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Everything You Ever Wanted to Know About Black Holes

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FAECIASP/NASA/Conicet of Argentina/Getty Images

They’re probably the weirdest—and certainly the most puzzling—objects in the universe. And yet black holes are oddly familiar, figuring prominently in pop culture (both Matthew McConaughey and Homer Simpson have had perilous encounters with them). It also seems they’re always in the news—as they were last year when an Israeli researcher created an artificial black hole (sort of) in his laboratory. (Read on to learn more about that.) But what exactly is the nature of this bizarre phenomenon? Here's what we know … and don't know.

WHAT IS A BLACK HOLE?

A black hole is a region of space in which gravity exerts such an enormous pull that nothing—not even light—can escape. That’s the simple definition of a black hole. But if you talk to a physicist, they’ll also describe a black hole as a region of very severely curved space-time—so sharply curved, in fact, that it’s “pinched off,” so to speak, from the rest of the universe.

This idea of curved space-time goes back to the work of Einstein. It was just over 100 years ago that Einstein put forward his theory of gravity, known as the general theory of relativity. According to the theory, matter curves, or distorts, the very fabric of space. A small object like Earth causes only a small amount of distortion; a star like our Sun causes more warping. And what about a very heavy, dense object? According to Einstein’s theory, if you squeeze enough mass into a small enough space, it will undergo a collapse, forming a black hole; the amount of warping will become infinite.

The boundary of the black hole is known as the “event horizon”—the point of no return. Matter that crosses the event horizon can never return to the outside. In this sense, the inside of a black hole is not even a part of our universe: Whatever might be happening there, we can never know about, since no signal from the inside can ever reach the outside. According to general relativity, the center of a black hole will contain a “singularity”—a point of infinite density and of infinitely curved space-time.

HOW DO YOU MAKE ONE?

Black holes come in different sizes. When a sufficiently massive star exhausts its nuclear fuel supply—that is, when it can no longer produce energy by means of a fusion reaction in its core—it explodes (this is called a supernova, in which the star sheds material from its outer layers); the remaining core then contracts, due to gravity. If the star was more than about 20 times as massive as the Sun, then nothing can stop this contraction, and the star collapses until it’s smaller than its own event horizon, becoming a black hole. These are called stellar-mass black holes, since their masses are on par with the masses of stars. But there are also giant black holes, with masses equal to that of millions of stars. These “supermassive” black holes are believed to be located in the centers of most galaxies, including our own Milky Way; theorists believe they evolved together with the galaxies that harbor them. There’s also speculation that microscopic or “primordial” black holes may have been created at the time of the big bang.

HOW DO WE KNOW THEY EXIST?

An artist's impression of a binary system containing a stellar-mass black hole pulling gas away from a companion star on the right. Image credit: NASA/CXC/M.Weiss

 
Since black holes emit no light, there’s no way to see them directly. However, astronomers have been able to infer their existence based on observations of ordinary stars that orbit a black hole as part of a binary star system. Sometimes the black hole “swallows” material from the companion star. As this material swirls around the black hole, it heats up due to friction; as a result it emits X-rays, which can be detected from Earth. (The X-rays are emitted before the material crosses the black hole’s event horizon.) This is how the first black hole to be detected, known as Cygnus X-1, was found.

WHAT WOULD HAPPEN IF YOU FELL IN ONE?

The answer, quite literally, depends on who you ask. Because black holes stretch time as well as space, an astronaut unlucky enough to fall into the hole sees something quite different from what an observer watching from a safe distance would observe. From the point of view of the unlucky astronaut, things do not go well. In the case of a stellar-mass black hole, she’ll feel something called tidal forces—the unequal pulling on her feet compared to her head (assuming she enters the hole feet-first). The astronaut would be stretched out like spaghetti, as Stephen Hawking has vividly put it. In the case of a supermassive black hole, tidal forces at the event horizon are less severe; the astronaut may not feel anything unusual is happening as she crosses it. Nonetheless, she is doomed; as she approaches the singularity, the tidal forces will inevitably rip her apart, before she is crushed into oblivion.

But the view from the outside is quite different. Because of the time-stretching—physicists call it “time dilation”—an observer located far from the event horizon never actually sees the astronaut meet her doom. Instead, we see her get ever-closer to the event horizon, but never crossing it. If we could see her watch, we’d see it ticking more and more slowly. She would end up “frozen” on the edge of the black hole. There is no right or wrong answer to the question of “How is the astronaut doing?” It really does depend on your frame of reference.

WOULD YOU BE ABLE TO GET BACK OUT?

The short answer is, probably not. But physicists have speculated about the existence of “wormholes”—a kind of tunnel through space-time connecting one black hole to another. When Carl Sagan was working on his novel Contact, he asked physicist Kip Thorne to suggest a method by which the story’s heroine might quickly travel from the Earth to the star Vega (some 26 light-years away); Thorne considered the matter, eventually suggesting that a wormhole might do the trick. That was good enough for Sagan’s book (later made into a movie starring Jodie Foster)—but as Thorne would later acknowledge, wormholes are a highly speculative idea, and he doubts that wormholes will actually be found in our universe. (Thorne would again lend his expertise to movie-makers for the 2014 film Interstellar, where black holes play a central role.)

