ASTR 1210 (O'Connell) Study Guide 23

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LIFE IN THE UNIVERSE

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Are we alone? OR: Are there billions of advanced lifeforms in the universe?

From the history of science, it's obvious that human beings have a deep urge to understand nature and the universe around them. We are the first species on our planet to have the desire and capacity to confront questions on a cosmic scale. The first recorded serious explorations were made over 2500 years ago, and since then we have invested tremendous effort into investigating basic questions about the mechanisms of nature and the place of humans in the universe.

Modern science has made great progress by insisting on strict standards of evidence. And, after 500 years, we now have a very good, if still preliminary, understanding of many aspects of the universe. The length of time it took to get meaningful answers indicates how difficult it is to obtain reliable knowledge on this scale.

One of the biggest unanswered questions concerns the presence of other lifeforms beyond the Earth. In fact, the most powerful impact of astronomy on popular thinking concerns something that it hasn't discovered yet and possibly never will: aliens.

So, in this lecture we examine the questions of whether there is life elsewhere in the cosmos, where it might be, and what forms it might take.

A. Perspective

The two questions at the top of the page frame the possibilities for the existence of other advanced species in the universe.

When you combine the ideas of life and the universe you reach a stunning conclusion no matter which way you argue:

• There are billions of stars in each galaxy and about 100 billion galaxies within reach of our telescopes. (See Study Guide 2.)
  The total number of stars in the observable universe is of order 10,000,000,000,000,000,000,000 (or 10²²).

• Therefore:
  • If you believe Earth and humanity are special, uniquely fortunate, or specially created (the "pre-Copernican" view), then we are ALONE in this unimaginably vast cosmos.

  • On the other hand, if you believe we are average (the "Copernican" view), then the universe TEEMS with billions of advanced lifeforms.

Most astronomers subscribe to the latter view---that conditions on Earth are typical (or at least not uncommon)---which implies that extraterrestrial life is widespread and that there are many advanced lifeforms.

This is not just a modern point of view. It goes back to the ancient Greeks and was historically advocated by many other writers, e.g. Huygens, the discoverer of Saturn's moon Titan. See this translation of his book Cosmotheoros.

"To consider the Earth as the only populated world in infinite space is as absurd as to assert that in an entire field of millet, only one grain will grow."                                         --- Metrodorus of Chios (ca. 300 BC) "What a wonderful and amazing Scheme have we here of the magnificent Vastness of the Universe! So many Suns, so many Earths..."                                         --- Christiaan Huygens (1698)

A revolution in prospects for astrobiology

Several remarkable discoveries lend credence to this picture:
1. 1995+: The discovery of planetary systems around nearby stars (see Guide 11). We have discovered hundreds of other planetary systems in the last 25 years. Many, if not most, stars like the Sun have planets. Despite the technical challenges, we have already identified many Earth-size candidate planets.
   In April 2014, the discovery of Kepler 186-f, the first Earth-sized planet (only 10% larger) in the "habitable zone" around another star, was announced.

2. 1996: Evidence suggesting the presence of fossil microorganisms on Mars (see Guide 17). This claim is highly controversial. But right or wrong, it has stimulated the development of vigorous research in the rapidly growing field of "Astrobiology."

3. The discovery of water-rich environments on Mars (in the past) and under the surfaces of Europa and Enceladus. And the strange and unique but promising hydrocarbon-rich surface of Titan.

4. The discovery by radio astronomers of many complex organic molecules in interstellar clouds. The list of these has been rapidly growing over the past 20 years. There are over 220 species of interstellar molecules, many containing more than 8 atoms. These include the amino acid glycine, the RNA base uracil, and the sugar glycolaldehyde. The basic building blocks of biological organisms are widespread in our Galaxy.

These breakthroughs, coming after centuries of speculation, have changed the whole character of the study of exterrestrial life.

B. Life on Earth

Just as our everyday "common-sense" perspective provides no clue to the scale of the real universe, our naive assumptions about the nature of life on Earth, as they existed at the beginning of the scientific age, have been shattered by the evidence:

Age: Life on Earth is ancient. It has thrived for at least 3 billion years, according to the fossil record, and the character of lifeforms has changed radically throughout that time.
Human beings, on the other hand, are brand new. The genus Homo, which includes modern humans, has existed for only about 2.5 million years, or 0.06% of the age of the Earth. Homo sapiens emerged only about 250,000 years ago.

