Going back in time may seem like an impossibility, but so did air travel at one point. According to Mark Thompson, the astronomy guy from BBC's The One Show, it's all a matter of side-stepping Einstein's theory of relativity.
As Thompson explains in a special guest article written for Discovery, time travel is already possible. It has been achieved. It's over and done with. We can all go home now, or several moments later than now, all thanks to the concept of time dilation.
Time dilation is a phenomenon that is describe in Einstein's theory of relativity, in which a stationary object moves faster through time than a moving object.
Thompson uses the example of an experiment that's been performed with atomic clocks to illustrate the phenomenon. One atomic clock was left on the ground, while another was taken up in a high speed aircraft. When the clocks were united, less time had passed on the one that got the plane ride.
Basically, the faster we move, the slower time passes. If we travel fast enough, we go backwards. Simple enough, right?
Unfortunately there is a fly in the ointment, an effect of Relativity is that the faster you go, the heavier you become and as you reach the speed of light, you become infinitely heavy! To accelerate something that is infinitely heavy requires an infinite amount of energy. Therein lies the problem. There isn't an infinite amount of energy in the Universe and if there was, it's a little selfish of you to use it all in your fanciful flight backward in time!
Mr. Thompson might not be counting the theory of relative caring, which states that the closer I get to traveling back in time the less I care about what other people think of my methods, but this is not a problem that can be solved with sass.
This is a job for quantum physics!
There may however be an alternative. Einstein's theory's of relativity only precludes travel at the speed of light, he never said anything about travel faster than light. Of course it will still take an incredible amount of energy, but crucially not an infinite amount.
Thompson is talking about quantum tunneling. It's the process used by electrons to move between two points without traversing the space between. It's pretty much the basis for the quantum communication system used in Mass Effect 2 that allowed the Illusive Man to communicate with the Normandy at all times.