Gravitational waves - Ripples in Spacetime !

What are Gravitational Waves?

Gravitational waves are 'ripples' in space-time caused by some of the most violent and energetic processes in the Universe. 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 undulating space-time would propagate in all directions away from the source. These cosmic ripples would travel at the speed of light, carrying with them information about their origins, as well as clues to the nature of gravity itself.

The strongest gravitational waves are produced by cataclysmic events such as colliding black holes, supernovae, and colliding neutron stars. Other waves are predicted to be caused by the rotation of neutron stars that are not perfect spheres, and possibly even the remnants of gravitational radiation created by the Big Bang.

The animation below illustrates how gravitational waves are emitted by two neutron stars as they orbit each other and then coalesce (credit: NASA/Goddard Space Flight Center). Note that gravitational waves themselves are invisible. They are made visible here to illustrate their propagation away from the source.

Though Einstein predicted the existence of gravitational waves in 1916, the first proof of their existence didn't arrive until 1974, 20 years after his death. In that year, two astronomers using the Arecibo Radio Observatory in Puerto Rico discovered a binary pulsar, exactly the type of system that general relativity predicted should radiate gravitational waves. Knowing that this discovery could be used to test Einstein's audacious prediction, astronomers began measuring how the stars' orbits changed over time. After eight years of observations, they determined that the stars were getting closer to each other at precisely the rate predicted by general relativity if they were emitting gravitational waves. For a more detailed discussion of this discovery and work, see Look Deeper.

Since then, many astronomers have studied pulsar radio-emissions (pulsars are neutron stars that emit beams of radio waves) and found similar effects, further confirming the existence of gravitational waves. But these confirmations had always come indirectly or mathematically and not through direct contact.

All of this changed on September 14, 2015, when LIGO physically sensed the undulations in spacetime caused by gravitational waves generated by two colliding black holes 1.3 billion light-years away. LIGO's discovery will go down in history as one of humanity's greatest scientific achievements.

While the processes that generate gravitational waves can be extremely violent and destructive, by the time the waves reach Earth they are thousands of billions of times smaller! In fact, by the time gravitational waves from LIGO's first detection reached us, the amount of space-time wobbling they generated was a 1000 times smaller than the nucleus of an atom! Such inconceivably small measurements are what LIGO was designed to make. To learn how LIGO achieves this seemingly impossible task, visit LIGO's Interferometer.

Sources and Types of Gravitational Waves: 

Every massive object that accelerates produces gravitational waves. This includes humans, cars, airplanes etc., but the masses and accelerations of objects on Earth are far too small to make gravitational waves big enough to detect with our instruments. To find big enough gravitational waves, we have to look far outside of our own solar system.

Continuous Gravitational Waves:Continuous gravitational waves are thought to be produced by a single spinning massive object like a neutron star

Compact Binary Inspiral Gravitational Waves:The next class of gravitational waves LIGO is hunting for is Compact Binary Inspiral gravitational waves. So far, all of the objects LIGO has detected fall into this category. Compact binary inspiral gravitational waves are produced by orbiting pairs of massive and dense ("compact") objects like white dwarf stars, black holes, and neutron stars.

Stochastic Gravitational Waves:Astronomers predict that there are so few significant sources of continuous or binary inspiral gravitational waves in the Universe that LIGO doesn't worry about the possibility of more than one passing by Earth at the same time (producing confusing signals in the detectors). However, we do presume that many small gravitational waves are passing by from all over the Universe all the time, and that they are mixed together at random. These small waves from every direction make up what is called a “Stochastic Signal”, so called because the word, 'stochastic' means having a random pattern that may be analyzed statistically but not predicted precisely.

Burst Gravitational Waves:The search for 'burst gravitational waves' is truly a search for the unexpected—both because LIGO has yet to detect them, and because there are still so many unknowns that we really don’t know what to expect! For example, sometimes we don’t know enough about the physics of a system to predict how gravitational waves from that source will appear. 

LIGO currently consists of two interferometers, each with two 4 km (2.5 mile) long arms arranged in the shape of an “L”. These instruments act as 'antennae' to detect gravitational waves.LIGO stands for "Laser Interferometer Gravitational wave Observatory". It is the world's largest gravitational wave observatory and a marvel of precision engineering. Comprising two enormous laser interferometers located 3000 kilometers apart, LIGO exploits the physical properties of light and of space itself to detect and understand the origins of gravitational waves.

                                                     

For more details Visit : LIGO Caltech