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सेमिनार: अंग्रेज़ी सीखने का अवसर (23 सितंबर: दोपहर 3 बजे)
Gravitational Waves: Einstein's Legacy
Mar 05, 2016

Gravitational Waves (GW) were the last prediction of Albert Einstein’s theory of relativity. The first direct detection of gravitational waves was announced on 11 February by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO). 

Using LIGO's twin giant detectors—one in Livingston, Louisiana, and the other in Hanford, Washington—researchers measured ripples in space-time produced by a collision between two black holes. This is the first major detection by LIGO experiments after more than a decade in operation. The new discovery is truly incredible science and marks three milestones for physics:

1. Direct detection of gravitational waves.

2. The first detection of a binary black hole system.

3. The most convincing evidence to date that nature’s black holes are the objects as predicted by Einstein’s theory. 

• According to Newtonian physics, gravity is a force which makes two bodies with mass attracts each other. 

• Einstein in the year 1915, with his radical General theory of relativity, gave a complete new perspective of gravity. 

• The concept is mathematical and quite sophisticated, but can be defined simply as “Gravity is not a force as such, but a curvature caused in the spacetime fabric due to the presence of an object with mass.”

The denser the mass is, the greater the curvature of spacetime. As objects with mass move around in spacetime, the curvature also changes to the trail of the moving masses. At times, accelerating objects generate changes in this curvature, which propagate outwards in a wave-like manner. These propagating phenomena are known as gravitational waves. These are outflowing fluctuations of expanding and contracting space time. Not all movements create Gravitational waves; you need to change the Quadrupole moment of a mass distribution. It means motion should not be perfectly spherically symmetric or cylindrically symmetric.

• LIGO have detected signals of gravitational waves from two merging black holes 1.3 billion years ago! 

• These G-waves propagate at the speed of light, unlike the Newtonian gravity which propagates at infinite speed. 

• This speed limit comes from the fact that speed of light is built into Einstein’s field equations. Therefore, for all massless things this is the ultimate speed limit. 

• Since G-waves are distortions in spacetime, if it passes through a free standing body, the body may experience rhythmic stretching or shortening, without an unbalanced external force! 

• There is one entity that is absolute, a universal constant—the speed of light ‘c’, LIGO uses this property of Light (using LASERs) to measure minute changes in the spacetime continuum. 

• The precision that LIGO requires for this kind of detection can be compared to measuring the distance between Milky Way and Andromeda galaxy (2.5 million Light years away) to the scale of the width of a hair.     

Gravity, being a very very weak force, when compared to nuclear force or electrostatic force, one needs really really massive object or something accelerating at a high rate to have G-waves capable of being detected. Gravitational waves carry energy away from their sources and, in the case of orbiting bodies, this is associated with an inspiral or decrease in orbit. Higher acceleration generates powerful gravitational waves, since these huge systems generating G-waves are millions of light years away; the power that reaches on earth is minuscule.

The historic detection of gravitational waves will open up new frontier for understanding of universe. This discovery is sure to usher a new era in Gravitational wave astronomy and will enable us in finding answers to fundamental questions on the origin of the universe, on how a primordial singularity Big Banged into the vast vistas of the cosmos.

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