If an advanced civilisation has sprung up on Earth, it is reasonable to assume that it should have happened thousands of times in our galaxy and millions of times in the universe. However, we have not managed to make contact with, or find evidence of a single intelligent extraterrestrial, and have not found even one alien cell. The Fermi Paradox asks, where are they? These are the possible answers.
The Dark Forest hypothesis is a scary scenario outlined in the Chinese science fiction author Liu Cixin’s book titled Dark Forest. Essentially, all advanced civilisations consider it best not to broadcast their existence, as a self-preservation measure, to prevent any attacks by even more advanced civilisations. If we have not already found any signals from aliens, and they exist, it means that they must have very good reason to not send out these signals, and it is prudent for humans to be cautious too.
The planetarium hypothesis is a paper by science fiction author Stephen Baxter, that is a rather innovative solution to the Fermi Paradox. According to the hypothesis, the universe that the human race is allowed to see is actually a simulation or illusion crafted by a technologically superior species. We do not see any aliens because we are not allowed to. Think of it as a Matrix-like artificial reality, for the entire planet.
According to the Rare Earth hypothesis, the particular conditions that allow life to flourish on Earth are rarer than say what the Drake equation predicts. We need the influence of a gas giant in the outer solar system, to act as a guardian planet, a kind of shield against meteorite impacts. We also need the emergence of eukaryotic life and the diversification seen during the Cambrian evolution. Essentially, intelligent life is incredibly rare.
While Earth actively scans the skies for signs of artificial radio signals, it very rarely broadcasts any of its own. It is an extremely rare event that a radio signal is sent directed towards a particular star, containing information that can be clearly interpreted as artificial. According to the SETI paradox, all advanced civilisations are the same. Everyone is listening with their ears cupped, but nobody is talking.
According to the water world hypothesis, most of the planets in habitable zones around their host stars have more water on the surface. Earth is an outlier case with enough land for space-faring civilizations to emerge. While life is common in the galaxy, none of them have invented fire, rockets or radio telescopes, simply because they all have flippers, fins or tentacles, and no hands.
According to the Zoo Hypothesis, the aliens are deliberately enforcing a policy of wait and watch for humans. They do not want to contaminate us with alien ideas, code or viruses. The civilisation or civilisations may be waiting for humans to reach a particular milestone of technological progress, before interfering with our social, cultural, technological and biological evolution.
According to this solution, while intelligent life has emerged in many planets, it may be that they just have not become technologically advanced yet. These kinds of civilisations would be extremely hard to detect over interstellar distances, and can only be done through direct probes sent to the target planets by humans.
The threat of mutually assured destruction hangs over the Earth, ever since the stockpiling of nuclear weapons began after World War II. Eventually, a sequence of events may lead to people pushing those buttons and launching those missiles. It may be that any advanced civilisation eventually ends up destroying itself, and because of this, we have not been able to find any aliens yet.
Homo sapiens is the only advanced species on the planet, even though there have been many homo species that had the potential to develop an advanced civilisation. On a galactic scale, it may be that any technologically advanced civilisation finds it prudent to cull competition, before it emerges, advances and becomes a threat.
Even if you discount the man-made disaster, there have been five major extinction events on Earth. Over geological time scales, an extinction level event is bound to happen periodically. It may just be that all over the galaxy, life simply has not been able to become advanced enough to escape these periodic extinction events.
A mathematically stronger version of the Drake Equation called the Statistical Drake Equation was used to figure out how many advanced alien species can exist. That comes out to around 4,590 civilisations in the Milky Way, which are on average 28,845 light years apart. The distance may just be too large to allow for interstellar communication, and our current technologies are not sufficient to detect civilisations over such large distances.
Aliens might just be too alien, made up of dust in interplanetary nebula, or choosing to live in space stations at a distance from planets that are subject to periodic extinction events. They may have achieved transcendence or singularity, collected into a hive mind, and have very little interest in physical reality. It is hard enough to understand alien cultures on Earth itself, so extraterrestrial life could have very different motivations and goals. They might have use signals that are too slow or too fast for us to process, and their signals may appear as noise to us.
Given the trajectory of human technology, it is very likely that brain-computer interfaces will allow humans to achieve a kind of virtual immortality, by uploading consciousness into computers. ET may well exist in the cloud, and we need to connect to this alien internet to find them. A planet might not be the best place to look for advanced civilisations.
One of the primary approaches of SETI is to use radio telescopes to scan the skies for signs of artificial radio signals. However, mankind has itself moved on from what is now considered a primitive technology. If other civilisations are anything like us, then there is a narrow window of less than a hundred years, when radio waves from a civilisation can be detected by anyone else. Extraterrestrials may be using technologies to communicate that we cannot detect.
Humanity has been able to detect signs of extraterrestrials for less than a hundred years. This narrow window is not sufficient to conclude that the search has not been fruitful. According to this theory, given more time, our radio telescopes will detect exactly what we are looking for - a clear artificial signal aimed at the Earth by another advanced civilisation.
According to this theory, life may be common enough among the ice worlds and habitable zone planets in the galaxy. However, the emergence of intelligent life is an extremely rare event, and in that man may well be unique. According to the theory, while the emergence of complex life forms is common enough, the emergence of intelligence is not.
We may assume that the relentless march of civilisation means that an advanced race spreads across its own star system, and then jumps to planets in orbit around nearby stars. Even humans have not achieved this in practice, and we simply do not know if enough resources are available for such relentless expansion. There may be other insurmountable barriers that we may not be aware of.
The Fermi Paradox indicates that advanced alien life is not commonplace in the galaxy, or we would have already encountered it. This means that there is some kind of filter that prevents most civilisations from reaching the stars. We do not know what this may be. It could be either behind us or in front of us in human history. We need the right star system, a self-replicating molecule such as RNA, the emergence or of prokaryotic then eukaryotic life, the advent of intelligence, technological sophistication all to happen. If any of these steps do not happen, then the ET cannot be advanced.
The governments have made contact with ET, and are hiding them from us. If you get abducted, do let us know by dropping in a mail to email@example.com.
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