Fermi paradox

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The Fermi Paradox is a train of thought by the physicist Enrico Fermi from 1950. Fermi assumed that there is extraterrestrial intelligence that can sustain technically highly developed civilizations for millions of years. During this period of time, it should be possible to colonize entire galaxies using interstellar and intergalactic space travel - and in all likelihood this should already have happened. The fact that the search for traces of extraterrestrial life has so far been unsuccessful seemed paradoxical to him and as an indication to question either the assumptions or the observations.

prehistory

Enrico Fermi discussed alleged UFO sightings and a cartoon from The New Yorker magazine about aliens on his way to lunch at Los Alamos National Laboratory in 1950 with Edward Teller , Emil Konopinski and Herbert York and finally asked: "Where is everybody?" to observe neither spaceships of other space dwellers nor other traces of extraterrestrial technology?

A detailed scientific examination of the problem began in the early 1970s with studies by Michael H. Hart , which is why the term Fermi-Hart paradox is also used.

Basic consideration

The core of the Fermi paradox is the following:

Due to the age of the universe and its high number of stars, life should also be spread outside the earth, provided that the formation of life on earth was not an unusual process ( rare earth hypothesis ).

Based on the assumption that the earth is not an absolute exception among the planets and that our solar system is not unique (in the sense that it contains intelligent living beings), there is the (so far not quantifiable) possibility that further technical civilizations in our galaxy exist. The galaxy is approximately 100,000 light years wide. A sub-light drive, like that of the Icarus project , with a speed factor of 0.01 to 0.1 the speed of light, would take about 1 to 10 million years for this route (straight ahead and without intermediate stops). The galaxy is about 10 billion years old. It would have been possible to cross it a thousand times over during its existence.

If there is even a single civilization capable of interstellar colonization in the Milky Way , then the entire galaxy could be completely colonized within a few million years. The Milky Way is now much older than the necessary 20 to 40 million years; consequently, extraterrestrial civilizations should exist everywhere in our galactic neighborhood. So far, however, no evidence of extraterrestrial civilizations has been found.

The paradox can be summarized as follows:

"The widespread belief that there are many technically advanced civilizations in our universe, combined with our observations suggesting the opposite, is paradoxical and suggests that either our understanding or our observations are flawed or incomplete."

In short, if they exist, why aren't they here?

Closely related to the Fermi paradox is the Drake equation , which is used to estimate the likelihood of the simultaneous existence of other civilizations in the Milky Way. However, since most of the parameters of the Drake equation are unknown so far , it can hardly contribute anything to the solution of the paradox.

Attempts to explain

There are several approaches to explaining the paradox; A distinction can be made between arguments in principle (which, due to the laws of nature, apply equally to all civilizations) and weak arguments.

The problem with the weak arguments is that Fermi assumes that only a single civilization is necessary to cause the paradox described. However, this civilization must maintain its basic properties over geological time. Weak arguments, however, do not represent any obstacles in principle. We would therefore have to assume that all civilizations develop similarly and therefore fail because of the same obstacles.

The following passage from the work The Fermi Paradox: An Approach Based on Percolation Theory by Geoffrey A. Landis has been translated accordingly:

“Proposed solutions to the Fermi Paradox either completely negate the possibility of extraterrestrial civilizations, an assumption that has not yet been proven, or accept the possibility of extraterrestrial technical civilizations and suggest explanations as to why they nevertheless did not colonize the Milky Way.
The explanations include the suggestion that such civilizations collapse or self-destruct, run out of resources, choose not to colonize or colonize but consciously ignore us.
The problem with these explanations is that they all assume similar motives for civilizations over extremely long periods of time. If even a single civilization decides to colonize the Milky Way, these attempts to explain must fail. "

Principal arguments

The Unusual Earth Hypothesis / We Are Actually Alone

One line of thought is that multicellular life is extraordinarily rare in the universe because Earth-like planets are potentially rare. There were many unlikely coincidences that made life on earth possible. Such coincidences are the position of the sun in the Milky Way (radiation), the position of the earth in the solar system (temperature), the existence of a relatively large moon (stabilization of the earth's axis ), etc.

Ultimately, in this attempt at an explanation, the parameters of the Drake equation are chosen in such a way that only one civilization exists in our galaxy - ours. In this respect, the Fermi paradox loses its paradoxical character because one of the basic assumptions is already rejected.

