Rather than contending with the long delays a radio dialogue would suffer, a probe housing an artificial intelligence would seek out an alien civilization to carry on a close-range communication with the discovered civilization. The findings of such a probe would still have to be transmitted to the home civilization at light speed, but an information-gathering dialogue could be conducted in real time.
Direct exploration of the Solar System has yielded no evidence indicating a visit by aliens or their probes. Detailed exploration of areas of the Solar System where resources would be plentiful may yet produce evidence of alien exploration,   though the entirety of the Solar System is vast and difficult to investigate. Attempts to signal, attract, or activate hypothetical Bracewell probes in Earth's vicinity have not succeeded.
In , Freeman Dyson observed that every developing human civilization constantly increases its energy consumption, and, he conjectured, a civilization might try to harness a large part of the energy produced by a star. He proposed that a Dyson sphere could be a possible means: a shell or cloud of objects enclosing a star to absorb and utilize as much radiant energy as possible.
Such a feat of astroengineering would drastically alter the observed spectrum of the star involved, changing it at least partly from the normal emission lines of a natural stellar atmosphere to those of black-body radiation , probably with a peak in the infrared.
Dyson speculated that advanced alien civilizations might be detected by examining the spectra of stars and searching for such an altered spectrum. There have been some attempts to find evidence of the existence of Dyson spheres that would alter the spectra of their core stars. Those who think that intelligent extraterrestrial life is nearly impossible argue that the conditions needed for the evolution of life—or at least the evolution of biological complexity —are rare or even unique to Earth. Under this assumption, called the rare Earth hypothesis , a rejection of the mediocrity principle , complex multicellular life is regarded as exceedingly unusual.
The Rare Earth hypothesis argues that the evolution of biological complexity requires a host of fortuitous circumstances, such as a galactic habitable zone , a star and planet s having the requisite conditions, such as enough of a continuous habitable zone , the advantage of a giant guardian like Jupiter and a large moon , conditions needed to ensure the planet has a magnetosphere and plate tectonics , the chemistry of the lithosphere , atmosphere , and oceans, the role of "evolutionary pumps" such as massive glaciation and rare bolide impacts.
And perhaps most importantly, advanced life needs whatever it was that led to the transition of some prokaryotic cells to eukaryotic cells , sexual reproduction and the Cambrian explosion. In his book Wonderful Life , Stephen Jay Gould suggested that if the "tape of life" were rewound to the time of the Cambrian explosion, and one or two tweaks made, human beings most probably never would have evolved.
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On the other hand, other thinkers such as Fontana, Buss, and Kauffman have written about the self-organizing properties of life. It is possible that even if complex life is common, intelligence and consequently civilizations is not. This is sometimes referred to as the "algae vs. Life on Earth has existed for 4 billion years, but intelligent life has only arisen with the genus Homo about 3 million years ago. For most of its existence, Earth was a wild planet. If other uninhabited planets developed similarly to Earth, they are more likely not to bear intelligent life yet.
Charles Lineweaver states that when considering any extreme trait in an animal, intermediate stages do not necessarily produce "inevitable" outcomes. For example, large brains are no more "inevitable," or convergent, than are the long noses of animals such as aardvarks and elephants.
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Humans, apes, whales, dolphins, octopi, and squids are among the small group of definite or probable intelligence here on Earth. It may be that while alien species with intelligence exist, they are primitive or have not reached the level of technological advancement necessary to communicate. Along with non-intelligent life, such civilizations would be also very difficult for us to detect,  short of a visit by a probe, a trip that would take hundreds of thousands of years with current technology.
This hypothesis presumes that other water-bearing planets must have much smaller landmasses than Earth, with terrestrial life being less likely. And the Earth may be anomalous. It may be that because we are so close to our sun, we have an anomalously oxygen-rich atmosphere, and we have anomalously little ocean for a water world. In other words, 32 percent continental mass may be high among water worlds.
This is the argument that technological civilizations may usually or invariably destroy themselves before or shortly after developing radio or spaceflight technology. Possible means of annihilation are many,  including war, accidental environmental contamination or damage, synthetic life like mirror life ,  resource depletion, climate change ,  or poorly designed artificial intelligence. This general theme is explored both in fiction and in scientific hypothesizing. Using extinct civilizations such as Easter Island Rapa Nui as models, a study conducted in posited that climate change induced by "energy intensive" civilizations may prevent sustainability within such civilizations, thus explaining the paradoxical lack of evidence for intelligent extraterrestrial life.
He writes, "There are many stories of islands whose men were almost wiped out—sometimes by internal strife, and sometimes by invading males from other islands.
Another hypothesis is that an intelligent species beyond a certain point of technological capability will destroy other intelligent species as they appear, perhaps by using self-replicating probes. Science fiction writer Fred Saberhagen has explored this idea in his Berserker series, as has physicist Gregory Benford.
