11 - E=mc²

Summary

Does the inertia of a body depend on its energy content?

Albert Einstein, title of his first paper on E=mc²

Energy, mass, and momentum reviewed

Easily the most famous result of relativity theory is the equation E=mc². In this chapter, we learn where the equation comes from, what it means, and what it does not mean. To do so, we need to be clear about what certain terms in physics mean, and so this section reviews the essential concepts. If you have not yet read appendix A, “Energy,” or have not done so recently, please read it before proceeding any farther in this chapter. Then this section will indeed be a review.

Because this section is a review, the topics are presented in telegraphic style.

Energy: the ability to do work, for example, to lift a weight. Energy is an attribute of a physical object or of whatever is contained in a specified region of space.

As figure 11.1 shows, a moving object can be used to lift a weight. Thus there is energy associated with motion, called “kinetic” energy, from the Greek verb “kinein,” meaning “to move.”

The higher the object starts, the more kinetic energy it has when it hits. Thus the greater the height, the more potential for kinetic energy at the bottom. Physics introduces the idea of a “gravitational potential energy,” an energy associated with position in the earth's gravitational field.