In physics and classical mechanics, the three-body problem is the problem of taking the initial positions and velocities (or momenta) of three point masses and solving for their subsequent motion according to Newton's laws of motion and Newton's law of universal gravitation. The three-body problem is a special case of the n-body problem. Unlike two-body problems, no general closed-form solution exists, as the resulting dynamical system is chaotic for most initial conditions, and numerical methods are generally required.
Historically, the first specific three-body problem to receive extended study was the one involving the Moon, the Earth, and the Sun. In an extended modern sense, a three-body problem is any problem in classical mechanics or quantum mechanics that models the motion of three particles.
Mathematical description
The
mathematical statement of the three-body problem can be given in terms
of the Newtonian equations of motion for vector positions of three gravitationally interacting bodies with masses :
where is the gravitational constant. This is a set of 9 second-order differential equations. The problem can also be stated equivalently in the Hamiltonian formalism, in which case it is described by a set of 18 first-order differential equations, one for each component of the positions and momenta :
where is the Hamiltonian:
In this case is simply the total energy of the system, gravitational plus kinetic.
Restricted three-body problem
In the restricted three-body problem,
a body of negligible mass (the "planetoid") moves under the influence
of two massive bodies. Having negligible mass, the planetoid exerts no
force on the two massive bodies, which can therefore be described in
terms of a two-body motion. Usually this two-body motion is taken to
consist of circular orbits around the center of mass, and the planetoid is assumed to move in the plane defined by the circular orbits.
The restricted three-body problem is easier to analyze
theoretically than the full problem. It is of practical interest as well
since it accurately describes many real-world problems, the most
important example being the Earth-Moon-Sun system. For these reasons, it
has occupied an important role in the historical development of the
three-body problem.
Mathematically, the problem is stated as follows. Let be the masses of the two massive bodies, with (planar) coordinates and , and let
be the coordinates of the planetoid. For simplicity, choose units such
that the distance between the two massive bodies, as well as the
gravitational constant, are both equal to . Then, the motion of the planetoid is given by
where . In this form the equations of motion carry an explicit time dependence through the coordinates .
However, this time dependence can be removed through a transformation
to a rotating reference frame, which is an important simplification in
any subsequent analysis.
Solutions
General solution
There is no general analytical solution to the three-body problem given by simple algebraic expressions and integrals. Moreover, the motion of three bodies is generally non-repeating, except in special cases.
On the other hand, in 1912 the Finnish mathematician Karl Fritiof Sundman proved that there exists a series solution in powers of t1/3 for the 3-body problem. This series converges for all real t,
except for initial conditions corresponding to zero angular momentum.
(In practice the latter restriction is insignificant since such initial
conditions are rare, having Lebesgue measure zero.)
An important issue in proving this result is the fact that the
radius of convergence for this series is determined by the distance to
the nearest singularity. Therefore, it is necessary to study the
possible singularities of the 3-body problems. As it will be briefly
discussed below, the only singularities in the 3-body problem are binary
collisions (collisions between two particles at an instant) and triple
collisions (collisions between three particles at an instant).
Collisions, whether binary or triple (in fact, any number), are
somewhat improbable, since it has been shown that they correspond to a
set of initial conditions of measure zero. However, there is no
criterion known to be put on the initial state in order to avoid
collisions for the corresponding solution. So Sundman's strategy
consisted of the following steps:
- Using an appropriate change of variables to continue analyzing the solution beyond the binary collision, in a process known as regularization.
- Proving that triple collisions only occur when the angular momentum L vanishes. By restricting the initial data to L ≠ 0, he removed all real singularities from the transformed equations for the 3-body problem.
- Showing that if L ≠ 0, then not only can there be no triple collision, but the system is strictly bounded away from a triple collision. This implies, by using Cauchy's existence theorem for differential equations, that there are no complex singularities in a strip (depending on the value of L) in the complex plane centered around the real axis (shades of Kovalevskaya).
