Discovering gravitational waves would be a huge deal for physics, cosmology, and our understanding of the universe at large. But if you’re not a scientist studying one of the aforementioned fields, it’s possible you’ve never heard of these mysterious ripples. What the heck are gravitational waves, and why have physicists been struggling to find them for a century? Moreover, why should we care?
Simply put, gravitational waves are vibrations in the fabric of the universe—light-speed ripples in spacetime itself, caused by such epically violent events as exploding stars and black hole mergers. Thanks to inconceivably large, violent, and distant celestial happenings, the atoms that make up everything from the stars in the sky to the human beings on Earth are shaking a tiny bit, all the time.
Krauss does not work with the Advanced Laser Interferometer Gravitational Wave Observatory, or LIGO, which is searching for ripples in the fabric of space and time.
But he tweeted on Monday about the apparent shoring up of rumor he’d heard some months ago, that LIGO scientists were writing up a paper on gravitational waves they had discovered using US-based detectors.
“My earlier rumor about LIGO has been confirmed by independent sources. Stay tuned! Gravitational waves may have been discovered!! Exciting,” Krauss tweeted.
His message has since between retweeted 1,800 times.
What are gravitational waves?
But how can space ripple? According to Einstein’s general theory of relativity, spacetime isn’t a void, but rather a four-dimensional “fabric,” which can be pushed or pulled as objects move through it. These distortions are the real cause of gravitational attraction. One famous way of visualizing this is to take a taut rubber sheet and place a heavy object on it. That object will cause the sheet to sag around it. If you place a smaller object near the first one, it will fall toward the larger object. A star exerts a pull on planets and other celestial bodies in the same manner.
What are gravitational waves and why do they matter?If gravitational waves have been discovered, astronomers could use them to observe the cosmos in a way that has been impossible to date. “We would have a new window on the universe,” Krauss said. “Gravitational waves are generated in the most exotic, strange locations in nature, such as at the edge of black holes at the beginning of time. We are pretty certain they exist, but we’ve not been able to use them to probe the universe.”
Einstein predicted that the waves would be produced in extremely violent events, such as collisions between two black holes. As gravitational waves spread out, they compress and stretch spacetime. The ripples could potentially be picked up by laser beams that measure minute changes in the lengths of two 4km-long pipes at the Ligo facilities.
Engineers occasionally add synthetic signals to the Ligo data to test the equipment. While these are designed to mimic gravitational wave signals, Krauss said he had heard explicitly that the signal had not been added artificially.
“I don’t know if the rumour is solid,” Krauss told the Guardian. “If I don’t hear anything in the next two months, I’ll conclude it was false.”
The discovery would open a new window on the universe by showing scientists for the first time that gravitational waves exist, in places such as the edge of black holes at the beginning of time, filling in a major gap in our understanding of how the universe was born.
A team of scientists on a project called BICEP2 (Background Imaging of Cosmic Extragalactic Polarization) announced in 2014 that they had discovered these very ripples in space time, but soon admitted that their findings may have been just galactic dust.
However, scientists have yet to confirm this theory with observational evidence, which is why LIGO is so important.
“The detection of gravitational waves would be a game changer for astronomers in the field,” Clifford Will, a distinguished profess of physics at the University of Florida who studied under famed astrophysicist Kip Thorne told Business Insider in 2015. “We would be able to test aspects of general relativity that have not been tested.”
Not only that, the ability to observe gravitational waves would open a whole new frontier of astronomy. The same way that astronomers today use light waves to study the universe, they could also use gravitational waves to see cosmic objects — such as colliding black holes — like never before.
How to snag a gravitational wave
LIGO first began sniffing the skies for gravitational waves in 2002. And between 2002 and 2010, the $620 million experiment came up empty handed.
To better the odds, engineers began upgrading LIGO to make it 10 times more sensitive to gravitational waves. Last September, scientists turned the new-and-improved machine on and began taking data with, what is now called Advanced LIGO.
The way Advanced LIGO works is that it consists of two identical machines that are located 1,865 miles apart — one is in Livingston, Louisiana and the other is in Hanford, Washington.
At each detector, there are two equally-long tunnels that have a mirror at the end (one of the mirrors is shown in the image above). Scientists split a laser beam in two and then fire each half down one of the two tunnels. When the beams reflects off the mirror, the two beams should return at the same time, since they’re both traveling at the speed of light.
However, if a gravitational wave passes through the detector the same time the laser is traveling through the two tunnels, there will be a slight difference in time when the first half of the beam returns compared to the other half.
