From the perspective of science, time travel was impossible in Newton’s universe, where time was seen as an arrow.  Once fired, it could never deviate from its past.  One second on Earth was one second throughout the universe.  This conception was overthrown by Einstein, who showed that time was more like a river that meandered across the universe, speeding up and slowing down as it snaked across stars and galaxies.  Therefore, one second on Earth is not absolute, and time varies when we move around the universe.

According to Einstein’s special theory of relativity, the faster a rocket moves, the more time slows down inside the rocket.  This makes time travel to the future possible, something that has been experimentally verified millions of times.  If an astronaut were to travel near the speed of light, it might take him only one minute to reach the nearest stars.  Four years would have elapsed on the Earth, but for him only one minute would have passed, because time would have slowed down inside the rocket ship.  Therefore, he would have traveled four years into the future, as experienced here on Earth.

The first serious attempt to explore time travel in fiction was H.G. Wells’ classic The Time Machine, in which the hero is sent hundreds of thousands of years into the future.  This journey is actually possible, and is consistent with the laws of physics.  All that would be necessary is a machine capable of traveling at extremely high speed. 

http://www.students.stedwards.edu/wgreine/time_machine2.jpg

The time machine used in the film adaptation of H.G. Wells’ novel. 

A real time machine would need to travel in space in order to travel in time.

Our astronauts actually take a short trip into the future every time they go into outer space.  As they travel at 18,000 miles per hour above the Earth, their clocks beat a tiny bit slower than the clocks on Earth.  Hence, after a yearlong mission on the space station, they have actually journeyed a fraction of a second into the future by the time they return to Earth.  However, traveling at the “slow” speed of 18,000 miles per hour won’t get you very far into the future.  The world record for time travel is currently held by Russian cosmonaut Sergei Krikalev, who orbited for 803 days and was hurled 0.02 seconds into the future.  Along the same lines, riding in a time machine made from a modified DeLorean going 88 miles per hour will take you into the future, but the difference between time experienced by a passenger and an observer on Earth will be even more negligible than the 0.02 seconds Krikalev traveled.

Sergei Krikalev, the current record holder for time travel

http://upload.wikimedia.org/wikipedia/commons/1/1b/Sergei_Konstantinovich_Krikalev.jpg