The gravitational field of a massive particle can be represented by straight lines spreading out radially from the center of the particle. This gives us a picture showing the origin of the inverse square law introduced in Unit 3. The density of field lines decreases as the square of the distance from the particle, so the gravitational attraction falls off as the distance squared. But what if there is a compact extra dimension hidden from our view? In that case, the gravitational field will look like this picture when viewed from distances large compared to the size of the extra dimension, but will fall off faster on distances shorter than the extra dimension's size. (Unit: 4)