The day humanity opened its ears to the cosmos

More than a billion years ago, two black holes, each weighing approximately thirty times the mass of our Sun, collided with one another.

The final moments of their collision lasted a few seconds. Right before, they were orbiting one another at ten percent of the speed of light. They merged in a collision so violent that space itself stretched and bent, and time slowed and sped up—sending a ripple throughout the universe.

In September last year the wave reached Earth. It was the day after the LIGO gravitational wave detector began a new round of experiments in sears of such a ripple. The detection, 100 years after Albert Einstein first predicted the existence of gravitational waves, represents a fundamental turning point in how humanity will observe the external universe, forever.

For thousands of years, humankind has observed the heavens with light. But we have been deaf to the subtle vibrations in the fabric of space and time that wash continuously over us. Now we have opened our ears. The LIGO collaboration converted the final moments of the collision of the black holes into a frequency audible to human ears. You can listen to the short clip here.

Einstein’s prediction of gravitational waves was based on a fundamental revision of our conception of space and time. He argued that space is not simply a static background where light and matter move, and that time does not simply tick along at the same rate everywhere and always. Instead, according to Einstein, space and time themselves bend according to the presence of mass.

As an analogy, think of a flat, two-dimensional elastic sheet that is held tight. The sheet plays the role of space and time in Einstein’s picture. Now imagine placing heavy marbles on the sheet. The sheet bends inward—by more or less for larger or smaller marbles. According to Einstein, gravity makes massive objects attract because, in this picture, around heavy particles the sheet bends inwards, drawing in any nearby objects. And, if you take one corner of the sheet and wiggle it quickly, the sheet will seek to overcome the build-up of stress locally by propagating a wave—just as with the gravitational waves in Einstein’s picture.

A graphical analogy of the curvature of space and time by massive objects. According to Einstein, gravity arises from the curvature space and time around the Earth as pictured here. Credit: Mysid

Einstein’s radical has truly bizarre implications. Another of his predictions is that light itself is bent by gravity—a prediction that has found stunning confirmation in countless images in the century since. Pictured below is a “horseshoe” galaxy. A galaxy in the centre has “lensed” the light from a far more distant, blue galaxy behind it, so that the latter appears to be in a ring.

A horseshoe galaxy, taken by the Hubble telescope. The red galaxy in the middle has curved the light from another galaxy behind it so that the latter appears to be in a ring all around it. The phenomenon is known as gravitational lensing: one of the beautiful consequences of Einstein’s theory. Credit: NASA

But Einstein knew that the waves would be difficult, if not impossible, to detect. Gravity is a very weak force: it is far weaker than the other forces, such as electricity. That means that it takes a huge mass, moving very quickly—like two black holes—to bend space and time enough to produce a large enough wave for us to detect.

The LIGO detector which made the discovery last September was the most precise experiment ever conducted, in the history of mankind. As the wave passed through, it stretched the detector by less than the width of a proton. In terms of relative accuracy, the detector essentially had to measure the width of the Milky Way galaxy to within the length of a pencil eraser.

Already, LIGO announced in June that it has detected a second gravitational wave. Now that we have begun listening to the sounds of the universe, we will not stop. Entire areas of the cosmos that have until now been excluded from study, because light cannot reach us from them, have been opened up. An array of gravitational wave detectors modelled on the same principles as LIGO are now being built. One senses that our generation is living through a key historical moment in astronomy that will be more fully appreciated by future generations to come.