Anthropic_principle

In [[physics]] and [[cosmology]], the '''anthropic principle''' states that humans should take into account the constraints that human existence as observers imposes on the sort of [[universe]] that could be observed. Originally proposed as a rule of reasoning, the term has since been extended to cover supposed "superlaws" that in various ways ''require'' the universe to support intelligent life, usually assumed to be carbon-based, and occasionally to be specifically human beings. '''Anthropic reasoning''' involves assessing these constraints by analyzing the properties of universes with different [[dimensionless physical constant | fundamental parameters]] or [[laws of physics]] from the current one, and has frequently concluded that essential structures, from [[atomic nucleus | atomic nuclei]] to the whole universe, depend, for stability, on delicate balances between different [[fundamental forces]]; balances which occur only in a small minority of possible universes — so that ours seems to be [[fine-tuned universe | fine-tuned]] for life. Anthropic reasoning also attempts to explain and quantify this fine tuning. Within the [[scientific community]] the usual approach is to invoke [[selection effects]] from a real [[multiverse | ensemble of alternate universes]], which cause an '''anthropic bias''' in what can be observed; competing strategies, occasionally also called anthropic, include [[intelligent design]].

The anthropic principle has led to more than a little confusion and controversy, partly because several distinct ideas carry this label. All versions of the principle have been accused of providing simplistic explanations which undermine the search for a deeper physical understanding of the universe. The invocation of either [[multiple universes]] or an intelligent designer are highly controversial, and both ideas have been criticized by some as being presently [[Falsifiability|untestable]], and therefore not within the purview of contemporary science.

==Anthropic coincidences==
{{main|fine-tuned universe}}

[[Robert Dicke]]<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}}</ref> noted that the [[age of the universe]] as seen by living observers is not random, but is constrained by biological factors that require it to be roughly a "golden age".<ref>{{cite book | authorlink = Paul Davies | author = Davies, P. | title = The Goldilocks Enigma | publisher = Allen Lane| year = 2006 | isbn = 0713998830}} </ref> Ten times younger, and there would not have been time for sufficient interstellar levels of carbon to build up by [[nucleosynthesis]]; but ten times older, and the golden age of main sequence stars and stable planetary systems would have already come to an end. This explained away a rough coincidence between large dimensionless numbers constructed from the constants of physics and the age of the universe, which had inspired [[Dirac large numbers hypothesis|Dirac's varying-G theory]].

Later, Dicke reasoned that the density of matter in the universe must be almost exactly the [[critical density]] needed to stop the universe from recollapsing (the "Dicke coincidences" argument, see article on [[Dicke]]). It seems he was wrong: latest estimates are that matter has about 30% of the 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}}</ref> gave an anthropic explanation: he noted that the cosmological constant has a remarkably low value, some 120 orders of magnitude smaller than expected from particle physics (often described as the worst prediction in physics). However, if the cosmological constant was more than about 10 times the observed value, the universe would suffer catastrophic [[Cosmic inflation | inflation]], preventing the formation of stars, and, presumably, life.

The observed values of the dimensionless parameters (such as the [[fine-structure constant]]) that govern the [[Fundamental interaction|four forces of nature]] are [[fine-tuned universe|finely balanced]]. A slight increase in the [[strong nuclear force]] would bind the dineutron and the diproton and all the hydrogen in the early universe would have been converted to helium. There would be no water or the long-lived stable stars that are essential for the development of life. Similar relationships are evident in each of the four force strengths. If they are modified sufficiently the universe's structure and capacity for life is greatly affected.
A list of cosmological, chemical and physical "anthropic coincidences" is given by [[Hugh Ross (creationist)|Hugh Ross]]<ref>Ross, H., web site: [http://www.reasons.org/resources/apologetics/design.shtml#The_Universe_as_a_Fit_Habitat Design and the Anthropic Principle, section "The Universe as a Fit Habitat"]</ref>.