Although this hypothesis is widely regarded as compellingly convincing, others contradict the rarity of Earth-like planets (which is suggested by the increasing number of discovered exoplanets ) or claim that complex life does not necessarily require Earth-like conditions in order to develop (see carbon chauvinism ).

A special form of this argument assumes that the development of higher intelligence in the course of evolution is extremely unlikely. For example, none of the complex forms of life that existed on earth in the past are known to have emerged or existed of higher intelligence.

Impossibility of interstellar colonization / communication

The prerequisite of the Fermi paradox, namely a civilization that is capable of interstellar colonization, may in principle not be met. Under these circumstances there could be several technical civilizations in the Milky Way, but they are too far apart spatially to influence one another ( illustration of distances ).

To illustrate this, the distance between our sun and the nearest star, Proxima Centauri , is mentioned as an example , which, even at almost the speed of light, could only be reached after a few years. Since it is not possible to exceed the speed of light according to the current state of knowledge, some questions arise:

  • Could a civilization (still) find the resources to reach strange star systems as soon as a situation arises that makes such an undertaking appear worthwhile or even necessary?
  • What temporal offset in communication must populations in different star systems be able to accept in order to have the cohesion necessary for a civilization?

Distribution pattern / civilizational diffusion

According to an approach by Geoffrey A. Landis, the colonization of the galaxy can be investigated using the percolation theory as a process similar to diffusion . Landis assumes two premises:

  1. Each civilization is maximally able to colonize direct neighboring systems in a limited area.
  2. Each colony can develop into a colonizing civilization with a certain probability or a stagnant (or non-colonizing) civilization.

Under these circumstances, the galaxy would not be evenly populated, rather "bubbles" would form, which are bordered by stagnant colonies. No further colonization would then take place within these bubbles. Conversely, there could also be bubbles with a high “density of civilization”. The relationship between these bubbles is significantly influenced by the likelihood of successful colonization and the development towards a colonizing or stagnating type of civilization.

  • If it is below a limit value , colonization will stop after a finite number of colonies.
  • If it is above the limit , almost the entire galaxy will be filled, with the exception of a few small bubbles.
  • When it is close to the limit , the galaxy is criss-crossed by a fractal structure in which both large populated and unpopulated areas exist. We would then live in an unpopulated area.

Self-extinction

Nuclear explosion 1000 times Hiroshima strength ( Operation Castle )

According to the argument of self-extinction, it is in the nature of technical civilizations to destroy themselves. According to Stephen Hawking , the emergence of life is probable and the development of intelligence possible, but at a certain point it would become unstable, so that (unintentional) self-extinction would occur. Possible triggers could (currently) z. B. be: nuclear war , genetically modified viruses and uncontrollable greenhouse effect . The technology philosopher Nick Bostrom also sees the danger of nanobots or an explosively developing superintelligence , each of which can no longer be controlled by humans, as well as unknown imponderables .

"[T] he most likely explanation of negative results, after a comprehensive and resourceful search, is that societies destroy themselves before they are advanced enough to establish a high-power radio transmitting service."

"The most likely explanation for negative results after an extensive and well-equipped search is that societies self-destruct before they are sufficiently advanced to establish high-performance radio communications."

- Carl Sagan

Astrophysical explanation

Gamma ray bursts (GRBs) are considered to be the most energetic phenomena in the universe. They can radiate the amount of energy released over the entire lifespan of a star in seconds in the form of focused jets . Even over light years away, these can trigger mass extinctions and the annihilation of higher life forms on the planets in their cones of rays. With the appropriate frequency, this effect could be responsible for an extensive sterilization of the cosmic living conditions. The probability of a GRB destroying the living conditions of the earth has been 50% for the past 500 million years. According to an estimate based on the Swift data (Tsvi Piran et al.) , Galaxies that formed in the first five billion years of the universe ( z  > 0.5) should generally develop into more complex forms of life due to intensive GRB activity have been impossible. Of the galaxies that exist today, only one in ten lives - as we know it - is said to have had a long enough time to develop undisturbed - but especially in larger galaxies than our Milky Way.

