# Tag Info

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Not all nonlinear systems are chaotic. However a chaotic system is necessarily nonlinear. There doesn't exists a definition for chaos but using the one given by Strogatz, ref 1: Chaos is aperiodic long-termed behavior in a deterministic system that exhibits sensitive dependence on initial conditions. Like explained in the text: aperiodic long-termed ...

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There are nonlinear versions of the Schrodinger equation that are completely irrelevant to your question. These are like the Gross-Pitaevski equation, they are nonlinear classical field equations that describe the flow of a self-interacting superfluid or BEC. These equations have nothing to do with the evolution of probability amplitudes, and I will not ...

8

Numerical simulations are not always meaningful, as chaos theory belongs to the large subject of dynamical systems theory. Although the definitions differ, chaos generally occurs in three contexts: Sensitive dependence on initial conditions (SDIC). The set is topologically transitive. Periodic points are dense in the set. Think of two particles having ...

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The interplay of Hamiltonian and Lagrangian theory is based on the following general identities, where $L$ is the Lagrangian function of the system, $$\dot{q}^k = \frac{\partial H}{\partial p_k}\:,\qquad(1)$$ $$\frac{\partial L}{\partial q^k} = -\frac{\partial H}{\partial q^k}\:.\qquad(2)$$ Above, the RH sides are functions of $t,q,p$ whereas the LH sides ...

7

If you want to generalize a potential to a class that's broader than the simple $\frac12 k_2 x^2$, it is tempting as a first step to include a small perturbation of the form $\frac13k_3x^3$. Unfortunately, this drastically changes the structure of the potential, because it becomes unbounded from below. Thus, you might get a slightly perturbed behaviour ...

5

@Ron Maimon has given the canonical answer to this: the wavefunction is probabilities, and to preserve probabilities one must have a linear equation (indeed, also a norm-preserving evolution operator). I offer another viewpoint, in the style of how Einstein thought about relativity, i.e. two postulates. The postulate is that it is not possible to solve ...

5

Three different points of views on essentially the same thing: Chaotic systems are not only sensitive to numerical errors, but also to any other small perturbations, such as dynamical noise, which may simulate real conditions. Though tiny perturbations affect the detailled, microscopic future of a system, its qualitative dynamics is unaffected. And the ...

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The essential idea of a Poincaré map is to boil down the way you represent a dynamical system. For this, the system has to have certain properties, namely to return to some region in it’s state space from time to time. This is fulfilled if the dynamics is periodic, but it also works with chaotic dynamics. To give a simple example, instead of analysing the ...

4

Since the rate of change of $x$ is the same as the rate of change of $y$ you really only a single equation of with one variable. We write, $$x = y + c$$ where the constant $c$ is determined by your initial conditions, $$c = x (0) - y (0)$$ (in your case it is the difference between the ...

4

It is an interesting problem. Usually you would find the infinitesimal oscillations by setting $p_i = b + \delta p_i$, $q_i = a + \delta q_i$ and expanding the Hamiltonian to second order in the $\delta p_i$, $\delta q_i$. Here though, this doesn't work as you just get the same Hamiltonian, $$H = \sum_{i,j} M_{ij} \sqrt{(\delta p_i - \delta p_j)^2 + (\delta ... 4 1D Burger's equation is not meant to model a physical phenomenon. Rather, it is a simplification of homogeneous incompressible Navier-Stokes equations that preserves (some of) its mathematical structure: the non-linear convection term and the second order derivative of viscous forces. It was initially intended as a useful simplification to try to ... 4 A linear system is one whose dynamics obeys linear differential equations, in contrast with those that are non-linear whose dynamics obeys non-linear differential equations. So if the dyanmics of the variable x(t) obeys a a differential equation$$f\left(x(t),\frac{d}{dt}x(t),\dots,\frac{d^n}{dt^n}x(t),t\right)=0,$$if x_1(t) and x_2(t) are differente ... 4 (1) In general, what is meant by non-linear system in classical mechanics? A linear system is described by a set of differential equations that are a linear combination of the dependent variable and its derivatives. Some examples of linear systems in classical mechanics: A damped harmonic oscillator,$$m \frac{d^2 x(t)}{dt^2} + c \frac{d x(t)}{dt} + k ...

3

In the action formalism a linear Euler-Lagrange (EL) equation corresponds to a quadratic action, i.e. an action which is quadratic in the dynamical field variables of the theory. On the other hand, self-coupling or interaction terms in the action correspond to cubic or higher terms. Such terms leads to non-linear EL equations. See also this related Phys.SE ...

