When Galileo first introduced the telescope in the 1600s, astronomers gained the ability to view parts of the universe that were invisible to the naked eye. This led to centuries of discovery—as telescopes advanced, they exposed new planets, galaxies and even a glimpse of the very early universe. Last September, scientists gained yet another invaluable tool: the ability to hear the cosmos through gravitational waves.

Artwork by Sandbox Studio, Chicago with Lexi Fodor

Ripples in space-time Newton described gravity as a force. Thinking about gravity this way can explain most of the phenomena that happens here on Earth. For example, the force of gravity acting on an apple makes it fall from a tree onto an unsuspecting person sitting below it. However, to understand gravity on a cosmic scale, we need to turn to Einstein, who described gravity as the bending of space-time itself. Some physicists describe this process using a bowling ball and a blanket. Imagine space-time as a blanket. A bowling ball placed at the center of the blanket bends the fabric around it. The heavier an object is, the further it sinks. As you move the ball along the fabric, it produces ripples, much like a boat travelling through water. “The curvature is what makes the Earth orbit the sun—the sun is a bowling ball in a fabric and it's that bending in the fabric that makes the Earth go around,” explains Gabriela González, the spokesperson for the Laser Interferometer Gravitational-Wave Observatory (LIGO) collaboration. Everything with mass—planets, stars and people—pulls on the fabric of space-time and produces gravitational waves as they move through space. These are passing through us all time, but they are much too weak to detect. To find these elusive signals, physicists built LIGO, twin observatories in Louisiana and Washington. At each L-shaped detector, a laser beam is split and sent down two four-kilometer arms. The beams reflect off the mirrors at each end and travel back to reunite. A passing gravitational wave slightly alters the relative lengths of the arms, shifting the path of the laser beam, creating a change that physicists can detect. Unlike telescopes, which are pointed toward very specific parts of the sky, detectors like LIGO scan a much larger area of the universe and hear sources from all directions. “Gravitational waves detectors are like microphones,” says Laura Nuttall, a postdoctoral researcher at Syracuse University.

Artwork by Sandbox Studio, Chicago with Lexi Fodor

First detections On the morning of September 14, 2015, a gravitational wave from two black holes that collided 1.3 billion years ago passed through the two LIGO detectors, and an automatic alert system pinged LIGO scientists around the world. “It took us a good part of the day to convince ourselves that this was not a drill,” González says. Because LIGO was still preparing for an observing run—researchers were still running tests and diagnostics during the day—they needed to conduct a large number of checks and analyses to make sure the signal was real. Months later, once researchers had meticulously checked the data for errors or noise (such as lightning or earthquakes) the LIGO collaboration announced to the world that they had finally reached a long-anticipated goal: Almost 100 years after Einstein first predicted their existence, scientists had detected gravitational waves. A few months after the first signal arrived, LIGO detected yet another black hole collision. “Finding a second one proves that there's a population of sources that will produce detectible gravitational waves,” Nuttall says. “We are actually an observatory now.”

Artwork by Sandbox Studio, Chicago with Lexi Fodor