On Thu, Feb 6, 2014 at 1:45 AM, meekerdb <[email protected]> wrote:
> On 2/5/2014 9:47 PM, Jesse Mazer wrote: > > > > On Wed, Feb 5, 2014 at 7:38 PM, meekerdb <[email protected]> wrote: > >> On 2/5/2014 9:31 AM, Jesse Mazer wrote: >> >> --question 1 dealt with the question of how YOU would define p-time >> simultaneity in a cosmological model where there's no way to slice the 4D >> spacetime into a series of 3D surfaces such that the density of matter is >> perfectly uniform on each slice (and that uniform can be characterized by >> the parameter Omega), unlike in the simple FLRW model where matter is >> assumed to be distributed in this perfectly uniform way. >> >> >> I don't see that perfect uniformity is necessary. We have calculated our >> epoch relative to the CMB as 13.8By. I assume any other scientific species >> in the universe could do the same and so say whether they were 'at the same >> time' as measured by expansion of the cosmos. I don't see how the >> existence of galaxies and galaxy clusters precludes this kind of >> measurement. >> > > > Using the CMB may give an approximate answer, but would you argue it > could distinguish between different simultaneity definitions that agree > approximately when averaged over large scales, but disagree somewhat about > the details of simultaneity in highly curved regions? For example, could > the CMB be used to define a unique definition of simultaneity in the > neighborhood of a black hole (where coordinate systems like Schwarzschild > coordinates and Eddington-Finkelstein coordinates and Kruskal-Szekeres > coordinates give very different definitions of simultaneity)? Edgar isn't > just claiming some approximate pragmatic truth about simultaneity, he's > claiming an absolute and exact truth about simultaneity in all > circumstances, I was asking if he thinks this truth can be empirically > determined to arbitrary precision even in principle, and if so what > empirical observations would be used. > > > Of course it can't give great precision because the recombination event > must have had significant duration. But aside from all the practical > problems I don't see a problem in principle. From the CMB to a given > 4-point in the universe there is a world line that is longest and that > length can be used as a t-label for that point. > Ah, thanks, I had been thinking of it in terms of the idea that the observer always moves in such a way that the CMB doesn't appear redshifted or blueshifted (I don't know if that's consistently possible in a universe with a lot of localized changes in curvature), rather than the worldline of greatest proper time from the surface of last scattering. Jesse -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/groups/opt_out.
