Talk:Modified Newtonian Dynamics

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The Modified Newtonian Dynamics 11:18 am [Added copyright notices] . . . . . 80.11.172.xxx And nobody's reacting ? --Taw

Well, we should expect a reaction tomorow, but I don't think anything is actually wrong with the text of the copyright notice. Everything on the wikipedia is copyrighted, we just license it on the terms of the GNU FDL, as long as the author is not specifically revoking those rights, it is perfectly acceptable that he mentions on the page that the text is copyrighted. But that's just my opinion. MRC

Adding a copyright notice seems right to me. How could I take such a picture of M51 myself? BTW, any reason all quotes are question marks now? dlebansais

Yes, because you copied the text from a Microsoft Windows document with "curly quotes", which are not valid ISO-8859-1 characters. Please see Wiki special characters for how to do these correctly. --LDC

Hopefully, everything is fixed now. dlebansais

I added the reason why most astrophysicists are unlikely to take MOND as the first explanation for odd galaxy rotation curves.

As an astrophysicist, I do not think the "principle of least astronishment" is actually the reason why MOND is not favored. I added what I believe is the widely-held opinion.

As far as I can see, the maths (as presented here) do not make sense. Surely there is no way a paper could have been published with this glaring weakness, so I am guessing the problem is in the Wikipedia article.

"In the every day world, a is greater than a0 for all physical effects, therefore µ(a/a0)=1 and F=ma as usual."

Now µ is defined as equalling either 1 (when x > 1) or x (when x < 1). Now these equations imply that on Earth x < 1 (and hence µ < 1). (a/a0 must be large, as a is greater than a0, so µ must be small to get it to equal 1). In this case x*a (and hence µ * a) must actually be pretty close to the value of a0 so as to cancel out and produce F=ma.

Why??? What determines the value of x (and hence µ) in this case, so that it cancels out so conveniently for the "on earth" situation? I can create any number of formulae which can do anything I want if I don't have to obey any rules determining the values of the variables.

Basically it is saying "Well on Earth, x (arbitrarily chosen) times a is equal to a0 (an undefined constant) so everything works out just fine!" That's not particularly rigorous.

Obviously the theory MUST conform to the net f=ma result on Earth, lest it be laughed off. But this conformity seems VERY contrived at the moment, and that really weakens the rest of the argument. I'm not criticising the theory, but as a person who knows nothing of this particular subject, yet is able to follow the maths, I have a problem with this as a Wikipedia article. - MMGB

Added an introduction which links to the Los Alamos preprint archive for papers on mond

To respond to the criticism above, just let me tell you first that I'm no expert either, and that any technical question should be directed to M. Milgrom himself.

Now, "In the every day world, a is greater than a0 for all physical effects" is something I have not verified myself, but I cannot imagine any Earth-bound system, even considered isolated, in which a component undergoes an acceleration smaller than a0. It is true in atoms and molecules, for instance.

This being said, and assumed true, means that in any system you study, the typical acceleration will be greater than a0. As a consequence, the value of µ is always 1 for these systems, and nothing is "cancelled out". F=ma*1, that's all.

If you can design an experiment in which a is less than a0, where you can measure it and the corresponding inertia, please tell M. Milgrom immediately. But remember that you experiment will be done inside the gravitational field of the Sun. It will never be an isolated system in this respect.

dlebansais

There was no technical question - I'm not inquiring about the theory - I'm simply pointing out a flaw in the article: a0 is not defined. So based on the information presented here I can easily design an experiment where a0 is greater than a - simply because at the moment I can set a0 to any damn value I choose. This will be the case until a better explanation of the nature and definition of a0 is presented.

Now if this theory was published in a peer-reviewed journal, there is no way such essential aspects were omitted. I am complaining that the information presented here fails to make any sense, because 2 variables (which are critical to the end result) have not been defined. I am not commenting in any way on the theory itself.

A recent edit changed

it was expected that stars at the edge would move much slower than those near the bulge.

to

it was expected that stars at the edge would have an orbital period much larger than those near the bulge.

Both of these statements are correct, so I didn't revert the change. But the reason presented for the change

objects in larger orbits actually move faster, they just take longer to orbit

is incorrect. The velocity of an orbiting object is proportional to 1/√R, where R is the radius of the orbit (let's assume a circle). The angular velocity is proportional to 1/R3/2. Both the velocity and the angular velocity thus decrease as R increases. --AxelBoldt