The commonly held belief that the universe has many technologically advanced civilizations combined with our observations that suggest otherwise, appears to be paradoxical, suggesting that either our understanding or our observations are flawed or incomplete.
Many people would like to believe that extraterrestrial life exists and that there are many planets in our own galaxy that harbor life. The idea that life is common everywhere and propelled from star to star by the pressure of starlight was proposed by Svante Arrhenius, who called it "panspermia," but this theory is now in disfavor.
Some believe that our current knowledge of both chemistry and of biology strongly indicates that life is an exceptionally improbable thing to arise spontaneously. "Strong life" proponents counter that because life arose on Earth as soon as the crust cooled, life itself must be intrinsically linked with terrestrial planet formation. Current data on this issue seems to support this second view or a related hypothesis that life originated elsewhere within the solar system and was transported to the Earth by a meteorite. The fact that signs of life on Earth seem to be present almost as soon as it cooled enough to support it, that life has been found in a variety of environments once thought incapable of supporting it, that planet formation seems to be fairly common, and that conditions to support bacterial life seem to exist elsewhere in our own solar system all support the position that life should be fairly common. A statistical analysis that treats the question of life arising on a planet like winning a lottery—and generalizing from the special case that, on the only terrestrial world we have seen, the lottery was won—some astrobiologists have concluded that there seems to be at least a one-in-eight chance per billion years of "appropriate" conditions that life will form.
As for the Earthly origin of life, it now seems fairly certain that it began within our solar system. The hard radiation of interstellar space coupled with the extremely low probability that any extra-solar rocks capable of protecting life in the harsh inter-stellar environment have ever struck the Earth seem to indicate that, if terrestrial life originated elsewhere, it would almost certainly have to have been carried here on purpose. It is possible that life was brought here, but, if so, it becomes difficult to explain why the first forms of life were simple, single-celled life instead of further up the evolutionary chain.
A widely-accepted view is that terrestrial life originated on the Earth itself. Lately, there has been increasingly more support for an idea first mentioned by Lord Kelvin— that life first came about on Mars and was transported to Earth by a meteorite. This latter position is defended on the basis that conditions which might support Earth-compatible life existed within a relatively short distance hundreds of millions of years before the Earth cooled. The more improbable that one deems life beginning spontaneously, the more likely it becomes that life arose first on Mars.
The issue of whether intelligent life develops as readily as simpler forms is still an open question.
An emerging line of thought even argues that multicellular life may be exceedingly rare in the universe because of a probable rarity of Earth-like planets. This line of reasoning has been dubbed the Rare Earth hypothesis and relies on that fact that many improbable coincidences converged to make complex life on Earth possible.
Spiral arms have many novas, and the radiation from them is believed inimical to higher life. The solar system is in a very special orbit within the galaxy. It is a nearly perfect circular orbit, 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. Many scientists believe it was formed by a rare collision between the young Earth and a Mars-sized body 4,450 million years ago. The collision had to occur at an exact angle; too direct and Earth would have been obliterated, too shallow and the Mars-sized body would have been deflected. This giant impact sent much of the felsic rich mantle of Earth into orbit. The removal of light-rock types (felsic rock) allowed for the formation of the first ocean basins (which are heavier (mafic) rock). The impact spun the Earth. Lunar tides stabilize the Earth's axis. The axis of rotation of a sphere is unstable, and if the Earth's axis varied, the weather would vary dramatically over periods too short for animals to evolve. Lunar tides also have helped heat the mantle. The molten mantle generates the magnetic field of the Earth. The magnetic field shields the Earth's air from the solar wind, which would otherwise acclerate light molecules away, sapping the air and water over a period of a few million years.
Furthermore, the presence of different crustal rock types allows for the existence of plate tectonics, which recycles limestone into biologically-active carbon dioxide.
This is just part of the Rare Earth hypothesis.
Those people who believe in the more optimistic assumptions used in the Drake equation proposed by Dr. Frank Drake and the even more optimistic assumptions given by Dr. Carl Sagan, add that intelligent life is also 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 civilisations in the Universe. However those people who adhere to the premise of the Fermi Paradox 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 Galaxy. 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 collection - which is 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.
Our solar system if seen from a radio telescope within a few tens of light years away would seem unusual for the huge amount of radio waves being emitted from what appears to be an otherwise unremarkable main sequence star. One can presume that a similar solar system civilization nearby would be immediately characterized as unusual by us.
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 seem to indicate that we are the only radio-transmitting species in at least that portion of our part of the Galaxy that has been surveyed; there are no known main sequence stars with unusually bright radio emissions. In addition, to date, the majority of the extrasolar planetary systems that have been found appear to be harsh environments for advanced life-forms.
Some people contend that these results probably have a significant amount of sampling error: that we are searching the wrong radio frequencies in SETI and that we can more easily find planetary systems with planetary orbits and configurations that are less stable than our own. Still other people contend that we are probably the only spacefaring species in at least our galaxy; otherwise we would be awash in their radio transmissions, and have already been overrun by early colonization efforts.
