Introduction
14th of September, 2015 marks the day when first observation of gravitational waves was made by humans. The observation was made at the LIGO facilities. This observation was officially announced on 11th of February, 2016. The discovery and observation is one of the greatest achievements of Science in the history of mankind.
I will tell you the story of this unique event here. It is the story of unimaginable human ambition and ingenuity. It is the story of man peeking into designs of the gods, hearing their whispers and lifting the veil.
1916: The journey begins with Einstein.
The greatest mind of our times and perhaps of all times, proposed the special theory of relativity in 1905. Nearly a decade later he had moved to the General theory of Relativity which he published in 1916. He interpreted gravity as a function or consequence of space time curvature and the perturbations of this fabric caused by mass and/or energy. It is the perturbation in space time fabric thus that governs the movement of earth around the sun.
Einstein predicted Gravitational waves in 1916.
sourceEarly work on Gravitational Wave detection.
There were many notable efforts in the early days but none more notable than the Physicist Joseph Weber.. He pioneered the efforts to detect Gravitational waves.
sourceAlthough Weber reported seeing evidence of gravitational waves, his claims were later discredited. To some he was a maverick who had many methodological flaws in his experiments. His idea of gravitational waves passing through the large aluminum cylinder and causing that energy to be transmitted to the overlying fixed bar and then release of the energy in the form of sound was his basic notion for the invention. It is also known as the Weber resonance bar. Despite his claims of success, others could not reproduce his results. Weber was rather non accommodating of the critics. Having reviewed this history, I see Weber as a pioneer in this field. Science has a way of building upon failures.source.
LIGO:( Laser Interferometer Gravitational Wave Observatory) and Advanced LIGO. A success story.
First, let’s make note of the fact that in 2017 Kip Thorne, Rainer Weiss and Barry Barish received the Nobel prize for contributions to LIGO and the first detection of gravitational waves. Researching for this post, what I came across makes me stand in awe of the efforts of the LIGO scientists and technicians. Each and every one of them deserves great recognition in their own respect.
It was 1980 when the National Science foundation allocated funds for research and design into building LIGO facility.
This is preceded by tremendous theoretical and experimental work by Weiss, Thorne, Drever, Braginsky and many others. A brief history of this can be found here.
It was 1989 when the initial proposal was for LIGO construction was put forward. With great foresight, from the beginning the plan was to develop 2 separate LIGO sites as pair of initial interferometers with enough sensitivity that may have a chance to detect gravitational waves and later develop ADVANCED LIGO with improved technologies with far greater chances of detecting gravitational waves. Hanford, Washington and Livingston, Louisiana were the 2 chosen sites. Construction and equipment installation lasted from 1994-2002. Operating since then the initial interferometers did not detect any gravitational waves. From 2008- 2014 advanced components were constructed and installed and LIGO became ADVANCED LIGO. Operations stated in May of 2014 and the rest is history. source.
sourceThe idea behind LIGO.
Gravitational waves originating from massive and colossal cosmic events travel outwards from the event in travel through spacetime fabric causing ripples in the fabric somewhat akin to ripples going outwards in a water pond when an object is dropped in it. These waves when passing through a body compress it in one direction and stretch it in another. If this stretching and compression of spacetime can be measured, then you have detected the presence of gravitational waves as a cause of it.
LIGO, in the most simplistic terms, employs a laser generator throwing out a laser beam through a splitter. The splitter puts the beam out in 2 directions at 90 degrees to each other. These beams travel through a 4 km tunnel in each arm and are reflected back to a photodiode detector. Interference of these beams causes a particular intensity signal. Change in interference pattern intensity indicates , under controlled conditions, expansion and compression of spacetime through which the lasers are travelling and thus the detection of gravitational waves.
sourceThe marvel of LIGO.
source source source
Only a few highlights of the technological marvels are mentioned here.
Increasing the reach.
LIGO’s transformation into Advanced LIGO improved the detection reach of the interferometers by 10 folds. The interferometers could now listen for gravitational waves 10 times farther than the original LIGO. That equates to 1000 times more volume of space that could be listened to, increasing detection chances greatly.
Measuring the smallest distance ever attempted.
Calculations prove the the first ever gravitational wave detected was caused by the merger of 2 spinning black holes. The event occurred 1.3 billion years ago. As the merger happened an enormous amount of energy was released as gravitational waves. It is estimated that briefly the energy output was greater than the electromagnetic energy output of all stars in the known Universe combined. As the wave passes through earth it creates a very very small change in spacetime fabric. That change is 1 part in 1021. This small change is easier ( if such a word can ever be used in anything related to LIGO) over a larger distance. This is why the arms are 4 Km long. Even at this distance the changes in spacetime within the LIGO arms is 10-18 meters. This distance is impossible for a human mind to visualize. Many analogies have been given. The distance is about 1/10000 the width of a proton. Decades of theory and experiments have gone into how to reliably make this measurement. A few interesting technical challenges are discussed below.
Curvature of the earth.
An important factor to consider when building a 4 km arm. It required the most precise concrete pouring and leveling.
Optics
LIGO mirrors are the smoothest mirrors created. Even with each arm at 4 km long, it is not long enough to detect gravitational waves. Fabry Perot cavities solved this problem. There are mirrors installed near the beam splitter that reflect light back and forth through the 4 km chamber. It is done 280 times before beams are merged. This allows for an effective length of 1120 km for the LIGO arms.
Isolating the system from interference and noise.
A passing by car, seismic activity and and other simple day to day activities and occurrences will interfere with measurements. Ingenious designs houses the components by silica threads via a series of pendulum like attachments. The whole housing and it’s movements are actively managed as well to minimize interference.
The laser
You are measuring interference of wavelength of 2 laser beams or one beam split in two. Thus you need the purest light. In other words you need a laser with only one wavelength. There are many filtering and corrective mechanisms at LIGO that achieve this laser stabilization. When measuring scales this small, many uncertainties come into play. To decrease this quantum uncertainty you need a large number of photons hitting the mirrors. This is why a very very high powered laser of 1 mega watt is used.
Decreasing interference in the arms.
Lasers and measurements will be affected by the air present in the arms. The air has to be vacuumed out and then chambers heated to expel the rest. The pressure achieved in the arms is one trillionth of an atmosphere.
Why 2 sites?
A signal observed near simultaneously at 2 sites that are adequately far apart from each other rules out the possibility of a false signal produced at one site by local mechanisms and interference.
A Gravitational wave is detected. How do we get the rest of the information?
How come the smartest folks in the world come to the conclusion that it was 1.3 billion years ago that 2 large spinning black holes collided and as the merged into one large black hole, they set off gravitational waves that were detected on earth?
Matthew Evans ( MIT) presents this beautifully in his lecture that are available online
So the waveform is analyzed first and going back through the equations of relativity to figure the mass of the source. Different masses produce different signals. The source parameters are solved inversely by analysis of waveform and backtracking relativity equations.
Some estimate of the distance to the event, final black hole spin speed are calculated via MCMC code ( Markov chain Monte Carlo) through inverse problem solving.
A new era
The story has just begun. There is a new way to look at and listen to the universe. Instruments will get better and better. Will we see the Big Bang and hear it? What other marvels will we discover? We are in for ride of our lives. Many thanks to the collective genius of those who got humanity this far.
Disclaimer: I have spent days researching and reading for this post. I have made all possible effort to make this information accurate. The story of the LIGO effort is the story of human passion, fallibility, endurance and courage. It is life affirming. It is difficult to fathom that those that can see so far exist amongst us. I welcome any corrections and constructive criticism.
N. Inayet MD, FCCP.