I don't know how to imagine open sets in Minkowski spacetime. I have seen that there are many diffrent ways of constructing them — that's OK. But for example. which construction do people mean in the case of the Reeh-Schlieder theorem?
In such an example, do I have to imagine balls in 4-dimensional Euclidean spacetime (simply treating time as additional axis) and forget about lightcones and stuff?