In 1916, Albert Einstein based on his General Theory of Relativity, predicted the existence of ripples of disturbances in the space-time, like waves of gravity pushing things around, by undetectable amounts, due to accelerated mass. These came to be known as the Gravitational Waves or the G-Waves. The General Theory of Relativity, in itself was a revolutionary idea, in face of the Classical Newton gravity, and hence was met with fierce opposition.
With time the understanding of the idea grew and with verification of its other predictions, like bending of light due to a massive object, this theory gained the acceptance of brains all over the world. There was yet to be an observation that would verify the ‘Gravitational Waves’.
It was not until 1974, that this prediction would be attested by Taylor and Hulse after their observation of a binary pulsar, which lead them to an indirect proof of gravitational waves and a Nobel Prize in 1993. A first direct detection of these waves was first announced on 11th February, 2016, at Washington, and was done using the Advanced LIGO (LASER Interferometer Gravitational-Wave Observatory) which consists of two interferometers: one in Hanford, Washington and other in Livingstone Louisiana, operating in unison to detect gravitational waves.
It was for this, that the Nobel Prize in Physics, for the year 2017 was awarded to Reiner Weiss, Barry C. Barish and Kip Thorne for their ‘decisive contribution to the LIGO detector and observation of Gravitational Waves’. The Nobel can be awarded to a maximum of three laureates and they were the pioneers of the idea but it is important to acknowledge the cumulative effort of thousands of scientists and engineers from all over the world which were also a part of this game changing experiment, that opens new windows to observational astronomy and cosmology, leaving us with enormous amounts of ideas to explore.
But first things first: What exactly are Gravitational waves?
Let’s draw out an analogy between electromagnetism and gravity. We are aware that when an electric charge is in an accelerated motion it emits electromagnetic waves: ripples of distortion in the electromagnetic field travelling through space and time transversely, carrying an associated energy with them, and interacting with matter in its path. Similarly, the source of gravitational interactions is mass. Thus, when a mass accelerates it loses its gravitational energy in form of ripples of distortion in the gravitational field travelling, pushing and pulling matter on their way, through space-time, carrying energy transversely, travelling at the speed of light in vacuum. Now, the gravitational interaction, as we know is much weaker than the electromagnetic interaction (approximately 1038 times weaker), so consequently the energy carried by the g-waves is negligible in comparison to the electromagnetic waves, making them extremely difficult to observe. For instance, as the Earth travels around the Sun in its curved trajectory, it emits gravitational waves. But for the entire Earth, that gravitational wave output amounts to a few hundred watts, not enough to ever be detected. Our sun also emits gravitational waves just as it emits electromagnetic waves, but in comparison to roughly 400 million trillion megawatts it emits as heat and light, it only emits about 79 megawatts in gravitational waves; again the amount is too low to be detected.
Fortunately, we have much stronger and much massive sources of gravitational waves. One such source, which was discovered by a Joseph Taylor and Russel Hulse, in 1974 was a binary pulsar i.e. a pulsar orbiting another star, which they were almost sure was a neutron star. The exciting measurement in this system was the observation that the two stars' orbits are shrinking at a rate of 1 cm/day. This shrinkage was caused by the loss of orbital energy due to gravitational radiation, which is a travelling ripple in space-time that was predicted by Einstein's General Relativity Theory but never previously verified.
Observations showed that the pulsar orbit was shrinking at exactly the rate that general relativity predicted it would, if gravity waves existed and were carrying away the expected amount of energy. It was this result that lead the astrophysicists to be secure about the Gravitational Waves and provided them with the confidence required to go forward with direct measurement with large detectors as LIGO.
LIGO Begins: The Origin Story!
Starting in 1960’s American and Soviet scientists conceived the basic ideas of LASER interferometry for detection of gravitational waves. In 1967, Rainer Weiss published his analysis of interferometer use, and in the next year Kip Thorne initiated theoretical development at Caltech. Many prototype interferometers were proposed in next two decades, but they failed to acquire funding or to make any further progress technically. Meanwhile, in 1980’s the National Science Foundation (NSF) funded a study in large interferometry lead by MIT, and Caltech constructed a forty meter prototype. Under pressure from NSF, these premier institutes came together to lead the LIGO initiative. In 1994, when Barry Barish took over as laboratory director, LIGO was told that it was its last shot at attaining funding, but a revised theoretical, budget, and project plan was successful in obtaining the green signal and funding. The project, at 395 million USD, broke first ground in late 1994 and the construction neared completion in 1997.
Initial run of LIGO from 2002 to 2010 detected no such gravitational waves. So, it was closed for further advancements in the equipment, increasing its sensitivity by a certain orders of magnitude. It was not until September 2015 that LIGO/a LIGO would begin the second phase.
