Scientists Detect a Gravitational Waves Theorized over a Century Ago
The field of Astrophysics just leveled up
Hiya!
I’ve always been curious about what surrounds us, just out of sight. After all, our vision is good, but there are more things around us we can’t see than we can. Imagine if we could see WiFi, microwaves, ultraviolet waves, electromagnetic waves, sound waves, or even gravitational waves. While we can’t naturally see any of them, they swirl around and through us all the time. Yet, we know about them, and we’ve invented the technology allowing us to locate and measure them.
However, some have proven easier to detect than others. Gravitational waves, in particular, have posed quite a challenge ever since Einstein predicted their existence over a century ago. But now, thanks to some recent breakthroughs, all that is changing. Not only have scientists discovered new evidence of gravitational waves, but they also say some are as big as the Milky Way and someday could be used to create a new map of the Universe.
What are Gravitational Waves?
That all sounds interesting, but what exactly are gravitational waves, and how do they form? Well, the leading theory is that gravitational waves form when two black holes orbit each other as their respective galaxies merge. These waves pulse through the fabric of spacetime, similar to how a rock creates ripples when tossed in water.
Einstein predicted the existence of gravitational waves in 1916 as part of his general theory of relativity, and in 2015, researchers at the Laser Interferometer Gravitational-Wave Observatory (LIGO) scientific collaboration detected the first proof of Einstein’s gravitational waves when their sensors registered high-frequency gravitational waves linked to the merging of two black holes roughly 30 times larger than the Sun — each.
The find was a big deal and provided a path forward for more researchers to join the search for Cosmic gravitational waves. However, LIGO scientists could only detect waves on the higher frequency side of the gravitational wave spectrum, mostly from smaller black holes. Experts needed a different approach to detect waves on the lower end of the spectrum, which are thought to come from supermassive black holes.
New Approach
Since LIGO’s discovery, the international group of researchers making up the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) was the first to think of a new way to detect these more massive waves that worked. Their latest research was published as a series of papers in The Astrophysical Journal Letters on June 29, 2023.
Simply put, the scientists had the brilliant idea of turning the galaxy itself into a type of instrument and taking advantage of some of its strangeness.
Jeff Hazboun, an astrophysicist at Oregon State University and one of the nearly 100 members of NANOGrav, said that, in a sense, it’s as if “We get to hack the galaxy.”
In other words, rather than trying to detect the gravitational waves directly, the NANOGrav crew decided to monitor the effects the waves would have on pulsars — super-dense and rapidly spinning cores of dead neutron stars. These pulsars are about the size of a city, but they shoot out beams of radio emissions as they spin hundreds of times a second. NANOGrav member Thankful Cromartie from Cornell University explains:
"Each time their [emissions] beam crosses our line of sight, we see a pulse signal. These pulses arrive at stunningly regular intervals."
It’s these timely intervals that the researchers can use to predict precisely when a pulse should reach Earth. Then, they look for deviations from that prediction, no matter how slight. Hazboun clarifies:
"And if that pulse is a little bit late or a little bit early, then we may be able to attribute that to a gravitational wave passing through.”
He goes on to say that gravitational waves stretch or compress the fabric of spacetime, so when one travels through the 70 pulses the team measured over 15 years, it should change the distance the pulses have to travel to arrive at Earth.
The Results
They discovered just what they expected — a pattern of deviations in the pulsar beams between their predictions of when the pulses should arrive at Earth and when they actually arrived. This discrepancy in the timing suggests that gravitational waves, or some other phenomena, are jostling the fabric of spacetime and disrupting the travel time of objects the waves pass through.
Finding evidence of these mega-gravitational waves is one thing, but figuring out the direction they’re coming from is a new problem to solve. Hazboun said it’s challenging to know which direction a wave’s source is and compares it to visiting the ocean.
You can easily see the direction the waves are rolling in and when they hit the shore while standing on the beach looking at the water from afar, but this becomes more difficult when you’re out swimming around in choppy water. Right now, Hazboun says, our planet is swimming in a choppy sea of gravitational waves.
Still, the researchers have a decent idea that the waves are coming from supermassive black holes as they orbit each other — or, at least, supermassive black holes are one source. Some scientists theorize all sorts of possibilities that could create similar types of gravitational waves, including dark matter, cosmic strings, and even activity from soon after the Big Bang.
What’s Next?
The team at NANOGrav plans to combine their research with other groups investigating similar efforts, so within a couple of years, they might be able to determine the direction of gravitational waves.
Astrophysics at Northwestern University and member of NANOGrav, Luke Kelley, explains that as we learn more and define the sources, directions, and sizes of these waves, we can create better maps of the Universe.
“Just like we can create maps of the night sky using electromagnetic radiation, we can now create maps of the sky using gravitation waves. We expect to be able to detect whether the gravitational wave background is stronger in one part of the sky than another.”
Not only would that be really cool, but such maps would also help astrophysicists better understand what phenomena are responsible for creating these waves. Calculations suggest Supermassive black holes are one source, but it’s possible that other events also create the waves. Maybe even something we’ve yet to imagine, something that could change the fundamentals of physics as we know it. But we’ll have to wait to find out.
Perspective Shift
Human ingenuity is truly astounding. It might take some time, but we have an uncanny ability to explore the world beyond our senses. Because of technology, we know that we only see a tiny sliver of the full-color spectrum, and billions of invisible-to-us subatomic particles called neutrinos pass through our bodies every second. Things we lived thousands of years being oblivious to. Now, we can add gravitational waves to the list of invisible forces surrounding us.
The kid in me wonders whether these gravitational waves affect us at all. I mean, probably not, but if they influence how time is perceived (such as throwing off the predicted arrival times of the pulses), could our experience of time also be affected? Would we even be aware of it if it did? I doubt we can possibly know for sure, but it sounds like the making of a sci-fi novel.
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You stirred my imagination and educated me-as you often do. Thanks!