[Marxism] Fwd: A history of dark matter | Ars Technica
meisner at xs4all.nl
Wed Feb 8 17:42:16 MST 2017
On 2017-02-07 23:23, Louis Proyect via Marxism wrote:
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
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:
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:
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!
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