Here is a further resource which explains the physics of virtual particle anti-particle pair creation rather well. In particular the Feynman diagram in my post above is explained well in a style that doen't shirk on the physics. No math to follow. There are parenthetical hints to those who have the math.
Virtual Particles: What are they? | Of Particular Significance
It looks like an interesting blog. I came across it when looking for the graph above. A link to getting started as a reader of this blog is here:
New? Start Here | Of Particular Significance
So, in response to gwangi's question, it sounds like the morph is allowed if the recombination time is sufficiently short such that it cannot be observed. To complete the picture we need to know how the uncertainty principle applies to the recombination time of virtual particles.
Very nice link,
Hope many check it out.
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Now this is all a little academic since it refers to a process that is not observable. But how this is used is that the Feynman graph would be summed over all possible durations of the virtual pair. For the shorter durations a larger energy conservation violation would be allowed, but it would have a lower probability. Once one does this summing process correctly to sompute something observable, energy conservation is restored. The low probability energy states that violate energy conservation can be higher energy or lower energy than the conserved energy which has higher probability.