On 14 September 2015 the Laser Interferometer Gravitational-Wave Observatory (LIGO) succeeded in detecting gravitational waves for the first time, the existence of which had been predicted by Einstein's General Theory of Relativity a century ago. The two instruments that detected the waves (one in the state of Washington, the other one in Louisiana) consisted of two enormous interferometers, with 'arms' that are 4 km long. It is the only technology able to obtain the necessary incredible precision: the device can measure a contraction of its arm of one thousandth of a proton diameter (10-~(18) m). It is less well known, however, that Einstein's ideas were tested interferometric ally much earlier. In the 1920s, when Einstein frequented the Netherlands on a regular basis, he personally took note of the sophisticated way in which the phlegmatic Amsterdam experimental physics professor Pieter Zeeman (1865-1943) conducted measurements that supported Einstein's Theory of Special Relativity using interferometric setups (although these measurements failed to provide incontrovertible evidence, as we will demonstrate below). Zeeman's setups have largely survived and became part of the collection of Rijksmuseum Boerhaave in Leiden, Netherlands. This article will discuss these artefacts, and the story behind them.
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