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| In [[astrophysics]] and [[cosmology]], the '''anthropic principle''' ({{ety|gre|anthropos|human}}) is the [[philosophy|philosophical consideration]] that observations of the physical [[Universe]] must be compatible with the [[conscious]] [[life]] that observes it. Some proponents of the anthropic principle reason that it explains why the Universe has the [[age of universe|age]] and the [[fundamental physical constant]]s necessary to accommodate conscious life. As a result, they believe it is unremarkable that the universe's fundamental constants happen to fall within the [[Fine-tuned Universe|narrow range thought to be compatible with life]].<ref>[http://abyss.uoregon.edu/~js/cosmo/lectures/lec24.html Anthropic Principle ] [http://abyss.uoregon.edu/~js/ James Schombert, Department of Physics at University of Oregon]</ref>
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| The strong anthropic principle (SAP) as explained by [[John D. Barrow|Barrow]] and [[Frank Tipler|Tipler]] (see [[Weak anthropic principle|variants]]) states that this is all the case because the Universe is compelled, in some sense, for conscious life to eventually emerge. Critics of the SAP argue in favor of a weak anthropic principle (WAP) similar to the one defined by [[Brandon Carter]], which states that the [[Fine-tuned Universe|universe's ostensible fine tuning]] is the result of [[selection bias]]: i.e., only in a universe capable of eventually supporting life will there be living beings capable of observing any such fine tuning, while a universe less compatible with life will go unbeheld.
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| ==Definition and basis==
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| The principle was formulated as a response to a [[Fine-tuned Universe|series of observations]] that the laws of nature and parameters of the Universe take on values that are consistent with conditions for life as we know it rather than a set of values that would not be consistent with life on [[Earth]]. The anthropic principle states that this is a [[a priori and a posteriori|necessity]], because if life were impossible, no one would know it. That is, it must be possible to observe ''some'' Universe, and hence, the laws and constants of any such universe must accommodate that possibility.
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| The term ''[[wikt:anthropic|anthropic]]'' in "anthropic principle" has been argued <ref>Mosterín (2005) pp. 12 http://philsci-archive.pitt.edu/1658/"</ref> to be a [[misnomer]].<ref>"anthropic" means "of or pertaining to mankind or humans"</ref> While singling out our kind of carbon-based life, none of the finely tuned phenomena require [[human]] life or some kind of [[carbon chauvinism]].<ref>[http://www.colorado.edu/philosophy/vstenger/Cosmo/ant_encyc.pdf The Anthropic Principle], Victor J. Stenger</ref><ref>[http://books.google.co.uk/books?id=TZ5FLwnCTMAC&client=firefox-a Anthropic Bias], Nick Bostrom, p.6</ref> Any form of life or any form of heavy atom, stone, star or galaxy would do; nothing specifically human or anthropic is involved.
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| The anthropic principle has given rise to some confusion and controversy, partly because the phrase has been applied to several distinct ideas. All versions of the principle have been accused of discouraging the search for a deeper physical understanding of the universe. The anthropic principle is often criticized for lacking [[falsifiability]] and therefore critics of the anthropic principle may point out that the anthropic principle is a non-scientific concept, even though the weak anthropic principle, ''"conditions that are observed in the universe must allow the observer to exist",''<ref>[http://www.merriam-webster.com/dictionary/anthropic%20principle Merriam-Webster Online Dictionary]</ref> is "easy" to support in mathematics and philosophy, i.e. it is a [[tautology (logic)|tautology]] or [[truism]]<!--, albeit a "vacuously true" statement [removed for the time being] -->. However, building a substantive argument based on a tautological foundation is problematic. Stronger variants of the anthropic principle are not tautologies and thus make claims considered controversial by some and that are contingent upon empirical verification.<ref>[http://www.physics.sfsu.edu/~lwilliam/sota/anth/SAP_FAP.htm The Strong Anthropic Principle and the Final Anthropic Principle]</ref><ref>[http://firstpraxis.wordpress.com/2011/05/26/on-knowing-sagan-from-pale-blue-dot/ On Knowing, Sagan from Pale Blue Dot]</ref>
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| ==Anthropic coincidences==
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| {{Main|Fine-tuned Universe}}
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| In 1961, [[Robert Dicke]] noted that the [[age of the universe]], as seen by living observers, cannot be random.<ref>{{cite journal | authorlink = Robert Dicke |author = Dicke, R. H. | journal = Nature | title = Dirac's Cosmology and Mach's Principle | volume = 192 | pages = 440–441| year = 1961 | doi = 10.1038/192440a0|bibcode = 1961Natur.192..440D | issue=4801}}</ref> Instead, biological factors constrain the universe to be more or less in a "golden age," neither too young nor too old.<ref>{{cite book | authorlink = Paul Davies | author = Davies, P. | title = The Goldilocks Enigma | publisher = Allen Lane| year = 2006 | isbn = 0-7139-9883-0}}</ref> If the universe were one tenth as old as its present age, there would not have been sufficient time to build up appreciable levels of
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| [[metallicity]] (levels of elements besides [[hydrogen]] and [[helium]]) especially [[carbon]], by [[nucleosynthesis]]. Small rocky planets did not yet exist. If the universe were 10 times older than it actually is, most stars would be too old to remain on the [[main sequence]] and would have turned into [[white dwarf]]s, aside from the dimmest [[red dwarf]]s, and stable planetary systems would have already come to an end. Thus, Dicke explained the coincidence between large dimensionless numbers constructed from the constants of physics and the age of the universe, a coincidence which had inspired [[Dirac large numbers hypothesis|Dirac's varying-G theory]].
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| Dicke later reasoned that the density of matter in the universe must be almost exactly the [[Friedmann equations#Density parameter|critical density]] needed to prevent the [[Big Crunch]] (the "Dicke coincidences" [[argument]]). The most recent measurements may suggest that the observed density of [[baryon]]ic matter, and some theoretical predictions of the amount of [[dark matter]] account for about 30% of this critical density, with the rest contributed by a [[cosmological constant]]. [[Steven Weinberg]]<ref>{{cite journal | authorlink = Steven Weinberg | author = Weinberg, S. | title = Anthropic bound on the cosmological constant | journal = Physical Review Letters| volume = 59| pages = 2607–2610| year = 1987 | doi = 10.1103/PhysRevLett.59.2607 | pmid = 10035596 | issue = 22 | bibcode=1987PhRvL..59.2607W}}</ref> gave an anthropic explanation for this fact: he noted that the cosmological constant has a remarkably low value, some 120 [[orders of magnitude]] smaller than the value [[particle physics]] predicts (this has been described as the "worst prediction in physics").<ref>[http://www.newscientist.com/blog/space/2007/02/physicists-debate-nature-of-space-time.html New Scientist Space Blog: Physicists debate the nature of space-time - New Scientist<!-- Bot generated title -->]</ref> However, if the cosmological constant were only one order of magnitude larger than its observed value, the universe would suffer catastrophic [[Cosmic inflation|inflation]], which would preclude the formation of stars, and hence life.
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| The observed values of the [[dimensionless physical constant]]s (such as the [[fine-structure constant]]) governing the four [[fundamental interaction]]s are balanced as if [[fine-tuned universe|fine-tuned]] to permit the formation of commonly found matter and subsequently the emergence of life.<ref>[http://math.ucr.edu/home/baez/constants.html How Many Fundamental Constants Are There? ] [http://math.ucr.edu/home/baez/README.html John Baez, mathematical physicist. U. C. Riverside], April 22, 2011</ref> A slight increase in the [[strong nuclear force]] would bind the [[dineutron]] and the [[diproton]], and [[nuclear fusion]] would have converted all hydrogen in the early universe to [[helium]]. Water, as well as sufficiently long-lived stable stars, both essential for the emergence of life as we know it, would not exist. More generally, small changes in the relative strengths of the four fundamental interactions can greatly affect the universe's age, structure, and capacity for life.
