IMAGE CREDIT: Michael S. Williamson / The Washington Post
I am so excited to be back writing for SkySimplified, and am even more excited to report on one of the most exciting results in modern astrophysics. One day ago, the team at NANOGrav—the North American Nanohertz Observatory for Gravitational Waves—announced a seminal result within our greater pursuit of understanding the universe: the spacetime continuum is constantly rippling with gravitational waves.
Before we dive into how exciting this result is and how it came about, it is important to analyze what gravitational waves are, and why they matter so much. So let us violate relativity in order to prove it and journey back to the early 20th century.
When Einstein was developing his seminal theory of relativity, one of his most exciting predictions was the existence of gravitational waves, stretching and squishing within the fabric of spacetime caused by distortions within it. Of course, this makes a lot of sense if one is to follow the development of Einstein’s ideas; when you predict the existence of a brand-new material (the spacetime continuum) it is natural that the material will wiggle (as most materials do).
Let us now move in the other direction. If relativity predicts the existence of gravitational waves, proving that gravitational waves exist should serve as a proof of relativity. And when dealing with one-half of the theory of everything, conclusively discovering gravitational waves is akin to confirming that the grand mathematical structure of the cosmos that we have constructed solely through observations on our rock is correct. It is one of the greatest validations of one of the greatest theories that humankind has ever devised.
Perhaps most excitingly, gravitational wave discoveries further push the boundary of human knowledge into unimaginably beautiful realms. Understanding gravitational waves will provide a new way to look at the Big Bang and the start of the universe, giving us an unprecedented amount of detail in answering one of the most fundamental questions of humanity: where did we come from? If the structure of gravitational waves were to be known, advanced simulations could further provide the answer to the second more fundamental question of humanity: where are we going?
We are dealing with the fabric of reality, the space that we all interact with every day, and the time that we all perceive with every accelerating moment. Gravitational wave discoveries let us probe this fabric in ways that no other medium can offer. As gravitational waves are unaffected by the attenuation of photos, they can travel immense distances without losing any of their initial power, giving us the equivalent of a high-resolution snapshot of their origin. In a way, gravitational waves are some of the most powerful tools we can use to peer into the past.
And lastly, the existence of gravitational waves is unequivocal proof that gravity behaves like a wave. If we are to extend this idea to wave-particle duality, it offers strong evidence that gravity might further behave like a particle. Discovering this particle that gravity is made of would be the greatest breakthrough in modern astrophysics and quantum mechanics alike, as it would put to rest one of the greatest challenges in modern science: the development of a grand unified theory, the true theory of everything, a complete and comprehensive understanding of how everything in our universe works. And, more excitingly, potential ways that we can manipulate the building blocks of matter in ways that were never possible before…
I hope I have been able to impress the fascination behind gravitational waves and their relevance to our present and our future. The seminal discovery that NANOGrav made was proving that these waves are constantly existing, everywhere. Right now, you are bobbing in the great cosmic ocean, buoyed by the tiny ripples of gravitational waves.
In 2014, LIGO discovered the first evidence that gravitational waves exist. (Read my article to learn more!) But these were incidental events: when two massive objects would interact and collide, they would emit a spurt of gravitational waves, but the gravitational composition of the background of the universe was still unknown. Regardless, the gravity of this accomplishment warranted the 2017 Nobel Prize in Physics.
NANOGrav approached the problem of gravitational wave detection from an entirely different direction. While LIGO attempted to detect minute laser changes from high-frequency gravitational wave releases, NANOGrav studied low-frequency radio waves to determine whether the gravitational waves existed in the background of the universe.
Pulsars are some of the most fascinating objects in the universe and can be visualized as rapidly-spinning lighthouse balls, periodically sweeping their radio waves across the universe. This innate periodicity makes them highly suited to be indicators of their background at large; if you can detect changes within them, you can detect changes within their background. This was the strategy that the NANOGrav team used to detect the existence of resting gravitational waves.
Collecting data over a massive period of 15 years, the team analyzed a dataset of 67 pulsars from across the universe. If background gravitational waves existed—generated from the hum of ancient supermassive black holes on the receding edges of the universe—the pulsar’s radio waves would stretch and shrink in minutiae, deviating from their expected course. With a sensitive enough telescope, you could detect these distortions, and conclude the existence of these waves.
Drawing on one of the greatest techniques in modern astrophysics, NANOGrav created a massive interferometer consisting of linked radio telescopes, acting as one large, highly accurate telescope. Studying these pulsars, they were able to detect a deviation with almost 99.5% confidence.
This remarkable discovery ushers in a new era for astrophysics and cosmology as a whole. We are just now finding an entirely new way to look at the universe. Analogous to the invention of the scanning electron microscope and its impact on molecular biology, finding a new perspective on the cosmos will almost certainly revolutionize our pursuit of physics and our understanding of why the universe works the way it does. It is one of the most incredible findings of our time, and we will be seeing its dominoes expand for years to come.