“At 5:51 a.m. Sept. 14, 2015, something went ‘chirp.’”
The layperson can appreciate that description, considering the source is an astrophysicist explaining what happened when two black holes collided in space — a billion light years away.
In fact, that faint “chirp” has emitted perhaps the loudest blast among modern scientific discoveries: It fulfilled the last prediction of Einstein’s general theory of relativity, a framework envisioning that dense objects cause a distortion in spacetime, which is felt as gravity.
Sean McWilliams, assistant professor of physics and astronomy in the Eberly College of Arts and Sciences, is a member of the research team — LIGO, short for the Laser Interferometer Gravitational-wave Observatory — that cracked the code by detecting invisible ripples in spacetime. Their observation proves that energy travels in waves across space and time, leaving unique distortions along its path.
“General relativity told us that what we experience as gravity is a curving of spacetime,” McWilliams said. “The sun displaces the spacetime around it. Think of a bowling ball sitting on a trampoline. That causes objects to move in orbits around the indentation.”
Shortly after Einstein developed that theory in 1915, he argued that if spacetime were jostled violently enough, by massive objects being accelerated to tremendous velocities, this would generate waves in the fabric of spacetime itself, and these waves would propagate across the universe at the speed of light.
“Being a good physicist, Einstein did the rough calculations,” McWilliams said. “He believed that the waves would travel on their merry way and not interact with anything, and that it would be impossible to see or measure them. At that time, he didn’t know about black holes, and even once he learned about them, he didn’t believe they existed in nature.”
LIGO’s discovery last September was recorded at laser facilities in Louisiana and Washington state as part of a $1 billion-plus project funded by the National Science Foundation. McWilliams, a self-described coder, focused on simulating and modeling the gravitational-wave emission in order to detect the incoming signals and infer details about their source.
The collision of the two black holes — each one roughly as massive as 30 suns, but squeezed within the size of a major city — unravels a brand new world in which we can better understand the astrophysics of the universe, McWilliams said.
“We will learn how massive stars evolve,” he said, “not just in our own galaxy but out to tremendous distances. As you look out into the distance, you also look back in time. The signal that was detected was emitted 1.3 billion years ago at a time when the universe was 10 percent smaller than it is today.”
McWilliams isn’t the lone Mountaineer with a hand in the discovery. The LIGO Scientific Collaboration includes Zachariah Etienne, assistant professor of mathematics; Caleb Devine, a master’s mathematics student; and Belinda Cheeseboro, a physics and astronomy doctoral student.
Because he earned his bachelor’s degree in physics and knew how to code, Devine was alerted about the research opportunity. Fast-forward a year-and-a-half, and Devine has benefited from working closely with McWilliams and Etienne on speeding up software to estimate the parameters of the wave signals.
“The original software would have taken several decades to complete the analysis,” Devine said. “We sped it up by nearly 300-times the original.”
Devine recalls staying up until 4 and 5 a.m. tinkering with the software. The trio is now optimizing a third version of the software, which allows for any kind of black holes to be modeled, regardless of whether their spins are aligned, Devine explained.
Sounds like a solid starting point for Devine’s career ambition — “I just want to figure out things that nobody has thought of before.”
For McWilliams, the discovery is quite an accomplishment for the Pennsylvania native, who, by the way, received a round of applause from the 135 mostly freshman students when he walked into his physics 101 class the day after LIGO’s announcement.
“As a kid, I wanted to go to the stars and be an astronaut,” McWilliams said. “As I got more sophisticated in my thinking, I wanted to understand what we were seeing when we look at stars. Why are they so bright? I was always drawn to fundamental questions, and in high school, primarily through conversations with my father, I decided physics was the path to pursue. When I first started at Penn State, I learned about a faculty member who studied black holes, and I knew without a doubt that was for me.
"I haven’t looked back since."