Many Body Problem: Physically & Mathematically Explained

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Discussion Overview

The discussion revolves around the challenges of the many-body problem in physics, particularly in the context of Newtonian gravity. Participants explore the physical and mathematical aspects of defining equations for three bodies interacting under gravitational forces, addressing the nature of nonlinearity and the absence of closed-form solutions.

Discussion Character

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

Main Points Raised

  • One participant claims it is impossible to define a set of equations for three bodies under gravity, questioning the solvability of such equations.
  • Another participant asserts that while equations can be defined, they lack a closed-form solution and must be solved numerically, leading to chaotic behavior in nonlinear dynamical systems.
  • A participant seeks clarification on how the equations become nonlinear and why they do not have closed-form solutions.
  • One reply suggests that the terminology used in the discussion is important, distinguishing between the "many-body problem" and the "N-body problem," with the former typically referring to a larger number of interacting bodies.
  • Another participant mentions that celestial mechanics literature extensively covers the three-body problem and references historical solutions found by Lagrange and Euler.
  • A participant explains that the nonlinearity arises from the dependence of gravitational force on the distance between bodies, and notes that most nonlinear differential equations do not have closed-form solutions, except in specific cases like the one-body problem.

Areas of Agreement / Disagreement

Participants express differing views on the terminology and the nature of the many-body problem. There is no consensus on the solvability of the equations or the implications of their nonlinearity, indicating ongoing debate and exploration of the topic.

Contextual Notes

Some participants highlight the importance of precise terminology in discussing the many-body problem versus the N-body problem, which may affect the clarity of the discussion. Additionally, the complexity of nonlinear differential equations and their solutions remains a point of contention.

Ed Quanta
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Using Newtonian physics, I have heard that it is impossible to define an equation or set of equations that describe the motion of three bodies interacting under the force of gravity. Can anyone demonstrate this physically or mathematically? I don't see how the equations become unsolvable.
 
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You certainly can define equations which describe many body problems. But they do not have a closed from solution. They must be solved numerically and since they are describing a non linear dynamical system the solution is falls into chaotic behavior.
 
I apologize for using the wrong word "define". I guess what I am curious about is how the equation becomes nonlinear or rather does not have a closed form solution.
 
Some Googling turned up this page which seems to have some good infromation on the problem, along with some nice java applets (look under current projects).
 
Ed Quanta said:
Using Newtonian physics, I have heard that it is impossible to define an equation or set of equations that describe the motion of three bodies interacting under the force of gravity. Can anyone demonstrate this physically or mathematically? I don't see how the equations become unsolvable.

To make sure you don't get into a state of confusion, note that your question actually is more related to N-body problem. "Many-Body" problem typically is reserved for a "gazillion" body interaction where many-body techniques are used, especially in condensed matter physics (See, for example, G.D. Mahan's standard text "Many-Particle Physics").

So if you are googling, it is good to make sure one knows what terminology is appropriate for what one is looking for.

Zz.
 
I'm sure that at your library u can find at least one book on celestial mechanics (describing gravitational interactions with Newtonian theory).All books on celestial mechanics should exhaust the subject of 3 body-problem.

Oh,and one more thing,add to what Integral has said.Lagrange and Euler have found particular solutions to this problem (3-body interaction in Newtonian gravity). (check "Lagrange points"...:wink:).

As for terminology,"Many Body Problem" refers to what Zapper said...

Daniel.
 
"how the equation becomes nonlinear "

It doesn't "become" nonlinear because there are more than 2 bodies, it is already non linear because one of the dependent variables is r, the distance between the two bodies and the force (so second derivative of r) depends on 1/r2.

As for why it does not have a closed form solution: Almost ALL nonlinear differential equations do not have closed form solutions! One exception is the "one-body" problem where you assume one of the gravitating bodies is so massive compared with the other than it can be considered and unmoving source for the force. The two-body problem can be reduce to "one-body" by using the center of mass and treating each body separately.
 
Thanks again, you guys are a lot better than some of my professors.
 

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