When black holes met

The 2017 Nobel Prize for Physics

This article belongs to a series on the 2017 Nobel Prizes

It isn’t often that one discovery can prove an aspect of General Relativity correct and allow us to observe things that had previously been invisible, but the discovery of gravitational waves did just that. So it’s fitting that the 2017 Nobel Prize in Physics has been awarded to three pioneers who contributed to the first direct detection of these waves: Rainer Weiss, Barry C. Barish, and Kip S. Thorne. In order to make this discovery, their team spent decades surmounting incredible technological challenges to measure something much smaller than an atom. Now, barely two years after their first detection, the technologies that Weiss, Barish and Thorne spearheaded have already lived up to their potential to make new discoveries about our universe.

Gravitational waves are exactly what they sound like: deformations, or waves, created by gravity, which move through the universe. Imagine that you are standing on the shore of a lake. Far out, two large boats smash into one another, creating ripples in the water. These ripples spread outward across the surface, changing its shape as they pass and shrinking as they spread, until they reach you on the shore. Gravitational waves are a bit like those ripples. As with the ripples, the larger the objects creating them, the larger the initial waves will be, and the further away from them the observer is, the smaller those waves will become.

These ripples in space-time are a consequence of Einstein’s theory of relativity, but that doesn’t mean their existence was always obvious. Even Einstein himself changed his mind over whether or not they actually existed, and died undecided on the matter, having barely discussed them in the forty years since he’d first considered their existence. Even though they flowed naturally from the theories he’d invented, he had doubts something so strange could actually be real.

But others stepped in where Einstein feared to walk. Physicists became convinced that the waves did exist by the late 1950s, and began trying to detect them in the 1960s. They were indirectly detected in 1978 by astronomers observing pulsars (a discovery that was part of the Nobel Prize for Physics in 1993), but it took almost forty more years to detect them here on Earth.

The delay wasn’t due to lack of trying, or because the waves are rare. It was because gravitational waves are so small it took time for technology to become precise enough to measure their very existence. The Laser Interferometer Gravitational-Wave Observatory, or LIGO, detectors behind the discovery were first opened in 2002, and went through two upgrades, each time increasing their precision (and therefore the wave size they could detect).

The last upgrade was completed in 2015, and managed to finally make the detectors sensitive enough to detect gravitational waves (other projects had already spent decades looking for the waves). The newly upgraded LIGO detected the first directly observed gravitational waves two days into its operation. This was done so soon the detectors were technically still in testing, and the researchers initially thought the signal was a test, not real data.

This initial doubt brings up an important point: this discovery wasn’t actually made by three people. Over a thousand scientists from 100 institutions in 18 countries are part of the LIGO Scientific Collaboration. This is the group that does everything from designing gravitational wave detectors through to operating the detectors and analyzing the data. The Nobel Prize requires that only three people are awarded the prize, and while Weiss, Barish, and Thorne played pivotal roles in inventing and executing this method of detection, every member of the collaboration contributed to the discovery. These types of discoveries are now group efforts, with large numbers of scientists around the world combining efforts (and funding) to unearth the true nature of our universe. Perhaps one day the Nobel Committee with find a better way of acknowledging this aspect of physics research, but until then a few (typically old, white men) will have to stand in for the teams that change how we understand reality.

Since 2015m gravitational waves have been detected five times in two years. The first four were all caused by black holes. This isn’t a coincidence. Although everything with mass can create gravitational waves, most of them are so small they cannot be detected. You can jump up and down right next to LIGO all you want, but the waves you create will be too small for it to detect. On the other hand, black holes are massive, and their interactions with one another create waves so large they can be measured, even when they’re billions of light-years away.

This is the promise of gravitational waves: They can let us sense things we cannot see, like black holes and dark matter. That might not sound like a big deal, but it is. Almost everything we know about the universe beyond earth comes from seeing, which depends upon light. With a few minor exceptions, astronomers cannot observe the universe in any way but looking out into the sky. They’ve been aided over the years by new technologies. The first telescopes allowed us to see in more detail than naked eyes could, and newer telescopes have let us see things not only magnified, but also in wavelengths of light humans cannot see from radio waves to x-rays  and beyond our atmosphere, but these still rely almost entirely on light. The problem is, not everything can be detected with light. Gravitational waves offer a new way of looking at anything with mass, from the Big Bang to dark matter.

As recently as two weeks ago, the LIGO Scientific Collaboration has demonstrated that this was more than just hype. They published a landmark new observation confirming many theories about two astronomical enigmas: supernovae and gamma ray bursts. This time the gravitational waves didn’t come alone. The detected waves were used alongside observations from telescopes to create a fuller view of an event in another galaxy, over a hundred million light years away. It’s time to stop talking about gravitational waves as the future of astronomy, because that future is already here.

Sarah Leach is studying for an MSc in Science Communication at Imperial College London

Banner Image: Merging binary black hole, NASA

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