On 2/7/2014 6:43 PM, [email protected] wrote:
On Saturday, February 8, 2014 12:44:30 AM UTC, Brent wrote:
On 2/7/2014 2:45 PM, [email protected] <javascript:> wrote:
On Friday, February 7, 2014 10:30:06 PM UTC, Brent wrote:
On 2/7/2014 1:37 PM, [email protected] wrote:
On Monday, February 3, 2014 6:27:14 AM UTC, Brent wrote:
On 2/2/2014 10:12 PM, [email protected] wrote:
Namely that however you jig it, there's still going to be huge
spacetime
distortion representing the sun and a tiny one representing the
earth,
which - I thought - had to bias the objectively true relation
between the
sun and the earth for the earth being gravitationally dominated by
the
sun not the other way around.
So the question was whether one could just consider the Sun,
calculate the
spacetime metric due to its mass, and then calculate the orbit of
the
Earth as an inertial path in that metric? In that case of course
the
answer is no. The metric has to take into account the mass of the
Earth
as well as the Sun. Just as in Newtonian theory, the Sun and the
Earth,
and Jupiter and the other planets all move around their mutual
center-of-mass. The center-of-mass is roughly near the surface of
the Sun
on the side nearest Jupiter.
Brent
Hi Brent - Sure, but is it ok to look at the sun and the earth together,
purely as their geometry's in spacetime, and say "that one goes around
that
one" to the same sort of accuracy we can say the earth goes around the
sun
without invoking relativity. Or is doing that running foul of the basic
principles in play of relativity ?
I'm not sure what you're asking. The corrections to a simple
"stationary Sun,
orbiting Earth" due to Jupiter are much bigger than any GR effect.
Brent
if you're willing to keep trying I would also like to...but please be
assured I
will understand anytime you no longer want to. So trying again here, I
would first
recap one sense of what this has been about....
- I related an experience I had a few years back, when some people much more
knowledgeable than I in relativity...at least factually speaking...argued
that
relativity was a fundamentally different way of explaining the universe
(with no
dispute from me so far)....such that, you literally could not sensibly talk
about
frame-free visualizations. I responded that I thought that was only true up
to a
point, because for example, the spacetime geometry of a much larger and much
smaller object, could be visualized as a single landscape for the purpose
of saying
something very general, like "that one goes around that one".
Which they explicitly then ruled out.
Of course, this statement was always untrue strictly speaking because both
go round
a common centre of mass. But the question here, is:
- was that more knowledgeable source correct
- was I correct
- was one but not the other exclusively correct, if so which one
- was there a good chance neither were correct, and you'd have to see what
was
actually said to be sure
- Is what I am saying to you now, which I think is a fairly good capture of
what I
said then, correct .....so far as it goes
My friend Bill Jefferys who used to teach astronomy at Univ of Texas says
it doesn't
matter what reference frame you choose as far as the physical events go, it
just
makes the math harder or easier. So sometimes you use a geocentric frame
and
sometimes a heliocentric frame and so forth...
Brent
but can you throw the relative character out of the window, and speak of an
'absolute landscape' implied by relativity theory that is made up of all the gravity
wells, that definitely suggests a 'reality' that goes beyond the principle of
equivalence. In that, the landscape shows what's big and what's small, so the relation
between them, in one big picture.
I ask this in context that acknowledges it wouldn't be any good for resolving much
useable knowledge about the world. In fact none at all, save that reality does
imply such a reality is really there.
True. Although this "absolute landscape" would look different according to one's
perspective (position, motion, etc) it hopefully represents the intersubjective agreement
of all possible observers. If you express this landscape only in terms of invariants, not
coordinates and not relative to a particular observer or frame, then you could call it
"absolute", but as you note that tends to be a messy representation to work with, so we
tend to use some particular representation; the same way we navigate with flat map
projections instead of charting courses on globes.
Brent
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