For more than a decade, a set of high-tech, universe-scouring contraptions sat gathering dust in Washington and Louisiana. For many long years, these gizmos, intended to reveal secrets of the cosmos, had simply lain dormant – but in 2015 they finally sprang into life. That year, the machines detected signs of an extraordinary catastrophe in space. And not only was this a profound discovery, it was also a development that defied the expectations of one of history’s greatest scientific minds. No less famous a thinker than Albert Einstein had thought such a thing would be impossible.
This story begins over a century ago, back in Einstein’s own lifetime. The year was 1916, a time when the physicist was formulating his theories on the nature of space and gravity. As a result of his work, Einstein came to consider the possibility that pulses of energy could surge through space. Today, we call these ripples “gravitational waves.”
Einstein’s theorizing was telling him that gravitational waves existed, but he wasn’t actually certain of his own predictions. In later years, in fact, he seemed to have a crisis of confidence of sorts: he even published work challenging his original ideas about the proposed phenomenon. Many other scientists, though, seemed to be more confident in the great physicist’s initial musings.
During the 1990s, many decades after Einstein’s death, a project was instigated to try and find out how accurate his ideas had really been. Scientists built a facility known as the Laser Interferometer Gravitational-Wave Observatory (LIGO), which involves two sophisticated measuring devices constructed on separate sites. One of these contraptions operates in a place called Livingston in Louisiana, while the other is in Hanford, Washington.
During the initial 13 years of the LIGO project’s operations, neither detector managed to record anything of note. But beginning in 2015 they finally started to yield some amazing results. Disturbances identified as gravitational waves, originating from a distance of 1.3 billion light years from the Earth, were recorded by the devices, apparently confirming Einstein’s original prediction about their existence.
Following the initial breakthrough, more and more discoveries about these gravitational waves were made. And the things they’ve revealed about the universe have ultimately been quite startling, uprooting a fair amount of conventional thinking in astronomy. Some of the findings have decidedly proven Einstein wrong, while others have implied the existence of something in our universe previously thought to be impossible.
The notion of gravitational waves first occurred to Einstein as a result of his work on the General Theory of Relativity. This, of course, was part of his series of revolutionary scientific achievements that totally changed our understanding of physics. Before relativity – to be precise his Special Theory of Relativity – space and time were understood to be rigid, but the scientist’s thought experiments demonstrated to us that this was emphatically not the case.
Einstein’s later conceptualization of general relativity also tells us about the nature of gravity. Now, in our everyday lives you and I know it as an unseen force that makes things fall down. Intellectually, we might understand it as something which draws one entity towards another. But Einsteinian general relativity explains the phenomenon as the curvature of space itself. These bends are caused by mass, and while all objects exert gravitational pull, it’s most obvious with planetary and stellar bodies with very large masses, such as stars.
A good example to think about is the centre of our own solar system, the Sun. This star has a significant mass, meaning that the space around it is bent. We might compare this to a weighty ball sitting on a blanket, with the sheet changing its shape because of the ball. We can see the effect of this phenomenon in the way that the planets of the solar system orbit the Sun. Essentially, the planets are moving in Newtonian straight lines – but those lines are warped into a continuous loop by the mass of the Sun.
Thanks to Einstein, we also understand that gravity has an impact on time. In our daily lives, time seems like a steady, linear thing. But general relativity showed how gravity changes its behavior. Believe it or not, if somebody stands at the peak of a big mountain, time actually moves imperceptibly quicker for them than for someone standing at the mountain’s foot. Basically, the greater the gravitational force, the slower time moves.
So, Einstein’s General Theory of Relativity stated that gravity warped space and time. And in 1916 the physicist took his ideas a step further, suggesting that if two massive, spinning entities in space approached each other, then waves should emanate as a result. We can compare this to the way that ripples on a calm lake travel outwards away from the spot that you drop a stone into it.
With this line of thinking Einstein was predicting the existence of gravitational waves, which are essentially ripples pulsing through space. These waves would result from incredibly intense celestial events, such as when stars blow up or when black holes collide. These would then move through the universe at the speed of light.
