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Hypothetical types of biochemistry

Hypothetical types of biochemistry are forms of biochemistry agreed 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, though 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 of interest in synthetic biology and 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. 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
A shadow biosphere is a hypothetical microbial biosphere of Earth that uses radically different biochemical and molecular processes than currently known life. 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 using 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.

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, 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 elements 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. Sagan used the term "carbon chauvinism" for such an assumption. He regarded silicon and germanium as conceivable alternatives to carbon (other plausible elements include but are not limited to palladium and titanium); but, on the other hand, he noted that carbon does seem more chemically versatile and is more abundant in the cosmos). Norman Horowitz devised the experiments to determine whether life might exist on Mars that were carried out by the Viking Lander of 1976, the first U.S. mission to successfully land a probe on the surface of Mars. Horowitz argued that the great versatility of the carbon atom makes it the element most likely to provide solutions, even exotic solutions, to the problems of survival on other planets.  He considered that there was only a remote possibility that non-carbon life forms could exist with genetic information systems capable of self-replication and the ability to evolve and adapt.

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

However, silicon has several drawbacks as a carbon alternative. Carbon is ten times more cosmically abundant than silicon, and its chemistry appears naturally more complex. By 1998, astronomers had identified 84 carbon-containing molecules in the interstellar medium, but only 8 containing silicon, of which half also included carbon. Even though Earth and other terrestrial planets are exceptionally silicon-rich and carbon-poor (silicon is roughly 925 times more abundant in Earth's crust than carbon), terrestrial life bases itself on carbon. It may eschew silicon because silicon compounds are less varied, unstable in the presence of water, or block the flow of heat.

Relative to carbon, silicon has a much larger atomic radius, and forms much weaker covalent bonds to atoms — except oxygen and fluorine, with which it forms very strong bonds. Almost no multiple bonds to silicon are stable, although silicon does exhibit varied coordination number. Silanes, silicon analogues to the alkanes, react rapidly with water, and long-chain silanes spontaneously decompose. Consequently, most terrestrial silicon is "locked up" in silica, and not a wide variety of biogenic precursors.

Silicones, which alternate between silicon and oxygen atoms, are much more stable than silanes, and may even be more stable than the equivalent hydrocarbons in sulfuric acid-rich extraterrestrial environments. Alternatively, the weak bonds in silicon compounds may help maintain a rapid pace of life at cryogenic temperatures. Polysilanols, the silicon homologues to sugars, are among the few compounds soluble in liquid nitrogen.

All known silicon macromolecules are artificial polymers, and so "monotonous compared with the combinatorial universe of organic macromolecules". Even so, some Earth life uses biogenic silica: diatoms' silicate skeletons. A. G. Cairns-Smith hypothesized that silicate minerals in water played a crucial role in abiogenesis, in that biogenic carbon compounds formed around their crystal structures. Although not observed in nature, carbon–silicon bonds have been added to biochemistry under directed evolution (artificial selection): a cytochrome c protein from Rhodothermus marinus has been engineered to catalyze new carbon–silicon bonds between hydrosilanes and diazo compounds.

Other exotic element-based biochemistries

 * Boranes are dangerously explosive in Earth's atmosphere, but would be more stable in a reducing atmosphere. 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; 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. 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.
 * 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. 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. 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). 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 biochemistry in place of phosphorus in the structure of their DNA. 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.

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. 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. This claim was heavily criticized almost immediately after publication for the perceived lack of appropriate controls. 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". 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. Alternatively, it was suggested that GFAJ-1 cells grow by recycling phosphate from degraded ribosomes, rather than by replacing it with arsenate.

Non-water solvents
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, and by the astrobiological committee chaired by John A. Baross. Solvents discussed by the Baross committee include ammonia, sulfuric acid, formamide, hydrocarbons, and (at temperatures much lower than Earth's) liquid nitrogen, or hydrogen in the form of a supercritical fluid.

Water as a solvent limits the forms biochemistry can take. For example, Steven Benner, proposes the polyelectrolyte theory of the gene that claims that for a genetic biopolymer such as, DNA, to function in water, it requires repeated ionic charges. If water is not required for life, these limits on genetic biopolymers are removed.

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

Some of the properties of water that are important for life processes include:
 * A complexity which leads to a large number of permutations of possible reaction paths including acid–base chemistry, H+ cations, OH− anions, hydrogen bonding, van der Waals bonding, dipole–dipole and other polar interactions, aqueous solvent cages, and hydrolysis. This complexity offers a large number of pathways for evolution to produce life, many other solvents have dramatically fewer possible reactions, which severely limits evolution.
 * Thermodynamic stability: the free energy of formation of liquid water is low enough (−237.24 kJ/mol) that water undergoes few reactions. Other solvents are highly reactive, particularly with oxygen.
 * Water does not combust in oxygen because it is already the combustion product of hydrogen with oxygen. Most alternative solvents are not stable in an oxygen-rich atmosphere, so it is highly unlikely that those liquids could support aerobic life.
 * A large temperature range over which it is liquid.
 * High solubility of oxygen and carbon dioxide at room temperature supporting the evolution of aerobic aquatic plant and animal life.
 * A high heat capacity (leading to higher environmental temperature stability).
 * Water is a room-temperature liquid leading to a large population of quantum transition states required to overcome reaction barriers. Cryogenic liquids (such as liquid methane) have exponentially lower transition state populations which are needed for life based on chemical reactions. This leads to chemical reaction rates which may be so slow as to preclude the development of any life based on chemical reactions.
 * Spectroscopic transparency allowing solar radiation to penetrate several meters into the liquid (or solid), greatly aiding the evolution of aquatic life.
 * A large heat of vaporization leading to stable lakes and oceans.
 * The ability to dissolve a wide variety of compounds.
 * The solid (ice) has lower density than the liquid, so ice floats on the 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 vapor 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. For instance, water ice has a high albedo, 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.

