Ok you my good sir make this make sense. Sorry for the confusion about my magnetics anology btw. I find this kind of stuff fascinating.
On Mon, Apr 1, 2019 at 3:14 AM Eric Smith <desm...@santafe.edu> wrote: > Hi Glen and Gil, > > What you have below, Glen, is right I think. To begin with the summary, > and put the TLDR afterward, it looks like these diffuse-galaxy results say > that gravity stays a clean theory, and we need to identify the origin and > nature of dark matter as a separate thing. While a hard problem, it is a > problem that respects the structure of physics as we have been using it. > Gravity is gravity, we can treat matter as “living on it” at all the > energies where we have ever done physics, and we need to figure out how > they unify, which we don’t really have a theory for at all, but which we > have good reason to believe only comes into play at extremely high > energies. > > The longer version: > > We know Einstein’s GR not only changes the picture of gravity from > Newton’s, but for comparable predictions (locations and rates of orbits, > their stability, etc.) it requires corrections from Newtonian gravity in > the strong-field regime. The calculations become complicated and hard to > do with pencil and paper, but this is okay, because once it is in its > geometric language, the Einstein version is in a conceptual sense “cleaner” > than the Newtonian version. > > The above view says that Newton becomes a better and better approximation > to Einstein the weaker the field gets. On the whole, galaxy dynamics on > the large scale is governed by very weak fields. So for the > radius-dependence of orbital velocities to deviate far from the Newtonian > prediction (as they do in most known galaxies) requires either ordinary > gravitation with out-of-the-ordinary matter, or a _different_ deviation > from Newton, which would exist in the weak-field limit, but only become > visible on very large scales. Since Einstein -> Newton in the very weak > field limit, the latter possibility would require a deviation from Einstein > too. I am not sure that could be done conceptually “cleanly” in the same > way GR is clean. > > So to find that the diffuse galaxies lacking dark matter go back to > orbital predictions that converge to weak-field Einstein with no Dark > Matter, which is also weak-field Newton with no DM, favors the > interpretation that gravity really is just gravity, and that we have to > figure out where some additional matter is coming from, just as the > accelerating expansion tells us we have to figure out where some “Dark > energy” is coming from. The cosmological constant is an important lynchpin > because it is the only observation about the structure of the vacuum for > which we really don’t have a “theory” at all. Anything else we can measure > is handled well by standard model physics, though with still some > unexplained parameters. > > In a way, this result is the one that could have been expected. There are > now lots of images from gravitational lensing that show “clouds” of DM > off-center from galaxies that we can see in the visible. This especially > happens when galaxies collide. So DM was behaving like matter already, and > it is not very surprising to see that maybe it could be all-but-stripped > from a galaxy, leaving only a scattering of visible matter. It would not > surprise me if at some point somebody can show that it was a long-ago > collision that did this stripping, and much later the diffuse ball of stars > re-settled to an ellipsoid. > > Keep in mind, in all of this, that the strong-field limit of GR is getting > better and better constrained with the gravitational-wave detections, in > addition to all the astrophysical stuff that it has successfully modeled > for decades. So some muddying of GR that only shows up at weak fields > would be strange. > > Finally, n.b. that my understanding of this doesn’t qualify as > professional — I got off the train too soon. But I think everything I have > said above is a correct account. > > All best, > > Eric > > > > On Apr 1, 2019, at 5:55 PM, glen∈ℂ <geprope...@gmail.com> wrote: > > > > I'm not sure how magnetism plays into all this. But it is interesting > that these are ultra diffuse galaxies. Maybe there is something wrong with > how we extrapolate the rules in flat space to the rules in very bent space, > where everything gets so weird. It seems (to me) that a regular galaxy > would be more like a colloidal solution, with lots of little clumps of bent > space (heavy things like brown dwarves[†] and such). Such a pock-marked, > bristly, region of space must be more difficult to model than something > relatively well-behaved like an ultra diffuse galaxy. Right? In the > vicinity of "almost singularities" (very heavy objects), any measurement or > calculation error will have more of an impact on the result. > > > > [†] I forgot to turn off the real-time spell checker on this new-to-me > computer and, lo and behold, "dwarves" is not the plural of "dwarf"! WTF? > https://grammarist.com/usage/dwarfs-dwarves/ tells me it's a neologism > popularized by Tolkien. So, by using it, I'm wearing my Dork on my sleeve. > > > > On 3/31/19 4:24 PM, Gillian Densmore wrote: > >> So it's possible that what we think of as dark matter could be more to > do > >> with a whole lot of magnets/lots and lots and lots of gravity energy and > >> makes things go weird? > >> Or weirder? > >> On Sun, Mar 31, 2019 at 3:25 PM glen <geprope...@gmail.com> wrote: > >>> > >>> > https://news.yale.edu/2019/03/29/new-studies-confirm-existence-galaxies-almost-no-dark-matter > >>> > >>>> The finding was highly significant because it showed that dark matter > is > >>> not always associated with traditional matter on a galactic scale. It > also > >>> ruled out several theories that said dark matter is not a substance > but a > >>> manifestation of the laws of gravity on a cosmic scale. > >>> -- > >>> glen > > > > ============================================================ > > FRIAM Applied Complexity Group listserv > > Meets Fridays 9a-11:30 at cafe at St. John's College > > to unsubscribe http://redfish.com/mailman/listinfo/friam_redfish.com > > archives back to 2003: http://friam.471366.n2.nabble.com/ > > FRIAM-COMIC http://friam-comic.blogspot.com/ by Dr. Strangelove > > > ============================================================ > FRIAM Applied Complexity Group listserv > Meets Fridays 9a-11:30 at cafe at St. John's College > to unsubscribe http://redfish.com/mailman/listinfo/friam_redfish.com > archives back to 2003: http://friam.471366.n2.nabble.com/ > FRIAM-COMIC <http://friam.471366.n2.nabble.com/FRIAM-COMIC> > http://friam-comic.blogspot.com/ by Dr. Strangelove >
============================================================ FRIAM Applied Complexity Group listserv Meets Fridays 9a-11:30 at cafe at St. John's College to unsubscribe http://redfish.com/mailman/listinfo/friam_redfish.com archives back to 2003: http://friam.471366.n2.nabble.com/ FRIAM-COMIC http://friam-comic.blogspot.com/ by Dr. Strangelove
