of life on earth.

In the history of life on Earth we do know that once life itself started it was here to stay. Very few of the catastrophes that faced Earth over the 4 billion year timespan of life were bad enough to completely sterilize the planet. In that respect, simple life on the level of microbes may be very common in the universe, with reasonably good chances it can evolve more than once on a given planet, independently, and on one or more planets, moons or even asteroids in a given solar system by natural chemical processes. The formation of amino acids and precursors to nucleic acids are shown to form in asteroids, the atmosphere of Titan and on the early earth in as demonstrated in Urey-Miller type experiments. Similar reactions could occur in hydrothermal vents. 

These facts make Europa, one of Jupiter's larger moons, the strongest possibility of an inhabited world other than the Earth. The strong tidal resonance between Europa, Jupiter and another moon of Jupiter, Io, means that Europa experiences a continuous amount of stretching and compressing in its orbit around Jupiter. This stretching and compressing creates a similar effect to if you stretch and compress dough or clay, it heats up. This internal heating creates an energy source within Europa. This heat source could  among other things, melt the ice to a considerable depth, creating an ocean world, coated in a shell of ice, with more liquid water than all of the ocean's of the Earth combined. 

Not only does the observational evidence support a subsurface ocean, due to the movement of the fault lines in ice sheets being seen as akin to the movement of sea ice on Earth, but the presence of oxygen in the ice itself has been shown to exist. The oceans of Europa are probably rich in oxygen due to the interaction of UV rays on the ice crystals which produces Hydrogen Peroxide which decomposes naturally into oxygen gas over time. Scientists had detected atomic oxygen emission from Europa since the Galileo mission to Jupiter in the 1990's. 

Europa's molecular oxygen atmosphere is very tenuous, with a surface pressure about 10^−11 that of the Earth's atmosphere at sea level.

However, the movement of the ice sheets over one another over thousands of years should have given the oceans below the ice a rich supply of oxygen. Similar processes could have happened on the Earth's past environments during the "Snowball Earth" epoch known to have existed before the Cambrian Explosion.

             Artist's rendition of "Snowball Earth", which occurred between 3.5-2.4 billion years ago

The build up of oxygen from years of UV radiation striking ice before earth had sufficient atmospheric oxygen to form an ozone layer would have enriched the planet-wide glaciers with trapped oxygen. The subsequent thawing period after "Snowball Earth" is then speculated to have helped generate enough oxygen in the atmosphere to generate an adequate ozone layer by releasing this trapped oxygen. The presence of an atmosphere with increased oxygen also allowed single celled-organisms, such as cyanobacteria, which had evolved oxygenic photosynthesis to become dominant by wiping out their oxygen intolerant competitors. The dominance of cyanobacteria to fix carbon dioxide from the atmosphere and produce oxygen started roughly 2.4 billion years ago. By producing more gaseous oxygen as a by-product of photosynthesis, cyanobacteria are thought to have converted the early reducing atmosphere into an oxidizing one, which dramatically changed the composition of life forms on Earth by stimulating biodiversity and leading to the near-extinction of oxygen-intolerant organisms. This eventually led to the emergence of complex life on Earth during the Cambrian explosion, which started to happen around 850 million years ago.

This active ice tectonics on Europa has not only created the train track-like chasms seen on its surface, but images of water plumes erupting on Europa taken from the Hubble Space Telescope have confirmed that there is a heat source on Europa that can, at the very least, move liquid water. 

These findings may also allow future space probes to examine such plumes for organic molecules, perhaps dissolved in the water. The discovery of volatile organics molecules in such plumes, in particular ones which can only remain stable in liquid water, would be the strongest evidence for biochemistry occurring beyond the earth yet discovered and would be the closest thing to finding a presently living extraterrestrial organism.

The strategy of searching for water, both on the surface of planets and in their atmospheres, is by far the best start in the search for life as we know it. Other solvents may exist for the molecules of life under extreme conditions, such as the lakes and seas of liquid methane and ethane on Titan, but liquid water has a far greater range of supporting and assisting the chemical reactions associated with biological activity than any other solvent.

Mars, often considered the most likely planet for human habitation other than the Earth, now seems capable of supporting liquid water flows in certain regions at certain times. In craters and canyons along the equator of the planet, salt water trapped in the soil can melt and flow down the surface before being lost by evaporation by the thin Martian atmosphere or being absorbed again by the permeable sandy soil.

