Fermi paradox

The Fermi paradox is a paradox proposed by physicist Enrico Fermi that questions the probability of finding intelligent extraterrestrial life. More specifically, it deals with attempts to answer one of the most profound questions of all time: "Are we (human beings) the only technologically advanced civilization in the Universe?". The vast number of stars in the galaxy suggests that we should not expect such contact to be rare.

Fermi questioned this conclusion. He asked that if there were a multitude of advanced extraterrestrial civilizations in our galaxy, the Milky Way, then, "Where are they? Why haven't we seen any traces of intelligent extraterrestrial life, such as probes, spacecraft or transmissions?" Those who adhere to the premise behind the Fermi paradox often refer to that premise as the Fermi principle.

The paradox can therefore be summed up as follows: The commonly held belief that the universe has many technologically advanced civilizations, combined with our observations that suggest otherwise, is paradoxical, suggesting that either our understanding or our observations are flawed or incomplete.

The Rare Earth hypothesis

An emerging line of thought argues that multicellular life may be exceedingly rare in the universe because of a possible rarity of Earth-like planets. The argument is that many improbable coincidences converged to make complex life on Earth possible. A few examples of such conditions follow.

Spiral arms of galaxies have many novae, and the radiation from them is believed inimical to higher life. The solar system is in a very special orbit within the Milky Way (our galaxy); it orbits in nearly a perfect circle, at a distance in which the solar system moves at the same speed as the shock waves forming the spiral arms. The Earth has been between spiral arms for hundreds of millions of years, more than thirty galactic orbits, almost all of the time there has been higher life on Earth.

Another crucial item is the Moon. The popular Giant impact theory asserts that it was formed by a rare collision between the young Earth and a Mars-sized body, usually referred to as Orpheus or Theia, approximately 4.45 billion years ago. The collision had to occur at a precise angle, as a direct hit would have destroyed the Earth, and a shallow hit would have deflected the Mars-sized body.

The moon is important because its gravitational pull creates tides that stabilize Earth's axis. Without this stability, its variation, known as precession of the equinoxes, could cause weather to vary so dramatically that it could potentially suppress the more complex forms of life. The heat generated by the Earth/Theia impact, as well as subsequent Lunar tides, may have also significantly contributed to the total heat budget of the Earth's interior, thereby both strengthening and prolonging the life of the dynamos that generate Earth's magnetic field Template:Mn. This field protects the conditions conducive to Earth life by preventing the solar wind from stripping away too much of the planet's atmosphere, as well as by deflecting much of the gene-damaging high-energy radiation away from the planet's surface.

Although many consider the Rare Earth hypothesis compelling, others disagree with the scarcity calculation or say complex life may not require exclusively Earth-like conditions in order to evolve (see Evolving the Alien: The Science of Extraterrestrial Life and Alternative biochemistry). Furthermore, in response to the Great Impact theory, regardless of whether the impact occurred, the premise that the creation of the moon was a singular event cannot be proven.

The Drake equation

The Drake equation was formulated by Dr. Frank Drake for estimating the number of extraterrestrial civilizations with which we might come in contact. Subject to the values inserted into this formula, the Drake equation seems to imply that we should not expect such contact to be extremely rare. Those people who believe in the more optimistic assumptions used in the Drake equation and the even more optimistic assumptions given by Dr. Carl Sagan, add that intelligent life should be common in the Universe. Some state that by making what they feel are reasonable assumptions and arguments we can ascertain that if life is possible at all, then the universe is so vast that it should not only be possible, but almost certain that there are large numbers of extraterrestrial civilizations in the Universe. However Fermi principle adherents believe that, due to a lack of evidence to the contrary, in all probability, humans (as a technologically advanced species) are effectively alone in at least our part of the Milky Way. They further say that since we cannot yet determine the variables of the Drake Equation with any real confidence, we cannot determine the numbers of extraterrestrial civilizations based solely on this equation. We must therefore, they argue, rely on data, which are only now beginning to be collected in a significant manner. Only then can we even begin to presume what the values of each of the variables in the Drake equation are, they say.

Current data

Our solar system, if seen from a radio telescope within a few tens of light years, would appear to have unusually intense radio wave emissions for an otherwise unremarkable main sequence star. The increased intensity is caused by broadcasters' use of this part of the electromagnetic spectrum. One can presume that our civilization would immediately characterize as unusual any similar output by a nearby star.

