"You see, our universe is kind of like a finely tuned machine. Scientists have found that there are about 20 numbers, 20 fundamental constants of nature that give the universe the characteristics we see today. These are numbers like how much an electron weighs, the strength of gravity, the electromagnetic force and the strong and weak forces. Now, as long as we set the dials on our universe machine to precisely the right values for each of these 20 numbers, the machine produces the universe we know and love.
But if we change the numbers by adjusting the settings on this machine even a little bit... the consequences are dramatic.
For example, if I increase the strength of the electromagnetic force, atoms repel one other more strongly, so the nuclear furnaces that make stars shine break down. The stars, including our sun, fizzle out, and the universe as we know it disappears.
So what exactly, in nature, sets the values of these 20 constants so precisely? Well the answer could be the extra dimensions in string theory. That is, the tiny, curled up, six-dimensional shapes predicted by the theory cause one string to vibrate in precisely the right way to produce what we see as a photon and another string to vibrate in a different way producing an electron. So according to string theory, these miniscule extra-dimensional shapes really may determine all the constants of nature, keeping the cosmic symphony of strings in tune."
~ Brian Greene, from The String's the Thing (Hour 2 of Nova's The Elegant Universe)