Editing Anthropic principle (section)

== 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 Collins & Hawking (1973) characterized Carter's then-unpublished big idea as the postulate that "there is not one universe but a whole infinite ensemble of universes with all possible initial conditions".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: "...&nbsp;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.