Dark Matter

[juanrga]

The problem with MOND is that it is a phenomenological theory that uses arbitrary functions and some new constant to fit observations. That is why in some cases it goes wrong.

 

MOND goes wrong only when it is not understood or when it misapplied beyond its scope. MOND was born from phenomenological (as most of laws of physics) but MOND is now derived from first principles. Several theories derived MOND. Moreover, MOND does not really use any new constant to fit observations (Milgrom's constant is related to c and Hubble H).

Edited May 26, 2012 by juanrga

[Alberto]

MOND goes wrong only when it is not understood or when it misapplied beyond its scope. MOND was born from phenomenological (as most of laws of physics) but MOND is now derived from first principles. Several theories derived MOND. Moreover, MOND does not really use any new constant to fit observations (Milgrom's constant is related to c and Hubble H).

 

OK, as you like. But I would only appreciate, if you mention those first principles. I mean, not the statement that the Newton law should be changed (because of the observations), but the reasons why it should be changed for exactly the form it obtains in MOND. I'm also interested in "several theories" deriving MOND, and it would be really geat if you mention two or three of them. Though if you mean the Beckenstein approach, one can say that it possesses the same drawback as other alternatives to the GRT. As to the constant, Milgrom does mention that it is rather close to the cH product, but no reason for that is suggested and no role of c and H in MOND is explained.

[juanrga]

OK, as you like. But I would only appreciate, if you mention those first principles. I mean, not the statement that the Newton law should be changed (because of the observations), but the reasons why it should be changed for exactly the form it obtains in MOND. I'm also interested in "several theories" deriving MOND, and it would be really geat if you mention two or three of them. Though if you mean the Beckenstein approach, one can say that it possesses the same drawback as other alternatives to the GRT. As to the constant, Milgrom does mention that it is rather close to the cH product, but no reason for that is suggested and no role of c and H in MOND is explained.

 

Different authors advance/suggest different interpretations using different theories. Some authors think that non-newtonian effects are due to changes in inertia other think that the effects are due to changes in the gravitational interaction. If my memory does not fail Milgrom likes the inertia explanation, but I like the other. There are many theories. Some of the theories are AQUAL, PCG, TeVeS...

[Alberto]

Different authors advance/suggest different interpretations using different theories. Some authors think that non-newtonian effects are due to changes in inertia other think that the effects are due to changes in the gravitational interaction. If my memory does not fail Milgrom likes the inertia explanation, but I like the other. There are many theories. Some of the theories are AQUAL, PCG, TeVeS...

 

 

"Different authors" and "many theories" don't tell me too much. Aside from AQUAL, PCG, TeVeS, one can also peep into 'Alternative theories of gravitation' in Wikipedia and find others. Discussing WHO PREFERS WHAT - either change in inertia or change in gravitational interaction - is also not very fruitful, because science is neither a question of personal taste nor a question settled by a Parliament majority (though, unfortunately, the current mainstream resembles this ruling party very much ). If one just chooses, say, a law of gravitation to be SUCH&SUCH, it doesn't mean that we have got a theory, even if it describes the observations. This is NOT first principles. Those extra fields that one can find in most of the alternatives (including the covariantized MOND) are hardly better than dark matter. Actually, it is just it that those fields describe.

 

The first principles could be the principle of relativity or the principle of equivalence, then they could lead to the geometrization... But since the way it was done in GRT caused the descrepancies on the galactic scale, and, consequently, the introduction of huge dark matter with the property of non-observability, it means that there is a need for another set of principles. By the way, the anisotropic geometrodynamics which I mentioned before uses equivalence first and relativity only afterwords and only as a limit case. That is, it starts with the world full of gravitating masses and only then looks what would be if there is no masses. And not vice versa as it is usually done. It seems logical and attractive, especially when I find no DM in the end. The law of gravitation has also changed there, but not because the author decided that it should be 'like that' or used the observations. It just TURNED OUT that it SHOULD be slightly different in a specific manner, if we follow the line of speculations stemming from the chosen first principles. The cH coincidence is explained there too.

[juanrga]

The first principles could be the principle of relativity or the principle of equivalence, then they could lead to the geometrization... But since the way it was done in GRT caused the descrepancies on the galactic scale, and, consequently, the introduction of huge dark matter with the property of non-observability, it means that there is a need for another set of principles. By the way, the anisotropic geometrodynamics which I mentioned before uses equivalence first and relativity only afterwords and only as a limit case. That is, it starts with the world full of gravitating masses and only then looks what would be if there is no masses. And not vice versa as it is usually done. It seems logical and attractive, especially when I find no DM in the end. The law of gravitation has also changed there, but not because the author decided that it should be 'like that' or used the observations. It just TURNED OUT that it SHOULD be slightly different in a specific manner, if we follow the line of speculations stemming from the chosen first principles. The cH coincidence is explained there too.

 

Interesting work, he gives an order of magnitude cH (but does not gives the value of a₀) and he assumes a geometrisation principle, which is not valid for the general potentials, but interesting.

Edited May 31, 2012 by juanrga

[Alberto]

Interesting work, he gives an order of magnitude cH (but does not gives the value of a₀) and he assumes a geometrisation principle, which is not valid for the general potentials, but interesting.

 

 

Which work have you read? The book?

Would you kindly mention what do you mean by a0 and general potentials? If a0 is a kind of MOND constant, it cannot have the exact value there, because it is a theory and not a fitting sample. I'm also a bit doubtful about the impossibility of geometrization. Could you write a couple of words about it? It could throw a new light.

[Yaptro]

Oh... Dark matter...

https://iversity.org/c/25?r=6358a - might be interesting. It's a free online course "Dark matter in galaxies"

[Perkinsjc12]

I am not a man of Physics so my post is more philosophical than scientific. But it occurred to me that all observations are made in the present so a logical frame of reference for the passage of time would also be the present (at least philosophically if not mathematically). With such a frame of reference, an object would not "move" forward in time, but rather it's past would propagate (?) further back in time from the present. And if two objects can occupy the same space as long as they are separated by time, then how much time need elapse before this could occur. If it is infinitesimal then could an object which is infinitesimally separated from the present, time wise still have observable effects in the present. I've been trying to reconcile the observations in quantum physics where a photon can seemingly go back in time and change it's properties from a particle to a wave and vice-versa. And if time does indeed propagate negatively from the present when viewed from a present frame of reference, then it might be possible that an object can exert a force on the present while existing ever so slightly in the past. Could that be dark matter? Could that be plausible mathematically?

[antwis06]

what if dark matter was a knot in space. if "space" had a constant extremely low gravitational pull witch had knotted its self some how like a piece of string that would explain the increase in gravitational pull and no change in mass because it would be gravity pulling and gravity.