LIGO’s groundbreaking discovery vindicates Einstein and lets us tune in to the rhythm of the universe.
For the first time ever we have heard the sound of gravitational waves as they ripple through spacetime. R. Hurt/Caltech-JPL (public domain)
"Ladies and gentlemen, we have detected gravitational waves. We did it!"
When David Reitze, executive director of the Laser Interferometer Gravitational-Wave Observatory Laboratory (LIGO), announced this at the press conference, my heart stopped for a moment. It was still early in the morning in Pasadena, at the live event at Caltech, but we could barely contain our excitement, breaking into applause and spraying champagne. We toasted the breakthrough in physics and astronomy, the great success of this worldwide collaboration, and the amazing people we were working with, regardless of whether we knew each other or not. The party for gravitational waves was on all over the world, no matter where you were and what the time it was.
As an astrophysics student, this has to be one of the most exciting moments in my whole life. As one of the thousands working in the LIGO Scientific Collaboration, I could not be more proud of this marvellous achievement.
One hundred years ago, Albert Einstein predicted that gravitational waves should exist according to his field equations of general relativity, which mathematically explain how gravity works. He also pointed out that such ‘ripples’ in the fabric of spacetime, traveling at the speed of light, were extremely tiny. However, physicists, scientists and engineers have never stopped searching for direct, observational evidence of gravitational waves. Now, we have proof: Einstein was right.
A new wave
The fabric of spacetime is warped by things having mass. The more massive the objects are, the more the spacetime gets distorted by gravity. When the masses accelerate, the distortion is changed, causing gravitational waves — like the waves that start from a stone dropped into a tranquil lake. When a gravitational wave passes by, it stretches space in one direction, and squeezes it in the other, but these distortions are extremely tiny even over large distances. For example, if you were measuring the distance between our solar system and the nearest star (over four light years away), space would be distorted about the width of a human hair. But how can we possibly measure such an incredibly small change in length?
According to Einstein, it’s the presence of mass that distorts the space around it, creating gravity. Mysid/Wikimedia Commons (CC BY-SA 3.0)
As we know the speed of light, when it travels from one point to another, we can determine the distance it has travelled. If the space has been stretched or squeezed, like when a gravitational wave passes through an area, the light will take a different amount of time to travel back and forth, so as to produce an interference pattern, from which we can precisely know how much the distance has been altered. We need something very large to magnify the tiny fraction of the distance being changed. That’s where LIGO steps in.
The construction of two LIGO detectors (one in Livingston, Louisiana, and the other in Hanford, Washington) was approved in 1992 by the National Science Foundation. More than one detector is required to confirm true gravitational wave signals by ruling out the possibility of localised false alarms. The initial LIGO started its first observation run in 2002, and concluded in 2010 for the installation of Advanced LIGO.
Just like waves in water cause an interference pattern, so too do waves of light. These patterns give you important information about the waves, such as the distance each have travelled. Scott Robinson/Flickr (CC BY 2.0)
After all the upgrading, reconstruction, and engineering testing, Advanced LIGO has reached a sensitivity much greater than the initial LIGO, and is expected to only get better. Laser beams reflect back and forth along the four kilometre long, L-shaped arms. If the length of LIGO’s arms is stretched or squeezed due to the presence of gravitational waves, this will alter the interference pattern in the detectors. As the arms are so long, it is possible to measure a change in length of about 10 -19 metres, equivalent to ten-thousandth the width of a proton.
The day the physical world stood still
14 September 2015. That day might have been just a normal Monday for most of people, but it certainly wasn’t a silent one in the LIGO Scientific Collaboration. The Advanced LIGO detectors had finished their upgrades, and started to operate in observation mode, listening carefully to the universe. The automated computer programs were monitoring the data, looking for any possible gravitational wave pattern. An alert was triggered because of a high signal-to-noise-ratio “sound” received at both detectors, which was so loud, and so beautiful, that the LIGO scientists could hardly believe it was truly coming from an astrophysical collision event.
The signal LIGO received matches up remarkably well with the predicted curve of a black hole merger using Einstein’s equations. Caltech/MIT/LIGO Laboratory (public domain)
The LIGO system works by sometimes putting in fake gravitational wave signals in order to promote rigorous testing of the extremely complicated instruments. So the scientists working here did not get their hopes up, in case this was just a test signal. But even after trying to reject it, step-by-step the signal passed rigorous data analysis and examination, eventually proving itself a true signal from one of the most catastrophic events in the universe — the explosive collision of two massive black holes. It was given the name “GW” (gravitational wave), and called “GW150914”.
An astronomical symphony
Let’s look back in time 1.3 billion years. Two black holes, each around 30 times the mass of the Sun, are orbiting each other and gradually losing energy over billions of years. They get closer and closer, until they finally collide at about half the speed of light. For that spectacular instant, the energy of three times the mass of the Sun bursts out, raising a tsunami in the fabric of spacetime, emitted as a gravitational wave. That wave travelled for 1.3 billion years until it arrived at Earth, bringing us a beautiful, dramatically rising tone over just 0.2 seconds long. And this is what LIGO heard. We heard it! LIGO heard it!
The violent end to a black hole merger – the “chirp”.
This is not just a sound; it’s a message. By decoding the gravitational wave signal and estimating the property parameters of the source, we actually “see” how the two black holes died, giving birth to a new one. It is just like a vivid movie, telling a story that happened long before humans emerged on Earth. We never stop listening and searching. In future observations of Advanced LIGO, we will have a collection of such detections, which gives us the chance to study these monster binary black holes more deeply, in the hope of understanding how they spin and precess, as well as how they emerge in the first place. Not yet, but soon.
This isn’t even the whole story of detecting gravitational waves. For hundreds of years, astronomers studied the universe utilising all kinds of electromagnetic waves, such as visible light and radio waves. Now that we have heard the first sound of gravitational waves, we’ve opened up new possibilities in astrophysics and astronomy. As LIGO improves further in sensitivity, we will hear more of them, and promisingly we’ll hear other sounds from a range of astrophysical sources, such as neutron stars, supernovae and cosmic strings. The truly amazing part is that we might even discover things we never expected. A new era of gravitational-wave astronomy has begun, allowing us to hear the beautiful rhythm of the universe, an intoxicating symphony.
Edited by Bryone Scott, Tessa Evans and Nicola McCaskill, and sponsored by Luke Weston