Simultaneously, an Italy based Laser Interferometer, VIRGO also started working in 2015, actively to detect gravitational waves. To the delight of involved scientists, it detected its first signal on 14th September 2015, emerging due to a merger of two massive black holes having 29 and 36 times the Solar Mass, which merged into a super massive black hole having 62x Solar Mass, which happened in a corner of the universe 1.3 Billion light years away. This lead to dissipation of gravitational energy, form the merger, equivalent to 3 solar masses. These results were published on 11th February 2016, in which LIGO Scientific Collaboration along with VIRGO collaborationconfirmed the first direct detection of Gravitational Waves. As of now, till November 2017, LIGO has announced four more detection of similar signals.
The Anatomy of LIGO/aLIGO
The LIGO consists of two large interferometers: one in Hanford, Washington and another in Livingstone, Louisiana separated by 10 milliseconds of light travel time (approx. 2400 miles). Each primary interferometer consists of two 4 km beam lines orthogonal to each other carrying a test mass to form a power recycled Michelson Interferometer. A pre-stabilized Nd:YAG laser source emits a beam of 20 W power of wavelength 1064 nm, which through a beam splitter sends the beam to the two arms. By the use of partially reflecting mirrors,Fabry-Perot resonance cavities are formed in both the arms, increasing the effective length of the path travelled by light beam. After approximately 280 trips down the 4km arms the light beams recombine at the beam splitter. The equipment is kept such that, these two beams are out of phase and interfere destructively and no light arrives at the photo-diode.
Now, when a gravitational wave passes through the interferometer, it disturbs the test masses in both the arms, shortening and lengthening the arms by a very small distance causing the beams to become slightly less out of phase, causing resonance and some light is detected at the photodiode. The results from both the interferometers were compared and analyzed to be found similar, hence proving that the movement in the test masses were due to a common distortion, not due to any seismic or human activity.
Each of these test masses had an extremely sensitive sensors to monitor any form of motion up to one attometer(10-18 -10-19 m). The sensors could measure a displacement of a 10000th of a proton. This is equal to measuring the distance to Alpha Centauri with a precision of a hair strand. With this extreme sensitivity, came a drawback: the signal could be disturbed by smallest of seismic activity or even traffic! To nullify the effect, the test masses are equipped with active and passive damping measures. Active damping works similarly to the noise cancelling headphones; a sensor is attached to measure the surrounding noise, and the computer informs the device to move in form so as to cancel the noise. Secondly, the system prevents any motion that is not countered by the active system from reaching the test mass. The test mass (the mirrors) are suspended by a 4 stage pendulum called as the Quad. They are held by a 0.4 mm think fused silica glass fibers. Four vibration damping masses are present in the pendulum which absorb the vibration. The "Main Chain" side faces the laser beam, while the "Reaction mass" side helps to keep the test mass steady from noise not associated with astrophysical sources. Thanks to Inertia, the sheer weight of these masses also contribute to damp the vibration.
So any distortion measured in test mass is now only due to distortion in space-time due to gravitational waves.
Alright, so Einstein was correct; but why are Gravitational waves so important?
Gravitational waves will usher in a new era in astronomy. Most of the astronomy done in the past has relied on different forms of electromagnetic radiation (visible light, radio waves, X-rays, etc.), but electromagnetic waves are easily reflected and absorbed by any matter that may be between their source and us. Even when light from the universe is observed, it is often transformed during its journey through the universe. For example, when light passes through gas clouds or the Earth's atmosphere, certain components of the light will be absorbed and cannot then be observed.
Gravitational waves will change astronomy because the universe is nearly transparent to them: intervening matter and gravitational fields neither absorb nor reflect the gravitational waves to any significant degree. Humans will be able to observe astrophysical objects that would have otherwise been obscured, as well as the inner mechanisms of phenomena that do not produce light. For example, if stochastic gravitational waves are truly from the first moments after the Big Bang, then not only will we observe farther back into the history of the universe than we ever have before, but we will also be seeing these signals as they were when they were originally produced. The physics that went into the creation of a gravitational wave is encoded in the wave itself. To extract this information, gravitational wave detectors will act very much like radios—just as radios extract the music that is encoded on the radio waves they receive, LIGO will receive gravitational waves that will then be decoded to extract information on their physical origin. It is in this sense that LIGO truly is an observatory, even though it houses no traditional telescopes.
However, the data analysis that is required to search for gravitational waves is much greater than that associated with traditional optical telescopes, so real -time detection of gravitational waves will usually not be possible. Therefore, LIGO creates a recorded history of the detector data. This provides an advantage when cooperating with traditional observatories, because LIGO has a ‘rewind’ feature that telescopes do not. Consider a supernova that is only observed after the initial onset of the explosion. LIGO researchers can go back through the data to search for gravitational waves around the start time of the supernova.
- Arnav Pushkar
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