In article ,
alistair wrote:
Modified Newtonian Dynamics

MOND basically says that if you double the distance of a star
from the galactic centre, then you half the force of gravity, instead
of quartering it as Newton's inverse square law would say.
This is what a physical theory needs to explain.

This is not accurate. John Baez's description of MOND is much
closer to what the theory actually says.

John's description was essentially this: if you define a_N to be
the Newtonian acceleration

a_N = F / m

then the actual acceleration of an object is

a = a_N if a_N a_0
a = sqrt(a_N a_0) if a a_0

Here a_0 is some fundamental constant.

He pointed out, quite correctly, that it's ugly for a fundamental
law to be split into cases like that. Last time I checked,
the MOND people weren't dogmatic about this exact form. They
considered smooth functional relationships between a and a_N.
I think that as long as the relationship approaches the above behavior
in the limits,

a - a_N when a_N a_0
a - sqrt(a_N a_0) when a_N a_0

the MOND people are satisfied. So I guess something like

a = sqrt(a_N (a_N + a_0))

might do the trick.

Personally, I can't get past my theorist's objections to MOND. It
doesn't play well at all with general relativity, and I just don't
believe that general relativity is completely on the wrong track. But
of course the issue should be settled observationally, not based on
theoretical prejudice (however well-justified!).

-Ted

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