A spacecraft flies away from earth with a speed of 4.8 million meter per second relative to the earth, and then returns at the same speed. The spacecraft carries a clock that has been synchronised with a clock on earth, When the spacecraft returns to its starting point 1 year later as measured by the clock on earth, what is the difference in elapsed times in the two clocks?
Ok, so the problem for me here is determining the proper time, I figured that since the spacecraft is accelerating the correct proper time $t$ should be the time on earth ( 1 year ) and $ t' $ should be the time elapsed on the rocket. But when I use the formula for time dilation this way I get the wrong answer however if I assume that the proper time is the one on the spacecraft I get the right answer. My question is why is the proper time the one on the spaceship