This chapter starts with four pages of algebra, that is designated as "Can Be Omitted." It translates the classical concepts of force, momentum, and energy into relativistic terms.
We included this chapter in the book because, out of these results came the principle of the equivalence of mass and energy, which surprised even Einstein.
When his theory led him to suspect that there may be this equivalence of energy and mass, Einstein confided to a friend, Conrad Habicht, "...This thought is both amusing and attractive, but whether or not the good Lord laughs at me concerning this notion and has led me around by the nose -- that I cannot know."
"Equivalence" means they are the same. It does not mean that one can be converted into the other. They do not need to be converted; they are both mass and energy at the same time.
Imagine that there is a box that is green on one side and red on the other.>
The box is both green and red, depending on which way you look at it.
We are accustomed to looking at the mass aspect of objects, and to look at the energy aspect of quantities like heat and motion or the stretch of a spring.
You don't convert a green box into a red one by walking from the green side around to the red side.
It does indeed seem like such a strange thing. Mass is a bunch of stuff; energy is what you have when you run. How can these be the same?
The equation for this equivalence is derived in this chapter. It is the famous equation, E = mc2, where c is the speed of light. This is an equivalence equation, not a conversion equation.
Here are some things that are true and one that is not:
Onward to Chap 12: The Lorentz Transformation
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