Time Travel

To actually travel to the past or future, one must first construct a wormhole.  Wormholes contain negative energy, which hasn't been discovered or controlled to date, therefore making the shenanigans you see in Doctor Who impossible to complete in real life.

     Then there are time paradoxes.  Depending what you do in the past could trigger events in the present.

     For example:  I travel back to when the dinosaurs roamed the earth and killed a butterfly.  By doing this, I could cause a zombie apocalypse.

     The joys of sci-fi.