Each of these hypothesized explanations is essentially an argument for decreasing the value of one or more of the terms in the Drake equation. The arguments are not, in general, mutually exclusive. For example, it could be that both life is rare, and technical civilizations are short lived, or many other combinations of the explanations below. Solutions to the Fermi Paradox can be grouped in three categories:
We Are Alone
No other civilizations have arisen
Those who believe that extraterrestrial intelligent life does not exist argue that the conditions needed for life—or at least complex life—to evolve are rare or even unique to Earth. This is known as the Rare Earth hypothesis, which attempts to resolve the Fermi paradox by rejecting the mediocrity principle, and asserting that Earth is not typical, but unusual or even unique. 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. It is possible that complex life may evolve through other mechanisms than those found specifically here on Earth, but the fact that in the history of life on the Earth only one species has developed a civilization to the point of being capable of space flight and radio technology; or, more basically, abstract ideas such as music, art, or religion lends more credence to the idea of technologically advanced civilizations being rare in the universe.
Even if the conditions needed for life might be common in the universe, that the formation of life itself, a complex array of molecules that are capable simultaneously of reproduction, of extraction of base components from the environment, and of obtaining energy in a form that life can use to maintain the reaction (or the initial abiogenesis on a potential life-bearing planet), might ultimately be very rare. Additionally, in the nondirectional meandering from initial life to humans, other low-probability happenings may have been the transition from prokaryotic cells to eukaryotic cells (with separate nucleus, organelles, specialization, and a cytoskeleton allowing the cell to take on different shapes) and the transition from single-cellular life to multicellular life, which was recorded in the Cambrian Explosion of 530 mya when significant numbers of organisms had evolved hard body parts, although multicellular life perhaps first started to evolve a couple of hundred million years before that. Remember, for most of Earth’s history, there have only been single-celled creatures.
It is also possible that intelligence is common, but industrial civilization is not. For example, the rise of industrialism on Earth was driven by the presence of convenient energy sources such as fossil fuels. If such energy sources are rare or nonexistent elsewhere, then it may be far more difficult for an intelligent race to advance technologically to the point where we could communicate with them. There may also be other unique factors on which our civilization is dependent. Or, on a water world, where the intelligent creatures are something like dolphins, it may be difficult to set fire, forge metals and build tools.
Another possibility is that Earth is the first planet in the Milky Way on which industrial civilization has arisen.However, critics note that according to current understanding, many Earth-like planets were created many billions of years prior to Earth, so this explanation required repudiation of the mediocrity principle.
Advanced civilizations are bound to decay
This is the argument that technological civilizations may usually or invariably destroy themselves before or shortly after developing radio or space flight technology. Possible means of annihilation include nuclear war, biological warfare or accidental contamination, nanotechnological catastrophe, ill-advised physics experiments, a badly programmed super-intelligence, or a Malthusian catastrophe after the deterioration of a planet’s ecosphere. In 1966 Sagan and Shklovskii suggested that technological civilizations will either tend to destroy themselves within a century of developing interstellar communicative capability or master their self-destructive tendencies and survive for billion-year timescales.
This argument does not require the civilization to entirely self-destruct, only to become once again non-technological. Most of ancient Indu philosophy is build around the idea of expension and decay of civilizations before they can reach the status of deities (mastering time, life and space).
Life is periodically destroyed by naturally occurring events
On Earth, there have been numerous major extinction events that destroyed the majority of complex species alive at the time. The extinction of the dinosaurs is the best known example. These are believed to be caused by events such as impact from a large meteorite, massive volcano eruptions, or astronomical events such as gamma ray bursts. It may be the case that such extinction events are common throughout the universe and periodically destroy intelligent life (or at least destroy their civilizations) before the species is able to develop the technology to communicate with other species.
Inflation theory and the Youngness Argument
Cosmologist Alan Guth proposed a multi-verse solution to the Fermi Paradox. In this theory, using the synchronous gauge probability distribution, young universes exceedingly outnumber older ones (by a factor of e1037 for every second of age).
