/sci/ - Science & Math

Only if simultaneity is the uniqueness of a hypersurface of what can have more if want.

no srsly

You're asking if isotropy and homogeneity imply that every hypersurface is unique? That doesn't make any sense, as they don't have anything to do with eachother. Reformulate your question please.

Well, I read this from william lane craig:

>... Friedman's spatiotemporal structure includes an absolute reference frame, the frame of homogeneity and isotropy. It has often been commented by physicists that Einstein's GTR thus reintroduced the relations of absolute simultaneity that his STR denied. In fact, the idea that the hypersurface of homogeneity is the privileged frame, which determines absolute temporal and spatial measurements, has been part of graduate textbooks on General Relativity or cosmology going back to the 20s.

He's trying to argue for absoluteness of the simultaneity of events in the universe, so he can argue the A-theory of time. I was wondering if this hypersurface of iso and homo was the only such hypersurface of simultaneity. If not, then Craig's claim would be akin to saying the existence of a torsion free connection (levi-cevita) implies the absoluteness of parallelity on manifolds, which is obviously false. Just because one connection has nice properties doesn't mean its privileged over others.

bump

>>1914469

first of all, wtf is the "A-theory of time?" and second of all,

>the idea that the hypersurface of homogeneity is the privileged frame, which determines absolute temporal and spatial measurements, has been part of graduate textbooks on General Relativity or cosmology going back to the 20s

lol no

FUCKIN' GENERAL RELATIVITY HOW DOES IT WORK

>>1914560
The A-theory of time proposes that the past and future are not as ontologically real as the present, if real at all. This is also called a 'tensed' theory of time, in that it holds that the events which exist are fundamentally tensed. The B-theory of time proposes that there is no unique present moment, and that all events objectively exist; tense is not a fundamental property of their existence. This is called the tenseless theory of time.

If events in spacetime, according to our best physical theories, cannot be uniquely given a description of which are simultaneous and which aren't, then the A-theory is destroyed. This is the case in special relativity, but Craig argues it is not the case in general relativity because of the Friedman metric as solution to the Einstein equations (preferred by cosmologists, he says) offers an absolute frame of reference (that of isotropy and homogeneity). Wiliam Lane Craig argues the A-theory of time, so that it makes more sense to say 'the universe began to exist' in his Kalaam cosmological argument for the existence of God. He even states that his premises doesn't hold under the B-theory of time.

Could I get some kind of reference or explanation so that I know for certain whether or not Craig is accurately representing general relativity or distorting it? Also, it'd be nice to get a mathematical definition of 'surface of simultaneity' so that I might be able to try out my own knowledge of differential geometry and tensor calculus to the task.