From minutephysics, host Henry Reich explains six ways you can travel through time - three easy ways, and three slightly more complex ones:
They range from the implicit (standing around, doing nothing) to the slightly more complicated (constructing an infinitely long super-dense spinning cylinder that could only exist with a theoretical form of energy that isn't stable in our universe). Also, wormholes.
Scientists are pretty confident that while relativistic time travel is within humanity's grasp (and really, more of a pain-in-the-butt side effect of extremely fast travel), the kind of technology that could result in your classic science fiction time machine is well nigh impossible. While relativistic time travel is capable of sending you thousands of years into the future, there's no mechanism to bring you back to current day. Outside of science fiction, effect can't precede cause. That means that much like Futurama's Fry, any traveler into the future would be stuck there, long after all of their loved ones had grown old and died, and after culture, politics, and society had long since evolved beyond their wildest imaginations:
But maybe you could get sent into an alternate universe with a different timeline! And then kill Hitler. Or, travel so far into the future that you end up in the past, then pat yourself on the back for being so clever.