Functional basis: random chemical interactions, governed by natural physical principles.
In contradiction to all expectations through about 1900, no special "vital force" is required. The laws of physics and chemistry that govern the structure and functioning of organic systems are exactly the same as those that govern inorganic systems.

Unity: Despite extraordinary external diversity, there is only one type of terrestrial life at the molecular level
• A small set of basic chemical building blocks (large molecules constructed from abundant atoms like H,C,N,O, and a few others)

• The key constituent is carbon, which offers 4 chemical bonds as the basis for forming complex molecules

• Nucleic acids (DNA, RNA): These long strand molecules provide an enormous capacity for information storage and chemical control for molecular reproduction

  
  

350 million years of evolution on Earth: characteristic terrestrial lifeforms
from the Cambrian (500 Myr ago, left) to the Jurassic (150 Myr ago, right) periods

C. Evolution

The proliferation and diversification of lifeforms is produced by evolution through natural selection for better adapted types (Darwin, Wallace 1858)

The basic principle of evolution is very simple: those organisms that are best adapted by their genetic makeup to a given environment thrive and pass those favorable genetics on to subsequent generations. Dispersion in genetic material in each generation is guaranteed by chemical mutations. Mutations that are accidentally favorable are amplified by natural selection and propagate forward into modified species. On Earth, there have been hundreds of millions of generations preceding the familiar lifeforms we see around us today. The consequent adaptation to an immense variety of environmental niches is profound.

Despite the raging "creationism," "intelligent design," and other controversies surrounding Darwinian evolution that get prominence in the media, and sometimes state legislatures, biological evolution is as well established a basic fact of science as any other, e.g. that Earth is a planet or that the Sun is a star.

All the basic "predictions" of Darwinian evolution have been thoroughly confirmed in the last 150 years of biology, physics, astronomy, geophysics, and paleontology. The evidence is overwhelming.

• The fossil record is now fabulously rich (250,000 species)---vastly more so than in Darwin's day---and clearly shows the radical change of lifeforms on Earth through time. Huge fossil collections exist for epochs between 10,000 and 500,000,000 years ago.

• Geological age-dating by radioactive isotopes is now highly accurate and graphically reveals the enormous stretch of time over which evolution on Earth has been working.

• Genetics, the mechanism by which inherited characteristics are passed from one generation to the next, was mysterious in Darwin's time but is now thoroughly understood, as are the causes of the mutations that engender changes in species.

• DNA mapping has recently demonstrated astonishing chemical similarities between humans and other higher lifeforms. We share 98.8% of our genes with chimpanzees.
  It is now clear that humans are part of a continuum of life on Earth, not a special class.
  This is probably the most stringest test of evolutionary biology ever made, and it passed with flying colors. Darwin, Huxley, and the other leading biologists up to 75 years ago could not have imagined how precise or conclusive such chemical tests of the evolutionary principle could be.

The astronomical evidence for evolution of the universe and its contents over a period of 10-15 billion years is as strong as, but entirely independent of, the biological & paleontological evidence for evolution of life on Earth. See Study Guide 2 and links therein.

• An apparent contradiction between the age of the Sun, in Darwin's time thought to be only about 30 million years, and the geological/fossil time-scale (much older) was resolved when physicists realized (ca. 1940) that the source of the Sun's energy was nuclear fusion, for which there is abundant fuel to maintain the Sun's enormous luminosity for much longer periods. The Sun's age is now estimated to be about 5 billion years, and the solar and geological time-scales are now in good agreement.

• Detailed measurements of the brightnesses and colors of stars in star clusters permit us to age-date their formation times. These range from about 10 million years for clusters in active star-forming regions to 13 billion years in the case of "globular clusters" like M80.

• By using powerful telescopes to study the Hubble Deep Field and other distant regions, astronomers can observe the universe as it was billions of years ago. No other field of science is able to make such direct observations of the distant past. Not only can we see the past, we also can determine how different the contents of the universe were at earlier times and trace how they change with "lookback time." There is no doubt that the universe has evolved.

• Spacecraft observations of the cosmic background radiation from the Big Bang have recently determined the age of the universe to be 13.7 billion years.

Controversies over the reality of evolution are confined to political, religious, & education circles.