One of the best known examples of anthropic reasoning used in the prediction of cosmological phenomena was by [[Fred Hoyle|Fred Hoyle]]. He calculated and then reasoned that there must be an excited state at an energy of 7.6 million electron volts in the nucleus of carbon-12 since he, Fred Hoyle, a life form based upon carbon molecules, existed, then the resonance must also exist to create the carbon. <ref>
University of Birmingham [[http://www.np.ph.bham.ac.uk/research/anthropic.htm|Life, Bent Chains and the Anthropic Principle]]</ref> <ref>Rev. Mod. Phys. 29 (1957) 547 </ref>

==Origin of the anthropic principle==
The phrase "anthropic principle" was coined by the theoretical astrophysicist [[Brandon Carter]], in his contribution to a 1973 [[Kraków]] [[symposium]] honouring [[Copernicus]]'s 500th birthday. Carter articulated the Anthropic Principle as a reaction to overuse of the [[Copernican Principle]], which states that we are not at a special position in the Universe. As Carter says, "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 | date = 1974 | location = Dordrecht |title = Large Number Coincidences and the Anthropic Principle in Cosmology | pages = 291-298}}</ref> Carter was particularly reacting against the use of the Copernican principle to justify the [[Perfect Cosmological Principle]], that all large regions ''and times'' in the universe must be statistically identical. This principle underlay the [[steady-state theory]] which had recently been proved wrong by unequivocal evidence for radical changes in the Universe with time (starting with the [[Big Bang]]).

Carter defined two forms of the Anthropic principle, a "weak" one which referred only to anthropic selection of privileged space-time locations in the universe, and a more controversial "strong" form which referred to the fundamental parameters of physics.

[[Roger Penrose]] explains the weak form: {{quotation| 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 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 plenty of other places and other 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 or infinite collection of universes; what is now called a [[multiverse]] ("world ensemble" is Carter's term), in which the parameters (and perhaps the laws of physics) do vary from universe to universe. 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 as a pay-off it offers at least a partial answer to a question which seems to be out of the reach of normal science: "why do the fundamental laws take that particular form and not another?"

Since Carter's original paper, the term "Anthropic Principle" has been extended to cover a number of ideas which are different in important ways from the ones he espoused. Particular confusion was caused by the influential book ''The Anthropic Cosmological Principle'' by [[John D. Barrow]] and [[Frank Tipler]],<ref>{{cite book| author=[[John D. Barrow | Barrow J. D. ]] and [[Frank J. Tipler| Tipler, F. J.]]| year=1986| title=The Anthropic Cosmological Principle| publisher=Oxford Univ. Press| isbn = 0-19-282147-4}}</ref> which makes a very different distinction between "weak" and "strong" anthropic principle, as discussed in the next section.

Brandon Carter was not the first to invoke some form of the anthropic principle. For instance, Dicke wrote in 1957 that: "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 }}</ref> [[Alfred Russel Wallace]] anticipated the anthropic principle as long ago as 1903: "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> The anthropic principle is perhaps even echoed by [[Karl Marx]]'s theory of [[historical materialism]]: "The first premise of all human history is, of course, the existence of living human individuals. Thus the first fact to be established is the physical organization of these individuals and their consequent relation to the rest of nature."<ref>{{cite book | author = Marx, Karl | authorlink = Karl Marx | title = The German Ideology, Chapter 1 | year = 1845 | url = http://www.marxists.org/archive/marx/works/1845/german-ideology/ch01a.htm#a2}}</ref>

==Variants of the anthropic principle==
* Carter's '''Weak anthropic principle (WAP)''': ''"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" is a ''space-time'' position.

* Carter's '''Strong anthropic principle (SAP)''': ''"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'."'' The Latin tag ("I think, therefore the world is such [as it is]") makes it clear that "must" indicates a deduction from the fact of our existence; the statement is thus a [[truism]].

