15.38***Asan observer moves through space with position x(t), the four-vector (x(t),ct) traces apath through space-time called the observer's world line. Consider two events that occur at points Pand Qin space-time. Show that if,as measured by the observer, the two events occur at the sametime t, then the line joining P and Qis orthogonal to the observer's world line at the time t; that is,(xP-xQ)*dx=0, where dx joins two neighboring points on the world line at times t and t+dt.