Radio and observational data have for several decades been collected and analyzed by such projects as Project Ozma, the Search for Extraterrestrial Intelligence (SETI), and the various projects searching for extrasolar planets. So far the SETI data show no known main sequence stars with unusually bright radio emissions; this would seem to indicate that we are the only radio-transmitting species in at least that portion of the Galaxy that has been surveyed.

Some contend that these observations are not conclusive since the radio transmission technologies most likely to be detected at interstellar distances, such as the high power continuous wave carrier used in 20th-century (pre Digital) television, are already obsolescent and will soon be replaced with far less detectable technologies, including spread spectrum, fiber optic cable, direct broadcast satellites and low power cellular communication. More advanced civilizations might use communication systems even more efficient and undetectable, with tightly focused emissions (e.g. laser or maser) used to communicate between stars. In this case, we could only detect the communications if we happened to be directly between connected stars — an extremely improbable prospect.

In addition, the majority of the extrasolar planetary systems that have been discovered to date appear to be harsh environments for advanced life-forms. This may be sampling error since we can more easily find planetary systems with orbits and configurations that are less hospitable to life than our own.

The argument over the premise behind the Fermi paradox

E.T. phone home

Some of those who subscribe to the Fermi principle state that given enough time to develop, the radio transmissions of any sufficiently advanced civilization will begin to outshine their parent star in the radio part of the spectrum. Since the use of the electromagnetic spectrum for information transmission is relatively cheap and easy, one would expect any technological civilization to take advantage of at least a part of this spectrum during their development. Despite actively searching for extraterrestrial signals for almost 30 years with projects such as SETI, and passively listening to radio static for nearly 100 years, we have yet to detect any confirmed alien broadcasts. Nor have we observed any main sequence stars with unusual electromagnetic radio signatures that might indicate a technological civilization.

Those that believe the galaxy has many technologically advanced civilizations counter that the extraterrestrials may simply be using a medium other than radio, or hiding their transmissions. Proponents of the Fermi principle say that this could very well be so, but only if there are very few such civilizations in both space and time and only if they very quickly abandon radio as a means of data transmission. Either way, they say, if there were many of these civilizations their transmissions would make a large impact on at least some part of the electromagnetic spectrum for at least a small part of their development. They further state that if there were as many advanced extraterrestrial civilizations as Drake and Sagan have estimated, then their presence would be made obvious by their transmissions. The fact that we have been able to receive and produce these transmissions for only a tiny fraction of our history allows only a weak lower-bound on the minimum separation of civilizations in time and space to be inferred through radio astronomy results. However, Sagan postulated many long-lived civilizations with vastly greater outputs; the lower bounds on the distance to such "Type III" civilizations (see Kardashev scale) can be set much higher.Template:Mn

The anthropic principle

Similar to the Rare Earth hypothesis is the anthropic principle, the idea that the universe is "fine-tuned" to fit life as we know it. The principle states that because life on Earth could not exist if any one of many parameters of the physical universe were slightly modified, it would seem that humans have a unique advantage over any other form of intelligent life. This makes it conceivable that they are the unique intelligent species. There are two variations on this principle: the strong and weak. The former is a more deterministic reading which asserts that the universe exists in its current form to provide the end result of human life, while the latter asserts that the human race exists simply as a result of the parameters being as they are.

Critics argue that the anthropic principle is essentially a tautology; life as we know it may not exist if things were different, but a different sort of life might exist in its place, for whatever reason, depending on which side of the anthropic fence you sit on (so to speak). Even more striking are several 2004 publications of Stephen Hawking, which assert that there is a 98% chance that a universe of a type such as ours will come from a Big Bang.

Freeman Dyson's contribution

Popularized by Dr. Freeman Dyson, a Dyson Sphere is an opaque shell around a star. Such a shell would be created by advanced alien civilizations that wished to harness as much of the radiant energy of their sun as possible. The exact design of the Dyson sphere was not specified; it could consist of billions of independent solar collectors and space habitats or be a single unified structure, but in any case it would be made of solid matter and would intercept most of the star's emitted light to re-radiate as waste heat. A star surrounded by a Dyson sphere would thus emit a distinctive black body spectrum without the strong emission lines that incandescent stellar plasma exhibits, probably with its peak unusually far into the infrared for a star of its size. With this speculation, he advised astronomers to search the night sky for unusually colored stars, which, he postulated, could only signify highly advanced and intelligent life. No such stars have yet been found.