They Exist But Have Not Communicated With Us
It may be that technological extraterrestrial civilizations exist, but that human beings cannot communicate with them because of constraints: problems of scale or of technology; because they do not wish to communicate or their nature is simply too alien for meaningful communication, or perhaps even be recognized as technology.
Communication is impossible due to problems of scale
It may be that non-colonizing technologically capable alien civilizations exist, but that they are simply too far apart for meaningful two-way communication. If two civilizations are separated by several thousand light years, it is very possible that one or both cultures may become extinct before meaningful dialogue can be established.
The problem of distance is compounded by the fact that timescales affording a “window of opportunity” for detection or contact might be quite small. Advanced civilizations may periodically arise and fall throughout our galaxy, but this may be such a rare event, relatively speaking, that the odds of two or more such civilizations existing at the same time are low. There may have been intelligent civilizations in the galaxy before the emergence of intelligence on Earth, and there may be intelligent civilizations after its extinction, but it is possible that human beings are the only intelligent civilization in existence now. The term “now” is somewhat complicated by the finite speed of light and the nature of spacetime under relativity. Assuming that an extraterrestrial intelligence is not able to travel to our vicinity at faster-than-light speeds, in order to detect an intelligence 1,000 light-years distant, that intelligence will need to have been active 1,000 years ago. Strictly speaking, only the portions of the universe lying within the past light cone of Earth need be considered, since any civilizations outside it could not be detected.
Human beings have not been searching long enough
Humanity’s ability to detect and comprehend intelligent extraterrestrial life has existed for only a very brief period—from 1937 onwards, if the invention of the radio telescope is taken as the dividing line—and Homo sapiens is a geologically recent species. The whole period of modern human existence to date (about 200,000 years) is a very brief period on a cosmological scale, while radio transmissions have only been propagated since 1895. Thus it remains possible that human beings have neither been searching long enough to find other intelligences, nor been in existence long enough to be found.
One million years ago there would have been no humans for any extraterrestrial emissaries to meet. For each further step back in time, there would have been increasingly fewer indications to such emissaries that intelligent life 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 interpreted by early human cultures as supernatural entities.
Communication is impossible for technical reasons
There are some assumptions that underlie the SETI search programs that may cause searchers to miss signals that are present. Extraterrestrials might also use frequencies that scientists have decided are unlikely to carry signals, or do not penetrate our atmosphere, or use modulation strategies that are not being looked for. The signals might be at a datarate that is too fast for our electronics to handle, or too slow to be recognised as attempts at communication.
The greatest problem is the sheer size of the radio search needed to look for signals (effectively spanning the entire visible universe), the limited amount of resources committed to SETI, and the sensitivity of modern instruments. SETI estimates, for instance, that with a radio telescope as sensitive as the Arecibo Observatory, Earth’s television and radio broadcasts would only be detectable at distances up to 0.3 light years. Clearly detecting an Earth type civilization at great distances is difficult. A signal is much easier to detect if the signal energy is focused in either a narrow range of frequencies (Narrowband transmissions), and/or directed at a specific part of the sky. Such signals can be detected at ranges of hundreds to tens of thousands of light-years distance. However this means that detectors must be listening to an appropriate range of frequencies, and be in that region of space to which the beam is being sent. Many SETI searches, starting with the venerable Project Cyclops, go so far as to assume that extraterrestrial civilizations will be broadcasting a deliberate signal (like the Arecibo message), in order to be found.
Thus to detect alien civilizations through their radio emissions, Earth observers either need more sensitive instruments or must hope for fortuitous circumstances: that the broadband radio emissions of alien radio technology are much stronger than our own; that one of SETI’s programs is listening to the correct frequencies from the right regions of space; or that aliens are sending focused transmissions such as the Arecibo message in our general direction.
