Re: Gravity Question

OG son,you should listen to Newton -

"I likewise call attractions and impulses, in the same sense,
accelerative, and motive; and use the words attraction, impulse or
propensity of any sort towards a centre, promiscuously, and
indifferently, one for another; considering those forces not
physically, but mathematically: wherefore, the reader is not to
imagine, that by those words, I anywhere take upon me to define the
kind, or the manner of any action, the causes or the physical reason
thereof, or that I attribute forces, in a true and physical sense, to
certain centres (which are only mathematical points); when at any time
I happen to speak of centres as attracting, or as endued with
attractive powers."

http://members.tripod.com/~gravitee/definitions.htm#time

Now Isaac's big achievement is that he replaced 'Geometric' with the
word 'mathematical' and you should really take to heart what the man
was saying and especially as he moves from terrestial ballistics to
planetary motions.

Take your time and absorb that the Principia is written in the language
of geometry and I assure you it is a very,very poor work in that
respect,at least in astronomical matters.It 'predictive' power owes
much to the constellational geometry of the calendrically driven
Ra/Dec system but then again you ,as a follower of Newton,are generally
lost in such matters.





OG wrote:
"Martin Brown" <|||newspam|||@nezumi.demon.co.uk> wrote in message
news:1156839414.327802.195560@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

OG wrote:
"Mark McIntyre" <markmcintyre@xxxxxxxxxxx> wrote in message
news:enl6f21njsb4a9qmg0h86g2to4td4dik47@xxxxxxxxxx
On Mon, 28 Aug 2006 20:03:29 GMT, in uk.sci.astronomy , "Pete"
<petenfay@xxxxxxxxxxxxxxxx> wrote:

Its a puzzle to me but I have to ask;
If the Sun vanished in an instant would all the orbiting objects
instantly
fly off into the universe or would they fly off one by one starting
with
the
nearest one to the Sun. Newtonian gravity or special relativity?

This seems to be an "is gravity instantaneous or constrained"
question. My understanding is that it is effectively instantaneous,
so everything would head off at a tangent to its orbit, as as the
moment that it became evident the sun had disappeared. The tricky part
would be confirming whether this actually happened or not.

I agree. Newtonian mechanics and special relativity do not mix well.
The crucial point here is that unless gravity acts instantaneously in
the Newtonian world view we would see the planets spiralling into the
sun since they would always feel a retarding force from the place where
the sun was a time R/c ago (where R is the planets orbital distance
from the sun). This is demonstrably not the case. The force or gravity
is always exactly a radial one.

SR is not sufficient to handle gravitation reliably.

Another point of view.
This seems to be an "is gravity instantaneous or constrained" question.
My
understanding is that it is propagated at a maximum speed of c; so
everything would head off at a tangent to its orbit only as the
information
that the Sun no longer existed reached them. i.e. their movement changed
at
the moment that it became evident the sun had disappeared. The tricky
part
would be confirming whether this actually happened or not.

We can rule this out because in Newtonian dynamics the planets behave
exactly as if the force of gravity is propogated instantaneously.

We can certainly rule out Newtonian dynamics, but we knew that already.

The GR interpretation is that space-time itself is curved so that
nothing moves between the sun and planet to acheive this result.
Planets follow a natural shortest path. If you were able to make the
sun disappear totally you would violate so many other laws of physics
that all bets are off.

Thought experiment
If we had a very massive particle and its antiparticle close together but
kept apart (exercise for the reader); at sufficient distance from their
barycentre they would (to first order measurements) distort space as though
a single mass, and the motion of a test particle would be evidence of the
mass of the pair.
Now let the two particles annihilate each other producing a spherical shell
of Gamma rays which move out at speed c. The question being asked above is
when does the test particle's 'natural path' change from curving around the
' central mass' to movement in a 'straight' line.

My understanding of GR is that its the mass/energy metric that determines
the curvature of the metric, so the curvature at the location of the test
particle doesn't change until the gamma rays propagate to the distance of
the test particle; i.e. while the energy is 'inside' the orbit, the orbit is
as if the mass is at the centre, but as soon as the energy is outside the
orbit, the space is flat within the sphere.

Ergo - the situation as described in the OP would be as though gravity
propagates at speed c.

.

Relevant Pages

  • Re: Doubts about relativity dogmas
    ... It is not a myth - Newtonian Gravity is wrong. ... If original theory was wrong, taking the Newtonian limit of GR would ... of general relativity is erroneously stated as if the speed of light c ...
    (sci.physics.relativity)
  • Re: Poincare conjecture
    ... Absolute and relative space,at least in Newtonian terms,represent his ... planetary orbital motion is Not seen directly from Earth when it is. ... Again,the creation of the AU through celestial sphere geometry by ... motions such as planetary motion around the Sun and simultaneously ...
    (sci.physics.relativity)
  • Re: Doubts about relativity dogmas
    ... It is not a myth - Newtonian Gravity is wrong. ... If original theory was wrong, taking the Newtonian limit of GR would ... of general relativity is erroneously stated as if the speed of light c ...
    (sci.physics.relativity)
  • Re: Shapiro delay deduced from newtonian gravity?
    ... what about a shapiro delay under newtonian ... Does newtonian gravity also predict half the ... deflection angle for light and that Newtonian gravity ...
    (sci.physics.relativity)
  • Re: Poincare conjecture
    ... However, you mentioned the principles, as I would be very glad to mention to ... Absolute and relative space,at least in Newtonian terms,represent his ... planetary orbital motion is Not seen directly from Earth when it is. ... Finally by what arguments do you prove that the centre of the Sun ...
    (sci.physics.relativity)