A priori mathematical argument

According to a consideration by the astrophysicist J. Richard Gott , the probability speaks against the thesis that galaxies are usually largely colonized, since then almost all living beings would be members of such supercivilizations. If such civilizations existed, then for statistical reasons it would be rather unlikely to have been born as a member of a comparatively small, young and not yet colonized civilization like ours. This purely mathematical consideration is equivalent to the so-called Doomsday argument and does not allow any statement about the existence of extraterrestrial life, but merely states that if such life should exist, it is most likely not to colonize. This dissolves Fermi's paradox, since J. Gott negates his basic assumption.

The big filter

The concept of the "big filter" tries to combine some of the arguments mentioned above. It says that there is some kind of filter, bottleneck or barrier until reaching a level of civilization that is associated with interstellar space travel. This barrier would be a challenge or threat that would make it extremely difficult to reach this level. It could e.g. B. be that the development of higher intelligence in the course of evolution is exceptionally rare (see We are actually alone ). In this case, humanity would be the only life form that has so far managed to overcome the Great Filter. Another possibility is that with the advancement and expansion of a civilization developments are automatically connected that normally lead to the extinction of this civilization (see self- extinction ). In this case, other civilizations would have failed so far at the great filter and humanity would still have the great filter ahead of them. A possible and plausible as well as illustrative form of the Great Filter is the Kessler Syndrome , a scenario calculated by NASA scientist Donald Kessler, according to which the increasing density of objects in near-earth orbit leads to a self-reinforcing probability of collision, which makes near-earth space an impenetrable barrier could do.

Berserker, Deadly Probes and Dark Forest Theory

The deadly probes or berserker theory (named after Fred Saberhagen's berserker saga) assumes that extraterrestrial civilizations send artificial probes (English probes , possibly self-replicating) into space, which wipe out other civilizations. An intentional extinction takes place in order to eliminate potential enemies or competitors for resources at an early stage so that they no longer stand in the way of the colonization of space. A variant of this theory is that the probes get out of control and then destroy their creators as well. Another variant assumes that the extinction is more or less (un) intentional and a by-product of a terraforming process in which the original ecosystem of the terraformed planet is destroyed.

Thematically the dark forest theory, named after Liu Cixin's book The Dark Forest, aims in a similar direction . According to this, extraterrestrial civilizations do exist and could possibly communicate with each other. However, they behave as inconspicuously as possible and camouflage themselves so as not to attract the attention of other, hostile species. Similar to the process of camouflage in biology, the risk of predation or extinction is to be reduced. The universe is like a dark forest full of predators, and the best survival strategy is camouflage.

Weak arguments

Lack of interest

Even if there is the technical possibility of interstellar communication or colonization, the question arises as to whether a civilization has any economic or philosophical interest in using this technology. So far, our civilization has not made great efforts to consciously send out signals (see Messages to Aliens , List of Interstellar Radio Messages ), and human space travel is largely limited to the sending of probes. Even interplanetary space flights that are possible in principle are questioned in terms of their economic and scientific sense.

Lack of visibility

Sending radio signals for communication is relatively inefficient. If all civilizations switch to more efficient communication methods within a short period of time, the proportion of radio emissions through which a civilization would make itself felt decreases.

It has also been suggested that a result of information theory could explain the lack of discernible signals. Information theory states that a maximally compressed message is indistinguishable from background noise for those who do not know the compression algorithm. SETI, on the other hand, only searches for the simplest of all signals, an unmodulated sinusoid . The basic assumption of SETI is the willingness of other forms of life to communicate clearly with an easily discovered signal. For these reasons, today's search methods would simply overlook a highly compressed transmission.

The summer sleep hypothesis takes up ideas from Freeman Dyson and refers to a well-advanced civilization for which information processing is essential. This requires energy, but it is finite. As the universe expands at an inflationary rate , it gets colder and the de-sitter model can apply. Taking into account the Landauer principle, it is obvious that information processing is more efficient by an astronomical factor after cooling. An advanced civilization will therefore go to sleep at a time when the environment is still warm in order to benefit from the cold environment at a much later point in time.

They exist - we just missed them

This hypothesis is based on the assumption that all visiting civilizations will stagnate or become extinct in the long term instead of expanding. This cannot be ruled out, because the entire duration of human existence is so short on a cosmological scale that even the survival of our species over hundreds of thousands of years changes little. As a result, civilizations could simply be too far apart in time and space to meet. This hypothesis contradicts the possibility of Von Neumann probes , which could have a much longer lifespan than their original civilization. A civilization that sends out Von Neumann probes could also limit their reproduction so that a maximum of one probe would be assigned to each star system. This probe would only reproduce when its own life expired. You could also be stationary as buoys and z. B. only send out a weak signal.

