Universes and Planets

The other day, Ulf Danielsson from Uppsala’s Dept. of Theoretical Physics gave a one-hour talk for astronomers, titled “Strings in the Sky”. I will not recap the contents, because that would only reveal how little I know of that subject matter. Instead, let me pick out one certain aspect and metaphore that I found enlightening.

It seems that the dream of finding in string theory a theory for everything that explains the lesser laws as well as the size of natural constants, has been greatly dicouraged by the finding that there are far too many string theories around and that they allows for many (not all) kinds of universes. The fact that ours is well suited for forming stars, galaxies and eventually life, may therefore just be a selection effect in the sense that we would not be able to exist in most of the landscape of possible universes. This kind of reasoning is called the anthropic priciple and is far from new, although hardly accepted by some and touching deep philosophical questions.

The analogy used by Ulf to support that this, in spite of not being testable, is nothing unscientific, were planets and Kepler’s laws. In the beginning, Kepler tried to explain the distances of the planets from the sun by some principle and failed. The famous laws he found instead account for how planets move around stars and leave space for an infinite amount of different planetary systems. No deep reason is behind the fact that Earth is at exactly this distance from the sun. It may be (and probably is) different in other solar systems and they might well be unsuitable for life.

In the same way, other universes may work differently and it is only natural that we observe one that is suited for us. We have measured the distance from Earth to the sun and we have measured the fundamental constants of our universe. Nobody asks for a derivation of the former and as far as I understood it, less and less cosmologists have hope in finding one for the latter.