Lonely black holes revealed by passing gravitational waves

Enlarge / 22 January 2019, Lower Saxony, Sarstedt: Lower Saxony's science minister Björn Thümler (l, CDU) and Karsten Danzmann, director at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) are in the GEO600 detector. The German-British GEO600 detector in Ruthe near Sarstedt south of Hanover is a highly sensitive measuring device and has contributed to the spectacular detection of gravitational waves. The proof of gravitational waves was awarded the Nobel Prize in 2017. Photo: Julian Stratenschulte/dpa (Photo by Julian Stratenschulte/picture alliance via Getty Images) (credit: Picture Alliance | Getty Images)
Today is a good day: I learned something new. Many of you have probably heard of gravitational lensing of light (if not, don’t worry, I will inflict an explanation on you below). But it never occurred to me that gravitational waves are also subject to gravitational lensing. Not only that, but unexpectedly, the current generation of gravitational wave observatories might be able to observe gravitational lensing. If this turns out to be the case, all sorts of small, heavy dark objects might be revealed.
Specsavers don’t stock these lenses
One of Einstein’s predictions for general relativity was that light would be bent by a gravitational field. In effect, every planet, star, and galaxy bends light to a greater or lesser degree. We have used this effect to our advantage: distant galaxies provide lenses that allow us to see objects far beyond the galaxy itself. In some cases, distant objects appear as a ring around the lensing galaxy.
This effect is due to mass' distortion of space-time, and it will happen to any waves that pass nearby. Gravitational waves are just another form of wave, and, yes, they are also subject to gravitational fields from nearby masses. That means that a source of gravitational waves that passes through a lens should produce a similar ring of gravitational wave images as well.
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