They exist - we are ignored

This assumption assumes that there is a consensus among all civilizations in our neighborhood to avoid contact. This speculation is sometimes referred to as the “zoo hypothesis”. More recently, it has also been referred to as the "Aurora Effect", named after Kim Stanley Robinson's novel Aurora.

The obligation to not interfere and the consequences of disregarding it is the subject of the 1964 novel It is not easy to be a god by Arkadi and Boris Strugazki .

The science fiction author Gene Roddenberry formulated this in Star Trek as a " supreme directive ", a strict non-interference requirement, at least as long as a civilization has not crossed a defined threshold, e.g. B. the technological ability to travel interstellar.

Considerations to camouflage ourselves speak for the possibility of other, more technically more developed societies, to be able to do this much better.

They exist - we ignore them

It is assumed that extraterrestrial civilizations have already attempted contact, but these have been ignored by science, or could not be noticed, or are kept secret by one or more governments. It has been used in a number of science fiction novels and films, including The Hitchhiker's Guide to the Galaxy, and is also represented in interpretations of UFO sightings, conspiracy theories and by supporters of various pseudosciences (see e.g. pre-astronautics ). It is often assumed that today's technology is not able to record, process or decipher extraterrestrial communication attempts, or that a signal is not recognized as a communication attempt.

"Hart-Tipler argument"

Since the term Fermi paradox is misleading, the recognized expert Robert H. Gray suggests the term "Hart-Tipler argument" in honor of Michael H. Hart and Frank J. Tipler . Hart came up with the pointed formulation, generally recognized as the Fermi paradox, “They are not here; therefore they do not exist ". Tipler added self-replicating machines to the problem .

See also

literature

  • Stephen Webb: If the Universe Is Teeming with Aliens… WHERE IS EVERYBODY ?: Seventy-Five Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life. 2nd edition Springer, Cham 2015, ISBN 978-3-319-13235-8 .
  • Marshall T. Savage: The millennial project - colonizing the galaxy in eight easy steps. Little Brown, Boston 1994, ISBN 0-316-77165-1 .
  • Ian Crawford: Where Are They? Maybe we are alone in the galaxy after all. In: Ufo Evidence. July 20, 2000. Retrieved November 9, 2017.
  • Eric M. Jones: Where is everybody? An Account of Fermi's Question. In: Los Alamos Technical Report. March 1985, fas.org (PDF; 1.1 MB) Federation of American Scientists ; Retrieved March 2, 2011.
  • Anders Sandberg, Eric Drexler, Toby Ord: Dissolving the Fermi Paradox. In: Popular Physics. June 2018, (PDF; 255.9 kB); accessed on June 29, 2018.
  • Duncan H. Forgan: Solving Fermi's Paradox. Cambridge University Press, Cambridge 2019, ISBN 978-1-107-16365-2 .
  • Milan M. Cirkovic: The Great Silence: Science and Philosophy of Fermi's Paradox , Oxford University Press (May 10, 2018), ISBN 978-0-19-964630-2 .

