 In physics , one of the most important experiments of the late 19th century was the famous " failed experiment " of Michelson and Morley which provided evidence for special relativity . Recent repetitions of the Michelson – Morley experiment perform heterodyne measurements of beat frequencies of crossed cryogenic optical resonators . Fig 7 illustrates a resonator experiment performed by Müller et al. in 2003 . Two optical resonators constructed from crystalline sapphire , controlling the frequencies of two lasers , were set at right angles within a helium <unk> . A frequency comparator measured the beat frequency of the combined outputs of the two resonators . As of 2009 , the precision by which anisotropy of the speed of light can be excluded in resonator experiments is at the 10 − 17 level . 
