The theory of relativity encompasses two mutual theories by Albert Einstein, including, special relativity and general relativity. The theory of relativity tells us that space and time are not absolute, they are perceived by all observers equally. Instead, space and time which are actually 2 perceptions of a single 4D entity are perceived differently by different observers. Everything which is based on space or time measurements, such as length, velocity, duration of an event etc, becomes also relative. The theory of special relativity was formed by Albert Einstein in 1905. Special relativity is an explanation of how speed affects mass, time, and space. The theory includes a way for the speed of light to define the relationship between energy and matter — small amounts of mass (m) can be interchangeable with enormous amounts of energy (E), as defined by the classic equation E = mc^2. One of the many significances of Einstein's special relativity work is that time moves relative to the observer. An object in motion experiences time dilation, meaning that when an object is moving very fast it experiences time more slowly than when it is at rest. But special relativity mostly pertains to extremely large distances, speeds, and objects, uniting them in a "smooth" model of the universe. Events in special relativity react in a direct, specific, and local consequence. Compared to quantum mechanics, events occur in quantum leaps that have indefinite outcomes. Researchers have united quantum mechanics and special relativity and have come up with a quantum field theory. General relativity was formed in 1915 by Albert Einstein. General relativity is based on the idea that massive objects cause a distortion in space-time. The more massive an object, the more it warps the space around it. Researchers tried to connect quantum mechanics and general relativity, and they consider it to be one of the great unsolved problems in physics. General relativity says that instead of being an invisible force that attracts objects to one another, gravity is a curving or warping of space. The stronger the gravity field is, the slower time goes. And as an effect, space also bends in the presence of a strong field of gravity.