Given that the gravitational wave detected by LIGO was a very weak echo of a very distant event, could it have been "deviated" and distorted on its way here by a sufficiently massive black hole in between?

Would a black hole, since it traps even light, and since gravitational waves travel at the speed of light, absorb some portion of the gravitational wave and leave a "shadow" in its propagation such that if a black hole was between your instrument and an incoming gravitational wave you would have no way to detect it?

If so wouldn't the mass distribution in the Universe look less homogeneous at larger scales (as observed), since gravitational waves would not deform space uniformly and would instead need to "equalize" by forming voids like the Boötes void, and filament structures like the Pisces–Cetus Supercluster Complex or Sloan Great Wall?

If not, how could a gravitational wave from a distant event possibly affect the gravity well of a black hole closer to the observer? It doesn't seem possible to me that the attenuated ripple in space time from an event so far away could pass right through the gravity well of a closer local black hole like a galactic supermassive black hole.


1 Answer 1


Firstly, gravitational waves (GWs) are not an echo - we measure the direct signal.

The process you describe here is known as gravitational lensing, the deviation of (usually) light rays due to massive objects between the source and the observer. This also applies to GWs. The result will be similar - the direction of the waves can be changed, resulting in a distorted image of the source and possibly a 'brighter' image (stronger signal) due to 'gravitational focussing'.

However, LIGO and Virgo do not image GWs, they detect point sources. Hence, there is no distorted image, but the 'luminosity' of the signal may be affected, resulting in a (somewhat) misjudged distance (although they use a 'luminosity distance', which is not necessarily the same as a current physical separation).

Just as in the case of light, a black hole could trap some of the GW signal that would have reached the observer (gravitons that fall directly into the black hole), if it is directly between the source and the observer, but the gravitational focussing makes it likely that the resulting signal will be stronger rather than weaker.

  • $\begingroup$ Wait, "they detect point sources" not so much as "the analysis assumes point sources*. If there were a known lensing or distorting effect along the way, that could probably be added to the simulation. Currently it may not make a difference or may correlate with other fitted parameters and so be impossible to resolve at this time, but that's not the same as saying it can't happen. $\endgroup$
    – uhoh
    Dec 26, 2019 at 14:03
  • $\begingroup$ LIGO, like a single-dish radio telescope has only a single "pixel", a "point-sink". A RT can scan an extended source to generate an image, but LIGO, while "directional", can't be scanned during the short (~0.1 sec) event. $\endgroup$
    – user38715
    Jan 2, 2020 at 4:11

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