They are not important among active scientists, who accept evolution as a foundation of modern science. Scientists involve themselves in these controversies only insofar as they try to protect the integrity of their disciplines and of science education from political intrusion.
In these debates, evolution is often mistakenly conflated with the question of the origin of life on Earth. Scientists do not presently have a good understanding of how (or even whether) life originated on Earth. That discussion is hypothetical. The evidence for evolution (that is, for a change over time) of lifeforms on Earth, however, is entirely independent of the question of life's origins. You do not have to know where a sapling came from in order to know that a tree is growing.

Most anti-evolution arguments are conceptually medieval. You can disregard evolution only if you are prepared to disregard the rest of modern science and scientific thinking. Beware of those who urge you to do this.

D. Origin of Life on Earth?

As just noted, we do not have a good understanding of how life originated on Earth. Given what we do know, however, it is plausible that the chain of life began at the molecular level with molecular evolution from simple, abundant, pre-organic chemicals.

• Probably in oceans, a dense medium with vast quantities of basic raw materials.

• Biosynthesis requires sufficient sources of energy, but these are abundant in the early Earth environment: solar radiation, vulcanism, ocean vents, lightning, etc.

• The plausibility of early molecular evolution was demonstrated in a classic 1953 laboratory simulation: the Miller-Urey experiment. Starting from common pre-biotic ingredients thought to be characteristic of the early Earth, sugars, amino acids, and DNA bases were generated in only a few days. This showed how easy it is to start a chemical synthesis sequence that can lead to organisms.

• Note that once it begins, molecular evolution is subject through natural selection to the same kind of acceleration toward survivable and proliferating forms as are biological organisms. Molecular autocatalysis probably plays an important role here.

• Discoveries of "extremophiles" --- organisms thriving under unexpectedly harsh conditions of temperature, pressure, or acidity --- demonstrate how robust and adaptable simple life forms can be. The best-known examples are bacteria that live at temperatures of 120C or pressures of 1000 bars in submarine hydrothermal vents or deep ocean trenches.

• For 3/4 of its history, life on Earth consisted only of very simple organisms, entirely unlike the profusion of advanced types seen today. Numerically, the simple organisms, starting with bacteria and viruses, vastly outnumber the more complex types even now.

An alternative to a terrestrial origin for life is panspermia: the seeding of Earth from an external source, accidental or deliberate. Remarkably, this possibility was first suggested by the Greek philosopher Anaxagoras around 450 BC.

• We have discovered that organic molecules, produced by natural chemical processes, are present in large quantities even in ostensibly hostile cosmic environments such as meteorites or interstellar clouds. These could be the building blocks for biogenesis throughout our Galaxy.

• An obvious way for lifeforms to propagate in a given planetary system is through comets or meteoroids, as the "SNC" meteorites from Mars vividly demonstrate. Meteoroids could have spread Mars life to Earth or vice versa. We could be Martians!

• The notion of panspermia simply pushes the question of how life originated back one level. Panspermia has occasionally been very controversial -- for example, when astronomers Chandra Wickramasinghe and Fred Hoyle suggested that infall of interplanetary dust grains carrying nucleic acids might have caused disease outbreaks on Earth. But at present we cannot determine whether life originated on Earth itself or came here from elsewhere.
  Introducing the possibility of panspermia doesn't, unfortunately, resolve the important question of whether early terrestrial conditions were favorable to the origin of Earth-like lifeforms. If life came here from elsewhere, it could have been because the early Earth was hostile to generating lifeforms. Or, Earth could have been perfectly hospitable to biogenesis, but the external seeding occurred before it could take place.

• For more information on panspermia, click here.

The "habitable zones" for Earth-like planets surrounding four different types of stars are shown in light blue;
"F" stars are more massive and hotter than the Sun, "K" and "M" are less massive and cooler.
The volume of the habitable zone increases for hotter stars.

E. Life Elsewhere in Our Solar System

Are there plausible biospheres elsewhere in the Solar System?

Requirements

• Raw materials;
  
• A dense medium (preferably water, but other liquids/dense gases are possible);
  
• A protected environment, maintained in the appropriate temperature/pressure range;
  
• Energy sources; primarily sunlight, but volcanic vents, lightning, interior heat, etc. are alternatives;
  
• Sufficient time (100 Myr-1 Gyr?)

