I Galaxies as systems extended from the solar system

AI Thread Summary
Galaxies do not adhere to Kepler's laws, which are applicable only to two-body systems, and are instead modeled using Newton's laws. The discussion highlights that galactic behavior is fundamentally Newtonian, though it may require adjustments for dark matter and general relativistic effects. The small sample size of the Solar System raises questions about the validity of extrapolating its dynamics to galaxies. Modified Newtonian Dynamics (MOND) is mentioned as one potential framework, but it is noted that there are multiple modified theories available. The conversation emphasizes the need for further understanding of dark matter and alternative gravitational models.
Martyn Arthur
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The application of Kepler's third law to galaxies
Is it a big assumption that Galaxies should follow Kepler's third law with rotation speeds decreasing with distance from the centre. Is the small tet of the behaviour of the Solar System not too small an example to make such a substantial assumption?
Thanks
Martyn
 
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Martyn Arthur said:
Is it a big assumption that Galaxies should follow Kepler's third law
They don't follow Kepler's laws. Those only apply to two masses.

They are modelled as following Newton's laws (GR corrections are too small to worry about).
Martyn Arthur said:
Is the small tet of the behaviour of the Solar System not too small an example to make such a substantial assumption?
With the caveat that you mean Newton's laws not Kepler's the answer is:

1 - no, galactic behaviour is purely Newtonian, we just need to work out what dark matter is.

2 - sort of, we need to include general relativistic corrections to a Newtonian model.

3 - yes, all we need to do is work out how the modified theory, Modified Newtonian Dynamics (MOND), works.

Take your pick which is the correct one...
 
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Martyn
 
Ibix said:
They don't follow Kepler's laws. Those only apply to two masses.
Technically they apply to problems of central motion in a Kepler potential (proportional to 1/r). The second law applies more generally to any central potential as it is related to conservation of angular momentum. However, the first and third laws are contingent on the problem actually being a Kepler central potential.

The gravitational two-body problem of course reduces to a Kepler central potential problem once the center of mass motion is factored out.
 
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Ibix said:
They don't follow Kepler's laws. Those only apply to two masses.

They are modelled as following Newton's laws (GR corrections are too small to worry about).

With the caveat that you mean Newton's laws not Kepler's the answer is:

1 - no, galactic behaviour is purely Newtonian, we just need to work out what dark matter is.

2 - sort of, we need to include general relativistic corrections to a Newtonian model.

3 - yes, all we need to do is work out how the modified theory, Modified Newtonian Dynamics (MOND), works.

Take your pick which is the correct one...
There is more than one modified theory. MOND is not the only game in town and is at best a "toy-model".
 
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