The Little Chirp by Delaney Hall
This story came to 99% Invisible from one of our listeners — Dr. Carrie Nugent. She is an asteroid scientist at CalTech and she has her own podcast called Space Pod. Our listeners write in all the time with story ideas and this one stood out. To understand the story, though, takes a bit of context.
Earlier this year there was a huge discovery in astronomy. Scientists with the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected a signal. What their machines registered was effectively the sound of two black holes colliding and unleashing the energy of a billion trillion suns.
The signal they detected was faint because the black holes collided a billion years ago and the resulting gravitational wave had been traveling through space for all of those years, getting fainter and fainter along the way. By the time it reached us here on earth, it sounded like a little chirp. But despite all this, scientists managed to detect it.
Their discovery was enormously significant because it’s the first direct evidence of gravitational waves, which are ripples in space-time. These were predicted more than 100 years ago by Einstein in his general theory of relativity, which upended the known rules of physics. Instead of a static framework, Einstein theorized that matter and energy could actually distort the geometry of the universe, creating ripples of gravity. The gravitational waves LIGO detected, so many years later, confirmed his theory with empirical data.
But as Dr. Nugent, our listener, pointed out to us, there is a design angle that has not really been covered as much: the instrument that detected the sound is amazing and extremely sensitive. It’s also incredibly complex and difficult to describe on the radio. But there’s one element we wanted to focus on: how LIGO scientists went about minimizing environmental noise that might interfere with the signal they were trying to detect.
First, they isolated the instrument’s detectors in giant vacuums that would largely protect them from the vibrations and noises of the outside world. Then, on top of that, the experiment employed two environmental monitoring experts. Their job was to figure out how outside sound (rumbling trucks, falling trees, thunderstorms, howling wolves) might interfere with the experiment. These monitors played recordings near the detectors and documented their effects. They even had a staff member ride away on his motorcycle to determine if and how the rumbling motor would impact their measurements.
Over time, the monitoring team generated a huge library of what different kinds of sonic interference might do to the experiment. That way, when the genuine signal appeared they would know it was the real deal.
It is hard to convey how crazy it is that they were able to detect the signal from the black hole collision. It’s as if the scientists at LIGO measured the distance from earth to the nearest star outside our solar system to within the width of a human hair.
Have a question or story for 99% Invisible? We will be doing a follow-up to this episode with additional mini-stories based on listener inquiries and ideas. Feel free to ask or suggest something in a comment below!