Energy equals mass times the speed of light squared. This is the famous equation of Albert Einstein. It(51)to the category of the theory of relativity, and it equates energy with mass. All things are made up of atoms. When(52)of an atom travels at almost the speed of light(53)we put more energy into it to increase the speed, it begins to in crease in mass. The energy that makes it travel fast cannot make it travel(54)the speed of light—nothing(55)light can travel that fast- so the energy goes into the thing(56)and increases its mass. Energy(57)into mass. Why is the theory called the theory of relativity? A thing that is relative depends upon(58)else to identify it or to define it. In relativity theory we identify or define mass, time, and length(59)to the speed of light. When something is at rest, it looks(60)in length. However, when it travels at almost the speed of light, it becomes(61). Time also changes. However, the change in time is(62)to the change in length. The length of a thing becomes short(63)time becomes long. If you want to(64)young, relative to a friend, take a trip in a spaceship that travels at(65)the speed of light. And, although time and heartbeat seem ordinary to you in the spaceship, when you return, look at your friend Relative to you, he or she is old.