Quite different definitions of these terms were offered by Barrow<ref>{{cite journal | author = Barrow, J. | title = Anthropic Definitions | journal = Quarterly Journal of the Royal Astronomical Society | volume = 24| pages = 146-153}}</ref>, and variants of these were used in his book with Tipler<ref>Barrow & Tipler's section on definitions is quoted verbatim at [http://www.dhushara.com/book/quantcos/anth/anth.htm ''Genesis of Eden Diversity Encyclopedia'']</ref>:
* Barrow and Tipler's '''Weak anthropic principle''': ''"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 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> 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 number of [[spacetime#8|dimension]]s in the universe, and the [[cosmological constant]] — just the topics that Carter reserves for the SAP.
* Barrow and Tipler's '''Strong anthropic principle''': ''"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> This looks very similar to Carter's SAP, but for Barrow and Tipler, unlike Carter, the "must" is an imperative, as shown by their three possible elaborations of the SAP:<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]] dressed in the garb of contemporary cosmology. It implies that the purpose of the universe is to give rise to intelligent life, with the laws of nature and their fundamental constants set to ensure that life as we know it will emerge and evolve.
** "''Observers are necessary to bring the Universe into being''." Barrow and Tipler believe that this can be validly inferred from [[quantum mechanics]], as has long been suggested by [[John Archibald Wheeler]] (his "participatory universe").
**"''An ensemble of other different universes is necessary for the existence of our Universe''." Contrast this with Carter, who merely says that an ensemble of universes is necessary for the SAP to count as an explanation.

The first of these has of course been welcomed by proponents of [[intelligent design]]. Carter has protested that such [[Teleology|teleological]] readings "are quite different from, and even contradictory with, what I intended".<ref>Carter, B. 2004, "[http://arxiv.org/gr-qc/0606117 Anthropic Principle in Cosmology.]" Paper presented at the conference "Cosmology: Facts and Problems," hosted by the [[College de France]].</ref> The Barrow and Tipler SAP has also been rejected as a fundamental misreading of Carter by the philosophers [[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 | date = 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> 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 (called "observation" selection effects by Bostrom) — the necessity for observers to exist in order to get a result. He writes:
{{quotation | 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 effects are to be taken into account. | ''Anthropic Bias'', Introduction. <ref>Bostrom, N. (2002), op. cit.</ref>}}
* Bostrom's '''Strong Self-Sampling Assumption (SSSA)''': ''Each observer-moment should reason as if it were randomly selected from the class of all observer-moments in its reference class.'' 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 a perfect choice.
*Playwright and Novelist [[Michael Frayn]] describes a position which expresses one 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>The Human Touch Michael Frayn Faber & Faber ISBN 0571232175''</ref></blockquote>

== Character of Anthropic reasoning ==
Much confusion has been caused by Carter's decision to focus on a rather tautological component of his ideas. In fact anthropic reasoning is of interest to scientists mainly for what is only implicit in the formal definitions, namely that we should consider seriously universes with different values of the "fundamental parameters" — that is, the [[dimensionless physical constant]]s and the dimensionless initial conditions at the [[Big Bang]]. Carter (and others) argue that life as we know it would not be possible in most of these; in other words, that the universe is [[fine-tuned universe | fine tuned]]. Collins & Hawking (1973) characterise 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}}</ref> If this is granted, the anthropic principle provides a plausible explanation for the fine tuning of our universe: ''typical'' universes are not fine-tuned, but, if there are enough universes, a small proportion 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. In this sense it is in direct opposition to design arguments.

