To fully understand the twin paradox let's first look at the Einstein's Theory of special relativity. According to this theory, time isn't the same to all reference frames across the universe. From special relativity we obtain a general result which is: Δt0 is known as the proper time, Δt is the dilated time interval, u is the velocity of the reference frame and c is the velocity of light . According to this equation, an observer O measures a longer time interval than O' measures. The observer O' is at rest relative to a device that produces a time interval Δt0 . F or this observer, the beginning and end of the time interval occur at the same location, and so the interval Δt0 is known as the proper time. An observer O , relative to whom O′ is in motion, measures a longer time interval Δt for the same device. The dilated time interval Δt is always longer than the proper time interval Δt0 , no matter what the magnitude or direction of u .