From Einstein to LIGO:
Gravitational Wave Detection
When Einstein published his theory of relativity, it served as a turning point for physicists and astronomers looking to unlock the secrets of the universe.
Einstein drew the universe as a grid of space and time, bent by every planet and star.
The more massive a planet or star is, the more it bends the grid.
Think about this spacetime grid as a sheet of cloth, pulled tightly from each corner.
If you place a tennis ball on the cloth, it will cause the cloth to bend around it. If you place a bowling ball on the cloth instead, it will bend even more, thanks to the ball’s weight. Roll something light around the bowling ball and it will naturally “orbit”—and in this way, Einstein successfully modeled astronomical orbits.
This is only the beginning to Einstein’s relativity, a concept as foreign as it is powerful. Einstein’s field equations, which claim that matter bends spacetime (as visualized by the spacetime cloth), explain why massive objects have a gravitational pull with near-perfect accuracy.
But taken one step further, Einstein’s strange models predict some extremely peculiar phenomena.
As two black holes pull at each other, their immensities cause each other to spin. As they get closer and closer, they orbit faster and faster. When they’re near enough, they start spinning so fast that spacetime can barely keep up with them. Their rotations actually cause ripples in spacetime. These ripples are called “gravitational waves.”
Despite the sheer mass of these objects, the waves are so tiny that their detection was thought to be the destiny of physicists many generations from now. Yet on September 14th, 2015 at 5:51 AM EST, the Laser Interferometer Gravitational Wave Observatory1 (LIGO) made its first gravitational wave detection, from a collision of two black holes that occurred 1.3 billion years ago 2.
LIGO is a unique observatory because it operates through two connected observatories that act in tandem.
They are separated by thousands of miles, with one located in Hanford, Washington, and the other in Livingston, Louisiana.
Gravitational waves travel through the universe and can theoretically be detected from anywhere on the globe.
LIGO ensures that it is detecting a gravitational wave by making sure the detection occurs at both locations. A detection made at just one location is classified as a local disturbance.
To minimize the chances of this happening, the observatories are located in areas far from traffic, railways, and human interaction 3. Even a pack of joggers could disturb these fine-tuned detectors.
Although the detectors are generally modeled off of small, simple designs from over a century ago (“Michelson-Morley interferometers”), there are two major upgrades, along with a plethora of smaller ones, that allow LIGO to reach an unprecedented level of sensitivity.
The first upgrade is size; the larger the detector, the more sensitive it is. LIGO’s facilities have ‘arms’ that stretch four kilometers long. When the facility detects a wave, it uses special mirrors on each end of the arms to bounce the detected wave back and forth, in the same way that light bounces off your closet mirror.
This effectively amplifies the arms three hundred times for more accurate detection.
The second upgrade is the use of new mirror technology. LIGO uses all sorts of mirrors to function, including the ones that make the wave bounce back and forth to improve signal.
For example, to increase the precision of the detection, LIGO uses Signal Recycling Mirrors. After a wave travels through the arms of the observatory, it would usually end up at a final detecting device.
Rather, by placing a Power Recycling Mirror in the system, it makes sure that most of the wave doesn’t reach the detector and is instead ‘recycled’ back into the arms for better signal detection afterwards 4.
Since the first gravitational wave detection in 2015, LIGO has successfully detected many more waves from major black hole collisions over the course of three observing periods. New observatories have started to spring up all over the world, such as KAGRA in Japan, which is smaller than LIGO but has more advanced technology 5.
Talks of creating a LIGO facility in India have been in the works, expanding LIGO’s current capabilities and making the project a worldwide collaborative effort 6.
This creates a promising future for gravitational wave detection and for our understanding of the universe itself.
Written by Karina Thanawala, Edited by Angelina Zhang & Alexander Fleiss