Habitable Zone

For a particular type of parent body (terrestrial planet, Jovian planet, Jovian satellite, comet), we can define a habitable zone as those distances from the parent star for which that type can offer a comfortable, long-lived biosphere, neither too hot nor too cold, for life to develop. For Earth-like life, we believe that biospheres require the presence of liquid water.
In the solar system, the habitable zone for Earth-like planets with open bodies of liquid water on their surfaces ranges from about 0.9 AU to 1.4 AU, covering the Earth's orbit (of course) but not reaching to either Venus or Mars. See the diagram above.
Here is a brief discussion of the factors entering the determination of an Earth-like habitable zone and a chart that shows where a number of the exoplanets now known fall. (From Sky & Telescope, March 2017.)
For stars hotter or brighter than the Sun, the zone would be farther from the star and larger in volume; and the converse for cooler or fainter stars.
The interest in searching for exoplanets around "cool dwarf" stars, such as the "K" and "M" stars shown in the diagram above, arises from the fact that their close-in habitable zones are more easily probed with current planet detection techniques than those of brighter stars (see Study Guide 11). They are also the most numerous types of stars.

Because their atmospheres or interiors can be much warmer than their surfaces and harbor large volumes of liquid water, the habitable zone for Jovian-type planets or their icy satellites would be much larger. Whether life could thrive in such environments, however, is unknown.

Possible biospheres on the outer satellites: (Left) The icy surface of Europa (pseudocolor, Galileo mission);
(Middle) The water vapor geysers of Enceladus (pseudocolor, Cassini mission);
(Right) The hydrocarbon-rich surface of Titan, as viewed by the descending Huygens probe.

Possibilities

1. Venus: Its high-temperature, high-pressure, and corrosive atmosphere is sufficient to sterilize the surface of Venus of all Earth-like life, so it was long ignored as a possible biosphere. But in September 2020, astronomers announced the tentative discovery of the molecule phosphine (PH₃) in the cloud layers of Venus' atmosphere. On Earth, there is no known non-biological source for phosphine; so it was suggested that there are microorganisms thriving in Venus' atmosphere. Within six weeks, other groups cast doubt on the detection by reanalyzing the same datasets. However this debate turns out, it has been useful in reminding us that there are regions in Venus' atmosphere that offer temperate conditions and could possibly harbor floating lifeforms.

2. Mars: There is plausible evidence for a biosphere > 1 Gyr ago with abundant water (see Guide16). SNC meteorites provide some evidence for microorganisms. The surface is probably too cold and dry now for life, and the absence of ozone in the present-day atmosphere allows damaging solar UV flux to penetrate to the surface. but there could be warm underground water reservoirs that are more promising.

3. Jupiter, Saturn atmospheres: Results from the Galileo probe (1995), which sampled the outermost layers of Jupiter's atmosphere, were not promising but don't exclude a biosphere.

4. The satellites Europa (J) and Enceladus (S) each have evidence for a liquid water reservoir/ocean lying beneath the visible crust of ice. These are probably the most promising sites for bio-exploration after Mars. Ganymede, Jupiter's largest moon, has just recently been shown to contain a subsurface ocean.

5. Titan (S) has an extraordinary hydrocarbon-rich atmosphere and surface, which is a possible biosphere. The Cassini mission demonstrated the presence of liquid hydrocarbons (methane, ethane) on Titan, and some scientists think "methanogenic" lifeforms might exist there. Most believe the very low temperatures (-180 degrees C) would preclude living organisms on the surface. As in the case of Europa, Ganymede, and Enceladus, however, deep reservoirs with more favorable temperatures are possible. Here is a prospectus for exobiology on Titan.

6. Comet nuclei: these icy bodies, often with a coating of organic molecules, could act as "portable reservoirs" of organisms; however, temperatures are normally very low.

The existence of advanced lifeforms anywhere else in our Solar System would astonish most scientists but is not completely impossible. On the other hand, primitive lifeforms are certainly possible in several settings, but remote detection is unlikely. We must search "in situ," which implies complicated and costly space missions.

F. Intelligent Life Elsewhere

Warning

This is a fascinating but virtually 100% speculative subject. There is a paucity of facts, understanding, and imagination, and that leads to a wealth of conjecture and controversy.

