Einstein

Albert Einstein's theory of special relativity is the closest thing we have to "real" time travel. The theory goes that if a man travels away from his twin at close to the speed of light, when he returns (also at close to the speed of light) he will find that he twin has aged more than he has. Because all processes — chemical, biological, mechanical, etc. — are constrained by the speed of light, the traveling twin's biological aging would slow as would the clocks aboard the vehicle in which he's traveling.