Naive Question regarding Galaxy Rotation Curves

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Discussion Overview

The discussion revolves around the concept of galaxy rotation curves, the implications of dark matter, and the relationship between mass and velocity in the context of special and general relativity. Participants explore the necessity of dark matter in explaining observed galactic dynamics and question the relevance of relativistic mass in this framework.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants note that the visible mass in spiral galaxies is insufficient to account for observed orbital velocities, leading to the introduction of dark matter with a proposed mass ratio of 5:1 relative to visible matter.
  • One participant questions whether the velocity-mass relationship from special relativity applies in general relativity and how this might affect models of galaxies with and without dark matter.
  • Another participant argues that relative motion does not alter the dynamics of a system, using the example of a coffee mug to illustrate that mass does not increase in a way that affects gravitational attraction in a practical sense.
  • There is a suggestion to abandon the concept of relativistic mass, with references to discussions in the forum about its outdated status.
  • A participant seeks clarification on the definition of mass in modern physics compared to classical mechanics, expressing confusion about the implications of relativistic effects on mass.
  • Participants discuss the implications of high relative velocities on the ability to accelerate further, noting that this is frame-dependent.

Areas of Agreement / Disagreement

Participants generally agree on the insufficiency of visible mass to explain galaxy rotation curves and the necessity of dark matter, but there is no consensus on the relevance of relativistic mass or the implications of velocity on mass in the context of general relativity.

Contextual Notes

There are unresolved questions regarding the application of relativistic mass in general relativity and the implications of high velocities on mass and gravitational dynamics. The discussion also highlights differing views on the definition and relevance of mass in modern physics.

Loudzoo
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Many apologies in advance if this question is ridiculous or if it has already been answered on another thread. I've searched and searched through the forums and haven't found the answer - please do direct me accordingly if that's possible. If not - please help!

Preamble:
We know from calculating Galaxy rotation curves that the visible mass in spiral galaxies is too low to be consistent with the observed orbital rotation velocity of galaxies.
Introducing the notion of dark matter to provide the "missing mass" solves the problem. Calculations demonstrate that dark matter would need to exist in a mass ratio of 5:1 to visible matter to explain galactic orbital rotation velocities.

In Special Relativity the mass of an object increases with velocity. According to the relativity calculators suggested by Marcus a relative velocity of 0.98c would increase the mass of an object by approx 5 times.

Question:
1) Does this velocity-mass relationship apply in general relativity? If so . . .
2) Would we expect to observe differences between (a) a model of a galaxy "at rest" with a dark to visible matter mass ratio of 5:1, and (b) a model of a visibly identical galaxy with velocity 0.98c (relative to our observational point of reference) and no dark matter?

Thank so much in advance for your help.
 
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Loudzoo said:
Would we expect to observe differences between (a) a model of a galaxy "at rest" with a dark to visible matter mass ratio of 5:1, and (b) a model of a visibly identical galaxy with velocity 0.98c (relative to our observational point of reference) and no dark matter?
Yes. For one, the relative motion can't change the dynamics of the system. For example, take an object in your hand, like a coffe mug. At this very moment, you and the mug are traveling at 0.999999c relative to some frame of reference. Do you feel the cup suddenly grow in mass and attract you to itself with its great gravitational potential, while at the same time you are crushed by the increadibly-inflated mass of the Earth?

Furthermore, the galactic rotation curves need dark matter to explain not simply due to missing mass, but also due to the way the missing mass needs to be distributed. Otherwise you could just postulate that the central black hole is more massive than we thought and voila! Dark matter needs to be in a halo-like structure around galaxies to make the curves work.

By the way, best drop the whole idea of relativistic mass altogether. The forum is full of posts detailing why it is an outdated concept.
 
Thank you very much Bandersnatch. I had no idea relativistic mass was such an outmoded concept. It explains why few ask questions such as mine. I found this thread (www.physicsforums.com/threads/relativistic-mass.642188/) but if you know of one with a clearer critique one that would be much appreciated!

On that thread - is the following statement correct?
"Physicists are not trying to change the definition of mass as implicit given by Newton's p=m·v. With this definition mass was invariant in classical mechanics. In modern physics the definition of mass has been changed to make it invariant in relativity too."

With your coffee cup example I understand you wouldn't witness any change in the mass of the mug (or yourself) - you'd still be able to lift it to your mouth to drink from it. Furthermore you and the cup would not merge together under immense gravitational forces.

However you're ability to accelerate closer to the speed of light relative to the other frame of reference would be impeded by your already large relative velocity would it not?
 
Loudzoo said:
but if you know of one with a clearer critique one that would be much appreciated!
Check this thread from the relativity FAQ:
https://www.physicsforums.com/threads/what-is-relativistic-mass-and-why-is-it-not-used-much.783220/

Loudzoo said:
However you're ability to accelerate closer to the speed of light relative to the other frame of reference would be impeded by your already large relative velocity would it not?
Only from the point of view of that other frame of reference.
 
Last edited by a moderator:
Perfect - thank you very much.
 

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