why is it impossible to travel in time?

imagine if all your life you lived in a little black box. you've only known space to be able to extend its reaches till the walls of the box.

then one day someone breaks a hole into the box, and connects an MRT track to it. "in this you can go beyond the box", someone tells you, and extends a hand to you.

you take that hand, and before you know it, you're whizzing along land through space.
...

i know, the situations are hardly parallel, but the thought just occured to me on the train. watching the surroundings run past, a picture of me going through time suddenly popped up. thats how it always looks in sci-fi shows isnt it? >.<