Gravitomagnetic time delay
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In according to general relativity, a branch of physics, a massive spinning body endowed with angular momentum S will alter the space-time fabric around it in such a way that several effects on moving test particles and propagating electromagnetic waves occur.
In particular, the direction of motion with respect to the sense of rotation of the central body is relevant because co-and counter-propagating waves carry a "gravitomagnetic" time delay ΔtGM which could be, in principle, be measured  if S is known.
On the contrary, if the validity of general relativity is assumed, it is possible to use ΔtGM to measure S. Such effect must not be confused with the much larger Shapiro time delay  ΔtGE induced by the "gravitoelectric", Schwarzschild-like component of the gravitational field of a planet of mass M considered non-rotating. Contrary to the quite small ΔtGM, the Shapiro time delay has been accurately measured in several radar-ranging experiments with solar system interplanetary spacecraft.
- G.W. Richter, R.A. Matzner (1983). "Second-order contributions to gravitational deflection of light in the parametrized post-Newtonian formalism". Phys. Rev. D 26 (Austin, Texas). doi:10.1103/PhysRevD.26.1219.
- A. Tartaglia, M.L. Ruggiero (2004). "Gravitomagnetic Measurement of the Angular Momentum of Celestial Bodies". General Relativity and Gravitation 36 (Kluwer Academic Publishers-Plenum). doi:10.1023/B:GERG.0000010476.58203.b6.
- A. Tartaglia, M.L. Ruggiero (2002). "Angular Momentum Effects in Michelson–Morley Type Experiments". General Relativity and Gravitation 34 (Kluwer Academic Publishers-Plenum). doi:10.1023/A:1020022717216.
- I.I. Shapiro (1964). "Fourth Test of General Relativity". Phys. Rev. Lett. 13 (Lexington, Massachusetts). doi:10.1103/PhysRevLett.13.789.