The first theory of relativity is Classical relativity. It proposes that objects move relative to each other. The earth, sun and the galaxy are in constant motion and hence all objects move relatively. Time dilation occurs together with length contraction resulting in the speed of light being constant for all. Einstein combined these two entities in his concept of ‘space-time’. Time travels slowly for objects that move closer to the speed of light and stops completely for those that move at the speed of light. Hence, time should go backwards if one were to travel at a speed greater than light. However, no object can yet travel at the speed of light. As an object starts reaching the speed of light, it increases in mass, thereby requiring greater energy to travel. This explains why we cannot go back in time. Einstein also theorized that objects warped space-time around it, making it curved, which in turn resulted in gravitational attraction between objects. This attribute has been used in many modern day technologies.