Myths
Absolute Space
Points of View
Twin Paradox
Einstein's Train
Andromeda Paradox
Time in Spacetime
Clocks & Time
Velocity of Light
Clocks & Time
It is said that clocks tell proper time, but what is proper time, and do clocks always tell it?
Proper time is a name given to a "timelike" spacetime interval between two events. It is called "proper time" because it is frame independent, and to distinguish it from the "coordinate time", which is time interval that an observer in an arbitrary rest frame would experience between those events.
However as we discover here, clocks only tell the proper time for events that are co-located. I.e. If there is any spatial distance between the events, the clock time will not match the proper time for those events. So, in fact, clocks actually tell the coordinate time. Hence it is more accurate to say that "a clock tells the coordinate time of its rest frame, which matches the proper time for spatially co-located events".
Furthermore, as coordinate time is what is displayed by clocks, it follows that coordinate time is "real" in the sense that it correlates to the actual passage of time that an observer would experience in a given frame of reference.
