On the Determination of Proper Time
  Through the analysis of the definition of the duration of proper time of a particle given by the length of its world line, we show that there is no transitivity of the coordinate time function derived from the definition, so there exists an ambiguity in the determination of the duration of the proper time for the particle. Its physical consequence is illustrated with quantum measurement effect. 
Bing Hurl, Zhi-Yong Wang Hai-Dong Zhang