The theorem mentioned is Nielsen-Ninomiya. A good analogy is aliasing in signal processing. It's not that you can't define a theory and compute it. It's that un

The theorem mentioned is Nielsen-Ninomiya. A good analogy is aliasing in signal processing. It's not that you can't define a theory and compute it. It's that under certain conditions, you get more than one electron, you will get electrons that behave like electron does but they are different, and they are an artifact of the discretization. This is similar to aliasing in signal processing. A high frequency signal behaving like low frequency. And there are solutions to this problem, but it's hard to reason which one is the "right" solution.