One year ago, exactly a century after Einstein introduced his General Theory of Relativity, a Columbia student started up a machine that aimed to probe the deepest mysteries Einstein’s theory proposed.
On Thursday, the LIGO (Laser Interferometer Gravitational-Wave Observatory) collaboration of over 1000 scientists worldwide—including many from Columbia—will announce the state of its hunt for gravitational waves. The first direct detection of gravitational waves would likely be a candidate for a Nobel Prize because of the difficulty of discovery and knowledge to be gained in observing them.
Imperceptible to all but the most powerful instruments, gravitational waves are created when the motion of massive objects like black holes distorts space. These waves emanate outward like ripples on the surface of a pond, passing invisibly through stars and planets and the void between galaxies.
Although gravitational waves were predicted only one year after the introduction of the theory of General Relativity, scientists have only ever indirectly observed the waves. This first indirect detection, by Hulse and Taylor in 1974, was of the decaying orbit of two neutron stars in a binary system. Battered by gravitational waves, one neutron star gradually fell out of its orbit.
Hulse and Taylor’s indirect detection confirmed the existence of gravitational waves, but told little to nothing of their character and properties—factors which could be used to investigate the nature of gravity and the universe.
And so the hunt for a direct detection began.
At Columbia, the search for gravitational waves has been led by associate professor of physics Szabolcs Marka, who is also a principal investigator in LIGO. Marka and his team have been involved with LIGO for almost 20 years now, helping construct its instruments, analyze the data, and design future experiments.
To illustrate the difference between indirect detection and direct detection, LIGO’s goal, Marka uses an analogy.
“One of your friends tells you that one of his friends had seen the Mona Lisa and it’s pretty cool. That’s indirect viewing of the Mona Lisa,” Marka said. “When you go to the museum and you have some luck and you can stand in front of the Mona Lisa alone, as long as you want—that’s direct detection.”
This view is the prize that LIGO has been after since its inception as a project in the ‘90s.
After its first run, which lasted from 2002 to 2010 without the detection of any gravitational waves, LIGO underwent a five-year-long upgrade designed to radically improve its capabilities. Advanced LIGO, or adLIGO, operates out of two almost identical facilities in Louisiana and Washington.
The use of two identical facilities allows scientists to use triangulation to detect the source of the gravitational waves. However, finding a source is the least of the problems scientists face.
Because the experiment is simply in contact with the Earth, scientists need to be sure that there is no seismic activity interfering with results. While this is not a problem for some experiments, the level of precision that adLIGO operates at is the equivalent of measuring the distance between the earth and the moon down to the length of two protons.
This level of precision is necessary because gravitational waves leave only infinitesimal traces to mark their passing.
To detect the waves, adLIGO uses a basic interferometer design: A laser is split into two beams that head off at 90 degree angles until they bounce off mirrors to recombine before entering a light detector. Because the beams are adjusted to be 180 degrees out of phase from one another, they normally cancel out destructively so that no light reaches the detector.
When a gravitational wave passes through adLIGO, an arm of the apparatus, which is four kilometers long, changes in length by a minute amount. This change in length would alter the amount of time it takes light from one arm to reach the detector, so that the beams would no longer perfectly cancel out. By measuring light received by the detector, scientists can observe and determine the characteristics of gravitational waves.
LIGO’s upgrade was a complete overhaul that removed just about every component of the experimental apparatus besides the vacuum and replaced them with new and improved versions. The Columbia team contributed a new timing system, without which the experiment would completely cease to work.
Recent media have used a variety of numbers to express just how much better adLIGO is, which Marka said is often misleading.
“It is true that at 100 hertz Advanced LIGO is three times better than initial LIGO ever was,” Marka said. “But the most important factor is that there are frequency ranges that were not accessible in initial LIGO, mainly between 10 and 80 hertz.
These low frequencies are particularly important because the heaviest of celestial objects, like black holes, only produce gravitational waves in this range. Adding this lower range of frequencies opened a whole new region of possible discoveries for adLIGO.
If this upgrade has resulted in the direct detection of gravitational waves, it could herald a future in which gravitational waves are used to study the universe.
“Direct detection is not the discovery of gravitational waves … it is opening the door to many many fundamental discoveries in physics,” Marka said.
The door to the examination of black holes, Einstein’s General Relativity, and other previously hard-to- or impossible-to-study cosmological phenomena could be blown right open with direct detection of these waves.
“Detection is something that will change how we think about the cosmos forever,” Marka said.