Relativity Lite

To c or Not to c  | 7 If your twin rides around in a spaceship at 111,600 miles/second for 40 years, 50 years will have passed for you when she returns! That may seem strange to you, but she is travel- ing at about 400 trillion miles/hour, something not exactly within your normal range of experience. Suppose your twin slows way down, to 1/10th of the speed of light (see figure 4). Her speed is v = 0.1 c , so the width of the rectangle is 1/10 of 10 cm or 1 cm. v t c t c τ c t Figure 4. The sequence of fours steps for v = 0.1 c . As you rotate the rectangle, you notice that c t = 10 cm is not much longer than c τ = 9.95 cm . This simply shows that the dilation of coordinate time (1.01 in this case) becomes unnoticeable at velocities that are small compared to c . So suppose your twin drives around at 70 miles/hour or 0.019 miles/second (figure 5). That is v = c /1000000 so the width of the rectangle should be 10 cm/1000000, or 80,000