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Thursday, August 16, 2018

Technology in science fiction

From Wikipedia, the free encyclopedia
A conceptional science fiction prosthetic/robotic arm.

Technology in science fiction examines the possibilities and implications of new technological concepts. Authors have taken, or created, new innovations and technologies, and elaborated on what they might be and how they might be used. This exchange goes in both directions – sometimes the technology appears first in science fiction, then becomes reality (such as space travel) and other times the real technology comes first, and science fiction authors speculate about how it might be used, and how it might affect the human condition. Likewise, the accuracy of the technology portrayed spans a wide range – sometimes it is existing technology, sometimes it is a physically realistic portrayal of a far-out technology, and sometimes it is simply a plot device that looks scientific, but has no basis in science. Examples drawn from space travel in science fiction include:
  • Realistic case: Space suits. These are almost always based on existing suits, or near-term extrapolation of their capabilities.
  • Extrapolation: Travel within the Solar System. As of 2018, humans have only travelled in Earth orbit or from Earth to Moon and back. However, travelling within the Solar System violates no physical principles.
  • Plot device: faster-than-light drive. It is unsupported by physics as we know it, but needed for galaxy-wide or intergalactic plots with human lifespans.

Fictional technologies that have since been realized

Almost every new technology that becomes practical was previously used in science fiction. The following are a few examples, from a very large set:

Transparent Aluminum as featured in the Star Trek universe has since become a reality[1] as Aluminium oxynitride (ALONtm), patented in 1985,[2] and as different from metallic aluminum as rust is from iron. Rather than being used as transparent blast shielding as in the fictional Enterprise class starships, this transparent ceramic is used, as the chemically similar (and similarly expensive) corundum (crystalline aluminum oxide) has long been used, in tough windows.

Tractor/Repulsor Beams have been realized as Laser-based Optical tweezers,[3] and more recently as a pair of Bessel beams.[4] These instruments use the radiation from the laser beam to manipulate microscopic particles in what is called an "optical trap"[5][6] along the length of the beam as desired.[7]

Fictional Tractor beams have been prominently used in the Star Wars universe and in the Star Trek universe. In an early scene of Star Wars: A New Hope a large spaceship uses such a beam to seize a small one, in order to capture the protagonists.

Artificial Vision/Prosthetic Eyes Visual prosthesis has been a topic of experiments since the late 20th century.

Notable characters using artificial vision include all characters from the Ghost in the Shell series who use prosthetic bodies e.g. Batou's ranger eyes, Saito's left eye, and Motoko Kusanagi's artificial eyes, Geordi LaForge from the Star Trek: The Next Generation series who made use of a VISOR and later; ocular implants, RoboCop from the RoboCop series, Spike Spiegel from the Cowboy Bebop anime series, and the Illusive Man from the Mass Effect series of videogames.

Tricorder The Lab-On-a-Chip Application Development Portable Test System (LOCAD-PTS) used by astronauts on the International Space Station is designed specifically to biochemical molecules with the purpose of "identifying microbes on space station services" through use of the Gram Staining Method.[8]

Though less advanced than the fictional tricorder of the Star Trek series, the LOCAD-PTS is useful for quickly identifying bacteria and fungi on the International Space Station without having to send samples back to Earth,[9] thus risking contamination or degradation.[10] Fungi have proven to be a hazard if left unchecked on the space station as they managed to decompose some Russian electronics.[11]

The Tricorder featured in the Star Trek universe was capable of measuring almost anything, from the chemical composition of explosives, to the life signs of a dying humanoid. The LOCAD-PTS does not differentiate between live and dead test material yet.[12]

Spaceflight

Since the principles of rocketry were worked out in the early 20th century, writers have used straightforward extrapolation to support stories of interplanetary exploration, colonization, conquest and so forth.

With new developments in space exploration and technology the idea of space exploration became a reality. Though many writers explored space travel before these events and inventions, the reality of new technologies and the evidence that space exploration was now possible opened new doors to create more fantastical ideas of space travel. Many Science Fiction topics are born from reality, but turn these new technologies to create imagined realities, thus creating Science Fiction in itself.

The Saturn V carrying Apollo 11 to the Moon, blasting off on July 16, 1969

1903 – The Wright brothers invented the first motored and manned airplane, launching the age of human flight

1920s – Robert Goddard and Wernher von Braun developed liquid-fueled rockets, later applied as the V2 in war. Fictional spaceships of the 1950s were typically shaped like the V2. Later long range missiles influenced later fiction.

The Space Race between the US and Soviet Union inspired more precise depiction of technology already under development.

The launch of the first man-made object to orbit Earth; USSR's Sputnik 1 (October 4, 1957)
Space stations, first presented in crude form by The Brick Moon, were popularized in the 1960s by books agitating for further development. Those little resembled the Salyut 1 or later actual stations.  2001: A Space Odyssey (film) presented the "rotating wheel space station" of the 1960s but few others did. The long-running fictional Deep Space Nine (space station) and Babylon 5 (space station) little resembled any of the above.

Faster than light

Galactic-scale stories usually call for interstellar travel in human lifetimes, which is not supported by existing science, so this technology is more speculative.

E.E. "Doc" Smith: Skylark and Lensman series (1920s). In this series, ships are "inertia-less"; this Inertialess drive makes travel effortless at huge multiples of the speed of light.

Hyperspace commonly designates one class of technology, where infinite speeds are possible; a ship may jump to hyper space or star drive "clutching at the very fabric of time itself" thus making travel that would normally take thousands of years possible in no time at all

Mechanical life/Androids/Robots

While now (as of 2017) there are companies that are fully devoted to creating robots and artificial intelligence, these ideas were long present in science fiction before they started to become real technology. Mechanical and artificial characters were derived both from extrapolations of real engineering efforts, and from the whims and imaginations of the authors. This technology has given writers, as well as other forms of art, the inspiration to create non-human characters.

