自牛顿以来，每个小学生都知道，苹果从树上掉下是因为存在引力。人们毫不怀疑地信奉着这条理论。然而，引力究竟是什么？它以什么状态存在？没有人能回答，也没有人能感知到它，我们只看到它带来的后果：苹果往下掉，地球绕太阳转……。牛顿把一个神秘现象——下落的苹果，换 Since Newton, every elementary schooler knew that Apple fell from the tree because of gravity. People believe this theory without doubt. However, what is gravitation? In what state does it exist? No one can answer, and no one can perceive it. We only see the consequences of it: Apple falls and the earth turns around the sun. Newton puts a mysterious phenomenon, the fallen apple, in exchange