On 25 September 2015 scientists at the LIGO experiment detected something that no human had ever seen before: a gravitational wave. This wave was emitted by two black holes that lived and died more than a billion years ago. Each of the black holes was around thirty times as massive as our own sun, and when they merged they gave out so much energy that they temporarily outshone every star in the observable Universe put together. However, the energy released from this cataclysmic event was not in the form of light, but in form of gravitational waves. The existence of gravitational waves was predicted by Einstein in 1916, but it took the amazing technological and scientific breakthroughs of the LIGO team in order to observe it, almost a century later. This was the first time in history that humans were able to see distant astrophysical events without light, and what we saw was one of the most incredible events that occurs in nature – two black holes colliding.
“Seeing without light” may seem like a nonsensical phrase, but it’s very close to what actually happened in the LIGO detectors. This is because the attractive part of the gravitational field, with which we are very familiar, is not the only thing that exists in Einstein’s theory of gravity. There also exist wave-like solutions to his gravitational field equations. So while light is the wave-like disturbance that results from the electromagnetic interaction, a gravitational wave is the propagating disturbance that results from Einstein’s gravity. We’re not generally aware of the existence of gravitational waves, because their amplitude is very small in the relatively calm and stable environment of the solar system. But the theory is clear; gravitational waves should exist in nature, they should get emitted from massive bodies in relative motion, and they travel through space at the speed of light.
So, if we can build detectors that are sensitive enough to detect gravitational waves then we should be able to use them to “see” distant astrophysical events, in a way that‘s almost directly analogous to the way that ordinary telescopes let us “see” using light. And while normal telescopes are excellent for viewing stars and galaxies, gravitational wave detectors are excellent for viewing black holes. The LIGO detection in September 2015 was the first of this new type of observation to be made, but it certainly will not be the last. In fact, the LIGO team have already announced a second detection that occurred on Boxing Day 2015. It seems inevitable that more events will follow, and that a new field of astronomy will be born. This is a particularly exciting prospect because observations of merging black holes gives us information not just about the life and death of stars, but also about the details of how Einstein’s theory of gravity operates in the most extreme of environments.
The amount of work that went into the detection by LIGO is almost as amazing as the detection itself. Many, many scientists and engineers have spent decades building and operating one of the most sensitive pieces of scientific equipment that has ever existed. The detectors in the LIGO experiment are so sensitive they can measure disturbances that cause its internal mirrors to move by less than a thousandth of the width of a proton (about a billionth of a billionth of metre). The technical and scientific work needed to reach this mind-boggling level of precision is immense, as is the theoretical and mathematical efforts that were required to make sense of the signal it measured. Nevertheless, this huge amount of industry has ultimately all been worthwhile: we now have first light on an entirely new and exciting way to view the Universe, through gravity itself.
Featured image credit: An artist’s depiction of the binary star series, J0806 by unknown. Public domain via Wikimedia Commons.