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| ==Origin==
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| The phrase "anthropic principle" first appeared in [[Brandon Carter]]'s contribution to a 1973 [[Kraków]] [[symposium]] honouring [[Nicolaus Copernicus|Copernicus's]] 500th birthday. Carter, a theoretical astrophysicist, articulated the Anthropic Principle in reaction to the [[Copernican Principle]], which states that humans do not occupy a privileged position in the [[Universe]]. As Carter said: "Although our situation is not necessarily ''central'', it is inevitably privileged to some extent."<ref>{{cite conference|first=B.| last = Carter | authorlink = Brandon Carter | booktitle =IAU Symposium 63: Confrontation of Cosmological Theories with Observational Data | publisher =Reidel | year = 1974 | location = Dordrecht |title = Large Number Coincidences and the Anthropic Principle in Cosmology | pages = 291–298}}</ref> Specifically, Carter disagreed with using the Copernican principle to justify the [[Perfect Cosmological Principle]], which states that all large regions ''and times'' in the universe must be statistically identical. The latter principle underlay the [[steady-state theory]], which had recently been falsified by the 1965 discovery of the [[cosmic microwave background radiation]]. This discovery was unequivocal evidence that the universe has changed radically over time (for example, via the [[Big Bang]]).
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| Carter defined two forms of the Anthropic Principle, a "weak" one which referred only to anthropic selection of privileged [[spacetime]] locations in the universe, and a more controversial "strong" form which addressed the values of the [[fundamental constant]]s of physics.
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| [[Roger Penrose]] explained the weak form as follows:
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| {{quote|"The argument can be used to explain why the conditions happen to be just right for the existence of (intelligent) life on the earth at the present time. For if they were not just right, then we should not have found ourselves to be here now, but somewhere else, at some other appropriate time. This principle was used very effectively by Brandon Carter and [[Robert Dicke]] to resolve an issue that had puzzled physicists for a good many years. The issue concerned various striking numerical relations that are observed to hold between the physical constants (the [[gravitational constant]], the mass of the [[proton]], the [[age of the universe]], etc.). A puzzling aspect of this was that some of the relations hold only at the present epoch in the earth's history, so we appear, coincidentally, to be living at a very special time (give or take a few million years!). This was later explained, by Carter and Dicke, by the fact that this epoch coincided with the lifetime of what are called [[main-sequence]] stars, such as the sun. At any other epoch, so the argument ran, there would be no intelligent life around in order to measure the physical constants in question — so the coincidence had to hold, simply because there would be [[Sapience|intelligent life]] around only at the particular time that the coincidence did hold!"|''[[The Emperor's New Mind]], Chapter 10''}}
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| One reason this is plausible is that there are many other places and times in which we can imagine finding ourselves. But when applying the strong principle, we only have one Universe, with one set of fundamental parameters, so what exactly is the point being made? Carter offers two possibilities: First, we can use our own existence to make "predictions" about the parameters. But second, "as a last resort", we can convert these predictions into ''explanations'' by assuming that there ''is'' more than one Universe, in fact a large and possibly infinite collection of universes, something that is now called a [[multiverse]] ("world ensemble" was Carter's term), in which the parameters (and perhaps the laws of physics) vary across universes. The strong principle then becomes an example of a [[selection effect]], exactly analogous to the weak principle. Postulating a multiverse is certainly a radical step, but taking it could provide at least a partial answer to a question which had seemed to be out of the reach of normal science: "why do the [[list of laws of science|fundamental laws of physics]] take the particular form we observe and not another?"
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| Since Carter's 1973 paper, the term "Anthropic Principle" has been extended to cover a number of ideas which differ in important ways from those he espoused. Particular confusion was caused in 1986 by the book ''The Anthropic Cosmological Principle'' by [[John D. Barrow]] and [[Frank Tipler]],<ref>{{BarrowTipler1986}}</ref> published that year which distinguished between "weak" and "strong" anthropic principle in a way very different from Carter's, as discussed in the next section.
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| Carter was not the first to invoke some form of the anthropic principle. In fact, the [[evolutionary biologist]] [[Alfred Russel Wallace]] anticipated the anthropic principle as long ago as 1904: "Such a vast and complex universe as that which we know exists around us, may have been absolutely required ... in order to produce a world that should be precisely adapted in every detail for the orderly development of life culminating in man."<ref>{{cite book| authorlink = Alfred R. Wallace | author = Wallace, A. R. | year=1904| title=Man's place in the universe: a study of the results of scientific research in relation to the unity or plurality of worlds, 4th ed| location=London| publisher=George Bell & Sons| pages= 256–7}}</ref> In 1957, [[Robert Dicke]] wrote: "The age of the Universe 'now' is not random but conditioned by biological factors ... [changes in the values of the fundamental constants of physics] would preclude the existence of man to consider the problem."<ref>{{cite journal | last = Dicke | first = R. H. | title = Gravitation without a Principle of Equivalence |journal = Reviews of Modern Physics |volume = 29 | year = 1957 | pages = 363–376 |doi=10.1103/RevModPhys.29.363 | bibcode=1957RvMP...29..363D | issue = 3}}</ref>
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| ==Variants==
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| '''Weak anthropic principle (WAP)''' ([[Brandon Carter|Carter]]): "we must be prepared to take account of the fact that our location in the universe is ''necessarily'' privileged to the extent of being compatible with our existence as observers." Note that for Carter, "location" refers to our location in time as well as space.
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| '''Strong anthropic principle (SAP)''' (Carter): "the Universe (and hence the [[Dimensionless physical constant|fundamental parameters]] on which it depends) must be such as to admit the creation of observers within it at some stage. To paraphrase [[Descartes]], ''cogito ergo mundus talis est''."<br>The Latin tag ("I think, therefore the world is such [as it is]") makes it clear that "must" indicates a [[deductive reasoning|deduction]] from the fact of our existence; the statement is thus a [[truism]].
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| In their 1986 book, ''The Anthropic Cosmological Principle'', [[John D. Barrow|John Barrow]] and [[Frank Tipler]] depart from Carter and define the WAP and SAP as follows:<ref>{{cite journal | author = Barrow, John D. | title = Anthropic Definitions | journal = Quarterly Journal of the Royal Astronomical Society | volume = 24| pages = 146–53 | doi = | year = 1997 | bibcode=1983QJRAS..24..146B }}</ref><ref>Barrow & Tipler's definitions are quoted verbatim at ''[http://www.dhushara.com/book/quantcos/anth/anth.htm Genesis of Eden Diversity Encyclopedia.]''</ref>
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| '''Weak anthropic principle (WAP)''' (Barrow and Tipler): "The observed values of all physical and [[cosmological]] quantities are not equally probable but they take on values restricted by the requirement that there exist sites where [[carbon-based life]] can [[evolution|evolve]] and by the requirements that the Universe be old enough for it to have already done so."<ref>Barrow and Tipler 1986: 16.</ref><br>Unlike Carter they restrict the principle to carbon-based life, rather than just "observers." A more important difference is that they apply the WAP to the [[fundamental physical constant]]s, such as the [[fine structure constant]], the number of [[Spacetime#Privileged character of 3+1 spacetime|spacetime dimensions]], and the [[cosmological constant]] —, topics that fall under Carter's SAP.
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| '''Strong anthropic principle (SAP)''' (Barrow and Tipler): "The Universe must have those properties which allow life to develop within it at some stage in its history."<ref>Barrow and Tipler 1986: 21.</ref><br>This looks very similar to Carter's SAP, but unlike the case with Carter's SAP, the "must" is an imperative, as shown by the following three possible elaborations of the SAP, each proposed by Barrow and Tipler:<ref>Barrow and Tipler 1986: 22.</ref>
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| * "There exists one possible Universe 'designed' with the goal of generating and sustaining 'observers'."
