A particle of rest mass m is dropped from the top of the tower
and falls freely with acceleration g [ see Figure 5.1 ]. It
reaches the ground
with a velocity 
, so its total energy E,
as measured by an observer at the foot of the tower is
Suppose the observer has some magical method of converting all this
energy into a photon of the same energy [ this is a thought
experiment after all! ]. Upon its arrival at the top of the tower with
energy 
the photon is again magically changed into a particle of
rest mass 
. It must
be that 
; otherwise,
perpetual motion could result, so
. We therefore obtain:
and since 
and 
we find:
We therefore predict that a photon climbing in the earths gravitational field will lose energy and will consequently be redshifted. The redshift is:
This was tested by Pound and Snider
in 1965 using
the Mossbauer effect [ photons from atomic decay peak sharply at a particular
frequency ]. They measured the redshift experienced by a 14.4 Kev
rays from the decay of 
Fe in climbing up a 20 m
tower by determining the speed at which a detector at the top must
be moved in order to maximize the detection rate i.e. the velocity
blueshift balances the gravitational
redshift . They found:
This experimental verification of Einstein's thought experiment is
a death- blow of one's chances
of finding a simple special relativistic theory of gravity!