 Suppose we wish to measure a single fermion. We model this as a wavefunction that zips by the locations of each fermion in our measurement device, interacting locally with the individual fermions in the device as it goes (Figure 1.5.2). The interaction is set up in such a way that every fermion is passed in finite time, and such that after the process is completed, the measurement device indicates what the original state of the fermion being measured was. In particular, suppose the single fermion begins in a \(z\)-spin-up state. Then, after it has zipped by each of the infinite fermions, they will all be found in the \(z\)-spin-up state.  If the single fermion begins in a \(z\)-spin-down state, then the infinite collection of fermions would all be \(z\)-spin-down. What if the single fermion was in a superposition? Then the infinite collection of fermions would contain some mixture of \(z\)-spin up and \(z\)-spin down states.