What are Gravitational Waves?
Gravitational waves are 'ripples' in the fabric of 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 distorted space would radiate from the source (like the movement of waves away from a stone thrown into a pond). Furthermore, 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.
The strongest gravitational waves are produced by catastrophic events such as colliding black holes, the collapse of stellar cores (supernovae), coalescing neutron stars or white dwarf stars, the slightly wobbly rotation of neutron stars that are not perfect spheres, and possibly even the remnants of gravitational radiation created by the birth of the Universe.
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)
Though gravitational waves were predicted to exist in 1916, actual proof of their existence wouldn't arrive until 1974, 20 years after Einstein's death. In that year, two astronomers working at the Arecibo Radio Observatory in Puerto Rico discovered a binary pulsar--two extremely dense and heavy stars in orbit around each other. This was exactly the type of system that, according to general relativity, should radiate gravitational waves. Knowing that this discovery could be used to test Einstein's audacious prediction, astronomers began measuring how the period of the stars' orbit 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 (which would remove energy from the system and cause the stars to get closer and closer together). This system has been monitored for over 40 years and the observed changes in the orbit agree so well with general relativity there is no doubt that it is emitting gravitational waves. For a more detailed discussion of this discovery and work, see Look Deeper.
Since then, many astronomers have studied the timing of pulsar radio-emissions and found similar effects, further confirming the existence of gravitational waves. But these confirmations had always come indirectly or mathematically and not through actual 'physical' contact.
All of this changed on September 14, 2015, when LIGO physically sensed the distortions in spacetime caused by passing gravitational waves generated by two colliding black holes nearly 1.3 billion light years away! LIGO's discovery will go down in history as one of humanity's greatest scientific achievements.
Lucky for us here on Earth, while the processes that generate gravitational waves can be extremely violent and destructive, by the time the waves reach Earth they are 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 thousands of times smaller than the nucleus of an atom! Such inconceivably small measurements are what LIGO was designed to make. To find out how LIGO can achieve this task, visit LIGO's Interferometer.