5 Things We Know About Gravitational Waves—And 2 That Are a Mystery

An illustration showing the merger of two black holes and the gravitational waves that ripple outward as the black holes spiral toward each other.
An illustration showing the merger of two black holes and the gravitational waves that ripple outward as the black holes spiral toward each other.
LIGO/T. Pyle

Gravitational waves, first detected in fall 2015 and then again a few months later, are making headlines this week following the detection of a third pair of colliding black holes. This particular duo is located a whopping 3 billion light years from Earth, making it the most distant source of gravitational waves discovered so far.

The signal from this latest black hole merger tripped the detectors at the twin LIGO facilities on January 4 of this year (the acronym stands for Laser Interferometer Gravitational-wave Observatory). The newly created black hole—the result of this latest cosmic collision—weighs in at about 49 times the mass of the Sun, putting it in-between the two earlier black hole collisions that LIGO recorded, in terms of size. There’s now ample evidence that black holes can weigh more than 20 solar masses—a finding that challenges the traditional understanding of black hole formation. “These are objects we didn’t know existed before LIGO detected them,” David Shoemaker, an MIT physicist and spokesperson for the LIGO collaboration, said in a statement.

Gravitational waves are shaping up to be the hot new astronomical tool of the 21st century, offering glimpses into the universe’s darkest corners and providing insights into the workings of the cosmos that we can’t get by any other means. Here, then, are five things we know about these cosmic ripples, and a couple more things that we haven’t quite figured out yet:

1. THEY'D HAVE MADE EINSTEIN SMILE.

We knew, or at least strongly suspected, that gravitational waves existed long before their discovery in 2015. They were predicted by Einstein’s theory of gravity, known as general relativity, published just over 100 years ago. The first black hole mergers observed by LIGO produced tell-tale cosmic signatures that meshed perfectly with what Einstein’s theory predicted. But the black hole collision announced this week may yield yet another feather for Einstein’s cap. It involves something called “dispersion.” When waves of different wavelengths pass through a physical medium—like light passing through glass, for example—the rays of light diverge (this is the how a prism creates a rainbow). But Einstein’s theory says gravitational waves ought to be immune to this sort of dispersion—and this is exactly what the observations suggest, with this latest black hole merger providing the strongest confirmation so far. (This Einstein fellow was pretty bright!)

2. THEY'RE RIPPLES IN THE FABRIC OF SPACE-TIME.

According to Einstein’s theory, whenever a massive object is accelerated, it creates ripples in space-time. Typically, these cosmic disturbances are too small to notice; but when the objects are massive enough—a pair of colliding black holes, for example—then the signal may be large enough to trigger a “blip” at the LIGO detectors, the pair of gravitational wave laboratories located in Louisiana and in Washington state. Even with colliding black holes, however, the ripples are mind-bogglingly small: When a gravitational wave passes by, each 2.5-mile-long arm of the L-shaped LIGO detectors gets stretched and squeezed by a distance equivalent to just 1/1000th of the width of a proton.

3. THEY LET US "LISTEN" TO THE UNIVERSE.

At least in a figurative sense, gravitational waves let us “listen in” on some of the universe’s most violent happenings. In fact, the way that gravitational waves work is closely analogous to sound waves or water waves. In each case, you have a disturbance in a particular medium that causes waves to spread outward, in ever-increasing circles. (Sound waves are a disturbance in the air; water waves are a disturbance in water—and in the case of gravitational waves, it’s a disturbance in the fabric of space itself.) To “hear” gravitational waves, you just have to convert the signals received by LIGO into sound waves. So what do we actually hear? In the case of colliding black holes, it’s something like a cosmic “chirp”—a kind of whooping sound that progresses quickly from low pitch to high.

4. THEY'VE SHOWN US THAT YOU REALLY DON'T WANT TO GET TOO CLOSE TO A PAIR OF COLLIDING BLACK HOLES.

Thanks to gravitational waves, we’re learning a lot about that most mysterious of objects, the black hole. When two black holes collide, they form an even bigger black hole—but not quite as large as you’d expect from simply adding up the masses of the two original black holes. That’s because some of the mass gets converted into energy, via Einstein’s famous equation, E=mc2. The magnitude of the explosion is truly staggering.

