The latest Bioshock Infinite developer diary has Ken Levine discussing the ghostly tears in reality that can be found throughout Columbia. The theory is that there are hundreds of alternative, parallel versions of the floating city, each with its own slight differences. Your companion, Elizabeth, can use tears in reality to reach into these parallel universes and pull objects into your world. In fights you can ask her to materialise any one of a number of different tears, giving you extra cover, a helpful turret, or even a mob of friendly fighters.
In the last clip, we see that Elizabeth has the power to revive the dead, and change reality completely if she wills hard enough. It's a considerable step up from the occasional apparitions that would appear in the underwater city of Rapture in the first game. For more on Bioshock Infinite, check out the previous dev diary showcasing Columbia's Sky-line firefights, or read our E3 impressions.