Concerning the importance of preffered time coordinates

[femtofranco]

I have been reading around in general relativity, and see that, for Quantum Field Theory in Curved Spacetime, the notion of a preferred time coordinate comes up quite often (to distinguish positive frequencies). Why must the coordinate be 'preffered,' i.e. why must it be a consequence of the manifold's structure.

I understand this may sound like a stupid question, but any spacetime manifold admits at least one timelike (or two null coordinates) coordinate - why not simply use this?

 

[Bill_K]

I don't there's any fundamental reason, isn't it just a matter of practicality. The things you want to do, such as separate the wave equation to define positive frequencies, write down a Green's function, etc, can be carried out explicitly in cases where the time coordinate has simple global properties.