Moreover, many of the central topics in Einstein's theory of special relativity can be understood only using algebra. For example, it turns out if you are traveling on a spaceship near the speed of light time actually slows down for you relative to your friends back on Earth. In other words, if you were to fly in a spaceship near the speed of light for some time and then you returned to Earth, you would find that you had aged very little while your friends on Earth have aged a great deal! Albert Einstein coined this phenomenon "time dilation" and it can easily be calculated using only Algebra. This effect is not a theoretical effect - it has actually been measured many times. In fact, the GPS system of satellites in the sky that the military and police forces depend on must take into account the effects of time dilation or else the system would not work at all! Because the satellites are moving in orbit around the Earth at speeds much smaller than the speed of light, the time dilation involved is very small - but it must be accounted for or the system would not function.