Editing Anthropic principle (section)

== Observational evidence ==
No possible observational evidence bears on Carter's WAP, as it is merely advice to the scientist and asserts nothing debatable. The obvious test of Barrow's SAP, which says that the universe is "required" to support life, is to find evidence of life in universes other than ours. Any other universe is, by most definitions, unobservable (otherwise it would be included in ''our'' portion of ''this'' universe{{Undue weight inline|date=August 2023|reason=emphasis is probably undue when compared w/ MOS:DOUBT}}). Thus, in principle Barrow's SAP cannot be falsified by observing a universe in which an observer cannot exist.

Philosopher [[John Andrew Leslie|John Leslie]]<ref>Leslie, J. (1986) op. cit.</ref> states that the Carter SAP (with [[multiverse]]) predicts the following:
* Physical theory will evolve so as to strengthen the hypothesis that early [[phase transition]]s occur probabilistically rather than deterministically, in which case there will be no deep physical reason for the values of fundamental constants;
* Various theories for generating [[multiverse|multiple universes]] will prove robust;
* Evidence that the universe is [[fine tuned universe|fine tuned]] will continue to accumulate;
* No life with a [[alternative biochemistry|non-carbon chemistry]] will be discovered;
* Mathematical studies of [[galaxy formation]] will confirm that it is sensitive to the rate of [[expansion of the universe]].

Hogan<ref>{{Cite journal |author=Hogan, Craig |title=Why the universe is just so |journal=Reviews of Modern Physics |volume= 72 |year=2000 |pages=1149–1161 |doi=10.1103/RevModPhys.72.1149 |bibcode=2000RvMP...72.1149H |arxiv=astro-ph/9909295 |issue=4 |s2cid=14095249 }}</ref> has emphasised that it would be very strange if all fundamental constants were strictly determined, since this would leave us with no ready explanation for apparent fine tuning. In fact, humans might have to resort to something akin to Barrow and Tipler's SAP: there would be no option for such a universe ''not'' to support life.

Probabilistic predictions of parameter values can be made given:
# a particular multiverse with a "[[measure theory|measure]]", i.e. a well defined "density of universes" (so, for parameter ''X'', one can calculate the [[prior probability]] ''P''(''X''<sub>0</sub>) ''dX'' that ''X'' is in the range {{nowrap|''X''<sub>0</sub> < ''X'' < ''X''<sub>0</sub> + ''dX''}}), and
# an estimate of the number of observers in each universe, ''N''(''X'') (e.g., this might be taken as proportional to the number of stars in the universe).
The probability of observing value ''X'' is then proportional to {{nowrap|''N''(''X'') ''P''(''X'')}}. A generic feature of an analysis of this nature is that the expected values of the fundamental physical constants should not be "over-tuned", i.e. if there is some perfectly tuned predicted value (e.g. zero), the observed value need be no closer to that predicted value than what is required to make life possible. The small but finite value of the [[cosmological constant]] can be regarded as a successful prediction in this sense.

One thing that would ''not'' count as evidence for the anthropic principle is evidence  that the Earth or the [[Solar System]] occupied a privileged position in the universe, in violation of the [[Copernican principle]] (for possible counterevidence to this principle, see [[Copernican principle]]), unless there was some reason to think that that position was a [[necessary condition]] for our existence as observers.