3

As requested by the OP, I gather my points in an answer. Linear systems Linear systems are systems which are linear with respect to a physical quantity. Mathematically, their evolution can be written as a (possibly differential) equation. Examples: A linear spring is linear in the sense that is produces a force proportional to the displacement it ...

3

I take the core of the question to be Is it possible to do linear stability analysis on 2nd order differential equations by finding eigen values of Jacobian matrix? The answer is yes, but first you have to convert your second-order equations into first-order ones. This is actually pretty easy to do: every time you see a second derivative, e.g. ...

3

One of the simplest measurements you can do is to characterize the spatial mode coming out of the laser. This can be done with a simple photodiode and a scanning slit (optical chopper or razor blade mounted on a movable stage). I wrote a brief note on making such a measurement here. This isn't the most interesting of measurements if you aren't going to ...

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A classical paper on this is Weinberg's Testing Quantum Mechanics.

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Yes, there is investigation. Some random names on the field (more on the physics side, NO specific order): Carl Dettmann, Tamás Tél, Ott, Ying-Cheng Lai, Adilson Motter, Celso Grebogi, Holger Kantz, Alessandro Moura, Eduardo G. Altmann, etc, etc, etc. A quick search on some of these names should help you to find some recent papers on what is being done ...

3

The other answers address how to solve this analytically, but I like numerical solutions to things so here goes: $$\frac{d}{dt} \begin{bmatrix}x \\ y \end{bmatrix} = -\begin{bmatrix}kxy\\kxy\end{bmatrix}$$ which can be solved using any number of numerical methods. For simplicity, we can take the second-order Runge-Kutta method where $i$ is the time index. ...

3

Hints: Conclude that $y-x=c$ is a constant. Use separation of variables $-k\int \!\mathrm{d}t= \int \!\frac{\mathrm{d}x}{x(x+c)}$.

3

There is a normalized form, though it's properly called the dimensionless Euler equations. The way to do it is define: scale time $t_0$ scale density $\rho_0$ scale length $L_0$ and then derive the scales from these: $$v_0 = \frac{L_0}{t_0},\quad p_0=\rho_0v_0^2$$ NB: it is possible to use other combinations, but I find that these are often the ...

3

As the first question has received sufficient exposition, I would like to make a point with regard to the second one. First thing to understand is that integrability and non-linearity of a system are two different concepts. It is true though that all linear systems in classical mechanics (i.e those that are described by systems of linear equations, be them ...

2

I can think of a method, although it may require to compute $\mathbf{F}_0$ for a very large number of test points. It is based on Gauss's law for gravity $$\frac1{m_0}\oint_{S} \mathbf{F}\cdot\mathrm d\mathbf S=-4\pi GM_S$$ where $S$ is a closed surface and $M_S$ is the total mass contained inside it. So the idea would be to use some numerical scheme to ...

2

There is of course research going on. For example the field of billiards is still very active. People study e.g. billiards with non-trivial reflection rules. In general this is done in Math departments rather than Physics, although there are a lot of physicists working there.

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You might try measuring the temporal coherence of the laser. (This came to mind 'cause it's closely related to the PhD work I would have done if I didn't quit with an ABD :-( ). Basically, you split the beam with, e.g., a 50-50 splitter, and recombine one way or another to get an intereference pattern. Then use an "optical trombone" (the meaning should ...

2

However, what if the time series is multi-dimensional, indeed of the same dimension as the phase space, to begin with? Well, how would you know that your time series is of the same dimension as the phase space? Usually, because you already know the dynamical equations for your system (as for your pendulum). If you observe a real-life complex system, ...

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Clarifications and addition: It's true that not all nonlinear systems are chaotic, but that all chaotic systems are nonlinear (or infinite-dimensional linear). The sensitivity to initial conditions is an important point and the commenter raises a good question. Consider the Lorenz system in a non-chaotic parameter regime (or for that matter, any stable ...

1

Usually averages of any total time derivatives vanish. Easiest way to see this is to use the fact that steady state time averages and ensemble averages should be the same if the system is ergodic....therefore $$<dA/dt>= \lim_{T \to \infty} (1/T) \int_0^T dt dA/dt = \lim_{T \to \infty} (1/T) [A(T)-A(0)] =0$$ unless $A(t)$ is unbounded.

1

I suppose it is related to hyperbolic partial differential equations.

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