We know that the speed of light, the charge of the electron, the mass ratio of the electron to the proton, the strength of Newton’s G, the relative weakness of the weak nuclear force (equivalently, the mass of the W and Z bosons relative to the mass of the proton and electron) take specific values. If we change these values even slightly at the time of the big bang, then the Universe would not have developed into one that could support our life: The sun would burn up too quickly, the chemistry of the periodic table would differ and would not support DNA. Nucleosynthesis would not have created the elements we need in the first place, neutrinos would orbit atoms as well as electrons, black holes would be everywhere, etc. etc.
The question is, suppose there’s a 4D graph of different values of G,h,c,e – which points in that graph correspond to universes that could support life, or intelligent life? The intuition seems to be that life (or even a recognisable universe) can only exist in a relatively narrow range of values of G,h,c,e compared to all possible values. So, why do we have the values that we do have?
The Weak Anthropic Principle stats that, for all we know, there are parallel universes out there, inaccessible to us, where radioactivity was so strong that every atom decayed in the first minute. There are universes out there where anything larger than a mouse collapses into it’s own black hole. We don’t find ourselves in those universes because they could not have supported us. We must find ourselves in a universe that is compatible with life, a kind of post-selection, so the argument about what percentage of the total space of allowed values of G,h,c,e support life is moot.
The Strong Anthropic Principle says that there is something fundamentally necessary about intelligent life, and that there’s some reason why the universe needs to create intelligent life, and so the other universes whose values of G,h,c,e exclude the development of intelligent life are excluded from being called into existence by some basic principle of physics that we haven’t discovered yet in detail, but which the authors would already label the ‘Strong Anthropic Principle’.
Of itself, it’s an interesting observation. Is there some new principle in Physics that corresponds to an anthropic principle? Fundamental physics has been basically cut and dried since the 1980’s … we know the basic laws, as far as we can tell, and all the work now taking place boils down to working out the consequences of those laws, and making educated guesses about where the next discrepancy between experiment and theory might be found, and so where the best places to look for discrepancies might be. The Anthropic Principle is a contribution to the question: what next? where to look?
If you wanted to be cynical, you could define a Weak Anthropic Tenure Principle, which states that, if you give philosophers of science permanent life-long jobs, some of them will create baseless, untestable theories to justify their existence. A corresponding the Strong Anthropic Tenure Principle that would state that there exists a deep underlying mechanism (called the ‘search committee’), that excludes the academics who can’t create baseless, untestable theories from ever becoming philosophers of science in the first place.
The closest that Physicists get to discussing anthropic principles is perhaps: Why does DNA coil clockwise versus anticlockwise? Why do the stereoisomers of the molecules in Oranges and Lemons taste different, although they contain the same atoms in the same numbers, arranged in mirror image of each other?
It turns out that the weak nuclear force favours a particular handedness of molecules at the level of about 1 part in 1,000,000, and that we are made from the molecules that are 1/1,000,000 lower in energy …