Physicists have so far had a good deal of success with this broad project of unification. I have already described how Newton understood that what causes the apple to fall from the tree is the same universal force (gravity) that controls the motion of the heavenly bodies across the sky. This was not at all obvious at the time, even though it might seem so to us today. Before Newton, it was believed that objects fell to the ground because everything had a ‘tendency’ to move to its ‘natural’ place—towards the centre of the world—and that the motion of the Sun, Moon, planets, and stars was
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