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. Further, the mediocrity principle states that physical laws are the same throughout the Universe and the development of anything within the Universe has to follow these laws. 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. We have been actively searching for extraterrestrial signals for almost 30 years with projects such as SETI and have been passively listening to radio static for nearly 100. During this entire period we have yet to hear 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 they eventually choose to hide their transmissions for some unknown reason. This could very well be so, proponents of the Fermi Principle say, but only if there are very few such civilizations in both space and time and they very quickly abandon radio as a means of data transmission. Either way, they say, if there were very many of these civilizations their transmissions would make a large impact on at least some part of the electromagnetic spectrum for at least some part of their development. They further state, that if there are as many advanced extraterrestrial civilizations as Drake and Sagan have estimated, then their presence would be made obvious by their transmissions.
Those that believe the Fermi Principle also state that from the Anthropic Principle one can see as a logical fallacy the following statement: "With billions of galaxies and countless trillions of planets in the Universe, intelligent life must exist somewhere besides Earth. After all intelligent life happened here, so why not on many of the trillions of other worlds? It is illogical to think that we are the only one." With the Anthropic Principle, Fermi Principle adherents say, one can quickly point out that if a particular planet is the only planet out of the trillions that has intelligent life on it, it would be certain that the people there would assume that they couldn't be the only planet with intelligent life. They would think that, given the sheer numbers of other worlds, there must be others like themselves in the Universe. However, the Anthropic Principle makes it necessary to gather additional information before such an assumption could be made.
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 liness 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 changing 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 said that he thought that it would soon be possible for us to create an explorer-device which drew 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 own galaxy much older than our own would be within a range to have built and launched fleets of these automated exploration devices.
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 feat [1] - which is a tiny amount of time on even a geologic scale (not to mention a Galactic one).
Solutions to the Fermi Paradox can be grouped in three categories:
Those who attribute UFOs to alien spacecraft have a ready answer to the paradox. However, there is little mainstream support for this view.
Another possibility is that advanced civilizations either deliberately or accidentally hide evidence of their existence from humanity.
They might do so out of ethical concerns for primitive beings or a desire to encourage cultural diversity. This has the Uniformity of Motive flaw common to other Fermi solutions: assuming that all alien civilizations throughout time will behave in the same way.
Civilizations might be deliberately hiding themselves in order to avoid destruction from more advanced civilizations.
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 which is compressed maximally, is indistiguishable from white noise. The counterargument to this would be that even though as bandwidth becomes a bottleneck to communication, there ought still be some niche technologies which would not or could not strive to maximal data compression.
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 and Greg Bear's The Forge of God.
Technological civilizations may usually or invariably destroy themselves via nuclear war, biological warfare, grey goo or destroying their planet's ecosphere) before or shortly after developing radio or spaceflight technology (a very real possibility).
Another possible explanation is that advanced civilizations would construct multiple concentric Dyson Spheres around their stars, each one radiating less energy than the next smallest one, with the outermost sphere radiating energy at close to the background radiation. These would be essentially unobservable from any distance. See Matrioshka Brain.
Yet another is that all intelligent life inevitably evolves towards a technological singularity and quickly becomes unrecognizable to humanity in our present state.
Others argue that the conditions for life, or at least complex life, are rare. For instance, some hypotheses say that complex life required the stimulation of tides from Earth's moon to evolve, and the Moon is the result of freak occurrence, a body of a certain mass striking Earth at just the right angle to carve off the material and put it in a stable orbit.
Another possibilty is that ice ages, comet or meteor impacts, supernovae, gamma ray bursters or other catastrophic planetary or galactic events are so common that life rarely has the time to evolve. Alternately, these events may not be frequent enough on other planets and evolution is slowed because there aren't enough mass extinctions to encourage diversity.
Even if the conditions for life are common, the evolution of human-like intelligence, the invention of radio technology or interest in the exploration of outer space may be vanishingly rare.
All this draws us to a few possible conclusions, say adherents of the Fermi Principle: that as a technologically advanced species, we are alone in our part of the Cosmos. The simplest explanation, say adherents to the premise behind the Fermi Paradox, is the last one.
See also: Fermi problemThe Controversy
Is There Extraterrestrial Life?
The Rare Earth Hypothesis
The Drake Equation
Current Data
The Argument Over the Premise Behind the Fermi Paradox
ET Phone Home
The Anthropic Principle
Freeman Dyson's Contribution
Extraterrestrial Colonization
Several Possible Explanations
They Are Here
They Exist But Have Not Communicated With Us
They Do Not Exist
Conclusion
A common concept used in the scientific method to test the validity of certain ideas is Occam's Razor. To paraphrase, Occam's Razor states that the explanation for a given phenomenon that has the fewest assumptions should be preferred over more complicated ones.