Some adherents to the Fermi principle state that it is highly unlikely that all advanced civilizations would not eventually take full advantage of the power source of their home star, and in doing so change the electromagnetic signature of their sun.

Dr. Dyson also proposed a type of invention which he deemed likely to appear within the life-span of an intelligent civilization, the absence of which tends to support the Fermi principle. He believed it would soon be possible for us to create an explorer-device that would draw power from its surroundings to propel itself through the universe in search of intelligent life forms. Moreover, it would be possible to create versions of this device which could create and launch vast numbers of copies of itself by the process of machine reproduction. Even allowing for the realities of vast distances between stars and the relativistic speed-limit, if intelligent life were common, stars in our galaxy much older than our own would be within range to have built and launched fleets of these automated exploration devices. See Astrochicken, Von Neumann probe.

Extraterrestrial colonization

Adherents to the Fermi principle furthermore argue that, from what we know about life's ability to overcome scarcity and colonize new habitats on our own planet, we can reasonably assume that life elsewhere will follow similar principles. Given this, Fermi principle adherents state that any advanced civilization will almost certainly try to seek out new resources and colonize first their solar system, and then surrounding solar systems. Several writers have tried to estimate the amount of time it would take for such a civilization to colonize the entire galaxy. What they have determined is that it would take 5 to 50 million years to accomplish this featTemplate:Mn — a relatively small amount of time on a geological scale, let alone a cosmological one.

Possible explanations

A variety of solutions have been proposed in response to the Fermi paradox, which can generally be grouped as follows.

They exist, but cannot technically reach us

Many assumptions on the ability of an alien culture to colonize other stars, let alone come near the solar system, are based on the idea that it is technologically possible to travel at a rate exceeding the speed of light. However, the work of physicists since Albert Einstein all hold that the speed of light is the universal 'speed limit' for any intelligent creatures, and all possible solutions to this problem can only be addressed in purely theoretical terms. Given the vast distance between stars, as well as the highly specific conditions needed for intelligent life to develop, it would be thus unlikely that such disparate races would ever come into contact.

While some variations of this theme hold that an alien race can colonize other star systems while obeying the speed of light's limitation, current theoretical technology (such as Project Daedalus) requires many resources that any society would have trouble organizing (see below).

They exist and have arrived—but most people have yet to see them

Those who attribute UFOs to alien spacecraft have a ready answer to the paradox: that it is not unreasonable to believe that a lifeform intelligent enough to travel to our planet would also be sufficiently intelligent to exist here undetected. Alternately, we may have been detected at a distance and either a return message, or an alien emissary itself, is currently en route. The fact that they have not detected us sooner or we did not detect them first may be simple coincidence. Another possibility is that aliens have arrived and are observing us, but are debating when or whether to establish contact. Such observation could be conducted in a number of ways that would be very difficult to detect, for example via nanotechnology on Earth or passive monitoring from elsewhere.

Many UFO researchers and watchers argue that society as a whole is unfairly biased against claims of alien abduction, sightings, and encounters, and as a result may not be fully receptive to claims of proof that aliens have already visited our planet. Others use complex conspiracy theories to allege that evidence of alien visits is being concealed from the public by political elites who seek to hide the true extent of contact between aliens and humans. Scenarios such as these have been depicted in popular culture for decades, with recent favourites being The X-files television series, and the eponymous Men in Black, named for the hypothetical government agents who suppress knowledge of alien contact.

Some people believe that aliens are visiting by astral projection or similar techniques, rather than physically, and that such methods might be the most efficient way to communicate or observe others across interstellar distances.

These theories are criticized as highly speculative by many scientists.

They exist—but we have missed them

Some commentators (such as British science fiction author Stephen Baxter) have pointed out that humanity's ability to detect and comprehend intelligent extraterrestrial life has existed for only a very brief period—by the early 21st century, perhaps only a century at best—and that Homo sapiens itself is a recent species, given the apparent size and age of the universe.

According to this view, humanity has simply not been around sufficiently long to encounter alien life. For example, one million years ago—a relatively brief period in cosmological terms—there would have been no humans for alien emissaries to meet, as modern humans only appeared about 200,000 years ago. For each step back further in time, there would arguably have been increasingly fewer indications to such emissaries that intelligent life as we know it would develop on Earth. In a large and already ancient universe, a space-faring alien species may well have had many other more promising worlds to visit and revisit.

