If the portals in Portal could take you back in time, a) your mind would break, and b) it would look like this.
It's a video of a prototype made by game designer Arthur Lee, in which you can create portals by taking screenshots. Whatever you snapped is what you'll see through the portal. Where it gets braintingling, though, is that the portal will take you back to the time when you took that screenshot. In other words, the portals don't just fold space, they fold time as well. So that's nice.
As Mike Rose over at IndieGames.com points out , there's your Portal 3 right there.
