I think it's good to periodically stretch out the ol' brain....
I've posted about this in the past, but here's some more info: Science hopes to change events that have already occurred
Dating back to Newton's laws of motion, the equations of physics are generally "time symmetric" -- they work as well for processes running backward through time as forward. The situation got really strange in the early 20th century when Einstein devised his theory of relativity, with its four-dimensional fabric of space-time. In this model, our sense that history is unfolding is an illusion: The past, present and future all exist seamlessly in an unchanging "block" universe.
"If you have the block universe view, the future and the past are not any different, so there's no reason why you can't have causes from the future just as you have causes from the past," says David Miller of the Centre for Time at the University of Sydney in Australia.
With the advent of quantum mechanics in the 1920s, the relative timing of particles and events became even less relevant. "Real temporal order in general, for quantum mechanics, is not important," says Caslav Brukner, a physicist at the University of Vienna, Austria. By the 1940s, researchers were exploring the possibility of time-reversed phenomena. Richard Feynman lent credibility to the idea by proposing that particles such as positrons, the antimatter equivalent of electrons, are simply normal particles traveling backward in time. Feynman later expanded this idea with his mentor, John Wheeler of Princeton University. Together they worked out a theory of electrodynamics based on waves traveling forward and backward in time. Any proof of reverse causality, however, remained elusive.
The article goes on to talk about the proposed experiment to see if an action can affect something in the past....
Or, you know, it may turn out that it is impossible to alter the past and also impossible to alter the future. Fun....