Even if alien emissaries visited in more recent times, they may have been misinterpreted by early human cultures as supernatural entities.

This hypothesis depends upon all visiting civilizations ultimately stagnating or dying out, rather than expanding. However, this cannot be ruled out, since the whole period of modern human existence to date (about 200,000 years) is a very brief period on a cosmological scale, a position which changes little even if our species survives for many more hundreds of thousand of years. Even if intelligent life undergoes a continuous cycle of birth, extinction and rebirth across the universe, civilizations may simply be too far apart in either time or space to actually meet.

A more recent idea, called the fiber optic objection, observes that the use of broadcast technologies like radio for the transmission of information are fundamentally wasteful of energy and that advanced technological civilizations might not use them for that reason. Because broadcasts are radiated in all directions evenly, a large amount of power is needed for a transmitter to send messages any significant distance. Adherents of this concept observe that human technology is currently moving away from broadcast for long-distance communication and replacing it with wires, optical fibers, and focused electromagnetic technologies like aimed narrow-beam radio, microwave, or laser transmission. Most recent technologies that employ broadcasting, such as mobile phones and Wi-Fi networks, use very short-range transmitters to communicate with fixed stations that are themselves connected by wires or narrow beams. It is argued that this trend may make Earth itself nearly undetectable from space within a few decades, and that therefore most civilizations would only be detectable for a short period of time between the discovery of radio and the switch to more efficient technologies.

A variation on this concept has recently emerged as a result of growing awareness of the problem of peak oil. It has been pointed out that our civilization has been capable of interstellar radio communication for only a few decades. If it is correct that we are running out of fossil fuels, then it may only be a few more decades before energy becomes too expensive, and the necessary electronics and computers too difficult to manufacture, for us to continue the search. If the same conditions regarding energy supplies hold true for other civilizations, then it may be that a given civilization would only be able to transmit for a very short time. Therefore, unless two civilizations happen to be near each other and develop the ability to communicate at the same time—which is statistically unlikely—it would be virtually impossible for anyone to talk to anyone else. This explanation is not based purely on the exhaustion of fossil fuel because the energy output of the sun impinging on Earth is vastly greater than currently generated by capital energy consumption; it is argued that some form of technological regression consistently occurs preventing the exploitation of renewables.

They exist—but do not communicate with us

Another series of views is that advanced life either deliberately or accidentally hides evidence of its existence from humanity, or is otherwise unable to communicate with us. They may not communicate for ethical reasons, like the desire to encourage our independent development, or be deliberately incommunicado to avoid detection or destruction (also see They no longer exist below). It is also possible that Drake's belief that alien races would communicate with us is a fallacy. He assumed that beings should wish to exchange information, but this information may fundamentally be mutually incomprehensible. Additionally, what we would observe as an advanced, conscious civilization, may not even regard themselves as "beings" (i.e., have no concept of consciousness), and could regard us and/or themselves as merely chemical reactions, predictable or not, but not worth bothering too much about. Alternatively, they might consider us simply technologically inferior, as we see for example ants. Another possibility is that earth is under quarantine because of humankind's warlike tendencies. As it's often humoristically said: If there are intelligent beings out there and they have the power to reach us... then they surely are too intelligent to actually make any effort to reach us.

Another proposed idea is the so-called Zoo hypothesis that Earth is being monitored by advanced civilizations for study or for ethical purposes. This idea is similar to the Prime Directive of the fictional Star Trek television series. It is sometimes extrapolated from this idea that humanity needs to pass a certain ethical or technological boundary before we will be able to make contact with advanced alien life.

Another possible explanation is that advanced civilizations would construct multiple concentric Dyson Spheres around their stars, each one radiating less energy per area than the next smallest one, with the outermost sphere radiating at close to the background radiation. These would be essentially unobservable from any distance Template:Mn. This has the uniformity of motive flaw common to other Fermi solutions, of requiring all alien civilizations throughout time to behave in the same way.

