Saturday, 13 February 2016

Waves, Mirrors and a Discovery

An Optics student's charm about the latest discovery !
Arms of LIGO
The world of Science is celebrating the detection of a certain kind of ripple- Gravitational waves ! The waves were predicted by Albert Einstein in 1916 while he was working on one of the most beautiful theories in Physics called the General Theory of Relativity. Almost 100 years later, the waves were recently detected thus opening a new window to the universe. While the whole prediction and discovery is amazing, what enchants me the most is the core idea of the experimental setup.

The whole experimental setup at LIGO (Laser Interferometer Gravitational wave Observatory), U.S.A. equipped with the best in technology spans over a radius of 4 km. The setup is a grand and a giant version of the humble Michelson interferometer. Any student of Physics should be familiar with the term and anyone with a sense of how light reflects and combines should understand the idea pretty well.
Fringes from M.I
I came across the concept of the Michelson interferometer during an Optics demonstration at College and it has impressed me at different levels ever since. Till then, nothing had been more beautiful than white light splitting into seven colours, but that holy experiment changed everything. The setup is simple yet elegant- A beam of light is split into two, allowed to travel and bounce from two mirrors at right angles to each other and recombined. A detector is then used to see how the recombined light looks like. As the mirrors get adjusted/disturbed, the pattern keeps evolving. For the first time, I saw streaks of bright and dark fringes that seemed to magically emerge out of a couple of mirrors illuminated by a laser beam. They looked just spectacular!

A good long time ago, when I was still in high school, my mother bought me a laser pointer and I spent an entire summer playing with it. I observed two things- the light reached farther and the beam spot was small compared to the light from a torch. Ten years later, as a masters student, I had to send a slightly high powered laser beam into an M.I and understand the reason behind those observations. It took a couple of more years for me to appreciate the technical beauty of Holograms which remained a fantasy for long. We did an M.I experiment at the dead of night in a pitch dark room on a vibration free surface to produce a hologram. A number of trials failed miserably but the end result was aces. We widened our tired eyes to see if the hologram was actually there and it was. Another sight worth witnessing! Later, during one of the classes on Special Relativity, M.I again came into picture and this time, it had incarnated itself as a tool to measure the speed of light-A humongous number and a prodigious upper limit in space-time.

Setup at LIGO
Fast forward another couple of years, 2015 arrived with royalty. The year was special for two reasons. It was declared the International Year of Light by UNESCO and it also marked the 100th anniversary of the General Theory of Relativity. On one side, Science folks were celebrating the importance of light and light based technologies and on the other side, a remarkable discovery was being anticipated that would make Einstein's speculation into reality. On September 14, 2015 at 5:51 a.m. Eastern Daylight Time (09:51 UTC) , the signal was finally detected at LIGO observatories in Louisiana and Washington, U.S.A.

The signal, which is essentially a disturbance in the space time fabric came from the collision of two black holes that took place 1.3 billion years ago. According to General Relativity, two black holes lose energy as they spin around each other and eventually merge together to form a single massive black hole. The process which happens at half the speed of light is highlighted by the conversion of a portion of the combined mass into energy. This energy travels through the cosmic fabric as gravitational waves.

The LIGO interferometer at the core is a Michelson interferometer that has arms 4 km in length and suspension mirrors that weigh 40 kg each. More mirrors are placed along the arms to efficiently recycle power in the cavity to enhance the sensitivity. Light beam emerging from a 200 W near infrared laser travels a whopping distance of 1600 km (due to the cavity effects) before reaching the detector.This setup is about 144,000 times bigger than the M.I that was used to measure the speed of light in 1887. Just like the original M.I, the interference pattern would change if the mirror moves because of any disturbance.

Colliding Black holes
The radiation from the collided black holes hit the mirror and moved it by a distance as small as the width of an atomic nucleus. The resulting interference is at excellent agreement with the simulations and calculations of the General Theory of Relativity thereby confirming the genius' prediction. Though it was Einstein's contemplation, a whole bunch of scientists and engineers had been instrumental in the development of the idea over the past few decades. The discovery is certainly a big leap pushing science to a new era.

There it goes, a favourite experimental setup, BIGGER and BETTER !


  1. A little correction, Einstein didn't predict gravitational waves in his seminal paper in 1915 but later in 1916.