As astronomer Duncan Brown told Mental Floss last June: “When a nuclear bomb explodes, you’re converting about a gram of matter—about the weight of a thumb-tack—into energy. Here, you’re converting the equivalent of the mass of the Sun into energy, in a tiny fraction of a second.” The blast could produce more energy than all the stars in the universe—for a split-second.

5. THEY MIGHT BE POWERFUL ENOUGH TO KICK A BLACK HOLE OUT OF A GALAXY.

This spring, astronomers discovered a “rogue” black hole moving speedily away from a distant galaxy known as 3C186, located some 8 billion light years from Earth. The black hole is believed to weigh as much as 1 billion Suns—which means it must have received quite a kick, to set it in motion (its speed was determined to be around 5 million miles per hour, or a bit less than 1 percent of the speed of light). Astronomers have suggested that the necessary energy may have come from gravitational waves produced by a pair of very heavy black holes that collided near the galaxy’s center.

But there’s still plenty we’d like to know about gravitational waves—and about the objects they let us probe. For example …

6. WE DON'T KNOW IF GRAVITATIONAL WAVES CONTRIBUTE TO "DARK MATTER."

Most of the mass of the universe—about 85 percent—is stuff we can’t see; astronomers call this unseen material “dark matter.” Exactly what this dark stuff is has been the subject of intense debate for decades. The leading theory is that dark matter is made up of exotic particles created soon after the big bang. But some physicists have speculated that so-called “primordial black holes”—black holes created in the first second of the universe’s existence—might make up a significant fraction of the mysterious dark matter. The theorists who back this idea say that it could help to explain the unusually high masses of the black hole binary systems that LIGO has detected so far.

7. WE DON'T KNOW IF THEY ARE EVIDENCE OF DIMENSIONS BEYOND THE ONES WE PERCEIVE.

Particle physicists and cosmologists have long speculated about the existence of “extra dimensions” beyond the four we experience (three for space and one for time). It was hoped that experiments at the Large Hadron Collider would offer hints of these dimensions, but no such evidence has turned up so far. Some physicists, however, suggest that gravitational waves might provide a clue. They speculate that gravity could freely spread out over all of the dimensions, perhaps explaining why gravity is such a weak force (it’s by far the weakest of the four known forces in nature). Further, they say that the existence of extra dimensions would leave their mark on the gravitational waves that we measure here on Earth. So, stay tuned: It’s only been a bit more than a year since we first detected gravitational waves; no doubt they have much more to tell us about our universe.

10 Products for a Better Night's Sleep

Amazon/Comfort Spaces
Amazon/Comfort Spaces

Getting a full eight hours of sleep can be tough these days. If you’re having trouble catching enough Zzzs, consider giving these highly rated and recommended products a try.

1. Everlasting Comfort Pure Memory Foam Knee Pillow; $25

Everlasting Comfort Knee Pillow
Everlasting Comfort/Amazon

For side sleepers, keeping the spine, hips, and legs aligned is key to a good night’s rest—and a pain-free morning after. Everlasting Comfort’s memory foam knee pillow is ergonomically designed to fit between the knees or thighs to ensure proper alignment. One simple but game-changing feature is the removable strap, which you can fasten around one leg; this keeps the pillow in place even as you roll at night, meaning you don’t have to wake up to adjust it (or pick it up from your floor). Reviewers call the pillow “life-changing” and “the best knee pillow I’ve found.” Plus, it comes with two pairs of ear plugs.

Buy it: Amazon

2. Letsfit White Noise Machine; $21

Letsfit White Noise Machine
Letsfit/Amazon

White noise machines: They’re not just for babies! This Letsfit model—which is rated 4.7 out of five with nearly 3500 reviews—has 14 potential sleep soundtracks, including three white noise tracks, to better block out everything from sirens to birds that chirp enthusiastically at dawn (although there’s also a birds track, if that’s your thing). It also has a timer function and a night light.

