Einsteins special theory of relativity invented a new perspective for physicists to consider, a combination of space and time, called spacetime. This new theory had to take into account how motion was defined and described from a persons differing perspectives, as they were observing the motion. If alpha stays on the Earth, the speed of light is definitely moving faster than he is, so normal aging will occur. Beta, on the other hand, is in a rocket that is moving at a speed comparable to the speed of light, which is how time is measured, so time essentially "stands still" for beta. Theoretically, as is measured by the passing of light, beta will not age as long as the rocket is traveling at the speed of light. Realistically speaking, he will probably age right along with his twin brother alpha. Einsteins theories superceded those of Sir Isaac Newton, whose laws of motion and law of universal gravitation were held as the standard for the last 200 years.