We also know now that Mars clearly had an abundant amount of liquid water in the past and a thicker atmosphere with large amounts of vulcanism making the formation of life favorable in the past. 

Whether life exists now is uncertain and debated and needs future missions dedicated to resolving this question once and for all, such as ESA's ExoMars Rover which is designed specifically to search for signs of life under the Martian soil using a drill, which will collect soil samples and return them to the analysis laboratory within the rover itself.

Meanwhile, the ExoMars Trace Gas Orbiter which will look for signs of methane on Mars and try to access whether it is from biological or geological origin.

ExoMars will hopefully be launched in 2016 and may resolve some questions regarding life on Mars not answered since the beginnings of exploration of the planet.

Whatever the findings, lets not forget that our own germs on earth have been the first living astronauts to the planets, as it is highly likely given imperfect decontamination procedures that germs have hitched a ride to mars and probably other worlds too. In that case we should not be surprised if we do find life on Mars, what is surprising is if it did not originate from Earth.

On The Possibility of Complex Extraterrestrial Life:

As for the development of complex and eventually intelligent life existing beyond the earth (or on the earth for that matter) we are uncertain of how favorable multicellular life is and even less certain of how favorable intelligence is in the grand scheme of evolution. 

We can get some clues to answering this from Peripatric speciation theory, which is currently the best mode of explanation for how different species emerge in a given environment, which was founded by the American evolutionary biologist Ernst Mayr. 

American evolutionary biologist Ernst Mayr. 

Peripatric speciation is a form of speciation, the formation of new species through evolution. In this form, new species are formed in isolated peripheral populations such that the populations are prevented from exchanging genes. In his book Systematics and the Origin of Species (1942) Mayr wrote that a species is not just a group of morphologically similar individuals, but a group that can breed only among themselves, excluding all others. When populations within a species become isolated by geography, feeding strategy, mate selection, or other means, they may start to differ from other populations through genetic drift and natural selection, and over time may evolve into new species.  

Peripatric speciation is related to the founder effect, based on the fact that small living populations from a parent population may undergo selection bottlenecks based on the surrounding environmental conditions exerting a selection pressure for certain characteristics.

Based on important theoretical work on genetic drift theory, pioneered by people like Sewall Wright, Mayr argued that during a population decline of a species, such as in a near extinction event, there is a loss of genetic variation that occurs when a the new population is established by a very small number of individuals from the originally larger initial population who survived extinction. 

Genetic drift is then proposed to play a significant role in peripatric speciation, which can occur through random mutations in a given population that can over a reasonably short period of time spread throughout the population as certain dominant traits in a particular species. However the driving force for species creation itself is in fact the loss of the initial genetic variation, caused by some dramatic die-off event.

As a result of the loss of initial genetic variation, which occurs in all species in a die-off event such as a near-extinction, this creates a bottleneck for the existing genetic variations (alleles), that exist through random mutations, from the original population, which will compete with each other over new positions in a biological niche. 

The new population may be distinctively different, both genotypically and phenotypically, from the parent population from which it is derived. In extreme cases, the founder effect is thought to lead to the speciation and subsequent evolution of new species, as theorized by geneticist Alan R. Templeton and others. The most significant and rapid genetic reorganization occurs in extremely small populations that have been isolated (as on islands).

Natural selection, in comparison to genetic drift, operates over a longer period of time and allows existing species to form more specialised adaptations to their environment that could not be formed by random mutation within a population alone. The evolution of specialized eyes for example could not have been caused by a bottleneck of existing variations as the variations between existing eye structures in all the species on Earth are too large and evolved, in many ways, convergently with each other. The differences in the eyes of cephalopods and animals were not caused by a drift between variations in an existing structure and evolved independently of one another. This is different than the genetic drift that occurred between the populations of hominids that existed before one species, homo sapiens, out lasted the rest largely by a genetic drift caused by variations within closely related populations. 

