Can Quantum Physics Select a Here and Now?
Can Quantum Physics Select a Here and Now? Relativity does not abolish the local present. It abolishes the assumption that a local present automatically determines a global present throughout space. An ordinary quantum measurement is often described as selecting one outcome, or one branch, from a superposition. But could relativistic quantum physics contain a more ambitious operation—one that selects not only what happens, but also a spacetime “here and now” in which it happens? The short answer is that relativistic quantum theory contains several structures that come close, but no universally accepted sharp projection operator that performs both tasks. The closest operational object is a spacetime-indexed quantum instrument : a family of quantum operations whose outcomes may include both a detector result and a spacetime event. Objective-collapse theories can go further and treat such events as elements of physical reality, but they add structure beyond standard quantu...