The discussion with Will Happer about the pyrgeometer recorded in recent posts illuminates the distinction between state and and process in physics, with process connecting to transition between states. For the pyrgeometer the question is if it primarily/directly measures (a) temperature/state or (b) radiation/process. Happer says (b) and and I say (a), while a pyrgeometer manual states that (b) is computed/derived from (a) and so is not directly measured.
To get perspective, consider a 3d coordinate system by which we can measure/record the position of an object as a form of state, assuming it does not change position. This seems like a fairly straight forward thing to do.
Assume now that the body changes position with time, which brings in the notion of time to which we will return, and we then meet the concept of velocity as change of position vs change of time. It then seems natural to view velocity as a process variable since it involves transition between two different positions/states over some (infinitesimally small) change of time. If position is easy to measure, velocity seems to be more difficult since it involves measuring a small change of position over a small change of time.
So it seems natural to view position as a state variable and velocity as a process variable, with a natural connection of transition to change of time. Translated to the pyrgeometer it suggests that (a) is easy (state) and (b) difficult (process). Accordingly a pyrgeometer can be expected to be designed to measure (a). If simple works, why aim for difficult?
Similarly, conductive heat flux scaling with temperature gradient is a process variable analogous to radiative heat flux/radiation.
This brings up the question if there is any difference between space variable(s) and time variable? Yes, if it is natural to view position as a state variable, then it may be natural to view time as a process variable as something always in transition form one time instance to the next.
Thus there seems to be a clear distinction between space (state) and time (process) variable. If so, Einstein's key notion of space-time variable with time acting like a 4th space dimension does not seem to be natural, since it makes time a state variable instead of process variable and if you mess things up then you will have a mess.
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