What Are Gravitational Waves and Why Are They Important?

Theoretical physicist Lawrence Krauss had the science world buzzing with a January 11th tweet saying that “independent sources” have confirmed the discovery of gravitational waves at the Laser Interferometer Gravitational-Wave Observatory (LIGO) experiment.

There is still plenty of reason to be skeptical. We won’t know for certain if the discovery was made until we hear from the scientists who conducted the experiment. If gravitational waves are discovered, it would be a major advance for science and another feather in the cap of Albert Einstein, who predicted the phenomena some 100 years ago.

Although scientists are excited, most people are left scratching their heads at the mention of gravitational waves. So what exactly are these cosmic ripples in the fabric of space-time?


Graphical interpretation of gravitational waves from a binary black hole. Image credit: NASA/ JPL-Caltech

According to Einstein’s Theory of General Relativity, when objects move around in space the curvature of spacetime changes to reflect the change in location of the objects. Under certain conditions, accelerating objects can generate changes in the curvature of spacetime that propagate outwards at the speed of light in a wave form. These propagating ripples are known as gravitational waves. A similar effect happens if you drag your hand through a pool of water, the waves follow in its path and spread outward from the source.

To put it even simpler, Einstein thought of space not as an empty void, but as a dynamic four dimensional fabric. If any accelerating bodies pass through the dynamic substance, it should create ripples in the fabric. In theory, small ripples wouldn’t be able to be detected here on Earth. Only the most massive objects, moving at incredible speeds, can create gravitational waves that can reach Earth.

So how can we detect these elusive waves? As a gravitational wave passes a distant observer, that observer will find spacetime distorted by the propagating effect. Subsequently, distances between free objects increase and decrease rhythmically at the corresponding frequency of the wave – much like an anchored boat would sway over waves in water, slightly increasing and decreasing its distance with each wave.

The latest rumors center around the LIGO experiment. LIGO uses an interferometer to detect the rhythmic changes in spacetime caused by gravitational waves. The instrument splits a single laser beam into two and sends them both off perpendicular to each other. After bouncing off mirrors, the waves that make up the lasers should be in perfect alignment as they return. Any change in distance that each beam travels relative to its sibling could be caused by gravitational waves. Scientists predict the observed effects of gravitational waves on Earth to be incredibly small – they are looking for oscillations of roughly the width of an atoms nucleus, or 1 part in 10^20.

In nearly a decade of operation, the LIGO experiement found no concrete evidence of gravitational waves, although its recent upgrade to Advanced-LIGO in September of 2015 should give it a better shot.

In December of 2015, the European Space Agency successfully launched the LISA Pathfinder mission. LISA could be thought of as a space-faring LIGO experiment as it to uses an interferometer. Science operations are expected to ramp up later in 2016. Other experiments such as the North American Nanohertz Observatory for Gravitational Waves, or NANOGrav, is looking at bursts of radio waves from neutron stars called pulsars. These pulses are normally strictly timed, so if they arrive early or late, it could be because gravitational waves interfered with their journey to Earth.

So what’s the big deal – What’s the point of finding gravitational waves (besides pumping Einsteins tires)?

Consequently, gravitational waves could give us another way to look at the universe. Optical and radio telescopes rely on electromagnetic waves that are unable to penetrate certain regions that gravitational waves could. Astronomers hypothesize that gravitational waves could provide information about black holes and other exotic objects that modern technology simply can’t observe. Gravitational waves could also help unlock the mysteries of the early universe. Before the first ever stars, the Universe was opaque to electromagnetic radiation, so it is impossible with modern electromagnetic-based technology to observe the universe at that time. Harnessing gravitational waves could change that.

And finally, gravitational waves could also help scientists more accurately understand the fundamental laws of the universe. It would allow scientists to more thoroughly test Einstein’s Theory of General Relativity, possibly leading to new directions and discoveries.

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