Cheers! The Sound of a Black Hole Collision

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In space no one can hear you scream…

But it turns out that you can “hear” some things, and they “sound” like beer bottles clanking together.

September 14, 2015. If you mention this date to anyone who studies black holes or who studies gravity waves, they will most likely get excited (probably with a touch of jealousy). The reason we know this date is so important is because of the Laser Interferometer Gravitational-Wave Observatory (LIGO). On this date, the gravity waves from the collision of two black holes passed through Earth, and we would have never known, except that LIGO was “listening” for the sound.

For people like Dr. Roy Kilgard and Dr. Erin Macdonald (who worked for LIGO prior to the discovery), these results are amazing because they truly understand the full ramifications of a discovery like this. They see what door opens up for future research and how the data matches the mathematical models so beautifully. For the lay audience, however, there is still a reason to be excited about massive, super advanced scientific collaborations like this one.

LIGO gave the human race a new “sense.” It is natural to talk about seeing things in space; light (and all EM waves) comes from an object and our telescopes catch the light (and other EM waves) to form a picture. Pretty simple. But sound cannot go through space because space is empty (for all intents and purposes); so what does it mean to “hear” something happening in space. When two really massive things spin around each other very quickly, they send out waves of gravity. Gravity can move though space, so these waves eventually makes it to Earth, and we can convert the waves so we “hear” them (the process of detecting them is complicated but that is the basic idea). Oddly enough, in the case of black hole collisions, if you were to convert these gravity waves into sound waves, they are almost exactly the sound of beer bottles clanking together.

The gravity waves that LIGO detected were about one ten-thousandth the size of an atom (10–22 meter) and were moving at the speed of light (c ≈ 3.00 × 108 meters/second). Einstein, when he first theorized the existence of gravity waves, said that we would never be able to detect them because they are so small. For LIGO to be developed, engineering had to first be advanced, and these engineering advancements will have applications in other areas, which means technological advancements all around. This is exiting for tech consumers.

The way that the experiment works is that a laser is passed through a lens that splits the beam into two beams at a right angle, and each of those beams are passed back through the lens but they don’t split again (because of optics and how the lens was designed) and they are now traveling in the same direction. A detector then receives both parts of the laser beam and if one of the beams takes longer to get to the detector, there must have been a stretch in the space though which the beam traveled.

To test whether LIGO was working properly the first time it went online, a false-positive result was added into the data. The scientists were told that this would happen but not when. They were also told that they would not be informed whether what they found was fraudulent until after they wrote up the paper. There was a small chance that they would find real gravity waves prior to the fraudulent data being added. After the paper was written up and the scientists were informed that the data were not real (and they all cried), LIGO was turned off and made about ten times more powerful (which equated to a 1,000-fold increase in accuracy).

September 2015 came around, and LIGO was powered back up. Almost immediately, gravity waves were discovered and Einstein’s theory of general relativity was proven.

LIGO consisted of two laboratories on opposite ends of the United States. Currently, three more detectors are being built around the world, including one in the Southern Hemisphere, which will allow scientists to triangulate the origins of the gravity waves.

The panel included an in-depth discussion on black holes that outlined the difference in size between stellar back holes (between 4 and 20 stellar masses) and supper massive black holes (which can be on the order of billions of stellar masses). This was relevant to the panel because of two reasons: first, the black holes detected by LIGO were rather large for stellar black holes but far too small to be supermassive black holes. The second was that we knew the mass of the two black holes pre-merger and the mass of the combined black hole post-merger. The sum of the mass of two black holes differed from their combined mass by about three times the mass of our sun. Matter and energy cannot be created or destroyed, only converted to the other. The energy in the gravity wave was subtracted from the mass of the black holes (after accounting for conserved angular momentum the original black holes started with). Einstein’s famous equation e = mc2 describes the energy produced by the loss of mass the black holes underwent. The energy equals three times the mass of the sun times the speed of light squared (c2 ≈ 9.00 × 1016 meters2/second2). To put that in perspective, we were able to detect the gravity wave when the source was a billion light-years away.

Kilgard and Macdonald were energetic though the entire panel, and they were so much fun to listen to as they got excited about science. After the panel was over, they hung around for about 20 minutes to continue answering questions. Asked whether there were any applications of LIGO’s research that would be interesting to a lay audience, McDonald said warp drives. Now, she noted that these are not in our near future, but the detection of gravity waves gives scientists a jumping point to start thinking about how they might try to start to design them. They are one step closer to being science and not science fiction. She also said that she sometimes stays up at night thinking of how to design them.

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