Quantum Entanglement in Gauge Theories

Quantum entanglement between different spacetime regions tells us how strongly correlated the regions are. However, the basic definition of entanglement does not work in gauge theories, as the underlying Hilbert space is not a tensor product space. The Standard Model is a gauge theory (and so are other important theories), so it would be nice to have a suitable redefinition. This essay is about redefining entanglement entropy to mitigate this problem (and what works and what does not).