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| ::This can be seen as simply the classic [[design argument]] restated in the garb of contemporary [[cosmology]]. It implies that the purpose of the universe is to give rise to [[Sapience|intelligent life]], with the [[physical law|laws of nature]] and their [[fundamental physical constant]]s set to ensure that life as we know it will emerge and evolve.
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| * "Observers are necessary to bring the Universe into being."
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| ::Barrow and Tipler believe that this is a valid conclusion from [[quantum mechanics]], as [[John Archibald Wheeler]] has suggested, especially via his idea that information is the fundamental reality, see [[It from bit]], and his '''Participatory Anthropic Principle (PAP)''' which is an [[Interpretations of quantum mechanics#von Neumann/Wigner interpretation: consciousness causes the collapse | interpretation of quantum mechanics]] associated with the ideas of [[John von Neumann]] and [[Eugene Wigner]].
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| * "An ensemble of other different universes is necessary for the existence of our Universe."
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| ::By contrast, Carter merely says that an [[multiverse|ensemble of universes]] is necessary for the SAP to count as an explanation.
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| '''Modified anthropic principle (MAP)''' (Schmidhuber): The 'problem' of existence is only relevant to a species capable of formulating the question. Prior to ''Homo sapiens'' intellectual evolution to the point where the nature of the observed universe - and humans' place within same - spawned deep inquiry into its origins, the 'problem' simply did not exist.<ref name="RgenSchmidhuber">[[Jürgen Schmidhuber]], 2000, "[http://arxiv.org/abs/quant-ph/0011122 Algorithmic theories of everything.]"</ref>
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| The [[philosopher]]s [[John Andrew Leslie|John Leslie]]<ref>{{cite conference | author = Leslie, J. | authorlink = John Andrew Leslie | title = Probabilistic Phase Transitions and the Anthropic Principle | booktitle = Origin and Early History of the Universe: LIEGE 26 | pages = 439–444 | publisher = Knudsen | year = 1986 | url = http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1986LIACo..26..439L}}</ref> and [[Nick Bostrom]]<ref>{{cite book | last = Bostrom | first = N. | authorlink = Nick Bostrom | year = 2002 | title = Anthropic Bias: Observation Selection Effects in Science and Philosophy | publisher = Routledge | isbn = 0-415-93858-9}} 5 chapters available [http://www.anthropic-principle.com/book/ online.]</ref> reject the Barrow and Tipler SAP as a fundamental misreading of Carter. For Bostrom, Carter's anthropic principle just warns us to make allowance for '''anthropic bias''', that is, the bias created by anthropic [[selection effect]]s (which Bostrom calls "observation" selection effects) — the necessity for observers to exist in order to get a result. He writes:
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| {{quote|"Many 'anthropic principles' are simply confused. Some, especially those drawing inspiration from Brandon Carter's seminal papers, are sound, but... they are too weak to do any real scientific work. In particular, I argue that existing methodology does not permit any observational consequences to be derived from contemporary cosmological theories, though these theories quite plainly can be and are being tested empirically by astronomers. What is needed to bridge this methodological gap is a more adequate formulation of how observation [[selection effect]]s are to be taken into account."|''Anthropic Bias'', Introduction.|<ref>Bostrom, N. (2002), op. cit.</ref>}}
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| '''[[Self-Sampling Assumption|Strong self-sampling assumption (SSSA)]]''' ([[Nick Bostrom|Bostrom]]): "Each observer-moment should reason as if it were randomly selected from the class of all observer-moments in its [[reference class]]."<br> Analysing an observer's experience into a sequence of "observer-moments" helps avoid certain paradoxes; but the main ambiguity is the selection of the appropriate "reference class": for Carter's WAP this might correspond to all real or potential observer-moments in our universe; for the SAP, to all in the multiverse. Bostrom's mathematical development shows that choosing either too broad or too narrow a reference class leads to counter-intuitive results, but he is not able to prescribe an ideal choice.
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| According to [[Jürgen Schmidhuber]], the anthropic principle essentially just says that the [[conditional probability]] of finding yourself in a universe compatible with your existence is always 1. It does not allow for any additional nontrivial predictions such as "gravity won't change tomorrow." To gain more predictive power, additional assumptions on the [[prior distribution]] of [[multiverse|alternative universes]] are necessary.<ref name="RgenSchmidhuber" /><ref>[[Jürgen Schmidhuber]], 2002, "[http://www.idsia.ch/~juergen/speedprior.html The Speed Prior: A New Simplicity Measure Yielding Near-Optimal Computable Predictions.]" ''Proc. 15th Annual Conference on Computational Learning Theory'' (COLT 2002), Sydney, Australia, Lecture Notes in Artificial Intelligence. Springer: 216-28.</ref>
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| Playwright and novelist [[Michael Frayn]] describes a form of the Strong Anthropic Principle in his 2006 book ''The Human Touch'', which explores what he characterises as "the central oddity of the Universe":
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| {{quote|"It's this simple paradox. The Universe is very old and very large. Humankind, by comparison, is only a tiny disturbance in one small corner of it - and a very recent one. Yet the universe is only very large and very old because we are here to say it is... And yet, of course, we all know perfectly well that it is what it is whether we are here or not."|<ref>[[Michael Frayn]], ''The Human Touch''. Faber & Faber ISBN 0-571-23217-5</ref>}}
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| == Character of anthropic reasoning ==
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| Carter chose to focus on a tautological aspect of his ideas, which has resulted in much confusion. In fact, anthropic reasoning interests scientists because of something that is only implicit in the above formal definitions, namely that we should give serious consideration to there being other universes with different values of the "fundamental parameters" — that is, the dimensionless [[dimensionless physical constant|physical constants]] and initial conditions for the [[Big Bang]]. Carter and others have argued that life as we know it would not be possible in most such universes. In other words, the universe we are in is [[fine-tuned universe|fine tuned]] to permit life. Collins & Hawking (1973) characterized Carter's then-unpublished big idea as the postulate that "there is not one universe but a whole infinite ensemble of universes with all possible initial conditions".<ref>{{cite journal | author = Collins C. B., [[Stephen Hawking|Hawking, S. W.]] | title = Why is the universe isotropic? | journal = Astrophysical Journal | volume = 180 | pages = 317–334 | year = 1973 | doi = 10.1086/151965 | bibcode=1973ApJ...180..317C}}</ref> If this is granted, the anthropic principle provides a plausible explanation for the fine tuning of our universe: the "typical" universe is not fine-tuned, but given enough universes, a small fraction thereof will be capable of supporting intelligent life. Ours must be one of these, and so the observed fine tuning should be no cause for wonder.
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| Although philosophers have discussed related concepts for centuries, in the early 1970s the only genuine physical theory yielding a multiverse of sorts was the [[many-worlds interpretation]] of [[quantum mechanics]]. This would allow variation in initial conditions, but not in the truly fundamental constants. Since that time a number of mechanisms for producing a multiverse have been suggested: see the review by [[Max Tegmark]].<ref>{{cite journal | author = Tegmark, M. | authorlink = Max Tegmark | title = Is 'the theory of everything' merely the ultimate ensemble theory?| journal = Annals of Physics | volume = 270 | pages = 1–51| year = 1998 | doi = 10.1006/aphy.1998.5855|arxiv = gr-qc/9704009 |bibcode = 1998AnPhy.270....1T }}</ref> An important development in the 1980s was the combination of [[inflation theory]] with the hypothesis that some parameters are determined by [[symmetry breaking]] in the early universe, which allows parameters previously thought of as "fundamental constants" to vary over very large distances, thus eroding the distinction between Carter's weak and strong principles. At the beginning of the 21st century, the [[string landscape]] emerged as a mechanism for varying essentially all the constants, including the number of spatial dimensions.<ref>Strictly speaking, the number of non-compact dimensions, see [[String theory]].</ref>
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| The anthropic idea that fundamental parameters are selected from a multitude of different possibilities (each actual in some universe or other) contrasts with the traditional hope of physicists for a [[theory of everything]] having no free parameters: as [[Einstein]] said, "What really interests me is whether God had any choice in the creation of the world." In 2002, proponents of the leading candidate for a "theory of everything", [[string theory]], proclaimed "the end of the anthropic principle"<ref>{{cite journal | author = Kane, Gordon L., Perry, Malcolm J., and Zytkow, Anna N. | title=The Beginning of the End of the Anthropic Principle | journal = New Astronomy | volume = 7 | year = 2002 | pages = 45–53 | doi=10.1016/S1384-1076(01)00088-4 | arxiv=astro-ph/0001197|bibcode = 2002NewA....7...45K }}</ref> since there would be no free parameters to select. Ironically, string theory now seems to offer no hope of predicting fundamental parameters, and now some who advocate it invoke the anthropic principle as well (see [[#String theory|below]]).