The genetic similarities between modern humans as compared to other hominids is a striking example of species creation from a small evolutionary bottleneck. Anatomically modern humans almost certainly drifted from a parent lineage after a near-extinction event ~100,000 years ago. Thus the variety of "races" we see in modern homo sapiens is merely a case of the rapidly acting genetic drift, favoring certain dominant alleles in certain populations, i.e. dark skin, dark eyes, ect, but by no means having either the conditions, such as a near extinction event, to create a population bottleneck nor the time to create new species of humans all across the earth. This makes it impossible, if we are studying evolutionary biology correctly, to assume that any of the different tribes of modern humans have diverged into separate species in the last 100,000 years. Moreover, even our genetic drift is minimal as compared to other species, humans today have less genetic diversity on opposite ends of the earth than do Chimpanzee tribes on opposite ends of the Congo Basin. In terms of variation, humans across the earth are clones in comparison to most species.

As it turns out, in the long haul of the history of life on earth has shown us that the species with the highest probability of surviving an extinction event will be the ones with the most proficiency for mutation or the largest distributions in the biosphere. In evidence of his reasoning, Mayr himself pointed out that in terms of biological success, which is essentially measured by how many variations of a group of organisms are out there, the organisms that do quite well are those that mutate very quickly, like bacteria, or those that are stuck in a fixed ecological niche, like beetles. 

                                          Beetles are the most diverse group of insects.

Indeed, the most successful class of animal species are insects, which are not considered to be intelligent, at least not in a technical and developmental sense, and yet are more successful than any species of great ape or whale.

Eusocial insects, such as ants, wasps, bees and termites, by sheer numbers and role specification within a colony, display an emergent intelligence in organisation. However the eusocial insects evolved this ability by natural selection, not by development of individual intelligence needed to transmit knowledge via a cultural system. 

The organisation abilities of termites, for example, is based on simple chemical messages transmitted over the population which instinctively organize based on their gene expression, i.e. act as workers, soldiers, ect. This mass social organisation is necessary for the survival of every individual in the colony. 
This creates a superorganism, where every individual organism is a cell in an slightly constrained system. Individual members can survive on their own, as they still have internal structure themselves, however there are some restraints on individual members, particularly on their ability to reproduce. In all superorganisms, there must be a single parental lineage to control the organisational instinct via genetic and hormonal information. 
Because the constraints for survival are not as extreme as in the case of multicellular life, i.e. where there is division of single cells into a unit organ structure, there is more freedom on the system to remain a fluid and amoeboid-like mass of cellular subunits. This is represented in the movement of the colony, which is very much like a aggregate fluid than a compartmentalized unit. Moreover, the colonies these superorganisms make are also asymmetrical and hence require only a few constraints. 


The slight constraints allows the complex behavior of social insects to be simulated with relatively simple Turing machine cellular automatons on a computer, such as the famous "Langton's Ant" computer simulation:

                                                   Langton's Ant

Hence the strategy for survival for termites requires little genetic investment, not the need to evolve a relatively complex brain or culture, and has good payoffs with an individual termite colony being just as successful in its environment as a large herbivorous mammal. Moreover, termites have been doing this for millions of years, while thousands of competing mammal species have evolved and become extinct around them.

By no coincidence, flowering plants such as Orchids (angiosperms) represent the most successful class of land plants, as these have evolved the need to be pollinated by social insects, such as bees, some of which have evolved into very fixed niches themselves, often depending on a single species of insect for pollination.

Orchids are the most diverse, and thus most successful, class of land plants largely due to perfecting the niche of insect-assisted pollination

As far as we know, and we only have one example of one inhabited world in the universe, evolution by natural selection is the only game in town as regards natural biology. Hence we must conclude from the evidence that intelligent life, capable of producing technology like ours, in the universe must be, by natural selection at least*, evolutionarily unfavored, at best difficult to evolve (if not completely unnecessary) and therefore rare in the universe which seems to be consistent with the facts of life on earth.

Most life on earth that existed and indeed that ever existed was not intelligent. Intelligent species are also rare on the earth. We are also the only intelligent species to have evolved on the earth to achieve a technological civilization and it took well over half the time our planet has to even support life before the sun begins to enter its final stages on the main sequence (~5 billion years). Therefore the odds of finding the kind of creatures capable of piloting a flying saucer may be very rare indeed. 

Hence, If a species has evolved to survive on the scale of cosmic time it would be far more likely to have evolved a living strategy closer to the eusocial insects than on mammals if for no other reason than for the sheer level of evolutionary success of insects. 

