It‘s pretty simple. In places with high gravity, time goes slower. In fast moving containers (car, plane, spaceship), mass+gravity are also increased. This is t

It‘s pretty simple. In places with high gravity, time goes slower. In fast moving containers (car, plane, spaceship), mass+gravity are also increased. This is the reason why shining a light forward on a moving train doesn‘t make the photons the speed of light + the speed of the train. The speed of the light emitted from the flashlight is slowed down by the mass/gravitation increase through the movement of the train. This way, travelling to the future is pretty easy by the way. Just travel in a vehicle almost at the speed of light and the outside worlds time will move faster (relatively), since you are slowed down.