On Tue, 11 Apr 2017, Phillip Helbig wrote:
... (Eric Flesch) writes:
gravitational medium would modify or nullify the inverse square law.
How could it not? And that's pretty much all of my point.

... But the kind of dark matter we are talking about has little if
any interaction with baryonic matter.

Um, sorry Phil, but dark matter is supposed to account for (1) spiral
galaxy rotational profiles, and (2) "great attractors". Given the
existence of dark matter (for this argument), it definitely interacts
with baryonic matter. And if it does so, it must have a major effect
on the dynamics of galactic haloes, elliptical galaxies and globular
clusters. Maybe fish-in-water isn't the right analogy, but the
presence of a gravitational scalar diminishes (or nullifies) the
gravitational influence of near neighbours. So the boundary of our
solar system may simply be where our Sun's gravitational influence is
supplanted by that of the "dark matter" medium, with neighbouring
stars not in the equation. That's the idea, anyway.

I also don't think that it would work quantitatively. Can you show that
this idea (even neglecting the point I mentioned above) results in the
very simple MOND law?

I'm not up with MOND. Is it simple? Simple is good but not
sufficient. I expect that MOND is a "top down" system designed to
best account for observed behaviour, as opposed to a "bottom up"
system which builds on known physical law. A top-down system must, of
human necessity, be simple, else nobody will pay attention.

thanks, Eric


[[Mod. note --
1. I think Phillip Helbig was referring to dark matter having little if
any *non-gravitational* interaction with baryonic matter.

2. There's an extensive history of people studying what the consequences
of a gravitational scalar field might be. See, for example, section
5.3 of Clifford M Will, "Theory and Experiment in Gravitational Physics"
(Cambridge University Press, 1981, 1985), or section 3.3.2 of Will's
review paper "The Confrontation between General Relativity and
Experiment" (open-access at http://www.livingreviews.org/lrr-2014-4 ).
To summarize, compatability with various solar-system experiments
constrains the free parameters in these theories so as to give
results almost identical to general relativity.
-- jt]]