But how seriously can we take the multiverse? And which specific multiverse should we assume? — this question must be answered before any quantitative anthropic predictions can be made. Although philosophers have discussed related concepts for centuries, in the early 1970s the only genuine physical theory giving a multiverse of sorts was the [[many worlds interpretation]] of [[quantum mechanics]]. This would allow variation in initial conditions, but not in 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}}</ref>, and the [[multiverse (science)]] article. An important development in the 1980s was the combination of [[inflation theory]] with the idea 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, eroding the distinction between Carter's weak and strong principles. At the beginning of the 21st century, the concept of the [[string landscape]] gave a mechanism for varying essentially all the constants, including the number of spatial dimensions.<ref>strictly, 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]] with no free parameters: as [[Einstein]] said, "What really interests me is whether God had any choice in the creation of the world". Quite recently,
proponents of the leading candidate "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 | id={{arxiv|archive=astro-ph|id=0001197}} }}</ref> since there would be no free parameters to select. Ironically, string theory now seems to offer no hope of predicting fundamental parameters, and some of its exponents have resorted to invoking the anthropic principle (see [[#Anthropic principle in string theory|below]]).

Opponents of intelligent design are not limited to hypothesizing the existence of alternate universes: they may 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, 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.</ref> and Ikeda and Jefferys<ref>Ikeda, M. and Jeffreys, 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. This is discussed in more detail in [[fine-tuned universe]].

[[Paul Davies]] has discussed fine-tuning at length, and in his book ''The Goldilocks Enigma'' (2006) he summarises the current state of the debate in detail. He concludes by enumerating the alternative responses:

* ''A - The absurd universe'' - It just happens to be that way.
* ''B - The unique universe'' - There is a deep underlying unity in physics which necessitates the universe being this way. Some 'Theory of Everything' will explain why the various features of the Universe must have exactly the values that we see.
* ''C - The multiverse'' - Multiple Universes exist which have all possible combinations of characteristics, and we naturally find ourselves within the one that supports our existence.
* ''D - Intelligent Design'' - An intelligent Creator designed the Universe specifically to support complexity and the emergence of Intelligence.
* ''E - The life principle'' - There is an underlying principle that constrains the universe to evolve towards life and mind.
* ''F - The self-explaining universe'' - A closed explanatory or causal loop: 'perhaps only universes with a capacity for consciousness can exist'.
* ''G - The fake universe'' - We are living in a virtual reality simulation.

Omitted here is [[Lee Smolin]]'s model of [[cosmological natural selection]], also known as fecund universes, which proposes that universes have "offspring" which are more plentiful if they happen to have features common to our universe. Also see Gardner (2005).<ref>Gardner, James N., 2005, "[http://www.kurzweilai.net/meme/frame.html?main=/articles/art0647.html The Physical Constants as Biosignature: An anthropic retrodiction of the Selfish Biocosm Hypothesis,]" ''International Journal of Astrobiology''.</ref>

Clearly all of these resolve some aspects of the puzzle, and leave other questions unanswered. Followers of Carter would allow only option ''C'' as an anthropic explanation, whereas ''C'' through ''F'' are covered by different versions of Barrow and Tipler's SAP (and also ''G'' if considered a variant on ''D'').

The anthropic principle, at least as conceived by Carter, can be applied on scales much smaller than the whole universe. For instance 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 }}</ref> inverted the usual line of reasoning and pointed out that in 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 probably can allow only one or two low probability links. This conclusion has been disputed.<ref>{{cite journal | author = Feoli, A. and Rampone, S. | id = {{arxiv|archive=gr-qc|id=9812093}} | title = Is the Strong Anthropic Principle too weak? |journal = Nuovo Cim. | volume = B114 | year = 1999 | pages = 281–289 }}</ref>

==Observational evidence==
Can observational evidence be obtained for the anthropic principle, or rather for any particular version? Certainly not for Carter's WAP, which is merely good advice to the scientist and makes no debatable assertions. The obvious way to test Barrow's SAP, which says that the Universe is ''required'' to support life, is to check a number of different universes to see if they all supported life; but such a test is impossible because other universes are unobservable by definition.