Compared to the length light waves we see with our eyes, which are micrometers in size (about the width of a human hair), gravitational waves are huge. This is why the distance between each LIGO detector is over 1800 miles, because that’s about how long astronomers think a gravitational wave should be.
Therefore, if one detector observes a gravitational wave, it should mean the other detector should measure the same signal, offering immediate confirmation that the observation at the first detector isn’t a fluke.
Scientists at LIGO aren’t taking any chances with this experiment. Before they announcing a discovery, the data will have been fully vetted twice-over by their expert peers.
But if they do succeed, it will revolutionize astronomy as we know it.
How gravitational waves were discoveredI got to the office a few days ago to find an email in my inbox from UAA astronomer Travis Rector. The subject line read “LIGO.” The key sentence: “It’s hard to express how big a deal this is!”
That morning’s “gravitational wave” headlines had slipped right by me in my coffee-deprived, sleepy-earthling state. Interesting, one brain wave murmured, but no time to grasp it now.
Rector’s email was a wake-up call. He also mentioned that one of UAA’s physics professors, Katherine Rawlins, had worked at the Laser Interferometer Gravitational-Wave Observatory about a decade ago. There’s almost nothing better than to invite an expert to explain a topic they’re excited about; it’s vivid, like visiting a country instead of just reading about it.
Rawlins was willing. By the time we got to her building, she’d commandeered a storage room and set up a tabletop instrument called an interferometer. On a very small scale, it could demonstrate how LIGO detected that gravitational wave.
She shone a tiny red laser light onto a mirror, called a beam splitter. The light beam split in two. Each beam traveled a different path and hit a second mirror before bouncing back to the beam splitter and rejoining.
Light travels in waves. The crests and troughs that don’t match after a split beam rejoins are proof of interference. The now-famous gravitational wave interacted with the split beams located at both of the LIGO sites, one in Washington and one in Louisiana.
So how does this prove Einstein’s theory of general relativity?
One hundred hears ago, long before anybody knew about black holes or quantum physics, Einstein said space and time and gravity are intertwined.
So, explained Rawlins, “if you have a mass, the presence of that mass distorts the space around it. And if you make some sort of violent change to that mass, then the distortion in space and time will make a ripple that spreads out at the speed of light.”
That ripple was the gravitational wave. The violent change was the shuddering merger of two black holes that had been circling one another with increasing velocity and growing proximity, somewhere way out in the Southern Hemisphere.
LIGO’s recently released publication estimated one of the black holes was about 36 times the mass of our sun, and the other was 29 times the mass. Their collision and merger generated a new black hole 62 times our own sun’s mass. That left three solar masses unaccounted for; they were released as energy in a gravitational wave.
This very wave has been en route to us for more than 1 billion years. By the time it hit the twin LIGO beam splitters in Washington and Louisiana, it was reduced to a brief 10th of a second that was documented in the sound of a quick chirp.
Rawlins came to UAA via Yale and the University of Madison-Wisconsin. She grew up wanting to be an astronaut, and even applied once, but her eyesight kept her from continuing. As a young post-doctorate more than a decade ago, she worked “the owl shift” in LIGO’s instrument room from midnight to 8 a.m.
“Along with a detector expert who knew much more than me,” Rawlins said, “I would sit there surrounded by computer monitors, keeping an eye on the noise level of the detector and other diagnostics.”
Her main job was to scour data for signals that were consistent with an airplane flying over head — something you wouldn’t think would be worrisome to a scientific experiment. “But to this one, it is,” she said. “All kinds of technology holds these mirrors absolutely steady. It is the most precise machine I have ever heard of.” In fact, after she left LIGO, the project went through a major technological upgrade to increase its sensitivity. Now it’s called Advanced LIGO.
The irony is scientists had to build LIGO with no guarantee that a gravitational wave would come. “You turn on the detector, and you wait” for a big-enough cosmic event.
Rawlins said physicists have spent entire careers—decades and decades— "chasing the gravitational wave.” After a few years at LIGO, she was ready to move on. “Ironically, I thought searching for astrophysical neutrinos sounded more plausible,” she laughed. As it happens, Rawlins was part of an international research group, the IceCube Collaboration, which accomplished exactly that in 2013.
Scientific success is incredibly exciting, Rawlins said, a moment to relish. The small UAA physics department lit up with the report. Department colleague Tyler Spilhaus, who studies gravitational physics, shared the glow. The day the news hit, he was about to head into work when Facebook stopped him cold. An old research advisor had posted 64 links, all to LIGO, a sure sign something was up.
Rawlins woke up to a text from her mother that read: “LIGO is in the news.” And in a maternal nod to her daughter’s stint there, added, “I knew it when!”