Nevertheless we should not rule out some of the similarities between emergent behavior of social insects and the emergent behaviour of the dominant intelligence on planet earth, namely humans and the varied civilizations that emerged from collections of humans once the initial organisation capacity to provide stable agriculture to give a sustaining food supply that is permanent enough to provide a way to fuel humans to construct cities and eventually high technology. This itself gave access to new sources of energy that humans cannot assimilate themselves with their biology, such as raw heat and light energy, but provide biological growth as a consequence of being assimilated by our technology - the current human population is undoubtedly a product of technologies that have assimilated natural resources, fossil fuels being the primary one which generate heat energy followed by an often fluctuating balance of either renewable sources or nuclear power. Either way, all energy we assimilate via our technology requires natural resources. Hence to find the likelihood of an advanced civilization existing we must consider the availability and assimilation of such natural resources. 

On The Possibility of Intelligent Extraterrestrial Life:

Going back to the original query of "Where is Everybody?", if we were to assume that there are only a few thousand planets that can support intelligent life in the galaxy and a few hundred civilizations at any given time the question arises that if even a few, 100 say, have been able to survive for a given amount of time with radio technology then their signals will traverse the galaxy for as long as the civilization existed. If the civilization was just going to exist for a few hundred years out of the 10 billion year lifespan of the galaxy, our chances of detecting it would then be staggeringly small. If our civilization was to decline to the point of emitting only the most feeble radio transmissions, in a thousand years say, then we would not have even reached a broadcasting radius of 1/10,000 the size of the galaxy and, even worse, would have only been broadcasting for 1/10 million the age of the galaxy. So really, when we are talking about if there are civilizations out there in space we should really be asking, how long could a civilization like ours really be expected to last, a much more somber question perhaps but one which may be much more important.

When considering a lifetime of a civilization, like our own, which depends primarily on non-renewable fuels for sustenance such as fossil fuels and nuclear fission fuels we can make an estimate on how long it can survive by monitoring how much energy and resources it has available. Historically, it is uncommon for entire civilizations to be wiped out entirely by war itself (though there are exceptions) and are more likely wiped out by environmental change and resource depletion. 

Fortunately, if that's the right word to use, despite the brief tenure of humans on earth we do in fact have a relatively diverse compilation of civilizations that did overreach themselves by falling into "progress-traps" by which over exploitation of a particular resource or dependence on a particular technology caused over-exploitation of the environment and eventual decline. From Ancient Sumeria with its over-reliance on irrigation that led to water depletion of the Tigris and salinization of cropland, Ancient Egypt to its over-reliance on a singular, non-rotated source of farmland (the Nile delta and surrounding floodplains). 

Even Rome and Greece, although fell victim to military overreach, almost certainly failed to keep cohesion in their respective empires as they over-exploited a resource they depended on, namely slave labor. By relying on slavery, several advancements that could have held Rome and Greece together, perhaps even propelling it into high technology, were not accomplished and so the "barbarian" hordes were not opposed by the common, over-exploited everyday people, who were in some sort of social or physical bondage anyway, as they were no more worse off than under callous emperors and dictators.

The case of environmental disaster, war and human exploitation creating a fall of civilization was the case for the Maya and to a lesser extent the Chacoan peoples.

      Tikal Ruins, Guetamala. The Capital of the Mayan Civilization. This was the largest, most advanced society in the America's, before European colonization

Chaco Canyon Ruins. Constructed at the height of the Chacoan Civilization in northwestern New Mexico between 850 and 1150 AD

Both of these civilizations had a rich culture, particularly in astronomy and in timekeeping. The Maya had a calendar that calculated the Earth Year perfectly as 365.2420 days, rivaling the modern Georgian Calenders. A 584-day Venus cycle was also maintained, which tracked the helical risings of Venus. The Chacoan Civilization was the only culture, other than the Chinese, to have recorded the supernova which created what is now called the Crab Nebula. 

However advanced these civilizations were they could not kept their populations from growing out of control, could not conserve their use of limited resources and could not control a changing climate. As a result the people abandoned their civilization and became nomadic. 

