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Why Can't We See Evidence of Alien Life?

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The Fermi paradox (or Fermi’s paradox) is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilization and humanity’s lack of contact with, or evidence for, such civilizations. The basic points of the argument, made by physicists Enrico Fermi and Michael H. Hart, are:

The Sun is a young star. There are billions of stars in the galaxy that are billions of years older;

Some of these stars likely have Earth-like planets[2] which, if the Earth is typical, may develop intelligent life;

Presumably some of these civilizations will develop interstellar travel, as Earth seems likely to do;

At any practical pace of interstellar travel, the galaxy can be completely colonized in just a few tens of millions of years.

According to this line of thinking, the Earth should have already been colonized, or at least visited. But no convincing evidence of this exists. Furthermore, no confirmed signs of intelligence elsewhere have been spotted, either in our galaxy or the more than 80 billion other galaxies of the observable universe. Hence Fermi’s question “Where is everybody?”.

Stand by for an animated exploration of the famous Fermi Paradox. Given the vast number of planets in the universe, many much older than Earth, why haven’t we yet seen obvious signs of alien life? The potential answers to this question are numerous and intriguing, alarming and hopeful.


Probes, colonies, and other artifacts

As noted, given the size and age of the universe, and the relative rapidity at which dispersion of intelligent life can in principle occur, evidence of alien colonization attempts might plausibly be discovered. Evidence of exploration not containing extraterrestrial life, such as probes and information gathering devices, may also await discovery.

Some theoretical exploration techniques such as the Von Neumann probe (a self-replicating device) could exhaustively explore a galaxy the size of the Milky Way in as little as half a million years, with comparatively little investment in materials and energy relative to the results. If even a single civilization in the Milky Way attempted this, such probes could spread throughout the entire galaxy. Evidence of such probes might be found in the Solar System—perhaps in the asteroid belt where raw materials would be plentiful and easily accessed.[34]

Another possibility for contact with an alien probe—one that would be trying to find human beings—is an alien Bracewell probe. Such a device would be an autonomous space probe whose purpose is to seek out and communicate with alien civilizations (as opposed to Von Neumann probes, which are usually described as purely exploratory). These were proposed as an alternative to carrying a slow speed-of-light dialogue between vastly distant neighbours. 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.

Since the 1950s, direct exploration has been carried out on a small fraction of the Solar System and no evidence that it has ever been visited by alien colonists, or probes, has been uncovered. Detailed exploration of areas of the Solar System where resources would be plentiful—such as the asteroids, the Kuiper belt, the Oort cloud and the planetary ring systems—may yet produce evidence of alien exploration, though these regions are vast and difficult to investigate.

There have been preliminary efforts in this direction in the form of the SETA and SETV projects to search for extraterrestrial artifacts or other evidence of extraterrestrial visitation within the Solar System. There have also been attempts to signal, attract, or activate Bracewell probes in Earth’s local vicinity, including by scientists Robert Freitas and Francisco Valdes.  Many of the projects that fall under this umbrella are considered “fringe” science by astronomers and none of the projects has located any artifacts.

Should alien artifacts be discovered, even here on Earth, they may not be recognizable as such. The products of an alien mind and an advanced alien technology might not be perceptible or recognizable as artificial constructs. Exploratory devices in the form of bio-engineered life forms created through synthetic biology would presumably disintegrate after a point, leaving no evidence; an alien information gathering system based on molecular nanotechnology could be all around us at this very moment, completely undetected. The same might be true of civilizations that actively hide their investigations from us, for possible reasons described further in this article. Also, Clarke’s third law suggests that an alien civilization well in advance of humanity’s might have means of investigation that are not yet conceivable to human beings.

Advanced stellar-scale artifacts

Further information: Dyson sphere, Kardashev scale, Alderson disk, Matrioshka brain, Stellar engine

A variant of the speculative Dyson sphere. Such large scale artifacts would drastically alter the spectrum of a star.