The Drake Equation

The Drake Equation, named after astronomer Frank Drake, was an early (1961) "rough order of magnitude" estimate of the number of advanced technical civilizations in our Galaxy capable of undertaking interstellar communication. "Communication" implies the ability to transfer information but not necessarily to undertake travel between stars. The most basic form of communication involves electromagnetic radiation. Here is a simplified list of the elements in the Drake estimate based on our current understanding:
1. There are 100 billion stars in our Galaxy

2. Assume 0.001% - 5% of the stars have Earth-like planets in the habitable zone.
   Support? The recent detection of thousands of extra-solar planets. Even though many of these are Neptune-class or larger, rather than Earth-size, most astronomers expect that the fraction of stars with Earth-like planets will prove to be of order 5% or larger. This is at the high end of the range considered plausible over the last 50 years.
   The Kepler mission has identified many planet candidates with sizes near Earth's, most of them slightly larger. Some of these are in or near the habitable zones of their parent stars. Although the volume of the habitable zones of stars cooler than the Sun ("K and M dwarf stars") is smaller than ours, such stars are much more common in the Galaxy than are Sun-like stars, so these are receiving increased scrutiny. The TESS mission, launched in April 2018, should discover many more planets hosted by small stars near the Sun. The latest results are discussed in this article.

3. Assume all of those develop life leading to advanced civilizations (i.e. assume Earth is average)
   Support? "Earth is average" is the Copernican assumption, which has proved so successful in studies of the structure of our inorganic universe on the scale of the Solar System, our Galaxy, and extragalactic space. However, there is great controversy about whether this assumption applies to biology. We have very little intuition here, and some biologists would argue that the chances of developing technological species on a given Earth-like planet are very small.

4. Assume the communication phase lasts 10,000 years.
   Note: we have only recently entered this phase. It has been only 90 years since we developed commercial radio stations that could be detected over interplanetary distances. Our artificially generated electromagnetic radiation would be the most definitive marker of advanced lifeforms on this planet to an exernal observer. Human EM signals are now propagating out through the Galaxy, with the most distant signals being about 90 light years away.

5. Combining all of these factors ===> 10 - 10,000 communicating civilizations in our Galaxy

6. ===> Distance to nearest: 10,000 - 1000 light years

The quantities entering the Drake calculation under items (3) and (4) are highly uncertain and controversial. You can find elaborate treatments of our understanding of the important factors, and gaps therein, in the books listed under "optional reading" below.
One obvious source of bias is the danger of carbon or planetary chauvinism. For instance, it has often been suggested that silicon, which also offers 4 chemical bonds, could be utilized by lifeforms in place of carbon. The numbers we used above also assume that intelligent life can develop only on terrestrial planets. But there's no compelling reason that intelligent life would be confined to planets or even planetary systems. Why not a sentient interstellar gas cloud? This idea was explored in the famous science fiction novel The Black Cloud by astronomer Fred Hoyle. In the story, the super-being in the Cloud expresses surprise that intelligence could develop in so adverse an environment as the surface of a planet.

The bottom line is that, at present, no one can exclude the possibility that a large number (10,000!) of advanced civilizations reside in our Galaxy. The number is large because the Galaxy is huge.

Interstellar migration/exploration:

The estimated separation between advanced civilizations, 1000-10000 LY, is obviously tremendous. But it could be traversed with foreseeable technology on cosmically short time scales. (Remember that 10 million years is a "short time" in the cosmic context!) Even at propagation velocities far below the speed of light, stable civilizations could "rapidly" explore the Galaxy.
Here's a relevant, even amazing, fact: We human beings managed to launch four interstellar spacecraft in the first 20 years of the space age! The four Pioneer and Voyager spacecraft, sent to study the Jovian planets, will all leave the Solar System, as will the New Horizons Pluto mission.
The most distant manmade object, the Voyager 1 spacecraft, is now 11.6 billion miles (125 Astronomical Units) from Earth and officially entered interstellar space in September 2013.

There is a growing community of scientists and engineers making preliminary plans for interstellar travel. Above right is a painting of a popular starship design, a "Bussard interstellar ramjet."

