7 General Relativity
133
a well-known example that we have discussed in earlier chapters. However,
the enormous progress in computational technology since the latter half
of the 20 th Century has facilitated an alternative approach. Physicists now
use numerical techniques and fast computers to obtain the trajectories of
the planets and space probes with the desired accuracy. Likewise, numerical techniques are now being employed in “numerical relativity”, which is
a burgeoning research field.
Being as it is, a theory of gravity, General Relativity is of most importance in domains where gravity overweighs the other forces of nature, i.e. in
the cosmos at large. In this domain, experimentation, as it usually pertains
to physics, is very difficult, and astronomers largely rely on observations of
events over which they have no control. Nevertheless, by chance, natural
phenomena do arise from time to time that provide an opportunity to subject
the predictions of General Relativity to observational test. We shall discuss
some examples of these in the following Sections.
7.6 Further Observational Evidence for General
Relativity
Changes in Orbit of Mercury
Since the time of Johannes Kepler (d. 1630) it has been known that the orbits
of the planets about the sun follow an elliptical path. Newton showed how
his law of gravity explained this phenomenon. However, closer observation
revealed that the major axis 8 of the orbit of mercury rotates slowly about the
sun. The effect, much exaggerated, is shown below in Fig. 7.9.
The vast majority of this precession is accounted for by Newton’s theory,
and the effects of the outer planets (see Appendix 7.1). However, there
remains a 40 arcseconds/century discrepancy, which is very well explained
by General Relativity, when one includes the distortion of space–time caused
by the gravitational mass of the sun.
The explanation of the hitherto mysterious anomaly in Mercury’s orbit,
combined with Eddington’s observation of the bending of light by gravity,
remained virtually the only experimental tests of General Relativity for the
best part of half a century. In 1916 Einstein had proposed three possible
tests of his theory [6]. The third, known as the gravitational redshift, was not
carried out successfully until 1954.
8 An ellipse has two axes of symmetry. The larger of these is known as the “major axis”.
133
a well-known example that we have discussed in earlier chapters. However,
the enormous progress in computational technology since the latter half
of the 20 th Century has facilitated an alternative approach. Physicists now
use numerical techniques and fast computers to obtain the trajectories of
the planets and space probes with the desired accuracy. Likewise, numerical techniques are now being employed in “numerical relativity”, which is
a burgeoning research field.
Being as it is, a theory of gravity, General Relativity is of most importance in domains where gravity overweighs the other forces of nature, i.e. in
the cosmos at large. In this domain, experimentation, as it usually pertains
to physics, is very difficult, and astronomers largely rely on observations of
events over which they have no control. Nevertheless, by chance, natural
phenomena do arise from time to time that provide an opportunity to subject
the predictions of General Relativity to observational test. We shall discuss
some examples of these in the following Sections.
7.6 Further Observational Evidence for General
Relativity
Changes in Orbit of Mercury
Since the time of Johannes Kepler (d. 1630) it has been known that the orbits
of the planets about the sun follow an elliptical path. Newton showed how
his law of gravity explained this phenomenon. However, closer observation
revealed that the major axis 8 of the orbit of mercury rotates slowly about the
sun. The effect, much exaggerated, is shown below in Fig. 7.9.
The vast majority of this precession is accounted for by Newton’s theory,
and the effects of the outer planets (see Appendix 7.1). However, there
remains a 40 arcseconds/century discrepancy, which is very well explained
by General Relativity, when one includes the distortion of space–time caused
by the gravitational mass of the sun.
The explanation of the hitherto mysterious anomaly in Mercury’s orbit,
combined with Eddington’s observation of the bending of light by gravity,
remained virtually the only experimental tests of General Relativity for the
best part of half a century. In 1916 Einstein had proposed three possible
tests of his theory [6]. The third, known as the gravitational redshift, was not
carried out successfully until 1954.
8 An ellipse has two axes of symmetry. The larger of these is known as the “major axis”.