One Hundred Years of Gravitational Waves

We all are familiar with the force of gravity. But, where do the gravitational waves fit in the picture?

On September 14, 2015 at 5:51 am ET, Laser Interferometer Gravitational-wave Observatory (LIGO), detected the gravitational waves for the first time. This discovery confirmed one of the major predictions of Einstein's general theory of relativity, which will definitely pave the way for a new era of observational astrophysics.

The general theory of relativity, published a hundred years ago, describes spacetime as a single dynamic entity and predicts the bizarre phenomena, like black holes and gravitational waves. In other words, Einstein discarded Newtonian notion of the independent nature of space and time and argued that space is not a mere canvas upon which all the events unfold by the mysterious force of gravity, as envisioned by Newton.

The general theory of relativity also predicts that a strong gravitational field, similar to that of the sun, "warps" spacetime and even light is deflected by gravity. In 1919, the British astrophysicist Arthur Eddington measured the bending of star light around the sun during a solar eclipse as predicted by the general theory of relativity -- it was a clear vindication of the validity of this theory. There have been other verifications of general theory of relativity since the days of Eddington.

In fact, the seemingly abstract concept of warping of spacetime produces noticeable effects, such as time dilation that has been measured precisely. The general theory of relativity predicts that the warping spacetime will be larger in stronger gravitational fields. Apparently, clocks on the ground (closer to earth) will run more slowly than the clocks, say, aboard the GPS satellites orbiting the earth.

Einstein's equations calculate this time difference to be about 38 microseconds per day, and these relativistic effects were taken into account by adding the required mathematical corrections into the system when the GPS satellites were deployed. The general theory of relativity (linked to special theory of relativity) explains even such microscopic anomalies about space and time so beautifully, and they together form the foundation of modern physics.

Another prediction of general theory of relativity is the newly detected gravitational waves. Physicists often describe gravitational waves as "ripples on space-time," caused by moving masses similar to the electromagnetic waves caused by moving charges. Like a pebble dropped in a pond can generate waves propagating in all directions across the water, cataclysmic events such as the collisions of black holes will send out energy propagating through the fabric of the spacetime in the form of gravitational waves.

LIGO scientists estimate that this particular event detected on earth in last September took place about 1.3 billion years ago. The researchers confirmed that these waves were generated by two massive black holes as they spiraled each other until merged together sending these ripples that washed across our planet at the speed of light about five months ago. Indeed, these waves are the voices of stellar ghosts (black holes) as they ripped each other apart in the far corners of the universe and that message was sent to us over the cosmic network.

Astronomers rely on different forms of electromagnetic waves to observe the universe. For example, radio telescopes are used in astronomy to explore the universe by detecting radio signals emitted by different objects. However, we learned to communicate with them -- think of mobile communication that is entirely dependent on radio waves. The detection of gravitational waves marked a triumphant moment in scientific history and will provide a completely new way of looking at the universe.

What will the coming years bring to physics? A new wave from the cosmic ocean, or, perhaps an ocean itself.