Sir Isaac Newton was a physicist who discovered the famous Newton's Law of Motion. He also discovered the Theory of Gravity, according to which all the objects in the universe attracts all other objects with a force which is proportional to the product of their masses and inversely proportional to the square of the distance between them. He believed that space and time are separate entity. Albert Einstein built up on Newtonian physics and postulated the Theory of Special Relativity. He stated that the speed of light in vacuum is constant ( c = 299,792 km/s) for everyone, no matter how fast observer or the light source are traveling. Suppose two trains are moving parallelly in the same direction. The speed of the first train is 50 km/hr and speed of the second train is 100 km/hr. When a person from the first train measures the speed of the second train then he finds it to be moving at 50 km/hr (100 km/hr - 50 km/hr). In contrast, when a person travels in a spacecraft at 0.5 c ...