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

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Common Misconceptions About Dreams

If your nightmares look like Henry Fuseli's The Nightmare (1781), we're so sorry.
If your nightmares look like Henry Fuseli's The Nightmare (1781), we're so sorry.
Henry Fuseli, Detroit Institute of Arts, Wikimedia Commons // Public Domain

Contrary to what Ebenezer Scrooge initially thought in Charles Dickens's A Christmas Carol, nightmares aren’t merely a side effect of eating cheese before bed. In fact, studies have shown that pre-sleep snacks—dairy or otherwise—probably don’t influence your dreams at all. But even if you can’t blame your latest wacky snooze vision on last night’s midnight helping of chicken nuggets, you can try to trace it back to some stressor from your daily life.

Since dream interpretation isn’t an exact science—and sleep in general is one of science’s murkier territories—quite a few myths have arisen about what, why, and how we dream. In this episode of Misconceptions, Mental Floss's own Justin Dodd walks us through some of the more common fallacies about dreams. (And if you’re convinced you never dream, well, he has some news for you on that front, too.)

For more videos like this one, subscribe to the Mental Floss YouTube channel here.