[[John Andrew Leslie| John Leslie]]<ref>Leslie, J. (1986) op. cit.</ref> makes a number of predictions from the point of view of the Carter SAP (with multiverse):
* Developments in physics will strengthen the idea that early phase transitions occur probabilistically rather than deterministically [so there will not be a deep physical reason for the values of fundamental constants].
* Various methods for generating multiple universes will survive theoretical investigation.
* Claims of fine tuning will be borne out.
* Attempts to discover exotic (non-carbon-based) life will repeatedly fail.
* Mathematical studies of galaxy formation will confirm that it does depend delicately on expansion rate.

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}}</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 like 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 (i) a particular multiverse with a "measure", i.e. a well defined "density of universes" (so, for parameter ''X'', one can calculate the prior probability ''P(X0) dX'' that ''X'' is in the range ''X0 < X < X0 + dX''), and (ii) 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). The probability of observing value ''X'' is then proportional to ''N(X) P(X)''. (A more sophisticated analysis is offered by [[Nick Bostrom]]<ref>Bostrom (2002), op. cit.</ref>). A generic feature is that the expected values should not be "over tuned", i.e. if there is some perfectly tuned value (e.g. zero) we don't expect to be much closer to it than needed to allow life. The small but (apparently) finite value of the [[cosmological constant]] is often regarded as a successful prediction in this sense.

One thing that would ''not'' count as evidence for the Anthropic principle is anti-Copernican evidence that the Earth or the Solar System were literally in a special position in the universe (for possible hints of this see [[Copernican principle]]), unless there was some reason to think that said position was a necessary condition for our existence as observers.

== ''The Anthropic Cosmological Principle''==
The most thorough extant study of the '''anthropic principle''' is the [[controversial book]] ''The Anthropic Cosmological Principle'' by [[John D. Barrow]], a cosmologist, and [[Frank J. Tipler]], a mathematical physicist. This book contains an extensive review of the relevant [[history of ideas]], because its authors believe that the anthropic principle has important antecedents in the notions of [[intelligent design]], the philosophies 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 Hicks (1883).<ref>{{cite book| last=Hicks| first= L. E.| year=1883| title=A Critique of Design Arguments| location=New York| publisher=Scribner's}}</ref>

Barrow and Tipler set out in great detail the numerous anthropic coincidences and constraints which have been derived over the years, including many of their own original results. While the book is primarily a work of theoretical physics, it also discusses a variety of related topics in [[chemistry]] and [[earth science]]. They also use the anthropic principle to reject the possibility of [[extraterrestrial intelligence]].

Seeing little sense in a principle that requires intelligent life to arise but then lets it go extinct, they propose yet another version:
* '''[[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>

They state that while the FAP is a valid physical statement, it is also "closely connected with moral values". FAP places strong constraints on the structure of the Universe, 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 = 0385467982}}</ref> One such constraint is that the universe must end in a [[big crunch]], which seems unlikely in view of the more recent discovery of [[dark energy]]. 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)''': ''"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>

In fairness, the authors warn at the start that it should not be assumed that they are committed to the ideas they describe, and they specifically call their SAP and FAP "quite speculative".

==Criticisms==

A common criticism of Carter's SAP is that it is an easy get-out 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 = 0198519737 | year =1989}} Chapter 10.</ref>

Some applications of the anthropic principle have been criticized as an [[argument by lack of imagination]] for assuming that the only possible chemistry of life is one based on carbon compounds and liquid water (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}}</ref> The range of [[fundamental physical constant]]s allowing evolution of carbon-based life may also be much less restrictive than proposed.<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 | pages = 035006}} [http://arxiv.org/abs/hep-ph/hep-ph/0604027 preprint]</ref> propose a [[Weakless Universe]] in which the [[Weak nuclear force]] is eliminated apparently without significant effect, provided some adjustments are made in the other forces. However, some of the fine-tuned details of our universe would rule out complex structures of any kind — stars, planets, galaxies etc — if violated.