Early fiction about mechanical life

C-3PO of Star Wars

Definitions

Artificial Intelligence (also known as machine intelligence and often abbreviated as AI) is intelligence exhibited by any manufactured (not grown) system. The term is often applied to general-purpose computers and also in the field of scientific investigation into the theory and practical application of AI.

A robot is an electro-mechanical or bio-mechanical device or group of devices that can perform autonomous or preprogrammed tasks.

An android is a robot made to resemble a human, usually both in appearance and behavior. The word derives from the Greek andr-, " meaning "man, male", and the suffix -eides, used to mean "of the species; alike" (from eidos "species").

A cyborg is a cybernetic organism which adds to or enhances its abilities by using technology.

Early timeline of real world technology

  • 1957: Applied Physics Laboratory AIS begins with focus on learning machines and self-organizing systems.
  • 1961: MINOS 1 First perceptron machine, responds to a pattern of binary inputs using weights.
  • 1966: Artificial Intelligence Center is formed
  • 1966-1972: Shakey the Robot First autonomous mobile robots, controlled from radio and TV links.
  • 1968: A* Algorithm Graph-searching algorithm used to route planning solver for navigation.
  • 1969: STRIPS Planning engine for Shakey.
  • 1969: QA3 and QA4 Automated problem solving.

ESP/Psychic powers/Psi phenomena

With new developments in science and technology helping to study and promote parapsychology or Psi Phenomena, many SF writers felt the need to incorporate and elaborate on these subjects in their stories. While technology helped the investigation into Psi Phenomena it also created questions that many SF writers chose to answer, through their stories, in their own unique way. If we look at some of the examples of Psi Phenomena prominent in stories, they may have stemmed from how science would take this experimentation with Psi Phenomena and use it. In Stephen King's "The Dead Zone", we see how precognition was used to affect political candidates. The idea that someone could harness this power and use it for good or evil was one that many SF writer's elaborated on. In "The Foreign Hand Tie" by Randall Garret espionage takes on a new form via telepathy through twins. When science and technology can be used to anchor something in reality, via experimentation or exploration, and yield results, it creates controversy that society may fear or even fantasize about. Throughout SF history, Psi Phenomena can be seen to be used for good and evil, and through new science and technological discoveries, this genre then becomes more real and more elaborate.

Terms commonly used

  • Telepathy: the ability to read minds
  • Precognition: the ability to see the future
  • Telekinesis: the ability to move objects with mental force (Psychokinesis (PK for short) or "mind over matter")
  • Teleportation: the ability to move oneself from one place to the other, or back and forward in time
  • Telempathy: Emotion-reading
  • Remote viewing/Clairvoyance/Scrying: the ability for seeing things not actually before your eyes
  • Psychometry: the ability to sense what has touched a certain physical object or the imprint it has left behind
  • Bilocation: the ability to be in two places at the same time.
  • Pyrokinesis: the capability to start fires by mental action alone

Writers to mention these topics

Brief history of psi phenomena in science

While ESP and belief in other powers were, in the beginning, mainly fueled by superstitions, religion and tradition, the dawn of science brought about a way to analyze and study these supposed "powers" giving them an anchor in reality. The Scientific Revolution featured ideas that life should be "led by reason" and that, "the universe as a mechanistic, deterministic system could eventually be known accurately and fully through observation and reason". While new science and technology gave rise to skepticism towards the existence of psi phenomena, it also gave way for new technologies to be applied in either proving or disproving such phenomena. One of the first experimental approaches to Psi Phenomena started in the 1930s and was conducted under the direction of J.B. Rhine (1895–1980). Rhine popularized the now famous methodology of card guessing and dice rolling experiments in a laboratory in attempt to find statistical validation for ESP. In 1957 the Parapsychological Association was formed at the preeminent society for parapsychology. Openness to new parapsychology studies and occult phenomena continued to rise in the 1970s.

Technological developments

  • The Random Number Generator
  • Ganzfeld Experiment: homogenous, unpatterened, sensory stimulation to produce an effect similar to sensory deprivation
  • Development of statistical tools by R. A. Fisher in the 1920s

Timeline of probable influences

E. Dawson Rogers hopes to gain new respectability for spiritualism and founds Society for Psychical Research in 1882

Government investigations into parapsychology: Project Star Gate, formed in 1970 with cooperation from the Central Intelligence Agency and Defense Intelligence Agency, investigates remote viewing, sees nothing useful

Visitors from other planets

Extraterrestrial life is a familiar topic in fiction. In the centuries since astronomers discovered that planets are worlds, people have speculated on the possibility of life existing there, though xenobiology has remained a science without a subject. However, people from afar, or alien creatures with various powers and purposes, provided fresh new material for fiction. Some stories were about friendly visitors who got along with humans, such as the aliens in the Keroro Gunsou series, when they give up on attempting to take over planet Earth. Others made alien invasion their theme, as in the 1898 novel, War of the Worlds. Meteorites have long shown that foreign bodies sometimes enter Earth's atmosphere, and the term "flying saucer" was coined in 1947.[13] Several science fiction novels used them.