An even worse fate was the outcome of Easter Island which, like our own Spaceship Earth, was truly isolated and could not even be reduced to a nomadic culture. The Rapa Nui People, the native Polynesian inhabitants of Easter Island, depended entirely on a non-renewable supply of trees to build their homes and construct fishing boats but in the space of a few hundred years, unregulated population growth and demand had completely deforested the island and with no wood to construct boats to leave their culture was doomed to extinction. 

Rapa Nui or "Easter Island" was the stage of a truly isolated culture that collapsed due to environmental destruction 

Without fish, the islanders began to decimate native bird populations for sustenance. However after even birds became scarce the original culture began to devolve into occultism. "The Birdman Cult" began to replace the original practice of construction of the Moai, or "Stone Heads", on top of the Ahu stone pillars which are now known to have been integral in the practice of ancestor worship on the island. The practice of building or even maintaining the Moai stone heads was abandoned for strange practices including, in a strange foreshadowing coincidence, egg hunting on Easter Island.  

The purpose of the birdman contest was to obtain the first egg of the season from the offshore islet Motu Nui. Contestants descended the sheer cliffs of Orongo and swam to Motu Nui where they awaited the coming of the birds. Having procured an egg, the contestant swam back and presented it to his sponsor, who then was declared birdman for that year, an important status position. During this period the original culture of Easter Island was in the last days of disintegration, the Moai had almost all been toppled by natural disasters such as earthquakes, storms or deliberate destruction by individual clans destroying rival clans statues. These rival clans had been once united under single leadership but the lack of food and resources created weak, ineffective leaders and as such power moved away from common interests into the hands of the singularly powerful warrior class. 

This inter-clan warfare made the islanders lose their common identity and made them vulnerable to the slave trade of the 1800s by invaders who, unlike them, had plenty of ships to come and go as they pleased. The remnants of society was vulnerable and the converting process of the alien religion of Christianity and this did not take long to spread among the leaderless, sick and desperate islanders. Native Easter Islanders lost their identity as first their style of clothing and soon their tattoos and body paint were banned by the new Christian proscriptions. The history of their ancestors was destroyed: artwork, buildings, sacred objects; leaving little record of the islanders' former lives. They were then subjected to forceful removal from their native lands and made to reside on a much smaller portion of the island while the rest was used for farming. Eventually all pure Rapa Nui people died out. The fate was complete extinction of their culture.

Rapa Nui Moai "Stone Heads" with the Milky Way and Large Magellanic Cloud (Small Magellanic Cloud is half hidden behind the 2nd statue from the right) - Does this illustrate how the fate of Easter Island might be the same for all independent civilizations in the universe? 

Is collapse or self-destruction true of all isolated civilizations, even ones with high technology? Do all, isolated civilizations eventually destroy themselves or is the universe filled with highly advanced, interstellar nomads who have outgrown their previous civilization? Is this one or other (or neither) scenarios where our current, technical, global civilization will find itself? 

From these case studies it may be accurate to measure a societies half-life by quantifying how much energy and resources it can continue to use. In the same spirit as the Drake, Equation, these are merely simple back-of-the-envelope estimates of a lifetime of a civilization.

Given current estimates for fossil fuels such as gas, oil and coal, we have enough coal deposits to last another 200 years however climate change caused by fossil fuel use would reduce our civilization's lifespan by approximately the same amount of years; this makes sense, as when we actually run out of coal the climate will have changed and with no infrastructure to use renewable we will have essentially no energy source and so our civilization will recede and fizzle out. This could therefore happen much sooner than 200 years and may even begin to happen before the mid 21st century.

Uranium and Thorium deposits, used correctly, could extend our lifetime for using nuclear fission reactions for power for about  8,500 years. However, given the energy returned on energy invested (EROEI) for fission it will reduce this lifetime by about 1/4.

A civilization that depends on nuclear fission must also insure that it uses a safe form of fission energy, such as thorium reactors, so as to not allow for weapons to be developed easily as a by-product, which can cause the civilization to self-destruct by nuclear war. 

A civilization like humanity's, if using thorium for power could optimistically allow our current technological civilization to last for about 2000 years. Adding renewable energy sources such as solar, wind and geothermal power could extend this to a 10,000 year lifespan but only if the civilization makes the transition to sustainable energy before it destroys itself. It is not entirely clear if human technological civilization will, or can, make the transition away from its dependence on non-renewable energy sources such as fossil fuels before it destroys itself as the manner in which cities and global trade has been set up in highly centralized, top-down approach, means that whenever the primary energy source, namely fossil fuels, is in decline full destabilization occurs with no real backup mechanism to continue development. In many ways it does seem that once oil runs out, then our current civilization, or at least our current way of running it, will be over to be replaced with, under the best case scenario, a far more decentralized and diverse global community. Under the worst case scenario our entire global technological civilization could either decline in growth or completely collapse.

