Editing Anthropic principle (section)

== 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 interpretation of quantum mechanics associated with the ideas of [[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>}}