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Imagine a double pendulum consisting of two pieces with different constant lengths and constant masses. This double pendulum dissipates no energy.

  1. When you let it go, you can vary the two angles in the pendulum and the angular momenta of the pieces or the potential energy of the DP. When will the strange attractor pattern ("ordered" chaos) arise for the DP?
  2. If so, how does it look like (in 3d)?

Other examples of a strange attractor are the Rössler attractor, which can be associated with the chaotic behavior of a dripping tap or a fibrillating heart, or the Lorenz attractor with its now quite iconic butterfly shape. They all follow for certain values in a set of non-linear coupled differential equations, which in the Lorenz attractor are the rate of convection (in 2d) on the x-axis and the temperature on the y- and z-axis The form of the attractor can be seen in this 3d space.
The coupled pendulum has four coupled differential equations (corresponding to the two angles and two angular momenta), but I couldn't find how the strange attractor looks like (which is impossible for four variables) in 3d.

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I assume that you are talking about an undamped undriven double pendulum.

In this case the motion exhibited by the double pendulum may be chaotic (depending on the intial conditions, lengths of the pendulum arms and masses), but it exhibits no attractors in the sense that trajectories converge to a certain invariant set (the attractor) in phase space. Instead all initial conditions are on some invariant set. For a reason, see this question of mine. So, it’s strange, but no strange attractor.

Does this pendulum, when you let it go, which can be done in many ways but let's keep the total potential energy always equal, always describe a figure that after a certain elapsed time resembles a part of a circle with a radius that's equal to the sum of the lengths of the two pendulums?

I understand the dynamics you are describing as one where both pendulum arms are always aligned and the pendulum behaves like a single pendulum. This corresponds to a regular, non-chaotic attractor. However, as already said, the double pendulum (as assumed above) does not have attractors at all. For a suited small excitation of both pendulums, you get a behaviour close to what you are describing, but it immediately starts to exhibit that motion – not just after a certain time.

To make such a dynamics happen, you would, e.g., have to have some friction within the double pendulum’s joint and nowhere else. (If you have friction elsewhere, the pendulum has a single fixed point as an attractor, namely its resting position.)

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  • $\begingroup$ I understand that if you take many different initial situations (all with the same initial potential energy (for simplicity lets say it´s less than the maximum potential energy possible) for the double pendulum), the path that the end of the pendulum traces out, never ends in the same situation (unlike a ball in a sink that always ends up in the sink, no matter the initial conditions of the ball. But isn´t the end of the pendulum tracing out a figure that after a long time wil start taking a shape, like the butterfly figure (order in chaos), no matter what the initial conditions are? $\endgroup$ – Deschele Schilder Feb 5 '16 at 17:11
  • $\begingroup$ the path that the end of the pendulum traces out, never ends in the same situation – That’s not entirely correct: There are initial conditions that provide periodic solutions. — But isn´t the end of the pendulum tracing out a figure that after a long time wil start taking a shape […], no matter what the initial conditions are? – Yes, it will describe some shape, but this is not related to an attractor, simply because it does not attract. Also, this shape depends on the initial conditons. For example, it will be fundamentally different for chaotic and periodic dynamics. $\endgroup$ – Wrzlprmft Feb 5 '16 at 18:07
  • $\begingroup$ To make such a dynamics happen, you would, e.g., have to have some friction within the double pendulum’s joint and nowhere else. Why do have to have friction to make the dynamics of the DP happen? $\endgroup$ – Deschele Schilder Aug 21 at 6:46
  • $\begingroup$ @descheleschilder: You need friction to make that specific dynamics (full circle with maximum radius) happen. You also need friction (or some other dissipation of energy) to have an attractor. $\endgroup$ – Wrzlprmft Aug 21 at 7:27
  • $\begingroup$ Okay. But why you need friction to make the dynamics happen or to let the system have a strange attractor? $\endgroup$ – Deschele Schilder Aug 21 at 7:39

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