Carter's SAP and Barrow and Tipler's WAP are [[truism]]s or [[tautology | tautologies]], stating something not obvious to everyone yet trivially true. As such they are often criticised as an elaborate way of saying "if things were different, they would be different", which explains nothing. But discussion of anthropic principles implicitly posits that our ability to ponder cosmology at all is contingent on one or more fundamental physical parameters having numerical values falling within quite a narrow range, which is ''not'' a tautology, and neither is the postulate of an actual multiverse. Moreover, working out the consequences of a change in the fundamental parameters for our existence is far from trivial, and, as we have seen, can lead to quite unexpected constraints.

Critics of the Barrow and Tipler SAP claim that it is neither testable nor falsifiable, and thus is not [[scientific method|science]]. The same criticism has been leveled against the multiverse idea, although we have seen that proponents argue that it does make falsifiable predictions, albeit not very strong ones. A modified version of this criticism is that such calculations are in practice impossible because 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; moreover the prior distribution of universes as a function of parameters is too easy to modify 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 |pages = 201301 | year = 2006}} See also this [http://www.physorg.com/news83924839.html physics news story]</ref>

Carter himself<ref>e.g. Carter (2004) op. cit.</ref> has frequently regretted his own choice of the word ''anthropic'' as conveying the misleading impression that the principle involves humans specifically, rather than intelligent observers in general. 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 too grandiose for a relatively straightforward application of [[selection effects]].

Steven Jay Gould
<ref>{{cite conference| last=Gould| first=Steven Jay| year 1998| title=Clear Thinking in the Sciences |booktitle=Lectures at Harvard University}}</ref>
<ref>{{cite book| last=Gould| first=Steven Jay | year= 2002 | title=Why People Believe Weird Things: Pseudoscience, Superstition, and Other Confusions of Our Time }}</ref>, Michael Schermer <ref>{{cite book| last=Schermer| first=Michael | year= 2007 | title=Why Darwin Matters}}</ref>
and others have observed that known causes and effects seem to have been reversed in the Anthropic Principle. Dr. Gould compared the claim that the universe is fine-tuned for the benefit of our kind of life to claiming that hotdogs were originally made long and narrow so that they would fit modern hotdog buns, or that ships had been invented to provide homes for barnacles. These critics cite the vast store of physical and evolutionary evidence which shows that life has been fine-tuned by the universe, through natural selection, to match the conditions in which life exists. Fossil, genetic and other biological evidence abundantly supports the observation that life adapts to physics, not the other way around.

The paleophysicist Caroline Miller has 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> <blockquote>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 non-anthropocentric universe, there is no need for multiple universes or supernatural entities to explain life as we know it.</blockquote>

Paul Davies contends (http://www.abc.net.au/science/bigquestions/) that this criticism incorrectly characterises the claim being made. He argues that the claim is that the physical nature of the universe has to be fine tuned in a number of ways to permit any sort of life to exist, which can then adapt to its conditions - as one example, 'for life to exist there must be atoms'; and that if any one of several physical constants were different by small amounts atoms would not exist.

== Anthropic principle in cosmic inflation ==
{{main|Cosmic Inflation}}

A critique of cosmic inflation, questioning the very premise of the theory, was offered by Don N. Page<ref>{{cite journal | first = D.N. | last = Page | year= 1983 | title = Inflation does not explain time asymmetry | journal = Nature | volume = 304 | pages = 39}}</ref> who emphasized the point that initial conditions which made it possible that a thermodynamic [[arrow of time]] in a [[Big Bang]] type of theory must necessarily include a low [[entropy]] initial state of the Universe and therefore to be extremely improbable. The critique was rebutted by [[Paul Davies]]<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 | pages = 524}}</ref> who used an inflationary version of the anthropic principle. While accepting the premise that the initial state of the visible Universe (originally a microscopic amount of space before the inflation) had to possess a very low entropy value — due to random quantum fluctuations — to account for the observed thermodynamic arrow of time, he deemed it not a problem of the theory but an advantage. That the tiny patch of space from which our observable Universe grew had to be extremely orderly, to allow inflation resulting in a universe with an arrow of time, makes it unnecessary to adopt any ''ad-hoc'' hypotheses about the initial entropy state which are necessary in other Big Bang theories.