It has also been proposed that a fundamental information theoretical axiom might be behind the lack of recognized signals. Information theory states that a message that is compressed maximally is indistinguishable from white noise to those ignorant of the compression algorithm. However, the radio telescope SETI searches to date all assume the simplest form of signal, a radio beacon, which is a pure sine wave with no modulation, other than that due to relative motion. The assumption is that the alien civilisation is intentionally broadcasting an easily detectable unmodulated beacon signal to facilitate contact.

The radio searches to date would completely miss very intense highly compressed broadcasts spread across the spectrum. The counterargument to this is again uniformity of motive; for the "compression" answer to suffice there can be no niche technologies optimized on lines other than data compression.

Another possibility is that the evolution of computing technologies leads first to the enhancement and/or replacement of biological organisms by cybernetic entities. The advancement of computer capacities so vastly increases the rate of evolutionary change that a Technological singularity occurs and eventually the entire material substrate of a solar system is converted into a Matrioshka Brain [1]. At that point any form of photonic communication would have an extremely low capacity compared to that of the Matrioshka Brains. Simply put, advanced civilizations would consider such communications as a waste of time, like trying to drink a malt through a microscopically small pipette.

They exist and communicate—but we are not listening or dismiss the evidence

Another series of views, which tend to be disregarded by contemporary science, consider that alien entities have been communicating with humans throughout history, but for any number of reasons we are unable to scientifically detect these attempts; or that the accounts of communication that have been reported are generally dismissed by scientists for sociological reasons.

For example, radio telescopes may simply not be facing in the direction from which signals are arriving, or listening on the appropriate frequencies, or using the appropriate demodulation. Alternatively, intelligent life may be using an esoteric, highly advanced or other non-conventional method of communication which we are unable (or perhaps unwilling) to detect or interpret.

These views do not necessarily assert that aliens are physically present on Earth. Rather, these views suggest that, for instance, if there is a communication medium that is more effective than electromagnetism over interstellar distances, e.g. a form of nonlocal 'psychic' communication, then physical travel would be superfluous and unnecessary, thereby offering a solution to Fermi's paradox.

For example, some have interpreted the accounts of mystics, shamans, schizophrenics, and channelers as evidence for a type of ongoing communication. In support of this view it has been posited that if the brain can somehow operate using quantum mechanical processes, as proposed by Roger Penrose and Stuart Hameroff, then in principle a type of nonlocal communication may be possible, which may then be enhanced or facilitated by traditional shamanic, meditation, or kundalini yoga techniques.

In addition, Terence McKenna and others who have experimented with Dimethyltryptamine (DMT) experiences have suggested that aliens (perhaps originating from other 'dimensions' or parallel universes), are communicating with humanity through unidentified processes which are activated by DMT. Proponents of this view believe that such experiences may have occurred, in the past as well as the present, amongst certain individuals such as tribal shamans, as DMT is endogenously synthesized in the brain's pineal gland, or can be ingested by shamans as the drug ayahuasca. It is suggested that the ability to utilise DMT for this purpose of contacting alien intelligences may have conferred evolutionary advantages by way of natural selection (e.g. by inspiring shamans to lead their tribes to undiscovered lands, such as by crossing the Bering Strait), or out of life-threatening situations (such as in the case of the shaman Black Elk).

Although classical communication in the context of information theory is not possible using quantum nonlocal correlations, supporters of this view believe that it may explain the 'garbled', associative, and inspirational nature of the 'messages' recorded in the world's religious and anthropological history.

The concept of biology as a basis for communication with alien intelligence has certain analogies with the field of neurotheology, which studies biology as a basis for spirituality and transcendental experiences.

That aliens live among us and knowledge of this is suppressed is a common feature in many conspiracy theories, for example those regarding reptilian humanoids. A comical suggestion along these lines, illustrated in such films as Evil Alien Conquerors, is that aliens live among us but are morons and passed off as insane or eccentric. Another example of this in fiction would be the so-called "mice" in Douglas Adams' classic the "Hitchhiker's Guide to the Galaxy". Mr. Adams also talks about another superior being here on earth in the version "So Long, and Thanks for all the Fish".

