I haven't tried it yet (hopefully will in a few days), but assuming

V(new) = V(old) + Acceleration*Delta_Time
P(new) = P(old) + V(new)*Delta_Time

I cannot see too much trouble with negative Delta_Time values so long as the 
Acceleration term is constant. Environmental collisions shouldn't be too much of 
a problem either. Am I missing something here?

As for saving particle data for each frame, I do this sometimes to make 
re-rendering a slow-parsing animation faster (eg. if I want to change how the 
particles are textured). I can typically save data for 1500-2500 particles before 
the data files get larger than a 320X240 frame saved in an uncompressed format. I 
don't see this as a major problem considering hard disk sizes nowadays.

MJL

Rune wrote:
> Slowing down to a stop I can understand, but I can't see how you can
> make it run backwards, without having to recalculate everything from
> scratch in those frames that run backwards (which would parse very
> slowly). Unless of course you save the data for each calculation step so
> that you have at once the data for the entire simulation, as opposed to
> just for one frame at a time. But that can require quite big amounts of
> space for simulations with many particles...

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