Early writers

Other terms

  • NTI: Non-Terrestrial Intelligence (A term for alien life that dwells in the oceans or otherwise not on land)
  • UFO: Unidentified Flying Object
  • Flying Saucer: A certain kind of space ship

Timeline of non science fiction influences

  • Antiquity onward: Philosophers have debated the existence of extraterrestrial life.[14]
  • 1609: Galileo, using a telescope to observe the heavens, discovers that planets are other worlds.[15]
  • 1877: Italian astronomer Giovanni Schiaparelli reported the appearance of certain long, thin lines he called canali, meaning channels in Italian.
  • early 1900s: Astronomer Percival Lowell, a science popularizer, wrote the books "Mars" (1895), "Mars and Its Canals" (1906), and "Mars As the Abode of Life" (1908). This was considered science at the time, not fiction, but has been shown to be incorrect by modern missions to Mars.[16]
  • 2009: NASA's Kepler mission shows that an assumption of science fiction, that planets are common throughout the galaxy, is in fact true.[17]

Parallel worlds

The notion of parallel worlds have always intrigued different types of genres, especially the science fiction aspect. Many authors have used the idea of travelling back into prehistoric times or traveling forwards to an unknown universe. The idea of entering a world that has not been touched or that has evolved into a new incomprehensible parallel, makes people ponder about what it could looks like or what it could be. Authors have used this notion of an alternate reality and have created their own worlds that have given readers a different view of alternate worlds.

Early writers

Definition

Parallel Universe Parallel universe or alternate reality in science fiction and fantasy is a self-contained separate reality coexisting with our own

Other terms

Multiverse Set of many universes. There are many specific uses of the concept, as well as systems in which a multiverse is proposed to exist in.

Parallel universe alternate universes, worlds, realities and dimensions in fiction.

Alternate reality alternate universes, worlds, realities and dimensions in fiction.

Alternate future is a possible future which never comes to pass, typically because someone travels back into the past and alters it so that the events of the alternate future cannot occur.

Early timeline

Gravitational distortions caused by a black hole in front of the Large Magellanic Cloud (artistic interpretation Provided by: Black hole )
  • 1905: Albert Einstein Proposes Special theory of Relativity
  • 1905: Albert Einstein's special theory of relativity shows that space and time are relative, not absolute, and that time is actually a fourth dimension within what he calls "space-time."
  • 1916: Einstein discovers that space-time is curved.
  • 1920s: Heisenberg, Schrödinger, and Dirac reformulate mechanics into Quantum Mechanics, based on the Uncertainty Principle.
  • 1922: Kaluza–Klein theory combined Einstein's General Relativity and Maxwell's electromagnetic field theory in 5 dimensions.
  • 1937: Mathematician Kurt Gödel proposes that the universe itself may be a time machine.
  • 1949: Gödel demonstrates mathematically that pathways through time are consistent with general relativity (see Gödel metric).
  • 1967: U.S. physicist John Wheeler invents the name "black hole" to describe singularities in space and time.

Invisibility

INVISIBILITY

The idea of being unseen and hence undetectable has fascinated mankind for generations. This concept has generated scientific pursuit towards defying our physical parameters. Many authors have toyed with the idea of gaining invisibility via both science-based and fictional means. Invisibility in the actual scientific world will be a very difficult achievement, one that will involve much more complication than we have begun to delve into. Further technological developments bring us closer to our goal, while also broadening the horizon for science fiction authors performing thought experiments on the topic of invisibility.

Mythology and folklore precursors

Many myths and legends include gods, spirits, angels, and demons that are often invisible or can choose to become invisible at will.
  • One of the first stories to explore the idea of invisibility was in Plato's The Republic. A peasant finds a ring in the tomb of a dead king that allows him to become invisible. He enters the palace, seduces the queen, and plots to kill the present king, showing that power such as invisibility corrupts.
  • Perseus, the Greek mythic hero who helped establish the Twelve Olympians, was equipped with a cap of invisibility to kill Medusa.

Early writers

  • H. G. Wells wrote The Invisible Man (1897) which was the first science fiction novel to explore the idea of invisibility. The invisible man is a scientist named Griffin who theorizes that if a person's refractive index is changed to exactly that of air and his body does not absorb or reflect light, then he will not be visible. He successfully carries out this procedure on himself, but cannot become visible again, leading to mental instability.
  • J. R. R. Tolkien wrote The Lord of the Rings series which revolves around the function of a ring that renders the user invisible. Unfortunately, it had an evil influence with negative effects on the wearer's actions.
  • Douglas Adams wrote The Hitchhiker's Guide to the Galaxy (1978) novels which encompass a humorous concept of a field which makes people believe the object in question is "somebody else's problem" and therefore do not see it. This concept as explained in the book, bases off of a statement to the effect that actual invisibility is impossible and that the field is merely a way to make something close to being invisible by actually making it hard to notice deliberately.
  • Philip K. Dick wrote in his 1974 novel A Scanner Darkly of a "scramble suit." This is a flexible sheath covering the body of the wearer with a reflective/refractive coating on the inside surface that transfers the camouflaging pattern- projected by a holographic lens mounted on the wearer's head- onto the outside surface of the sheath causing a camouflage-like invisibility.

Definition

Invisibility is a term that is usually used as a fantasy or science fiction term where objects are literally made unseeable by magical or technological means.

Invisibility in science fiction

There is an undeniable link between science fact and the ideas that emerge in science fiction. Science fiction authors are inspired by actual scientific and technological discoveries, but allow themselves the freedom to project the possible future course of these discoveries and their potential impact on society, perhaps only weakly bound to the facts.

Invisibility in fiction

Authors are faced with obstacles presented by the realities of actual technology, however fiction allows a window for the opportunity of inventing completely imaginary technologies to move their storyline forward and maybe even still explore the outcomes of such power.
  • Magic objects such as rings and cloaks can be worn to grant the wearer permanent invisibility.
  • Spells and potions can be used or cast upon people or objects granting temporary invisibility.

