Why Consider MOND?
Most simply, because the data demand it.
MOND makes unique predictions about the relation between the
mass distribution and kinematics. These predictions are confirmed
with surprising accuracy in the observed rotation curves of observed galaxies,
a phenomenon which is exceptionally well established observationally
(Sanders & McGaugh 2002).
I fail to see how this can be any more of an accident than the success of
purely Newtonian gravity within the solar system. If dark matter is
correct, this should not happen.
As a matter of the philosophy of science, one guiding principle which I
try to follow is that if a theory makes testable predictions, and those
predictions come true, then we are obliged to give the theory due credit,
even if we don't like it. This has been precisely my experience with MOND.
I came to the subject a True Believer in dark matter, but it was MOND that
nailed the predictions for the LSB galaxies I
was studying (McGaugh & de Blok 1998), not any flavor of dark matter theory.
So what am I suppose to conclude?
That the theory that got its predictions right is wrong because
that was my preconception?
The situation calls to mind that at the birth of Newtonian gravity.
Recall Newton's original statement of his Universal Law
"Everything happens... as if the force between two bodies is directly
proportional to the product of their masses and inversely proportional
to the square of the distance between them."
Note the important caveat that Newton made: as if.
That is exactly the situation in galaxies. Everything happens
as if MOND were the effective force law. Now, it may be
that somehow dark matter always results in the observed MONDian
phenomenology. But it seems sensible to at least consider the
theory that got it right in the first place.
Indeed, it is incumbent on any dark matter based theory to explain
the observed phenomena. There exists no plausible explanation for
MONDian behavior based on conventional cold dark matter.
Indeed, there is only one paper in the scientific literature
that makes a quasi-legitimate attempt to do so (van den Bosch & Dalcanton 2000),
and it appears to be nothing more than the sort of radical fine-tuning
I warned would be necessary (McGaugh & de Blok 1998).
Even straigthforward attempts to interpret the implications of the
observed phenomenology in terms of dark matter (e.g., Sanders & Begeman 1994;
Brada & Milgrom 1999; McGaugh 2004) have simply been ignored. The
entire field seems to be suffering from a massive case of cognitive
dissonance: the facts don't compute, so must be rejected.
Another guiding principle I try to follow as a practicing scientist is
to advocate strongly for a theory before I reject it. I have been through
this exercise many times trying to reconcile the CDM predicted NFW halo with
the data. I've tried very hard many times to salvage it. It just can't be
done... at least not without simply disregarding significant chunks of the
data. Once we engage in this sort of reality denial, then of course anything
is possible (like the earth being flat and only 6,000 years old). Now, I've
often been tempted to reject MOND, and can name any number of reasons why we
may be obliged to do just that. But that does not automatically guarantee
that our ideas about dark matter are correct, nor does it aleviate the
burden of explaining the observed MONDian phenomenology.
If I obey my principle, and work as
hard in favor of MOND as I do in favor of dark matter, I've consistently found
I have to work less hard to salvage MOND. Most people do not seem to share
this experience, but I suspect that has more to do with their preconceptions
than any legitimate scientific argument: they simply have not tried to act on
behalf of MOND. Indeed, the number of misconceptions and myths about the
theory which many respectable scientists still seem to harbor is shocking.
I understand that it can be painful and time consuming to learn about a
theory that is new to you, especially one you've been
trained to despise. However, I would submit that an objective scientific
conclusion can not be reached from a base of willful ignorance.
There is plenty of precedence in the history of science for the present
situation. Analogies abound in the works of Kuhn and Popper. Indeed,
even the origin of gravity itself was similar.
Newton received a certain amount of abuse for his law of gravity.
Famous are the criticisms of Leibniz and Huygens:
How can there be action at a distance?
This notion seemed so silly at the time that Leibniz described it as
"occult" and Huygens rejected it as "absurd".
Under this pressure, Newton himself apparently felt obliged to reject the
notion as a real physical possibility, saying
[Action at a distance is] "so great an absurdity that I believe no man
who has, in philosophical matters, a competent faculty of thinking,
can ever fall into it".
I often feel obliged to make analogous statements about MOND just to
get my colleagues to listen to what it might mean about dark matter,
let alone consider MOND as a real physical possibility.
A compelling physical basis for MOND is still lacking. But then,
it took Newton 20 years to realize there was a good geometric reason
for the inverse square law, and centuries to develop our modern undestanding
of gravity. These things only seem crystal clear with the benefit of hindsight.
So it no doubt shall be with MOND, whatever the underlying physics.