Arago’s
Inadvertent

Test of Relativity

Greg Gbur

Portrait of Arago.

Wikimedia Commons/Engraving by A.V. Sixdeniers from a painting by H. Scheffer

Nearly 100 years before Einstein’s theory of special relativity, François Arago
unknowingly found the first experimental evidence for it.

The year 2010 marked an unusual and little known milestone in the history of physics: the 200-year anniversary of the first experimental evidence for Einstein’s special relativity. In 1810, French physicist François Arago (1786- 1853) attempted and failed to measure expected variations in the speed of light coming from distant stars. Of course, Einstein’s theory of special relativity, which postulates the constancy of the vacuum speed of light, would not be proposed until 1905. Arago didn’t discover relativity, but his experiment nevertheless had a remarkable influence on 19th century physics and a role in the acceptance of the wave theory of light.

The years leading up to Arago’s
experiment were trying ones for the
young researcher. In 1806, at age 20, he
undertook a trip to Spain with Jean-
Baptiste Biot on behalf of the Académie
des Sciences to perform mountaintop
triangulation measurements of the
curvature of the Earth. Things went
so smoothly that Biot soon returned
to France and left Arago in charge. In
late 1807, Napoleon invaded Spain, and
angry Spanish locals assumed that the
Frenchman shining mysterious lights at
the mountain’s peak was signaling the
French army.

While at the Academie, Arago focused his attention on answering a question that had been weighing on astronomer’s minds: Is the speed of light from all stars constant?

imprisoned on the island of Ibiza for
his own protection. However, when
he was presented news accounts of his
own execution, he read between the
lines and escaped, heading to Algiers
and then catching a ship back towards
safety in Marseilles. The Algerian ship
was intercepted by the Spanish, though,
and Arago was imprisoned a second
time on the Spanish coast. When
the Algierian government forced the
prisoners’ release, the ship made again
for Marseilles, only to be blown by an
intense windstorm further down the
African coast. In 1809, after a danger-
ous overland trip back to Algiers, Arago
finally returned to France—where he
then spent weeks in quarantine.