Bertrand's theorem

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In classical mechanics, Bertrand's theorem states that only two types of potentials produce stable, closed orbits: an inverse-square force such as the gravitational or electrostatic potential

[V(\mathbf) = \frac]

and the radial harmonic oscillator potential

[V(\mathbf) = \frac kr^]

Contents

1 General Preliminaries
2 Bertrand's theorem
3 Inverse-square force (Kepler problem)
4 Radial harmonic oscillator
5 Reference

General Preliminaries

All attractive central forces can produce closed, circular orbits; all that's required is to match the force with the centripetal force requirement and calculate the required angular velocity. Non-central forces (i.e., those that depend on the angular variables as well as the radius) do not produce circular orbits in general, so we ignore them here.

The equation of motion for the radius [r] of a particle of mass [m] moving in a central potential [V(r)] is given by Lagrange's equations

[m\fracr}} - mr \omega^ = m\fracr}} - \frac}} = -\frac]

where [\omega \equiv \frac] and the angular momentum [L = mr^\omega] is conserved. For illustration, the first term on the left-hand side is zero for circular orbits, and the applied inwards force [\frac] equals the centripetal force requirement [mr \omega^], as expected.

The angular-momentum definition allows a change of independent variable from [t] to [\theta]

[\frac = \frac} \frac]

giving the new equation of motion

[\frac} \frac \left( \frac} \frac \right)- \frac}} = -\frac]

This equation becomes quasilinear on making the change of variables [u \equiv \frac] and multiplying both sides by [\frac}}]

[\fracu}} + u = -\frac} \frac V(1/u)]

Bertrand's theorem

As noted above, all central forces can produce closed, circular orbits given an appropriate initial velocity. However, these orbits need not be stable (i.e., remain in orbit indefinitely if some radial velocity is introduced) nor closed (repeatedly returning to exactly the same path). Here we show that stable, exactly closed orbits can be produced only with an inverse-square force or radial harmonic oscillator potential (a necessary condition). In the following sections, we show that those force laws do produce stable, exactly closed orbits (a sufficient condition).

For brevity, we introduce the function [J(u)] into the equation for [u]

[\fracu}} + u = J(u) \equiv -\frac} \frac V(1/u) = -\fracu^} f(1/u)]

where [f] represents the radial force. The criterion for perfectly circular motion at a radius [r_] is that the first term on the left-hand side be zero

[

u_ = J(u_)]

where [u_ \equiv 1/r_].

The next step is to consider the equation for [u] under small perturbations [\eta \equiv u - u_] from perfectly circular orbits. On the right-hand side, the [J] function can be expanded in a standard Taylor series

[J(u) \approx u_ + \eta J^(u_) + \frac \eta^ J^(u_) + \frac \eta^ J^(u_) + \ldots]

Substituting this expansion into the equation for [u] and subtracting the constant terms yields

[\frac\eta}} + \eta = \eta J^(u_) + \frac \eta^ J^(u_) + \frac \eta^ J^(u_) \ldots]

which can be written as

[\frac\eta}} + \beta^ \eta = \frac \eta^ J^(u_) + \frac \eta^ J^(u_) \ldots]

where [\beta^ \equiv 1 - J^(u_)] is a constant. [\beta^] must be non-negative; otherwise, the radius of the orbit would vary exponentially away from its initial radius. (The solution [\beta=0] corresponds to a perfectly circular orbit.) If the right-hand side may be neglected (i.e., for very small perturbations), the solutions are

[\eta(\theta) = h_ \cos \beta\theta]

where the amplitude [h_] is a constant of integration. For the orbits to be closed, [\beta] must be a rational number. What's more, it must be the same rational number for all radii, since [\beta] cannot change continuously; the rational numbers are totally disconnected from one another. Since the defining equations

[J^(u_) \equiv -2 + \frac})} \frac = 1 - \beta^ ]

must hold for any value of [u_], we can write

[\frac = \left( \beta^ - 3 \right) \frac]

which implies that the force must follow a power law

[f(r) = - \frac}}]

