Again, the question of why comes up. Why is it that there are three extended dimensions and not four, five, six, or more? One answer to this question is known as the anthropic principle. In essence, it means that things are the way they are to allow for the existence of life. In two space dimensions, animals wouldn't be able to have a digestive track because it would split their body in half. In four or greater dimensions, the strength of gravity would decrease more rapidly than in three, making the orbits of planets very unstable. They would either spiral into the sun or be flung out of orbit. Scientists conjecture that three space dimensions and one time is the only combination in which life can exist. If we weren't alive, we would not be here to ask the question, "Why are things the way they are?"

It turns out that there is another explanation for the existence of three visible spacial dimensions in our universe. This explanation was put forward by Robert Brandenberger and Cumrun Vafa. They said that at the beginning of the universe, all the dimensions were curled up and wrapped around by strings. When a wrapped string and its antistring partner meet, they annihilate each other to form an unwrapped string. If this happened enough times, a curled-up dimension would no longer be restricted and would unfold. They went on to say that this only happened in the first three spacial dimensions because in the other dimensions the wrapped string - antistring pairs were unlikely to collide. Consider two points in one-dimensional space. If they move around, they are very likely to come into contact with one another. They are less likely to meet in two dimensions, and even less in three. This continues, with it being more difficult for the points to collide in higher dimensions. In more or less the same way, Vafa and Brandenberger realized, after three dimensions not enough wrapped strings were annihilated to allow these dimensions to extend.