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In Topology, there is considerable effort given to describing what makes two points "close" to each other. The answer is always to define a closeness function appropriate to the space, and then to apply the function to one fixed point and two test points; the function will determine which one of them is "closer" to the fixed point. As you can see this is ...


In quantum field theory, the casuality is manifested as a requirement imposed on the Heisenberg fields $\varphi(x)$: they must be locally commutable:$$[\varphi(x),\varphi(y)]=0 $$ when $x-y$ are spacelike. The aim is to make one measurement cannot affect the other which is outside the lightcone of the first one. The casuality can also give a condition for ...

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