When two black holes slam together they don't make a sound.
And yet this is what we hear if we listen
closely,
Let’s listen again
That “chirp” is what we heard
from two black holes
that slammed together
about a billion light-years from Earth.
The tone rises
as they spiral closer
together,
and abruptly stops when they merge.
But sound
can’t travel through the vacuum of space.
So what exactly are we hearing?
Each of these black holes
weighs as much as several stars;
hefty enough
that as they pass through space
they make waves—
gravitational waves, specifically.
These waves are undulations
in the fabric of spacetime
that fan outwards at the speed of light,
like ripples on a cosmic pond.
Albert Einstein predicted this phenomenon
in 1916,
based on his theory of
general relativity,
but was skeptical
that gravitational waves
could ever be detected.
Even the strong ones
from colliding
black holes produce ripples roughly
a thousandth the size of a proton.
It took almost a
century for
scientists to prove him wrong.
Today they have built–and
are expanding–a global
network of observatories
that has so far detected
gravitational waves
from about 100 cosmic collisions
Recorded, analyzed,
and converted to sound, each one’s
jostling of spacetime
becomes its own distinctive, data-rich
“chirp.”
That long, low buildup
is a sign of a slower,
more sedate merger
from relatively lightweight
in-spiraling black holes.
A more abrupt chirp, like this:
is a sign
of a faster merger
of heavier black holes
in this case
a pair that combined
to form one over 80 times
the mass of our Sun.
after a long hiatus
for upgrades
and the COVID
pandemic, the world’s
gravitational-wave observatories
are tuning back in
to this celestial symphony.
Gravitational wave observatories
don’t have mirrors or lenses
like normal telescopes.
Instead
they use lasers beamed down long tunnels,
laid out like L’s,
with two arms
laying flat against the ground.
between mirrors at the ends of each arm,
the lasers act like extremely sensitive
violin strings,
As gravitational waves pass
through one laser's path,
They repeatedly stretch
and contract the space.
Similarly, when a violinist uses vibrato,
she shortens and lengthens her string.
Convert all this laser vibrato to sound
and you can even hear black holes collide
with a chirp.
Checking between these
observatories confirms
each event is more than random noise;
if the same ripples appear in each one,
they must come from somewhere in the sky.
a wave’s exact arrival
time in each arm of an observatory–and
at each different observatory
around the world–helps
pinpoint the wave’s direction
and source location.
And the chirps
picked up by these detectors
can sometimes
reveal much more
than the final moments
of merging massive bodies.
If astronomers get lucky enough to detect
both gravitational waves
and light from some celestial smash-up
as happened in 2017 with a neutron-star
merger called GW170817
that rich dataset
allows them to measure the expansion
rate of the universe
and perform better tests of Einstein’s
general relativity.
GW170817 even showed scientists
how much gold, platinum and other
heavy metals
these sorts of high-energy
explosions hurl into the cosmos.
Currently, there are
93 confirmed mergers.
the next 18 months, astrophysicists
hope to double their catalog of crashes
turning once
rare chirps into a cosmic chorus.
A growing soundscape of the universe's
most epochal
and otherwise silent collisions.