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 bar of pressure, it can indeed exist in liquid form over a wide temperature range.

Ammonia
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. 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.

Numerous chemical reactions are possible in an ammonia solution, and liquid ammonia has chemical similarities with water. 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).

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−). Compared to water, however, ammonia is more inclined to accept an H+ ion, and less inclined to donate one; it is a stronger nucleophile. 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. 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.

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. Ammonia is also flammable in oxygen and could not exist sustainably in an environment suitable for aerobic metabolism.

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 a pressure designated as normal pressure, and between 0 and 100 °C. When also held to normal pressure, ammonia's melting and boiling points are -78 °C and -33 °C respectively. Because chemical reactions generally proceed more slowly at lower temperatures, ammonia-based life existing in this set of conditions might metabolize more slowly and evolve more slowly than life on Earth. 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.

Another set of conditions where ammonia is liquid at Earth-like temperatures would involve it being at a much higher pressure. For example, at 60 atm ammonia melts at -77 °C and boils at 98 °C.

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.

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

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. 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. 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. 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. He noted that even a non-biological catalyst effective at 95 K would in itself be a startling discovery.

Azotosome
A hypothetical cell membrane termed an azotosome, capable of functioning in liquid methane in Titan conditions was computer-modeled in an article 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. 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. Later studies questioned whether acrylonitrile would be able to self-assemble into azotozomes.

Hydrogen fluoride
Hydrogen fluoride (HF), like water, is a polar molecule, and due to its polarity it can dissolve many ionic compounds. At atmospheric pressure, its melting point is 189.15 K, and its boiling point is 292.69 K; 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 and Carl Sagan.

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. 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.

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

Hydrogen sulfide
Hydrogen sulfide is the closest chemical analog to water, but is less polar and is a weaker inorganic solvent. Hydrogen sulfide is quite plentiful on Jupiter's moon Io and may be in liquid form a short distance below the surface; astrobiologist Dirk Schulze-Makuch has suggested it as a possible solvent for life there. On a planet with hydrogen sulfide oceans, the source of the hydrogen sulfide could come from volcanoes, 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 on 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 silica and 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, 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 have lower melting points than silica. Feinberg and 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.

Other solvents or cosolvents
Other solvents sometimes proposed:
 * Supercritical fluids: supercritical carbon dioxide and supercritical hydrogen.
 * Simple hydrogen compounds: hydrogen chloride.
 * More complex compounds: sulfuric acid, formamide, methanol.
 * Very-low-temperature fluids: liquid nitrogen and hydrogen.
 * High-temperature liquids: sodium chloride.

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 slowly decomposes. 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.

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

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.

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. These studies indicate that blue plants would be unlikely; however yellow or red plants may be relatively common.

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. 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, 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. 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.

Alanine world and hypothetical alternatives
The genetic code may have evolved during the transition from the RNA world to a protein world. The Alanine World Hypothesis postulates that the evolution of the genetic code (the so-called GC phase ) started with only four basic amino acids: alanine, glycine, proline and ornithine (now arginine). The evolution of the genetic code ended with 20 proteinogenic amino acids. From a chemical point of view, most of them are Alanine-derivatives particularly suitable for the construction of α-helices and β-sheets – basic secondary structural elements of modern proteins. Direct evidence of this is an experimental procedure in molecular biology known as alanine scanning.

A hypothetical "Proline World" would create a possible alternative life with the genetic code based on the proline chemical scaffold as the protein backbone. Similarly, a "Glycine World" and "Ornithine World" are also conceivable, but nature has chosen none of them. Evolution of life with Proline, Glycine, or Ornithine as the basic structure for protein-like polymers (foldamers) would lead to parallel biological worlds. They would have morphologically radically different body plans and genetics from the living organisms of the known biosphere.

Dusty 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. 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...helical grain structure reproduction".

Cosmic necklace-based
In 2020, Luis A. Anchordoqu and Eugene M. Chudnovsky of the City University of New York hypothesized that cosmic necklace-based life composed of magnetic monopoles connected by cosmic strings could evolve inside stars. This would be achieved by a stretching of cosmic strings due to the star's intense gravity, thus allowing it to take on more complex forms and potentially form structures similar to the RNA and DNA structures found within carbon-based life. As such, it is theoretically possible that such beings could eventually become intelligent and construct a civilization using the power generated by the star's nuclear fusion. Because such use would use up part of the star's energy output, the luminosity would also fall. For this reason, it is thought that such life might exist inside stars observed to be cooling faster or dimmer than current cosmological models predict.

Life on a neutron star
Frank Drake suggested in 1973 that intelligent life could inhabit neutron stars. Physical models in 1973 implied that Drake's creatures would be microscopic.

