Relativity Lite

46 | Relativity Lite as you start upward and you will notice that you feel very heavy. As you near your destina- tion, also walk around the elevator and you will feel like you are on Mars. Figure 2a. Acceleration due to thrusters. Figure 2b. Acceleration due to gravity. This is called the principle of equivalence . Thus, the path of a light beam near a gravita- tionally attractive body must be curved just as is the light beam a passenger sees passing through an accelerating spaceship. One would like to illustrate the full four-dimensionality of this bending, but in a two-­ dimensional book, that is difficult to do. Artists have projected a sense of depth onto two-dimensional media by using vanishing points ever since Florentine architect Filippo Brunelleschi illustrated the Baptistery in Florence in 1415, but attempting to project four-­ dimensions onto two just leaves us with a mess. The solution seems to be to toss out one of the three spatial dimensions (north, west, or up), replace it with the time dimension, and project that trio onto the two dimensions of this book, as illustrated in figure 3.