Ziggy Stardust
Absolutely Sane
Think about dutchfire's last sentence. No matter how advanced it is, a computer's calculation is based on the interaction of elementary particles. Without approximation of any kind, any computer can at most simulate itself - not a whole universe that also happens to include a copy of itself.
The idea is that it doesn't represent the current Universe but as it was in the past. The past being the time we live in.