Anthropic Principle

In General > s.a. constants; cosmology; observational cosmology; relativistic cosmology.
* History: The idea was proposed by Dicke in 1961, and the term coined by Brandon Carter in 1974; Has been a focus of controversy (even intense antipathy in some quarters) ever since; It has been claimed that it represents a departure from Copernicanism in cosmology.
* Idea: The claim that the ability to support life, and possibly intelligence, is a valid constraint on physical theories; It is like a modern version of the argument by design, as opposed to causality.
* Applications: Dimensionality of spacetime (three spatial dimensions are required for the stability of planetary orbits, and more than one time dimension would destroy causality – Tegmark); Considerations re constants of nature and contents of the universe.

Versions > s.a. multiverse; quantum cosmology.
* Weak: Relatively uncontroversial; Accepts the laws of nature and the values of the physical constants as given and claims that the existence of life then imposes a selection effect on where and when we observe the universe; For example, the current age of the universe cannot be less than the nuclear-burning time of a massive star, and cannot be much older than this (because the stars would have all burned out); Life can only exist when the universe has roughly its observed age; This is a logical consequence of our existence.
* Strong: The presence of observers imposes constraints on values of physical constants; Could be used to infer the existence of a creator who tailor-made the universe for our benefit, or in conjunction with the idea of an ensemble of universes generated by early-universe processes with different values of the constants; The idea is that we live in one that is conducive to life – highly speculative, but it makes the strong anthropic principle more palatable from a physical point of view, since it just becomes an aspect of the weak version.
* Refined: The observer is "weighted" according to the amount of information processed.
* Other versions: They differ partly on the issue of which observers are important in the selection process for universes (ants? extraterrestrials? only Homo sapiens?); Participatory; Final (Intelligent life, not necessarily human, will exist forever; Puts tight bounds on the future of the universe; advocated by Tipler).
* Views: (Stoeger) The weak anthropic principle is a logical necessity, but the strong version only makes sense if variations in initial conditions, values of the constants, or laws of nature allow some scope for anthropic selection; The multiverse proposal may accommodate this possibility; (Smolin) It is only justifiable if one has a theory that independently predicts the existence of different universes, and such a theory, to be scientific, must be falsifiable, such as his natural selection theory.
@ References: Hawking in(82) [weak]; Feoli & Rampone NCB(99)gq/98; Cirkovic & Bostrom ASS(00)gq/99 [final]; Kamenshchik & Teryaev a0705 [mesoscopic, and many-worlds quantum theory].
> Related topics: see Civilizations; foundations of quantum mechanics [micro-anthropic principle]; Large Number Hypothesis; solar system [comets]; spacetime topology.

References > s.a. cosmological-constant problem; information; neutrino.
@ General: Davies 82; Rosen AJP(85)apr; Rosen AJP(88)may; Thirring APH(88); Carter in(89); Abramowicz & Ellis Nat(89)feb; Gribbin & Rees NS(90)jan; Balashov AJP(91)dec-RL; Demaret & Lambert 94; Linde in(03)ht/02; Carter gq/06-in.
@ I: Gale SA(81)dec; Barrow & Tipler 86; Carr pw(01)oct.; Davies 06
@ Overview, status: Hogan RMP(00)ap/99; Müller ap/01; Bettini phy/04-in [history]; Cirkovic AAT(04)-ap/05 [epistemological].
@ Assessments: Smolin ht/04-in [no falsifiable predictions]; Hetesi & Balázs APPB(06)ap.
@ And fundamental physics: Agrawal et al PRL(98); Kane et al NA(02)ap/00 [strings]; Bjorken PRD(03)ht/02 [standard model]; Hartle in(04)gq [and quantum gravity]; Page PLB(09) [proton mass and charge]; Jenkins a0906 [fermion masses].
@ And cosmology: Roush SHPMP(03) [and cosmological principle]; Vilenkin in(07)ap/04 [cosmological constant prediction]; Wilczek hp/04-in [multiverse and dark matter]; Freivogel a0810 [dark matter abundance]; Ellis & Smolin a0901 [WAP and string theory, predicition of < 0]; > s.a. inflationary models.
@ Other related topics: Cirkovic SAJ(00)ap [and infinite past], FS(02)phy/01 [entropy fluctuation], AAT(03)ap [ancient origins]; Susskind ht/04 [and supersymmetry breaking]; Carter a0708-in [objective and subjective time]; Maor et al a0812 [and causal entropic principle].

"The anthropic principle is the duct tape of cosmology" – Rocky Kolb

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