Re: Is gravity relative?


<brentlmt@xxxxxxx> wrote in message
news:09ddf125-08ac-44b5-b626-d4d1c8e24e58@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Apr 8, 2:02 pm, Tom Roberts <tjroberts...@xxxxxxxxxxxxx> wrote:
PD wrote:
We don't. We see and feel the sun in the same location, where it was 8
minutes ago.

In the instantaneously-comoving inertial frame of an observer on earth,
the "gravitational force" from the sun points approximately at the sun,
where the sun is located right now (not 8 minutes ago). This
approximation is exceedingly good, far better than the 8-minute
difference in the sun's position wrt this frame. (This frame is moving
at 30 km/s wrt the sun, and is not rotating.)

In GR, changes in gravitation propagate with speed c, but
"gravitational force" [*] is not a central force; the sun's
location, velocity, and acceleration (wrt this frame) are
essentially extrapolated from 8 minutes ago; the
result is quite close to the location it has now, and the
force points to this extrapolated position of the sun. Note
that a similar thing happens for E&M fields, except the
extrapolation is to lower order.

Light, on the other hand, propagates along a null geodesic
from where it was emitted, and light observed on earth was
emitted from the sun 8 minutes ago. Here the null geodesic
is quite accurately a straight line.

[*] In the relevant approximation to GR.

Such an earthbound observer sees the sun where it was 8 minutes ago, but
feels its gravity from exceedingly close to where it is right now. These
are not the same direction.

Tom Roberts

OK, let me try a different example.
Let's say that Androcles and I are hanging out in space. There is a
small
gravitational attraction between us (and the ONLY kind of
attraction).
Along comes an asteroid and smashes into Anrocles. Now if we assume a
purely inelastic colision, how does GR resolve the fact that I don't
see
Androcles become one with the asteroid until sometime after it happens
(depending on distance) but according to GR I would feel his
gravitational
center change at the moment of impact? This would imply that his
gravity well
is propegating faster than light.

Thanks

*donning asbestos jammies*

================================================
Let's be a little more scientific.

First we accept for the sake of argument that E = mc^2, the sun and other
stars are losing mass as they radiate energy. We'll postulate that.

Next we'll postulate that we can detect the Moon's gravity. We spin
the Earth in the Moon's field and watch sea water levels rise and fall.
These we call tides. We can even detect the Sun's gravitational field
this way, we have spring tides and neap tides.
http://easytide.ukho.gov.uk/EASYTIDE/EasyTide/SelectPort.aspx

The nearest star to our solar system is Proxima Centauri, about 4 light
years away. What we'll do is make that go nova, so that it suddenly
loses ALL its mass in one huge burst of light. This is the biggest
negative going gravity pulse imaginable (as well as being the biggest
atomic bomb imaginable) but never mind that, your question is
basically how long will it take for us to detect the loss of the gravity
(which must vanish along with its mass) of Proxima Centauri,
instantly or 4 years?
Ok...
Let me ask you this, since you are wearing asbestos jammies.
How much of Proxima Centauri's gravity can you detect RIGHT NOW,
and what hope do you think Gravity Probe A or B has of detecting any
change in gravity from a star or pulsar that is at least 250 times further
away (inverse square law 1/(250 * 250) = 0.000016) than Proxima
Centauri and is not blowing itself to smithereens to create the biggest
negative going gravity pulse imaginable?

This is not an ant on an elephant's arse, this is not even a microbe
on an elephant's arse, this a microbe on Jupiter that you want to
detect.

As to gravity "propagating", it doesn't. Falling deeper into a
gravitational field and asking how fast gravity propagates is like
diving into a swimming pool and asking how fast getting wet
propagates. The field is there, surrounding the mass. It is not
radiating from it. If the mass moves, the field moves with it.
If the Sun is losing mass then the field is getting weaker.


.

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