Everybody knows that light travels in straight lines, but while that
is its natural tendency light can be deflected by lenses, mirrors, and by
gravitational fields. Newtonian mechanics predicts that a particle
traveling at the speed of light which just grazes the edge of the
Sun will be deflected by 0.875 seconds of arc. That means that the
image we see of a star will be displaced away from the Sun by this
angle. The figure below shows this with the black showing the situation
when the Sun is not close to the star. When the Sun is nearly blocking
the star its image is deflected outward giving the red image. This
Newtonian model also predicts that the gravitational attraction of the
Sun will make light travel faster close to the Sun, so according to
Newton the deflected
light arrives before the undeflected light. The figure shows the red light
pulse arriving before the black light pulse.
Of course the travel
time for starlight is very hard to measure, and the deflection of
starlight can only be measured during a total eclipse of the Sun.
The deflection angle is actually very small, and in the figure it has
been increased by a factor of nearly 10,000 for clarity.
Einstein predicts that light will be delayed instead of accelerated
when passing close to the Sun.
Notice in the figure above that the green light pulse arrives after the
black light pulse.
This effect is closely related to
the deflection of starlight. Since
times can be measured to much greater accuracy than arcsecond angles,
the greatest accuracy on this effect is now given by measuring the time delay
instead of the angle. In order to measure the time delay one needs a
a spacecraft behind the Sun instead of a star.
This was first done by Irwin Shapiro
In a very real sense, the delay experienced by light passing a massive object
is responsible for the deflection of the light. The figure below shows a
bundle of rays passing the Sun at various distances. The rays are always
perpendicular to the wavefronts which mark the set of points with constant travel
time from the star. In order to bend the light toward the star one needs to
delay the wavefront near the star.
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© 2004 Edward L. Wright. Last modified 29 Dec 2004