General relativity
${\displaystyle G_{\mu \nu }+\Lambda g_{\mu \nu }={\kappa }T_{\mu \nu }}$
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According to general relativity, 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.^[1]

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 Δt_GM which could be, in principle, be measured^[2][3] if S is known.

On the contrary, if the validity of general relativity is assumed, it is possible to use Δt_GM to measure S. Such effect must not be confused with the much larger Shapiro time delay^[4] Δt_GE induced by the "gravitoelectric" Schwarzschild-like component of the gravitational field of a planet of mass M considered non-rotating. Unlike the small Δt_GM, the Shapiro time delay has been accurately measured in several radar-ranging experiments with Solar System interplanetary spacecraft.

See also

• Introduction to general relativity
• Gravitomagnetic clock effect

References