A species might undertake such extermination out of expansionist motives, greed, paranoia, or aggression. In , cosmologist Edward Harrison argued that such behavior would be an act of prudence: an intelligent species that has overcome its own self-destructive tendencies might view any other species bent on galactic expansion as a threat. New life might commonly die out due to runaway heating or cooling on their fledgling planets.
These are thought to have been caused by events such as impact from a large meteorite, massive volcanic eruptions, or astronomical events such as gamma-ray bursts. It may be that non-colonizing technologically capable alien civilizations exist, but that they are simply too far apart for meaningful two-way communication. Human searches may be able to detect their existence, but communication will remain impossible because of distance. In this case at least one partner in the exchange may obtain meaningful information.
Alternatively, a civilization may simply broadcast its knowledge, and leave it to the receiver to make what they may of it. This is similar to the transmission of information from ancient civilizations to the present,  and humanity has undertaken similar activities like the Arecibo message , which could transfer information about Earth's intelligent species, even if it never yields a response or does not yield a response in time for humanity to receive it.
It is possible that observational signatures of self-destroyed civilizations could be detected, depending on the destruction scenario and the timing of our observation relative to it. A related speculation by Sagan and Newman suggests that if other civilizations exist, and are transmitting and exploring, their signals and probes simply have not arrived yet. This is a tiny fraction of the lifespan of a galaxy under ordinary assumptions, so the likelihood that we are in the midst of this transition is considered low in the paradox.
Some SETI skeptics may also believe that we are at a very special point of time. Specifically, that we are in a transitional period from no space-faring societies to one space-faring society, namely that of human beings. Many speculations about the ability of an alien culture to colonize other star systems are based on the idea that interstellar travel is technologically feasible.
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While the current understanding of physics rules out the possibility of faster-than-light travel, it appears that there are no major theoretical barriers to the construction of "slow" interstellar ships, even though the engineering required is considerably beyond our present capabilities. This idea underlies the concept of the Von Neumann probe and the Bracewell probe as a potential evidence of extraterrestrial intelligence. It is possible, however, that present scientific knowledge cannot properly gauge the feasibility and costs of such interstellar colonization.
Theoretical barriers may not yet be understood, and the resources needed may be so great as to make it unlikely that any civilization could afford to attempt it. Even if interstellar travel and colonization are possible, they may be difficult, leading to a colonization model based on percolation theory. Colonization may thus occur in "clusters", with large areas remaining uncolonized at any one time. Some colonization scenarios predict spherical expansion across star systems, with continued expansion coming from the systems just previously settled.
It has been suggested that this would cause a strong selection process among the colonization front favoring cultural or biological adaptations to living in starships or space habitats. As a result, they may forgo living on planets.
This may result in the destruction of terrestrial planets in these systems for use as building materials, thus preventing the development of life on those worlds. Or, they may have an ethic of protection for "nursery worlds", and protect them in a similar fashion to the zoo hypothesis. If human-capability constructs in a machine such as mind uploading are possible, and it is possible to transfer such constructs over vast distances and rebuild on a remote machine, then it might not make a strong economic sense to travel the galaxy by spaceflight.
After the first civilization have physically explored or colonized the galaxy, as well as sent such machines for easy explorations, then the subsequent civilizations, after having contacted the first, may find it cheaper, faster, and easier to explore the galaxy through intelligent construct transfers to the machines built by the first civilization, which is cheaper than spaceflight by a factor of 10 8 However, since a star system needs only one such remote machine, and the communication is most likely highly directed, transmitted at high-frequencies and at a minimal power to be economical, such signals would be hard to detect from Earth.
Humanity's ability to detect intelligent extraterrestrial life has existed for only a very brief period—from 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 is a very brief period on a cosmological scale, and radio transmissions have only been propagated since Thus, it remains possible that human beings have neither existed long enough nor made themselves sufficiently detectable to be found by extraterrestrial intelligence.
There are some assumptions that underlie the SETI programs that may cause searchers to miss signals that are present. Extraterrestrials might, for example, transmit signals that have a very high or low data rate, or employ unconventional in our terms frequencies , which would make them hard to distinguish from background noise. Signals might be sent from non- main sequence star systems that we search with lower priority; current programs assume that most alien life will be orbiting Sun-like stars.
The greatest challenge is the sheer size of the radio search needed to look for signals effectively spanning the entire observable 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.
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A signal is much easier to detect if the signal energy is limited to either a narrow range of frequencies, or directed at a specific part of the sky. Such signals could be detected at ranges of hundreds to tens of thousands of light-years distance. Many SETI searches 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 fortunate 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 deliberately sending focused transmissions in our general direction.
It may be that alien civilizations are detectable through their radio emissions for only a short time, reducing the likelihood of spotting them. The usual assumption is that civilizations outgrow radio through technological advancement. Such uses may remain visible even after broadcast emission is replaced by less observable technology.