They key point towards growth is the energy available to allow the civilization to not only function but expand. The so-called "new" renewable energy sources that we are trying more and more to use to allow for a sustainable civilization on earth, and perhaps beyond, have in fact always been available, such as solar and wind power, and it is just that we have only recently, through technology, begun to trap what was once considered to be nebulous, almost elemental, energy sources in the forms of light, heat and movement. However the growth of the technology to do this has depended on research and development that has been itself fueled by access to cheap and non-renewable fossil fuels for as long as it has been economically viable to fuel our civilization now and plan ahead for when the fuels inevitably run out so that we can move onto a different energy source, namely solar, wind and safe forms of nuclear energy. The question is, have we used the time and energy given to us by using fossil fuels wisely to prepare for when they are not there anymore? This is something which can only be answered in retrospect. 

Assuming that some form of nuclear fusion energy that occurs in stellar conditions is possible to a civilization like our own in the future, and that deuterium enrichment and extraction becomes energetically viable to do for energy production (perhaps being related to the hydrogen and solar energy economy) then fusion power could also be considered as an energy source to expand the power available for growth. In seawater, 1 out of every 6500 Hydrogen atoms in H2O is a deuterium isotope of hydrogen. About 1 cubic kilometer of completely deuterium seawater, so-called "Heavy Water" could meet our civilizations power demands for  single year. 

However, as explained in a previous article on fusion, the deuterium-tritium fuel cycle is not energetically favorable for power production due in large part to most of the energy escaping in the form of neutrons which cannot be converted into power. Hence, Aneutronic fusion, with Helium-3 say, really is the only game in town as regards to fusion reactors actually producing useable energy. Since Helium-3 is only present in abundant quantities in space-based sources, such as lunar soil and the lower atmospheres of gas giants, then a space-based civilization must already exist beforehand. 

If there was a viable solar and hydrogen economy formed, which would convert water into hydrogen for use as fuel for spacecraft, this would kickstart a feedback loop whereby missions to rich Helium-3 sources, namely gas giant planets, would allow for fusion to start as a by-product of the hydrogen and space-based economy of an advanced civilization. The Helium-3, used for fusion reactors, could then in principle create faster spacecraft, driven by nuclear fusion thrusters, which would in turn lead to faster extraction of Helium-3 which would then lead to further expansion to other Helium-3 sources and so a feedback is created towards expansion of the civilization across space. If that could happen, then the civilization could last for very long eras indeed, anywhere from between tens of thousands years to hundreds of thousands of years.

If there are intelligent beings, or their descendants, that have continuously through their development and acquisition of more and more energy employed radio technology in their society for thousands of years, then we will stumble upon a signal sooner or later when we continue to do conventional radio astronomy, detecting natural radio sources such as Pulsars, Radio galaxies and the hyperfine splitting spectral line of neutral hydrogen at 1420.40575 MHz from the Reinionzation Epoch after The Big Bang. Fundamental research and practical research will mean that nations will continue to build bigger and more sensitive equipment for listening to radio sources in the universe.

 The Low-Frequency Array, LOFAR, is the largest connected radio telescope ever built. All data processing is  performed by a Blue GeneP superomputer situated in the Netherlands.

It is here that Fermi's Paradox arises because if we do not receive any signals then intelligent life must be so staggeringly rare in the universe then by all probability, as intelligent life forms ourselves, we should not even be here asking the question. Moreover, if life is so staggeringly rare in the universe then how can we have plausible and tested explanations for biology, shouldn't we then have concluded independently of this paradox that life is highly improbable? This would be a seeming contradiction as again we are surrounded by life on a planet that has no significantly special physics behind it. As far as we know the rocky planets all formed at around the same time and under similar conditions so why should the earth be special where life is concerned?