They no longer exist—or we do not exist for long enough

Yet another idea is that all intelligent life inevitably destroys itself or evolves towards a technological singularity and quickly becomes unrecognizable to humanity in our present state. An intriguing idea related to this is that technological singularity may reflect a Universal inflection point in the development of information processing capability. The development of information processing, and the existence of information itself, appears to be a fundamental product woven into the fabric of the Universe, just like gravity or the strong nuclear force. Therefore, information processing capability may be acted upon by ongoing developments in the ever-expanding Universe, and something like a technological singularity may simply be the product of the triggering of one or several metastable states, like the condensation of matter when a critical temperature had been reached in the early Universe. Similarly, in the technological singularity, all of the matter in the universe can be imagined to be turning into a giant information-processing machine or living being of some sort, or a society of overwhelmingly complex computers or self-enhanced biological organisms. Post-singularity, the universe is no longer a collection of dead rocks and inanimate matter, but is becoming one giant living thing, with every atom put to work as part of this living entity. In regards to extraterrestrial civilizations, a look out into space with telescopes is simply a glimpse at a rear-view mirror. Deep space observation provides no direct observation about the present state of the Universe, especially in terms of where evolution and development (i.e. technological singularity) will take us in the future. In other words, the very first civilizations Universe-wide could all be developing at the same time (now) due to the aforementioned hypothetical metastable trigger in information processing capability. Due to the limited speed of light, there is no way to see what's happening, save here on the Earth.

Science fiction authors have proposed another possible explanation — that someone, or something, is destroying intelligent life in the universe as fast as it is created. This theme can be found in novels such as Frederik Pohl's Heechee novels, Fred Saberhagen's Berserker novels, Alastair Reynolds' Revelation Space novels, Greg Bear's novel The Forge of God, Arthur C. Clarke's Childhood's End, Ian Douglas's series The Heritage Trilogy, K.A. Applegate's novel The Ellimist Chronicles and Jack McDevitt's novel The Engines of God.

Another alternative is that they have disappeared following a technological singularity, or that they simply destroy themselves. Life on Earth, and intelligent life on Earth, evolved as a result of the competition for scarce resources. The evolutionary psychology that developed during this struggle has left its mark on our characters, and left human beings subject to involuntary, instinctual drives to consume resources and to breed. It seems likely that intelligent life on other planets evolved subject to similar constraints, and as such pessimism about their long term viability is a justifiable position.

Technological civilizations may usually or invariably destroy themselves (via nuclear war, biological warfare, grey goo, or in a Malthusian catastrophe after destroying their planet's ecosphere) before or shortly after developing radio or spaceflight technology. This general theme is explored in The Mote in God's Eye by Larry Niven and Jerry Pournelle, which has as its central premise a civilisation that overtaxes its resource base and cyclically self-destructs, but which tries to preserve its culture from one cycle to the next.

It would be anthropocentric to suggest that humanity is immune to any of these fates. Therefore another view is that we ourselves do not exist long enough to encounter alien life. Indeed, there are probabilistic arguments which suggest that our end may occur sooner rather than later. See Doomsday argument.

They do not exist – yet

Theoretically, in any given plurality of intelligent races, one of them has to be first. To avoid their equivalent of anthropocentricity, any such first civilization would likely avoid the possibility that it is, in fact, the first civilization, always assuming that there is some older civilisation somewhere even if it came into contact with numerous younger ones.

If humans are, in fact, the first such civilization, we would appear to be the only such civilisation, but other species may be still evolving or at lower levels of technology or motivation. We may encounter them in the future as we achieve the ability to reach them or as they develop technology sufficient to reach us.

An interesting twist on this involves a common science fiction postulate that some ancient race seeded the universe with life. Presumably that race evolved life on its own. If microbes have survived on any of our space exploration devices (perhaps forced into a natural cryogenic state), they may at some point provide the seeds for life in other worlds.

They never existed

Others argue that the prerequisites for the establishment of life, or at least complex life, are rare in the universe. This returns us to the Rare Earth hypothesis (see section above), which implies that the rise of complex life on Earth required a series of seemingly improbable events, such as the existence of a large moon to provide essential gravitational effects, or the presence of a large planet like Jupiter to shepherd stray asteroids and comets to protect earth from catastophic colision, all of which coincidences being necessary for the evolution of complex life here. Lending some support to this hypothesis is the leading scientific theory for the creation of the Moon, the Giant impact theory, which posits that the Moon was formed as the result of a singular occurrence, whereby a body of a certain mass struck Earth at just the right angle and velocity to carve off sufficient material, which would eventually enter stable orbit and form the Moon. Some consider that the improbability of the occurrence of such an event is magnified by the relative proximity of the massive planets to the inner solar system (too far and the impact rate greatly increases for the inner planets; too close and their orbits are greatly disrupted). The likelihood of complex life evolving as a result of the conjunction of all these events and effects is seen by supporters of the Rare Earth hypothesis as being so unlikely that the evolution of complex life elsewhere must be exceedingly rare.

