by Cate Pitterle, ’20
Recent discoveries in astronomy could change the scientific world forever.
Almost every space observatory in the world collects light, but some objects in the universe—such as black holes—can’t be seen through these methods, so scientists created revolutionary technology that allowed them to “hear” the universe—and just recently, they got their wish with the discovery of gravitational waves.
The waves, distortions of space-time caused by the collision of huge astronomical bodies, can be described as something like ripples in a pond. Scientists first observed them in September 2015, using the Laser Interferometer and Gravitational-wave Observatory (LIGO) detectors in Livingston, LA and Hanford, WT, to detect the collision, or merger, of two black holes.
The discovery baffled and excited scientists around the world. “It’s mind-boggling,” said David Reitze, executive director of the LIGO Scientific Collaboration. “As we open a new window into astronomy, we may see things we’ve never seen before.”
The waves have already confirmed the existence of a binary black hole system where black holes rotate around each other, and they’ve given insight into the speed of the universe’s expansion—a phenomenon scientists have been trying to explain for decades.
“What’s really exciting is what comes next,” Reitze added, discussing future possibilities for LIGO. “Four-hundred years ago, Galileo turned a telescope to the sky and opened the era of modern observational astronomy. I think we’re doing something equally important here today.”
A Long Time Coming
Though gravitational waves were detected only recently, their existence was predicted over 100 years ago by one of the world’s most famous scientists, Albert Einstein, in his theory of general relativity. Through mathematics, he showed that the collision of large bodies in space would trigger “waves” of gravity that would ripple out from the source.
“[Einstein’s] equations have worked so well, in ways he never could have imagined,” Rainer Weiss, co-founder of LIGO and professor at MIT, said at the time. “The field equations and the whole history of general relativity have been complicated. Here suddenly we have something we can grab onto and say, ‘Einstein was right.’”
The discovery was the last part of Einstein’s theory to be proven by scientists. Now, with the confirmation of gravitational waves’ presence, Einstein’s entire theory is proven.
But what makes Einstein’s predictions so incredible is that he predicted the presence of gravitational waves long before the technology for LIGO became available—in fact, his predictions came over 50 years before the technologies for detecting such a wave started development.
“All of this technology wasn’t available to Einstein,” Weiss said. “I bet he would’ve invented LIGO.”
“Let’s Take a Chair”
After the initial discovery, the observations of gravitational waves continued. Since that fateful day in 2015, LIGO has made three more observations of black hole mergers. On the fourth detection, the Virgo observatory in Italy joined in with its own recordings of the event, but the true prize was gained with a major discovery on August 17.
That was the day when astronomers observed the collision of two massive neutron stars. Unlike regular stars, neutron stars are dead—they’re the cores that remain when stars burst in the cosmos, an event similar to the creation of black holes. But while black holes are the remains of large supernovae, neutron stars are the remnants of small stars. Consisting almost entirely of neutrons, they are the densest matter in the universe.
The two neutron stars observed spiraled around each other for millions of years before smashing into each other and generating an explosion that reached Earth 3 billion years later.
Before August, every detection of gravitational waves was a result of the collision of black holes; however, this most recent observation, as it was created by stars, made the resulting fireball visible to both gravitational wave observatories and regular light telescopes.
Scientists working at both LIGO detectors as well as the Virgo detector in Italy picked up the signal for a gravitational wave and soon narrowed their search to the merger of two neutron stars.
The first observation of the event was sent to scientists by LIGO, and the signal lasted 100 seconds or nearly two minutes—500 times longer than the signal emitted from the merger of black holes. It quickly became clear that this observation was a new phenomenon. Just 1.7 seconds after the initial wave observation by LIGO, the Fermi gamma ray space telescope recorded a gamma ray burst.
The cause of gamma ray bursts is unknown, but scientists had long predicted the collision of neutron stars could be a cause.
Salvatore Vitale, an assistant professor of physics at MIT, was one of the scientists observing the waves. He said of the gamma ray detection, “You want the gamma ray burst to come after the gravitational waves because first you have to smash the objects together, then the material is warmed up, and then you get the radiation. So you would expect to see the gravitational waves first.” And that is just what happened.
Scientists soon linked the gamma ray burst to the detected gravitational waves, and they decided that the collision came between two neutron stars.
It also proved Einstein’s theory that light and gravitational waves travel at the same speed. Though the collision happened 3 billion years ago, the signals came in not even two seconds apart.
In an interview, Vitale described his feelings when things began to click. “Then it was, like… ‘Okay. Oookay…let’s take a chair… and sit down…’” he said.
The Work Continues
After this latest discovery, individual groups soon wrote papers on the subject and published them all around the world, in publications and magazines such as Science and Nature. A main paper was also compiled and submitted to The Astrophysical Journal Letters, with some sources saying it has 4600 authors—about a third of the world’s astronomers.
But the work is not yet done. LIGO is currently offline for updates, which scientists are confident will make LIGO’s detection system even more streamlined. “With further improvements, I think we’ll begin to detect tens of these events per year,” said Bangalore Sathyaprakash, a physicist at Penn State and LIGO collaborator. “And that will be an exciting era.”
The work done by LIGO and Virgo have also inspired scientists around the world to build gravitational wave observatories of their own. In Western Japan, production for a new observatory called KAGRA (Kamioka Gravitational-wave Detector) is underway. Another detector, GEO600, located near Hannover, Germany, is also working and shares data with LIGO.
The scientists’ efforts have also inspired praise. LIGO scientists have earned a variety of awards over the past couple years, including the Gruber Prize in Cosmology and the Special Breakthrough Prize in Fundamental Physics, both of which were awarded to Weiss as well as fellow collaborators.
The dramatic discoveries also earned three scientists the ultimate award: the Nobel Prize in Physics. On October 3, the Nobel Committee awarded one-half of the prize to Weiss, and fellow LIGO co-founders Barry Barish and Kip Thorne of Caltech shared the other half.
Barish said the prize is “a win for Einstein, and a very big one.” He added, “LIGO is a prime example of what couldn’t be done before.”
“It’s a win for the human race as a whole,” said Thorne in an interview. “These gravitational waves will be powerful ways for the human race to explore the universe.”
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