Hence, [J] must have the general form

[J(u) = \frac} u^}]

For more general deviations from circularity (i.e., when we cannot neglect the higher order terms in the Taylor expansion of [J]), [\eta] may be expanded in a Fourier series, e.g.,

[\eta(\theta) = h_ + h_ \cos \beta \theta + h_ \cos 2\beta \theta + h_ \cos 3\beta \theta + \ldots]

Substituting this solution into both sides of the equation for [\eta] and equating the coefficients belonging to the same frequency yields the system of equations

[h_ = h_^ \frac(u_)}}]

[h_ = -h_^ \frac(u_)}}]

[h_ = -\frac} \left[ h_h_ frac(u_)} +h_^ frac(u_)} right]]

and, most importantly,

[ \left( 2 h_ h_ + h_ h_ \right) \frac(u_)} + h_^ \frac(u_)} = 0]

This last equation, when combined with the equation for [J] in terms of [\beta], yields the main result of Bertrand's theorem

[\beta^ \left( 1 - \beta^ \right) \left( 4 - \beta^ \right) = 0]

Hence, the only potentials that can produce stable, closed, non-circular orbits are the inverse-square force law ([\beta = 1]) and the radial harmonic oscillator potential ([\beta = 2]). The solution [\beta = 0] corresponds to perfectly circular orbits, as noted above.

Inverse-square force (Kepler problem)

For an inverse-square force law such as the gravitational or electrostatic potential, the potential can be written

[V(\mathbf) = \frac = -ku]

The orbit [u(\theta)] can be derived from the general equation

[\fracu}} + u = -\frac} \frac V(1/u) = \frac}]

whose solution is the constant [\frac}] plus a simple sinusoid

[

u \equiv \frac = \frac} \left[ 1 + e cos left( theta - theta_right) right]]

where [e] (the eccentricity) and [\theta_] (the phase offset) are constants of integration.

This is the general formula for a conic section that has one focus at the origin; [e=0] corresponds to a circle, [e<1] corresponds to an ellipse, [e=1] corresponds to a parabola, and [e>1] corresponds to a hyperbola. The eccentricity [e] is related to the total energy [E] (cf. the Laplace-Runge-Lenz vector)

[

e = \sqrt}m}}]

Comparing these formulae shows that [E<0] corresponds to an ellipse, [E=0] corresponds to a parabola, and [E>0] corresponds to a hyperbola. In particular, [E=-\fracm}}] for perfectly circular orbits.

Radial harmonic oscillator

To solve for the orbit under a radial harmonic oscillator potential, it's easier to work in components [\mathbf = (x, y, z)]. The potential energy can be written

[V(\mathbf) = \frac kr^ = \frac k \left( x^ + y^ + z^\right)]

The equation of motion for a particle of mass [m] is given by three independent Lagrange's equations

[\fracx}} + \omega_^ x = 0]

[\fracy}} + \omega_^ y = 0]

[\fracz}} + \omega_^ z = 0]

where the constant [\omega_^ \equiv \frac] must be positive (i.e., [k>0]) to ensure bounded, closed orbits; otherwise, the particle will fly off to infinity. The solutions of these simple harmonic oscillator equations are all similar

[x = A_ \cos \left(\omega_ t + \phi_ \right)]

[y = A_ \cos \left(\omega_ t + \phi_ \right)]

[z = A_ \cos \left(\omega_ t + \phi_ \right)]

where the positive constants [A_], [A_] and [A_] represent the amplitudes of the oscillations and the angles [\phi_], [\phi_] and [\phi_] represent their phases. The resulting orbit [\mathbf(t) = \left[ x(t), y(y), z(t) right]] is closed because it repeats exactly after a period

[T \equiv \frac}]

The system is also stable because small perturbations in the amplitudes and phases cause correspondingly small changes in the overall orbit.

Reference

Goldstein H. (1980) Classical Mechanics, 2nd. ed., Addison-Wesley. ISBN 0201029189

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