Here is a Shockwave movie illustrating the
deflection
of light in a gravitational field, and
here is a Java applet illustrating the
shift
in
star position produced by the Sun's gravitational field.
Gravitational Lensing
One important
consequence of
Einstein's gravitation
is that gravitational masses can alter the direction of light and cause
lensing effects. We have already seen a weak example of this when we discussed the slight
bending of starlight that can be observed near the Sun in a total solar eclipse (see the animations
in the adjacent box).
However, the bending of light can become much more dramatic when large masses are involved, as in clusters of
galaxies, or when there are
very high concentrations of mass, as for neutron stars or black holes. We have already discussed
these effects for neutron stars and black holes. Here we address large-scale lensing effects produced by
galaxies, clusters of galaxies, and their associated dark matter.