Gravitational-wave astronomy will change our understanding of the universe

A revolution in astronomy occurred on September 14, 2015, when scientists observed the collision of the two black holes for the first time ever by directly detecting gravitational waves emitted as the collision occurred.

Using the twin LIGO interferometers located in Livingston, Louisiana and Hanford, Washington, USA, faint fluctuations in the fabric of space and time emitted during the black hole collision were detected, heralding the dawn of gravitational-wave astronomy.

This revolution was a century in the making, coming just 50 years after the first detectors were built to search for them, and almost exactly 100 years after Einstein first predicted them in his revolutionary general theory of relativity, which also changed the way we understand gravity.

Einstein himself thought gravitational waves were too feeble to be detected, and at one point even believed that they were a mathematical artifact of general relativity. Their detection was only made possible through the tremendous scientific and technological advances of the 20th century, including lasers and optics, high vacuum systems, high performance computers, and servo control systems.

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The most precise distance measurement ever made

The faint ripples in space-time were detected using laser interferometry, one of the most precise methods ever devised for measuring tiny changes in distance.

General relativity predicts that gravitational waves stretch and squeeze space-time in a very predictable way, which lends itself naturally to detection by interferometry.

In scientific terms, gravitational waves are strains, or changes in length per unit length, which occur perpendicularly to the direction that the waves are traveling. You can see this process in action in the video at the top of this article.

The major challenge to overcome is one of scale. The LIGO interferometers have arms that are 4 kilometers long, and for that distance the predicted magnitude of the change is approximately one-thousandth of the diameter of a proton. This means that the relative change in length of the arms produced by a gravitational wave is less than one part in one thousands of one billionths of one billionths. To put that in perspective, it is the equivalent of measuring the distance from the center of the earth to the center of the sun with the precision of the diameter of just one atom.

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Watch the entire event in the video above (Video: Science & Cocktails)

A new kind of astronomy

The detection of gravitational waves should rightly be considered both a confirmation of one of the last remaining predictions of general relativity and a triumph of science and technology. But in reality, it is much, much more than that.

Gravitational waveforms carry unique information about the physical characteristics of the sources that produced them. Einstein’s theory predicts that any accelerating object should produce gravitational waves, but in practice detectable gravitational waves can only be produced by the most energetic and violent events in the universe.

The black holes that LIGO detected each possessed roughly 30 times the entire mass of the sun, colliding with each other approximately 1.3 billion light years away while traveling at half the speed of light.

From the collision emerged a new black hole more than 60 solar masses, with the mass-energy equivalent of three times the mass of the sun radiated away in gravitational waves (from E = mc2).

During the final fraction of a second, the power emitted exceeded the entire power output of the universe in the electromagnetic spectrum by more than ten times. We reconstructed this last fraction of a second of the collision, using the data from the two waveforms measured by the two LIGO detectors, which you can see in the video below.    

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So what does the future hold?

This discovery has truly profound implications. Gravitational waves provide unique information about the most energetic astrophysical events. They reveal incomparable insights into the nature of gravity, matter, space, and time.

As an example, a pair of ultradense neutron stars colliding with each other will produce not only gravitational waves but also a huge burst of electromagnetic radiation ranging from gamma rays to radio waves, allowing for joint observations between gravitational-wave detectors and astronomical telescopes. 

Gravitational waves should also be produced as massive stars burn through their nuclear fuel and undergo a violent gravitational implosion to produce a supernova, providing clues to the underlying physical processes which drive the explosion. 

Soon, a global ground-based network of gravitational-wave detectors will be constantly searching the sky, with LIGO and Virgo (a detector in Italy) as well as detectors under construction in Japan and India coming on line in the next five to ten years.

Further in the future, Europe is leading the effort to develop the Einstein Telescope, a completely new gravitational-wave observatory which will be ten times as sensitive as the current detectors and allow us to peer almost to the edges of the gravitational-wave universe.

Like electromagnetic waves, gravitational waves come in different wavelengths. Other types of detectors are also looking for longer wavelength gravitational waves. Using radio telescopes, researchers are using the precise timing of pulsars to detect supermassive black hole collisions.

Plans are moving forward for LISA, a detector in space capable of detecting intermediate mass black hole collisions and probing phase transitions in the early universe.

And ultrasensitive infrared detectors are making the most exact measurements of the polarization of the cosmic microwave background to detect the faint whispers of gravitational waves produced by the Big Bang.

LIGO has opened a completely new window onto the cosmos. What new, exciting, and unanticipated events we will observe over the coming years and decades will no doubt fundamentally change our understanding of the universe.

LIGO detected gravitational waves emitted from the collision of two black holes, merging to form a larger and new black hole, the first ever detection of its kind. This slow motion computer simulation accurately depicts the dynamics of the final fraction of a second of the initial black holes as they orbit around each other and eventually coalesce to produce the final black hole. (Video: Image Credit: Simulating eXtreme Spacetimes (SXS) Project (Caltech/Cornell))

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• David Reitze