Scientists who have published on this topic
Scientists who have considered possible alternatives to carbon-water biochemistry include:
 * J. B. S. Haldane (1892–1964), a geneticist noted for his work on abiogenesis.
 * V. Axel Firsoff (1910–1981), British astronomer.
 * Isaac Asimov (1920–1992), biochemist and science fiction writer.
 * Fred Hoyle (1915–2001), astronomer and science fiction writer.
 * Norman Horowitz (1915–2005) Caltech geneticist who devised the first experiments carried out to detect life on Mars.
 * George C. Pimentel (1922–1989), American chemist, University of California, Berkeley.
 * Peter Sneath (1923–2011), microbiologist, author of the book Planets and Life.
 * Gerald Feinberg (1933–1992), physicist and Robert Shapiro (1935–2011), chemist, co-authors of the book Life Beyond Earth.
 * Carl Sagan (1934–1996), astronomer, science popularizer, and SETI proponent.
 * Jonathan Lunine (born 1959), American planetary scientist and physicist.
 * Robert Freitas (born 1952), specialist in nano-technology and nano-medicine.
 * John Baross (born 1940), oceanographer and astrobiologist, who chaired a committee of scientists under the United States National Research Council that published a report on life's limiting conditions in 2007.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Hypothetical types of biochemistry


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Talk:Hypothetical types of biochemistry

Untitled
Since the name of the page has changed, you can now find the peer review archive here: Peer_review/March_2005.

Suggestion
Suggestion: Replace the nanotechnology link with one to the more-to-the-point less-hypey article on molecular assembler which uses only credible sources...

Relationship between Water loss and Leaf Structure
in strand one,we studied the internal and external structure of leaves. However,the structure of the leaves of different species of plants are not all the same.Leaves of one species may differ from those of another in respect of one or more of the following:

•their general shape and size •the arrangement of leaves in relation to each other •the thickness of the cuticle •the number and position of stomata

All the above structure differencesaffect the rate at which water vapour is lost by the plant by the process of transpiration. For example:

•The shapes and sizes of the leaves of some plants are altered so as to reduce the surface area exposed to the sun,thus reducing the transpiration rate e.g. in pinus,the leaves are needle-shaped.

•The leaves of some plants are situated close to each other so that they overlap and shade each other,ths reducing the transpiration rate.

•In some plants e.g. Nurium oleander,hairs occur on the leaves.This reduces air movement and also reflects the lightfalling on the leaves,thus reducing transpiration rate.

•The cuticle of the leaves and other aerial parts may be thickened to reduce water vapour loss through transpiration.

•The number of stomata may be reduced to cut down water loss.In some plants e.g. Hąķęą and Pìņųş, the stomata may be sunken so as to accumulate water vapour  molecules in sunken pockets, not exposed to direct sunlight. Jelmani (talk) 17:47, 20 May 2024 (UTC)

 * um... what does this have to do with hypothetical biochemistry?... Tamtrible (talk) 12:18, 11 June 2024 (UTC)

title: Talk:Hypothetical types of biochemistry


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Creation myth

A creation myth or cosmogonic myth is a type of cosmogony, a symbolic narrative of how the world began and how people first came to inhabit it. While in popular usage the term myth often refers to false or fanciful stories, members of cultures often ascribe varying degrees of truth to their creation myths. In the society in which it is told, a creation myth is usually regarded as conveying profound truths – metaphorically, symbolically, historically, or literally. They are commonly, although not always, considered cosmogonical myths – that is, they describe the ordering of the cosmos from a state of chaos or amorphousness.

Creation myths often share several features. They often are considered sacred accounts and can be found in nearly all known religious traditions. They are all stories with a plot and characters who are either deities, human-like figures, or animals, who often speak and transform easily. They are often set in a dim and nonspecific past that historian of religion Mircea Eliade termed in illo tempore ('at that time'). Creation myths address questions deeply meaningful to the society that shares them, revealing their central worldview and the framework for the self-identity of the culture and individual in a universal context.

Creation myths develop in oral traditions and therefore typically have multiple versions; found throughout human culture, they are the most common form of myth.

Definitions
Creation myth definitions from modern references:
 * A "symbolic narrative of the beginning of the world as understood in a particular tradition and community. Creation myths are of central importance for the valuation of the world, for the orientation of humans in the universe, and for the basic patterns of life and culture."
 * "Creation myths tell us how things began. All cultures have creation myths; they are our primary myths, the first stage in what might be called the psychic life of the species. As cultures, we identify ourselves through the collective dreams we call creation myths, or cosmogonies. ... Creation myths explain in metaphorical terms our sense of who we are in the context of the world, and in so doing they reveal our real priorities, as well as our real prejudices. Our images of creation say a great deal about who we are."
 * A "philosophical and theological elaboration of the primal myth of creation within a religious community. The term myth here refers to the imaginative expression in narrative form of what is experienced or apprehended as basic reality ... The term creation refers to the beginning of things, whether by the will and act of a transcendent being, by emanation from some ultimate source, or in any other way."

Religion professor Mircea Eliade defined the word myth in terms of creation: Myth narrates a sacred history; it relates an event that took place in primordial Time, the fabled time of the "beginnings." In other words, myth tells how, through the deeds of Supernatural Beings, a reality came into existence, be it the whole of reality, the Cosmos, or only a fragment of reality – an island, a species of plant, a particular kind of human behavior, an institution.

Meaning and function

Creation myths have been around since ancient history and have served important societal roles. Over 100 "distinct" ones have been discovered. All creation myths are in one sense etiological because they attempt to explain how the world formed and where humanity came from. Myths attempt to explain the unknown and sometimes teach a lesson.

Ethnologists and anthropologists who study origin myths say that in the modern context theologians try to discern humanity's meaning from revealed truths and scientists investigate cosmology with the tools of empiricism and rationality, but creation myths define human reality in very different terms. In the past, historians of religion and other students of myth thought of such stories as forms of primitive or early-stage science or religion and analyzed them in a literal or logical sense. Today, however, they are seen as symbolic narratives which must be understood in terms of their own cultural context. Charles Long writes: "The beings referred to in the myth – gods, animals, plants – are forms of power grasped existentially. The myths should not be understood as attempts to work out a rational explanation of deity."

