““This unification of forces and motion has a simple consequence. In a particle theory, you can freely add all kinds of forces, so there is nothing to prevent a proliferation of constants describing the workings of each force. But in string theory, there can be only two fundamental constants. One, called the string tension, describes how much energy is contained per unit-length of string. The other, called the string coupling constant, is a number denoting the probability of a string breakdown into two strings, thus giving rise to a force; as it is a probability, it is a simple number, without units. All the other constants in physics must be related to these two numbers. For example, Newton's gravitational constant turns out to be related to the product of their values.””
