Listen to the amazing ‘chirp’ of two merging black holes

When two black holes collide, they don’t make a sound. And yet this is what we hear when we listen carefully.

[CLIP: Black hole “chirp”]

Let’s listen again.

[CLIP: Black hole “chirp”]

That “chirp” is what we heard from two black holes slamming into each other about a billion light-years from Earth. The tone rises as they get closer together and stops abruptly when they merge.

But sound cannot travel through the vacuum of space. So what exactly are we hearing?

Each of these black holes weighs as much as several stars; heavy enough that they make waves as they travel through space – specifically gravitational waves.

These waves are undulations in the fabric of spacetime that fan out at the speed of light, like ripples on a cosmic pond.

Albert Einstein predicted this phenomenon in 1916, based on his general theory of relativity, but was skeptical that gravitational waves could ever be detected.

Even the strong ones from colliding black holes produce ripples about a thousandth the size of a proton.

It took scientists nearly a century to prove him wrong.

Today, they’ve built — and are expanding — a global network of observatories that has detected gravitational waves from about 100 cosmic collisions to date.

Recorded, analyzed and converted to sound, each jostling of spacetime becomes its own distinctive, data-rich “chirp”.

Let’s listen again:

[CLIP: Black hole “chirp”]

That long, low build is a sign of a slower, quieter merger of relatively light inspiratory black holes.

A more abrupt beep, like this: [CLIP: Black hole “chirp”]

is a sign of a faster merger of more massive black holes…

In this case, a pair that combined to form more than 80 times the mass of our sun.

Now, after a long hiatus for upgrades and the COVID pandemic, the world’s gravitational wave observatories are once again tuning in to this celestial symphony

Gravitational wave observatories do not have mirrors or lenses like normal telescopes.

Instead, they use lasers beamed through long tunnels laid out like that of an L, with two arms flat against the ground.

The lasers bounce between mirrors at the ends of each arm and act like violin strings, producing slightly different frequencies as their paths through space are stretched or contracted by passing gravitational waves.

Shorten the laser and just like a violin string you get a higher pitch. Lengthen it and you get a lower pitch. Turn all this laser vibrato into sound, and you can even hear black holes collide with a “chirp”.

Scientists around the world have built a number of these laser-based ears to listen for gravitational waves from cosmic sources.

One, called LIGO, has two detectors: one in Hanford, Washington, and another in Livingston, Louisiana.

There is also the VIRGO observatory near Pisa, Italy, and the KAGRA detector in Hida, Japan.

Another detector is planned to be built in India.

Cross-checking between these observatories confirms that each event is more than random noise; if the same ripples appear in each, they must be coming from the sky somewhere.

Timestamping a wave’s exact arrival time at each arm of an observatory — and at every observatory around the world — helps determine the wave’s direction and source location.

And the beeps picked up by these detectors can sometimes reveal much more than the final moments of massive bodies merging.

If astronomers are lucky enough to detect both gravitational waves and light from some celestial collision, as happened in 2017 with a neutron star merger called GW170817.

With that rich dataset, they can measure the expansion rate of the universe and perform better tests of Einstein’s general theory of relativity.

GW170817 even showed scientists how much gold, platinum and other heavy metals these kinds of high-energy explosions hurl into the cosmos.

Currently there are 93 confirmed mergers. Over the next 18 months, astrophysicists hope to double their catalog of crashes, turning once-rare peeps into a cosmic refrain — a growing soundscape of the universe’s most groundbreaking — and incidentally quiet — collisions.

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