The Gravity of the Situation

By Olivia Gebhardt


On February 11th, the world was rocked by science–literally. In a small classroom at Johns Hopkins University, about 20 physics majors watched a live announcement about the discovery of gravitational waves.

The press conference played on the pull-down projector screen as students put away their notebooks, realizing their teacher wanted them to understand the science purely for benefit. Some students had brought muffins or cups of yogurt, flying through their typical morning routine in order to guarantee themselves a seat.

The students bristled with anticipation, understanding the event must be significant since the teacher sacrificed a lecture in order to watch it. Many remembered BICEP2, the US-led telescope near the South Pole, that claimed in March 2014 to have witnessed gravitational waves only to find their peculiar results were caused by cosmic dust. “I remember thinking it was really exciting,” Halley Cromley, a sophomore Physics major in the class, said. “I knew gravitational waves were something scientists had been trying to detect for a while.”

Through descriptions the conference painted a picture of two black holes, many millions of galaxies away, dancing around each other and finally combining in space 1.3 billion years ago. As they did, they released a ripple through the universe, distorting space-time and producing quick shudders that traveled through all kinds of matter for millions of years.

Space-time is curved in the presence of mass, like a ball on a stretched out bed sheet, and this warping produces what we call gravity.

These waves finally reached Earth, and were detected on September 14th, 2015 by the Laser Interferometer Gravitational-wave Observatory detectors (LIGO) project.

LIGO consists of two separate facilities, one stationed in Livingston, Louisiana and the second in Hanford, Washington. Built twenty-two years ago, LIGO is an experiment aimed at physically verifying the nuances of gravity proposed by Einstein’s general theory of relativity. Put simply, the theory reveals that space-time is curved in the presence of mass, like a ball on a stretched out bed sheet, and this warping produces what we call gravity. When two extremely massive objects interact in this space-time fabric, they stretch and compress the universe around them, creating ripples large enough that humans can measure.

Predicted in 1916 by Albert Einstein, gravitational ripples through space-time proved exceedingly hard to find. Five decades passed before a University of Maryland professor named Joseph Weber attempted to detect the waves. With multiple scientific failures in the field, Kip Thorne, Ronald Drever, and Rainer Weiss approached the National Science Foundation to fund construction of LIGO with little hope of success. Astonishingly, LIGO became the first significantly sensitive of detectors with the possibility of revealing gravitational waves.

To accomplish this great task, LIGO used light measurement, timing how long lasers took to start at one point, travel the distance of the device, and back. From this, scientists calculated the distance the light traveled. To detect waves, which propagate in all directions, each device is L-shaped, propped by platforms in order to make the sides, each 4 kilometers long, completely flat. Experimenters ran a laser source from the L’s corner, down one side, back to the center, down the other wing, and back. With this method, scientists were able to measure extremely minute fractional changes in distance by constantly running the laser and checking for anomalies.

“We can hear gravitational waves.”

A little before 10 on a September morning, Coordinated Universal Time, scientists in Livingston saw a slight squiggle appear in the continuous measurements. Seven milliseconds later, the same signal appeared at the Hanford detector, some nineteen hundred miles away. Through computer simulations, the team extracted an extraordinary amount of data from less than a second of information. In total, the measurement amounted to a change in distance the length of one one-thousandth of a proton’s diameter, an inconceivably small length. The two disturbances matched almost perfectly with solutions to Einstein’s theory describing the coalescence of two black holes. Scientists found the masses of the black holes (thirty-six and twenty-nine times the sun’s mass), the moment they merged, their distance from earth, and their orbital speed. The most extraordinary fact, however, might have been that the frequency of gravitational waves is within the human hearing spectrum. The chirp was too faint to hear by itself but with simple audio processing, the sounds of the universe came to life.

“We can hear gravitational waves,” said Gabriela González, LIGO Scientific Collaboration Spokesperson, during the conference. “We’re not only going to be seeing the universe, we are going to be listening to it.”

Dr. Julian Krolik, the professor of the class and a theoretical astrophysicist (specializing in black holes), sat perched on a desk in the back of the room while the announcement played. His hands propped up his head as he looked towards the screen with pure excitement. Occasionally he would provide more details about the conference to his students, seeing as he is Hopkins’ resident expert on gravitational waves.

Although his experiments are one step removed from the space-time distortions, Krolik has been collaborating with scientists hunting the ripples since around 2010. He became interested in detailed calculations of what happens to matter near ordinary black holes in the late 1990s and was later approached by groups attempting to detect gravitational waves. Tired or running codes on new parameters such as mass and distance, wave experimenters decided to contact theoretical astrophysicists since black holes are generally surrounded by other matter. The teams needed collaborators who knew how to model the behavior of gas and light in the presence of a black hole. The hope was the massive objects interacting to create the gravitational radiations would heat the gas around and make it shine brightly, if even only briefly. In this instance, ordinary telescopes would be able to detect the event itself.

“Maybe, if we’re lucky, we can also detect light at the same time as the gravitational detector observatories,” Krolik said.

After the conference finished, the students left the small classroom and continued to their next class of the day, discussing the significance of the event. “As a student it just makes me more excited about physics,” Cromley said. “This is a big step forward for the science community.”

By later that day, the news had spread to the rest of campus. Sathvik Namburar, a sophomore Public Health major on the pre-medical track, did not think the average Hopkins student truly understood the experiment but hoped they generally recognized its importance. “I think it’s quite significant because it’s kind of the final nail in the coffin for Newtonian theories, which basically said space and time did not change,” he said.

Krolik believes students should be more curious and appreciate LIGO for its ingenuity. “It’s a wonderful human cultural achievement,” he said. “It takes a tremendous effort to understand things that we weren’t fundamentally built to deal with.”