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On 2017-02-07 23:23, Louis Proyect via Marxism wrote:
https://arstechnica.com/science/2017/02/a-history-of-dark-matter/
Well this was a fairly interesting look back at the discovery and search for so-called dark matter, by a history of science journalist but not someone working as a physicist or astronomer. It has a couple of shortcomings I'd like to point out and expand upon.
First, of greatest interest to me, but also possibly of the most crucial importance (we'll find out in coming years!) is the article's failure to mention the theory of emergent gravity advanced (especially) by Erik Verlinde (University of Amsterdam) which, if true, provides a simpler explanation for the observations for which the idea of dark matter was ever postulated. It is only because of the unquestioning acceptance of the law of gravitation as currently formulated, that one hears that "there is very strong evidence for the existence of dark matter". But unlike any other accepted form of matter or energy, ALL evidence for dark matter is based on a single (assumed) property: observed gravitational fields which are attributed to it. No other type of evidence for it has been found, even after decades of experiments. Verlinde's theory of gravity, instead, predicts the observed gravitational field without invoking the hypothetical "dark matter." I'll stop there, since I recently wrote a longer post to the list on the subject:
http://lists.csbs.utah.edu/pipermail/marxism/2016-December/021296.html
The article Louis posted on the history of dark matter does (briefly) mention alternate theories of gravity called MOND (Modified Newtonian Dynamics) which likewise explain the observed gravitational fields without supposing any "dark matter." But Verlinde's theory has the beauty that it derives what we call gravity as an emergent property predictable by some very deep reasoning involving quantum mechanics, string theory, and thermodynamic concepts. Unlike the MOND theories, it wasn't "designed" to account for the observed gravitation, but happens to do so without requiring any free parameters (additional assumptions). MOND eliminates one free parameter (dark matter) but requires assuming another (a parameter of the alternative gravitational law). On the face of it, a theory requiring one less free parameter (assumption), like Verlinde's theory, is preferred to a more elaborate theory (one with additional assumptions, free parameters) as codified in "Occam's razor". But of course there is much more to it. Anyway, the article's mentioning of MOND but not Verlinde's theory is regrettable.
In fact on the page of the article there is a link (but not introduced by the article's author, I'm pretty sure) to an article on the same website on the subject of Verlinde's theory:
https://arstechnica.com/science/2017/01/new-ideas-on-gravity-would-vanquish-dark-matter/
Also, here is another popular article that I didn't point to last time, regarding the agreement of Verlinde's gravity model with observations of gravitational lensing around 30000 galaxies:
https://www.newscientist.com/article/2116446-first-test-of-rival-to-einsteins-gravity-kills-off-dark-matter/
I saw these results plotted at a talk recently, and the data fit both standard gravity with dark matter, and Verlinde's prediction, about equally well. There were 4 graphs for different cases of galaxy size I believe, on which the observational points were plotted on top of the theories' predictions. The big difference, though, was that Verlinde's gravity was just computed and fit the data, whereas the dark matter fit required fitting a free parameter for each of the 4 graphs (thus not just a single number!). Any scientist would be highly impressed with an observational agreement to a theory derived only from basics!
Changing the subject now, the other thing about the article Louis posted I don't like is that it started out by talking about "dark matter" historically, but there it was really talking about cold matter, matter that doesn't radiate simply because it is cold (unlike stars, which radiate because they are hot). Maybe that history was added by the author to reach a word-count for the publisher; I don't know. But it's disconnected from the modern search for "dark matter" which (as the article states) was begun in the 1960's following the observations by the astronomer Vera Rubin, whose obituary Louis also posted recently. That modern version of "dark matter" should more accurately be called "transparent, invisible, non-interacting matter". Rather than cold matter, which is dark (non-emitting) but would still reflect light and block light (thus visible as a silhouette), the modern poorly-named "dark matter" is dark because it has almost no interaction (other than gravitational) with ordinary matter or electromagnetic energy. That is why it is so very very difficult to observe.
Or because it doesn't exist! - Jeff _________________________________________________________ Full posting guidelines at: http://www.marxmail.org/sub.htm Set your options at: http://lists.csbs.utah.edu/options/marxism/archive%40mail-archive.com