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| The modern form of a [[design argument]] is put forth by [[Intelligent design]]. Proponents of intelligent design often cite the [[Fine-tuned Universe|fine-tuning]] observations that (in part) preceded the formulation of the anthropic principle by Carter as a proof of an intelligent designer. Opponents of intelligent design are not limited to those who hypothesize that other universes exist; they may also argue, anti-anthropically, that the universe is less fine-tuned than often claimed, or that accepting fine tuning as a brute fact is less astonishing than the idea of an intelligent creator. Furthermore, even accepting fine tuning, [[Elliott Sober|Sober]] (2005)<ref>Sober, Elliott, 2005, "[http://philosophy.wisc.edu/sober/black-da.pdf The Design Argument]{{dead link|date=March 2013}}" in Mann, W. E., ed., ''The Blackwell Guide to the Philosophy of Religion''. Blackwell Publishers.</ref> and Ikeda and [[William H. Jefferys|Jefferys]],<ref>Ikeda, M. and Jefferys, W., "The Anthropic Principle Does Not Support Supernaturalism," in ''The Improbability of God,'' Michael Martin and Ricki Monnier, Editors, pp. 150-166. Amherst, N.Y.: Prometheus Press. ISBN 1-59102-381-5</ref><ref>Ikeda, M. and Jefferys, W. (2006). Unpublished FAQ "[http://quasar.as.utexas.edu/anthropic.html The Anthropic Principle Does Not Support Supernaturalism.]"</ref> argue that the Anthropic Principle as conventionally stated actually undermines intelligent design; see [[fine-tuned universe]].
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| [[Paul Davies]]'s book ''[[The Goldilocks Enigma]]'' (2006) reviews the current state of the fine tuning debate in detail, and concludes by enumerating the following responses to that debate:
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| #The absurd universe: Our universe just happens to be the way it is.
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| #The unique universe: There is a deep underlying unity in physics which necessitates the Universe being the way it is. Some [[Theory of Everything]] will explain why the various features of the Universe must have exactly the values that we see.
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| #The multiverse: Multiple universes exist, having all possible combinations of characteristics, and we inevitably find ourselves within a universe that allows us to exist.
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| #Intelligent Design: A creator designed the Universe with the purpose of supporting complexity and the emergence of intelligence.
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| #The life principle: There is an underlying principle that constrains the Universe to evolve towards life and mind.
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| #The self-explaining universe: A closed explanatory or causal loop: "perhaps only universes with a capacity for consciousness can exist." This is [[John Archibald Wheeler|Wheeler's]] Participatory Anthropic Principle (PAP).
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| #The fake universe :We live inside a [[virtual reality simulation]].
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| Omitted here is [[Lee Smolin]]'s model of [[Fecund universes theory|cosmological natural selection]], also known as "fecund universes," which proposes that universes have "offspring" which are more plentiful if they resemble our universe. Also see Gardner (2005).<ref>Gardner, James N., 2005, "[http://www.kurzweilai.net/the-physical-constants-as-biosignature-an-anthropic-retrodiction-of-the-selfish-biocosm-hypothesis The Physical Constants as Biosignature: An anthropic retrodiction of the Selfish Biocosm Hypothesis,]" ''[[International Journal of Astrobiology]]''.</ref>
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| Clearly each of these hypotheses resolve some aspects of the puzzle, while leaving others unanswered. Followers of Carter would admit only option 3 as an anthropic explanation, whereas 3 through 6 are covered by different versions of Barrow and Tipler's SAP (which would also include 7 if it is considered a variant of 4, as in Tipler 1994).
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| The anthropic principle, at least as Carter conceived it, can be applied on scales much smaller than the whole universe. For example, Carter (1983)<ref>{{cite journal | author = Carter, B. | title = The anthropic principle and its implications for biological evolution | journal = Philosophical Transactions of the Royal Society | volume = A310 | pages = 347–363 | year = 1983 | doi = 10.1098/rsta.1983.0096 | last2 = McCrea | first2 = W. H. | issue = 1512 |bibcode = 1983RSPTA.310..347C }}</ref> inverted the usual line of reasoning and pointed out that when interpreting the evolutionary record, one must take into account [[cosmological]] and [[astrophysical]] considerations. With this in mind, Carter concluded that given the best estimates of the [[age of the universe]], the evolutionary chain culminating in ''[[Homo sapiens]]'' probably admits only one or two low probability links. [[Antonio Feoli]] and [[Salvatore Rampone]] dispute this conclusion, arguing instead that the estimated size of our universe and the number of planets in it allows for a higher bound, so that there is no need to invoke intelligent design to explain evolution.
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| <ref>{{cite journal | author = Feoli, A. and Rampone, S. | title = Is the Strong Anthropic Principle too weak? |journal = Nuovo Cim. | volume = B114 | year = 1999 | pages = 281–289 | doi =| arxiv=gr-qc/9812093 | bibcode = 1999NCimB.114..281F | last2 = Rampone }}</ref>
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| ==Observational evidence==
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| No possible observational evidence bears on Carter's WAP, as it is merely advice to the scientist and asserts nothing debatable. The obvious test of Barrow's SAP, which says that the Universe is "required" to support life, is to find evidence of life in universes other than ours. Any other universe is, by most definitions, unobservable (otherwise it would be included in ''our'' portion of ''this'' universe). Thus, in principle Barrow's SAP cannot be falsified by observing a universe in which an observer cannot exist.
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| Philosopher [[John Andrew Leslie|John Leslie]]<ref>Leslie, J. (1986) op. cit.</ref> states that the Carter SAP (with [[multiverse]]) predicts the following:
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| * Physical theory will evolve so as to strengthen the hypothesis that early [[phase transition]]s occur probabilistically rather than deterministically, in which case there will be no deep physical reason for the values of fundamental constants;
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| * Various theories for generating [[multiverse|multiple universes]] will prove robust;
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| * Evidence that the universe is [[fine tuned universe|fine tuned]] will continue to accumulate;
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| * No life with a [[alternative biochemistry|non-carbon chemistry]] will be discovered;
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| * Mathematical studies of [[galaxy formation]] will confirm that it is sensitive to the rate of [[expansion of the universe]].
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| Hogan<ref>{{cite journal | author = Hogan, Craig | title = Why is the universe just so? | journal = Reviews of Modern Physics | volume = 72 | year = 2000 | pages = 1149–1161 | doi = 10.1103/RevModPhys.72.1149 | bibcode=2000RvMP...72.1149H|arxiv = astro-ph/9909295 | issue = 4 }}</ref> has emphasised that it would be very strange if all fundamental constants were strictly determined, since this would leave us with no ready explanation for apparent fine tuning. In fact we might have to resort to something akin to Barrow and Tipler's SAP: there would be no option for such a universe ''not'' to support life.
