作替换以考虑电磁场的存在后,才具有局域规范不变性
http://webcache.googleusercontent.com/search?q=cache:29yK2PddWacJ:vega.bac.pku.edu.cn/rxxu/teach/atom/lecture%2520notes/18_sm.pdf+%22%E5%9C%BA%E9%87%8F%CF%88%E4%B8%8D%E5%8F%AF%E8%A7%82%E6%B5%8B%22&hl=zh-CN