Einstein saw from his theory that gravitational waves could exist, but he didn’t think that we’d ever be able to prove it. After all, the ripples would have to travel across huge expanses before ever reaching Earth. They’d therefore be extremely weak by this point, meaning that we’d need incredibly sensitive instruments to pick up on them over the everyday noises of our planet. Einstein didn’t foresee such equipment ever existing.
But as it turns out, the great physicist was wrong. Beginning back in the 1970s – a couple of decades after Einstein’s death – we got closer to confirming the existence of gravitational waves. It was thanks to the work of scientists trying to learn more about entities known as pulsars. What in heaven’s name, I hear you ask, is a pulsar? Isn’t it a kind of watch? Well, in everyday life, maybe, but to astronomers the word pulsar describes a very specific kind of star, quite small and incredibly dense.
More specifically, pulsars are a type of neutron star, which come into being when a stellar body with a mass many times greater than our Sun finally burns out and collapses. Ordinarily, a colossal explosion known as a supernova will result from this process. A neutron star is what then emerges from this catastrophe. Some of these dense objects rotate and emit a stream of high-energy radiation from their poles, which periodically point directly at Earth. We call such objects pulsars.
Neutron stars and pulsars are fascinating objects. And when scientists were studying two particular pulsars during the 1970s, they noticed something astonishing. These two entities were spinning around each other – and they were getting closer and closer to colliding. If this was the case, the scientists reasoned, then the pulsars must be emitting energy – and it was likely that it had to take the form of gravitational waves.
This was an incredible theory, but it still didn’t prove Einstein wrong. After all, we hadn’t actually detected these gravitational waves firsthand. But decades later this feat was finally achieved. In 2015 the LIGO project, with its separate instruments in Louisiana and Washington, proved Einstein’s prediction about the existence of gravitational waves correct. Conversely, it proved his doubts that we’d ever directly pick them up wrong.
The first incarnation of the LIGO instruments became operational in 1999 with concerted attempts to find Einstein’s predicted waves beginning in earnest in 2002. This initial phase of work lasted for eight whole years, but it failed to register any gravitational pulses in that time. Having said that, the experience allowed the scientists involved to hone their efforts ahead of the next phase.
From 2010 to 2014 the LIGO equipment was modernized, essentially turning the apparatus into an instrument ten times more sensitive. This, in turn, meant that the instruments could pick up on gravitational waves emanating from sources much further away. The odds of making a discovery, then, were much higher.
The LIGO project sparked back into life with its updated technology in September 2015. And it wasn’t at all long before it made the significant breakthrough that had previously eluded it. In fact, it took only a few days of operations before gravitational waves were directly detected for the first time in history.
These pulses had originated from a hugely dramatic event far away in space. Around 1.3 billion light years from the Earth, a pair of black holes had smashed into each other and coalesced. If you’re struggling with the distances involved, you’re probably not alone. To try and give you some context, one light year is equivalent to roughly 6 million million miles. Although it was so distant, this unthinkably violent cosmic event sent out the gravitational waves which were picked up by LIGO, thus proving Einstein wrong in his thinking that we’d never be able to do so.
The LIGO researchers use the intuitive term “chirps” to describe the detection of such gravitational waves. This is because the vibrations caused by the pulses give off sounds when they hit the detection equipment. And as the two black holes approached one another, the waves gave off noises with an ever-higher pitch – or chirp.
This discovery in 2015 was an immense development in the field of gravitational waves, but it was just the beginning. Another find was soon registered in 2017 by LIGO and a similar project in Italy called Virgo. This time, gravitational waves arising from a pair of neutron stars crashing together were recorded.
As discoveries go, you could even say that this particular find was golden. You see, it shed light on something tangible that we can actually see on our own planet. Basically, researchers believe that the intensity of the impact when neutron stars hit each other leads to huge amounts of precious metals such as silver and gold being created.
It was once theorized that gold was created when stars die and consequently explode. But now it’s been suggested that heavy metals like this come into being when neutron stars crash together and cause a type of explosion known as a kilonova. Got any gold jewelry? Well, it can be quite humbling to think that your precious keepsake may well have been spawned by a gigantic cataclysm in space.
After researchers noted the gravitational waves from the neutron star collision, yet more were detected. This time, the ripples had come into being as a result of a black hole devouring a neutron star. With this discovery, scientists had now noted gravitational waves originating from three different events. To give a quick recap: firstly, after two black holes combined; secondly after two neutron stars combined and thirdly after a neutron star was engulfed by a black hole.
