Enlightenment, knowledge, ignorance, temptation

Modified version of summary talk at the symposium Expectations of a Final Theory at Trinity College, Cambridge, 4 September 2005 A new Zeitgeist Our previous ‘Rees-fest’ Anthropic Arguments in Fundamental Physics and Cosmology at Cambridge in 2001 had much in common with this one, in terms of the problems discussed and the approach to them. Then, as now, the central concerns were apparent conspiracies among fundamental parameters of physics and cosmology that appear necessary to ensure the emergence of life. Then, as now, the main approach was to consider the possibility that significant observational selection effects are at work, even for the determination of superficially fundamental, universal parameters. That approach is loosely referred to as anthropic reasoning, which in turn is often loosely phrased as the anthropic principle: the parameters of physics and cosmology have the values they do in order that intelligent life capable of observing those values can emerge. That formulation upsets many scientists, and rightly so, since it smacks of irrational mysticism. On the other hand, it is simply a fact that intelligent observers are located only in a miniscule fraction of space, and in places with special properties. As a trivial consequence, probabilities conditioned on the presence of observers will differ grossly from probabilities per unit volume. Much finer distinctions are possible and useful; but I trust that this word to the wise is enough to it make clear that we should not turn away from straightforward logic just because it can be made to sound, when stated sloppily, like irrational mysticism.