While creation myths are not literal explications, they do serve to define an orientation of humanity in the world in terms of a birth story. They provide the basis of a worldview that reaffirms and guides how people relate to the natural world, to any assumed spiritual world, and to each other. A creation myth acts as a cornerstone for distinguishing primary reality from relative reality, the origin and nature of being from non-being. In this sense cosmogonic myths serve as a philosophy of life – but one expressed and conveyed through symbol rather than through systematic reason. And in this sense they go beyond etiological myths (which explain specific features in religious rites, natural phenomena, or cultural life). Creation myths also help to orient human beings in the world, giving them a sense of their place in the world and the regard that they must have for humans and nature.

Historian David Christian has summarised issues common to multiple creation myths:

"How did everything begin? This is the first question faced by any creation myth and ... answering it remains tricky. ... Each beginning seems to presuppose an earlier beginning. ... Instead of meeting a single starting point, we encounter an infinity of them, each of which poses the same problem. ... There are no entirely satisfactory solutions to this dilemma. What we have to find is not a solution but some way of dealing with the mystery .... And we have to do so using words. The words we reach for, from God to gravity, are inadequate to the task. So we have to use language poetically or symbolically; and such language, whether used by a scientist, a poet, or a shaman, can easily be misunderstood."

Classification

Mythologists have applied various schemes to classify creation myths found throughout human cultures. Eliade and his colleague Charles Long developed a classification based on some common motifs that reappear in stories the world over. The classification identifies five basic types:

 * Creation ex nihilo in which the creation is through the thought, word, dream, or bodily secretions of a divine being.
 * Earth-diver creation in which a diver, usually a bird or amphibian sent by a creator, plunges to the seabed through a primordial ocean to bring up sand or mud which develops into a terrestrial world.
 * Emergence myths in which progenitors pass through a series of worlds and metamorphoses until reaching the present world.
 * Creation by the dismemberment of a primordial being.
 * Creation by the splitting or ordering of a primordial unity such as the cracking of a cosmic egg or a bringing order from chaos.

Marta Weigle further developed and refined this typology to highlight nine themes, adding elements such as deus faber, a creation crafted by a deity, creation from the work of two creators working together or against each other, creation from sacrifice and creation from division/conjugation, accretion/conjunction, or secretion.

An alternative system based on six recurring narrative themes was designed by Raymond Van Over:
 * Primeval abyss, an infinite expanse of waters or space
 * Originator deity which is awakened or an eternal entity within the abyss
 * Originator deity poised above the abyss
 * Cosmic egg or embryo
 * Originator deity creating life through sound or word
 * Life generating from the corpse or dismembered parts of an originator deity

Ex nihilo

The myth that God created the world out of nothing – ex nihilo – is central today to Judaism, Christianity, and Islam, and the medieval Jewish philosopher Maimonides felt it was the only concept that the three religions shared. Nonetheless, the concept is not found in the entire Hebrew Bible. The authors of Genesis 1 were concerned not with the origins of matter (the material which God formed into the habitable cosmos), but with assigning roles so that the cosmos should function. In the early 2nd century CE, early Christian scholars were beginning to see a tension between the idea of world-formation and the omnipotence of God, and by the beginning of the 3rd century creation ex nihilo had become a fundamental tenet of Christian theology.

Ex nihilo creation is found in creation stories from ancient Egypt, the Rig Veda, and many animistic cultures in Africa, Asia, Oceania, and North America. In most of these stories, the world is brought into being by the speech, dream, breath, or pure thought of a creator but creation ex nihilo may also take place through a creator's bodily secretions.

The literal translation of the phrase ex nihilo is "from nothing" but in many creation myths the line is blurred whether the creative act would be better classified as a creation ex nihilo or creation from chaos. In ex nihilo creation myths, the potential and the substance of creation springs from within the creator. Such a creator may or may not be existing in physical surroundings such as darkness or water, but does not create the world from them, whereas in creation from chaos the substance used for creation is pre-existing within the unformed void.

Creation from chaos
In creation from chaos myths, there is nothing initially but a formless, shapeless expanse. In these stories the word "chaos" means "disorder", and this formless expanse, which is also sometimes called a void or an abyss, contains the material with which the created world will be made. Chaos may be described as having the consistency of vapor or water, dimensionless, and sometimes salty or muddy. These myths associate chaos with evil and oblivion, in contrast to "order" (cosmos) which is the good. The act of creation is the bringing of order from disorder, and in many of these cultures it is believed that at some point the forces preserving order and form will weaken and the world will once again be engulfed into the abyss. One example is the Genesis creation narrative from the first chapter of the Book of Genesis.

World parent
There are two types of world parent myths, both describing a separation or splitting of a primeval entity, the world parent or parents. One form describes the primeval state as an eternal union of two parents, and the creation takes place when the two are pulled apart. The two parents are commonly identified as Sky (usually male) and Earth (usually female), who were so tightly bound to each other in the primeval state that no offspring could emerge. These myths often depict creation as the result of a sexual union and serve as genealogical record of the deities born from it.

In the second form of world parent myths, creation itself springs from dismembered parts of the body of the primeval being. Often, in these stories, the limbs, hair, blood, bones, or organs of the primeval being are somehow severed or sacrificed to transform into sky, earth, animal or plant life, and other worldly features. These myths tend to emphasize creative forces as animistic in nature rather than sexual, and depict the sacred as the elemental and integral component of the natural world. One example of this is the Norse creation myth described in Völuspá, the first poem of Gylfaginning.

