The Carter constant is a conserved quantity for motion around black holes in the general relativistic formulation of gravity. Carter's constant was derived for a spinning, charged black hole by Australian theoretical physicist Brandon Carter in 1968. Carter's constant along with the energy, axial angular momentum, and particle rest mass provide the four conserved quantities necessary to uniquely determine all orbits in the Kerr–Newman spacetime (even those of charged particles).
Carter noticed that the Hamiltonian for motion in Kerr spacetime was separable in Boyer–Lindquist coordinates, allowing the constants of such motion to be easily identified using Hamilton-Jacobi theory. The Carter constant can be written as follows:
where is the latitudinal component of the particle's angular momentum, is the energy of the particle, is the particle's axial angular momentum, is the rest mass of the particle, and is the spin parameter of the black hole. Because functions of conserved quantities are also conserved, any function of and the three other constants of the motion can be used as a fourth constant in place of . This results in some confusion as to the form of Carter's constant. For example it is sometimes more convenient to use:
in place of . The quantity is useful because it is always non-negative. In general any fourth conserved quantity for motion in the Kerr family of spacetimes may be referred to as "Carter's constant".
As generated by a Killing tensor
Noether's theorem states that all conserved quantities are related to spacetime symmetries. Carter's constant is related to a higher order symmetry of the Kerr metric generated by a second order Killing tensor field (different than used above). In component form:
where are the components of the metric tensor and and are the components of the principal null vectors:
The spherical symmetry of the Schwarzschild metric for non-spinning black holes allows one to reduce the problem of finding the trajectories of particles to three dimensions. In this case one only needs , , and to determine the motion; however, the symmetry leading to Carter's constant still exists. Carter's constant for Schwarzschild space is:
By a rotation of coordinates we can put any orbit in the plane so . In this case , the square of the orbital angular momentum.
- Kerr metric
- Kerr–Newman metric
- Boyer–Lindquist coordinates
- Hamilton–Jacobi equation
- Euler's three-body problem
- Carter, Brandon (1968). "Global structure of the Kerr family of gravitational fields". Physical Review. 174 (5): 1559&ndash, 1571. Bibcode:1968PhRv..174.1559C. doi:10.1103/PhysRev.174.1559.
- Misner, Charles W.; Thorne, Kip S.; Wheeler, John Archibald (1973). Gravitation. New York: W. H. Freeman and Co. p. 899. ISBN 0-7167-0334-3.