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{{Short description|Hypothesis about sapient life and the universe}}
{{Redirect|Anthropic bias|the book by Nick Bostrom|Anthropic Bias}}

The '''anthropic principle''', also known as the "observation selection effect",<ref>{{Cite journal |last=Bostrom |first=Nick |year=2008 |title=Where are they? Why I hope the search for extraterrestrial life finds nothing |url=https://www.nickbostrom.com/extraterrestrial.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.nickbostrom.com/extraterrestrial.pdf |archive-date=2022-10-09 |url-status=live |journal=Technology Review |volume=2008 |pages=72–77}}</ref> is the hypothesis, first proposed in 1957 by [[Robert Dicke]], that the range of possible observations that could be made about the universe is limited by the fact that observations could happen only in a universe capable of developing intelligent life.<ref>{{Cite web |url=http://www.anthropic-principle.com/ |title=Was the Universe made for us? |last=Bostrom |first=Nick |date=9 February 2020 |website=anthropic-principle.com |quote=The data we collect about the Universe is filtered not only by our instruments' limitations, but also by the precondition that somebody be there to 'have' the data yielded by the instruments (and to build the instruments in the first place). }}</ref> Proponents of the anthropic principle argue that it explains why the universe has the [[age of the universe|age]] and the [[dimensionless physical constant|fundamental physical constant]]s necessary to accommodate conscious life, since if either had been different, no one would have been around to make observations. Anthropic reasoning is often used to deal with the idea that the universe seems to be [[Fine-tuned universe|finely tuned for the existence of life]].<ref>{{Cite web |url=http://abyss.uoregon.edu/~js/cosmo/lectures/lec24.html |author=James Schombert |publisher=Department of Physics at University of Oregon |title=Anthropic principle |access-date=2012-04-26 |archive-date=2012-04-28 |archive-url=https://web.archive.org/web/20120428004546/http://abyss.uoregon.edu/~js/cosmo/lectures/lec24.html |url-status=dead }}</ref>

There are many different formulations of the anthropic principle. Philosopher [[Nick Bostrom]] counts them at thirty, but the underlying principles can be divided into "weak" and "strong" forms, depending on the types of cosmological claims they entail. The '''weak anthropic principle''' ('''WAP'''), as defined by [[Brandon Carter]], states that the universe's ostensible fine tuning is the result of [[selection bias]] (specifically [[survivorship bias]]). Most such arguments draw upon some notion of the [[multiverse]] for there to be a [[statistical population]] of universes from which to select. However, a single vast universe is sufficient for most forms of the WAP that do not specifically deal with fine tuning. Carter distinguished the WAP from the '''strong anthropic principle''' ('''SAP'''), which considers the universe in some sense compelled to eventually have conscious and [[wikt:sapient|sapient]] life [[emergence|emerge]] within it.<ref name="auto">{{Cite web |url=https://www.britannica.com/science/anthropic-principle/Forms-of-the-anthropic-principle#ref1078369 |title=Forms of the anthropic principle |website=britannica.com |access-date=4 August 2022 }}</ref><ref>{{Cite web |url=https://www.thoughtco.com/what-is-the-anthropic-principle-2698848 |title=What is the anthropic principle? |website= thoughtco.com |access-date=4 August 2022 }}</ref> A form of the latter known as the '''participatory anthropic principle''', articulated by [[John Archibald Wheeler]], suggests on the basis of [[quantum mechanics]] that the universe, as a condition of its existence, must be observed, thus implying one or more observers. Stronger yet is the '''final anthropic principle''' ('''FAP'''), proposed by [[John D. Barrow]] and [[Frank Tipler]], which views the universe's structure as expressible by [[bit]]s of [[information]] in such a way that [[Information processing (psychology)|information processing]] is inevitable and eternal.<ref name="auto"/>

==Definition and basis==

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 have values that are consistent with conditions for life as it is known rather than values that would not be consistent with life on [[Earth]].  The anthropic principle states that this is an '' a posteriori'' [[a priori and a posteriori|necessity]], because if life were impossible, no living entity would be there to observe it, and thus it would not be known. That is, it must be possible to observe ''some'' universe, and hence, the laws and constants of any such universe must accommodate that possibility.

The term ''[[wikt:anthropic|anthropic]]'' in "anthropic principle" has been argued<ref>Mosterín J., (2005), ''Antropic explanations in Cosmology'', in  Hajek, Valdés & Westerstahl (eds.), ''Proceedings of the 12th international congress of logic, Methodology and philosophy of science''; http://philsci-archive.pitt.edu/1658/"</ref> to be a [[misnomer]].<ref group=note>"anthropic" means "of or pertaining to mankind or humans".</ref>  While singling out the currently-observable kind of carbon-based life, none of the finely tuned phenomena require [[human]] life or some kind of [[carbon chauvinism]].<ref>{{Cite book |last1=Stenger |first1=Victor J. |author-link1=Victor J. Stenger |editor1-last=Flynn |editor1-first=Tom |editor1-link=Tom Flynn (author) |title=The new encyclopedia of unbelief |year=2007 |publisher=Prometheus books |isbn=9781615922802 |pages=65–70 |chapter-url=https://books.google.com/books?id=fsZ26vQxJKMC&pg=PA65 |language=en |chapter=The anthropic principle }}</ref><ref>{{Harvnb|Bostrom|2002|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.<ref>{{Cite journal |last=Smith |first=Quentin |date=September 1994 |title=Anthropic explanations in cosmology |url=http://dx.doi.org/10.1080/00048409412346161 |journal=Australasian Journal of Philosophy |volume=72 |issue=3 |pages=371–382 |doi=10.1080/00048409412346161 |issn=0004-8402 }}</ref>