== Anthropic principle in string theory ==
{{main|String theory landscape}}

[[String theory]] predicts a large number of possible universes, called the backgrounds or vacua. The set of these universes or vacua is often called the "[[multiverse (science)|multiverse]]" or "[[String theory landscape|anthropic landscape]]" or "string landscape". [[Leonard Susskind]] has argued that the existence of a large number of vacua puts the anthropic reasoning on firm ground; only universes with the remarkable properties sufficient to allow observers to exist are beheld while a possibly much larger set of universes without such properties go utterly unnoted. [[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 = 0521848415}} [http://arxiv.org/abs/hep-th/0511037 preprint]</ref> refers to the Anthropic Principle as a "turning point" in modern science since, applied to the string landscape, it "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. and Rees, M. J. | title = Anthropic reasoning | journal = Science| volume = 309 | pages = 1022 | date = 2003}}</ref> respond that various ingredients of well-accepted theories will never be testable, and that the test of a physical theory is not that every aspect of it should be observable and/or testable, but rather that enough is observable and testable to give confidence in the theory's correctness.

== See also ==

* [[Big Bounce]]
* [[Doomsday argument]]
* [[Final anthropic principle]]
* [[Inverse gambler's fallacy]]
* [[Meduso-anthropic principle]]
* [[metaphysical naturalism]]
* [[Nick Bostrom]]
* [[Principle of mediocrity]]
* [[Rare Earth hypothesis]]

== Footnotes ==

{{reflist|3}}

== References ==


{{refbegin|2}}
*[[John D. Barrow | Barrow J. D. ]] and [[Frank J. Tipler| Tipler, F. J.]] (1986) ''The Anthropic Cosmological Principle''. Oxford Univ. Press. [[ISBN]] 0-19-282147-4.
* {{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}}
* {{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}}
* {{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 = }}
* {{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}}
* {{cite book | last = Hawking | first = Stephen W. |authorlink = Stephen Hawking | year = 1988 | title = A Brief History of Time | pages = p.174 | location = New York | publisher = Bantam Books | id = ISBN 0-553-34614-8 }}
* [[Victor J. Stenger | Stenger, Victor J.]] (1999), "Anthropic design," ''The Skeptical Inquirer 23'' (August 31 1999): 40-43
*{{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 = 0521785219}}
* {{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}} A simple anthropic argument for why there are 3 spatial and 1 temporal dimensions.
* {{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}}
* {{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}} Shows that some of the common criticisms of AP based on its relationship with numerology or the theological Design Argument are wrong.
* {{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.}}
* {{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 = 978-0809095230}}
{{refend}}

== External links ==

* [[Nick Bostrom]]: [http://www.anthropic-principle.com web site] devoted to the Anthropic Principle.
* 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] Kavli-CERCA Conference Video Archive.
* Tobin, Paul N., 2000, "[http://www.geocities.com/paulntobin/finetuned.html Is the Universe Fine-Tuned for Life?]" A critique of the Anthropic Principle from an [[atheist]] viewpoint.
* "[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].
* [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.]
* [http://www.epicidiot.com/evo_cre/13cards.htm Evolutionary Probability and Fine Tuning]
* [http://www.epicidiot.com/evo_cre/vr_privileged_planet.htm Critical review of "The Privileged Planet"]
* [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 resumé of the Anthropic Cosmological Principle.]" A critique of Barrow & Tipler.
* [[Paul Davies]]: [http://www.wpr.org/book/070415a.html Cosmic Jackpot] Interview about the Anthropic Principle (starts at 40 min), 15 May 2007.

[[Category:Cosmology]]
[[Category:Physical cosmology]]
[[Category:Philosophical concepts]]

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