In short, it is possible to build a time machine, but just because building something is possible, however, does not make it easy or likely. Albert Einstein's Theory of Relativity states that time and space cannot be separated but are dependent upon each other. Therefore, a time traveler must move through both time and space. For example, if a traveler went back in time six months but did not travel in space, he might step out of the time machine into outer space, because the Earth would be on the other side of the sun. Spacetime is the entity through which a time machine must travel.
Time travel is possible. The faster an object moves, the slower it ages compared to a stationary object. If the time machine moves close to the speed of light into outer space, it will return to Earth many years into the future.
When people gaze at the stars, they are seeing into the past, because it took light years for the light to reach Earth. In theory, the person in the time machine would travel through time but not age. Mankind has yet to build an engine that will propel a rocket that can carry a human at the speed of light or a rocket that would withstand the pressure and the heat that would result from doing so.
Science fiction aficionados are familiar with wormholes as a means not only for traveling through time but also for bridging the gaps between two distant places. In the Star Trek: Deep Space 9 television series, spaceships travel through wormholes in order to reach far corners of other universes. Wormholes can be better understood when one visualizes a piece of paper with a hole punched in either end. The distance between the holes is reduced by bending the paper and placing the holes one on top of the other. The small space between the two holes is the wormhole.
Black holes in space have an increased gravitational pull. Objects in space, such as comets, asteroids and even entire solar systems are being sucked into black holes. The gravitational pull of the black hole eventually bends light and changes — slows down — the speed at which light travels. If Einstein is correct that nothing can travel faster than the speed of light, then time itself has slowed down near the mouth of the black hole.
If a man were to fly a rocket or spaceship near the mouth of the black hole, he would find himself in a time machine, because the time in which he is traveling moves more slowly than the time in rest of the universe. As he pulls his ship away from the black hole, time would have moved faster away from the black hole, and he would, presumably, be in the future.