Every time a new telescope has been turned on, we have learned something fundamental about our universe.
Galileo’s first telescope saw the moons of Jupiter and forever destroyed the idea that the Earth was at the center of the universe. Edwin Hubble used the Mount Wilson Observatory in 1917 to show that other galaxies exist and that the Milky Way was but one of many.
The Vela satellites orbiting the Earth in the 1960s were designed to detect the gamma rays that accompany a nuclear explosion. They worked for that purpose, but they also discovered gamma ray bursts from space, which was eventually identified as the explosion of a star so violent that they could be seen across the entire universe.
Wednesday, scientists made a momentous announcement resulting from the latest form of “telescopes” — detectors of gravitational waves. And the scientists instrumental in the detection have now been awarded the Nobel Prize.
What they revealed is the observation of a cosmic calamity.
Literally a long time ago and in a galaxy far, far away, two black holes, locked for eons in a dance of death, finally slammed into one another. Over the course of a few milliseconds, energy equivalent to the mass of three stars the size of our sun was released as gravitational waves that roared across the cosmos.
Gravitational waves occur when the fabric of space and time are distorted by the movement of large masses. Their existence was predicted in 1916 by Albert Einstein. In this announcement, black holes with the masses of 31 and 25 solar masses merged into a larger black hole with a mass 53 times that of our sun.
For that brief instant, the gravitational energy emitted by that collision outshined all of the light emitted by all the galaxies throughout the known universe. After traveling for about 1.8 billion light years, the death scream of these two ancient stars passed through the Earth.
On August 14, three detectors recorded the passage of these gravitational waves. Two detectors in the United States — one in Hanford, Washington, and the other in Livingston, Louisiana — are called the Laser Interferometer Gravitational-Wave Observatory, or LIGO. The other detector, located near Pisa, Italy, is called Virgo.
All three detectors are L-shaped, with each leg being about two miles long. Using lasers and mirrors, these phenomenal pieces of scientific equipment are able to measure tiny changes in the length of the legs of the detectors and identify the passage of gravitational waves.
In February 2016, scientists announced that the first direct observation of gravitational waves had been made using just the two LIGO detectors. That was followed by a second announcement in June 2016. Because Virgo was undergoing an extensive upgrade, it was not operating during these first observations.
Virgo’s upgrades were completed and the facility became operational August 1, and thus it also recorded the gravitational waves of August 14.
The addition of a third facility is an enormous improvement in capability. Like seismographs on Earth, gravitational wave detectors are non-directional and individually cannot determine the location from which the gravitational waves originated. However, by employing multiple detectors and carefully recording the arrival time of gravitational waves at each detector, scientists can triangulate and vastly improve the directional precision of the measurement.
By including Virgo with the two LIGO measurements, scientists’ measurements of the location in the sky from which the waves originated was improved tenfold. A proposed additional facility in India called Indigo that is an exact copy of the LIGO equipment will result in an even greater improvement if it is built.
So why are gravitational wave observatories interesting?
Well, the simplest answer is that they can verify that Einstein’s theory of general relativity is right, but that’s actually not a very satisfying one. There have already been many other tests of general relativity, including the simple fact that the GPS on your phone would simply not work if the theory were not correct.
A better answer involves astronomy. Black holes are just that — black. They are the corpses of dead stars, so massive and compact that not even light can escape them. They literally cannot be seen, and before LIGO came online, their existence could only be inferred by their gravitational effect on their neighbors or because of light (often X-rays) emitted by hot gas falling into the black hole.
But an isolated black hole is invisible. It interacts via gravity and, even then, it only emits gravitational radiation when it is moving. So detectors like LIGO or Virgo are the only way to see them. They are essentially black hole telescopes.
With even just a few observations of gravitational waves, the LIGO measurements have already perplexed scientists. Prior to 2016, astronomers thought that there were two classes of black holes: stellar-class black holes, with masses no more than about 10 times that of our sun, and massive, monstrous black holes at the center of galaxies with masses in the range of hundreds of thousands to billions of solar masses.
Black holes with masses in the range of 30 solar masses or so were unexpected. And yet, that’s just what LIGO (and now LIGO plus Virgo) have observed.
If history teaches us anything, it’s that a new telescope means we should expect the unexpected. Studying gravitational waves will teach us something that can’t be observed in any other way. There’s no way to know what we’ll learn. But I am positive that it will be fascinating.
Correction: An earlier version of this article said that scientists made the first direct observation of gravitational waves in February, 2016. In fact, that was the month they announced the observation, which had been made in September, 2015.
