Three Body Problem: Dark Matter's Impact

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Discussion Overview

The discussion revolves around the classification of galaxies in the context of the three-body problem, particularly considering the role of dark matter. Participants explore whether galaxies can be treated as a three-body problem or if they are better understood as many-body systems, while also discussing the complexities of solving the three-body problem in general.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • Some participants argue that three galaxies held together by dark matter do not constitute a three-body problem due to their loose structure, suggesting that the Sun, Earth, and Moon serve as a better example.
  • Others propose that if galaxies are sufficiently distant from each other, their relative motions could be analyzed as a three-body problem, provided there are no other galaxies or dark matter concentrations nearby.
  • A participant mentions that galaxies might be treated as fluid dynamical problems due to their many-body nature, drawing an analogy between gravity and van der Waals forces.
  • Historical approaches to modeling galaxies included analytical methods, but current practices favor numerical simulations, as highlighted by references to the Toomre brothers' work.
  • One participant presents a hypothetical scenario involving a satellite orbiting Earth, discussing the forces acting on it and proposing a simplified equation related to the three-body problem.
  • There is a discussion about the existence of solutions to the three-body problem, with some participants asserting that numerical methods are the only viable approach, while others question whether an analytical solution might still be possible.
  • Concerns are raised regarding the accuracy of numerical integrations due to truncation and round-off errors, contrasting them with the desire for analytical solutions that avoid such issues.

Areas of Agreement / Disagreement

Participants express differing views on whether galaxies can be treated as a three-body problem, with no consensus reached. There are also varying opinions on the nature of solutions to the three-body problem, particularly regarding the feasibility of analytical versus numerical methods.

Contextual Notes

Some discussions reference the limitations of analytical solutions in the context of chaotic systems and the need for precise definitions of perturbations, indicating that the discussion is constrained by these unresolved complexities.

  • #31
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  • #32
Very interesting web sites. Nice simulations!

The unrestricted simulations done by my program are interactive. You can edit the values of the initial conditions of the bodies and observe their amended trajectories.
 
  • #33
Hi khavel,
Your simulations work incrementally, right?
As in each step is calculated from the previous step?
And the smaller the time increment, the more accurate the simulation, right??
 
  • #34
Quote from khavel's site (http://www.grevytpress.com/enbody.pdf ):
In an n-body gravitational system, the trajectory of anybody generally depends on the masses, positions, and velocities of all remaining bodies.
To determine the trajectories of all bodies, we set a time interval deltaT for the recalculation. Obviously, the shorter the time interval results in more precise calculation of the trajectories. At the beginning of the time interval we know the masses, positions, and velocities of all bodies. During time interval deltaT we calculate for each body the sum of accelerations imparted by gravitation of all other bodies, from their masses and positions. Then, for each body, we respectively integrate the sums of the accelerations over time interval deltaT, to obtain the increment of its velocity. We add the increment of its velocity to its previous velocity, to obtain its new velocity. Then we integrate its new velocity over time interval deltaT, to obtain the increment of its position. We add the increment of its position to its previous position, to obtain its new position. Since acceleration, velocity, and position are vectors, all calculations are done with their x and y components.
We keep repeating all calculations, over successive time intervals deltaT, using the new positions and velocities of all bodies. This method of calculation is valid for any number of bodies. Needless to say, the complexity of calculation increases with the number of bodies.

'nuff said. Khavel has produced an incremental numerical simulation, not an analytic one. Nothing new here.
 
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