Lorenz attractor

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A plot of the trajectory Lorenz system for values ρ=28, σ = 10, β = 8/3

The Lorenz attractor, named for Edward N. Lorenz, is a 3-dimensional structure corresponding to the long-term behavior of a chaotic flow, noted for its butterfly shape. The map shows how the state of a dynamical system (the three variables of a three-dimensional system) evolves over time in a complex, non-repeating pattern.

1 Overview
2 Equations
3 Butterfly effect
4 Rayleigh number
5 See also
6 References
7 External links

[edit] Overview

A trajectory of Lorenz's equations, rendered as a metal wire to show direction and 3D structure

The attractor itself, and the equations from which it is derived, were introduced by Edward Lorenz in 1963, who derived it from the simplified equations of convection rolls arising in the equations of the atmosphere.

From a technical standpoint, the system is nonlinear, three-dimensional and deterministic. In 2001 it was proven by Warwick Tucker that for a certain set of parameters the system exhibits chaotic behavior and displays what is today called a strange attractor. The strange attractor in this case is a fractal of Hausdorff dimension between 2 and 3. Grassberger (1983) has estimated the Hausdorff dimension to be 2.06 ± 0.01 and the correlation dimension to be 2.05 ± 0.01.

The system arises in lasers, dynamos, and specific waterwheels. ^[1]

[edit] Equations

Trajectory with scales added

The equations that govern the Lorenz attractor are:

$\frac{dx}{dt} = \sigma (y - x)$

$\frac{dy}{dt} = x (\rho - z) - y$

$\frac{dz}{dt} = xy - \beta z$

where $σ$ is called the Prandtl number and $ρ$ is called the Rayleigh number. All $σ$ , $ρ$ , $β$ > 0, but usually $σ$ = 10, $β$ = 8/3 and $ρ$ is varied. The system exhibits chaotic behavior for $ρ$ = 28 but displays knotted periodic orbits for other values of $ρ$ . For example, with $ρ = 99.96$ it becomes a T(3,2) torus knot.

[edit] Butterfly effect

Butterfly effect

Time t=1 (Enlarge) Time t=2 (Enlarge) Time t=3 (Enlarge)

These figures — made using ρ=28, σ = 10 and β = 8/3 — show three time segments of the 3-D evolution of 2 trajectories (one in blue, the other in yellow) in the Lorenz attractor starting at two initial points that differ only by 10^-5 in the x-coordinate. Initially, the two trajectories seem coincident (only the yellow one can be seen, as it is drawn over the blue one) but, after some time, the divergence is obvious.

Java animation of the Lorenz attractor shows the continuous evolution.

Butterfly effect
Time t=1 (Enlarge)	Time t=2 (Enlarge)	Time t=3 (Enlarge)

These figures — made using ρ=28, σ = 10 and β = 8/3 — show three time segments of the 3-D evolution of 2 trajectories (one in blue, the other in yellow) in the Lorenz attractor starting at two initial points that differ only by 10^-5 in the x-coordinate. Initially, the two trajectories seem coincident (only the yellow one can be seen, as it is drawn over the blue one) but, after some time, the divergence is obvious.
Java animation of the Lorenz attractor shows the continuous evolution.

[edit] Rayleigh number

The Lorenz attractor for different values of ρ

ρ=14, σ=10, β=8/3 (Enlarge) ρ=13, σ=10, β=8/3 (Enlarge)

ρ=15, σ=10, β=8/3 (Enlarge) ρ=28, σ=10, β=8/3 (Enlarge)

For small values of ρ, the system is stable and evolves to one of two fixed point attractors. When ρ is larger than 24.28, the fixed points become repulsors and the trajectory is repelled by them in a very complex way, evolving without ever crossing itself.

Java animation showing evolution for different values of ρ

The Lorenz attractor for different values of ρ

ρ=14, σ=10, β=8/3 (Enlarge)	ρ=13, σ=10, β=8/3 (Enlarge)

ρ=15, σ=10, β=8/3 (Enlarge)	ρ=28, σ=10, β=8/3 (Enlarge)
For small values of ρ, the system is stable and evolves to one of two fixed point attractors. When ρ is larger than 24.28, the fixed points become repulsors and the trajectory is repelled by them in a very complex way, evolving without ever crossing itself.
Java animation showing evolution for different values of ρ