More hypothetically, advanced alien civilizations evolve beyond broadcasting at all in the electromagnetic spectrum and communicate by principles of physics we don’t yet understand. Some scientists have hypothesized that advanced civilizations may send neutrino signals. If such signals exist they could be detectable by neutrino detectors that are now under construction. Unless two civilizations happen to be near each other and develop the ability to communicate at the same time it would be virtually impossible for any one civilization to “talk” to another
It may be that alien civilizations are detectable through their radio emissions for only a short time, reducing the likelihood of spotting them. There are two possibilities in this regard: civilizations outgrow radio through technological advance or, conversely, resource depletion cuts short the time in which a species broadcasts.
They tend to experience a technological singularity
Another possibility is that technological civilizations invariably experience a technological singularity and attain a posthuman (or more properly, post-biological) character. Theoretical civilizations of this sort may have altered drastically enough to render communication impossible. The intelligences of a post-singularity civilization might require more information exchange than is possible through interstellar communication, for example. Or perhaps any information humanity might provide would appear elementary, and thus they do not try to communicate, any more than human beings attempt to talk to ants—even though we do ascribe a form of intelligence to them.
The evidence is being suppressed
It is theoretically possible that SETI groups are not reporting positive detections, or governments have been blocking extraterrestrial signals or suppressing publication of detections, perhaps in response to National Security and Trade Interests from the potential use of advanced extraterrestrial technology or weapons. Claims that this has already happened are common in the popular press, ut the scientists involved report the opposite experience – the press becomes informed and interested in a potential detection even before a signal can be confirmed.
They refuse to interact with us
It is possible that the belief that alien races would communicate with the human species is merely an assumption, and that alien civilizations may not wish to communicate, even if they have the technical ability. A particular reason that alien civilizations may choose not to communicate is the so-called Zoo hypothesis: the idea that alien civilizations avoid contact with Earth so as not to interfere with our development, or to preserve an isolated “zoo or wilderness area”.
Many other reasons that an alien race might avoid contact have been proposed. Aliens might only choose to allow contact once the human race has passed certain ethical, political, or technological standards, e.g., ending poverty/war or being able to master interstellar travel.
Such advanced civilizations may 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.
They are too different from us
Alien psychologies may simply be too different to communicate with human beings, to understand the concept of communication, or to even be interested in other lifeforms at all, and so they may be unable or unwilling to make the attempt. Human mathematics, language, tool use, and other concepts and communicative capacity may be parochial to Earth and not shared by other life.
For example, in Contact, Carl Sagan briefly speculated that an alien species might have a thought process orders of magnitude slower (or faster) than humans. Such a species could conceivably speak so slowly that it requires years to say even a simple phrase like “Hello”. A message broadcast by that species might well seem like random background noise to humans, and therefore go undetected.
They are non-technological
It is not clear that a civilization of intelligent beings must be technological. If alien species do not develop technology, because it is difficult in its environment, because it chooses not to, or for any other reason, they will be very hard for human beings to detect. Intelligence alone, as opposed to life, is not necessarily visible across interstellar distances.
They Are Here
It may be that intelligent alien life forms not only exist, but are already present here on Earth. They are not detected because they do not wish it, human beings are technically unable to, or because societies refuse to admit to the evidence. Several variations of this idea have been proposed:
Carl Sagan and Iosif Shklovsky argued for serious consideration of “paleocontact” with extraterrestrials in the early historical era, and for examination of myths and religious lore for evidence of such contact. Sagan and Shklovsky noted that many or most religions were founded by men who claimed contact with supernatural entities who bestowed wisdom, guidance and technology, citing the fish-god Oannes as a particularly salient example. On this hypothesis, there is in fact ample evidence of alien visitation – it is simply not recognized as such.
It is possible that a life form technologically advanced enough to travel to Earth might also be sufficiently advanced to exist here undetected. In this view, the aliens have arrived on Earth, or in our solar system, and are observing the planet, while concealing their presence. Observation could conceivably be conducted in a number of ways that would be very difficult to detect. For example, a complex system of microscopic monitoring devices constructed via molecular nanotechnology could be deployed on Earth and remain undetected, or sophisticated instruments could conduct passive monitoring from elsewhere.