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 its critics 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 anthropic principle]. ''Merriam-Webster online dictionary''.</ref> is "easy" to support in mathematics and philosophy (i.e., it is a [[tautology (logic)|tautology]] or [[truism]]).  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>{{Cite web |url=http://www.physics.sfsu.edu/~lwilliam/sota/anth/SAP_FAP.htm |title=The strong anthropic principle and the final anthropic principle |access-date=2011-08-03 |archive-date=2019-07-29 |archive-url=https://web.archive.org/web/20190729215113/http://www.physics.sfsu.edu/~lwilliam/sota/anth/SAP_FAP.htm |url-status=dead }}</ref><ref>{{Cite web |url=http://firstpraxis.wordpress.com/2011/05/26/on-knowing-sagan-from-pale-blue-dot/ |title=On knowing, Sagan from Pale blue dot }}</ref>

==Anthropic 'coincidences'==
{{Intelligent Design}}
{{Main|Fine-tuned universe}}
In 1961, [[Robert Dicke]] noted that the [[age of the universe]], as seen by living observers, cannot be random.<ref>{{Cite journal |author-link=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 |s2cid=4196678 }}</ref> Instead, biological factors constrain the universe to be more or less in a "golden age", neither too young nor too old.<ref name="Davies-2006">{{Cite book |author-link=Paul Davies |author=Davies, P. |title=The Goldilocks enigma |publisher=Allen Lane |year=2006 |isbn=978-0-7139-9883-2 }}</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 [[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 that inspired [[Dirac large numbers hypothesis|Dirac's varying-''G'' theory]].

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 |author-link=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 "[[Cosmological constant problem|worst prediction in physics]]").<ref>{{Cite web |url=https://www.newscientist.com/blog/space/2007/02/physicists-debate-nature-of-space-time.html |title=New scientist space blog: Physicists debate the nature of space-time - New scientist<!-- Bot generated title -->}}</ref> However, if the cosmological constant were only several orders 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.

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 interaction]] (up to 50% for some authors<ref>{{Cite journal |last1=MacDonald |first1=J. |last2=Mullan |first2=D. J. |date=2009-08-12 |title=Big bang nucleosynthesis: The strong nuclear force meets the weak anthropic principle |url=https://link.aps.org/doi/10.1103/PhysRevD.80.043507 |journal=Physical Review D |language=en |volume=80 |issue=4 |page=043507 |doi=10.1103/PhysRevD.80.043507 |issn=1550-7998|arxiv=0904.1807 |bibcode=2009PhRvD..80d3507M |s2cid=119203730 }}</ref>) would bind the [[dineutron]] and the [[diproton]] and convert all hydrogen in the early universe to helium;<ref name=focus>{{Cite web |date=August 2010 |publisher=Focus |last=Couchman |first=D. |title=The strong nuclear force as an example of fine tuning for life |url=http://www.focus.org.uk/strongforce.php |access-date=15 July 2019 }}</ref> likewise, an increase in the [[weak interaction]] also would convert all hydrogen to helium. Water, as well as sufficiently long-lived stable stars, both essential for the emergence of life as it is known, would not exist.<ref name=leslie2>{{Cite book |last=Leslie |first=J. |title=Universes |year=1989 |isbn=0-203-05318-4 |url=https://books.google.com/books?id=dEeEAgAAQBAJ |publisher=Routledge <!--page=-->}}</ref> 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.

==Origin==
{{Section rewrite|date=August 2023}}
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]]. Carter said: "Although our situation is not necessarily ''central'', it is inevitably privileged to some extent."<ref name="carter1973">{{Cite conference |first=B. |last=Carter |author-link=Brandon Carter |book-title=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 |volume=63 |pages=291–298 |doi=10.1017/S0074180900235638 |doi-access=free }}; republished online by Cambridge University Press (7 Feb 2017)</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 underlies 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]]).

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 that addressed the values of the fundamental constants of physics.

[[Roger Penrose]] explained the weak form as follows:

{{Blockquote|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, the argument ran, there would be no intelligent life around 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}}

One reason this is plausible is that there are many other places and times in which humans could have evolved. But when applying the strong principle, there is only one universe, with one set of fundamental parameters, so what exactly is the point being made? Carter offers two possibilities: First, humans can use their own existence to make "predictions" about the parameters. But second, "as a last resort", humans 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 the [[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 seemingly 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?"

Since Carter's 1973 paper, the term ''anthropic principle'' has been extended to cover a number of ideas that differ in important ways from his. Particular confusion was caused by the 1986 book ''The Anthropic Cosmological Principle'' by [[John D. Barrow]] and [[Frank Tipler]],<ref>{{BarrowTipler1986}}</ref> which distinguished between a "weak" and "strong" anthropic principle in a way very different from Carter's, as discussed in the next section.

