Did you feel it?
On May 21, 2019, the mass of eight suns disappeared. In a universe like ours, in which mass and energy are conserved, mass cannot disappear without consequences: and so it went that, as two distant black holes merged, the entire universe vibrated. A powerful gravitational shockwave expanded outwards from the merger, expanding out for billions of years before passing through Earth. On that day, every cell in your body stretched and compressed in four quick successions, as did the atoms of everything else on Earth and in our solar system.
You might not have noticed, but scientists did: three gravitational wave observatories strategically located around the planet — observatories which do not resemble traditional optical telescopes, but rather, long laser beams in dark rooms — saw their lasers jiggle just enough to detect this black hole merger.
That humans are able to measure such distant events in the universe with relative precision is one of the marvels of modern science. This particular merger happened some 16 billion light-years from us, or 17 percent the width of the known universe. Until recently, such phenomenally distant astronomical events were typically a mystery to astronomers. It is only because of the advent of gravitational wave astronomy, a very new field within observational astronomy, that our eye on the universe has expanded.
Gravitational waves are ripples in the fabric of space and time that are produced after two black holes collide with each other. Acclaimed physicist Albert Einstein first theorized about the existence of gravitational waves in 1916, and after being discovered a century later, astronomers have applied this knowledge to achieve the previously unthinkable, such as observing a black hole devouring a neutron star. Science news headlines regularly tout how gravitational waves are allowing scientists to do new things like peer inside neutron stars and discover the wobbliest black hole ever detected.
Yet what exactly are gravitational waves? Could humanity's newfound ability to observe them really be as much of a game-changer as headlines suggest? And to what extent is the excitement over gravitational waves substantive, and to what extent is it mere hype?
To answer the first question — what are gravitational waves — it is useful to first understand gravity itself.
As Montana State University physics professor Dr. Neil Cornish explained to Salon, Einstein's general theory of relativity was "fairly radical in its rewriting of gravity" because it replaced the idea of gravity as some kind of force with gravity as simply being space and time.
"There is no gravitational force in Einstein's theory," Cornish pointed out. "It's just that we live in a space-time that's curved and shaped by the matter and energy inside it." Because black holes are the collapsed remnants of former stars, they are massive, and when they collide with each other they produce measurable gravitational waves.
"As they orbited, they were like mallets banging on a drum," Levin recalled to Salon.
But gravitational waves were not detected definitively until 2015, at the Laser Interferometer Gravitational-Wave Observatory (LIGO) — two facilities located in Washington state and Louisiana which, together, can measure the direction and strength of gravitational waves passing through Earth. The two facilities were opened in 2002, and operated for years without finding any results; only in 2015 were engineers able to refine their precision enough to detect the tiny perturbations at an atomic level that define gravitational waves. 2015 marked the confirmation of what was predicted a century earlier by Albert Einstein.
The confirmation of Einstein's theory was a milestone in the history of modern science — and, according to Barnard College physics and astronomy Dr. Janna Levin, the big moment of discovery in 2015 was "very cinematic."
"As they orbited, they were like mallets banging on a drum," Levin recalled to Salon about the binary black hole merger that yielded the confirmed gravitational waves. "The drum is space-time, and they created ripples and sounds, technically sounds in the same way that an electric guitar plays sound or a drum plays sounds, but in the shape of space-time right before they coalesced, merged and quieted down."
She added that "there are many impressive things about this phenomenon," among them that it emitted the most energy detected by humans since the Big Bang itself. Yet for it to travel for all of those years at the speed of light, only to arrive at Earth at the perfect moment to be detected in 2015 "to be recorded by this instrument that had been devised over the span of a hundred years" was, to say the least, "fascinating."
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Cornish also used music to illustrate gravitational waves.
"When you're producing sound waves with a guitar, a cello or a violin, the distance between the peaks on the sound waves is roughly the same size based on the object that produces it," Cornish explained. "In the same way that you can tell just by listening, you know, is this a guitar or is this a drum or a tuba? The same goes for these collisions" between black holes and other cosmic objects, all of which produce different types of gravitational waves.
Yet how much can this really transform our knowledge of science?
"I like that kind of question. It's punchy," Dr. Rana X. Adhikari, a professor of physics at the California Institute of Technology, told Salon by email. Adhikari said that when it comes to assessing the utility of gravitational waves to future scientific endeavors, it is easier to describe quality than quantity.
"The kind of information you get from gravity is just very different from what you get with other kinds of astronomy."
"I can tell you a little bit more qualitatively though," Adhikari told Salon. "The kind of information you get from gravity is just very different from what you get with other kinds of astronomy."
As an analogy, Adhikari compared it to the relationship between light and sound. While we can process different colors with our eyes, a person singing while wearing a yellow shirt will sound the same as a person singing while wearing a blue shirt. You need a different instrument to measure the singing. In that same sense, "gravity tell us about things that are obscured by light, like black holes. The same goes for neutron stars. Those are really interesting things, because we've never studied the inside of those. Gravity is probably our only probe that gets into the heart of a neutron star to tell us what is happening."
Cornish also told Salon that our ability to detect gravitational waves will indeed be quite useful to current and future astronomers.
"We're actually able to extract very detailed information because the movement of the mass is directly reflected in these oscillations that we pick up in these ripples of gravity," Cornish explained. Instead of merely inferring, gravitational waves permit direct measurements. "That's how we're able to confidently say, 'Okay, we've detected a black hole of this mass because the actual size of the black hole changes the wavelengths, and inversely the frequency of that wave.' So a bigger black hole, just like a bigger instrument plays a lower tone, we're able to extract a whole lot of information from these signals."
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