Ok in my class we just started reviewing relativity and I had a really hard time grasping it. My teacher says that time moves slower as you get closer to light speed, which I understand because time is kind of a made up variable (right?). The one thing I really have a hard time understanding is this. Imagine two clocks, one on Earth and one in a spaceship. The space ship travels at 0.5 x the speed of light and travels far out into space and comes back. Why when the clock in the spaceship comes back it becomes unsynchronized with the clock that was stationary on Earth. From my understanding the physical mechanics in the clock stay the same no matter how fast or far it travels. So even if it goes the speed of light for a really long time the second hand of the clock will still move forward every second. So even when it comes back, taking out any variable of part failures, the distance the hands of the clock moved should be the same. My teacher says different though. Can anyone help?