Timeline of possible influences

  • 17th century the refractive index was developed. Major advances near the end of the 19th century raised author's awareness.
  • 1670s Emitting or reflecting light outside the wavelength range of visible light would result in a human-shaped black hole which would be completely opaque.
  • 1930s Chroma key began to develop which is the removal of color from one image to reveal another image "behind it." The removed color becomes transparent, which is also called "color keying."
  • 1938 Stealth technology began to develop. It is used with aircraft, ships, and missiles, in order to make them less visible to certain detection methods.
  • 1966 An enemy in a Star Trek episode uses a Cloaking device. Other fiction has used a cloak of invisibility.
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Hypothetical types of biochemistry

From Wikipedia, the free encyclopedia
 
False-color Cassini radar mosaic of Titan's north polar region; the blue areas are lakes of liquid hydrocarbons

"The existence of lakes of liquid hydrocarbons on Titan opens up the possibility for solvents and energy sources that are alternatives to those in our biosphere and that might support novel life forms altogether different from those on Earth."—NASA Astrobiology Roadmap 2008[1]

Hypothetical types of biochemistry are forms of biochemistry speculated to be scientifically viable but not proven to exist at this time. The kinds of living organisms currently known on Earth all use carbon compounds for basic structural and metabolic functions, water as a solvent, and DNA or RNA to define and control their form. If life exists on other planets or moons, it may be chemically similar; it is also possible that there are organisms with quite different chemistries—for instance, involving other classes of carbon compounds, compounds of another element, or another solvent in place of water.

The possibility of life-forms being based on "alternative" biochemistries is the topic of an ongoing scientific discussion, informed by what is known about extraterrestrial environments and about the chemical behaviour of various elements and compounds. It is also a common subject in science fiction.

The element silicon has been much discussed as a hypothetical alternative to carbon. Silicon is in the same group as carbon on the periodic table and, like carbon, it is tetravalent, although the silicon analogs of organic compounds are generally less stable. Hypothetical alternatives to water include ammonia, which, like water, is a polar molecule, and cosmically abundant; and non-polar hydrocarbon solvents such as methane and ethane, which are known to exist in liquid form on the surface of Titan.

Shadow biosphere

The Arecibo message (1974) sent information into space about basic chemistry of Earth life.

A shadow biosphere is a hypothetical microbial biosphere of Earth that uses radically different biochemical and molecular processes than currently known life.[4][5] Although life on Earth is relatively well-studied, the shadow biosphere may still remain unnoticed because the exploration of the microbial world targets primarily the biochemistry of the macro-organisms.

Alternative-chirality biomolecules

Perhaps the least unusual alternative biochemistry would be one with differing chirality of its biomolecules. In known Earth-based life, amino acids are almost universally of the L form and sugars are of the D form. Molecules of opposite chirality have identical chemical properties to their mirrored forms, so life that used D amino acids or L sugars may be possible; molecules of such a chirality, however, would be incompatible with organisms using the opposing chirality molecules. Amino acids whose chirality is opposite to the norm are found on Earth, and these substances are generally thought to result from decay of organisms of normal chirality. However, physicist Paul Davies speculates that some of them might be products of "anti-chiral" life.[6]

It is questionable, however, whether such a biochemistry would be truly alien. Although it would certainly be an alternative stereochemistry, molecules that are overwhelmingly found in one enantiomer throughout the vast majority of organisms can nonetheless often be found in another enantiomer in different (often basal) organisms such as in comparisons between members of Archaea and other domains,[citation needed] making it an open topic whether an alternative stereochemistry is truly novel.

Non-carbon-based biochemistries

On Earth, all known living things have a carbon-based structure and system. Scientists have speculated about the pros and cons of using atoms other than carbon to form the molecular structures necessary for life, but no one has proposed a theory employing such atoms to form all the necessary structures. However, as Carl Sagan argued, it is very difficult to be certain whether a statement that applies to all life on Earth will turn out to apply to all life throughout the universe.[7] Sagan used the term "carbon chauvinism" for such an assumption.[8] He regarded silicon and germanium as conceivable alternatives to carbon;[8] but, on the other hand, he noted that carbon does seem more chemically versatile and is more abundant in the cosmos.[9]

Silicon biochemistry

Structure of silane, analog of methane.
 
Structure of the silicone polydimethylsiloxane (PDMS).
 
Marine diatoms—carbon-based organisms that extract silicon from sea water, in the form of its oxide (silica) and incorporate it into their cell walls

The silicon atom has been much discussed as the basis for an alternative biochemical system, because silicon has many chemical properties similar to those of carbon and is in the same group of the periodic table, the carbon group. Like carbon, silicon can create molecules that are sufficiently large to carry biological information.[10]

However, silicon has several drawbacks as an alternative to carbon. Silicon, unlike carbon, lacks the ability to form chemical bonds with diverse types of atoms as is necessary for the chemical versatility required for metabolism. Elements creating organic functional groups with carbon include hydrogen, oxygen, nitrogen, phosphorus, sulfur, and metals such as iron, magnesium, and zinc. Silicon, on the other hand, interacts with very few other types of atoms.[10] Moreover, where it does interact with other atoms, silicon creates molecules that have been described as "monotonous compared with the combinatorial universe of organic macromolecules".[10] This is because silicon atoms are much bigger, having a larger mass and atomic radius, and so have difficulty forming double bonds (the double bonded carbon is part of the carbonyl group, a fundamental motif of bio-organic chemistry).