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| Probabilistic predictions of parameter values can be made given:
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| #a particular multiverse with a "[[measure theory|measure]]", i.e. a well defined "density of universes" (so, for parameter ''X'', one can calculate the [[prior probability]] ''P''(''X''<sub>0</sub>) ''dX'' that ''X'' is in the range ''X''<sub>0</sub> < ''X'' < ''X''<sub>0</sub> + ''dX''), and
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| #an estimate of the number of observers in each universe, ''N''(''X'') (e.g., this might be taken as proportional to the number of stars in the universe).
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| The probability of observing value ''X'' is then proportional to ''N''(''X'') ''P''(''X'').
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| (A more sophisticated analysis is that of [[Nick Bostrom]].)<ref>Bostrom (2002), op. cit.</ref> A generic feature of an analysis of this nature is that the expected values of the [[fundamental physical constant]]s should not be "over-tuned," i.e. if there is some perfectly tuned predicted value (e.g. zero), the observed value need be no closer to that predicted value than what is required to make life possible. The small but finite value of the [[cosmological constant]] can be regarded as a successful prediction in this sense.
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| One thing that would ''not'' count as evidence for the Anthropic Principle is evidence that the Earth or the [[solar system]] occupied a privileged position in the universe, in violation of the [[Copernican principle]] (for possible counterevidence to this principle, see [[Copernican principle]]), unless there was some reason to think that that position was a [[necessary condition]] for our existence as observers.
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| ==Applications of the principle==
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| {{Synthesis|date=December 2010}}
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| ===The nucleosynthesis of carbon-12===
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| [[Fred Hoyle]] may have invoked anthropic reasoning to predict an astrophysical phenomenon. He is said to have reasoned from the prevalence on earth of life forms whose chemistry was based on [[carbon-12]] [[atom]]s, that there must be an undiscovered [[resonance]] in the carbon-12 nucleus facilitating its synthesis in stellar interiors via the [[triple-alpha process]]. He then calculated the energy of this undiscovered resonance to be 7.6 million [[electron-volt]]s.<ref>
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| University of Birmingham [http://www.np.ph.bham.ac.uk/research/anthropic.htm Life, Bent Chains and the Anthropic Principle]{{dead link|date=March 2013}}</ref><ref>''Rev. Mod. Phys.'' 29 (1957) 547</ref> [[William Alfred Fowler|Willie Fowler]]'s research group soon found this resonance, and its measured energy was close to Hoyle's prediction.
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| However, a recently released paper argues that Hoyle did not use anthropic reasoning to make this prediction.<ref>Kragh, Helge (2010) When is a prediction anthropic? Fred Hoyle and the 7.65 MeV carbon resonance. http://philsci-archive.pitt.edu/5332/</ref>
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| === Cosmic inflation ===
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| {{Main|Cosmic inflation}}
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| [[Don Page (physicist)|Don Page]] criticized the entire theory of [[cosmic inflation]] as follows.<ref>{{cite journal | first = D.N. | last = Page | year= 1983 | title = Inflation does not explain time asymmetry | journal = Nature | volume = 304 | page = 39 | doi = 10.1038/304039a0|bibcode = 1983Natur.304...39P | issue=5921}}</ref> He emphasized that initial conditions which made possible a thermodynamic [[arrow of time]] in a universe with a [[Big Bang]] origin, must include the assumption that at the initial singularity, the [[entropy]] of the universe was low and therefore extremely improbable. [[Paul Davies]] rebutted this criticism by invoking an inflationary version of the anthropic principle.<ref>{{cite journal | first = P.C.W. | last = Davies |authorlink = Paul Davies | year = 1984 | title = Inflation to the universe and time asymmetry | journal = Nature | volume = 312 | page = 524 | doi = 10.1038/312524a0|bibcode = 1984Natur.312..524D | issue=5994}}</ref> While Davies accepted the premise that the initial state of the visible Universe (which filled a microscopic amount of space before inflating) had to possess a very low entropy value — due to random quantum fluctuations — to account for the observed thermodynamic arrow of time, he deemed this fact an advantage for the theory. That the tiny patch of space from which our observable Universe grew had to be extremely orderly, to allow the post-inflation universe to have an arrow of time, makes it unnecessary to adopt any "ad hoc" hypotheses about the initial entropy state, hypotheses other Big Bang theories require.
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| === String theory ===
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| {{Main|String theory landscape}}
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| [[String theory]] predicts a large number of possible universes, called the "backgrounds" or "vacua." The set of these vacua is often called the "[[multiverse]]" or "[[String theory landscape|anthropic landscape]]" or "string landscape." [[Leonard Susskind]] has argued that the existence of a large number of vacua puts anthropic reasoning on firm ground: only universes whose properties are such as to allow observers to exist are observed, while a possibly much larger set of universes lacking such properties go unnoticed.
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| [[Steven Weinberg]]<ref>{{cite conference | author = Weinberg, S. | authorlink = Steven Weinberg | year = 2007 | title = Living in the multiverse | booktitle = Universe or multiverse? | editor = B. Carr (ed) | publisher = Cambridge University Press | isbn = 0-521-84841-5}} [http://arxiv.org/abs/hep-th/0511037 preprint]</ref> believes the Anthropic Principle may be appropriated by [[cosmologist]]s committed to [[nontheism]], and refers to that Principle as a "turning point" in modern science because applying it to the string landscape "...may explain how the [[fundamental physical constant|constants of nature]] that we observe can take values suitable for life without being fine-tuned by a benevolent creator." Others, most notably [[David Gross]] but also [[Lubos Motl]], [[Peter Woit]], and [[Lee Smolin]], argue that this is not predictive. [[Max Tegmark]],<ref>Tegmark (1998) op. cit.</ref> [[Mario Livio]], and [[Martin Rees]]<ref>{{cite journal | author = Livio, M. and Rees, M. J. | title = Anthropic reasoning | journal = Science| volume = 309 | pages = 1022–3 | year = 2003 | doi = 10.1126/science.1111446 | pmid = 16099967 | issue = 5737|bibcode = 2005Sci...309.1022L }}</ref> argue that only some aspects of a physical theory need be observable and/or testable for the theory to be accepted, and that many well-accepted theories are far from completely testable at present.
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| [[Jürgen Schmidhuber]] (2000–2002) points out that [[Ray Solomonoff]]'s [[Solomonoff's theory of inductive inference|theory of universal inductive inference]] and its extensions already provide a framework for maximizing our confidence in any theory, given a limited sequence of physical observations, and some [[prior distribution]] on the set of possible explanations of the universe.
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| ===Ice density===
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| When water freezes into ice, the ice floats because ice is less dense than liquid water. This is one possible example of the anthropic principle, because if ice did not float, it might have been difficult or impossible for living organisms to have existed in water; without the insulating properties of a top ice layer, lakes and ponds would tend to freeze solid and thaw very little during warmer periods. This principle has been criticized as neglecting the existence of the tropical zone and other warmer climates.
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| Ice is unusual in that it is approximately 9% less dense than liquid water. Water is the only known [[nonmetal|non-metallic]] substance to expand when it freezes. The [[density]] of ice is 0.9167 g/cm<sup>3</sup> at 0°C, whereas water has a density of 0.9998 g/cm<sup>3</sup> at the same temperature. Liquid water is densest, essentially 1.00 g/cm<sup>3</sup>, at 4°C and becomes less dense as the water molecules begin to form the [[Hexagonal crystal system|hexagonal]] [[crystal]]s<ref>The word ''crystal'' derives from Greek word for [[frost]].</ref> of [[ice crystals|ice]] as the freezing point is reached. This is due to [[hydrogen bond]]ing dominating the intermolecular forces, which results in a packing of [[molecules]] less compact in the solid.