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 |author-link=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 |edition=4th |location=London |publisher=George Bell & Sons |pages= 256–257 |bibcode=1903mpus.book.....W }}</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>

[[Ludwig Boltzmann]] may have been one of the first in modern science to use anthropic reasoning. Prior to knowledge of the [[Big Bang]] Boltzmann's thermodynamic concepts painted a picture of a universe that had inexplicably low entropy. Boltzmann suggested several explanations, one of which relied on fluctuations that could produce pockets of low entropy or Boltzmann universes. While most of the universe is featureless in this model, to Boltzmann, it is unremarkable that humanity happens to inhabit a Boltzmann universe, as that is the only place where intelligent life could be.<ref name="carroll2017">{{Cite arXiv |eprint=1702.00850 |last1=Carroll |first1=Sean M. |title=Why Boltzmann brains are bad |year=2017 |class=hep-th }}</ref>{{Sfn|Bostrom|2002}}

==Variants==
{{Prose|date=August 2023|section}}
'''Weak anthropic principle (WAP)''' ([[Brandon Carter|Carter]]): "... our location in the universe is ''necessarily'' privileged to the extent of being compatible with our existence as observers."<ref name="carter1973"/> For Carter, "location" refers to our location in time as well as space.

'''Strong anthropic principle (SAP)''' (Carter): "[T]he 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]].

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=146–153 |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>

'''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 constants, such as the [[fine-structure constant]], the [[Anthropic principle#Dimensions of spacetime|number of spacetime dimensions]], and the [[cosmological constant]]—topics that fall under Carter's SAP.

'''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>
* "There exists one possible Universe 'designed' with the goal of generating and sustaining 'observers'."
::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 constants set to ensure that life emerges and evolves.
* "Observers are necessary to bring the Universe into being."
::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 [[Von Neumann–Wigner interpretation|interpretation of quantum mechanics]] associated with the ideas of [[John von Neumann]] and [[Eugene Wigner]].
* "An ensemble of other different universes is necessary for the existence of our Universe."
::By contrast, Carter merely says that an [[multiverse|ensemble of universes]] is necessary for the SAP to count as an explanation.

The [[philosopher]]s [[John Andrew Leslie|John Leslie]]<ref>{{Cite conference |author=Leslie, J. |author-link=John Andrew Leslie |title=Probabilistic phase transitions and the anthropic principle |book-title=Origin and early history of the Universe: LIEGE 26 | pages= 439–444 |publisher=Knudsen |year=1986 |bibcode=1986LIACo..26..439L }}</ref> and [[Nick Bostrom]]{{Sfn|Bostrom|2002}} 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:

{{Blockquote|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>}}

'''[[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.

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">{{Cite arXiv |eprint=quant-ph/0011122 |last1=Schmidhuber |first1=Juergen |title=Algorithmic theories of everything |year=2000 }}</ref><ref>[[Jürgen Schmidhuber]], 2002, "[http://www.idsia.ch/~juergen/speedprior.html The speed prior: A new simplicity measure yielding near-optimal computable predictions.]" ''Proceedings of 15th annual conference on computational learning theory'' (COLT 2002), Sydney, Australia, Lecture notes in artificial intelligence. Springer: 216–228.</ref>

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":

{{Blockquote|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>}}

==Character of anthropic reasoning==
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 humans should give serious consideration to there being other universes with different values of the "fundamental parameters"—that is, the [[dimensionless physical constant]]s and initial conditions for the [[Big Bang]]. Carter and others have argued that life would not be possible in most such universes. In other words, the universe humans live 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. |author-link=C. B. Collins |author2=Hawking, S. W. |author2-link=Stephen Hawking |title=Why is the universe isotropic? |journal=Astrophysical Journal |volume=180 |pages=317–334 |date=1973 |doi=10.1086/151965 |bibcode=1973ApJ...180..317C |doi-access=free }}</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 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.

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. |author-link=Max Tegmark |title=Is 'the theory of everything' merely the ultimate ensemble theory?| journal=Annals of Physics |volume=270 |issue=1 |pages=1–51 |year=1998 |doi=10.1006/aphy.1998.5855 |arxiv=gr-qc/9704009 |bibcode=1998AnPhy.270....1T |s2cid=41548734 }}</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 group=note>Strictly speaking, the number of non-compact dimensions, see [[String theory]].</ref>

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 [[Albert Einstein]] said: "What really interests me is whether God had any choice in the creation of the world." In 2002, some 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. |author2=Perry, Malcolm J. |author3=Zytkow, Anna N. |name-list-style=amp |title=The beginning of the end of the anthropic principle |journal=New Astronomy |volume=7 |issue=1 |date=2002 |pages=45–53  |doi=10.1016/S1384-1076(01)00088-4 |arxiv=astro-ph/0001197|bibcode=2002NewA....7...45K |s2cid=15749902 }}</ref> since there would be no free parameters to select. In 2003, however, [[Leonard Susskind]] stated: "...it seems plausible that the landscape is unimaginably large and diverse. This is the behavior that gives credence to the anthropic principle."<ref name="arXiv:hep-th/0302219">{{Cite journal |title=The anthropic landscape of string theory |arxiv=hep-th/0302219 |last=Susskind |first=Leonard |journal=The Davis Meeting on Cosmic Inflation |date=27 Feb 2003 |page=26 |bibcode=2003dmci.confE..26S }}</ref>

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]" in Mann, W. E., ed., ''The Blackwell guide to the philosophy of religion''. Blackwell publishers. {{Webarchive |url=https://web.archive.org/web/20110903095009/http://philosophy.wisc.edu/sober/black-da.pdf |date=September 3, 2011 }}</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.