There are several answers to solve this paradox, one is using the principle of mediocrity, which can be though of as follows:

We used to think that the natural world was in some sense centered around the desires and needs of human beings. Eventually we began to slowly think of ourselves as a component of the natural world, in that we are subject to its indifferent laws. We also initially thought that the universe was earth-centered, and that the laws of planetary and stellar motion was dependent on a permanently fixed earth, eventually however we then realized that the earth was not even the center of the solar system, let alone the universe which was later still realized to not be confined to a single galaxy.

Even today we are beginning to conceive of our own universe as being part of a multiverse, both in the context of quantum superposition and the extremes of general relativity. So, our position whenever defined as being special or unique in the universe has almost always been overturned to be a mediocre position that is a part of a much broader sequence of which we have currently only seen a small portion of, akin to our range of vision being limited to a small portion of the electromagnetic spectrum but with developments this apparently "special" range turned out to be mediocre with far more ways to possibly see than previously thought possible. 

With the existence of life then, currently seen as confined to a single world in a single range of forms, by the principle of mediocrity this is almost certainly due to be overturned at some point if enough time is given to find out new information, conduct new experiments, build new telescopes, etc.

Therefore by using the principle of mediocrity to prevent short-sighted assumptions, we could assume, based on the chemical abundance, range of conditions available for biochemical reactions and the number of earth-like planets discovered in a deent cross-section of star systems then we can there are probably many millions, maybe up to a billions, of habitable planets in the galaxy capable of supporting simple life, with a few hundred thousand of those having complex life at any given time and maybe up to 1,000 or so with intelligent life. This makes an estimate of a few tens to a few hundred space-faring civilizations a promising estimate.

What kinds of intelligence could exist in the galaxy is hard to predict, and by ignoring the principle of mediocrity we have had our fingers burnt several times by making elaborate predictions based on parochial knowledge, as Dr. Carl Sagan here explains in an interview with Sir Patrick Moore in a clip from "The Sky at Night" from 1974:

Of course even if there are 1,000 space faring civilizations in a given galaxy, ranging from societies that launch Voyager-like robotic craft to those that may construct Bussard Ramjets, Warp Drives, Dyson Spheres or Wormholes our chances of bumping into them or discovering their signals really depends on how long they live. The longer lasting the civilization, the more chances we have of us getting the chance to discover it and consequently the more advanced the civilization is going to be. In this sense, it seems that if we ever do discover an extraterrestrial civilization there is a higher probability that it would be in our technological future. We have only been broadcasting radio for ~100 years, so if a technological civilization can last 1000s of years, it would be more probable that we would pick up a transmission from much older and advanced culture than our own. This can be illustrated more clearly on a bell curve.

If advanced civilizations can survive for thousands of years, on a normalized probability distribution, it is then much more likely that we would receive a signal from a much older civilization if we listen out for them.

It is hard to imagine an event in the galaxy violent enough to destroy a species capable of making structures like Dyson spheres, hence they may very well exist for millions of years. Therefore there may be good reason to listen to signals from space.

Carl Sagan, along with many others, had argued that all technical civilizations, no matter how advanced or exotic, will have to speak in the language of science and mathematics if they are capable of developing and using radio technology in the first place. Moreover, it seems likely that a civilization will stumble upon radio technology for communication at some point in technical development, perhaps even very early, although whether or not radio waves are the golden standard of communication, with fiber optics and lasers being at least the compliment of radio wave communication technology on our own society, not to mention factors such as encryption that would block a lot of our own signals from alien ears. A civilization much ahead of ours could have advanced communication mediums such as neutrino channels or large scale quantum encryption protocols and other forms of information carrying media beyond our current technological ability. 

In any case, in another hundred years mankind's emitted radio signals will envelope a sphere, with the earth at the center, 200 light years in diameter, creating a radio sphere with a volume of 33,500,000 cubic light years. These may sound like impressive numbers but they are nothing compared to the vast expanses of the Milky Way Galaxy:

An illustration of what a radio bubble (yellow speck) from a civilization transmitting radio signals for 200 years looks like on a galactic scale. 

In this image, the square is 10kly x 10kly and the whole galaxy is 100kly x100kly. So a civilization transmitting for 100,000 years will have filled the square and a civilization transmitting for a million years will have filled the whole galaxy. 

