Our universe seems to be fine-tuned (for intelligent life), because any small changes in the 20 or so physical constants would make it very different, and presumably not hospitable to intelligent life. For example, stars would not be able to fuse hydrogen and helium if the electric charge of the electron had been only slightly different, or if the strength of the strong nuclear force had been only 2% greater diprotons would be stable and hydrogen would fuse too easily.
Another related source of astonishment is that the early universe had a very low entropy, a very unlikely situation: statistical mechanics says that the most probable state of matter has a very high entropy, a state where life is not possible. The universe is now going towards that state, according to the second law of thermodynamics, resulting eventually in heat death. This paradox can be treated in the same way as fine-tuning.
Does "fine-tuning" need an explanation ?
Some, like Stephen Jay Gould, believe that fine-tuning does not need any more explanation than a particular roll of dice that would result in a double six. Our universe had to have physical constants, and they just happen to be the ones that permit our existence. Moreover, any other combinations of physical constants may have resulted in a very different universe, one in which a human-like civilization may not emerge, but who is to say that it would not be hospitable to other, very different, forms of intelligent life?
What are the possible explanations ?
There are actually three broad types of explanations: the universe is not fine tuned, the intelligent designer, and the multiverse. While the latter two are not incompatible with each other, accepting one of them makes the other one less necessary.
- The universe is not fine tuned explanation questions whether the universe is in fact fine tuned. The fact that a universe with different physical constants might be inhospitable to life as we know it does not necessarily mean that it is inhospitable to any form of life, and there is no known way of actually experimentally verifying whether a universe allows for life or not.
- The multiverse explanation assumes the existence of a mechanism that has created many universes with different physical constants, some of which are hospitable to intelligent life. And because we are intelligent beings, we would happen to be in an hospitable one. This approach has led to a lot of research in the anthropic principle. This explanation has been of particular interest to particle physicists because theories of everything do apparently generate large numbers of universes in which the physical constants are random.
- The intelligent design explanation assumes the existence of a being, principle or mechanism with the purpose to create a universe with intelligent life. It would thus somehow set the physical constants as we know them. This agent could of course be a God-like being, but it need not be.
How can we choose between the competing explanations ?
The usual test for a satisfactory explanation is that it fits all known observations, and that it can make testable predictions. It remains to be seen whether the fine-tuned universe enigma can be resolved satisfactorily.
- John Barrow (2003) The Constants of Nature, Pantheon Books, ISBN 0375422218
- Nick Bostrom (2002) Anthropic Bias: Observation Selection Effects in Science and Philosophy, Routledge, New York, ISBN 0415938589