Buy it: Amazon

3. ECLIPSE Blackout Curtains; $16

Eclipse Black Out Curtains
Eclipse/Amazon

According to the National Sleep Foundation, too much light in a room when you’re trying to snooze is a recipe for sleep disaster. These understated polyester curtains from ECLIPSE block 99 percent of light and reduce noise—plus, they’ll help you save on energy costs. "Our neighbor leaves their backyard light on all night with what I can only guess is the same kind of bulb they use on a train headlight. It shines across their yard, through ours, straight at our bedroom window," one Amazon reviewer who purchased the curtains in black wrote. "These drapes block the light completely."

Buy it: Amazon

4. JALL Wake Up Light Sunrise Alarm Clock; $38

JALL Wake Up Light Sunrise Alarm Clock
JALL/Amazon

Being jarred awake by a blaring alarm clock can set the wrong mood for the rest of your day. Wake up in a more pleasant way with this clock, which gradually lights up between 10 percent and 100 percent in the 30 minutes before your alarm. You can choose between seven different colors and several natural sounds as well as a regular alarm beep, but why would you ever use that? “Since getting this clock my sleep has been much better,” one reviewer reported. “I wake up not feeling tired but refreshed.”

Buy it: Amazon

5. Philips SmartSleep Wake-Up Light; $200

Philips SmartSleep Wake-Up Light
Philips/Amazon

If you’re looking for an alarm clock with even more features, Philips’s SmartSleep Wake-Up Light is smartphone-enabled and equipped with an AmbiTrack sensor, which tracks things like bedroom temperature, humidity, and light levels, then gives recommendations for how you can get a better night’s rest.

Buy it: Amazon

6. Slumber Cloud Stratus Sheet Set; $159

Stratus sheets from Slumber Cloud.
Slumber Cloud

Being too hot or too cold can kill a good night’s sleep. The Good Housekeeping Institute rated these sheets—which are made with Outlast fibers engineered by NASA—as 2020’s best temperature-regulating sheets.

Buy it: SlumberCloud

7. Comfort Space Coolmax Sheet Set; $29-$40

Comfort Spaces Coolmax Sheets
Comfort Spaces/Amazon

If $159 sheets are out of your price range, the GHI recommends these sheets from Comfort Spaces, which are made with moisture-wicking Coolmax microfiber. Depending on the size you need, they range in price from $29 to $40.

Buy it: Amazon

8. Coop Home Goods Eden Memory Foam Pillow; $80

Coop Eden Pillow
Coop Home Goods/Amazon

This pillow—which has a 4.5-star rating on Amazon—is filled with memory foam scraps and microfiber, and comes with an extra half-pound of fill so you can add, or subtract, the amount in the pillow for ultimate comfort. As a bonus, the pillows are hypoallergenic, mite-resistant, and washable.

Buy it: Amazon

9. Baloo Weighted Blanket; $149-$169

Baloo Weighted Blanket
Baloo/Amazon

Though the science is still out on weighted blankets, some people swear by them. Wirecutter named this Baloo blanket the best, not in small part because, unlike many weighted blankets, it’s machine-washable and -dryable. It’s currently available in 12-pound ($149) twin size and 20-pound ($169) queen size. It’s rated 4.7 out of five stars on Amazon, with one reviewer reporting that “when it's spread out over you it just feels like a comfy, snuggly hug for your whole body … I've found it super relaxing for falling asleep the last few nights, and it looks nice on the end of the bed, too.” 

