edit: clarity.
Imagine you have a billiard table, on which there's a wormhole. If you roll a billiard ball into the wormhole on the right trajectory, it will emerge three seconds prior to entering the wormhole, hit its earlier self a solid blow, and its earlier self will never enter the wormhole, creating a paradox.
But when you try, it emerges from the wormhole on an altered trajectory, and strikes itself only a glancing blow. And why did it emerge with an altered trajectory? Because it was struck a glancing blow.
This is the Novikov self-consistency principle [1]. Physicists have been unable to find initial conditions that don't allow a consistent solution. On the other hand, nobody has proven that consistent solutions always exist. And sometimes there are lots of consistent solutions. (I wonder whether that's why quantum physics isn't deterministic, but I'm no physicist.)
[1] http://en.wikipedia.org/wiki/Novikov_self-consistency_princi...
Using the above analogy the message sent from the future (the man exists) to the past will prevent the grandfather to be killed just like the decaying atom, maybe the messages send back and forth from the future rewrites the events to prevent the paradox, something like backtracking until it works.
Imagine that you are in Stalinist Russia and win a minor scientific prize for a paper you wrote. There is a purge, and while you manage to not get deported to Siberia, the prize is reassigned to a less brilliant but more politically orthodox colleague, to the point that he is listed as first author of your paper.
He goes on to have a better career because of this paper, and by the end of the affair, nobody remembers that you wrote the paper to begin with.
If all observable effects of an events have been retconned, can that event be said to have occured?
Can a similar situation happen in physics, and if so, does it "count" if only macroscopic effects are reversed? (To keep with the analogy, in this case, everything but your memory has been changed, but your memory hasn't been - although it might over time, due to you wanting to avoid cognitive dissonance).