Gravitational Waves- Century’s biggest discovery

The collision of two black holes—an event detected for the first time ever by the LIGO—is seen in this still from a computer simulation. Source;Image Credit: SXS, Caltech


By Bijay Puri

Last month, Scientists from LIGO (Light Interferometers Gravitational Observatory) and NSF (National Science Foundation) announced that they have succeeded in detecting gravitational waves from the violent merging of two black holes in deep space. Gravitational waves are distortions or ‘ripples’ in the fabric of space-time caused by some of the most violent and energetic processes in the Universe.

Through this discovery, it is largely expected in the scientific community that we will be able to know about the early stages of universe, what happened during the big bang. Scientists are confident that the detection of gravitational wave is going to open door for many fundamentals principles which will have significant applications for human life.

Albert Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity. Einstein’s mathematics showed that massive accelerating objects (such as neutron stars or black holes orbiting each other) would disrupt space-time in such a way that ‘waves’ of distorted space would radiate from the source. These ripples would travel at the speed of light through the Universe, carrying with them information about their cataclysmic origins, as well as invaluable clues to the nature of gravity itself.


Einstein’s official 1921 portrait after receiving the Nobel Prize in Physics

LIGO team’s visualization of gravitational waves caused by two rapidly orbiting black holes in a binary system
LIGO team’s visualization of gravitational waves caused by two rapidly orbiting black holes in a binary system

History of Gravity and Gravitational Waves
In 17th century, Isaac Newton developed a universal law of gravity. He calculated that the attraction between two bodies was equal to the product of their masses divided by the square of the distance between them. It explains, motions of the planets around the sun, tides on earth and even predicted the motion and position of Neptune before its telescopic observation. Newton described gravity mathematically, but he didn’t understand the fundamental nature of this force.
In 1915, Albert Einstein came up with an explanation, The General Theory of Relativity. First he introduces the principle of equivalence (gravity and acceleration are the same thing). Then, he describes gravity geometrically, as a curvature created by matter in the fabric of space-time, where mass tells space how to curve and curved space tells matter how to move. Greater the mass of matter greater the curvature in spacetime. Objects in motion will move through space and time on the path of least resistance. A planet will orbit a star not because it is connected to the star by some kind of invisible wire, but because the space is curved around the star and the planet is moving in straight line in curved space-time.
After the final formulation of the field equations of general relativity, Einstein predicted the existence of gravitational waves. He found that the linearized weak-field equations had wave solutions: transverse waves of spatial strain that travel at the speed of light, generated by time variations of the mass quadrupole moment of the source. Einstein understood that gravitational-wave amplitudes would be remarkably small; moreover, until the Chapel Hill
Conference in 1957, there was significant debate about the physical reality of gravitational waves.
In 1974, the discovery of the binary pulsar system PSR B191316 by Hulse and Taylor and subsequent observations of its energy loss by Taylor and Weisberg demonstrated the existence of gravitational waves. Experiments to detect gravitational waves began with Weber and his resonant mass detectors in the 1960s, followed by an international network of cryogenic resonant detectors. Interferometric detectors were first suggested in the early 1960s and the 1970s. A study of the noise and performance of such detectors, and further concepts to improve them, led to proposals for long-baseline broadband laser interferometers with the potential for significantly increased sensitivity. Afterward the search of gravitational waves started in many countries.

The 4km-long arms of the LIGO Hanford Observatory in Washington State, USA Photo NASA

Timeline: Gravitational Wave Detectors

1979 NSF funds California Institute of Technology and MIT to develop design for LIGO.

1992 Sites in Washington and Louisiana selected for LIGO facilities; construction starts in 1994

1995 Construction starts on GEO600 gravitational wave detector in Germany, which partners with LIGO and starts taking data in 2002.

1996 Construction starts on VIRGO gravitational wave detector in Italy, which starts taking data in 2007.

2002–2010 Runs of initial LIGO—no detection of gravitational waves.

2007 LIGO and VIRGO teams agree to share data, forming a single global network of gravitational wave detectors.

2010–2015 LIGO detectors upgraded and become more sensitive.

2015 Advanced LIGO begins initial detection runs in September.

2016 NSF and LIGO team announce successful detection of gravitational waves.

Where did those Gravitational Waves come from?
Long ago, deep in space, two massive black holes: the ultrastrong gravitational fields left behind by gigantic stars that collapsed to infinitesimal points—slowly drew together. The stellar giant spiraled ever closer, until they whirled about each other at half the speed of light and finally merged. Based on the observed signals, LIGO scientists estimate that the black holes for this event were about 29 and 36 times the mass
of the sun, and the event took place 1.3 billion years ago. About 3 times the mass of the sun was converted into gravitational waves in a fraction of a second via E = mc2. The collision sent a shudder through the universe: ripples in the fabric of space and time. Six months ago (September 14, 2015 at 09:50:45), they were detected for the first time, fulfilling a 4-decade quest and century old prediction.

Merging of neutron stars; Image Source: NASA
Merging of neutron stars; Image Source: NASA

How did we detect them?
LIGO searches for distortions
in space time that indicates the passage of gravitational waves through the earth. The gravitational waves cause tiny but measurable distortions in the laser beams that cause a “changing interference pattern” in the photo sensors that read the laser reflections.
Gravitational-wave interferometer consist of laser beam which is splited into two light beams, travelling between pairs of mirrors down pipes running in perpendicular directions (L-shaped, with each arm 4 km).The effect of a passing gravitational wave should stretch space in one direction and shrink it in the direction that is at right angles, that would cause the mirrors to swing by tiny amounts, so that the distance between one pair of mirrors gets smaller, while the other gets larger, which changes the interference pattern. By using interference we can determine the tiny change in length so you can think of LIGO as the most sensitive ruler ever made. It detects changes in its length of about one thousandth the diameter of a proton – 10-18 meters.
LIGO is composed of two stations- Hanford observatory in Washington and the Livingston observatory nearly 3002km away in Louisiana. Gravitational-wave, travelling
at speed of light, will have difference of arrival time at two LIGO detector of up to 10 millisecond, that delay allows scientists to calculate the origin and direction of waves. If a candidate gravitational wave signal detected by both stations we can rule out false positives.
What can we do with gravitational waves?
Gravitational waves are our window into the early universe. The farthest we have ever seen is CMBR (cosmic microwave background radiation). The CMBR gives us a picture of the universe about 380,000 years after the big bang. We can’t see before photonic epoch by using conventional Electromagnetic Radiation. By gathering gravitational waves we will be able to see exactly what
happened before that.
Until now, we’ve explored the universe in just one way, using light—that is, electromagnetic waves. There are lots of things which do not interact electromagnetically, and monstrous object like black hole do not emit them. By analyzing their gravitational wave, we can detect and understand them. Gravitational waves could also offer insight into the motion of neutron stars, the formation and merger of massive black holes and the nature of space time and gravity.

Observation of Gravitational Waves from a Binary Black Hole Merger; Physical Review Letters, LIGO


1 Comment on Gravitational Waves- Century’s biggest discovery

  1. Since gravitational waves are not visible waves how can we see what happened before 380000 years of big bang with its help?

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