Buy it: Amazon 

10. Philips Smartsleep Snoring Relief Band; $200

Philips SmartSleep Snoring Relief Band
Philips/Amazon

Few things can disturb your slumber—and that of the ones you love—like loudly sawing logs. Philips’s Smartsleep Snoring Relief Band is designed for people who snore when they’re sleeping on their backs, and according to the company, 86 percent of people who used the band reported reduced snoring after a month. The device wraps around the torso and is equipped with a sensor that delivers vibrations if it detects you moving to sleep on your back; those vibrations stop when you roll onto your side. The next day, you can see how many hours you spent in bed, how many of those hours you spent on your back, and your response rate to the vibrations. The sensor has an algorithm that notes your response rate and tweaks the intensity of vibrations based on that. “This device works exactly as advertised,” one Amazon reviewer wrote. “I’d say it’s perfect.”

Buy it: Amazon

This article contains affiliate links to products selected by our editors. Mental Floss may receive a commission for purchases made through these links.

10 Facts About Argentine Ants

A pile of genetically-related Argentine ants
A pile of genetically-related Argentine ants
Marc Matteo, Flickr // CC BY-NC 2.0

A supercolony of invasive Argentine ants (Linepithema humile) stretches for 560 miles beneath California, from San Diego to San Francisco. The billions of Argentine ants are unlike other ants in many ways—and they are virtually indestructible. Along with their supercolonies in Europe, Japan, and Australia, L. humile’s global domination is rivaled only by that of human beings. Here’s what you should know about these prolific pests.

1. Argentine ant colonies are ruled by hundreds of queens.

Most ant colonies revolve around a single queen. Growing much larger than the worker drones, she is programmed to mate as quickly as possible, then to leave her nest of origin and establish a new one. In some species, a single queen can lay millions of eggs in a lifetime, producing an army of worker drones and future queens who will go off to build their own nests. But unlike most ants, Argentines are polygynous: Each nest contains multiple queens. In some, they can form up to 30 percent of the population.

2. Argentine ants move their nests frequently.

Nest types vary from ant species to ant species, but those who live in soil commonly dig tunnels and chambers deep into the earth that will protect the colony throughout the life of the queen. L. humile, though, is transient and ever shifting. Argentine ants frequently pack up their eggs and move the entire colony, queen and all, to a new nest, even when there is no apparent threat. Biologist Deborah Gordon told Ars Technica that the ants typically have 20 to 30 shallow nests at any one time, which can be built up in a matter of just weeks.

3. Argentine ants traveled the U.S. before settling down in California.

Argentine ants arrived in the United States from Northern Argentina in the late 19th century, when the first recorded Argentine ant was found in Louisiana in 1891. Researchers believe that the ants hitched a ride to North America in Argentinian shipments of coffee or sugar off-loaded at the Port of New Orleans. From there, they traveled—most likely by train—across the South and into California. Enticed by the Mediterranean climate, one similar to that of its original home in South America, the ants set up shop. By 1907, they’d displaced local native ants and begun their first steps towards total soil domination along 560 miles of California coastline.

4. California’s Argentine ants are more laid-back than their South American cousins.

In side-by-side comparisons of Argentine ants from their South American homeland and California, researchers have found that those from the West Coast are far more mellow than those from Argentina. In studies, it was typical for two ants from different nests to fight when placed in the same vial in Argentina, but in California, ants from different nests rarely fought, even when they were collected from locations several hundred miles apart.

A DNA study of ants from both locations in 2000 revealed a stark difference. In the ants from Argentina, microsatellites—short, uniquely patterned DNA sequences passed down from generation to generation—had more than twice as much variation as the microsatellites of the Californian ants. When two individuals from different nests in California were placed together, they recognized one another as family. The ants from Argentina didn’t, making them more likely to display territorial aggression.

The difference is rooted in the genetic bottleneck the ants encountered on their arrival to the Golden State over a century ago. According to biologist Neil D. Tsutsui, who conducted the DNA study, the ants in California today are all descendants of that founding colony. “It would be as if all of the people in the United States were descended from the Pilgrims who came here in 1620,” he told the Stanford Report in 2004. Instead of competing with one another, generation after generation has worked together to take out native ants and build an immense California colony.