Web links

Videos

Individual evidence

  1. ↑ Michael Michaud: Contact with Alien Civilizations - Our Hopes and Fears about Encountering Extraterrestrials. Springer, Berlin 2006, ISBN 0-387-28598-9 , p. 164, google books
  2. ^ EM Jones: Where is everybody. An account of Fermi's question. Technical Report, US Department of Energy, 1985, abstract , osti.gov (PDF), accessed October 12, 2013
  3. ↑ Michael H. Hart: An Explanation for the Absence of Extraterrestrials on Earth. In: The Quarterly Journal of the Royal Astronomical Society. 16, 1975, p. 128.
  4. ^ Paul Wesson: Cosmology, extraterrestrial intelligence, and a resolution of the Fermi-Hart paradox. In: Royal Astronomical Society, Quarterly Journal. Vol. 31, June 1992, ISSN  0035-8738 , pp. 161-170, bibcode : 1990QJRAS..31..161W
  5. ↑ The Fermi Question: No Paradox At All . centauri-dreams.org, accessed August 20, 2015
  6. ↑ Carl Sagan: Cosmos. Ballantine Books, New York 1985, ISBN 0-345-33135-4 .
  7. ^ Robert Gast: Dice games with ET Spektrum.de, June 27, 2013; Retrieved July 2, 2013
  8. ^ Claudius Gros: Expanding advanced civilizations in the universe. In: J.Br.Interplanet.Soc. 58, 2005, p. 108, arxiv : astro-ph / 0501119 .
  9. ↑ Geoffrey A. Landis: The Fermi Paradox: An Approach Based on Percolation Theory. (Third paragraph of the Introduction )
  10. ↑ Geoffrey A. Landis: The Fermi Paradox: An Approach Based on Percolation Theory.
  11. ↑ Stephen Hawking: Life in the Universe. (No longer available online.) In: Public Lectures. Archived from the original on April 11, 2006 ; Retrieved on February 13, 2017 (English): "A third possibility is that there is a reasonable probability for life to form, and to evolve to intelligent beings, in the external transmission phase. But at that point, the system becomes unstable, and the intelligent life destroys itself. "
  12. ↑ Nick Bostrom: Existential Risks - Analyzing Human Extinction Scenarios and Related Hazards. In: Journal of Evolution and Technology, Vol. 9, No. March 1 , 2002, accessed February 13, 2017 .
  13. ↑ Carl Sagan: Cosmic Search Vol. 1 No. 2 - The Quest for Extraterrestrial Intelligence. In: Cosmic Search Magazine. Smithsonian Institution, May 1978, accessed February 13, 2017 .
  14. ↑ Philipp Hummel: Extraterrestrial Life - The reason for our loneliness. Spektrum.de, December 10, 2014, accessed on May 8, 2015 .
  15. ↑ Gamma-ray bursts are a real threat to life . In: CERN Courier . Vol. 55, No. 01 (Jan / Feb), 2015, p. 15 ( fileburst.com [PDF; 7,8 MB ]).
  16. ↑ JR Gott: Implications of the Copernican principle for our future prospects . In: Nature. Vol. 363, 1993, pp. 315-319.
  17. ^ Fraser Cain, Universe Today: Watch: Humanity's Biggest Discovery Could Herald The Beginning of The End. In: Science Alert. Retrieved February 6, 2018 (UK English).
  18. ^ "The 'Great Silence': The Controversy Concerning Extraterrestrial Intelligent Life" , Quarterly Journal of the Royal Astronomical Society , Glen David Brin, Volume 24 : pp. 283-297, 3rd quarter of 1983 (received Sept. 1982).
  19. ↑ If the Universe Is Teeming with Aliens ... WHERE IS EVERYBODY ?: Seventy-Five Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life, Second Edition , Stephen Webb, foreword by Martin Rees, Heidelberg, New York, Dordrecht, London : Springer International Publishing, 2002, 2015.
  20. ↑ Up here we are - camouflage in space? - Zeit Online, March 7, 2018
  21. ↑ Anders Sandberg, Stuart Armstrong, Milan Cirkovic: That is not dead which can eternal lie: the aestivation hypothesis for resolving Fermi's paradox, 2017
  22. ↑ John A. Ball: The zoo hypothesis. In: Icarus. Volume 19, Issue 3, July 1973, pp. 347-349, doi: 10.1016 / 0019-1035 (73) 90111-5 .
  23. ↑ Alien civilizations may have explored the galaxy and visited Earth already, a new study says. We just haven't seen them recently. Retrieved September 12, 2019 .
  24. ↑ Cosmic game of hide and seek: Laser cloak is supposed to deceive dangerous aliens. Nobody knows if extraterrestrial civilizations are really benevolent to us. So maybe we shouldn't be found . Spektrum.de, with reference to: A Cloaking Device for Transiting Planets . oxfordjournals.org, March 30, 2016; accessed April 4, 2016
  25. ^ Robert H. Gray in the English language Wikipedia
  26. ↑ “At lunch, the famous physicist Enrico Fermi is said to have refuted the existence of ET. But that's neither back nor front ... ”, neither Fermi nor paradoxical. Where are they, the aliens? Spektrum.de, March 8, 2016. The Fermi Question: No Paradox At All . centauri-dreams.org, March 10, 2015; accessed April 4, 2016