Communication from beyond a cosmic grave

Podcast transcript:

This is a Northwestern News podcast. Coming up in this episode – communication from beyond a cosmic grave. The sound that proves Einstein’s theory of relativity and expands it to depths even he couldn’t have imagined.

Sound of first chirp

That is the sound of a stellar event that happened more than a billion years ago. Two black holes collide in space to create a new black hole. Full disclosure, the collision didn’t actually create that chirp. You’re not listening to a space echo of the crash. We’ll explain the sound in just a minute, but first, some background.

On September 14, 2015, LIGO detectors recorded evidence of gravitational waves for the first time in history. Gravitational waves are ripples in space-time, caused by things like star explosions or black hole collisions.

On December 26, 2015, LIGO detectors picked up a second ripple from gravitational waves, caused by a separate event.

Sound of second chirp

Kalogera: It confirms that this first detection was not a fluke. It confirms that there is a population of binary black holes that we never knew existed before these two discoveries.

That’s Northwestern University astrophysicist Vicky Kalogera. She’s a member of an international team of scientists involved in the LIGO Scientific Collaboration and one of the biggest discoveries in modern science. 

Although LIGO just detected gravitational waves for the first time, Einstein actually predicted their existence almost a century ago. Northwestern astrophysicist Shane Larson also works on the LIGO Scientific Collaboration. He says even Einstein couldn’t have known the genius of that prediction.

Larson: It took 41 years before astronomers and physicists actually understood what you had to do to go look for these waves that Einstein proposed should exist. The reason it took 40 years is because we were completely confused about what they were. We don’t have a physiological sense of what we should do to even look for these things. We’re not designed to do it. You hear us keep using words that are amenable to our senses – ‘we are seeing gravitational waves; we are hearing gravitational waves’ – because we’re used to describing the world in terms of the way our bodies interface with the world. But they’re not designed to interface with gravitational waves.

This is the first time scientists have been able to study space using something other than visual cues, like X-rays or telescopes. Kalogera says it’s easier to think about hearing gravitational waves rather than seeing them.

Kalogera: Telescopes are thought of and behave as pointing instruments. This is very much like our eyes. We turn our head, and we say, ‘I’m going to look there,’ or ‘I’m going to look left or look right.’ Gravitational wave detectors, though, they don’t act as pointing instruments. They act more like our ears. Our ears receive sounds from all over. Gravitational waves can come and be detected from all directions.

Gravitational waves are not sound waves, but then, why do we hear this as they pass the LIGO detectors?

Sound of second chirp

Well, that’s kind of a trick question. We don’t really hear that chirp. Scientists created that sound in a lab by lining up the frequency of the gravitational waves with the sound wave frequency that the human ear can hear. The length of the chirp and the pitch change depending on the mass of the black hole. 

Larson: Your smartphone has a whole bunch of electronic wiggles on it, that when it pushes it out through the headphone port, gives you Katy Perry or Led Zeppelin or whatever it is you listen to while you’re walking to work. Well, if I steal your smartphone, and I replace those wiggles with the LIGO wiggles, your phone is perfectly capable of pushing it out through the headphone jack and turning it into sound.

Scientists expect the LIGO detectors to pick up many more gravitational waves, each one different. Each detection adds a note to a space symphony written billions of years ago. It’s a sound that forever changes the way we see or, better yet, hear, the universe.

This is Kayla Stoner reporting a Northwestern News podcast. Special thanks to the California Institute of Technology and the LIGO Scientific Collaboration for providing the chirps for this story. 

Read more about gravitational waves on our website, Northwestern.edu.