So, thanks to our advances in technology, we’ve now made discoveries that even Einstein thought would be impossible to make. Some 50 likely gravitational waves have now been directly detected, a mere century since Einstein first expressed his doubts on the subject. In the great scheme of things, that’s really not a huge amount of time.
Of course, developing technology to the point where we are able to pick up gravitational waves hasn’t been easy: many issues have had to be ironed out. For instance, the instruments used in this research are incredibly sensitive, meaning that other, closer events might easily affect and distort readings. An example might be a vehicle passing nearby, which then registers on the devices. This is precisely why the LIGO project utilizes devices at two different locations. If they both pick up a signal at the same time, it’s probably a gravitational wave rather than something more arbitrary.
Scientists are frequently updating LIGO’s instruments, making them more effective. One recent alteration has further limited the chances of detecting false signals. And the upgrades have also meant that LIGO should be capable of finding waves across a greater expanse of space than ever before.
The new additions to the LIGO technology have quickly proven to be effective. Together with Italy’s Virgo project, as of December 2020 LIGO had recorded 39 potential gravitational waves over a six-month period in the wake of the upgrade. That breaks down to a new wave being picked up every five or so days. That’s a very promising rate, especially when we consider that gravitational waves were unconfirmed until so recently.
Frank Ohme is associated with the Max Planck Institute for Gravitational Physics, where he’s the head of a LIGO research team. In a statement, he’s reflected on the groundbreaking project. He said, “We’re getting a richer picture of the population of gravitational-wave sources. The masses of these objects span a very wide mass range, from about that of our sun to more than 90 times that. Some of them are closer to Earth, some of them are very far away.”
Another scientist named Stan Woosley has also been paying close attention to the work of LIGO. In particular, Dr. Woosley is interested in one of the project’s discoveries that implied the existence of a black hole 85 times heavier than our Sun. This colossal singularity is likely to have come into being after two separate black holes had merged. The thing is, though, Dr. Woosley had previously presumed that such an entity couldn’t possibly exist.
Speaking to website Business Insider about this latest revelation, Dr. Woosley didn’t mince his words. He remarked, “This is exactly what I predicted wasn’t there… We and a lot of other people will go back and look hard at our assumptions.” All in all, then, the data emanating from LIGO and similar projects look like they are upending scientists’ expectations.
And, of course, LIGO is far from the only project concerned with gravitational waves. The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) project, for instance, hopes to utilize these phenomena to unlock some of the secrets of the universe. Using the latest technology and theories, this group studies pulsars in order to pick up on gravitational waves.
NANOGrav’s work is a little different to LIGO’s in that the former is searching for a “gravitational wave background.” LIGO has detected gravitational waves that can be thought of as short, sharp bursts. The gravitational wave background sought by NANOGrav, meanwhile, would be a more sustained murmur of sound.
Theorized gravitational wave background events would be so colossally large that they would take a long time to travel through the Earth, compared to ordinary gravitational waves. In fact, it’s thought that it could even take several years to pass through our planet. But such a discovery would be groundbreaking. It could, for instance, tell us more about supermassive black holes in the middle of galaxies, and what happens when such behemoths hit one another.
NANOGrav is just one group of several that are involved in a global race to detect the gravitational wave background. Having said that, this “race” does involve a great deal of collaboration. In fact, NANOGrav is part of a network known as the International Pulsar Timing Array, which includes members from Australia and Europe.
NANOGrav’s approach involves observing pulsars with telescopes located on Earth. Basically, it’s hoped that by studying the light emitted from pulsars, the researchers can pinpoint the gravitational wave background. It won’t be an easy task, of course, and it may well take years before the project bears fruit in a conclusive way.
Having said that, however, the signs are looking good. The NANOGrav team has, in fact, already noted a signal that appears to be influencing the light coming from lots of their monitored pulsars. For the time being, we don’t know for sure if this is the gravitational wave background. But further research should reveal the truth down the line.
All in all, the prospects for the field of gravitational wave research are looking very bright indeed. The progress since LIGO first detected the phenomenon in 2015 has been swift, and who knows what secrets studies like this will reveal in the future? In all likelihood, the discoveries yet to come would probably shock even Einstein.