The Fermi "Paradox"

A famous question, now called the Fermi Paradox because it was first raised by physicist Enrico Fermi in the 1950's, is therefore "Where is everybody?"
What Fermi meant was that if interstellar expansion is possible for thousands of Galactic species, they should have reached here long ago. There ought to be aliens cluttering up the Solar System. We don't see them -- there is no credible evidence for alien visitation (see below) -- so are the estimates for the number of advanced civilizations and our expectations regarding interstellar travel badly wrong or naive?
Entire books have been written on possible solutions (all sheer speculation) to the problems raised by Fermi's question. I give my favorite answer below.

SETI = "Search for Extraterrestrial Intelligence"

The best alternative to interstellar travel to assay the prevalence of life in the nearby universe would be EM communication, probably in the radio or optical EM bands
Several passive listening, radio search programs are ongoing. Most sophisticated are those operated by the SETI Institute, once a well-financed NASA program but killed by Congressional scepticism and now running on private contributions.
When SETI programs were first conceived 50 years ago, it was assumed that we might most easily detect "leakage" radiation from civilizations employing radio- or TV-like broadcasting for communication. But we ourselves have backed away from radio/TV broadcasts, and today orders of magnitude more information is being conveyed over cable and fiber optic systems that don't radiate significantly. Therefore, if we do detect ETI through EM communications, it is more likely to be in the form of beacons deliberately established to attract attention rather than "leakage." These are also likely to be products of the most successful and advanced of (possibly) many Galactic civilizations.

G. Alien Artifacts?

Most people assume that if spacefaring aliens are active today in the vicinity of the Earth, or had arrived anytime over the last few 100 million years, there should be concrete evidence of that visitation -- artifacts of extraterrestrial intelligence. In fact, you can find many gigabytes on the Internet devoted to claims that there is "good" evidence for visitations. But, so far, these have not stood up under scrutiny.

Here are two prominent examples of a supposed alien presence in the Solar System:

• UFO's
  See Guide 18 and links therein. There is no credible evidence that an intelligent species (other than our own) is involved in the "Unidentified Flying Object" phenomenon. An overwhelmingly strong media influence governs the number and similarity of reports. Publicity can even provoke mild mass hysteria (e.g. alien abduction claims).

• The face on Mars
  This is a strange and interesting feature in the Cydonia region of Mars that was imaged (poorly) by the Viking spacecraft in the 1970's. In a partially-illuminated state, it looked like a gigantic, carved human head. One of the original Viking images is shown below (left panel). This elicited speculation about civilizations on Mars, the more responsible of which can be sampled here.
  In April 1998, the Mars Global Surveyor was retargeted to image the same region at much higher resolution. Several good images were returned, showing the fully illuminated "face" to be an unambiguously natural feature. See the right panel below. (Some of the intriguing features in the original image, like the "headdress," were actually data drop-outs.) Yet better images have been more recently obtained by the Mars Express orbiter. No other candidates for alien artifacts on Mars or other planets are credible either.
  You can find trenchant commentary by Phil Plait (the "Bad Astronomer") about the (needless) continuing controversy over this subject here.

Astronomers have also considered the possibility of detecting alien artifacts other than deliberate signals beyond the Solar System. For instance, an advanced civilization might have created a mega-structure to capture a large fraction of the energy output of their host star. Such a structure is called a Dyson Sphere, and its presence could be indicated by an unusual near-infrared flux excess not typical of normal stars. Deliberate searches among databases from infrared sky surveys have not yet turned up any viable candidates.

However, in 2015, a fascinating candidate for a Dyson-like structure emerged from the Kepler Mission exoplanet survey:

• Tabby Boyajian's Star
  Tabby's Star is an outwardly inconspicuous star in the large Kepler survey field. It shows a strange, non-repeating pattern of eclipses some of them much too large to be from planet-sized bodies. The star has received intense scrutiny, and it has recently been discovered to be declining slowly in (uneclipsed) brightness in a way that normal stars do not.
  It is very unlikely that an alien mega-structure is responsible for the weird properties of Tabby's Star, but it must be admitted that none of the more conventional explanations (e.g. debris from a planetary collision or a swarm of comets) explain the observations either. So far, the star is an astronomical mystery.

Evidence of alien technology, whether in the form of a communication signal or an artifact, would now be called a technosignature, and a 2018 NASA conference reviewed the status of work in this area.

H. The Recognition Chasm

There is a much more fundamental problem in communicating with alien civilizations than their distance from us. It presents major obstacles to even recognizing them in the first place.

