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The two-body problem in general relativity (or relativistic two-body problem) is the determination of the motion and gravitational field of two bodies as described by the field equations of general relativity. Solving the Kepler problem is essential to calculate the bending of light by gravity and the motion of a planet orbiting its sun
One starts with a high accuracy value for the position (x, y, z) and the velocity (v x, v y, v z) for each of the bodies involved. When also the mass of each body is known, the acceleration (a x, a y, a z) can be calculated from Newton's Law of Gravitation. Each body attracts each other body, the total acceleration being the sum of all these ...
Let x 1 and x 2 be the vector positions of the two bodies, and m 1 and m 2 be their masses. The goal is to determine the trajectories x 1 (t) and x 2 (t) for all times t, given the initial positions x 1 (t = 0) and x 2 (t = 0) and the initial velocities v 1 (t = 0) and v 2 (t = 0). When applied to the two masses, Newton's second law states that
where is the semimajor axis of the planet's orbit relative to the Sun; and are the masses of the planet and Sun, respectively. This simplification is sufficient to compute rough estimates of fuel requirements, and rough time-of-flight estimates, but it is not generally accurate enough to guide a spacecraft to its destination.
The last general constant of the motion is given by the conservation of energy H. Hence, every n-body problem has ten integrals of motion. Because T and U are homogeneous functions of degree 2 and −1, respectively, the equations of motion have a scaling invariance: if q i (t) is a solution, then so is λ −2/3 q i (λt) for any λ > 0. [18]
The red planet is stationary; the force F(r) is balanced by a repulsive inverse-cube force. A GIF version of this animation is found here. Figure 2: The radius r of the green and blue planets are the same, but their angular speed differs by a factor k. Examples of such orbits are shown in Figures 1 and 3–5.
He eventually summarized his results in the form of three laws of planetary motion. [2] What is now called the Kepler problem was first discussed by Isaac Newton as a major part of his Principia. His "Theorema I" begins with the first two of his three axioms or laws of motion and results in Kepler's second law of planetary motion. Next Newton ...
For example, in a system composed of one planet orbiting a star, the motion of the planet is determined by solving the field equations with the energy–momentum tensor the sum of that for the planet and the star. The gravitational field of the planet affects the total spacetime geometry and hence the motion of objects. It is therefore ...