Emergence
In emergence myths, humanity emerges from another world into the one they currently inhabit. The previous world is often considered the womb of the earth mother, and the process of emergence is likened to the act of giving birth. The role of midwife is usually played by a female deity, like the spider woman of several mythologies of Indigenous peoples in the Americas. Male characters rarely figure into these stories, and scholars often consider them in counterpoint to male-oriented creation myths, like those of the ex nihilo variety.

Emergence myths commonly describe the creation of people and/or supernatural beings as a staged ascent or metamorphosis from nascent forms through a series of subterranean worlds to arrive at their current place and form. Often the passage from one world or stage to the next is impelled by inner forces, a process of germination or gestation from earlier, embryonic forms. The genre is most commonly found in Native American cultures where the myths frequently link the final emergence of people from a hole opening to the underworld to stories about their subsequent migrations and eventual settlement in their current homelands.

Earth-diver
The earth-diver is a common character in various traditional creation myths. In these stories a supreme being usually sends an animal (most often a type of bird, but also crustaceans, insects, and fish in some narratives) into the primal waters to find bits of sand or mud with which to build habitable land. Some scholars interpret these myths psychologically while others interpret them cosmogonically. In both cases emphasis is placed on beginnings emanating from the depths.

Motif distribution
According to Gudmund Hatt and Tristram P. Coffin, Earth-diver myths are common in Native American folklore, among the following populations: Shoshone, Meskwaki, Blackfoot, Chipewyan, Newettee, Yokuts of California, Mandan, Hidatsa, Cheyenne, Arapaho, Ojibwe, Yuchi, and Cherokee.

American anthropologist Gladys Reichard located the distribution of the motif across "all parts of North America", save for "the extreme north, northeast, and southwest". In a 1977 study, anthropologist Victor Barnouw surmised that the earth-diver motif appeared in "hunting-gathering societies", mainly among northerly groups such as the Hare, Dogrib, Kaska, Beaver, Carrier, Chipewyan, Sarsi, Cree, and Montagnais.

Similar tales are also found among the Chukchi and Yukaghir, the Tatars, and many Finno-Ugric traditions, as well as among the Buryat and the Samoyed. In addition, the earth-diver motif also exists in narratives from Eastern Europe, namely Romani, Romanian, Slavic (namely, Bulgarian, Polish, Ukrainian, and Belarusian), and Lithuanian mythological traditions.

The pattern of distribution of these stories suggest they have a common origin in the eastern Asiatic coastal region, spreading as peoples migrated west into Siberia and east to the North American continent. However, there are examples of this mytheme found well outside of this boreal distribution pattern, for example the West African Yoruba creation myth of Ọbatala and Oduduwa.

Native American narrative
Characteristic of many Native American myths, earth-diver creation stories begin as beings and potential forms linger asleep or suspended in the primordial realm. The earth-diver is among the first of them to awaken and lay the necessary groundwork by building suitable lands where the coming creation will be able to live. In many cases, these stories will describe a series of failed attempts to make land before the solution is found.

Among the indigenous peoples of the Americas, the earth-diver cosmogony is attested in Iroquois mythology: a female sky deity falls from the heavens, and certain animals, the beaver, the otter, the duck, and the muskrat dive in the waters to fetch mud to construct an island.

In a similar story from the Seneca, people lived in a sky realm. One day, the chief's daughter was afflicted with a mysterious illness, and the only cure recommended for her (revealed in a dream) was to lie beside a tree and to have it be dug up. The people do so, but a man complains that the tree was their livelihood, and kicks the girl through the hole. She ends up falling from the sky to a world of only water, but is rescued by waterfowl. A turtle offers to bear her on its shell, but asked where would be a definitive dwelling place for her. They decide to create land, and the toad dives into the depths of the primal sea to get pieces of soil. The toad puts it on the turtle's back, which grows larger with every deposit of soil.

In another version from the Wyandot, the Wyandot lived in heaven. The daughter of the Big Chief (or Mighty Ruler) was sick, so the medicine man recommends that they dig up the wild apple tree that stands next to the Lodge of the Mighty Ruler, because the remedy is to be found on its roots. However, as the tree has been dug out, the ground begins to sink away, and the treetops catch and carry down the sick daughter with it. As the girl falls from the skies, two swans rescue her on their backs. The birds decide to summon all the Swimmers and the Water Tribes. Many volunteer to dive into the Great Water to fetch bits of earth from the bottom of the sea, but only the toad (female, in the story) is the one successful.

title: Creation myth


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Crucifix

A crucifix (from the Latin cruci fixus meaning '(one) fixed to a cross') is a cross with an image of Jesus on it, as distinct from a bare cross. The representation of Jesus himself on the cross is referred to in English as the corpus (Latin for 'body'). The crucifix emphasizes Jesus' sacrifice, including his death by crucifixion, which Christians believe brought about the redemption of mankind. Most crucifixes portray Jesus on a Latin cross, rather than a Tau cross or a Coptic cross.

The crucifix is a principal symbol for many groups of Christians, and one of the most common forms of the Crucifixion in the arts. It is especially important in the Catholic Church, and is also used in the Eastern Orthodox Church, most Oriental Orthodox Churches except the Armenian & Syriac Church, Lutheranism, Anglicanism. The symbol is less common in churches of other Protestant denominations, and in the Assyrian Church of the East and Armenian Apostolic Church, which prefer to use a cross without the figure of Jesus (the corpus).

Roman Catholics see the crucifix as the perfect fulfillment of that inferred by the serpent created by Moses in Numbers 21:8—9, called the Nehushtan. It was promised that those sinners who looked upon the Nehushtan would be healed. The section of Numbers about the Nehushtan is one of the readings on Exaltation of the Cross that occurs on September 14 in the Roman Catholic Church. It is paired with John 3:14–15 as the gospel reading. Taken together, these readings explain the striking front and center position of a large crucifix normally fixed above or behind a Catholic altar.

