Gravitational Waves Observed
It took nearly 1.5 billion years to arrive. It was here for less than two hundred milliseconds. And its presence moved a pair of 2.5-mile vacuum tubes a distance of 1/400th the diameter of a proton. Yet despite its incredibly short stay and the microscopic movement, it is enough for scientists to claim one of the most significant discoveries in the world of physics this century.
For the first time ever, a gravitational wave has been observed. A team of global researchers announced the finding on Thursday, February 11. The discovery comes 100 years after Albert Einstein predicted the existence of gravitational waves in his theory of general relativity.
Gravitational waves are ripples in the very same fabric of the universe that bend and distort space-time. They are produced during violent cosmic disturbances.
In this case, the observed wave was created when two black holes collided approximately a billion and a half years ago, sending a ripple hurtling through space at the speed of light. It arrived on September 14, 2015, and was detected by LIGO – the Laser Interferometer Gravitational-Wave Observatory — a National Science Foundation-funded physics experiment that has searched for waves for more than a decade.
The LIGO Scientific Collaboration includes two Georgia Tech College of Sciences faculty members, and their team of 10 postdoctoral fellows, graduate, and undergraduate students. One of them is School of Physics Associate Professor Laura Cadonati, who chairs LIGO’s Data Analysis Council.
In this edition of Tech+Knowledge+Y, Cadonati explains how the waves were observed and why they unlock more secrets of the universe. The confirmation of Einstein’s prediction opens a new window of the cosmos, one that will provide humanity with better clues of how the universe was created and continues to evolve.
Expectations, and Career Goals, Confirmed
Relief. That’s what Deirdre Shoemaker says she felt when she saw proof of the gravitational wave last fall. After all, the director of Georgia Tech’s Center for Relativistic Astrophysics had planned her entire research career around the discovery, churning out hundreds of computer simulations for something that no one could guarantee actually existed. Shoemaker, a member of the LIGO Scientific Collaboration, solves Einstein’s equations for the collision of two binary black holes. All she needed was a real gravitational wave to see if her predictions were true.
“Gravitational waves are a vibration of space-time propagating toward us. To say it unscientifically, it’s like banging my fist on a table — you would feel a vibration if you were holding on to the other end. Any non-uniformly accelerating mass causes a gravitational wave — even moving your arms back and forth. But only very compact objects moving rapidly have a chance of being detected.
The signal from LIGO caught a lot of people by surprise. Most people thought the first one detected would be from a binary neutron star (a pair of the most compact stars known to exist). But this signal was unmistakable. It was from two relatively large black holes, something we didn't expect based on our astrophysical understanding. LIGO knew within minutes that we had something big. And I couldn’t have been more excited.
I don’t study neutron stars. I study binary black holes. So when I saw the signal, I knew something that strong could only be from colliding black holes. Our Georgia Tech team played a direct and pivotal role in the analysis of the observed signal. When the wave was detected at LIGO, it was a combination of the actual signal and background noise. Once that signal was extracted, our team was able to compare it with hundreds of our simulations of binary black hole mergers. This helped us confirm that the signal indeed originated from two black holes, nearly equal in mass, that were spinning on their axis as they orbited and collided, forming a single, spinning black hole.
We also created a visualization (seen further down this page) of the collision. The colors represent the ripples in space-time created by the black hole merger. It’s actually what happened 1.5 billion years ago. It’s 20 years of research and two weeks of dedicated time on a supercomputer solving Einstein’s equations."
VIDEO: Why Are Gravitational Waves So Important? Georgia Tech's faculty and students played a role in the discovery and explain its historic significance. Click to play.
Another Piece of the Universal Puzzle
A day like this was just a dream when Pablo Laguna started his doctoral work in the early 1980s. A science driven by gravitational wave observations is now a reality for the chair of the School of Physics. It’s one of three pillars that support Georgia Tech’s Center for Relativistic Astrophysics (CRA), which he established in 2008.
“Gravitational waves, particle astrophysics, and high-energy astrophysics. These are the three pillars around which the CRA has assembled a research team comprising multi-messenger astrophysicists. It’s one of the fastest-growing astrophysical centers in the nation.
Why are we devoted to multi-messenger astrophysics? There are only three types of messengers that carry information about the cosmos: photons, or light, particles like neutrinos or cosmic rays and gravitational waves. Only centers like the CRA are uniquely positioned to enable research that cuts across these channels.
As LIGO’s observations become routine, our group and others will learn more about the populations of black holes and neutron stars. Those findings will provide us additional information about the lives of the stars that, after their death, leave behind black holes and neutron stars. In turn, these findings will shed light on the birth of the stars and the environments where they form, thus completing the life cycle of the universe.
LIGO’s detected wave, officially deemed GW150914, has given us the first snapshot of one of the most violent events in the universe — the collision of two black holes. It’s an essential piece of the puzzle that is our universe, a piece that allows my colleagues and I not only to test if Einstein was right, but also to investigate astrophysical phenomena where gravity has the strongest grip."
VIDEO: When Black Holes Collide. A binary by Georgia Tech's team including Matt Kinsey, Karan Jani and Michael Clark. Click to play. (No audio.)