it might be best to first detail my (admittedly limited) understanding of the LIGO experiment. a beam of light is split along two 4Km evacuated pipes then reflected back where their interference pattern is analysed for any disturbances in the interference pattern, any contraction or extension in the time the split beams took to travel will be detected in the interference pattern of the two beams when they return. when a gravitational wave passes it leads to a change in the length of one of the arms relative to the other by stretching or contracting the space that arm is in as a result one beam will return at a very slightly different time to the other. assuming that my understanding of the experiment is not flawed does this mean that to an outside observer the light along the contracted arm made the trip in less time than would have been made in normal circumstances by light and furthermore is this in any way similar to how the fictional alcubierre drive would function?

i'm not asking if the beams themselves travelled faster than light (i know everyone's tired of questions like that from rookie physics students like myself) just if to an outside observer it would appear that they made the trip in a time that would be less than normal i.e. if an outside observer not affected by the gravitational waves would theoretically see an arm of the experiment contract

  • $\begingroup$ Shouldn't the observer conclude that the arm has been shrinked or stretched? $\endgroup$
    – Alchimista
    Feb 2, 2019 at 15:13
  • 3
    $\begingroup$ Please use standard capitalization. $\endgroup$
    – user4552
    Feb 3, 2019 at 15:29

1 Answer 1


an outside observer not affected by the gravitational waves would theoretically see an arm of the experiment contract

LIGO-type interferometric antennas are just (an understatement if there is one) extremely sophisticated ways of detecting and measuring motions of mirrors placed at the extremes of both arms. In additions mirrors are to be insensitive to any other stress: microquakes, traffic vibrations, etc. In principle if you had two really rigid rulers 4 km long you could effect the same measurements: you would see an arm contracting and the other lengthening, taking turns.

When I wrote "rigid" I meant the same thing as your "outside observer not affected by the gravitational waves". A gravitational wave is nothing but an oscillating tide force, just like the one Moon (and Sun) act on Earth and whose effect we see as ocean's tides. There is no way to be unaffected by a gravitational wave, but a free body follows it unobstructed, whereas a rigidly constrained body resists it and shows only a minimal deformation. LIGO's mirrors are as free as possible and the laser rays "simply" detect their motion.

  • $\begingroup$ Maybe get rid of the stuff about rulers & rigidity, it's a bit confusing, and not really that relevant. The variation in the interference pattern lets us see when one arm of LIGO is longer or shorter than the other. Why bother bringing unphysical rulers into it? ;) $\endgroup$
    – PM 2Ring
    Feb 3, 2019 at 15:46

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