[[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:<ref name="Davies-2006" />{{rp|261–267}}

# The absurd universe: Our universe just happens to be the way it is.
# The unique universe: There is a deep underlying unity in physics that necessitates the Universe being the way it is. A [[Theory of Everything]] will explain why the various features of the Universe must have exactly the values that have been recorded.
# The multiverse: Multiple universes exist, having all possible combinations of characteristics, and humans inevitably find themselves within a universe that allows us to exist.
# Intelligent design: A creator designed the Universe with the purpose of supporting complexity and the emergence of intelligence.
# The life principle: There is an underlying principle that constrains the Universe to evolve towards life and mind.
# 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).
# The fake universe: Humans live inside a [[virtual reality simulation]].

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" that 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|International journal of astrobiology]]''.</ref>

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).

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 |s2cid=92330878 }}</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.

==Observational evidence==
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{{Undue-inline|date=August 2023|reason=emphasis is probably undue when compared w/ MOS:DOUBT}}). Thus, in principle Barrow's SAP cannot be falsified by observing a universe in which an observer cannot exist.

Philosopher [[John Andrew Leslie|John Leslie]]<ref>Leslie, J. (1986) op. cit.</ref> states that the Carter SAP (with [[multiverse]]) predicts the following:
* 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;
* Various theories for generating [[multiverse|multiple universes]] will prove robust;
* Evidence that the universe is [[fine tuned universe|fine tuned]] will continue to accumulate;
* No life with a [[alternative biochemistry|non-carbon chemistry]] will be discovered;
* Mathematical studies of [[galaxy formation]] will confirm that it is sensitive to the rate of [[expansion of the universe]].

Hogan<ref>{{Cite journal |author=Hogan, Craig |title=Why the universe is 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 |s2cid=14095249 }}</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, humans 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.

Probabilistic predictions of parameter values can be made given:
#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
#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).
The probability of observing value ''X'' is then proportional to ''N''(''X'') ''P''(''X''). A generic feature of an analysis of this nature is that the expected values of the fundamental physical constants 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.

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.

==Applications of the principle==

===The nucleosynthesis of carbon-12===
[[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]] nuclei, 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 [[electronvolt]]s.<ref>University of Birmingham [http://www.np.ph.bham.ac.uk/research/anthropic.htm Life, Bent chains and the anthropic principle] {{Webarchive |url=https://web.archive.org/web/20090927094126/http://www.np.ph.bham.ac.uk/research/anthropic.htm |date=September 27, 2009 }}</ref><ref>{{Cite journal |doi=10.1103/RevModPhys.29.547 |volume=29 |title=Synthesis of the elements in stars |year=1957 |journal=Reviews of Modern Physics |pages=547–650  |last1=Burbidge |first1=E. Margaret |issue=4 |bibcode=1957RvMP...29..547B |doi-access=free }}</ref> [[William Alfred Fowler|Willie Fowler]]'s research group soon found this resonance, and its measured energy was close to Hoyle's prediction.

However, in 2010 [[Helge Kragh]] argued that Hoyle did not use anthropic reasoning in making his prediction, since he made his prediction in 1953 and anthropic reasoning did not come into prominence until 1980. He called this an "anthropic myth," saying that Hoyle and others made an after-the-fact connection between carbon and life decades after the discovery of the resonance.

{{Blockquote| An investigation of the historical circumstances of the prediction and its subsequent experimental confirmation shows that Hoyle and his contemporaries did not associate the level in the carbon nucleus with life at all.<ref>{{Cite web |last1=Kragh |first1=Helge |author-link=Helge Kragh |title=When is a prediction anthropic? Fred Hoyle and the 7.65 MeV carbon resonance |url=http://philsci-archive.pitt.edu/5332/ |date=2010 |access-date=2 July 2019 }}</ref>}}

=== Cosmic inflation ===
{{Physical cosmology}}
{{Main|Cosmic inflation}}
[[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 |s2cid=4315730 }}</ref> He emphasized that initial conditions that 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 |author-link=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| s2cid=4307546 }}</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.

=== String theory ===
{{Main|String theory landscape}}

[[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.<ref name="arXiv:hep-th/0302219"/>

[[Steven Weinberg]]<ref>{{Cite conference |author=Weinberg, S. |author-link=Steven Weinberg |year=2007 |title=Living in the multiverse |book-title=Universe or multiverse? |editor=B. Carr |publisher=Cambridge University Press |isbn=978-0-521-84841-1 |arxiv=hep-th/0511037 |bibcode=2005hep.th...11037W }}</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 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. |author2=Rees, M. J. |name-list-style=amp |title=Anthropic reasoning |journal=Science |volume=309 |pages=1022–1023 |year=2003 |doi=10.1126/science.1111446 |pmid=16099967 |issue=5737 |bibcode=2005Sci...309.1022L |s2cid=40089857 }}</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.

[[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.