Silanes, which are chemical compounds of hydrogen and silicon that are analogous to the alkane hydrocarbons, are highly reactive with water, and long-chain silanes spontaneously decompose. Molecules incorporating polymers of alternating silicon and oxygen atoms instead of direct bonds between silicon, known collectively as silicones, are much more stable. It has been suggested that silicone-based chemicals would be more stable than equivalent hydrocarbons in a sulfuric-acid-rich environment, as is found in some extraterrestrial locations.[11]

Of the varieties of molecules identified in the interstellar medium as of 1998, 84 are based on carbon while only 8 are based on silicon.[12] Moreover, of those 8 compounds, four also include carbon within them. The cosmic abundance of carbon to silicon is roughly 10 to 1. This may suggest a greater variety of complex carbon compounds throughout the cosmos, providing less of a foundation on which to build silicon-based biologies, at least under the conditions prevalent on the surface of planets. Also, even though Earth and other terrestrial planets are exceptionally silicon-rich and carbon-poor (the relative abundance of silicon to carbon in Earth's crust is roughly 925:1), terrestrial life is carbon-based. The fact that carbon is used instead of silicon, may be evidence that silicon is poorly suited for biochemistry on Earth-like planets. Reasons for which may be that silicon is less versatile than carbon in forming compounds, that the compounds formed by silicon are unstable, and that it blocks the flow of heat.[13]

Even so, biogenic silica is used by some Earth life, such as the silicate skeletal structure of diatoms. According to the clay hypothesis of A. G. Cairns-Smith, silicate minerals in water played a crucial role in abiogenesis: they replicated their crystal structures, interacted with carbon compounds, and were the precursors of carbon-based life.

Although not observed in nature, carbon–silicon bonds have been added to biochemistry by using directed evolution (artificial selection). A heme containing cytochrome c protein from Rhodothermus marinus has been engineered using directed evolution to catalyze the formation of new carbon–silicon bonds between hydrosilanes and diazo compounds.[16]

Silicon compounds may possibly be biologically useful under temperatures or pressures different from the surface of a terrestrial planet, either in conjunction with or in a role less directly analogous to carbon. Polysilanols, the silicon compounds corresponding to sugars, are soluble in liquid nitrogen, suggesting that they could play a role in very low temperature biochemistry.

In cinematic and literary science fiction, at a moment when man-made machines cross from nonliving to living, it is often posited, this new form would be the first example of non-carbon-based life. Since the advent of the microprocessor in the late 1960s, these machines are often classed as computers (or computer-guided robots) and filed under "silicon-based life", even though the silicon backing matrix of these processors is not nearly as fundamental to their operation as carbon is for "wet life".

Other exotic element-based biochemistries

  • Boranes are dangerously explosive in Earth's atmosphere, but would be more stable in a reducing environment. However, boron's low cosmic abundance makes it less likely as a base for life than carbon.
  • Various metals, together with oxygen, can form very complex and thermally stable structures rivaling those of organic compounds;[citation needed] the heteropoly acids are one such family. Some metal oxides are also similar to carbon in their ability to form both nanotube structures and diamond-like crystals (such as cubic zirconia). Titanium, aluminium, magnesium, and iron are all more abundant in the Earth's crust than carbon. Metal-oxide-based life could therefore be a possibility under certain conditions, including those (such as high temperatures) at which carbon-based life would be unlikely. The Cronin group at Glasgow University reported self-assembly of tungsten polyoxometalates into cell-like spheres.[19] By modifying their metal oxide content, the spheres can acquire holes that act as porous membrane, selectively allowing chemicals in and out of the sphere according to size.[19]
  • Sulfur is also able to form long-chain molecules, but suffers from the same high-reactivity problems as phosphorus and silanes. The biological use of sulfur as an alternative to carbon is purely hypothetical, especially because sulfur usually forms only linear chains rather than branched ones. (The biological use of sulfur as an electron acceptor is widespread and can be traced back 3.5 billion years on Earth, thus predating the use of molecular oxygen.[20] Sulfur-reducing bacteria can utilize elemental sulfur instead of oxygen, reducing sulfur to hydrogen sulfide.)

Arsenic as an alternative to phosphorus

Arsenic, which is chemically similar to phosphorus, while poisonous for most life forms on Earth, is incorporated into the biochemistry of some organisms.[21] Some marine algae incorporate arsenic into complex organic molecules such as arsenosugars and arsenobetaines. Fungi and bacteria can produce volatile methylated arsenic compounds. Arsenate reduction and arsenite oxidation have been observed in microbes (Chrysiogenes arsenatis).[22] Additionally, some prokaryotes can use arsenate as a terminal electron acceptor during anaerobic growth and some can utilize arsenite as an electron donor to generate energy.
It has been speculated that the earliest life forms on Earth may have used arsenic in place of phosphorus in the structure of their DNA.[23] A common objection to this scenario is that arsenate esters are so much less stable to hydrolysis than corresponding phosphate esters that arsenic is poorly suited for this function.[24]

The authors of a 2010 geomicrobiology study, supported in part by NASA, have postulated that a bacterium, named GFAJ-1, collected in the sediments of Mono Lake in eastern California, can employ such 'arsenic DNA' when cultured without phosphorus.[25][26] They proposed that the bacterium may employ high levels of poly-β-hydroxybutyrate or other means to reduce the effective concentration of water and stabilize its arsenate esters.[26] This claim was heavily criticized almost immediately after publication for the perceived lack of appropriate controls.[27][28] Science writer Carl Zimmer contacted several scientists for an assessment: "I reached out to a dozen experts ... Almost unanimously, they think the NASA scientists have failed to make their case".[29] Other authors were unable to reproduce their results and showed that the study had issues with phosphate contamination, suggesting that the low amounts present could sustain extremophile lifeforms.[30] Alternatively, it was suggested that GFAJ-1 cells grow by recycling phosphate from degraded ribosomes, rather than by replacing it with arsenate.[31]

Non-water solvents

Carl Sagan speculated alien life might use ammonia, hydrocarbons or hydrogen fluoride instead of water.

