About those particles moving at the speed of light and colliding (now I seriously wonder if their relative speed will be 2*c..., should have payed more attention to physics at school :P) in 2): It depends a lot on how you handle that collision. The naive approach would be to move entity foo first and entity bar second and after each movement, check if their is any intersection with another collider and get how far inside they are in the direction of the previous movement and then move them back accordingly.
Now with a sequential approach, foo will always reach his destination, while bar will always get stuck inside and be moved back. When doing the updates multithreaded however, it sometimes could also be the other way around, resulting in different positions for both. Or in the worst case, when both do the collision check at the same time, the collision would probably not be detected and they would stay inside each other and generate black holes or something. The speed part however is the real problem. Sequentially foo would just be placed somewhere far away first and then the same for bar without any collision, even if they should collide. Multithreading wouldn´t really make a difference.
The solution some physics engines hide as continuous collision detection is to stretch foo and bars bounding boxes along the whole way they are supposed to go and make intersection tests of those two bounding boxes and if they cross and according to their real speed and size would collide there, foo and bar would be placed accordingly. This behaviour should then be pretty much the same in sequential and multithreaded implementations, but multithreaded will need to synchronize their desired speed inbetween or do the calculation based on the previous speed. Thinking about it, sequential would also have to determine the speed of both before moving...

Sid will tell you more.


Good for you, because if you did, I am still mindfucked on how it would work with our goals for Rayne, which do not include the world and domination!

PS: I btw think that we DO update the entities multithreaded, at least in theory... and not sure how we handle updates depending on each other...