Hi, I've seen quite a few textbooks and lectures start by saying something like "to solve a quantum system of N particles would take the best computer in the world, to solve a quantum system of (N+a small number) of particles would take a computer bigger than the universe..", where they give specific numbers. An example of this is Wen's book. My question is which SPECIFIC back of the envelope calculations are they using to get these numbers? It's not something as simple as the Hilbert space for spin growing like 2^N or something so what are the specific assumptions that are made when coming up with these numbers? Anyone know?