Listening to vibrations in space-time, with lasers
Can you see in the dark? Probably not, which is problem for astronomers, as at least 90% of the universe isn’t bouncing any light back towards our eyes or telescopes. Black holes, in fact, aren’t bouncing light anywhere, instead trapping that light from ever escaping their super-dense clumps of imploded mass. Somehow though, we’ve just observed two black holes crashing into each other. Since there was no light, we figured out a way to “listen” to them instead.
The vibration we detected was, on one hand, incredibly huge and powerful. The two black holes were 36- and 29- times the mass of our Sun, just packed into tighter balls. They were spiraling around each other, somewhat like a couple of large soap bubbles in a bathtub drain, until eventually they merged into a single black hole, releasing at least three Suns’ worth of energy as gravitational waves. Those waves were basically energy sending out ripples in space-time, warping and stretching it as they moved out from the black holes’ collision. As powerful as this event was close to the epicenter, it all took place over a billion miles away, over a billion years ago. Gravity in smaller doses isn’t nearly as powerful, so none of us felt this temporary warping as it passed through the Earth back in September, 2015.
The sound of disturbed lasers
Fortunately, humans didn’t have to feel this warping to be aware of it. Two facilities, collectively called the Laser Interferometer Gravitational-wave Observatory (LIGO), were running (although not officially on the job yet.) They each house a laser in a two-and-a-half mile long, L-shaped tube. The laser originates from the bend, being split down the length of each arm, where it is then reflected back to a single detection point. Without any disruptions, the light from each end of the tube should arrive synced up, or in phase, with each other. When a gravitational wave arrives, it would cause one end to compress slightly, while the other end is stretched as the ripple moves through space-time. Because the tubes are so long, it would create enough of a difference between the paths of the lasers that they would no longer arrive at the detector in phase. A movement as small as the diameter of a single proton could be recorded. The particular frequencies of September’s vibration happen to be audible, showing that what had once been a cataclysmic event a billion light years away was now moving by us as a small “chirp.”
Testing the data, models and tools
The sensitivity of these measurements has obviously been a concern. Albert Einstein first predicted gravitational waves 100 years ago as part of his theory of general relativity, but he didn’t think they’d ever be detectable. In 1969, physicist Joe Weber tried to detected them with a few aluminum cylinders that were meant to vibrate with passing gravitational waves, but too many false positives raised many doubts on the whole concept. The LIGO facilities were all built with these concerns in mind, starting with the idea of twin facilities in Washington and Louisiana to squelch noise from one local source or the other— any vibrations not seen in both lasers at once could be discounted.
Planners knew from the start that such complex tools would be difficult, and expected them to fail for their first years of operation. To successfully pick up the intended vibration while filtering out earthquakes, passing trucks, and even lightning as far away as Africa, initial designs needed lasers, vacuums, and other materials at specifications that hadn’t even been invented yet. While LIGO first opened in 2002, the goal was basically to figure out what parts would need upgrading. To test the team of over 1000 researchers involved, a group of four scientists were also tasked with faking signals, hoping to expose blind spots in how well people were sorting their information. After another round of upgrades in 2010, researchers were surprised that they picked up the first chirp only a few days of operation. Since September, they’ve been analyzing and reanalyzing the data, because with the most expensive measurement history, they want to make sure they truly measured what they thought they did.
New views of the universe
At this point, things are pretty clear. The data from the chirp not only holds up to scrutiny, but it’s revealed much about the event that started this set of waves in the first place, such as the size and nature of the merging black holes. Since that chirp, other, smaller waves have been detected as well. They’ve all been from within a specific spectrum of vibration, but other instruments should be coming online in the near future to help us listen to gravitational waves from other kinds of events. For example, the LISA Pathfinder spacecraft is set to see how well we can isolate a vibration from outer space, at the meeting and cancellation point between the Earth and Sun’s gravitational influence, to try to cut down on terrestrial interference.
In addition to historical milestones, like Einstein being proven right (while being wrong about the test,) this opens up a range of possibilities for astronomers, who are now poised to start learning about the universe in a whole new way. Being able to track objects in space through their gravitational influence will let us “hear” much more than we could hope to see, possibly sparking a “new era” of astronomy.
My first grader said: This totally changed how I think about space. Please tell me about the Big Bang soon.
I swear. I’m not making that up.
Source: Gravitational Waves Exist: The Inside Story of How Scientists Finally Found Them by Nicola Twilley, The New Yorker
