WHEN THE spaceprobe Voyager 1 flew past Saturn in 1980, it not only
observed the planet and its moons but also provided scientists with data
that confirms Einstein’s general theory of relativity. In 1915, Einstein
predicted that light loses energy climbing out of the ‘gravitational well’
of a massive body, such as a planet. The effect is known as the gravitational
red shift, because as light loses energy its frequency is reduced, making
it appear redder.

Voyager 1 was able to test the red shift effect when it flew past Saturn,
using radio signals sent back to Earth by a transmitter locked at a very
precise frequency. The spaceprobe was equipped with an ultrastable crystal
oscillator, known as the USO. This acted as a very precise standard, varying
its frequency by less than 5 parts in 1012. Electronic equipment on board
multiplied the frequency of the oscillator by 120 times, to provide the
frequency of the spaceprobe’s radio transmitter – 2.3 gigahertz. These signals
were then picked up by the radio telescopes of NASA’s Deep Space Network,
which was used to track and control both Voyager spaceprobes.

Now, long after the event, Timothy Krisher and his colleagues at the
Jet Propulsion Laboratory in Pasadena, California, have examined the record
of the signal from Voyager 1. The spaceprobe flew within 180 000 kilometres
of Saturn on 12 November 1980. They find that as the spaceprobe moved in
and out of the gravitational field of Saturn, its transmitter’s frequency
dipped. The dip was no more than a few hertz, but it was within 1 per cent
of the prediction of general relativity for a frequency of 2.3 gigahertz.
Fluctuations in the frequency of the transmitter – though very small – nevertheless
prevented the theory from being tested even more severely (Physical Review
Letters, vol 64, p 1322).

Krisher and his colleagues have yet to determine whether data from the
transmitter on board Voyager 2 can help them to test general relativity
further. It was badly affected by intense radiation as it passed by Jupiter
on its way to Saturn and so its data may not be as reliable as the data
from its predecessor. The current results may, however, rule out some alternatives
to Einstein’s theory.