Earth's Motion and Symmetry Breaking

Applications of the concept of symmetry to explain physical phenomena go all the way back to the ancient Greek philosophers, if not further. As we discussed in Chapter 1, the Greeks had already figured out that the Earth is a sphere. Moreover, they had known that it rotates about its axis, because all the stars at night seemed to revolve around the Northern star, Polaris, and they thought that would be rather unlikely and they postulated that instead the Earth is rotating but the stars are fixed. They also thought (incorrectly) that the center of the Earth is not moving because they thought if the Earth were moving the positions of stars would change, unlike what was seen night after night. The fact that the center of the Earth seemed stationary bothered them and they looked for an explanation of it. They knew about the rotational symmetry enjoyed by spheres. Based on symmetry considerations, their view of the heavens placed the Earth at the center of the universe. They then argued that since the Earth is at the center of the universe, there is no preferred direction in which to move: If it were to move, they reasoned, it would break the rotational symmetry. In order to preserve the rotational symmetry, they concluded, it better not move. This line of reasoning led to the picture that the center of the Earth is fixed at the center of the universe.