[[Zhi-Wei Wang]] and [[Samuel L. Braunstein]] proved that life's existence in the universe depends on various fundamental constants. It suggests that without a complete understanding of these constants, one might incorrectly perceive the universe as being intelligently designed for life. This perspective challenges the view that our universe is unique in its ability to support life.<ref>{{cite journal |last1=Wang |first1=Zhi-Wei |last2=Braunstein |first2=Samuel L. |year=2023 |title=Sciama's argument on life in a random universe and distinguishing apples from oranges |journal=Nature Astronomy |volume=7 |issue=2023 |pages=755–756 |doi=10.1038/s41550-023-02014-9 |arxiv=2109.10241 }}</ref>

{{Anchor|Spacetime}}

===Dimensions of spacetime===
[[File:Spacetime dimensionality.svg|thumb|300px|Properties of ''n''+''m''-dimensional spacetimes<ref>{{Cite journal |last=Tegmark |first=Max |date=1997-04-01 |title=On the dimensionality of spacetime |journal=Classical and Quantum Gravity |volume=14 |issue=4 |pages=L69–L75 |doi=10.1088/0264-9381/14/4/002 |arxiv=gr-qc/9702052 |bibcode=1997CQGra..14L..69T |s2cid=250904081 |issn=0264-9381}}</ref>]]

There are two kinds of dimensions: [[spatial dimension|spatial]] (bidirectional) and [[temporal dimension|temporal]] (unidirectional).<ref name="Skow2007">{{Cite journal |author-link=Bradford Skow|last1=Skow |first1=Bradford |title=What makes time different from space? |journal=Noûs |year=2007 |volume=41 |issue=2 |pages=227–252 |url=http://web.mit.edu/bskow/www/research/temporality.pdf |access-date=13 April 2018 |doi=10.1111/j.1468-0068.2007.00645.x |citeseerx=10.1.1.404.7853 |archive-url=https://web.archive.org/web/20160824025031/http://web.mit.edu/bskow/www/research/temporality.pdf |archive-date=2016-08-24 }}</ref> Let the number of spatial dimensions be ''N'' and the number of temporal dimensions be ''T''. That ''N'' = 3 and ''T'' = 1, setting aside the compactified dimensions invoked by [[string theory]] and undetectable to date, can be explained by appealing to the physical consequences of letting ''N'' differ from 3 and ''T'' differ from 1. The argument is often of an anthropic character and possibly the first of its kind, albeit before the complete concept came into vogue.

The implicit notion that the dimensionality of the universe is special is first attributed to [[Gottfried Wilhelm Leibniz]], who in the [[Discourse on Metaphysics]] suggested that the world is "[[wikiquote:Gottfried Leibniz|the one which is at the same time the simplest in hypothesis and the richest in phenomena]]".<ref name="Leibniz1686">{{Cite book |last1=Leibniz |first1=Gottfried |title=Die philosophischen schriften von Gottfried Wilhelm Leibniz |volume=4 |year=1880|publisher=Weidmann |pages=427–463 |chapter-url=https://en.wikisource.org/wiki/Discourse_on_Metaphysics |access-date=13 April 2018 |chapter=Discourse on metaphysics }}</ref> [[Immanuel Kant]] argued that 3-dimensional space was a consequence of the inverse square [[law of universal gravitation]]. While Kant's argument is historically important, [[John D. Barrow]] said that it "gets the punch-line back to front: it is the three-dimensionality of space that explains why we see inverse-square force laws in Nature, not vice-versa" (Barrow 2002:204).<ref group=note>This is because the law of gravitation (or any other [[inverse-square law]]) follows from the concept of [[flux]] and the proportional relationship of flux density and field strength. If ''N'' = 3, then 3-dimensional solid objects have surface areas proportional to the square of their size in any selected spatial dimension. In particular, a sphere of [[radius]] r has a surface area of 4''π''r{{Sup|2}}. More generally, in a space of ''N'' dimensions, the strength of the gravitational attraction between two bodies separated by a distance of r would be inversely proportional to r<sup>''N''−1</sup>.</ref>

In 1920, [[Paul Ehrenfest]] showed that if there is only a single time dimension and more 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 |author-link=Paul Ehrenfest |title=Welche Rolle spielt die Dreidimensionalität des Raumes in den Grundgesetzen der Physik? |trans-title=How do the fundamental laws of physics make manifest that space has 3 dimensions? |journal=Annalen der Physik |volume=61 |pages=440–446 |year=1920 |doi=10.1002/andp.19203660503 |issue=5 |bibcode=1920AnP...366..440E |url=https://zenodo.org/record/1424351 }}. 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 positive whole number, then wave impulses become distorted. In 1922, [[Hermann Weyl]] claimed that [[James Clerk Maxwell|Maxwell]]'s theory of [[electromagnetism]] can be expressed in terms of an action only for a four-dimensional manifold.<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. |title=Schwarzschild field in ''n'' dimensions and the dimensionality of space problem|journal=Nuovo Cimento |volume= 27|issue= 3|pages= 636–651|year=1963 |doi=10.1007/BF02784569 |bibcode=1963NCim...27..636T |s2cid=119683293 }}</ref>

[[Max Tegmark]] expands on the preceding argument in the following anthropic manner.<ref name="tegmark-dim">{{Cite journal |last=Tegmark |first=Max |author-link=Max Tegmark |title=On the dimensionality of spacetime |journal=Classical and Quantum Gravity |volume=14 |issue=4 |pages=L69–L75 |date=April 1997 |url=https://space.mit.edu/home/tegmark/dimensions.pdf |doi=10.1088/0264-9381/14/4/002 |access-date=2006-12-16 |arxiv=gr-qc/9702052 |bibcode=1997CQGra..14L..69T |s2cid=15694111 }}</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.)<ref name="tegmark-dim" /> Lastly, if ''N'' < 3, gravitation of any kind becomes problematic, and the universe would probably be too simple to contain observers. For example, when ''N'' < 3, [[nerve]]s cannot cross without intersecting.<ref name="tegmark-dim" /> Hence anthropic and other arguments rule out all cases except ''N'' = 3 and ''T'' = 1, which describes the world around us.

