Re: Beginner Questions

On May 27, 11:46 pm, Jim <jimaf...@xxxxxxxxx> wrote:
On May 27, 10:13 pm, Eric Gisse <jowr...@xxxxxxxxx> wrote:



On May 27, 8:32 pm, Jim <jimaf...@xxxxxxxxx> wrote:
[...]

Concerning my last question let me rephrase it this way: When using
"full blown GR" to determine planetary orbits do the calculations
include Newtonian gravitation in some form or another?

The only involvement of Newtonian gravitation in general relativity is
the comparison of GR and Newton in the weak field limit [which Bill
explained] to fix a constant in the field equations. Other than that
initial fixing, not involved at all.

Hope this is understandable to you because I am doing the best I can
with limited knowledge.

Eric, I'm afraid I do not understand the implications concerning your
statement about fixing a constant in the field equations.

The field equations are G_uv = k*T_uv. What G and T are is fairly
irrelevant at this juncture, what is important is that the constant,
k, is fixed via comparison with Newtonian gravitation in the weak
field limit.


Here is the problem that is bothering me: As the earth orbits the sun
its velocity is greater at perihelion than at aphelion. This is an
empirical fact. If I completely eliminate Newtonian attraction of
bodies from any consideration in GR and only consider the behavior of
a body in curved spacetime I cannot understand why the velocity of the
earth would be reduced at aphelion. What causes the earth to
decelerate in curved spacetime, there is no friction? I am assuming
that the gravitational field of curved spacetime is stronger at
perihelion that at aphelion. Why should a weaker gravitational field
cause the earth to decelerate?

Conservation of angular momentum is the reason. The further out you
orbit from a source, the slower you will go. This remains generally
true in GR, as in Newtonian mechanics.


Thanks to everyone for their reply and thanks for being patience with
me I know it if frustrating to converse with someone who does not have
command of the technical language.


.

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