Relativity Lite

64 | Relativity Lite distant future, or perhaps into another universe. Figure 12 shows a wormhole connecting two regions of the same universe. With only two spatial dimensions shown, one is free to imagine the space portion bent around a curve in the time dimension and back the way it came. Thus, a trip that would take the spaceship eons by traveling leftward through space on the lower two-dimensional sheet—from a star near the wormhole, up around the curve, and back rightward—toward another star near the other end of the wormhole on the upper two-dimensional sheet could, in principle, be accomplished in much less time by traveling through the wormhole to the other two-dimensional spatial sheet, with the two openings of the wormhole separated by a time interval. The wormhole thus connects two very distant spatial places. Figure 12. A wormhole connecting two distant regions of the same universe, with only two spatial dimensions shown. Unfortunately, this would appear to be a one-way trip. Going back through would put her further into the future. So even if we had some brave volunteers to test these hypotheses, we would never know the answer. GRAVITATIONAL WAVES In 1916, Einstein predicted that the motion of massive objects would cause spacetime rip- ples, or gravitational waves. Figure 13 shows a pair of black holes orbiting each other, and their stirring of spacetime locally sends out an expanding ripple of gravitational waves.