Re:A Physicist Who Denies That Dark Matter Exists
by Avshalom on Monday February 27, 2017 @10:47AM
(#13745020)
Also assuming gravity tends to 1/r at galactic distances makes no god damned geometric sense and would require like extra dimensions with funny topology for gravity to be traveling though.
Re:A Physicist Who Denies That Dark Matter Exists
by effie on Monday February 27, 2017 @02:46PM
(#13747272)
> would require like extra dimensions with funny topology for gravity to be traveling though.
Why? I think I can use simple force formula such as F=k/r without any idea of extra dimensions or topology.
Re:A Physicist Who Denies That Dark Matter Exists
by Avshalom on Monday February 27, 2017 @05:21PM
(#13748945)
Geometrically if a force radiates uniformly it either
A)has infinite density
B)drops off as 1/r^2
We know that gravity drops off as 1/r^2 for all cases except galaxy rotation curves so for it become 1/r for just this specific scale requires that from gravity's perspective the surface area of a sphere usually but doesn't always increase as r^2. Which in turn means it doesn't move through the 3 spacial dimensions that everything else does.
Re:A Physicist Who Denies That Dark Matter Exists
by jcranmer on Monday February 27, 2017 @07:59PM
(#13750210)
The electromagnetic force (specifically, the Coulomb interaction) is 1/r^2. The strong force is actually effectively constant at long ranges, since it becomes energetically favorable to create an antiquark/quark pair. The residual strong force is exp(-r)/r. The weak nuclear force is exp(-r)/r^2 (or exp(-r)/r? I can't really tell with the links on hand--I just know for sure there's an exp(-r) term in it). Gravity being 1/r wouldn't be a surprise given the other known forces.
Re:A Physicist Who Denies That Dark Matter Exists
by effie on Wednesday March 01, 2017 @03:17PM
(#13767871)
> it either
A)has infinite density
B)drops off as 1/r^2
How did you arrive at this? Which force has infinite density and where? k/r is finite except at the point where the source is, the formula is invalid there.
AIUI, assuming that gravity tends to 1/r at galactic distances "solves" the dark matter problem, for galactic rotation at least. The problem is that there are other phenomena that involve invoking dark matter that aren't explained by MOND. In short, changing gravity sounds appealing, but the math doesn't quite work out, just as xkcd commented about it (see https://xkcd.com/1758/ ).