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| ===Spacetime===
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| [[File:Spacetime dimensionality.svg|thumb|300px|Properties of ''n''+''m''-dimensional spacetimes]]
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| {{Main|Spacetime}}
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| In 1920, [[Paul Ehrenfest]] showed that if there is only one time dimension and greater than three spatial dimensions, the [[orbit]] of a [[planet]] about its sun cannot remain stable. The same is true of a star's orbit around the center of its [[galaxy]].<ref>{{Cite journal| last = Ehrenfest| first = Paul| authorlink = Paul Ehrenfest| title = How do the fundamental laws of physics make manifest that Space has 3 dimensions?|journal = Annalen der Physik| volume = 61| pages = 440| date = | year = 1920| doi = 10.1002/andp.19203660503|issue = 5 |bibcode = 1920AnP...366..440E}}. Also see Ehrenfest, P. (1917) "In what way does it become manifest in the fundamental laws of physics that space has three dimensions?" ''Proceedings of the Amsterdam Academy''20: 200.</ref> Ehrenfest also showed that if there are an even number of spatial dimensions, then the different parts of a [[wave]] impulse will travel at different speeds. If there are <math>5 + 2k</math> spatial dimensions, where ''k'' is a whole number, then wave impulses become distorted. In 1922, [[Hermann Weyl]] showed that [[James Clerk Maxwell|Maxwell]]'s theory of [[electromagnetism]] works only when with three dimensions of space and one of time.<ref>Weyl, H. (1922) ''Space, time, and matter''. Dover reprint: 284.</ref> Finally, Tangherlini showed in 1963 that when there are more than three spatial dimensions, electron [[atomic orbital|orbitals]] around nuclei cannot be stable; electrons would either fall into the [[atomic nucleus|nucleus]] or disperse.<ref>{{cite journal| last = Tangherlini| first = F. R.| authorlink = | title = Atoms in Higher Dimensions| journal = Nuovo Cimento| volume = 14 | issue = 27| pages = 636| publisher =| year= 1963| url = | doi = | id =| accessdate = }}</ref>
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| [[Max Tegmark]] expands on the preceding argument in the following [[anthropic principle|anthropic]] manner.<ref name="tegmark-dim">{{cite journal| last = Tegmark| first = Max| authorlink = Max Tegmark| title = On the dimensionality of spacetime| journal = Classical and Quantum Gravity| volume = 14 | issue = 4| pages = L69–L75|publisher =| date= April 1997| url = http://space.mit.edu/home/tegmark/dimensions.pdf| doi = 10.1088/0264-9381/14/4/002| id =| accessdate = 2006-12-16 |arxiv = gr-qc/9702052 |bibcode = 1997CQGra..14L..69T}}</ref> If''T'' differs from 1, the behavior of physical systems could not be predicted reliably from knowledge of the relevant [[partial differential equation]]s. In such a universe, intelligent life capable of manipulating technology could not emerge. Moreover, if ''T'' > 1, Tegmark maintains that [[proton]]s and [[electron]]s would be unstable and could decay into particles having greater mass than themselves (This is not a problem if the particles have a sufficiently low temperature).
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| ==''The Anthropic Cosmological Principle''==
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| A thorough extant study of the anthropic principle is the book ''The Anthropic Cosmological Principle'' by [[John D. Barrow]], a [[cosmologist]], and [[Frank J. Tipler]], a theosophist and mathematical physicist. This book sets out in detail the many known anthropic coincidences and constraints, including many found by its authors. While the book is primarily a work of theoretical [[astrophysics]], it also touches on [[quantum physics]], [[chemistry]], and [[earth science]]. An entire chapter argues that ''[[Homo sapiens]]'' is, with high probability, the only [[extraterrestrial intelligence|intelligent species]] in the [[Milky Way]].
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| The book begins with an extensive review of many topics in the [[history of ideas]] the authors deem relevant to the anthropic principle, because the authors believe that principle has important antecedents in the notions of [[teleology]] and [[intelligent design]]. They discuss the writings of [[Fichte]], [[Hegel]], [[Bergson]], and [[Alfred North Whitehead]], and the [[Omega Point]] cosmology of [[Teilhard de Chardin]]. Barrow and Tipler carefully distinguish [[teleology|teleological]] reasoning from ''eutaxiological'' reasoning; the former asserts that order must have a consequent purpose; the latter asserts more modestly that order must have a planned cause. They attribute this important but nearly always overlooked distinction to an obscure 1883 book by L. E. Hicks.<ref>{{cite book| last=Hicks| first= L. E.| year=1883| title=A Critique of Design Arguments| location=New York| publisher=Scribner's}}</ref>
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| Seeing little sense in a principle requiring intelligent life to emerge while remaining indifferent to the possibility of its eventual extinction, Barrow and Tipler propose the:
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| {{quote|"[[Final anthropic principle]] (FAP): Intelligent information-processing must come into existence in the Universe, and, once it comes into existence, it will never die out."|<ref>Barrow and Tipler 1986: 23</ref>}}
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| Barrow and Tipler submit that the FAP is both a valid physical statement and "closely connected with moral values." FAP places strong constraints on the structure of the [[universe]], constraints developed further in Tipler's ''The Physics of Immortality''.<ref>{{cite book | first = F. J. | last = Tipler |authorlink = Frank Tipler | title = The Physics of Immortality | publisher = DoubleDay | year = 1994 |isbn = 0-385-46798-2}}</ref> One such constraint is that the universe must end in a [[big crunch]], which seems unlikely in view of the tentative conclusions drawn since 1998 about [[dark energy]], based on observations of very distant [[supernova]]s.
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| In his review<ref>[[Martin Gardner|Gardner, M.]], "WAP, SAP, PAP, and FAP," ''The New York Review of Books 23'', No. 8 (May 8, 1986): 22-25.</ref> of Barrow and Tipler, [[Martin Gardner]] ridiculed the FAP by quoting the last two sentences of their book as defining a Completely Ridiculous Anthropic Principle (CRAP):
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| {{quote|"At the instant the [[Omega Point]] is reached, life will have gained control of ''all'' matter and forces not only in a single universe, but in all universes whose existence is logically possible; life will have spread into ''all'' spatial regions in all universes which could logically exist, and will have stored an infinite amount of information, including ''all'' bits of knowledge which it is logically possible to know. And this is the end."|<ref>Barrow and Tipler 1986: 677</ref>}}
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| ==Criticisms==
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| Carter has frequently regretted his own choice of the word "anthropic," because it conveys the misleading impression that the principle involves humans specifically, rather than intelligent observers in general.<ref>e.g. Carter (2004) op. cit.</ref> Others<ref>e.g. message from [[Martin Rees]] presented at the Kavli-CERCA conference (see video in External links)</ref> have criticised the word "principle" as being too grandiose to describe straightforward applications of [[selection effects]].
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| A common criticism of Carter's SAP is that it is an easy [[deus ex machina]] which discourages searches for physical explanations. To quote Penrose again: "it tends to be invoked by theorists whenever they do not have a good enough theory to explain the observed facts."<ref>{{cite book | author = Penrose, R. |authorlink = Roger Penrose | title = The Emperor's New Mind | publisher = Oxford University Press | isbn = 0-19-851973-7 | year =1989}} Chapter 10.</ref>
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| Carter's SAP and Barrow and Tipler's WAP have been dismissed as [[truism]]s or trivial [[tautology (logic)|tautologies]], that is, statements true solely by virtue of their [[logical form]] (the conclusion is identical to the premise) and not because a substantive claim is made and supported by observation of reality. As such, they are criticized as an elaborate way of saying "if things were different, they would be different," which is a valid statement, but does not make a claim of some factual alternative over another.
