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OK, in 100 words - here goes :-)
Event 1: An electron collides with a positron (the antimatter equivalent of an electron), annihilate each other and release a photon of electromagnetic energy.
Event 2: A photon of energy disintegrates into an electron-positron pair.
The cause of the photon in 1 is the collision, the cause of the particle pair in 2 is the disintegration - conventional causality.
Imagine you then reverse the flow of time and watch the events backwards. What would you see? First a collision of an electron and a positron (event 2 backwards), then the spontaneous disintegration of a photon (event 1 backwards).
Time-symmetry!
What this boils down to is that you can treat a positron as an electron travelling backwards in time. Similarly for other antiparticles. Anything travelling at the speed of light experiences no 'flow' of time - it's all the same instant to a photon.
So our positron/electron interaction was actually the collision between an electron going 'forwards' in time with one going 'backwards' in time - they just happened to collide in our 'now'.
That's imagining only one electron and one positron, but we know there's an enormous amount of stuff in our now (both matter and antimatter), all of it interacting and giving rise to what we perceive as causality.
If we accept that the interaction of things in our past have consequences in our now and our future, then - by this symmetry - there are large amounts of stuff in our future that are interacting and affecting our now and our past.
We only perceive an 'arrow of time' pointing from our past through our now to our future because of the laws of thermodynamics.
Where it starts to get really weird is at the now-now - because the now-now is affected both by the traditional causality of things that have happened in our now-past and by things that will happen in our now-future.
It's like us dropping a stone into the pond of our now-now and the ripples from the event spreading out symmetrically into the now-past and the now-future. If the ripples going into the now-past from our now-now event interact with ripples from a still earlier event (the stone we dropped in last week) which created its own ripples that have been travelling 'forwards' with us in our now-now, then what we do today can affect what we did last week, which changes the effect of last week's event in the now-now.
Clear? :-)
--
simon
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