※ 引述《WaitElevator (等不到電梯)》之銘言：
: In its own frame of reference, a proton take 5 min to cross the Milky Way
: galaxy which is about 10^5 light-year in diameter.
: a.What is the approximate energy of the proton in electronvolts?
: b.About how long would the proton take to cross the galaxy as measured by an
: observer in the galaxy's reference frame?
: 請問要如何計算能量?
: 質子速度要怎麼算?
假設 protone和銀河的相對速度為v
(記住，銀河看protone和protone看銀河的速度大小是一樣的)
銀河原長 L0=10^5 light-year
在protone的坐標中變成 L(v) [用lenth contraction算]
在protone的坐標中花的時間就是 L(v)/v = 5 min
即可解得v
之後要算什麼都簡單
另外要注意單位的換算
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