求對時間的二階導數，空間變量就進來了。用拉格朗日力學寫方程，就會有時間空間的混合導數。愛因斯坦就這麽順著寫，

來源: 蔣聞銘於 2024-11-07 18:14:40 [博客] [舊帖] [給我悄悄話] 本文已被閱讀：次

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