On the other hand, in view of creating [[black hole]]s from an ideal [[monatomic gas]] under its self-gravity, Wei-Xiang Feng showed that (3+1)-dimensional spacetime is the marginal dimensionality. Moreover, it is the unique [[dimensionality]] that can afford a "stable" gas sphere with a "positive" [[cosmological constant]]. However, a self-gravitating gas cannot be stably bound if the mass sphere is larger than ~10<sup>21</sup> solar masses, due to the small positivity of the cosmological constant observed.<ref>{{Cite journal |last=Feng |first=W.X. |date=2022-08-03 |title=Gravothermal phase transition, black holes and space dimensionality |url=https://journals.aps.org/prd/abstract/10.1103/PhysRevD.106.L041501 |journal=Physical Review D |volume=106 |issue=4 |pages=L041501 |arxiv=2207.14317 |doi=10.1103/PhysRevD.106.L041501 |bibcode=2022PhRvD.106d1501F |s2cid=251196731 }}</ref>

In 2019, James Scargill argued that complex life may be possible with two spatial dimensions. According to Scargill, a purely scalar theory of gravity may enable a local gravitational force, and 2D networks may be sufficient for complex neural networks.<ref>{{Cite journal |last=Scargill |first=J. H. C. |date=2020-02-26 |title=Existence of life in 2 + 1 dimensions |url=https://link.aps.org/doi/10.1103/PhysRevResearch.2.013217 |journal=Physical Review Research |volume=2 |issue=1 |pages=013217 |arxiv=1906.05336 |doi=10.1103/PhysRevResearch.2.013217 |bibcode=2020PhRvR...2a3217S |s2cid=211734117 }}</ref><ref>{{Cite web |title=Life could exist in a 2D universe (according to physics, anyway) |url=https://www.technologyreview.com/2019/06/20/102942/life-could-exists-in-a-2d-universe-according-to-physics-anyway/ |access-date=2021-06-16 |website=technologyreview.com |language=en}}</ref>

== Metaphysical interpretations ==
Some of the metaphysical disputes and speculations include, for example, attempts to back [[Pierre Teilhard de Chardin]]'s earlier interpretation of the universe as being Christ centered (compare [[Omega Point]]), expressing a ''creatio evolutiva'' instead the elder notion of ''creatio continua''.<ref name=":3">Johann.&nbsp;Dorschner. und.&nbsp;Ralph.&nbsp;Neuhäuser.&nbsp;Evolution. des. Kosmos. und. der. Punkt. omega, in Nikolaus Knoepffler,&nbsp;H. James Birx, Teilhard de Chardin, V&R unipress GmbH, 2005, p. 109 ff</ref> From a strictly secular, humanist perspective, it allows as well to put human beings back in the center, an anthropogenic shift in cosmology.<ref name=":3" />  [[Karl W. Giberson]]<ref>Giberson, Karl. "Anthropic principle: A postmodern creation myth?". ''Journal of interdisciplinary studies''. 9.1/2 (1997): 63–90.</ref> has laconically stated that{{Blockquote|text=What emerges is the&nbsp;suggestion&nbsp;that cosmology may at last be in possession of some raw material for a&nbsp;[[postmodern]] creation myth.|sign=[[Karl W. Giberson]]|source=}}

William Sims Bainbridge disagreed with de Chardin's optimism about a future Omega point at the end of history, arguing that logically, humans are trapped at the Omicron point, in the middle of the Greek alphabet rather than advancing to the end, because the universe does not need to have any characteristics that would support our further technical progress, if the anthropic principle merely requires it to be suitable for our evolution to this point.<ref>Bainbridge, William Sims, 1997, "The Omicron point: Sociological application of the anthropic theory, in ''Chaos and complexity in sociology: Myths, models and theory,'' Raymond E. Eve, Sara Horsfall, and Mary E. Lee, editors, pp. 91–101. Thousand Oaks, California: Sage publications. {{ISBN|0-7619-0890-0}}</ref>

=== ''The anthropic cosmological principle'' ===
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 [[cosmologist]] 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]].

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 [[eutaxiology|''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 |url=https://archive.org/details/critiqueofdesign00hick |location=New York |publisher=Scribner's }}</ref>

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 [[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>

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 |author-link=Frank Tipler |title=The physics of immortality |publisher=DoubleDay |year=1994 |isbn=978-0-385-46798-8 |url=https://archive.org/details/physicsofimmorta00fran }}</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.

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):

{{Blockquote|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 that it is logically possible to know. And this is the end.<ref>Barrow and Tipler 1986: 677</ref>}}

==Reception and controversies==
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]].

