Anthropic Principle - Observational Evidence

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), however in principle, Barrow's SAP cannot be falsified by observing a universe in which an observer cannot exist.

Philosopher John Leslie states that the Carter SAP (with multiverse) predicts the following:

• Physical theory will evolve so as to strengthen the hypothesis that early phase transitions 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 multiple universes will prove robust;
• Evidence that the universe is fine tuned will continue to accumulate;
• No life with a 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 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 we 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:

1. a particular multiverse with a "measure", i.e. a well defined "density of universes" (so, for parameter X, one can calculate the prior probability P(X₀) dX that X is in the range X₀ < X < X₀ + dX), and
2. 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 N(X) P(X). (A more sophisticated analysis is that of Nick Bostrom.) 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.