In addition to carbon compounds, all currently known terrestrial life also requires water as a solvent. This has led to discussions about whether water is the only liquid capable of filling that role. The idea that an extraterrestrial life-form might be based on a solvent other than water has been taken seriously in recent scientific literature by the biochemist Steven Benner,[32] and by the astrobiological committee chaired by John A. Baross.[33] Solvents discussed by the Baross committee include ammonia,[34] sulfuric acid,[35] formamide,[36] hydrocarbons,[36] and (at temperatures much lower than Earth's) liquid nitrogen, or hydrogen in the form of a supercritical fluid.

Carl Sagan once described himself as both a carbon chauvinist and a water chauvinist;[38] however on another occasion he said he was a carbon chauvinist but "not that much of a water chauvinist".[39] He speculated on hydrocarbons,[39]:11 hydrofluoric acid,[40] and ammonia[39][40] as possible alternatives to water.

Some of the properties of water that are important for life processes include a large temperature range over which it is liquid, a high heat capacity (useful for temperature regulation), a large heat of vaporization, and the ability to dissolve a wide variety of compounds. Water is also amphoteric, meaning it can donate and accept an H+ ion, allowing it to act as an acid or a base. This property is crucial in many organic and biochemical reactions, where water serves as a solvent, a reactant, or a product. There are other chemicals with similar properties that have sometimes been proposed as alternatives. Additionally, water has the unusual property of being less dense as a solid (ice) than as a liquid. This is why bodies of water freeze over but do not freeze solid (from the bottom up). If ice were denser than liquid water (as is true for nearly all other compounds), then large bodies of liquid would slowly freeze solid, which would not be conducive to the formation of life. Water as a compound is cosmically abundant, although much of it is in the form of vapour or ice. Subsurface liquid water is considered likely or possible on several of the outer moons: Enceladus (where geysers have been observed), Europa, Titan, and Ganymede. Earth and Titan are the only worlds currently known to have stable bodies of liquid on their surfaces.

Not all properties of water are necessarily advantageous for life, however.[41] For instance, water ice has a high albedo,[41] meaning that it reflects a significant quantity of light and heat from the Sun. During ice ages, as reflective ice builds up over the surface of the water, the effects of global cooling are increased.[41]

There are some properties that make certain compounds and elements much more favorable than others as solvents in a successful biosphere. The solvent must be able to exist in liquid equilibrium over a range of temperatures the planetary object would normally encounter. Because boiling points vary with the pressure, the question tends not to be does the prospective solvent remain liquid, but at what pressure. For example, hydrogen cyanide has a narrow liquid phase temperature range at 1 atmosphere, but in an atmosphere with the pressure of Venus, with 92 bars (91 atm) of pressure, it can indeed exist in liquid form over a wide temperature range.

Ammonia

Artist's conception of how a planet with ammonia-based life might look.

The ammonia molecule (NH3), like the water molecule, is abundant in the universe, being a compound of hydrogen (the simplest and most common element) with another very common element, nitrogen.[42] The possible role of liquid ammonia as an alternative solvent for life is an idea that goes back at least to 1954, when J.B.S. Haldane raised the topic at a symposium about life's origin.[43]

Numerous chemical reactions are possible in an ammonia solution, and liquid ammonia has chemical similarities with water.[42][44] Ammonia can dissolve most organic molecules at least as well as water does and, in addition, it is capable of dissolving many elemental metals. Haldane made the point that various common water-related organic compounds have ammonia-related analogs; for instance the ammonia-related amine group (-NH2) is analogous to the water-related hydroxyl group (-OH).[44]

Ammonia, like water, can either accept or donate an H+ ion. When ammonia accepts an H+, it forms the ammonium cation (NH4+), analogous to hydronium (H3O+). When it donates an H+ ion, it forms the amide anion (NH2−), analogous to the hydroxide anion (OH−).[34] Compared to water, however, ammonia is more inclined to accept an H+ ion, and less inclined to donate one; it is a stronger nucleophile.[34] Ammonia added to water functions as Arrhenius base: it increases the concentration of the anion hydroxide. Conversely, using a solvent system definition of acidity and basicity, water added to liquid ammonia functions as an acid, because it increases the concentration of the cation ammonium.[44] The carbonyl group (C=O), which is much used in terrestrial biochemistry, would not be stable in ammonia solution, but the analogous imine group (C=NH) could be used instead.[34]

However, ammonia has some problems as a basis for life. The hydrogen bonds between ammonia molecules are weaker than those in water, causing ammonia's heat of vaporization to be half that of water, its surface tension to be a third, and reducing its ability to concentrate non-polar molecules through a hydrophobic effect. Gerald Feinberg and Robert Shapiro have questioned whether ammonia could hold prebiotic molecules together well enough to allow the emergence of a self-reproducing system.[45] Ammonia is also flammable in oxygen, and could not exist sustainably in an environment suitable for aerobic metabolism.

Titan's theorized internal structure, subsurface ocean shown blue.

