[Marxism] Fwd: A history of dark matter | Ars Technica

Jeff meisner at xs4all.nl
Wed Feb 8 17:42:16 MST 2017


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







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