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Black Holes Make Waves!

February 13th, 2016

Artist conception of gravitational wave. Credits: R. Hurt/Caltech-JPL

by Jason Kendall

There was some amazing news a couple of days ago.  Researchers have seen gravitational waves from a pair of colliding black holes!

Now this sounds like the start of a science-fiction movie, but it’s the culmination of work that started from when your parents were children, over 40 years ago. And it’s something Albert Einstein predicted a century ago but wasn’t observed … until now.

The machine that saw the waves is huge, more than twice the length of the Brooklyn Bridge in New York City.  The machine known as LIGO (Laser Interferometric Gravitational wave Observatory) comes in pairs.  One is in Louisiana, and the other is in Washington state.

LIGO site in Washington. Photo credit: LIGO Laboratory

The idea is simple for the machines : take a laser beam and shine it at a see-through mirror, so some of the laser beam goes through and some is reflected.  That see-through mirror is tilted, like your car rear-view mirrors, so you see in a different direction.  The tilt of the mirror allows the two beams to go down a REALLY long tunnel (about 2 miles long!) and bounce off mirrors at the end of the tunnel and come right back to the see-through mirror.  Then the laser mixes back together and the scientists take a picture of the remixed beams.  The two tunnels go in different directions, and they are EXACTLY the same length.

Here’s the CRAZY thing .  A Gravitational Wave, when it passes through Earth will stretch one tunnel, while compressing the other.  The wave pulls and smushes Earth like a salt-water taffy machine you might see at a county fair.  What this means is that the beams no longer match up perfectly when we take a picture of them, so we would know that a wave passed through.

Hold on!   The Earth got squashed like a bug?  Not really.  The effect is incredibly small that was measured.  So small that you couldn’t measure it with the best ruler you could possibly make.  It’s like measuring the Moon being pushed away from Earth by a distance of the width of one atom.   Astronomers can measure the Moon’s distance to within a centimeter, but a hundred billion atoms can be lined up across a centimeter.  That’s TINY!  So, no one got hurt by the wave, nor will anyone ever get hurt by one.

So, what MADE the wave?   Well, that’s where it gets amazing.  You might have heard about Black Holes.  They are remnants of big dead stars.  They are so compact that not even light can escape from them.  Anything that gets too close, and falls in, can never come out, or even tell us what it’s like inside, because the message coming out would have to go faster than the speed of light.  Black holes that are formed from big dead stars are small things, measuring only a few miles across.  If somehow two of them get close enough, they can collide together and form a bigger black hole.  That’s what happened!

What’s neat about these waves is that you can tell what they were in roughly the same way you can tell the kind of instrument you’re listening to in a band or orchestra.  When you hear a trumpet play some music, it can play the exact same pitches as, say, a violin.  But, even though they are playing the same tune, you can tell the a trumpet from a violin from a guitar from a piano with ease.  In a very similar way, people have been making computer simulations of what two massive black holes would “sound like” if they collided.  They would sound different than two neutron stars or two white dwarfs or anything else.  So when the signal was seen back in September, 14, 2015, they just had to match the signal with the simulation, and they knew exactly what it was.

Now, what about the sound?   Sound is basically waves and vibrations in air.  But, we’ve always been told that since there is no air in space, you can’t hear anything.  That is exactly true.  Now, imagine instead of air, it is the very distance and time between things and events themselves that is vibrating.  The gravitational waves would make everything look like a bunch of fun-house mirrors, where a short person looks tall, and a tall person looks short.  But instead of it being in mirrors, it’s REALLY happening to everything around you, including you!

Sound has frequencies and wavelengths, and in the same way, so do these gravitational waves.  They are not played on a string or with a stick or with your breath, but with the very nature of space and time itself.  The pitches of the waves are the most interesting thing.  If it were being played on an instrument, you could actually hear them.  But, they are so very very very quiet, that it took a really big machine that was really sensitive to hear it.  Imagine you could hear every noise being created in a big city all at the same time.  Now imagine picking out from all the loud car horns, street repair, building work, yelling and every possible noise, that you pick out the sound of a single mouse scratching his whiskers deep in his little home.

But that mouse scratch that we now hear was once an enormous lion roar from the catastrophic destruction of two stellar beasts a billion years ago.

This is a whole new world.  We used to just have eyes in Astronomy.  Now, we have cosmic ears.

  Jason Kendall is the Astronomy liaison and an adjunct professor at William Paterson University in New Jersey. Thanks for making Space understandable and interesting, Jason!

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