Before Einstein came along, classic physicists thought mass and energy were separate things. With the now-famous E = MC2 equation, Einstein showed that mass and energy are interchangeable concepts, and relate to each other through the square of the speed of light. There are two categories of relativity: special relativity, and general relativity. In special relativity, Einstein also showed that the speed of light is the absolute fastest speed any object can travel in the universe, precisely at 299,792,458 meters per second. In general relativity, Einstein postured that mass not only creates a gravitational field around them, they actually bend the space time around it. The more massive the object, the greater the bend. In fact, some objects such as black holes are so massive that time actually slows down around it.