Western crucifixes usually have a three-dimensional corpus, but in Eastern Orthodoxy Jesus' body is normally painted on the cross, or in low relief. Strictly speaking, to be a crucifix, the cross must be three-dimensional, but this distinction is not always observed. An entire painting of the crucifixion of Jesus including a landscape background and other figures is not a crucifix either.

Large crucifixes high across the central axis of a church are known by the Old English term rood. By the Late Middle Ages these were a near-universal feature of Western churches, but they are now very rare. Modern Roman Catholic churches and many Lutheran churches often have a crucifix above the altar on the wall; for the celebration of Mass, the Roman Rite of the Catholic Church requires that "on or close to the altar there is to be a cross with a figure of Christ crucified".

Description
The standard, four-pointed Latin crucifix consists of an upright post or stipes and a single crosspiece to which the sufferer's arms were nailed. There may also be a short projecting nameplate, showing the letters INRI (Greek: INBI). The Russian Orthodox crucifix usually has an additional third crossbar, to which the feet are nailed, and which is angled upward toward the penitent thief Saint Dismas (to the viewer's left) and downward toward the impenitent thief Gestas (to the viewer's right). The corpus of Eastern crucifixes is normally a two-dimensional or low relief icon that shows Jesus as already dead, his face peaceful and somber. They are rarely three-dimensional figures as in the Western tradition, although these may be found where Western influences are strong, but are more typically icons painted on a piece of wood shaped to include the double-barred cross and perhaps the edge of Christ's hips and halo, and no background. More sculptural small crucifixes in metal relief are also used in Orthodoxy (see gallery examples), including as pectoral crosses and blessing crosses.

Western crucifixes may show Christ dead or alive, the presence of the spear wound in his ribs traditionally indicating that he is dead. In either case his face very often shows his suffering. In the Eastern Orthodox tradition he has normally been shown as dead since around the end of the period of Byzantine Iconoclasm. Eastern crucifixes have Jesus' two feet nailed side by side, rather than crossed one above the other, as Western crucifixes have shown them since around the 13th century. The crown of thorns is also generally absent in Eastern crucifixes, since the emphasis is not on Christ's suffering, but on his triumph over sin and death. The S-shaped position of Jesus' body on the cross is a Byzantine innovation of the late 10th century, though also found in the German Gero Cross of the same date. Probably more from Byzantine influence, it spread elsewhere in the West, especially to Italy, by the Romanesque period, though it was more usual in painting than sculpted crucifixes. It was in Italy that the emphasis was put on Jesus' suffering and realistic details, during a process of general humanization of Christ favored by the Franciscan order. During the 13th century the suffering Italian model (Christus patiens) triumphed over the traditional Byzantine one (Christus gloriosus) anywhere in Europe also due to the works of artists such as Giunta Pisano and Cimabue. Since the Renaissance the "S"-shape is generally much less pronounced. Eastern Christian blessing crosses will often have the Crucifixion depicted on one side, and the Resurrection on the other, illustrating Eastern Orthodox theology's understanding of the Crucifixion and Resurrection as two intimately related aspects of the same act of salvation.

Another, symbolic, depiction shows a triumphant Christ (Christus triumphans), clothed in robes, rather than stripped as for his execution, with arms raised, appearing to rise up from the cross, sometimes accompanied by "rays of light", or an aureole encircling his body. He may be robed as a prophet, crowned as a king, and vested in a stole as Great High Priest.

On some crucifixes a skull and crossbones are shown below the corpus, referring to Golgotha (Calvary), the site at which Jesus was crucified, which the Gospels say means in Hebrew "the place of the skull." Medieval tradition held that it was the burial-place of Adam and Eve, and that the cross of Christ was raised directly over Adam's skull, so many crucifixes manufactured in Catholic countries still show the skull and crossbones below the corpus.

Very large crucifixes have been built, the largest being the Cross in the Woods in Michigan, with a 31 ft high statue.

Usage
In the early Church, many Christians hung a cross on the eastern wall of their house in order to indicate the eastward direction of prayer. Prayer in front of a crucifix, which is seen as a sacramental, is often part of devotion for Christians, especially those worshipping in a church, also privately. The person may sit, stand, or kneel in front of the crucifix, sometimes looking at it in contemplation, or merely in front of it with head bowed or eyes closed. During the Middle Ages small crucifixes, generally hung on a wall, became normal in the personal cells or living quarters first of monks, then all clergy, followed by the homes of the laity, spreading down from the top of society as these became cheap enough for the average person to afford. Most towns had a large crucifix erected as a monument, or some other shrine at the crossroads of the town. Building on the ancient custom, many Catholics, Lutherans and Anglicans hang a crucifix inside their homes and also use the crucifix as a focal point of a home altar. The wealthy erected proprietary chapels as they could afford to do this.

Catholic (both Eastern and  Western), Eastern Orthodox, Oriental Orthodox, Anglican and Lutheran Christians generally use the crucifix in public religious services. They believe use of the crucifix is in keeping with the statement by Paul the Apostle in 1 Corinthians: "we preach Christ crucified, a stumbling block to Jews and folly to Gentiles, but to those who are called, both Jews and Greeks, Christ the power of God and the wisdom of God".

