In second grade, my teacher made a statement that literally shocked me to the core. I have not forgotten it after all these years. She said, "God so loved the Earth that he put the Earth just right from the Sun -- Not too far or the oceans would freeze over and not too close or the oceans would boil." This was an epiphany for me. I thought "That's right - The Earth IS just right from the Sun!" This was an amazing observation, my first exposure to an astronomical argument. I could see that there was some truth in her statement, since Mars is a frozen desert, and Venus is scorching hot. So the earth is in the Goldilocks Zone region of space, the right distance from the sun, just right for life.
But today, I can view my second grade teacher's statement from a different point of view. Today, astronomers have identified over 500 planets orbiting other stars, and they are all too close or too far from their mother star. Most of them, we think, cannot support life as we know it. So it is unnecessary to invoke God.
But now, cosmologists are facing this paradox again, but from a cosmic perspective. It turns out that the fundamental parameters of the universe appear to be perfectly "fine-tuned." For example, if the nuclear force were any stronger, the sun would have simply burned out billions of years ago, and if it were any weaker the sun wouldn't have ignited to begin with. The Nuclear Force is tuned Just Right. Similarly, if gravity were any stronger, the Universe would have most likely collapsed in on itself in a big crunch; and if it were any weaker, everything would have simply frozen over in a big freeze. The Gravitational Force is Just Right.
This begs the question of how many of the Goldilocks zones there actually are. If you begin to count them, you will soon realize that there are so many of these instances, it simply boggles the mind. The chance that our universe would be randomly placed in so many Goldilocks zones has been compared to a jet airliner being torn apart by a tornado and then suddenly reassembling itself by chance.
The paradox is: why does our universe reside in so many of these Goldilocks zones? Is it because God loved the universe so much that he chose to place it precisely in all these zones? Some theologians think so. They cannot believe that our universe is an accident. It almost appears as if the universe knew we were coming.
However, there is another interpretation. In the same way that astronomers have discovered over 500 (dead) solar systems, perhaps there are billions of parallel universes, most of them unsuitable for life. Our universe is special, only in the sense that it makes life possible for human beings who can contemplate this question. In many of these other universes, there is no intelligent life to ask this question. In these parallel universes, the nuclear force, the gravitational force, etc. are either too strong or too weak to allow for life. So it is a matter of luck that we happen to live in a universe compatible with life.
There are two philosophies that you can consider that are consistent with everything that we currently know and understand about the universe we live in. The first is the Copernican principle and the other is the Anthropic principle. The Copernican principle says that there really isn't anything special about humans or our place in the universe. There is nothing special about our existence in that we exist amongst billions of stars and perhaps millions of planets. We are puny and insignificant. The Anthropic principle is exactly the opposite in stating that we are indeed special, so special that we are among only a handful of universes that have intelligent life.
It turns out that all these philosophical questions have relevance today in the debate over string theory. String theory is supposed to be a theory of everything which can unify all physical laws. But the weakness of string theory is that it has many possible solutions, perhaps an infinite number of them. Since string theory is a theory of universes, it means that there are perhaps an infinite number of parallel universes. If so, then which one do we live in? It seems that string theory cannot predict which universe we occupy, since there is no principle to distinguish between them.
For example, the amount of dark energy in the universe is huge, making up 73% of all matter/energy in the universe. String theory can easily generate dark energy. But it can generate an infinite number of possible universes with different amounts of dark energy. So which universe is ours?
There is one school of thought that says that string theory, plus a version of the Anthropic Principle, can predict the properties of the universe, so everything is okay. This makes some scientists uneasy (since the Anthropic principle does not appear to be typical scientific principle, since it seems to have no predictive power.) However, this might be the ultimate resolution of the problem. String theory predicts an infinite number of universes, but we need some Anthropic principle to determine our universe.
(My own point of view, however, is that string theory is not in its final form. It has been evolving ever since it was discovered by accident in 1968. What we need, I think, is a higher version of the theory. This is what I am working on now. To be continued...)