🚀 go-pugleaf

micha@ecrc.de (Micha Meier) writes:
>I am always puzzled by this reasoning. Remember days before Einstein
>when people
>tried to find the reasons why the Mercure's orbit is not exactly what it
>should be? They also tried to invent hidden planets (i.e. 'dark matter')
>to explain this, although the problem was in the Newton's theory of gravity.
>IMHO, this is quite similar to the current ideas about the dark matter.
>Why is it more plausible that there is a huge amount of matter that we cannot
>detect rather than to suppose that our theories are not precise enough?

I understand your puzzlement but there are a few reasons why people
don't generally go for modified theories of gravity.  The
modifications necessary say that _Newtonian_ gravity is wrong
(usual GR is then out of the picture).  They usually involve
either a turnover of the force from 1/r^2 to 1/r past some large
distance R_0, or a similar turnover at very small accelerations,
a < a_0, where R_0 or a_0 are some new fundamental constant.  In 
some way this seems more "drastic" than the tiny GR fix to the precession
of the perihelion of Mercury.  Of course GR has its own drastic
effects but there is some kind of gut-feeling that this is too
drastic.

Other problems are: making such a theory relativistic, reconciling it
with all the observed evidence (Tremaine's article in Physics Today,
2/92, points out a problem with the R_0 theory; some people say they
have counterexamples to Milgrom's a_0 theory), and redoing everything
that has been done with Newtonian gravity with the new theory
(when this came up in sci.astro I invited proponents to try rederiving
all the galaxy stability results, etc.).

The upshot is that it's not a more precise theory which is required,
but a rather different theory.  And, in fact, there is evidence for
"dark matter" in the solar neighborhood (although some of this may
just be brown dwarfs, it's still dark matter).  It does seem to be
true that the amount of dark matter required increases for larger length
scales; I guess this implies that dark matter is more spread-out
than luminous matter.

Ben Weiner