QUASI-LOCAL MEASUREMENTS AND ORIENTATION IN BLACK-HOLE FIELDS

An observer travelling in the Kerr-Newman field can deduce the paramet ers of the host spacetime and those of his orbit from quasi-local meas urements only. A way to achieve this result is first to set oneself in to the orbit of an equatorial canonical (Carter) observer, and then me asure (i) the electric and magnetic fields, (ii) the linear velocities of the equatorial circular geodesics at a given radius (and possibly also those of geodesics which are purely latitudinal locally), (iii) o ne's thrust, (iv) the angular frequency of the 'Pharaoh's fan' and (v) the angular velocity of precession of comoving gyroscopes. We show th at the local experiments which are necessary to acquire all the desire d information can be performed in a general Kerr-Newman background and also in the special cases of Kerr, Reissner-Nordstrom and Schwarzschi ld spacetimes. We point out a close connection between the behaviour o f one of the observer's devices-the 'Pharaoh's fan'-and the Wilkins ef fect.