@Im>bE
The collision of one photon with another produces two photons. Neat, huh?
You wouldn't need to know everything to simulate the universe, just parts. That's why it's called a simulation. If it was perfect, it'd have followers and zealots. :-) You could really just get away with the broad effects of stuff that happens. Why simulate the orbits of our solar system exactly, when you can just worry about the total effect all the orbits have on the stuff a few light years away. Go far enough away and you can estimate an entire solar system's influence on the universe as a point.
What happens anywhere does affect everything else, sure, but not immediately. Light or gravity from anything happening 15 billion light years away will take 15 billion years to affect anything here. If you limit the scope of the simulation to only certain parts and eras, the stuff you need to worry about is smaller.
Physics is neat. :-D
