The
structure of a black hole is quiet simple. All you need to know is its mass,
electric charge, and rotation, and you know what the structure of space and
time around the black hole must be. But if you have two black holes orbiting
each other, then things get complicated.

Unlike a
single black hole, for which there is an exact solution to Einstein’s
equations, there is no exact solution for two black holes. It’s similar to the
three-body problem in Newtonian gravity. But that doesn’t mean astronomers
can’t figure things out, as a couple of recent studies show.

Although
Einstein’s equations don’t have an exact solution for a binary black hole
system, there are aspects of binary black holes that the equations predict. One
of these is known as spin-orbit resonance. When a black hole rotates, the
structure of space around it is twisted in the direction of rotation, known as
frame-dragging.

When two
black holes orbit each other closely, the frame-dragging of each black hole
affects the rotation of the other. As a result, the two black holes will tend
to enter a resonance, where the rotations either align in the same way
(parallel) or opposite (anti-parallel). If spin-orbit resonance is real, then
binary pairs should tend to have one of these orientations.

One recent
study suggests this is true. In it, the team looked at gravitational-wave data
from known black hole mergers and found that their rotations tend to be
parallel or anti-parallel. Given the small sample size, and the fact that black
hole binary rotations are never exactly aligned, there isn’t enough data to
confirm the effect, but the data we have points in that direction.

One of the
challenges to measuring black hole spin is that the signal is rather weak. The
gravitational waves we measure from distant black hole mergers are so faint that
it’s easy to get lost in the noise. Observatories such as LIGO and Virgo need
to make extremely sensitive measurements, and their data must be filtered
through computer models. Its the combination of data processing and computer
simulation that makes the mergers detectable. Adding spin to the mix makes
things even more difficult.

But in a
second paper, the team looked at how we could get better results. They found
that the signal for spin resonance is strongest when they are just about ready
to merge. That makes sense since that’s when they are closest together and when
frame-dragging is strongest. But currently, the rotation information for binary
black holes is found by looking at gravitational waves while they are still
orbiting each other. The team showed how models can analyze the near-merger
signal instead, getting much better results. By applying this new method to
black hole mergers, they should be able to confirm spin-orbit resonance in the
near future.

Gravitational-wave
astronomy is still a new field, and we’re still learning how to capture and
analyze the data. As these new studies show, gravitational waves hold a great
deal of information, and with a bit of digging there’s plenty more, we can
uncover.

**Reference:**

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