Hi guys, I'm confused with proper time. Because t=t0 * lambda, proper time t0 is usually smaller than the relativistic time right? Also, proper time is measured by an observer whose frame of reference is at rest right? (Can it be measured by an observer in an inertial frame of reference? Like one in constant speed). In the case of an observer on earth measuring the time taken of an airplane travelling at constant speed, which is the proper time to be taken? At first look, I wanted to take the proper time as the time taken by the observer at rest on earth but that would give me a greater time for the plane travelling, which is wrong since time runs slower in the plane (so it should have a smaller time quantity for a given time measured by a non-moving observer) if that is the case, then the plane should have the proper time right? Why is this so? Is my definition of proper time wrong? Also, if the plane is initially accelerating to reach its cruising constant speed, then how can I take it as my proper time? Thanks a lot!