Principle of Equivalence

Principle of Equivalence

Albert Einstein once said: “I was . . . in the patent office at Bern when all of a sudden a thought occurred to me: ‘If a person falls freely, he will not feel his own weight.’ I was startled. This simple thought made a deep impression on me. It impelled me toward a theory of gravitation.”

Thus Einstein tells us how he began to form his general theory of relativity. The fundamental postulate of this theory about gravitation (the gravitating of objects toward each other) is called the principle of quivalence,which says that gravitation and acceleration are equivalent. If a physicist were locked up in a small box as in Fig. 13-17, he would not be able to tell whether the box was at rest on Earth (and subject only to Earth’s gravitational force), as in Fig. 13-17a, or accelerating through interstellar space at 9.8 m/s² (and subject only to the force producing that acceleration), as in Fig. 13-17b. In both situations he would feel the same and would read the same value for his weight on a scale.

Moreover, if he watched an object fall past him, the object would have the same acceleration relative to him in both situations.

Fig. 13-17 (a) A physicist in a box resting on Earth sees a cantaloupe falling with acceleration a 9.8 m/s² .(b) If he and the box accelerate in deep space at 9.8 m/s², the cantaloupe has the same acceleration rela-tive to him. It is not possible, by doing experiments within the box, for the physicist to tell which situation he is in. For example, the platform scale on which he stands reads the same weight in both situations.