Not Just Waves but Black Holes (Go LIGO!!)


“This is the first direct detection of gravitational waves
and the first observation of a binary black hole merger.” This final line of a
paper published this morning in Physical Review Letters by the Laser
Interferometer Gravitaional-Wave Observatory (LIGO) collaboration sums up
today’s press conference announcement. 14 years after their search began and
100 years after Einstein predicted them, gravitational waves were detected.
This would be headline news on its own, but LIGO got in a second first,
highlighting the “observatory” part of their name:  they are the first to
observe a black hole merger. And just to add another interesting twist, no one
really thought we’d find black holes this size (they are the largest stellar black holes every observed). Let’s hope they have one heck
of a bottle of champagne in Hanford, WA and Livingston, LA today.  

Two black holes are entwined in a gravitational tango in this artist’s conception. Supermassive black holes at the hearts of galaxies are thought to form through the merging of smaller, yet still massive black holes, such as the ones depicted here. Image credit: NASA

So what exactly did LIGO do and why is everyone going so
crazy over it? (We even had an office-sponsored champagne toast at PhysicsCentral headquarters today, so you know it’s a big deal!) In 1915, Einstein published his paper on General Relativity. He theorized that we can look at the world not just as three dimensions but as four, with the fourth being time. It’s hard to discuss any of this without using a whole lot of analogies and I plan on doing just that, but I’ll try my best not to jump from trampolines to antennas too quickly. Without any mass, space-time would be one long flat sheet. But objects with mass warp that space-time sheet like a kid on a trampoline. And just like with a kid on a trampoline, the bigger the mass, the more space-time is warped. And one of Einstein’s huge breakthroughs was that warped space-time affects not just mass, but light, too. Scientists have seen this before through gravitational lensing. Here is the obligatory picture of the earth warping space-time. 

Earth warping space-time with its mass. Photo credit: NASA

Gravitational waves happen when the kid on the trampoline starts jumping, or in this case when two really big kids start jumping. Those “big kids” are two stellar black holes, created when stars collapse, about 30 times the mass of our sun. Before now, scientists were not aware stellar black holes could be that big. As they spiraled toward each other they gave off energy, or bounced on the trampoline of space-time.  When they finally collided the released a whole lot of energy, making very strong ripples in space-time, and LIGO was able to detect this. In the trampoline analogy, LIGO sits on the springs on the edge and measures the ripples as they come through. They detected that people were jumping and used how they jumped to determine what the people looked like—the frequency of the space-time ripples detected allow LIGO to figure out the size of the massive objects. General Relativity predicts different frequencies for different types of events. In this case the frequencies they saw could only be explained by two large, stellar black holes.
Diagram of LIGO detector and signal. Photo Credit: PRL
LIGO is a perfection of a very old technology, the interferometer. Because light is a wave, when it interacts with itself it interferes, sometimes destructively and sometimes constructively. The resulting interference pattern can be used to figure out how far the light has traveled. For more information about interference and to use it to measure the width of your hair, see PhysicsQuest: Spectra, the Original Laser Superhero Activity 2. The interferometer was first developed by Michelson and Morley in 1887 to measure the “aether wind” (spoiler alert, there is no aether wind, but there was a Nobel Prize). LIGO’s interferometer shines a very powerful laser beam down two perpendicular, 4km long arms, has it bounce of a very, very massive mirrors, and come back together where it started. It’s done in such a way that the two laser beams cancel each other out perfectly so a very sensitive light detector doesn’t see any signal. But if one of those mirrors moves even very, very slightly the laser light no longer destructively interferes and there’s a signal. LIGO is so sensitive that it can measure changes in distance of less than the width of a proton. Because gravitational waves affect very massive things, if one were to pass through the the long arms of LIGO it would even so slightly move the massive mirrors and the light detector would get a signal. The big problem those is that it’s not just gravitational waves that move the mirrors, it’s pretty much every passing truck. 
Photo Credit: PRL 
LIGO has the big job of figuring out what signals are trucks and what signals are actual gravitational waves. One big way to do this is to build more than one detector. Any local noise such as earthquakes and crashing waves will only be seen on one detector, not two. For everyone to be sure they saw an actual gravitational wave, each of the two detectors has to see the same signal at the same time. And then to be really, really sure it was a wave, the data go through many layers of removing background noise. It’s a complicated process and a science unto itself. After exhaustive analysis, LIGO scientists confirm that they have indeed seen a wave and that it was created by two stellar black holes merging. Here’s the signal they saw. The red line is what they’d predicted it would look like, the gray line is what they actually saw. It’s some pretty darn good agreement there. 
Photo Credit: PRL
This detection ushers in a whole new era of physics and how we can see our universe. LIGO did something many thought impossible—some didn’t think gravitational waves existed and many thought even if they did, we’d never have technology sensitive enough to detect them. But haters gonna hate and LIGO detected both black holes and gravitational waves in one go. We used to just see things with light, now we can see things with gravity. Go LIGO! 

If you’d like to get into more of the heavy physics, APS is providing free access to the LIGO paper here, and a fantastic summary of the findings here.

APS News has published a story on the findings and it can be found here
Tune in at 3pm today to see me sit down with LIGO scientist Lynn Cominsky to discuss what they’ve found and tweet your questions to us @physicscentral or with #LIGOLive

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