It is sometimes said that the physical structure of our universe is exquisitely fine-tuned. If the relative strengths of the physical forces (gravity, electrostatics, etc) were just slightly different, the universe would be a vastly different place, and life as we know it could not exist.
This claim is often used as an argument for the existence of an ultimate creator: God. When creating the universe, God, being benevolent, tweaked the physical forces such that life was made possible. It can't be an accident, after all?
Reading a book that made this argument, I started thinking. Is it really true that the universe is well designed for life?
If one is sitting in an armchair in a nice house, sipping tea, the idea that the universe is basically a nice place may seem reasonable. However, if one looks beyond the tea cup, doubts may start to creep in. Let's ignore all those human issues with poverty, disaster, disease, war, oppression and conflict. Instead, let's look at the Big Picture. What about life in the universe as a whole? How friendly is the universe to life?
I decided to make a simple calculation. Consider the volume of the entire universe: In how large a fraction of it could life possibly exist?
Let's start with the planet Earth. It is, after all, the only representative of a life-hosting planet we currently know about. The volume where life can exist consists of the air, the water, the surface, and some depth into the ground: the biosphere. A reasonable value for the thickness of Earth's biosphere is about 23 km. Given the radius of Earth at 6371 km, I compute the volume of the biosphere as a spherical shell:
Volume of biosphere = 1.2 * 1019 m3
I then consider the likely number of Earth-like planets potentially harbouring life in our galaxy, the Milky Way, as a representative for all galaxies, with the help of a fairly liberal estimate using Drake's equation:
Number of Earth-like planets in a galaxy = 40 * 109
The number of galaxies in the observable universe as estimated with data from the Hubble telescope is:
Number of galaxies = 1012
Given an rough value of the volume of the observable universe,
Volume of universe = 4 * 1080 m3
we can now compute the fraction of the volume of the universe that can harbour life:
0.0000000000000000000000000000000000001 %
Or, as a scientist would write it: 10-37 %. Of course, this is a very coarse estimate; it might be off by several orders of magnitude. I don't think that affects the conclusion, however.
Let me try to compare this value with something we know about: If we let the volume of the Earth represent the universe, then the entire life-compatible volume of it would be just a tiny fraction of a single grain of sand. In fact, about a fifty-millionth part of a grain of sand. This is not easy to envision, but I have not been able to find a better comparison.
I think this gives some perspective to the notion that the universe was designed for life. The universe is, in fact, overwhelmingly hostile to life, since 99.9999999999999999999999999999999999999 % of it cannot harbour life.
If God had the goal of designing a universe fine-tuned for life, it appears that the implementation is somewhat lacking. If I were God, I'd be grateful that I don't actually exist. It's less embarrassing that way.