Although it is possible that complex life may evolve through other mechanisms, the Rare Earth Hypothesis posits that the prerequisites of life as we know it seem to be rare. Bolstering this is the fact that in the extremely long history of life on the Earth only one sentient species has evolved that has the capability of space flight and developing technologies such as radio. So, even when conditions for complex life exist there does not appear (based on the only evidence we have) to be any inevitability that a sentient space-faring species will develop. However it cannot be conclusively proved or disproved, without a survey of the entire Cosmos, that the conditions which are believed responsible for the evolution of complex life on Earth have not occurred elsewhere.

It has also been proposed that even if the conditions for life are common, the probability of sentient life developing on more than one world is so vanishingly rare that in fact, such life has not yet developed outside our solar system. Further, it has been proposed that even if complex life is relatively common, such life may not be interested in the exploration of outer space or developing relevant technology such as radio.

Another possibility is that ice ages, comet or meteor impacts, supernovae, gamma ray bursts or other catastrophic planetary or galactic events are so common that they usually prevent life, let alone complex life, from evolving in the first place. Conversely, if evolution is sometimes aided or facilitated by such events, they may not occur at the right time or place, or at the right frequency.

An episode of the PBS television show 'Nova', entitled 'Death Star', discussed the hypothesis of physicist Arnon Dar regarding galactic sterilization due to gamma ray bursts.[2]

Conclusion

Many views and hypotheses have been put forward to explain, solve, or otherwise address the Fermi paradox. The scientific method provides useful principles and tools to assess the validity of these ideas in their own right and their value relative to each other.

For example, by using Occam's Razor (which generally states that for a given phenomenon the explanation that has the fewest assumptions should be preferred over more complicated ones; also see reductionism), adherents to the premise of the Fermi paradox claim that the simplest explanation would be that we are the only species in our part of the Cosmos that relies on carrier wave radio technology for communication.

Nonetheless, the realities of the vast distances between stars, let alone between galaxies, and the relativistic speed-limit, would seem to render the entire question of intelligent life elswhere very nearly moot.

References

The Millennial Project: Colonizing the Galaxy in 8 Easy Steps by Marshall T. Savage (Empyrean Pubishing; Denver; 1992) pages 341–354 ISBN 0-9633914-8-8
Scientific American — June 2000: "Where Are They? Maybe we are alone in the galaxy after all" by Ian Crawford

External links

The Fermi Paradox: An Approach Based on Percolation Theory by Geoffrey A. Landis
Space.com: Our Galaxy Should Be Teeming With Civilizations, But Where Are They? by Seth Shostak
The Possibilities of FTL: Or Fermi's Paradox Reconsidered by F.E. Freiheit IV
Fermi's Paradox (i.e. Where are They?) by James Schombert
Answering the Fermi Paradox: Exploring the Mechanisms of Universal Transcension by John Smart
The Great Filter — Are We Almost Past It? by Robin Hanson
Extraterrestrial Intelligence in the Solar System: Resolving the Fermi Paradox, which argues that our observations are incomplete, and There Is No Fermi Paradox, arguing that the paradox is based on a logical flaw, both by Robert Freitas
Beyond Kardaschev: Possible Answer to Fermi's Paradox by Paul Hughes
SETI and the Cosmic Quarantine Hypothesis by Steven Soter.

Notes

Template:Mnb These dynamos can only exist in circulating, charge-carrying, fluid portions of a spinning body. For the present-day Earth, this includes only the outer core. However, shortly after lunar formation, a significant portion of the Earth's mantle was also likely molten and probably contributed dramatically to the planet's total magnetic field.

Template:Mnb That is, if searching for a single century's transmissions at Earth-like power output we would only expect to pick up any signals at relatively short ranges and if those signals had been transmitted within the past century. This places only a weak constraint on the frequency of civilizations. However, applying this assumption about the civilizations from which signals would be received implies that radio transmissions last only 100 years at historic Earth power levels for every civilization: This was not the original speculation made by Drake and Sagan.

Template:Mnb Colonization speed calculation.

Template:Mnb See Matrioshka Brain ([3]).