After the Michelson-Morley (MM) experiment many scientists performed similar experiments using the original MM experimental setup, or different derivative experimental setups employing lasers and fiber optics. From MM calculations the length of the path traveled by the light along a single arm of the interferometer was important to the observation of the amount of the fringe shift. One of the subsequent Michelson experiments employed a path of light of over 30 meters (along a single arm) but also produced null result.

Which of the subsequent replications of the MM experiment (whether using fiber optic, laser, or free space interferometer) used the longest path of light along a single arm of the interferometer.

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    $\begingroup$ LIGO is 1200 km, but I don’t know if that is the longest $\endgroup$
    – Dale
    Nov 19, 2023 at 23:45
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    $\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$
    – Community Bot
    Nov 20, 2023 at 0:02

1 Answer 1


Experimental setup in Ligo uses replica of Michelson Morley interferometer with additional etalon used to increase the optical path length .The effective path length along each arm by adding etalon increases to about 1200 km from 4km.You can check this site for more details.

Ligo interferometer

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    $\begingroup$ Good answer but please may I suggest you check the effective length brought about by the use of etalons. I think it is a lot more than 4 km and indeed a quick search just now suggested values in the region of 1000 km (it's a big effect because the etalons have high Q, owing to high reflectivity). $\endgroup$ Nov 20, 2023 at 0:14
  • $\begingroup$ Thanks, I have changed my answer. $\endgroup$ Nov 21, 2023 at 13:38

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