Hence we can assume that if we ever do hear a message it will be most likely be from a civilization between 100,000 and 1,000,000 years old because that would be the optimum time period at which a civilization is likely to have transmitted across most of the galaxy. Such messages would be more ancient than all but the most primitive of mankind's archaeological findings and would almost certainly come from a dead culture if we take our previous assumption that an advanced, sustainable culture's lifespan is at best 10,000 years.

Therefore transmitting messages, written in scientific and mathematical text, in long lasting artifacts and mediums such as deep space probes or radio waves is the best way to contact an extraterrestrial civilization and hence the best way to receive contact should be to listen to very weak signals embedded in radio noise over a few important frequencies, perhaps associated with the physical and mathematical constants. 

Of course this would be just a way to get our immediate attention, most likely the data being transmitted from the extraterrestrial source will be embedded in some carrier wave working over a broadband of frequencies. This method would allow large amounts of data to be transmitted efficiently and, if transmitted over a broad spectrum of frequencies, would avoid the signal being scrambled by interstellar fields. This would make the true data being transmitted appear invisible to anyone searching over a single frequency at a time and with only moderate computer power at our disposal, alien signals may be too compressed for us to interpret correctly making the challenge almost impossible unless there is painstaking intent on the messenger to make the signal as easy to understand as possible. 

So it seems, in conclusion, that there are many problems with searching for signs of extraterrestrial intelligence and that perhaps science fiction has in some way spoiled us with exotic tales of alien invasions, first contacts, galactic imperialism and colonialism. In a sense, our ideas about life elsewhere in the universe is sometimes just an unsubtle way to take a different point of view about understanding life on our own planet which is, whether or not there is life elsewhere, unique and very precious in a universe which seems devoid of complex life. The greatest lesson we may then learn from the question of "where is everybody?" should then be a sense of respect for life on this planet, with stronger attempts needed to understand, communicate with and preserve lifeforms on this world. The fact that we have several intelligent beings on our own planet, dolphins, whales, great apes, elephants and even our fellow human beings which are shown indifference, callousness and cruelty should cause delay our attempts to search for life elsewhere and instead focus more on protecting life which we know is in trouble now. 

We know that complex species are vulnerable to extinction, in particular since they are often unable to mutate quickly or occupy the most varied of ecological roles. Even that our own great civilizations from antiquity were very fragile and far from immortal. So before we think we are ready to begin grand strategies to contact alien intelligence we should perhaps take the time to consider whether or not we going about this from the wrong point of view. Instead of focusing more and more on building bigger constructs that will probe deeper into space at the expense of much needed developments on earth, we should perhaps begin to get our own civilization in order in a way which keep our society and planet healthy that would allow us to continue the search for knowledge over a longer period of time. 

Many civilizations built large constructs to connect them to the heavens however they did not survive the real calamities facing them on earth which is an important and sobering lesson for us today. We may have advanced telescopes that can give us glimpses of the first galaxies and robotic spacecraft exploring the solar system but the fact that we have looming environmental crisis and the very real possibility of depletion of fossil fuels that power our society, including its scientific ventures, may see ourselves fall into decline as previous cultures did, advanced though they were. 

The only sense of "first contact" our culture has had with the alien-like cultures inhabiting our planet, from the Ancient Sumerians to the Maya are from their archaeological remains. The only knowledge we have of the alien-like monsters that inhabited the planet in its varied prehistoric forms, from the Cambrian to the Jurassic, is from their fossils and skeletal remains. In a sense the vast distances of space require that if we are to discover an alien signal from a true island civilization in the universe it is certain to be thousands if not millions of years old, the sender and perhaps the culture it was sent from being long dead. In a sense then, the answer of Fermi's Paradox is a fundamental lesson in archaeology, with our own radio transmissions and robotic spacecraft being valuable artifacts in the tomb of a mortal civilization.

The Voyager Golden Record, included on both Voyager 1 and 2 - an artifact designed to last for millions of years in the vacuum of space. Should it encounter extraterrestrial intelligence, instructions are etched on the cover for it to be read, displaying information in the form of physical, chemical and biological values, text, sounds and images in effect chosen to act as a time-capsule. There have been a few attempts to purposefully broadcast mankind's presence in either physical or radio form since the 1970's, however most of our broadcasts into space are from television and radio creating a very diverse picture of our mortal civilization that may be viewed millions of years from now in some distant corner of the galaxy by another civilization.