Appendices
277
this figure can be accounted for by Newton’s theory, and another 530 arcseconds/century is caused by perturbations to the orbit from the outer planets.
This leaves 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.
A.7.2 Gravitational Redshift of Light
What happens when light tries to escape from a really intense gravitational
field, such as that in the neighbourhood of a black hole? In Chap. 7,
we discussed the event horizon that surrounds a black hole, and saw that
nothing, not even light, can exit from within this region. Another way of
explaining this is to say that the gravitational redshift of light is so severe that
the light is redshifted to zero frequency. Zero frequency light, of course, does
not exist.
As mentioned in Chap. 7, the first reliable verification of the gravitational
redshift was made in 1954 by Daniel M. Popper [10]. He utilised the line
spectra (see Chap. 8) emitted by a hot gas, and measured the frequency of 42
hydrogen lines on 27 spectrograms of the white dwarf star, 40 Eridani B. He
obtained a frequency shift of 0.007 percent, which was within experimental
error of the value predicted by General Relativity.
So sensitive have modern experiments become that it is no longer necessary to use astronomical observations to test General Relativity. Instead,
atomic clocks have been flown around the world, and rockets launched tens
of kilometres into space to study the time dilation caused by gravity. Such
an experiment in 1976 found General Relativity to be accurate to within
0.007% [11].
A.7.3 Gravitational Lensing
Suppose that we have a situation where a distant star or galaxy lies behind a
region where the gravitational field is intense. Light passing through this field
will be deflected, and as a consequence the distant object will appear to lie in
a different location from its true position.
This effect, known as gravitational lensing, is illustrated in Fig. A.7. Light
from the source S passing above the massive object G will appear to be
coming from location A, while light passing below G will appear to originate at location B. Fig. A.7 is a two-dimensional representation of a scenario
277
this figure can be accounted for by Newton’s theory, and another 530 arcseconds/century is caused by perturbations to the orbit from the outer planets.
This leaves 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.
A.7.2 Gravitational Redshift of Light
What happens when light tries to escape from a really intense gravitational
field, such as that in the neighbourhood of a black hole? In Chap. 7,
we discussed the event horizon that surrounds a black hole, and saw that
nothing, not even light, can exit from within this region. Another way of
explaining this is to say that the gravitational redshift of light is so severe that
the light is redshifted to zero frequency. Zero frequency light, of course, does
not exist.
As mentioned in Chap. 7, the first reliable verification of the gravitational
redshift was made in 1954 by Daniel M. Popper [10]. He utilised the line
spectra (see Chap. 8) emitted by a hot gas, and measured the frequency of 42
hydrogen lines on 27 spectrograms of the white dwarf star, 40 Eridani B. He
obtained a frequency shift of 0.007 percent, which was within experimental
error of the value predicted by General Relativity.
So sensitive have modern experiments become that it is no longer necessary to use astronomical observations to test General Relativity. Instead,
atomic clocks have been flown around the world, and rockets launched tens
of kilometres into space to study the time dilation caused by gravity. Such
an experiment in 1976 found General Relativity to be accurate to within
0.007% [11].
A.7.3 Gravitational Lensing
Suppose that we have a situation where a distant star or galaxy lies behind a
region where the gravitational field is intense. Light passing through this field
will be deflected, and as a consequence the distant object will appear to lie in
a different location from its true position.
This effect, known as gravitational lensing, is illustrated in Fig. A.7. Light
from the source S passing above the massive object G will appear to be
coming from location A, while light passing below G will appear to originate at location B. Fig. A.7 is a two-dimensional representation of a scenario