The theory of relativity includes two interrelated theories by Albert Einstein: Special Theory of Relativity and General Theory of Relativity.
Simply put, the Special Theory says that the mass of an object depends on its speed. If a force acts on an object, it accelerates. But as it speeds up, more energy goes into increasing its mass and less into increasing its speed.
This prevents it from reaching the speed of light. One consequence is the equation E =square which says that mass and energy are interchangeable.
Though revolutionary, the Special Theory of Relativity was incomplete by itself as gravity was not accounted for.
Einstein remedied this in 1915 with his General Theory of Relativity. He replaced Newton’s space and time with a unified space-time. According to this, gravity was a property of space, not a force between bodies. The final form of general relativity was published in 1916.
Einstein’s theory revolutionized theoretical physics and astronomy.
Picture Credit : Google