For most spherical astronomical objects except black holes the distance of closest approach R will be very much greater than the mass M of the object, measured in units of length. To analyze light passing close to these objects we can validly apply the results of this section, the weak field approximation [15]. Motion of light passing close to a black hole is analyzed in Chapter 5 and in Section 10 of this project.

QUERY 12 Deflection of light by a neutron star. How great is the deflection of light that skims past the surface of a neutron star? Assume that a nonspinning neutron star has a mass 1.4 times the mass of Sun and a radius of 10 kilometers. What is the deflection of light that skims tangentially past its surface, according to equation [15]? Express your result in degrees. Do you trust this conclusion? That is, does this case satisfy approximations used in deriving equation [15]?

6 Comparison with Observation

How well do observations verify the prediction of light deflection by Sun given in equation [15]? Table 1 shows measurements using visible light. Note: Observed stars were at various angles from the eclipsed Sun. Figures in the fourth column are values recomputed for the deflection of a light ray that just grazes Sun's surface.

Table 1 Deflection of starlight by Sun; values deduced in various eclipses

Eclipse Date





of stars

(seconds of arc)

(end of project)

May 20,1919


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