3 ms·

No, I don't think that follows. It is plausible that a system could remain stable under perturbations to the dynamical laws. In their case they find that ther

No, I don't think that follows. It is plausible that a system could remain stable under perturbations to the dynamical laws. In their case they find that there seems to be no critical threshold for GR on the stability of the system. Presumably if you strengthened GR somewhat rather than weakening it you would find increased stability. (Though I don't know that they ran that experiment.) I should note that you could make these changes two ways: you could just suddenly change the dynamical rules or you could gradually change them (a so-called "adiabatic" change). Making a sudden change is more likely to result in an instability. In this paper they were looking at adiabatic changes.

raattgift 4y ago
Please forgive what I suspect is something of a rant. I'm not comfortable with the wording "strengthened GR" because there's an implicit assumption that one can straightforwardly map the authors' time-dependent parameter on the perihelion procession (their eqs (1), (2), and (3) in <https://arxiv.org/abs/2303.05567 https://arxiv.org/abs/2303.05567>) into a relativistic theory at all. The experience of various alternatives to GR (and in particular MONDian alternatives) suggests that assumption may be a bad one. What they are doing is essentially exploring a family of theories of gravitation (which may or may not solve the Einstein Field Equations, or even a comparable set of equations) where each theory in the family produces a different evolution of the orbit of a planet. General Relativity is one of the theories in that family. But how would you even get from eqns 1 & 2 with the parameter fixed at unity (and not varying in time) to the EFEs? What do the Einstein and stress-energy tensors look like? I do sympathize with the implied (and even partially-stated in §3.1) idea that each theory in the family of theories maps to a different set of initial (stress-energy) values for the planet (ignoring backreaction onto the metric) in standard GR, but I wouldn't bet much on that being right. The authors write about the (and in particular "some critical") general relativistic precession rate (or frequency), and I don't mind that wording.

raattgift 4y ago

Please forgive what I suspect is something of a rant. I'm not comfortable with the wording "strengthened GR" because there's an implicit assumption that one can straightforwardly map the authors' time-dependent parameter on the perihelion procession (their eqs (1), (2), and (3) in <https://arxiv.org/abs/2303.05567 https://arxiv.org/abs/2303.05567>) into a relativistic theory at all. The experience of various alternatives to GR (and in particular MONDian alternatives) suggests that assumption may be a bad one. What they are doing is essentially exploring a family of theories of gravitation (which may or may not solve the Einstein Field Equations, or even a comparable set of equations) where each theory in the family produces a different evolution of the orbit of a planet. General Relativity is one of the theories in that family. But how would you even get from eqns 1 & 2 with the parameter fixed at unity (and not varying in time) to the EFEs? What do the Einstein and stress-energy tensors look like? I do sympathize with the implied (and even partially-stated in §3.1) idea that each theory in the family of theories maps to a different set of initial (stress-energy) values for the planet (ignoring backreaction onto the metric) in standard GR, but I wouldn't bet much on that being right. The authors write about the (and in particular "some critical") general relativistic precession rate (or frequency), and I don't mind that wording.