Part of the reason for that is that LIGO isn&#x27;t exactly a Michaelson interferometer in that it has an extra pair of mirrors in each arm. If you look at this schematic [1] then in a traditional Michaelson interferometer you would only have the mirrors that are at the end of both arms.<p>With LIGO there is an extra set of mirrors within the arms this allows the light from the laser to bounce between them ~100 times or so increasing the effective path length greatly.<p>[1] <a href="https:&#x2F;&#x2F;www.nsf.gov&#x2F;news&#x2F;speeches&#x2F;colwell&#x2F;rc03_ligo&#x2F;img009.jpg" rel="nofollow">https:&#x2F;&#x2F;www.nsf.gov&#x2F;news&#x2F;speeches&#x2F;colwell&#x2F;rc03_ligo&#x2F;img009.j...</a>