In the West, altar crosses and processional crosses began to be crucifixes in the 11th century, which became general around the 14th century, as they became cheaper. The Roman Rite requires that "either on the altar or near it, there is to be a cross, with the figure of Christ crucified upon it, a cross clearly visible to the assembled people. It is desirable that such a cross should remain near the altar even outside of liturgical celebrations, so as to call to mind for the faithful the saving Passion of the Lord." The requirement of the altar cross was also mentioned in pre-1970 editions of the Roman Missal, though not in the original 1570 Roman Missal of Pope Pius V. The Rite of Funerals says that the Gospel Book, the Bible, or a cross (which will generally be in crucifix form) may be placed on the coffin for a Requiem Mass, but a second standing cross is not to be placed near the coffin if the altar cross can be easily seen from the body of the church.

Eastern Christian liturgical processions called crucessions include a cross or crucifix at their head. In the Eastern Orthodox Church, the crucifix is often placed above the iconostasis in the church. In the Russian Orthodox Church a large crucifix ("Golgotha") is placed behind the Holy Table (altar). During Matins of Good Friday, a large crucifix is taken in procession to the center of the church, where it is venerated by the faithful. Sometimes the soma (corpus) is removable and is taken off the crucifix at Vespers that evening during the Gospel lesson describing the Descent from the Cross. The empty cross may then remain in the centre of the church until the Paschal vigil (local practices vary). The blessing cross which the priest uses to bless the faithful at the dismissal will often have the crucifix on one side and an icon of the Resurrection of Jesus on the other, the side with the Resurrection being used on Sundays and during Paschaltide, and the crucifix on other days.

Exorcist Gabriele Amorth has stated that the crucifix is one of the most effective means of averting or opposing demons. In folklore, it is believed to ward off vampires, incubi, succubi, and other evils.

Modern anti-Christians have used an inverted (upside-down) crucifix when showing disdain for Jesus Christ or the Catholic Church which believes in his divinity. According to Christian tradition, Saint Peter was martyred by being crucified upside-down.

Protestant Reformation
In the Moravian Church, Nicolaus Zinzendorf had an experience in which he believed he encountered Jesus. Seeing a painting of a crucifix, Zinzendorf fell on his knees vowing to glorify Jesus after contemplating on the wounds of Christ and an inscription that stated "This is what I have done for you, what will you do for me?".

The Lutheran Churches retained the use of the crucifix, "justifying their continued use of medieval crucifixes with the same arguments employed since the Middle Ages, as is evident from the example of the altar of the Holy Cross in the Cistercian church of Doberan." Martin Luther did not object to them, and this was among his differences with Andreas Karlstadt as early as 1525. At the time of the Reformation, Luther retained the crucifix in the Lutheran Church and they remain the center of worship in Lutheran parishes across Europe. In the United States, however, Lutheranism came under the influence of Calvinism, and the plain cross came to be used in many churches. In contrast to the practice of the Moravian Church and Lutheran Churches, the early Reformed Churches rejected the use of the crucifix, and indeed the unadorned cross, along with other traditional religious imagery, as idolatrous. Calvin, considered to be the father of the Reformed Church, was violently opposed to both cross and crucifix. In England, the Royal Chapels of Elizabeth I were most unusual among local churches in retaining crucifixes, following the Queen's conservative tastes. These disappeared under her successor, James I, and their brief re-appearance in the early 1620s when James' heir was seeking a Spanish marriage was the subject of rumour and close observation by both Catholics and Protestants; when the match fell through they disappeared.

Modern
In 2005, a mother accused her daughter's school in Derby, England, of discriminating against Christians after the teenager was suspended for refusing to take off a crucifix necklace.

In 2008, a chapel in a prison in England replaced its crucifix and static altar with a cross and portable altar when it was renovated as a multi-faith chapel. Right-leaning media reported that the crucifix had been removed "in case it offends Muslims".

In 2008 in Spain, a local judge ordered crucifixes removed from public schools to settle a decades-old dispute over whether crucifixes should be displayed in public buildings in a non-confessional state.

On 18 March 2011, the European Court of Human Rights ruled in the Lautsi v. Italy case, that the requirement in Italian law that crucifixes be displayed in classrooms of state schools does not violate the European Convention on Human Rights. Crucifixes are common in most other Italian official buildings, including courts of law.

On 24 March 2011, the Constitutional Court of Peru ruled that the presence of crucifixes in courts of law does not violate the secular nature of the state.

title: Crucifix


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Copernican principle

In physical cosmology, the Copernican principle states that humans, on the Earth or in the Solar System, are not privileged observers of the universe, that observations from the Earth are representative of observations from the average position in the universe. Named for Copernican heliocentrism, it is a working assumption that arises from a modified cosmological extension of Copernicus' argument of a moving Earth.

Origin and implications
Hermann Bondi named the principle after Copernicus in the mid-20th century, although the principle itself dates back to the 16th-17th century paradigm shift away from the Ptolemaic system, which placed Earth at the center of the universe. Copernicus proposed that the motion of the planets could be explained by reference to an assumption that the Sun is centrally located and stationary in contrast to the geocentrism. He argued that the apparent retrograde motion of the planets is an illusion caused by Earth's movement around the Sun, which the Copernican model placed at the centre of the universe. Copernicus himself was mainly motivated by technical dissatisfaction with the earlier system and not by support for any mediocrity principle. Although the Copernican heliocentric model is often described as "demoting" Earth from its central role it had in the Ptolemaic geocentric model, it was successors to Copernicus, notably the 16th century Giordano Bruno, who adopted this new perspective. The Earth's central position had been interpreted as being in the "lowest and filthiest parts". Instead, as Galileo said, the Earth is part of the "dance of the stars" rather than the "sump where the universe's filth and ephemera collect". In the late 20th Century, Carl Sagan asked, "Who are we? We find that we live on an insignificant planet of a humdrum star lost in a galaxy tucked away in some forgotten corner of a universe in which there are far more galaxies than people."

