If the strong nuclear force which glues atomic nuclei together were only a few per cent stronger than it is, stars like the sun would exhaust their hydrogen fuel in less than a second. Our sun would have exploded long ago and there would be no life on Earth. If the weak nuclear force were a few per cent weaker, the heavy elements that make up most of our world wouldn't be here, and neither would you.
If gravity were a little weaker than it is, it would never have been able to crush the core of the sun sufficiently to ignite the nuclear reactions that create sunlight; a little stronger and, again, the sun would have burned all of its fuel billions of years ago. Once again, we could never have arisen.
Such instances of the fine-tuning of the laws of physics seem to abound. Many of the essential parameters of nature - the strengths of fundamental forces and the masses of fundamental particles - seem fixed at values that are "just right" for life to emerge. A whisker either way and we would not be here. It is as if the universe was made for us.
What are we to make of this? One possibility is that the universe was fine-tuned by a supreme being - God. Although many people like this explanation, scientists see no evidence that a supernatural entity is orchestrating the cosmos. The known laws of physics can explain the existence of the universe that we observe. To paraphrase astronomer Pierre-Simon Laplace when asked by Napoleon why his book Mécanique Céleste did not mention the creator: we have no need of that hypothesis.
Another possibility is that it simply couldn't be any other way. We find ourselves in a universe ruled by laws compatible with life because, well, how could we not?
This could seem to imply that our existence is an incredible slice of luck - of all the universes that could have existed, we got one capable of supporting intelligent life. But most physicists don't see it that way.
The most likely explanation for fine-tuning is possibly even more mind-expanding: that our universe is merely one of a vast ensemble of universes, each with different laws of physics. We find ourselves in one with laws suitable for life because, again, how could it be any other way?
The multiverse idea is not without theoretical backing. String theory, our best attempt yet at a theory of everything, predicts at least 10 (times by the power of 500) universes, each with different laws of physics. To put that number into perspective, there are an estimated 10 (times by the power of 25) grains of sand in the Sahara desert.
Another possibility is that there is nothing to explain. Some argue that the whole idea of fine-tuning is wrong. One vocal critic is Victor Stenger of the University of Colorado in Boulder, author of The Fallacy of Fine-tuning. His exhibit A concerns one of the pre-eminent examples of fine-tuning, the unlikeliness of the existence of anything other than hydrogen, helium and lithium.
All the heavy elements in your body, including carbon, nitrogen, oxygen and iron, were forged inside distant stars. In 1952, cosmologist Fred Hoyle argued that the existence of these elements depends on a huge cosmic coincidence. One of the key steps to their formation is the "triple alpha" process in which three helium nuclei fuse together to form a carbon-12 nucleus. For this reaction to occur, Hoyle proposed that the energy of the carbon-12 nucleus must be precisely equal to the combined energy of three helium nuclei at the typical temperature inside a red giant star. And so it is.
However, Stenger points out that in 1989 a team at the Technion-Israel Institute of Technology in Haifa showed that, actually, the carbon-12 energy level could have been significantly different and still resulted in the heavy elements required for life.
There are other problems with the fine-tuning argument. One is the fact that examples of fine-tuning are found by taking a single parameter - a force of nature, say, or a subatomic particle mass - and varying it while keeping everything else constant. This seems very unrealistic. The theory of everything, which alas we do not yet possess, is likely to show intimate connections between physical parameters. The effect of varying one may very well be compensated for by variations in another.
Then there is the fact that we only have one example of life to go on, so how can we be so sure that different laws could not give rise to some other living system capable of pondering its own existence?
One example of fine-tuning, however, remains difficult to dismiss: the accelerating expansion of the universe by dark energy. Quantum theory predicts that the strength of this mysterious force should be about 10120 times larger than the value we observe.
This discrepancy seems extraordinarily fortuitous. According to Nobel prizewinner Steven Weinberg, if dark energy were not so tiny, galaxies could never have formed and we would not be here. The explanation Weinberg grudgingly accepts is that we must live in a universe with a "just right" value for dark energy. "The dark energy is still the only quantity that appears to require a multiverse explanation," admits Weinberg. "I don't see much evidence of fine-tuning of any other physical constants."