Did you feel it?
On May 21, 2019, the mass of eight suns disappeared. In a universe like ours, where mass and energy are conserved, mass cannot disappear without consequences: this is how, when two distant black holes merged, the entire universe vibrated. A powerful gravitational shock wave traveled outward from the meltdown, spanning billions of years before slicing through Earth. That day, every cell in your body stretched and squeezed in four rapid successions, as did the atoms of everything else on Earth and in our solar system.
You may not have noticed it, but scientists have: three gravitational wave observatories strategically located around the planet – observatories that don’t look like traditional optical telescopes, but rather like long beams laser in dark rooms – saw their lasers wiggle just enough to detect this black hole. merger.
That humans are able to measure such distant events in the universe with relative accuracy is one of the marvels of modern science. This particular merger happened about 16 billion light-years away from us, or 17% of the width of the known universe. Until recently, such phenomenally distant astronomical events were generally a mystery to astronomers. It is only because of the advent of gravitational wave astronomy, a very new field within observational astronomy, that our view of the universe has broadened.
Gravitational waves are ripples in the fabric of space and time that occur after two black holes collide. Famous physicist Albert Einstein first theorized 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 enabling scientists to do new things, like look inside neutron stars and discover the most wonky black hole ever detected.
Yet what exactly are gravitational waves? Could humanity’s newfound ability to observe them really be a game-changer as the headlines suggest? And how substantial is the excitement of gravitational waves, and how much is it mere hype?
To answer the first question – what are gravitational waves – it helps to first understand gravity itself.
As Montana State University physics professor Dr. Neil Cornish explained to Salon, Einstein’s theory of general relativity was “quite radical in its rewriting of gravity” because it replaced the idea of gravity as a kind of force through 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 is bent and shaped by matter and the energy within it.” Because black holes are the collapsed remnants of ancient stars, they are massive and when they collide they produce measurable gravitational waves.
“When they were spinning in orbit, they were like mallets beating on a drum,” Levin recalled to Salon.
But gravitational waves weren’t definitely detected until 2015, at the Laser Interferometer Gravitational-Wave Observatory (LIGO) – two facilities in Washington state and Louisiana that together can measure the direction and strength of gravitational waves passing through the Earth. Both facilities were opened in 2002 and operated for years without finding results; it wasn’t until 2015 that engineers were able to hone their accuracy enough to detect the tiny atomic-level perturbations that define gravitational waves. 2015 marked the confirmation of what had been 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 beating on a drum,” Levin reminded Salon of the binary black hole merger that produced the confirmed gravitational waves. “The drum is space-time, and they created ripples and sounds, technically sounds the same way an electric guitar plays sound or a drum plays sounds, but in the form of a space-time just before they merge, coalesce and settle down.”
She added that “there are a lot of impressive things about this phenomenon”, among them the fact that it was emitting the most energy detected by humans since the Big Bang itself. Yet for it to travel all those years at the speed of light, only to arrive on Earth at the perfect time to be detected in 2015 “to be recorded by this instrument that had been designed over a period of a hundred years” was , to say the least, “fascinating”.
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Cornish also used music to illustrate gravitational waves.
“When you produce sound waves with a guitar, cello, or violin, the distance between the peaks of the sound waves are about the same size depending on the object producing it,” Cornish explained. “The same way you can tell just by listening, you know, is that a guitar or is that 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, to what extent can this really transform our knowledge of science?
“I love this kind of question. It’s hard-hitting,” Dr. Rana X. Adhikari, professor of physics at the California Institute of Technology, told Salon via email. Adhikari said that when it comes to assessing the usefulness of gravitational waves for future science projects, it’s easier to describe quality than quantity.
“The kind of information you get from gravity is just very different from what you get from other kinds of astronomy.”
“I can tell you a bit more qualitatively though,” Adhikari told Salon. “The kind of information you get from gravity is just very different from what you get from other kinds of astronomy.”
By analogy, Adhikari compared it to the relationship between light and sound. Although 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 vocals. Along the same lines, “gravity tells us about things that are obscured by light, like black holes. The same goes for neutron stars. These are really interesting things, because we’ve never studied starlight. Gravity is probably our only probe that penetrates the core of a neutron star to tell us what is going on there.”
Cornish also told Salon that our ability to detect gravitational waves will indeed be very useful to current and future astronomers.
“We’re actually able to extract very detailed information because the movement of mass is reflected directly in these oscillations that we pick up in these gravity ripples,” Cornish explained. Instead of just inferring, gravitational waves allow direct measurements. “That’s how we can confidently say, ‘Okay, we’ve detected a black hole of this mass, because the actual size of the black hole changes the wavelengths, and vice versa the frequency of that wave. So a bigger black hole, just like a bigger instrument plays a lower pitch, we’re able to extract a lot of information from those signals.”
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