5 reasons why you should care about the discovery of gravitational waves

Last week marked the historic announcement of the first detection of gravitational waves.

A big press conference was held, and physicists around the world celebrated.

The discovery was even compared to Galileo looking through a telescope for the first time.

So why all the fanfare? Why are gravitational waves such a huge deal? 

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1. Gravitational waves are an entirely new way of observing the universe

Astronomers observe the universe across the electromagnetic spectrum, from X-ray and ultraviolet through optical and down to radio frequencies. Emission in each of these frequency ranges provides different information and thus a different perspective on our astronomical points of interest.

For example, we know that there are millions of stars clustered toward the center of our Galaxy which emit mostly at optical wavelengths, but there is also a lot of dust near the Galactic center as well. So to study those dust-enshrouded stars, astronomers must observe them at either infrared wavelengths (where the dust emits) or radio wavelengths (which can penetrate through the dust more effectively than shorter optical wavelengths). All of these wavelengths offer a unique perspective on the universe, but they are all the same kind of light, electromagnetic radiation, and so behave in similar, understood ways.

Gravitational waves are an entirely new phenomenon different from anything on the electromagnetic spectrum. In 1915, Albert Einstein proposed a radically different way of looking at gravity with his theory of general relativity. Rather than thinking of gravity as a force pushing and pulling massive objects in different directions, he described gravity as being manifested in a curvature of spacetime. In other words, the space (and time) around a massive object is curved, which then dictates how passing objects can move through that space.

This may sound crazy, but we can actually observe many of the effects predicted by Einstein’s theory. For example, general relativity informs us that time passes more slowly by an ever so small margin down here on Earth than it does for GPS satellites in orbit, an effect known as time dilation, a result of the curvature of spacetime. Without adjusting for this small time difference in our satellite communications, we would never get to where we are trying to go.

A consequence of the general relativity framework is that when objects accelerate through this warping of spacetime, they produce ripples known as gravitational waves. These waves propagate through space, compressing it in one direction and stretching it in another.

The frequencies predicted for these fluctuations are within the human hearing range. We can hear gravitational waves and already scientists and artists have teamed up to explore other artistic interpretations of their sound.

So why did it take 100 years to detect them? These ripples are tiny, on order of a thousandth of the size of a proton nucleus, so we need a pretty violent event to occur to produce enough of them for us to detect. We also need, of course, a very sensitive detector.

2. The instrument that made the gravitational wave detection is the most precise measuring system ever built

To detect such tiny distortions in spacetime, physicists use a technique called laser interferometry. A focused beam of light is sent in different directions to bounce back and forth between two sets of mirrors before being sent to a detector. If a gravitational wave passes by the interferometer during all of this bouncing, the distance between the mirrors will change ever so slightly and this change will translate to a difference in the two signals as measured by the detector.

Not only is the signal from gravitational waves incredibly weak, there is also a significant amount of competing noise attempting to drown it out. To increase the detectability of such a signal against the background noise, the path the laser travels must be a long one. The Laser Interferometer Gravitational-Wave Observatory (LIGO), the instrument that made the historic detection, is four kilometers long on each side. The detectors are further suspended in the air in hopes of isolating the slightly faster wiggles due to gravitational waves from terrestrial interference.

To further fight back against false detections, LIGO has not one but two detectors: one in Hanford, Washington and the other in Livingston, Louisiana. Detecting the same signal at both, widely separated locations would mean that signal was likely not a local one. And that is exactly what happened on September 14, 2015—a signal with the precise characteristics predicted for gravitational waves was observed at both detectors only milliseconds apart.

3. We now know that massive black holes can merge to create even bigger black holes

Famous theoretical physicist and author Kip Thorne described the event that produced the detected gravitational waves as a "violent storm in the fabric of space and time".

Around 1.3 billion years ago, when multicellular life was just beginning here on Earth, two black holes orbiting each other began to close in on one another. As these dense objects got closer, they began to accelerate to nearly half the speed of light in the presence of their shared strong gravitational field - the perfect mix for producing gravitational waves.

From fitting the waveform of the gravitational wave detection and comparing it to simulations done with a supercomputer, astronomers can tell that the two black holes were originally 29 and 36 times the mass of our Sun. They merged to form a 62 solar mass black hole which means that an amount three times the mass of our Sun was emitted away as energy in the form of gravitational waves, all in the 20 milliseconds it took for the collision to happen! That’s a power output of 50 times greater than all of the power put out by all of the stars in the universe put together.

Before this first detection, astronomers were not even sure that mergers between black holes existed, and now the details of this particular event are known to a high degree of certainty.