• The mean age difference between two Galactic species is likely to be 100's of millions of years.

• Given favorable conditions, successful advanced lifeforms may have continued evolution and intellectual/technical development for a large fraction of that time. (Once they reach a certain level of development, they become immune to the more serious astronomical hazards for lifeforms, such as asteroid impacts and stellar evolution.)

• The age separation is much more important than the spatial separation
  ====> us : them ~ goldfish : us

• Since recognition and communication is possible only for cultures in close intellectual proximity,
  ====> Aliens would appear to be natural phenomena.
  So, my favorite answer to Fermi's question, "Where is everybody?" is:
  "They're here, but we don't recognize them."

I. Epilogue

A quote from Stanley Kubrick, director of "2001 -- A Space Ddyssey," on the possibilities for advanced lifeforms in the universe:

"Now, the Sun is by no means an old star, and its planets are mere children in cosmic age, so it seems likely that there are billions of planets in the universe not only where intelligent life is on a lower scale than man but other billions where it is approximately equal and others still where it is hundreds or thousands of millions of years in advance of us. When you think of the giant technological strides that man has made in a few millennia -- less than a microsecond in the chronology of the universe -- can you imagine the evolutionary development that much older life forms have taken? They may have progressed from biological species, which are fragile shells for the mind at best, into immortal machine entities -— and then, over innumerable eons, they could emerge from the chrysalis of matter transformed into beings of pure energy and spirit. Their potentialities would be limitless and their intelligence ungraspable by humans."

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Reading for this lecture:

Study Guide 23
Bennett textbook, Chapter 24
Optional Reading:
Are We Alone? by James Trefil & Robert Rood (Clemons Lib: QB 54.R55)
The Biological Universe: The Twentieth Century Extraterrestrial Life Debate and the Limits of Science by Steven J. Dick (SciEngr Lib: QB 54.D47 1996).
Rare Earth: Why Complex Life is Uncommon in the Universe by Peter Douglas Ward & Donald Brownlee. (QB54.W336.2000).
Lonely Planets: The Natural Philosophy of Alien Life by David Grinspoon
Life in the Universe by Jeffrey Bennett and Seth Shostak (SciEngr Lib: QH327.B45 2007).
Astrobiology: A Multi-Disciplinary Approach by Jonathan Lunine (SciEngr Lib: QH325.L86 2005)

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Web Links:

Talk.Origins (a thorough web discussion of evolution vs. creationism)

FAQs about the origin of life controversy

Science, Evolution, and Creationism (2008 report from the National Academy of Sciences)
Essay on Intelligent Design Creationism (Donald Simanek)
Good summaries of development of life on Earth:
Origin & Unicellular Organisms
Multicellular Organisms
The Age of Animals

NASA's Astrobiology Program
Encyclopedia of Astrobiology, Astronomy, & Spaceflight
Mars Global Surveyor Press Releases on "Face on Mars"
The SETI Institute

Big Picture Science (radio & podcasts)

"Archaeology, Anthropology, and Interstellar Communication" (NASA report by Douglas Vakoch, 2014)
The Planetary Society SETI Page
"The Great Silence" (about the Fermi Paradox, by David Brin)
Lecture on "The Eerie Silence" (about the Fermi Paradox and SETI, by Paul Davies)
Report of the NASA Workshop on the Search for Technosignatures (2018)
The Black Cloud, (online copy of the Fred Hoyle novel)
When will humanity first discover extraterrestrial life? (21 experts give their opinions)
Interpretations of "2001: A Space Odyssey": human contact with alien technology
Centauri Dreams (site devoted to the potential of interstellar exploration)
Initiative for Interstellar Studies (similar)
The 100-Year Starship Project
Breakthrough Starshot (project to send a small instrument package to Alpha Centauri)
Astronomy 1250: Alien Worlds
Astronomy 3420: Life Beyond the Earth
Guide 18: Life on Mars, War of the Worlds, and the UFO phenomenon (O'Connell)

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Last modified March 2023 by rwo

Text copyright © 1998-2023 Robert W. O'Connell. All rights reserved. Movie poster captured from the Internet Movie Data Base. Habitable zone drawing copyright © Brooks/Cole-Thomson. Starship painting by Don Davis. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1210 at the University of Virginia.