What Kind of Noise Annoys an Interferometer?


sound_of_gravity(Many thanks to NOVA’s Kate Becker for the title to this post...)

By now, you’ve no doubt heard the news that the Laser Interferometer Gravitational-wave Observatory (LIGO) detected gravitational waves, confirming yet another facet of Einstein’s general theory of relativity...one that he developed a hundred years ago, but was actually impossible to observe until now.

I’m excited about LIGO’s recording because it’s a brand new way of observing the universe – a way of hearing it! Rather than looking at cosmic events using various kinds of light, scientists are able to convert them into sound waves and listen to them via speakers.

On September 14th, 2015 they picked up the sound of otherwise invisible gravitational waves rippling through the fabric of space-time, an event that was apparently caused by the merger of two orbiting black holes 1.3 billion light years away.

As so nicely described in The New York Times article “Gravitational Waves Detected, Confirming Einstein’s Theory,” this event was so powerful it converted three solar masses’ worth of energy into gravity waves in one-fifth of a second. That’s the equivalent of 50 times the energy output of all of the stars in the universe combined! Yet by the time this wave passed through Earth, it was so small it only moved the LIGO equipment by a few thousands of the diameter of a proton. All the team heard was a quick ‘chirp’ at middle C.

I’ve found a few excellent articles about this incredible event and highly recommend that you spend a few minutes reading, looking and – of course – listening:

  • To prep, read “The Sounds of Empty Space.” This article by NASA introduces the concepts (prior to LIGO’s detection of the gravitational wave). Yes, it’s geared towards school-age kids, but we were all that young once...and it’s nice to have the basics down before you move on.
  • After that, go to National Geographic magazine’s “Found! Gravitational Waves, or a Wrinkle in Spacetime.” In this article, Nadia Drake manages to capture the drama of the cosmic collision. Best of all, you can hear the gravitational waves’ chirp.
  • And, finally, read “How LIGO Detected Gravitational Waves” by NOVA’s Kate Becker, which goes into more detail about the history of the two LIGO observatory locations and how they work together to let scientists hear the universe.

Amazingly, the LIGO team almost missed this event because they had discussed delaying the launch of Advanced LIGO (with improved ‘noise cancelling’ technology) by several days. So, in the end, this raises a philosophical question (sorry, but it’s hard to resist): If a gravitational wave chirps and there’s no LIGO to hear it, does it make a sound?



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