Harvard Tower Experiment
 |
In just 22.6 meters, the fractional gravitational
red shift given by
 is just 4.92 x
10^-15 , but the Mossbauer
effect with the 14.4 keV gamma ray from iron-57
has a high enough resolution to detect that difference. In the early
60's physicists Pound, Rebka,and Snyder at the Jefferson Physical
Laboratory at Harvard measured the shift to within 1% of the
predicted shift. |
By just using the expression for gravitational
potential energy near the Earth, and using the m in the relativistic
energy expression, the gain in energy for a photon which falls
distance h is
Comparing the energy shifts on the upward and downward paths gives a
predicted difference
The measured difference was
The success of this experiment owed much to the care of Pound and Rebka
in preparing the source. They electroplated cobalt-57 onto the surface of
a thin sheet of iron and then heated the combination at 1220 K for an
hour. The heat treatment caused the cobalt to diffuse into the iron to a
depth of about 300 nm or 1000 atomic spacings. The source was then mounted
on the cone of a loudspeaker driven at 10Hz to sweep the source velocity
in a sinusoidal variation. The detector was a thin sheet of iron about 14
micrometers thick which was also annealed. The heat treatments were found
to be crucial in obtaining high resolution. |
Index
General
relativity ideas
References
|