A common criticism of Carter's SAP is that it is an easy ''[[deus ex machina]]'' that discourages searches for physical explanations. To quote Penrose again: "[I]t 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. |author-link=Roger Penrose |title=The Emperor's new mind |url=https://archive.org/details/emperorsnewmindc00penr |url-access=registration |publisher=Oxford University Press |isbn=978-0-19-851973-7 |year=1989 |chapter=10 }}</ref>

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]] 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",{{Citation needed |date=September 2022 }} which is a valid statement, but does not make a claim of some factual alternative over another.

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<ref>[https://blogs.scientificamerican.com/guest-blog/are-parallel-universes-unscientific-nonsense-insider-tips-for-criticizing-the-multiverse/ Are parallel universes unscientific nonsense? Insider tips for criticizing the multiverse]  Tegmark, Max. February 4, 2014.</ref> that it does make falsifiable predictions. A modified version of this criticism is that humanity understands 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. |author2=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 |s2cid=27409290 }} See also this [http://www.physorg.com/news83924839.html news story.]</ref>

Many criticisms focus on versions of the strong anthropic principle, such as Barrow 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. Similarly, [[Stephen Jay Gould]],<ref>{{Cite conference |last=Gould |first=Stephen Jay |year=1998 |title=Clear thinking in the sciences |book-title=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 |publisher=W.H. Freeman |isbn=978-0-7167-3090-3  }}</ref> [[Michael Shermer]],<ref>{{Cite book |last=Shermer |first=Michael |year=2007 |title=Why Darwin matters |publisher=Macmillan |isbn=978-0-8050-8121-3 |url=https://archive.org/details/whydarwinmatters00mich }}</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 the universe, and not vice versa.

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=Carr, B. J. |author2=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 |s2cid=4363262 |author2-link=Martin Rees |author-link=Bernard Carr }}</ref> The range of [[dimensionless physical constant|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.| author-link=Victor Stenger |year=2000 |title=Timeless reality: Symmetry, simplicity, and multiple universes |publisher=Prometheus books |isbn=978-1-57392-859-5 }}</ref> For instance, Harnik et al.<ref>{{Cite journal |author=Harnik, R. |author2=Kribs, G. |author3=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 |s2cid=14340180 }}</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.

[[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 [[Lee Smolin#Cosmological natural selection|fecund universes]] theory, which assumes universes have "offspring" through the creation of [[black hole]]s whose offspring universes have values of physical constants that depend on those of the mother universe.<ref>{{Cite book |author=Lee Smolin |author-link=Lee Smolin |editor=Tyson, Neil deGrasse |editor-link=Neil deGrasse Tyson |editor2=Soter, Steve |editor2-link=Steven Soter |year=2001 |title=Cosmic horizons: Astronomy at the cutting edge |publisher=The New Press |isbn=978-1-56584-602-9 |pages=[https://archive.org/details/isbn_9781565846029/page/148 148–152] |url=https://archive.org/details/isbn_9781565846029/page/148 }}</ref>

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 |journal=American Philosophical Quarterly |volume=24 |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:

{{Blockquote|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.}}

== See also ==
{{Cols|colwidth=21em}}
* ''{{Annotated link|A Fórmula de Deus}}'' (discussing the anthropic principle)
* {{Annotated link|Anthropocentrism}}
* {{Annotated link|Arthur Schopenhauer}} (an immediate precursor of the idea)
* {{Annotated link|Big Bounce}}
* {{Annotated link|Copernican principle}}
* {{Annotated link|Doomsday argument}}
* {{Annotated link|Fermi paradox}}
* {{Annotated link|Goldilocks principle}}
* {{Annotated link|Great Filter}}
* {{Annotated link|Infinite monkey theorem}}
* {{Annotated link|Inverse gambler's fallacy}}
* {{Annotated link|Mathematical universe hypothesis}}
* {{Annotated link|Measure problem (cosmology)}}
* {{Annotated link|Mediocrity principle}}
* {{Annotated link|Metaphysical naturalism}}
* {{Annotated link|Neocatastrophism}}
* {{Annotated link|Fine-tuned universe}}
* {{Annotated link|Alejandro Jenkins#Quark mass and congeniality to life|Quark mass and congeniality to life}} (work of Alejandro Jenkins)
* {{Annotated link|Rare Earth hypothesis}}
* {{Annotated link|Sleeping Beauty problem}}
* {{Annotated link|Triple-alpha process}}
* {{Annotated link|Why there is anything at all}}
{{Colend}}