While the Copernican principle is derived from the negation of past assumptions, such as geocentrism, heliocentrism, or galactocentrism which state that humans are at the center of the universe, the Copernican principle is stronger than acentrism, which merely states that humans are not at the center of the universe. The Copernican principle assumes acentrism and also states that human observers or observations from Earth are representative of observations from the average position in the universe. Michael Rowan-Robinson emphasizes the Copernican principle as the threshold test for modern thought, asserting that: "It is evident that in the post-Copernican era of human history, no well-informed and rational person can imagine that the Earth occupies a unique position in the universe."

Most modern cosmology is based on the assumption that the cosmological principle is almost, but not exactly, true on the largest scales. The Copernican principle represents the irreducible philosophical assumption needed to justify this, when combined with the observations. If one assumes the Copernican principle and observes that the universe appears isotropic or the same in all directions from the vantage point of Earth, then one can infer that the universe is generally homogeneous or the same everywhere (at any given time) and is also isotropic about any given point. These two conditions make up the cosmological principle.

In practice, astronomers observe that the universe has heterogeneous or non-uniform structures up to the scale of galactic superclusters, filaments and great voids. In the current Lambda-CDM model, the predominant model of cosmology in the modern era, the universe is predicted to become more and more homogeneous and isotropic when observed on larger and larger scales, with little detectable structure on scales of more than about 260 million parsecs. However, recent evidence from galaxy clusters, quasars, and type Ia supernovae suggests that isotropy is violated on large scales. Furthermore, various large-scale structures have been discovered, such as the Clowes–Campusano LQG, the Sloan Great Wall, U1.11, the Huge-LQG, the Hercules–Corona Borealis Great Wall, and the Giant Arc, all which indicate that homogeneity might be violated.

On scales comparable to the radius of the observable universe, we see systematic changes with distance from Earth. For instance, at greater distances, galaxies contain more young stars and are less clustered, and quasars appear more numerous. If the Copernican principle is assumed, then it follows that this is evidence for the evolution of the universe with time: this distant light has taken most of the age of the universe to reach Earth and shows the universe when it was young. The most distant light of all, cosmic microwave background radiation, is isotropic to at least one part in a thousand.

Bondi and Thomas Gold used the Copernican principle to argue for the perfect cosmological principle which maintains that the universe is also homogeneous in time, and is the basis for the steady-state cosmology. However, this strongly conflicts with the evidence for cosmological evolution mentioned earlier: the universe has progressed from extremely different conditions at the Big Bang, and will continue to progress toward extremely different conditions, particularly under the rising influence of dark energy, apparently toward the Big Freeze or Big Rip.

Since the 1990s the term has been used (interchangeably with "the Copernicus method") for J. Richard Gott's Bayesian-inference-based prediction of duration of ongoing events, a generalized version of the Doomsday argument.

Tests of the principle
The Copernican principle has never been proven, and in the most general sense cannot be proven, but it is implicit in many modern theories of physics. Cosmological models are often derived with reference to the cosmological principle, slightly more general than the Copernican principle, and many tests of these models can be considered tests of the Copernican principle.

Historical
Before the term Copernican principle was even coined, past assumptions, such as geocentrism, heliocentrism, and galactocentrism, which state that Earth, the Solar System, or the Milky Way respectively were located at the center of the universe, were shown to be false. The Copernican Revolution dethroned Earth to just one of many planets orbiting the Sun. Proper motion was mentioned by Halley. William Herschel found that the Solar System is moving through space within our disk-shaped Milky Way galaxy. Edwin Hubble showed that the Milky Way galaxy is just one of many galaxies in the universe. Examination of the galaxy's position and motion in the universe led to the Big Bang theory and the whole of modern cosmology.

Modern tests
Recent and planned tests relevant to the cosmological and Copernican principles include:
 * time drift of cosmological redshifts;
 * modelling the local gravitational potential using reflection of cosmic microwave background (CMB) photons;
 * the redshift dependence of the luminosity of supernovae;
 * the kinetic Sunyaev–Zeldovich effect in relation to dark energy;
 * cosmic neutrino background;
 * the integrated Sachs–Wolfe effect
 * testing the isotropy and homogeneity of the CMB;
 * Some authors claim that the KBC Void violates the cosmological principle and thus the Copernican principle. However, other authors claim that the KBC void is consistent with the cosmological principle and the Copernican principle.

Physics without the principle
The standard model of cosmology, the Lambda-CDM model, assumes the Copernican principle and the more general cosmological principle. Some cosmologists and theoretical physicists have created models without the cosmological or Copernican principles to constrain the values of observational results, to address specific known issues in the Lambda-CDM model, and to propose tests to distinguish between current models and other possible models.

A prominent example in this context is inhomogeneous cosmology, to model the observed accelerating universe and cosmological constant. Instead of using the current accepted idea of dark energy, this model proposes the universe is much more inhomogeneous than currently assumed, and instead, we are in an extremely large low-density void. To match observations we would have to be very close to the centre of this void, immediately contradicting the Copernican principle.

While the Big Bang model in cosmology is sometimes said to derive from the Copernican principle in conjunction with redshift observations, the Big Bang model can still be assumed to be valid in absence of the Copernican principle, because the cosmic microwave background, primordial gas clouds, and the structure, evolution, and distribution of galaxies all provide evidence, independent of the Copernican principle, in favor of the Big Bang. However, the key tenets of the Big Bang model, such as the expansion of the universe, become assumptions themselves akin to the Copernican principle, rather than derived from the Copernican principle and observations.

title: Copernican principle