With so little context and no actual reference (there are a lot of university physics books and they have a lot of pages - which one, which page?) it's impossible to know. My guess - and it is just a guess - is that the author is stating that time dilation is not merely the delay in clock readings from light travel time (if I look at a clock one light second away, it'll read one second slow because I don't see it as it is now, but as it was when the light left it). It's the discrepancy between clock rates once you have corrected for that lag due to the finite speed of light (which is a changing quantity for a clock moving with respect to the observer).Zephaniah said:Can someone explain this sentence to me? "Any differences between Δt and proper time are not caused by differences in transit times from those space time points to an observer at rest in S." From University Physics book.