Sept. 14, 2015, could go down in history as the day the cosmos chirped. Astronomy will never be the same.
An international team of researchers announced on Thursday that on that date, they became the first group to detect a cosmic phenomenon that Albert Einstein first predicted 100 years ago: gravitational waves.
These waves radiate from their sources at the speed of light, stretching and squeezing the very fabric of space-time with predictable cadences.
The source, in this case: a pair of co-orbiting black holes some 1.3 billion light-years away, locked in an ever-shrinking spiral that yielded a new, more massive black hole when the two violently merged.
Each initial black hole packed more than 30 times the mass of the sun within a region roughly 93 miles across. They collided at half the speed of light.
At the moment of collision, the binary black holes released an enormous amount of energy, notes Kip Thorne, a theoretical physicist at the California Institute of Technology in Pasadena, and co-founder of the Laser Interferometer Gravitational-Wave Observatory (LIGO), which recorded the waves that the collision generated.
The collision lasted about 20 milliseconds. During that time, “the total power output in gravitational waves was 50 times greater than all of the power put out by all of the stars in the universe put together. It's unbelievable,” Dr. Thorne said during a press conference in Washington announcing the results.
The achievement “is monumental,” says Lawrence Krauss, a theoretical physicist and cosmologist at Arizona State University in Tempe. There are very few times in the history of astronomy where new windows open, and this is one of them, he says, adding, “This will be the astronomy of the 21st century.”
A formal version of the results also is being published in the journal Physical Review Letters.
Since Galileo first turned his simple telescope to the heavens, the cosmos has revealed itself via visible light, X-rays, radio waves, and other forms of electromagnetic radiation. Each yields fresh details about the makeup of the objects and the physical processes involved that produce the radiation.
Astrophysicists also have enlisted neutrinos – subatomic particles that rarely interact with matter – to reveal processes going on inside the sun and, in one case, the mechanism driving the explosion of a massive star in the Large Magellanic Cloud, a satellite galaxy to the Milky Way.
Gravitational waves open yet another window on the universe, focusing on objects and processes where gravity is extremely strong and where space-time under those conditions is poorly known. The objects in question include neutron stars and black holes, particularly those paired up in binary systems. There, two objects orbit their common center of mass like partners in a cosmic square dance.
One of these systems, some 21,000 light-years away in the constellation Aquila, yielded the first indirect evidence for gravitational waves. One of the objects was a pulsar, a rapidly spinning neutron star radiating energy in Earth's direction. The second object was another neutron star.
In 1974, Joseph H. Taylor Jr. and Russell A. Hulse detected the pair. By 1979, they showed that as the two objects spiraled toward each other, they were losing angular momentum in ways that dovetailed with predictions of Einstein’s theory of general relativity. The radiation leaves the system at the speed of light as gravitational waves. This garnered the two a Nobel Prize in 1993.
This new result is expected to do the same in short order.
For astrophysicists interested in the most violent events the cosmos can dish up, the news is welcome.
“This is quite exciting. I've been waiting for this for years,” says Edo Berger, a Harvard astrophysicist who studies high-energy events such as gamma-ray bursts.
Dr. Berger and colleagues are hunting members of one family of bursts that are generated by mergers: two neutron stars, which are known to exist in binary systems; or binary systems involving a neutron star and black hole, or two black holes.
Systems of two black holes are particularly elusive, he notes.
“We think these systems should exist because we see very massive stars in binary systems,” he says. “Those should explode and leave behind black holes. We have no way other than gravitational waves of seeing those systems because they produce no light.”
With Thursday's announcement, the existence of black hole binaries no longer is in doubt.
Beyond expectations for astrophysical discoveries, gravitational waves may also hold the potential to reveal important clues about the nature of gravity itself.
This has been a nagging thorn in the side of theoretical physicists working an another Big Idea – that the four basic forces of nature are low-energy remnants of what was a single force during the earliest fractions of a second after the big bang. These forces are gravity, electromagnetism, the strong force binding particles in the nuclei of atoms, and the weak force governing radioactive decay.
Physicists have been able to show through quantum theory that three of the four forces have a common ancestor. Gravity remains the stubborn holdout.
Through the direct study of gravitational waves, “we potentially can learn about the nature of gravity” in ways that might point to a quantum theory of gravity, speculates Dr. Krauss at Arizona State University.
Indeed, by comparing the details of the signal that LIGO picked up with high-precision calculations of the same event based on general relativity, it might be possible to see if the waves LIGO detected are distorted. Even a slight distortion of the real wave would be telling. That distortion could yield clues as to the mass of a hypothesized subatomic particle associated with quantum gravity, the graviton.
LIGO consists of two observatories – one near Livingston, La., the other near Hanford, Wash. Each facility uses lasers to measure subtle changes in the light's travel time as gravitational waves stretch and compress two perpendicular tunnels 4.5 miles long, explains Rainer Weiss, a physicist at the Massachusetts Institute of Technology in Cambridge and another LIGO co-founder.
As the lasers leave their sources, the beams are split so that the light travels down each tunnel simultaneously. Mirrors at the far ends reflect the beams back to be recombined at the splitter and sent to a detector.
The beams are tweaked so that if no gravity wave is passing by, the two beams' light waves will cancel each other out, leaving no signal on the detector. If a gravity wave passes through the neighborhood, the beams travel slightly different lengths. When their light is combined, the beams no longer cancel each other out, so they deposit light onto the detectors. The light flickers to match the pace of expansion and contraction that the gravity wave imparts to space-time.
The change in length that the team measured was 1/1000th the size of a proton.
If both facilities record the same event at essentially the same time and the wave patterns that the detectors produce on their readouts are the same, the LIGO team knows it has bagged gravitational waves.
LIGO is sensitive to gravity waves over a range of frequencies that coincide with those of sound and human hearing. This allows the researchers to produce sound recordings of the passing waves. In the case of last September's detection, the result is a brief chirp.
LIGO, which made its discovery during engineering tests that followed a significant upgrade, is operating only at about one third of its design sensitivity, which means it has plenty of room for more discoveries over wider distances, team members say.
“Every time we open up a new window we're surprised,” Krauss says. “The imagination of nature turns out to be far greater than the imagination of humans.”