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.