A biosphere based on ammonia would likely exist at temperatures or air pressures that are extremely unusual in relation to life on Earth. Life on Earth usually exists within the melting point and boiling point of water at normal pressure, between 0 °C (273 K) and 100 °C (373 K); at normal pressure ammonia's melting and boiling points are between −78 °C (195 K) and −33 °C (240 K). Chemical reactions generally proceed more slowly at a lower temperature. Therefore, ammonia-based life, if it exists, might metabolize more slowly and evolve more slowly than life on Earth.[46] On the other hand, lower temperatures could also enable living systems to use chemical species that would be too unstable at Earth temperatures to be useful.[42]

Ammonia could be a liquid at Earth-like temperatures, but at much higher pressures; for example, at 60 atm, ammonia melts at −77 °C (196 K) and boils at 98 °C (371 K).[34]

Ammonia and ammonia–water mixtures remain liquid at temperatures far below the freezing point of pure water, so such biochemistries might be well suited to planets and moons orbiting outside the water-based habitability zone. Such conditions could exist, for example, under the surface of Saturn's largest moon Titan.[47]

Methane and other hydrocarbons

Methane (CH4) is a simple hydrocarbon: that is, a compound of two of the most common elements in the cosmos, hydrogen and carbon. It has a cosmic abundance comparable with ammonia.[42] Hydrocarbons could act as a solvent over a wide range of temperatures, but would lack polarity. Isaac Asimov, the biochemist and science fiction writer, suggested in 1981 that poly-lipids could form a substitute for proteins in a non-polar solvent such as methane.[42] Lakes composed of a mixture of hydrocarbons, including methane and ethane, have been detected on the surface of Titan by the Cassini spacecraft.

There is debate about the effectiveness of methane and other hydrocarbons as a solvent for life compared to water or ammonia.[48][49][50] Water is a stronger solvent than the hydrocarbons, enabling easier transport of substances in a cell.[51] However, water is also more chemically reactive, and can break down large organic molecules through hydrolysis.[48] A life-form whose solvent was a hydrocarbon would not face the threat of its biomolecules being destroyed in this way.[48] Also, the water molecule's tendency to form strong hydrogen bonds can interfere with internal hydrogen bonding in complex organic molecules.[41] Life with a hydrocarbon solvent could make more use of hydrogen bonds within its biomolecules.[48] Moreover, the strength of hydrogen bonds within biomolecules would be appropriate to a low-temperature biochemistry.[48]

Astrobiologist Chris McKay has argued, on thermodynamic grounds, that if life does exist on Titan's surface, using hydrocarbons as a solvent, it is likely also to use the more complex hydrocarbons as an energy source by reacting them with hydrogen, reducing ethane and acetylene to methane.[52] Possible evidence for this form of life on Titan was identified in 2010 by Darrell Strobel of Johns Hopkins University; a greater abundance of molecular hydrogen in the upper atmospheric layers of Titan compared to the lower layers, arguing for a downward diffusion at a rate of roughly 1025 molecules per second and disappearance of hydrogen near Titan's surface. As Strobel noted, his findings were in line with the effects Chris McKay had predicted if methanogenic life-forms were present.[51][52][53] The same year, another study showed low levels of acetylene on Titan's surface, which were interpreted by Chris McKay as consistent with the hypothesis of organisms reducing acetylene to methane.[51] While restating the biological hypothesis, McKay cautioned that other explanations for the hydrogen and acetylene findings are to be considered more likely: the possibilities of yet unidentified physical or chemical processes (e.g. a non-living surface catalyst enabling acetylene to react with hydrogen), or flaws in the current models of material flow.[54] He noted that even a non-biological catalyst effective at 95 K would in itself be a startling discovery.[54]

Azotosome

A hypothetical cell membrane termed an azotosome capable of functioning in liquid methane in Titan conditions was computer-modeled in a paper published in February 2015. Composed of acrylonitrile, a small molecule containing carbon, hydrogen, and nitrogen, it is predicted to have stability and flexibility in liquid methane comparable to that of a phospholipid bilayer (the type of cell membrane possessed by all life on Earth) in liquid water.[55][56] An analysis of data obtained using the Atacama Large Millimeter / submillimeter Array (ALMA), completed in 2017, confirmed substantial amounts of acrylonitrile in Titan's atmosphere.[57][58]

Hydrogen fluoride

Hydrogen fluoride (HF), like water, is a polar molecule, and due to its polarity it can dissolve many ionic compounds. Its melting point is −84 °C and its boiling point is 19.54 °C (at atmospheric pressure); the difference between the two is a little more than 100 K. HF also makes hydrogen bonds with its neighbor molecules, as do water and ammonia. It has been considered as a possible solvent for life by scientists such as Peter Sneath[59] and Carl Sagan.[40]

HF is dangerous to the systems of molecules that Earth-life is made of, but certain other organic compounds, such as paraffin waxes, are stable with it.[40] Like water and ammonia, liquid hydrogen fluoride supports an acid-base chemistry. Using a solvent system definition of acidity and basicity, nitric acid functions as a base when it is added to liquid HF.[60]

However, hydrogen fluoride is cosmically rare, unlike water, ammonia, and methane.[61]

Hydrogen sulfide

Hydrogen sulfide is the closest chemical analog to water,[62] but is less polar and a weaker inorganic solvent.[63] Hydrogen sulfide is quite plentiful on Jupiter's moon Io, and may be in liquid form a short distance below the surface; and astrobiologist Dirk Schulze-Makuch has suggested it as a possible solvent for life there.[64] On a planet with hydrogen-sulfide oceans the source of the hydrogen sulfide could come from volcanos, in which case it could be mixed in with a bit of hydrogen fluoride, which could help dissolve minerals. Hydrogen sulfide life might use a mixture of carbon monoxide and carbon dioxide as their carbon source. They might produce and live off of sulfur monoxide, which is analogous to oxygen (O2). Hydrogen sulfide, like hydrogen cyanide and ammonia, suffers from the small temperature range where it is liquid, though that, like that of hydrogen cyanide and ammonia, increases with increasing pressure.