- Find a conformal transformation that maps this strip into the unit disc. For example, if s = t1/3 (the new variable after the regularization) and if |ln s| ≤ β, then this map is given by
This finishes the proof of Sundman's theorem.
Unfortunately, the corresponding series converges very slowly.
That is, obtaining a value of meaningful precision requires so many
terms that this solution is of little practical use. Indeed, in 1930,
David Beloriszky calculated that if Sundman's series were to be used for
astronomical observations, then the computations would involve at least
108000000 terms.
Special-case solutions
In 1767, Leonhard Euler found three families of periodic solutions in which the three masses are collinear at each instant.
In 1772, Lagrange
found a family of solutions in which the three masses form an
equilateral triangle at each instant. Together with Euler's collinear
solutions, these solutions form the central configurations for the three-body problem. These solutions are valid for any mass ratios, and the masses move on Keplerian ellipses. These four families are the only known solutions for which there are explicit analytic formulae. In the special case of the circular restricted three-body problem, these solutions, viewed in a frame rotating with the primaries, become points which are referred to as L1, L2, L3, L4, and L5, and called Lagrangian points, with L4 and L5 being symmetric instances of Lagrange's solution.
In work summarized in 1892–1899, Henri Poincaré
established the existence of an infinite number of periodic solutions
to the restricted three-body problem, together with techniques for
continuing these solutions into the general three-body problem.
In 1893, Meissel stated what is now called the Pythagorean
three-body problem: three masses in the ratio 3:4:5 are placed at rest
at the vertices of a 3:4:5 right triangle. Burrau further investigated this problem in 1913. In 1967 Victor Szebehely and C. Frederick Peters
established eventual escape for this problem using numerical
integration, while at the same time finding a nearby periodic solution.
In the 1970s, Michel Hénon and Roger A. Broucke
each found a set of solutions that form part of the same family of
solutions: the Broucke–Henon–Hadjidemetriou family. In this family the
three objects all have the same mass and can exhibit both retrograde and
direct forms. In some of Broucke's solutions two of the bodies follow
the same path.
In 1993, a zero angular momentum solution with three equal masses
moving around a figure-eight shape was discovered numerically by
physicist Cris Moore at the Santa Fe Institute. Its formal existence was later proved in 2000 by mathematicians Alain Chenciner and Richard Montgomery.
The solution has been shown numerically to be stable for small
perturbations of the mass and orbital parameters, which raises the
intriguing possibility that such orbits could be observed in the
physical universe. However, it has been argued that this occurrence is
unlikely since the domain of stability is small. For instance, the
probability of a binary-binary scattering event resulting in a figure-8 orbit has been estimated to be a small fraction of 1%.
In 2013, physicists Milovan Šuvakov and Veljko Dmitrašinović at
the Institute of Physics in Belgrade discovered 13 new families of
solutions for the equal-mass zero-angular-momentum three-body problem.
In 2015, physicist Ana Hudomal discovered 14 new families of
solutions for the equal-mass zero-angular-momentum three-body problem.
In 2017, researchers Xiaoming Li and Shijun Liao found 669 new
periodic orbits of the equal-mass zero-angular-momentum three-body
problem. This was followed in 2018 by an additional 1223 new solutions for a zero-momentum system of unequal masses.
In 2018, Li and Liao reported 234 solutions to the unequal-mass "free-fall" three body problem.
The free fall formulation of the three body problem starts with all
three bodies at rest. Because of this, the masses in a free-fall
configuration do not orbit in a closed "loop", but travel forwards and
backwards along an open "track".
Numerical approaches
Using a computer, the problem may be solved to arbitrarily high precision using numerical integration although high precision requires a large amount of CPU time. In 2019, Breen et al. announced a fast neural network solver, trained using a numerical integrator.
History
The gravitational problem of three bodies in its traditional sense dates in substance from 1687, when Isaac Newton published his "Principia" (Philosophiæ Naturalis Principia Mathematica).
