Re: relativity question re supernovae

In article <SFxIf.89922$4l5.54974@dukeread05>,
Cyberkatru <perapera77@xxxxxxxxx> wrote:
One of the first things that one learns from relativity is the idea that
simultaneity is not an absolute notion. *When* something happened, and
*where* it happened are dependent on the observers state of motion. Now our
state of motion is not special.
So how is it that astronomers so nonchalantly speak about a supernova
explosion that we see now, as really happening at the "same time" as some
other old event such as the earth moon system forming. (I am watching an
astronomy thing on the science channel right now).
They seems to speak as if every cosmic event was ordered into before and
after! Doesn't that violate the spirit of relativistic thinking?
Are they just picking some reference (fiducial) foliation by spacelike
3-surfaces perhaps picked out becuase of the leaves being approximately
totally geodesic or some other geometric fact coming from the presummed
solution of Einstein's equations being used?

You've got it precisely right. That's exactly what people are doing
when they say things like that.

The specific foliation that's being used is the one that arises
naturally from the fact that the Universe is approximately
homogeneous. That is, the observed homogeneity gives rise to a
preferred notion of "translation in space but not time," i.e., a
notion of simultaneity.

For the record, cosmologists do all know about this issue. Actual
calculations are always done with proper attention to relativistic
considerations like the relativity of simultaneity. We're just not
always very careful about stating things like that explicitly when
talking to outsiders.

-Ted

--
[E-mail me at name@xxxxxxxxxx, as opposed to name@xxxxxxxxxxxxxxxxxxx]

.

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