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Eric Gisse
#9
Jan26-06, 06:35 PM
P: n/a

John Bell wrote:
> Eric Gisse wrote:
> > John Bell wrote:
> > > Mathematically speaking, Einstein's general relativistic field
> > > equation admits two possibilities for the speed of propagation of
> > > gravitational fields/waves, + or - the speed of light (c) (i.e.
> > > propagation forwards or backwards in the dimension of time).
> > >
> > > Would I be correct in therefore concluding that the planetary orbits
> > > predicted by this field equation remain the same, irrespective of this
> > > mathematical sign ambiguity? If so, it appears to me that this might
> > > provide a conceptually simpler/alternative explanation as to why such
> > > orbits remain stable, to that given in the physics faq. (although
> > > clearly, the real reason for this stability is that Einstein derived
> > > the equation from the axiomatic foundations of the theory, under the
> > > constraints of energy and impulse conservation)

> >
> > [snip]
> >
> > Orbits in multi-body systems aren't stable, I have no idea why you
> > would think that they were. They are stable over human and geologic
> > timescales, but forever.

>
> Obviously, if you meant here *but NOT forever* . I was referring to
> relative stability as in the context of the physics FAQ, and expected
> that the readers would have the intelligence to appreciate this, and
> the generosity of spirit to grant me the same intelligence.
> >
> > I would love to see your justification for thinking a sign change in c
> > would manifest itself in altering the stability of orbits, because as
> > it stands what you said is absurd.

>
> Read the posting more carefully. I repeat:
> > > Would I be correct in therefore concluding that the planetary orbits
> > > predicted by this field equation remain the same, irrespective of this
> > > mathematical sign ambiguity?

> The reason for the question was that somebody else had made the
> opposite *absurd* assertion, at sci.physics.relativity, and I didn't
> trust that assertion.


Looks like I mis-read who was saying what. Sorry.


> >
> > Also, energy conservation is less than well-defined in general
> > relativity - energy is not an invariant, though there are ways to
> > define energy but not when formulating the theory.

>
> Einstein explicitly states in the authorised English translation of his
> popular exposition, that he constrained the solution to be consistent
> with the laws of conservation of energy and impulse. You say he didn't?


Beats me, I haven't read his works and don't really care to. I find
them a lot harder to read than the moden expositions on the subject.
That is also where my knowledge comes from, such as it is.

This isn't specifically directed at you, but I will never understand
why people focus on Einstein so much. I'm not arguing against him on
this or anything, I just don't understand why people don't seem to be
quite capable of seperating the man from the theory.

> >
> > Furthermore, there is no such thing as "impulse conservation" unless
> > you are referring to conservation of momentum - which isn't needed to
> > formulate theory, but is needed to make predictions using it.

>
> Again, your argument is with Einstein, not me. Einstein used the term
> impulse not momentum, in the passage to which I refer.


I'm going to make the guess that they are the same thing.

>
> Thank you, however, for appearing to confirm that my understanding is
> correct re: orbital invariance with sign reversal, despite your
> unconventional way of doing so.


:D

>
> John Bell