In Proposition 66 of Book 1 of the "Principia", and its 22 Corollaries,
Newton took the first steps in the definition and study of the problem
of the movements of three massive bodies subject to their mutually
perturbing gravitational attractions. In Propositions 25 to 35 of Book
3, Newton also took the first steps in applying his results of
Proposition 66 to the lunar theory, the motion of the Moon under the gravitational influence of the Earth and the Sun.
The physical problem was addressed by Amerigo Vespucci and subsequently by Galileo Galilei;
in 1499, Vespucci used knowledge of the position of the Moon to
determine his position in Brazil. It became of technical importance in
the 1720s, as an accurate solution would be applicable to navigation,
specifically for the determination of longitude at sea, solved in practice by John Harrison's invention of the marine chronometer. However the accuracy of the lunar theory was low, due to the perturbing effect of the Sun and planets on the motion of the Moon around the Earth.
Jean le Rond d'Alembert and Alexis Clairaut,
who developed a longstanding rivalry, both attempted to analyze the
problem in some degree of generality; they submitted their competing
first analyses to the Académie Royale des Sciences in 1747. It was in connection with their research, in Paris during the 1740s, that the name "three-body problem" (French: Problème des trois Corps)
began to be commonly used. An account published in 1761 by Jean le Rond
d'Alembert indicates that the name was first used in 1747.
Other problems involving three bodies
The term 'three-body problem' is sometimes used in the more general sense to refer to any physical problem
involving the interaction of three bodies.
A quantum mechanical analogue of the gravitational three-body problem in classical mechanics is the helium atom,
in which a helium nucleus and two electrons interact according to the inverse-square Coulomb interaction. Like the
gravitational three-body problem, the helium atom cannot be solved exactly.
In both classical and quantum mechanics, however, there exist nontrivial interaction laws besides the inverse-square force which
do lead to exact analytic three-body solutions. One such model consists of a combination
of harmonic attraction and a repulsive inverse-cube force. This model is considered nontrivial
since it is associated with a set of nonlinear differential equations containing singularities (compared with,
e.g., harmonic interactions alone, which lead to an easily solved system of linear differential equations). In
these two respects it is analogous to (insoluble) models having Coulomb interactions, and as a result
has been suggested as a tool for intuitively understanding physical systems like the helium atom.
The gravitational three-body problem has also been studied using general relativity. Physically, a relativistic
treatment becomes necessary in systems with very strong gravitational fields, such as near
the event horizon of a black hole. However, the relativistic problem
is considerably more difficult than in Newtonian mechanics, and sophisticated numerical techniques are required.
Even the full two-body problem (i.e. for arbitrary ratio of masses) does not have a rigorous analytic solution in general relativity.
n-body problem
The three-body problem is a special case of the n-body problem, which describes how n
objects will move under one of the physical forces, such as gravity.
These problems have a global analytical solution in the form of a
convergent power series, as was proven by Karl F. Sundman for n = 3 and by Qiudong Wang for n > 3. However, the Sundman and Wang series converge so slowly that they are useless for practical purposes; therefore, it is currently necessary to approximate solutions by numerical analysis in the form of numerical integration or, for some cases, classical trigonometric series approximations (see n-body simulation). Atomic systems, e.g. atoms, ions, and molecules, can be treated in terms of the quantum n-body problem. Among classical physical systems, the n-body problem usually refers to a galaxy or to a cluster of galaxies; planetary systems, such as stars, planets, and their satellites, can also be treated as n-body systems. Some applications are conveniently treated by perturbation
theory, in which the system is considered as a two-body problem plus
additional forces causing deviations from a hypothetical unperturbed
two-body trajectory.
In popular culture
- The problem is a plot device in the science fiction trilogy by Chinese author Cixin Liu, and its name has been used for both the first volume and the trilogy as a whole.