Anthropic Principle | Universe Today

Written by Jean Tate

The universe seems curiously fine-tuned to suit lifeforms like us humans. Why? Because if it weren't, we wouldn't be here to discover that! And that's the anthropic principle in a tweet.

As you might guess, the anthropic principle is not considered science by some scientists (it's philosophy), because (they say) it's not testable, even in principle. There are also several different variants, with names like weak anthropic principle, strong anthropic principle, and participatory anthropic principle. Oh, and it's a bit misleading … none of the coincidences (etc) studied in any variant have to do with us humans, or even carbon-based intelligent life, merely carbon-based life ('anthropo-' means 'man', or 'human').

So what are these coincidences, or fine-tuning?

The theories of physics, today, have astonishing explanatory and predictive power; just about everything ever observed in the universe (within the domain of astrophysics, physics, etc) can be accounted for by these theories. However, there are some 25 constants in these theories which seem arbitrary; they cannot be derived from any other theory. One example is the fine-structure constant, which describes the strength of the electromagnetic interaction; if it were even a few percent different, carbon-based life would be impossible, because carbon could not be produced in stars. Another example is the cosmological constant; if it were much larger, the universe would have undergone a big rip well before stars could have formed.

One way to look at the anthropic principle is as something rather obvious and not terribly interesting (of course we live in the goldilocks era, the universe is not too young – it takes time for enough carbon to be made and a main sequence star like our Sun to form, with its planetary system – and not too old – when main sequence stars like our Sun have all evolved into white dwarfs and no habitable planets are left!); another is to say that "observers are necessary to bring the universe into being" (Brandon Carter). Some say there is considerable confusion, which can be traced to a misunderstanding of observation selection effects (the philosopher Bostrom, for example). The term anthropic principle was first used – in cosmology – by Brandon Carter, in 1973; perhaps the most widely known book on the topic is Barrow and Tipler's 1986 "The Anthropic Cosmological Principle".

The anthropic principle comes up in several Universe Today stories, although it is not always actually named. It pops up in book reviews (such as The Cosmic Landscape, and Many Worlds in One), and in multi-verse stories (e.g. What If There Is Only One Universe?).

The Anthropic Principle is covered, briefly, in the Astronomy Cast episode The Fermi Paradox: Where Are All the Aliens?.

Filed under: Astronomy

Tags: Cosmology, life in the universe, The Universe