5. Argentine ants protect other insects in exchange for sweet, sweet honeydew.

Argentine ants
Two Argentine ants share a tiny blob of honeydew.
Davefoc, Wikimedia Commons // CC BY-SA 4.0

Argentine ants love to feed on sweet nectar, but flowers and suburban kitchens aren’t the only source of such desirable foodstuffs. Insects that feed on plant sap, like mealybugs, scales, and aphids, naturally excrete sugar-rich liquid “honeydew” from their butts. To secure a steady flow of the sticky-sweet substance, Argentine ants will fight off the predators of their insect chefs, including soldier beetles and midges. They’ll even relocate their honeydew producers to better food sources or microclimates to get the most they can out of their anal secretions.

6. The California Argetine ant supercolony is one-sixth the size of Southern Europe’s.

The California supercolony, which scientists have named the “Californian large,” is only the second-biggest conglomeration of Argentine ants in the world. The biggest colony is found along Southern Europe’s Mediterranean coast, where it stretches 3700 miles from northern Italy to the Atlantic coast of Spain. The ants, introduced around 80 years ago, now number in the billions. Smaller supercolonies also exist in Japan and Australia.

7. Argentine ants are second only to humans in their scale of world domination.

In 2009, researchers discovered that Argentine ants from three of the world’s largest supercolonies (Southern Europe, California, and Japan) are so closely related that they actually form a single mega-colony. The study, led by Eriki Sunamura from the University of Tokyo, found that when placed together, ants from the three supercolonies refused to fight. Instead, they rubbed antennae in greeting the way L. humile does when interacting with genetically-related individuals.

The researchers believe that the Argentine ant mega-colony isn’t just the largest insect colony ever identified; it rivals that of human colonization around the globe. Presenting their findings in the journal Insect Sociaux, they wrote, “the enormous extent of this population is paralleled only by human society.”

8. A mass execution of Argentine ant queens takes place every spring.

Each spring, just before mating season begins, worker ants go on a killing rampage and assassinate 90 percent of their queens. Entomologists aren’t sure exactly why the large-scale execution occurs, but one hypothesis, published in the Journal of Evolutionary Biology in 2001, suggests that it is a “spiteful behavior” to kill the queens that are less related, on average, to the workers.

In their study, researchers from the University of Lausanne hypothesized that Argentine ants are regularly separated from direct family members through free exchange among the nests. Before mating season begins each year, those that are genetically related band together to kill more distantly related queens. Doing so decreases the nest’s genetic diversity and allows it to be rebuilt with a queen who is directly related to the greatest majority of workers.

The study’s results were inconclusive and the question remained unanswered, yet researchers learned something unexpected in the process. Instead of finding genetic diversity among worker ants, those belonging to each nest were actually a homogenous population. Only the queens were genetic outliers with relatively few familial relationships in each nest.

9. Climate change is making Argentine ants more of a nuisance to humans.

Argentine ants thrive in a Mediterranean climate where winters are cool and wet and summers are warm and dry. When conditions are ideal, they largely keep to themselves, but when conditions are drought-like or extremely wet, the ants move indoors in search of more hospitable climes. Experts at survival, Argentine ants can find food or water that’s been left unguarded in just minutes.

With the climate crisis, conditions in California are becoming more extreme. Hot days, no longer relegated just to the summer months, are becoming more numerous and prolonged. Droughts are becoming more frequent. While these changes are unlikely to harm much of the California supercolony, they are likely to drive the residents of urban nests more frequently into people's homes, making the ants a major nuisance for residents from San Diego to San Francisco.

10. Argentine ants are almost impossible to eradicate.

Individual Argentine ants are easy enough to kill, but an Argentine ant colony is a different story. The California colony has no natural predators and, thanks to their high levels of cooperation and massive numbers, L. humile has effectively destroyed possible competitors and disrupted the ecological balance of native species in the process. Insecticides, which are unable to penetrate into the underground nests, aren’t particularly effective. And because the ants can pick up and move their entire nest so quickly, neither are household control measures such as ant bait. After just over a century in California, Argentine ants are now virtually invincible.