Out of interest I'm studying a book "A first course in general relativity" which is a great book in my opinion because it explains the subject very well. I'm a beginner though and I have a hard time understanding one particular thing mentioned quite early in the book. I'm attaching a scan of a small part of the text. I understand this part fine except for one thing. On the second page I'm attaching it says "A simple calculation shows this to be at t=sqrt((1-v²))". I dont understand how they arrive at this. Would someone be able to provide an explanation or some hints so that I will hopefully get it? It must be something pretty obvious because they say its simple but I just dont understand it at the moment. Thanks!