Mark Srednicki's Quantum Field Theory book has a very nice and often quoted passage in its introduction regarding two approaches to incorporating time in a rela

Mark Srednicki's Quantum Field Theory book has a very nice and often quoted passage in its introduction regarding two approaches to incorporating time in a relativistic theory. The problem is that non-relativistic QM treats space (position) as an operator while time as a sort of label. But relativity demands that space and time be on equal footing. So we can either make space a label (as is done in QFT), or time an operator (leading to string theory). The passage is long, but here is a Quora article that excerpts it: https://www.quora.com/Why-is-time-not-an-operator-in-quantum-mechanics-even-though-it-seemingly-is-an-observable https://www.quora.com/Why-is-time-not-an-operator-in-quantum...