16
M. R. SCHROEDER
three important experimental tests during Einstein's lifetime:
1) The perihelion motion of the orbit of the planet Mercury, which was
already known from earlier astronomical observations.
2) The bending of light waves near the sun, first observed during the total
eclipse of 1919 by the Eddington expedition.
3) The gravitational red shift, first seen in the light from massive stars, but
now measurable even on Earth herself using the ultrasensitive
Moessbauer effect.
A fourth effect inherent in Einstein's theory was not confirmed until fairly
recently: the slowing of electro-magnetic radiation in a gravitational field.1
This effect was observed by means of radar echoes from the planets Venus
and Mercury as they disappeared behind the sun as seen from the earth
("superior conjunction"). In that position, both the outgoing and returning
radar waves have to travel near (indeed around) the sun. Even after taking
plasma effects near the sun's surface and other factors into account, physicists
found an extra delay of 200 JIS - very close to the prediction of general
relativity [5].
Why was this measurement not done long ago? The reason is that the
echo energy from Mercury - exceedingly weak even when visible - drops to
10~27 of the outgoing energy as the planet slips behind the sun. The
astounding fact that reliable results have been obtained in spite of these
miniscule reflected energies is due mainly to the proper choice of the
transmitted sequence of radar pulses, based on primitive polynomials over
finite number fields.
In the long struggle to put his principle of general equivalence of different references
frames into proper mathematical clothing, Einstein discovered - as early as 1909 - that the
speed of light could not be constant (as in special relativity) but must depend on the
gravitational potential [ . Although he had no general theory then, Einstein found that, to
first order, c())) = C + |)/c , where c is the usual vacuum velocity of light in field-free
space (Note: j 0). Ironically, the slowing of radiation i n gravitational fields, although
appreciated very early, was not considered a testable proposition until the perfection of
radar technology, using Galois sequences, in the second half of this century. The reason for
this delay in testing the extra delay was, of course, that no one could picture himself (or
anyone else, for that matter) floating next to the sun, stopwatch in hand, clocking the
passing photons.
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