1. The problem statement, all variables and given/known data In a thought experiment, a train is moving at a speed of 0.95c relative to the ground. A pendulum attached to the ceiling of the train is set into oscillation. An observer T on the train and an observer G on the ground measure the period of oscillation of the pendulum. State and explain whether the pendulum period is a proper time interval for observer T, observer G or both T and G. 2. Relevant equations Lorentz transformations and special relativity equation? 3. The attempt at a solution I believe that T experiences the proper time period for the pendulum because we know that for points A and B that are static in space, a person stationary to either A or B measures the proper length and the person travelling from A to B at a speed of a fraction of light measures the proper time. However, since in this situation A and B are not stationary to G, they are stationary to T, therefore T measures the proper time for the oscillation period of the pendulum. Any help would be greatly appreciated. Thanks!