In 1959, Freeman Dyson observed that every developing human civilization constantly increases its energy consumption, and theoretically, a civilization of sufficient age would require all the energy produced by its star. The Dyson Sphere was the thought experiment that he derived as a solution: a shell or cloud of objects enclosing a star to harness 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 that of a black body radiation, probably with a peak in the infrared. Dyson himself speculated that advanced alien civilizations might be detected by examining the spectra of stars, searching for such an altered spectrum.

Since then, several other theoretical stellar-scale megastructures have been proposed, but the central idea remains that a highly advanced civilization—Type II or greater on the Kardashev scale—could alter its environment enough as to be detectable from interstellar distances.

However, such constructs may be more difficult to detect than originally thought. Dyson spheres might have different emission spectra depending on the desired internal environment; life based on high-temperature reactions may require a high temperature environment, with resulting “waste radiation” in the visible spectrum, not the infrared. Additionally, a variant of the Dyson sphere has been proposed which would be difficult to observe from any great distance; a Matrioshka brain is a series of concentric spheres, each radiating less energy per area than its inner neighbour. The outermost sphere of such a structure could be close to the temperature of the interstellar background radiation, and thus be all but invisible.

There have been some preliminary attempts to find evidence of the existence of Dyson spheres or other large Type-II or Type-III Kardashev scale artifacts that would alter the spectra of their core stars. These surveys have not located anything yet, though they are still incomplete. Similarly, direct observation of thousands of galaxies has shown no explicit evidence of artificial construction or modifications.

Explaining the paradox theoretically

Certain theoreticians accept that the apparent absence of evidence implies the absence of extraterrestrials and attempt to explain why. Others offer possible frameworks in which the silence may be explained without ruling out the possibility of such life, including assumptions about extraterrestrial behaviour and technology. 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 both that life is rare, and technical civilizations are short lived, or many other combinations of the explanations below.

Few, if any, other civilizations currently exist

One explanation is that the human civilization is alone (or very nearly so) in the galaxy. Several theories along these lines have been proposed, explaining why intelligent life might be either very rare, or very short lived. Implications of these hypotheses are examined as the Great Filter.

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    Total 5 comments
    • i_think_i_smell_a_rat

      why us earth dwellers think we’re so intelligent and have something to say when pondering a scope that is beyond us

      your title alone…”why can’t we see evidence of alien life”
      contemplate that some…perhaps by a pyramid in Egypt

      as much as we’ve advanced to see “how little we know” about all those galaxies once thought to be stars

      just where do we fit in all this…how important are we…how closed are our minds still to this day

      evidence is scattered all over this rock that this rock has been visited…some yet to be found under layers…or perhaps out in the open

    • Anonymous

      We can’t see them because they are either playing hide and seek, are Romulans or Klingons or have a cloaking device. Everybody knows this already.

    • Ozzie_Thinker

      There is overwhelming evidence that other intelligent life forms exist [other than man] whether terrestrial or ET. What man can’t deal with is why other intelligent do not venture into the mainstream. There are two possible reasons.

      1) Humans are horrible and they avoid us like the plague

      2) They remain hidden [largely] and “rule” by proxy indirectly meddling with our lives via human agents.

      The general concensus of the “alternative” community is that it is option 2 (though different measures are attributed to political logic).

      • Mayhem

        Eh are you serious or am I getting sucked into a leg pull here? Overwhelming might mean something different in your language but in mine they’d have to be doing starbucks regularly for the evidence to be overwhelming. Then again sometimes I just think too hard.

    • HereAmI

      If you start to walk in the wrong direction towards your destination, you will not get to it. This seems fairly straightforward and uncontroversial, does it not ?
      So if your very first thought is wrong, and you base all subsequent thoughts on it, you can develop as many theories as you like, but they will all be wrong. The fundamental mistake, hidden in a few words in this piece, is the following:
      “….. Some of these stars likely have Earth-like planets[2] which, if the Earth is typical, may develop intelligent life;….”
      Specifically, the last four words. The problem is that life does not develop, it is created.
      For a much more illuminating analysis of these issues, I strongly recommend that you go back to Square One, and begin your journey afresh.
      Here is how you do that.

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