當琴玄震動時形成駐波，這個時候它的相速是0.但是它的群速不是0.晶體中的光的相速可以超出光速。但群速不會。

所有跟帖： 

• This is hard to me since I am a mathematician/statistician -Euro-dollar- ♂ (241 bytes) () 11/07/2018 postreply 06:45:22

• String Theory is not mathematically rigorous in my understanding -Euro-dollar- ♂ (174 bytes) () 11/07/2018 postreply 08:27:17