Special Relativity does not agree with Newton's Law of Gravity that states when forces acted upon each other, it is in fact instantaneous. In order that these two theories can work together, the effects of gravity must move at or less than the speed of light and not instantly. The idea of time dilation can be drawn from this theory. This is due to the increased speed on the rocket, time slows down for the rocket-bound twin and hence he is younger. One other derivative from these postulates is that simultaneity is simply relative.
Simultaneity depends on someone s inertial frame of reference.
Events can no longer be said to occur at the same time. Previously, the conservation of mass and energy were separate entities, but now they are combined into one single law. In 1915, Einstein unveiled his Theory of General Relativity that dealt with gravitational forces in terms of an object's curvature. The background of this theory is four dimensional in what can be called space-time. The points on this sort of plane are referred to as events.