Re:A Physicist Who Denies That Dark Matter Exists
by pdonis on Monday February 27, 2017 @12:48PM
(#13746153)
> "If Newtonian physics can’t predict the fixed curves, perhaps we should fix Newton, instead of making up a whole new class of matter just to fit our measurements."
The problem with this is that we already know Newtonian physics is wrong. It makes incorrect predictions for planetary orbits in the solar system (the original wrong prediction was the motion of Mercury, but the perihelion shift that is not predicted by Newtonian physics but is predicted by general relativity has now been verified, AFAIK, for all the inner planets). So the way to "fix Newton" is to adopt general relativity instead.
And once you adopt general relativity, you can't just arbitrarily change the equation for "acceleration due to gravity" the way MOND does. That equation is not a free parameter in GR; it's derived from the Einstein Field Equation. So you would have to change the Einstein Field Equation, and that would change all of the predictions of GR, not just the prediction for galaxy rotation curves.
What I've just stated is the standard objection to MOND among physicists. Milgrom mentions relativistic formulations of MOND and says that this issue is "no longer a problem", but I think that's much too optimistic.
The problem with this is that we already know Newtonian physics is wrong. It makes incorrect predictions for planetary orbits in the solar system (the original wrong prediction was the motion of Mercury, but the perihelion shift that is not predicted by Newtonian physics but is predicted by general relativity has now been verified, AFAIK, for all the inner planets). So the way to "fix Newton" is to adopt general relativity instead.
And once you adopt general relativity, you can't just arbitrarily change the equation for "acceleration due to gravity" the way MOND does. That equation is not a free parameter in GR; it's derived from the Einstein Field Equation. So you would have to change the Einstein Field Equation, and that would change all of the predictions of GR, not just the prediction for galaxy rotation curves.
What I've just stated is the standard objection to MOND among physicists. Milgrom mentions relativistic formulations of MOND and says that this issue is "no longer a problem", but I think that's much too optimistic.