Editing Anthropic principle (section)

== Origin ==
{{Section rewrite|date=August 2023}}
The phrase "anthropic principle" first appeared in [[Brandon Carter]]'s contribution to a 1973 [[Kraków]] [[symposium]] honouring [[Nicolaus Copernicus|Copernicus's]] 500th birthday. Carter, a theoretical astrophysicist, articulated the Anthropic Principle in reaction to the [[Copernican Principle]], which states that humans do not occupy a privileged position in the [[Universe]]. Carter said: "Although our situation is not necessarily ''central'', it is inevitably privileged to some extent."<ref name="carter1973">{{Cite conference |first=B. |last=Carter |author-link=Brandon Carter |book-title=IAU symposium 63: Confrontation of cosmological theories with observational data |publisher=Reidel |year=1974 |location=Dordrecht |title=Large number coincidences and the anthropic principle in cosmology |volume=63 |pages=291–298 |doi=10.1017/S0074180900235638 |doi-access=free }}; republished online by Cambridge University Press (7 Feb 2017)</ref> Specifically, Carter disagreed with using the Copernican principle to justify the [[Perfect Cosmological Principle]], which states that all large regions ''and times'' in the universe must be statistically identical. The latter principle underlies the [[steady-state theory]], which had recently been falsified by the 1965 discovery of the [[cosmic microwave background radiation]]. This discovery was unequivocal evidence that the universe has changed radically over time (for example, via the [[Big Bang]]).{{citation needed|date=April 2024}}

Carter defined two forms of the anthropic principle, a "weak" one which referred only to anthropic selection of privileged [[spacetime]] locations in the universe, and a more controversial "strong" form that addressed the values of the fundamental constants of physics.

[[Roger Penrose]] explained the weak form as follows:

{{Blockquote|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, the argument ran, there would be no intelligent life around to measure the physical constants in question—so the coincidence had to hold, simply because there would be [[Sapience|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 many other places and times in which humans could have evolved. But when applying the strong principle, there is only one universe, with one set of fundamental parameters, so what exactly is the point being made? Carter offers two possibilities: First, humans can use their own existence to make "predictions" about the parameters. But second, "as a last resort", humans can convert these predictions into ''explanations'' by assuming that there ''is'' more than one universe, in fact a large and possibly infinite collection of universes, something that is now called the [[multiverse]] ("world ensemble" was Carter's term), in which the parameters (and perhaps the laws of physics) vary across universes. 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 taking it could provide at least a partial answer to a question seemingly out of the reach of normal science: "Why do the [[list of laws of science|fundamental laws of physics]] take the particular form we observe and not another?"

Since Carter's 1973 paper, the term ''anthropic principle'' has been extended to cover a number of ideas that differ in important ways from his. Particular confusion was caused by the 1986 book ''The Anthropic Cosmological Principle'' by [[John D. Barrow]] and [[Frank Tipler]],<ref>{{BarrowTipler1986}}</ref> which distinguished between a "weak" and "strong" anthropic principle in a way very different from Carter's, as discussed in the next section.

Carter was not the first to invoke some form of the anthropic principle. In fact, the [[evolutionary biologist]] [[Alfred Russel Wallace]] anticipated the anthropic principle as long ago as 1904: "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 |author-link=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 |edition=4th |location=London |publisher=George Bell & Sons |pages= 256–257 |bibcode=1903mpus.book.....W }}</ref> In 1957, [[Robert Dicke]] wrote: "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 |doi=10.1103/RevModPhys.29.363 |bibcode=1957RvMP...29..363D |issue=3 }}</ref>

[[Ludwig Boltzmann]] may have been one of the first in modern science to use anthropic reasoning. Prior to knowledge of the [[Big Bang]] Boltzmann's thermodynamic concepts painted a picture of a universe that had inexplicably low [[entropy]]. Boltzmann suggested several explanations, one of which relied on fluctuations that could produce pockets of low entropy or Boltzmann universes. While most of the universe is featureless in this model, to Boltzmann, it is unremarkable that humanity happens to inhabit a Boltzmann universe, as that is the only place where intelligent life could be.<ref name="carroll2017">{{Cite arXiv |eprint=1702.00850 |last1=Carroll |first1=Sean M. |title=Why Boltzmann brains are bad |year=2017 |class=hep-th }}</ref>{{Sfn|Bostrom|2002}}