== Notes ==
{{Reflist|group=note|30em}}

== Footnotes ==
{{Reflist|30em}}

== References ==
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{{Refbegin|30em}}
* {{BarrowTipler1986}}
* {{Citation |last=Bostrom |first=N. |author-link=Nick Bostrom |year=2002 |title=Anthropic bias: Observation selection effects in science and philosophy |publisher=Routledge |isbn=978-0-415-93858-7}} 5 chapters available [http://www.anthropic-principle.com/book/ online.] {{Webarchive |url=https://web.archive.org/web/20060826121429/http://www.anthropic-principle.com/book/  |date=2006-08-26 }}
* {{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 |arxiv=astro-ph/0306185 |bibcode=2002EM&P...91..243C |s2cid=17341587 }}
* {{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 |s2cid=6068309 }}
* {{Cite book |first=Simon |last=Conway Morris |author-link=Simon Conway Morris |date=2003 |title=Life's solution: Inevitable humans in a lonely Universe |url=https://archive.org/details/lifessolutionine01conw |url-access=registration |publisher=Cambridge University Press }}
* {{Cite journal |author=Craig, William Lane |author-link=William Lane Craig |date=1987 |title=Critical review of ''The anthropic cosmological principle'' |journal=International Philosophical Quarterly| volume=27 |issue=4 |pages=437–47 |doi=10.5840/ipq198727433}}
* {{Cite book |last=Hawking |first=Stephen W. |author-link=Stephen Hawking |date=1988 |title=A brief history of time |url=https://archive.org/details/briefhistoryofti00step_1 |url-access=registration |page=[https://archive.org/details/briefhistoryofti00step_1/page/174 174] |location=New York |publisher=Bantam books |isbn=978-0-553-34614-5 }}
* [[Victor J. Stenger|Stenger, Victor J.]] (1999), "Anthropic design," ''The skeptical inquirer 23'' (August 31, 1999): 40–43
* [[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.&nbsp;441–473. {{ISBN|1-904987-21-4}}.
*{{Cite book |author=Taylor |author2=Stuart Ross |date=1998 |title=Destiny or chance: Our Solar System and its place in the cosmos |publisher=Cambridge University Press |isbn=978-0-521-78521-1 }}
* {{Cite journal |author=Tegmark, Max |author-link=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 |doi=10.1088/0264-9381/14/4/002|arxiv=gr-qc/9702052 |bibcode=1997CQGra..14L..69T |issue=4 |s2cid=15694111 }} A simple anthropic argument for why there are 3 spatial and 1 temporal dimensions.
* {{Cite journal |author=Tipler, F. J. |author-link=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 |s2cid=119283361 }}
* {{Cite journal |author=Walker, M. A. |author2=Cirkovic, M. M. |name-list-style=amp |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 |citeseerx=10.1.1.212.2588 |s2cid=8804703 }} Shows that some of the common criticisms of anthropic principle based on its relationship with numerology or the theological design argument are wrong.
* {{Cite book |author=Ward, P. D. |author2=Brownlee, D. |name-list-style=amp |year=2000 |title=Rare Earth: Why complex life is uncommon in the Universe |publisher= Springer Verlag |isbn=978-0-387-98701-9 |title-link=Rare Earth hypothesis }}
* {{Cite book |author-link=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 |url=https://archive.org/details/manyworldsinoneo00vile }}
{{Refend}}

==External links==
{{Refbegin|30em}}
* [[Nick Bostrom]]: [http://www.anthropic-principle.com/ web site] devoted to the anthropic principle.
* Friederich, Simon. [https://plato.stanford.edu/entries/fine-tuning/ Fine-tuning], review article of the discussion about fine-tuning, highlighting the role of the anthropic principles.
* Gijsbers, Victor. (2000). [https://web.archive.org/web/20141102054638/http://www.positiveatheism.org/faq/anthropic.htm Theistic anthropic principle refuted] – Positive atheism magazine.
* Chown, Marcus, [http://space.mit.edu/home/tegmark/toe_frames.html Anything Goes], ''New scientist'', 6 June 1998. On Max Tegmark's work.
* [[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] {{Webarchive|url=https://web.archive.org/web/20200410165653/http://www.phys.cwru.edu/events/cerca_video_archive.php |date=2020-04-10 }} Kavli-CERCA conference video archive.
* Sober, Elliott R. 2009, "[http://sober.philosophy.wisc.edu/selected-papers/PS-2009-AbsenseOfEvidenceAndEvidenceOfAbsence.pdf Absence of evidence and evidence of absence – Evidential transitivity in connection with fossils, fishing, fine-tuning, and firing squads.]{{Dead link|date=May 2023 |bot=InternetArchiveBot |fix-attempted=yes }}"  Philosophical Studies, 2009, 143: 63–90.
* "[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.
* [[Leonard Susskind]] and [[Lee Smolin]] debate the [http://www.edge.org/3rd_culture/smolin_susskind04/smolin_susskind.html anthropic principle].
* [https://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.]
* [https://web.archive.org/web/20060502002523/http://www.epicidiot.com/evo_cre/13cards.htm Evolutionary probability and fine tuning]
* [https://archive.today/20130128125932/http://www.mathpages.com/home/kmath252/kmath252.htm Benevolent design and the anthropic principle] at MathPages
* [https://web.archive.org/web/20051223140116/http://www.epicidiot.com/evo_cre/vr_privileged_planet.htm Critical review of "The privileged planet"]
* [https://web.archive.org/web/20070227160115/http://www.ipod.org.uk/reality/reality_anthropic_principle.asp The anthropic principle] – a review.
* Berger, Daniel, 2002, "[http://www.bluffton.edu/~bergerd/essays/impert.html An impertinent résumé of the Anthropic cosmological principle.] {{Webarchive|url=https://web.archive.org/web/20160304201803/http://www.bluffton.edu/~bergerd/essays/impert.html |date=2016-03-04 }}" A critique of Barrow & Tipler.
* [[Jürgen Schmidhuber]]: [http://www.idsia.ch/~juergen/computeruniverse.html Papers on algorithmic theories of everything] and the anthropic principle's lack of predictive power.
* [[Paul Davies]]: [https://web.archive.org/web/20071030190754/http://www.wpr.org/book/070415a.html Cosmic jackpot] – Interview about the anthropic principle (starts at 40 min), 15 May 2007.
{{refend}}

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