  Experimental tests on `time dilation' began in 1938 with Ives and Stilwell's
work of the transverse Doppler effect due to atoms in inertial flight. Rossi
and Hall (1941) inaugurated the era of fast moving elementary particles that
dominated the scene until the discovery of the Mossbauer effect (1957). This
discovery suggested the use of photons emitted without recoil in crystalline
solids for testing both time dilation and gravitational red shift. Finally,
around 1970, Hafele and Keating dealt again with time dilation by sending
macroscopic atomic clocks around the Earth. The interpretations of these
experiments by experimenters have been characterized by the use of additional
hypotheses not necessary for the formal development of the theories under test
(the idea that all clocks measure proper time) or hypotheses completely
extraneous to the theories themselves (the idea that atoms are clocks). If
these assumptions are dropped, it turns out that the only experiments
concerning time dilation are those performed with elementary particles in
inertial flight. The historical and epistemological implications are discussed.
