4
$\begingroup$

It is well-known that Birkhoff's theorem and the classification of LTB spacetimes proves one version of Newton's shell theorem in the context of GR. Another statement in Newtonian gravity, often understood as a shell theorem(*), stipulates that a point mass $m_1$ will attract a homogeneous massive spherical shell $S_2$ as though all the mass of that shell were concentrated at the center of the shell (in the case where $m_1$ is not inside of $S_2$. If that point-mass does happen to be inside of $S_2$, it exerts no net force on the shell). Contrary to the first shell theorem, it is difficult to even formulate a version of the second shell-theorem in GR(**).

With these two shell-theorems, the Newtonian dynamics of the solar system is reduced to a system of interacting point-masses (provided the internal mass-distribution of the planets is almost radially symmetric).

My question: In Newtonian gravity, there is a generalization of the first shell theorem to the shells of a spheroidal shape (Laplace-Ivory theorem) and even more general algebraic surfaces -i.e. the gravitational force vanishes everywhere inside these shells-. Is something similar true and known in GR?

(*) I believe it was also derived by Newton.

(**) The Swiss-cheese cosmologies are testament to a version of the second shell-theorem.

$\endgroup$
4
  • $\begingroup$ You seem to be distinguishing two versions of the Newtonian shell theorem, but the distinction you're making seems either unclear or trivial. Is this even relevant here? It doesn't seem to me that the first two paragraphs are needed. $\endgroup$
    – user4552
    Commented Aug 15, 2018 at 13:51
  • $\begingroup$ The difference between the two shell-theorems is substantial (and it is regrettable that the second one is often omitted in today's curricula): both in their statement/meaning and their proof/verification (e.g. for the first shell theorem you can use Gauss+symmetry while the second shell theorem always involves -as far as I know- the calculation of an integral over the shell $S_2$ with no simplification via symmetry arguments. $\endgroup$
    – 5th decile
    Commented Aug 15, 2018 at 15:03
  • $\begingroup$ I agree that the two first paragraphs are a bit floklorish and not immediately to the point for my question. But on the other hand, my total post doesn't seem too long. So I find everything okay as things are stated now. (this folklore may inspire other people to ask other questions) $\endgroup$
    – 5th decile
    Commented Aug 15, 2018 at 15:07
  • $\begingroup$ Ok, suddenly it occurs to me that you might be able to prove the second shell theorem using the first shell theorem + Newton's third law $\endgroup$
    – 5th decile
    Commented Aug 15, 2018 at 15:12

0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.