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| Critics of the Barrow and Tipler SAP claim that it is neither testable nor falsifiable, and thus is not a [[scientific method|scientific statement]] but rather a philosophical one. The same criticism has been leveled against the hypothesis of a [[multiverse]], although some argue that it does make falsifiable predictions. A modified version of this criticism is that we understand so little about the emergence of life, especially intelligent life, that it is effectively impossible to calculate the number of observers in each universe. Also, the prior distribution of universes as a function of the fundamental constants is easily modified to get any desired result.<ref>{{cite journal | author = Starkman, G. D., Trotta, R. | title = Why Anthropic Reasoning Cannot Predict Λ | journal = Physical Review Letters | volume = 97 |page = 201301 | year = 2006 | doi = 10.1103/PhysRevLett.97.201301 | pmid = 17155671 | issue = 20 | bibcode=2006PhRvL..97t1301S|arxiv = astro-ph/0607227 }} See also this [http://www.physorg.com/news83924839.html news story.]</ref>
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| Many criticisms focus on versions of the Strong Anthropic Principle, such as Barrett and Tipler's ''anthropic cosmological principle'', which are [[Teleology|teleological]] notions that tend to describe the existence of life as a ''necessary prerequisite'' for the observable constants of physics. In a lecture titled "The Confusion of Cause and Effect in Bad Science," the paleophysicist Caroline Miller said:<ref>{{cite conference| last=Miller| first=Caroline| year =2006| title=The Confusion of Cause and Effect in Bad Science| booktitle=Lecture at Piffard University}}</ref> WARNING: this quotation is attributed to a talk given at Piffard College, but Piffard College seems not to exist.
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| {{quote|"The Anthropic Principle is based on the underlying belief that the universe was created for our benefit. Unfortunately for its adherents, all of the reality-based evidence at our disposal contradicts this belief. In a nonanthropocentric universe, there is no need for multiple universes or supernatural entities to explain life as we know it."}}
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| Similarly, [[Stephen Jay Gould]],<ref>{{cite conference| last=Gould| first=Stephen Jay| year =1998| title=Clear Thinking in the Sciences |booktitle=Lectures at Harvard University}}</ref><ref>{{cite book| last=Gould| first=Stephen Jay | year= 2002 | title=Why People Believe Weird Things: Pseudoscience, Superstition, and Other Confusions of Our Time| isbn=0-7167-3090-1 }}</ref> [[Michael Shermer]],<ref>{{cite book| last=Shermer| first=Michael | year= 2007 | title=Why Darwin Matters| isbn=0-8050-8121-6}}</ref> and others claim that the stronger versions of the Anthropic Principle seem to reverse known causes and effects. Gould compared the claim that the universe is fine-tuned for the benefit of our kind of life to saying that sausages were made long and narrow so that they could fit into modern hotdog buns, or saying that ships had been invented to house [[barnacle]]s. These critics cite the vast physical, fossil, genetic, and other biological evidence consistent with life having been [[fine-tuned universe|fine-tuned]] through [[natural selection]] to adapt to the physical and geophysical environment in which life exists. Life appears to have adapted to physics, and not vice versa.
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| Some applications of the anthropic principle have been criticized as an [[argument by lack of imagination]], for tacitly assuming that carbon compounds and water are the only possible chemistry of life (sometimes called "[[carbon chauvinism]]", see also [[alternative biochemistry]]).<ref>e.g. {{cite journal | author = [[Bernard Carr|Carr, B. J.]], [[Martin Rees|Rees, M. J.]] | title = The anthropic principle and the structure of the physical world | journal = Nature | volume = 278 | pages = 605–612| year = 1979 | doi = 10.1038/278605a0 | bibcode=1979Natur.278..605C | issue=5705}}</ref> The range of [[fundamental physical constant]]s consistent with the evolution of carbon-based life may also be wider than those who advocate a [[fine tuned universe]] have argued.<ref>{{cite book| last=Stenger| first=Victor J| authorlink = Victor Stenger | year= 2000| title=Timeless Reality: Symmetry, Simplicity, and Multiple Universes| publisher=Prometheus Books| isbn = 1-57392-859-3}}</ref> For instance, Harnik et al.<ref>{{cite journal | author = Harnik, R., Kribs, G., Perez, G. | year = 2006 | title =A Universe without Weak interactions | journal = Physical Review | volume = D74 | page = 035006 | doi =10.1103/PhysRevD.74.035006 |arxiv= hep-ph/0604027 | bibcode=2006PhRvD..74c5006H | issue = 3}}</ref> propose a [[weakless universe]] in which the [[weak nuclear force]] is eliminated. They show that this has no significant effect on the other [[fundamental interaction]]s, provided some adjustments are made in how those interactions work. However, if some of the fine-tuned details of our universe were violated, that would rule out complex structures of any kind — [[star]]s, [[planet]]s, [[galaxies]], etc.
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| [[Lee Smolin]] has offered a theory designed to improve on the lack of imagination that anthropic principles have been accused of. He puts forth his [[Fecund universes#Fecund universes|fecund universes]] theory, which assumes universes have "offspring" through the creation of [[black hole]]s, and that these offspring universes have values of physical constants that depend on these of the mother universe.<ref>{{cite book| author=[[Lee Smolin]] | editor=[[Neil deGrasse Tyson|Tyson, Neil deGrasse]] and [[Steven Soter|Soter, Steve]]| year=2001| title=Cosmic Horizons: Astronomy at the Cutting Edge | publisher= The New Press| isbn = 978-1-56584-602-9 | pages =148–152}}</ref>{{Self-published inline|date=May 2009}}{{Verify credibility|date=May 2009}} Some versions of the anthropic principle are only interesting if the range of physical constants that allow certain kinds of life are unlikely in a landscape of possible universes. But Lee Smolin assumes that conditions for carbon based life are similar to conditions for black hole creation, which would change the [[a priori distribution]] of universes such that universes containing life would be likely. In <ref>[http://www.edge.org/3rd_culture/smolin_susskind04/smolin_susskind.html Smolin vs. Susskind: The Anthropic Principle]</ref>{{Self-published inline|date=May 2009}}{{Verify credibility|date=May 2009}} the string theorist [[Leonard Susskind]] disagrees about some assumptions in [[Lee Smolin]]'s theory, while Smolin defends his theory.
