The way I'd once heard it explained is that you could think of those particle/anti-particle pairs as a single particle stuck in a time loop, but then again I am pretty sure that book was published in the late 80s or something and I haven't read it in years, so my memory could be way off on top of the facts being way off. Then presumably if you fuck with one and not the other, it just makes that time loop obnoxiously complicated.
I know what you mean, particle goes forward from creation point to annihilation point and the anti-particle it annihilates is itself going back in time, with (from the anti-particle POV) the later point in time is the point of creation and the earlier the point of annihilation.
It needn't be a 'loop', but it can still involve bidirectional (or effectively undeterminably-directioned) time...
Imagine the following (with limitations of ASCII graphics not helping, but let's give it a go anyway.
P+ -----------------*====== energy
/
/
/
/ P-
/
energy =======*------------- P+
(Noting that where I've labelled "+" could equally have been labelled "-", with the "-" changed to "+". So particle/anti-particle could as easily be anti-particle/particle.)
Scenario 1: Particle is merrily travelling forward in time (left to right). Spontaneous creation of a particle/anti-particle pair, from who-knows-what stock of energy, happens nearby. The anti-particle of this pair annihilates with the pre-existing particle, paying back all the energy it was given, or borrowed.
Scenario 1a: Pretty much the same, except that everything's travelling backwards in time (right to left). Who's to say which direction everything is moving? (You know, as well as the troll race in Pratchett books, there actually
is at least one human tribe (Amazonian?) who think of the future as being behind them and the past as in front of them... because you can see what's in the past but you can't see the future. Not that this is relevant, just saying.)
Scenario 2: Particle travels forward until it 'reflects' at a point. This might be due to hitting some stray energy that may (or may not) consider time to be reversed. I don't think the universe need care about this, but it's the upper lot of energy in this diagram. Anyway, Particle bounces back in time as an anti-particle, which then has another 'bounce' (the energy opposing this being the lower line of energy in the diagram) to travel forward again as a 'new' Anti-Anti-Particle. Or 'Particle'.
Redking: "But wait...if the particles poof into existence, and then collide a femtosecond later, doesn't the energy released violate the law of conservation? Or does the energy poof out of existence too? And if so, where does it go?"
However you consider it (one particle or three, in the above; or one particle in an eternal back-and-forth/two particles involved in self0re-annihilation in a more standard way), it's conservation of matter+energy Energy may becomes matter, then back to energy, and it all equals out. Or matter's matter (whatever direction it's going) and energy is eternal in the tapestry of time, as it plays its part.
Bauglir: "Then presumably if you fuck with one and not the other, it just makes that time loop obnoxiously complicated."
It's a time loop. (Or a time-bounce.) If you're "fucking with it", then you're part of it, already involved in the loop, as far as anybody observing from 'outside of time' would be concerned. At least that's
one way to look at it. The universe basically works. Anything that didn't wouldn't be part of the viable universe, so we'd never see it.