Lonely black holes revealed by passing gravitational waves
Ars Technica said: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.
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.
However, our gravitational wave observatories don’t have the spatial resolution to see that ring. Effectively, we “see” a single blurry object if it is subject to gravitational lensing.
That view, though, is a distinctly optical one. Gravitational waves are not light waves, and if you consider where they come from, the situation gets far more interesting.
Currently, the only gravitational waves that we can detect come from mergers—a tame name for an Earth-shattering collision—between objects like black holes and neutron stars. These are the sumo wrestlers of the Universe: stars like the Sun are too small to even get an audition in the gravitational wave show.
The signal that we get, shown below, is called a chirp. Essentially, it is a vibration that starts in the base register and ends in the treble. The signal only exists for a few seconds before the sources merge. And it is this combination of the signal being short-lived and chirping that gives us the opportunity to detect the presence of a gravitational lens.
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