To accurately simulate a universe,
it would have to have variables of
-position
-velocity
-direction
of all photons, electrons, neutrons, positrons,
and possibly any other small things flying around.
On top of that,
the simulator would have to know EXACTLY
what happens when these particles crash into eachother.
What they produce,
and what direction and velocity the end particle(s)
will get in the process.
This is neccesary
because everything that happens,
affects, and thereby make, the future.
Needless to say,
we can probably not make even a 90% accurate simulator
within the next 50 years.
Using piss'ees wont make it easier ofcourse. 