Silicon dioxide and silicates

Silicon dioxide, also known as glass, silica, or quartz, is very abundant in the universe and has a large temperature range where it is liquid. However, its melting point is 1,600 to 1,725 °C (2,912 to 3,137 °F), so it would be impossible to make organic compounds in that temperature, because all of them would decompose. Silicates are similar to silicon dioxide and some could have lower boiling points than silica. Gerald Feinberg and Robert Shapiro have suggested that molten silicate rock could serve as a liquid medium for organisms with a chemistry based on silicon, oxygen, and other elements such as aluminium.[65]

Other solvents or cosolvents

Sulfuric acid (H2SO4).

Other solvents sometimes proposed:
Sulfuric acid in liquid form is strongly polar. It remains liquid at higher temperatures than water, its liquid range being 10 °C to 337 °C at a pressure of 1 atm, although above 300 °C it will slowly decompose. Sulfuric acid is known to be abundant in the clouds of Venus, in the form of aerosol droplets. In a biochemistry that used sulfuric acid as a solvent, the alkene group (C=C), with two carbon atoms joined by a double bond, could function analogously to the carbonyl group (C=O) in water-based biochemistry.[35]

A proposal has been made that life on Mars may exist and be using a mixture of water and hydrogen peroxide as its solvent.[69] A 61.2% (by weight) mix of water and hydrogen peroxide has a freezing point of −56.5 °C, and also tends to super-cool rather than crystallize. It is also hygroscopic, an advantage in a water-scarce environment.[70][71]

Supercritical carbon dioxide has been proposed as a candidate for alternative biochemistry due to its ability to selectively dissolve organic compounds and assist the functioning of enzymes and because "super-Earth"- or "super-Venus"-type planets with dense high-pressure atmospheres may be common.[66]

Other speculations

Non-green photosynthesizers

Physicists have noted that, although photosynthesis on Earth generally involves green plants, a variety of other-colored plants could also support photosynthesis, essential for most life on Earth, and that other colors might be preferred in places that receive a different mix of stellar radiation than Earth.[72][73] These studies indicate that, although blue photosynthetic plants would be less likely, yellow or red plants are plausible.[73]

Variable environments

Many Earth plants and animals undergo major biochemical changes during their life cycles as a response to changing environmental conditions, for example, by having a spore or hibernation state that can be sustained for years or even millennia between more active life stages.[74] Thus, it would be biochemically possible to sustain life in environments that are only periodically consistent with life as we know it.

For example, frogs in cold climates can survive for extended periods of time with most of their body water in a frozen state,[74] whereas desert frogs in Australia can become inactive and dehydrate in dry periods, losing up to 75% of their fluids, yet return to life by rapidly rehydrating in wet periods.[75] Either type of frog would appear biochemically inactive (i.e. not living) during dormant periods to anyone lacking a sensitive means of detecting low levels of metabolism.

Nonplanetary life

Dust and plasma-based

In 2007, Vadim N. Tsytovich and colleagues proposed that lifelike behaviors could be exhibited by dust particles suspended in a plasma, under conditions that might exist in space.[76][77] Computer models showed that, when the dust became charged, the particles could self-organize into microscopic helical structures, and the authors offer "a rough sketch of a possible model of the helical grain structure reproduction".

Scientists who have published on this topic

Scientists who have considered possible alternatives to carbon-water biochemistry include:

In fiction

  • Alternate chirality: In Arthur C. Clarke's short story "Technical Error", there is an example of differing chirality.
  • The concept of reversed chirality also figured prominently in the plot of James Blish's Star Trek novel Spock Must Die!, where a transporter experiment gone awry ends up creating a duplicate Spock who turns out to be a perfect mirror-image of the original all the way down to the atomic level.
  • The eponymous organism in Michael Crichton's The Andromeda Strain is described as reproducing via the direct conversion of energy into matter.
  • Silicoids: John Clark, in the introduction to the 1952 shared-world anthology The Petrified Planet, outlined the biologies of the planet Uller, with a mixture of siloxane and silicone life, and of Niflheim, where metabolism is based on hydrofluoric acid and carbon tetrafluoride.
  • In the original Star Trek episode "The Devil in the Dark", a highly intelligent silicon-based creature called Horta, made almost entirely of pure rock, with eggs which take the form of silicon nodules scattered throughout the caverns and tunnels of its home planet. Subsequently, in the non-canonical Star Trek book 'The Romulan Way', another Horta is a junior officer in Starfleet.
  • In Star Trek: The Next Generation, the Crystalline Entity appeared in two episodes, "Datalore" and "Silicon Avatar". This was an enormous spacefaring crystal lattice that had taken thousands of lives in its quest for energy. It was destroyed before communications could be established.
  • In the Star Trek: The Next Generation episode "Home Soil" the Enterprise investigates the sabotage of a planetary terraforming station and the death of one of its members; these events are finally attributed to a completely non-organic, solar powered, saline thriving sentient life form.
  • In the Star Trek: Enterprise episode "Observer Effect" Ensign Sato and Commander Tucker are infected by a silicon-based virus, while being observed by a non-physical life forms called Organians testing humanity if they are intelligent enough to engage in first contact. A reference to The Andromeda Strain (film) was also made in this episode.
  • In the 1994 The X-Files episode "Firewalker", Mulder and Scully investigate a death in a remote research base and discover that a new silicon-based fungus found in the area may be affecting and killing the researchers.
  • The Orion's Arm Universe Project, an online collaborative science-fiction project, includes a number of extraterrestrial species with exotic biochemistries, including organisms based on low-temperature carbohydrate chemistry, organisms that consume and live within sulfuric acid, and organisms composed of structured magnetic flux tubes within neutron stars or gas giant cores.
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