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; It 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. Fine Tuning; 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 the values of physical constants; It 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; Ćirković & Bostrom ApSS(00)gq/99 [final]; Kamenshchik & Teryaev ONCP(08)-a0705, NQ-a1302 [mesoscopic, and many-worlds quantum theory].
> Related topics: see civilizations; foundations of quantum mechanics [micro-anthropic principle]; Large-Number Hypothesis; spacetime topology.

References > s.a. cosmological-constant problem.
@ General: Carter in(74); 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-conf.
@ I: Gale SA(81)dec; Barrow & Tipler 86; Carr pw(01)oct; Davies 06; Lewis & Barnes 16.
@ Overview, status: Hogan RMP(00)ap/99; Müller ap/01; Bettini phy/04-in [history]; Ćirković AAT(04)-ap/05 [epistemological].
@ Assessments: Smolin ht/04-ch [no falsifiable predictions]; Hetesi & Balázs APPB(06)ap; Schneider & Olum a1304; Lewis & Barnes a2104 [in defense of the anthropic principle].
@ 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 APPBS(09)-a0906 [fermion masses]; Gould a1207/ApJ [connection with the search for extrasolar life/intelligence]; Meißner IJMPE(14)-a1312-proc [chiral symmetry and fine-tuning of nuclear forces], ScBull(15)-a1409 [nuclear physics]; Page a1703 [anthropic estimates]; > s.a. spacetime dimensionality; standard model of particle physics [3 generations].
@ And cosmology: Roush SHPMP(03) [and cosmological principle]; Vilenkin in(07)ap/04 [cosmological constant prediction]; Wilczek hp/04-ch [multiverse and dark matter]; Freivogel JCAP(10)-a0810 [dark-matter abundance]; Ellis & Smolin a0901 [WAP and string theory, predicition of Λ < 0]; Loeb IJAB(14)-a1312 [argument against the anthropic explanation for a small cosmological constant]; Piran et al PRL(16)-a1508; > s.a. inflationary models.
@ And solar astronomy: Balbus PRS(14)-a1406 [anthropic consequences of equal lunar and solar angular radii].
@ Generalizations: Galantai a1501 [the universe as "roughly-tuned" for computation].
@ Related topics: Ćirković 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-conf [objective and subjective time]; Maor et al a0812 [and causal entropic principle]; Sivanandam a1002-FQXi [and the limits of physics]; Armstrong a1110 [anthropic problems and paradoxes, anthropic decision theory]; Benétreau-Dupin PhSc(15)-a1412 [and probabilistic reasoning].
> Other related topics: see fine-structure constant; information; many-worlds quantum theory; neutrino; solar system [comets].

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

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