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| The philosophers of cosmology [[John Earman]],<ref>{{cite journal | author = Earman John | year = 1987 | title = The SAP also rises: A critical examination of the anthropic principle | url = | journal = American Philosophical Quarterly | volume = 24 | issue = | pages = 307–317 }}</ref> [[Ernan McMullin]],<ref>McMullin, Ernan. (1994). "Fine-tuning the universe?" In M. Shale & G. Shields (ed.), ''Science, Technology, and Religious Ideas'', Lanham: University Press of America.</ref> and [[Jesús Mosterín]] contend that "in its weak version, the anthropic principle is a mere tautology, which does not allow us to explain anything or to predict anything that we did not already know. In its strong version, it is a gratuitous speculation".<ref>Mosterín, Jesús. (2005). Op. cit.</ref> A further criticism by Mosterín concerns the flawed "anthropic" inference from the assumption of an infinity of worlds to the existence of one like ours:
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| {{quote|“The suggestion that an infinity of objects characterized by certain numbers or properties implies the existence among them of objects with any combination of those numbers or characteristics [...] is mistaken. An infinity does not imply at all that any arrangement is present or repeated. [...] The assumption that all possible worlds are realized in an infinite universe is equivalent to the assertion that any infinite set of numbers contains all numbers (or at least all Gödel numbers of the [defining] sequences), which is obviously false.”}}
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| == See also ==
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| {{organize section|date=June 2013}}
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| * [[Big Bounce]]
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| * [[Biocentrism (theory of everything)]]
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| * [[Doomsday argument]]
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| * [[Final anthropic principle]]
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| * [[Fine-tuned Universe]]
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| * [[The Great Filter]]
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| * [[Infinite monkey theorem]]
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| * [[Inverse gambler's fallacy]]
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| * [[Mediocrity principle]]
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| * [[Metaphysical naturalism]]
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| * [[Neocatastrophism]]
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| * [[Nick Bostrom]]
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| * [[Puddle thinking]]
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| * [[Alejandro Jenkins#Quark mass and congeniality to life|Quark mass and congeniality to life]]
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| * [[Rare Earth hypothesis]]
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| * [[Selection bias]]
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| * [[Triple-alpha process#Discovery|Triple-alpha process]]
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| * [[Teleology]]
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| == Footnotes ==
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| {{reflist|colwidth=30em}}
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| == References ==
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| {{refbegin|2}}
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| * {{BarrowTipler1986}}
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| * {{cite journal | author = Cirkovic, M. M. | year = 2002 | title = On the First Anthropic Argument in Astrobiology | journal = Earth, Moon, and Planets |volume = 91 | pages = 243–254 | doi = 10.1023/A:1026266630823 | issue = 4}}
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| * {{cite journal | author = Cirkovic, M. M. | year = 2004 | title = The Anthropic Principle and the Duration of the Cosmological Past | journal = Astronomical and Astrophysical Transactions |volume = 23 | pages = 567–597 | doi = 10.1080/10556790412331335327|arxiv = astro-ph/0505005 |bibcode = 2004A&AT...23..567C | issue = 6 }}
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| * {{cite book | first = Simon | last = Conway Morris | authorlink = Simon Conway Morris | year = 2003 | title = Life's Solution: Inevitable Humans in a Lonely Universe | publisher=Cambridge University Press | isbn = }}
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| * {{cite journal | author = Craig, William Lane | authorlink = William Lane Craig | year = 1987 | title = Critical review of ''The Anthropic Cosmological Principle'' | journal = International Philosophical Ouarterly| volume = 27 | pages = 437–47}}
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| * {{cite book | last = Hawking | first = Stephen W. |authorlink = Stephen Hawking | year = 1988 | title = A Brief History of Time | page = 174 | location = New York | publisher = Bantam Books | isbn = 0-553-34614-8 }}
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| * [[Victor J. Stenger|Stenger, Victor J.]] (1999), "Anthropic design," ''The Skeptical Inquirer 23'' (August 31, 1999): 40-43
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| * [[Mosterín, Jesús]] (2005). "Anthropic Explanations in Cosmology." In P. Háyek, L. Valdés and D. Westerstahl (ed.), ''Logic, Methodology and Philosophy of Science, Proceedings of the 12th International Congress of the LMPS''. London: King's College Publications, pp. 441–473. ISBN 1-904987-21-4.
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| *{{cite book | author = Taylor, Stuart Ross | year = 1998 | title = Destiny or Chance: Our Solar System and Its Place in the Cosmos | publisher = Cambridge University Press| isbn = 0-521-78521-9}}
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| * {{cite journal | author = Tegmark, Max | authorlink = Max Tegmark | year = 1997 | url = http://space.mit.edu/home/tegmark/dimensions.html | title = On the dimensionality of spacetime | journal = Classical and Quantum Gravity |volume = 14 | pages = L69–L75
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| | doi = 10.1088/0264-9381/14/4/002|arxiv = gr-qc/9702052 |bibcode = 1997CQGra..14L..69T | issue = 4 }} A simple anthropic argument for why there are 3 spatial and 1 temporal dimensions.
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| * {{cite journal | author = Tipler, F. J. | authorlink = Frank J. Tipler | year = 2003 | title = Intelligent Life in Cosmology | journal = International Journal of Astrobiology| volume = 2 | pages = 141–48 | doi = 10.1017/S1473550403001526|bibcode = 2003IJAsB...2..141T | issue = 2 |arxiv = 0704.0058 }}
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| * {{cite journal | author = Walker, M. A., and Cirkovic, M. M. | year = 2006 | title = Anthropic Reasoning, Naturalism and the Contemporary Design Argument | journal = International Studies in the Philosophy of Science |volume = 20 | pages = 285–307 | doi = 10.1080/02698590600960945 | issue = 3}} Shows that some of the common criticisms of AP based on its relationship with numerology or the theological Design Argument are wrong.
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| * {{cite book | author = Ward, P. D., and Brownlee, D. | year = 2000 | title = [[Rare Earth hypothesis|Rare Earth]]: Why Complex Life is Uncommon in the Universe | publisher = Springer Verlag| isbn = 0-387-98701-0}}
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| * {{cite book | authorlink = Alex Vilenkin | first = Alex |last = Vilenkin | year = 2006 | title = Many Worlds in One: The Search for Other Universes | publisher = Hill and Wang | isbn = 978-0-8090-9523-0}}
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| {{refend}}
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| ==External links==
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| * Caner, Taslaman, [http://bigbang.ws/articles.asp?id=73 Anthropic Principle and Infinite Universes]{{dead link|date=March 2013}}
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| * [[Nick Bostrom]]: [http://www.anthropic-principle.com/ web site] devoted to the Anthropic Principle.
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| * Chown, Marcus, [http://space.mit.edu/home/tegmark/toe_frames.html Anything Goes,] ''New Scientist'', 6 June 1998. On Max Tegmark's work.
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| * [[Stephen Hawking]], [[Steven Weinberg]], [[Alexander Vilenkin]], [[David Gross]] and [[Lawrence Krauss]]: [http://www.phys.cwru.edu/events/cerca_video_archive.php Debate on Anthropic Reasoning] Kavli-CERCA Conference Video Archive.
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| * Sober, Elliott R. 2009, "Absence of Evidence and Evidence of Absence -- Evidential Transitivity in Connection with Fossils, Fishing, Fine-Tuning, and Firing Squads." Philosophical Studies, 2009, 143: 63-90.[http://philosophy.wisc.edu/sober/Absence%20-%20final%20in%20Phil%20stud.pdf]{{dead link|date=March 2013}}
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| * Tobin, Paul N., 2000, "[http://www.geocities.com/paulntobin/finetuned.html Is the Universe Fine-Tuned for Life?]{{dead link|date=March 2013}}" ([http://www.webcitation.org/5knILnytk Archived] 2009-10-25) A critique of the Anthropic Principle from an [[atheist]] viewpoint.
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| * "[http://www.starlarvae.org/Star_Larvae_Anthropic_Coincidence.html Anthropic Coincidence]"—the anthropic controversy as a segue to [[Lee Smolin]]'s theory of cosmological natural selection.
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| * [[Leonard Susskind]] and [[Lee Smolin]] debate the [http://www.edge.org/3rd_culture/smolin_susskind04/smolin_susskind.html Anthropic Principle].
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| * [http://arXiv.org/find/astro-ph/1/OR+ti:+AND+anthropic+principle+abs:+AND+anthropic+principle/0/1/0/all/0/1 debate among scientists on arxiv.org.]
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| * [http://www.epicidiot.com/evo_cre/13cards.htm Evolutionary Probability and Fine Tuning]
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| * [http://www.mathpages.com/home/kmath252/kmath252.htm Benevolent Design and the Anthropic Principle] at MathPages
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| * [http://www.epicidiot.com/evo_cre/vr_privileged_planet.htm Critical review of "The Privileged Planet"]
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| * [http://www.ipod.org.uk/reality/reality_anthropic_principle.asp The Anthropic Principle] - a review.
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| *Berger, Daniel, 2002, "[http://www.bluffton.edu/~bergerd/essays/impert.html An impertinent resumé of the Anthropic Cosmological Principle.]" A critique of Barrow & Tipler.
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| * [[Jürgen Schmidhuber]]: [http://www.idsia.ch/~juergen/computeruniverse.html Papers on algorithmic theories of everything] and the Anthropic Principle's lack of predictive power.
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| * [[Paul Davies]]: [http://www.wpr.org/book/070415a.html Cosmic Jackpot] Interview about the Anthropic Principle (starts at 40 min), 15 May 2007.
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| {{DEFAULTSORT:Anthropic Principle}}
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| [[Category:Physical cosmology]]
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| [[Category:Philosophical concepts]]
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| [[Category:Religion and science]]
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| [[Category:Principles]]
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| [[Category:Astronomical hypotheses]]
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| [[Category:Anthropic principle]]
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