HOW LONG DO THEY LAST?

Before the work of Stephen Hawking in the 1970s, for all we knew, black holes stuck around forever. But Hawking, together with physicist Jacob Beckenstein, showed that black holes actually emit a kind of radiation (now known as Hawking radiation). This radiation carries away energy, which means that, over very long time scales, black holes should simply evaporate away into nothingness. (Theorists who have crunched the numbers believe this process should take billions upon billions of years—the era of “black hole evaporation” lies in the far future; in comparison, our universe’s current age—about 14 billion years—is a mere blip.)

The announcement that Jeff Steinhauer, a physicist at the Technion-Israel Institute of Technology in Haifa, Israel, had created an artificial black hole analogue bears directly on the issue of black hole evaporation. Steinhauer’s experiment didn’t use gravity; instead, he used a tube filled with ultra-cold atoms in a peculiar state known as a Bose-Einstein condensate. Then he accelerated the atoms so that they were moving faster than sound (but actually still quite slow, since sound can only move slowly in such a condensate), creating an “acoustic” event horizon, as the researchers describe it. Think of it as swallowing sound rather than light, as a black hole does. The experiment produced more than just an event horizon—it produced the equivalent of Hawking radiation, Steinhauer says.

If the experiment holds up to scrutiny, it could be seen as bolstering the case for black hole evaporation. The physics community is reacting cautiously; Silke Weinfurter of the University of Nottingham, UK, told Nature: “This experiment … is really amazing, [but] it doesn’t prove that Hawking radiation exists around astrophysical black holes.”

Does it matter if black holes evaporate? If you’re a physicist, it does. The problem has to do with “information.” According to quantum mechanics, information—the numbers that describe how massive a particle is, how fast it’s spinning, and so on—can neither be created nor destroyed. But when something falls into a black hole, whatever information it contained would seem to disappear. Even worse, when the black hole evaporates, the Hawking radiation that’s emitted is all scrambled up; the original information is seemingly lost for good. Although a number of possible solutions have been put forward, this information loss paradox remains one of the most pressing problems in theoretical physics.

ARE THERE ANY NEW WAYS TO STUDY THEM?

Yes. In 2016, scientists announced the discovery of gravitational waves emitted by a pair of merging black holes (and, a few months later, a second pair of colliding black holes was announced). Gravitational waves are ripples in space-time; though predicted by general relativity, they eluded detection for a century, and were only successfully snagged with the completion of the LIGO detectors (Laser Interferometer Gravitational wave Observatory). As with the earlier kinds of observations, the evidence is indirect—we don’t actually see the black holes—but the strength and profile of these gravitational waves meshes perfectly with Einstein’s theory and with the known physics of black holes.

WHAT'S NEXT ON THE (EVENT) HORIZON?

The heart of the Milky Way. Soon—perhaps this year—we’ll get a glimpse of a black hole event horizon, thanks to the Event Horizon Telescope. It’s actually a globe-spanning array of radio telescopes, and its prime target will be the supermassive black hole at the center of our galaxy—an object known as Sagittarius A*. Because it’s so far from Earth (about 25,000 light-years), it appears as a mere pinprick in the sky; no single telescope has the resolving power to show what’s happening in any detail. But with the combined power of the entire array, astronomers expect to produce a detailed picture of the radiation emitted by gas and dust just before it crosses the black hole’s event horizon.

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iStock // Ekaterina Minaeva
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technology
Man Buys Two Metric Tons of LEGO Bricks; Sorts Them Via Machine Learning
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iStock // Ekaterina Minaeva

Jacques Mattheij made a small, but awesome, mistake. He went on eBay one evening and bid on a bunch of bulk LEGO brick auctions, then went to sleep. Upon waking, he discovered that he was the high bidder on many, and was now the proud owner of two tons of LEGO bricks. (This is about 4400 pounds.) He wrote, "[L]esson 1: if you win almost all bids you are bidding too high."

Mattheij had noticed that bulk, unsorted bricks sell for something like €10/kilogram, whereas sets are roughly €40/kg and rare parts go for up to €100/kg. Much of the value of the bricks is in their sorting. If he could reduce the entropy of these bins of unsorted bricks, he could make a tidy profit. While many people do this work by hand, the problem is enormous—just the kind of challenge for a computer. Mattheij writes:

There are 38000+ shapes and there are 100+ possible shades of color (you can roughly tell how old someone is by asking them what lego colors they remember from their youth).

In the following months, Mattheij built a proof-of-concept sorting system using, of course, LEGO. He broke the problem down into a series of sub-problems (including "feeding LEGO reliably from a hopper is surprisingly hard," one of those facts of nature that will stymie even the best system design). After tinkering with the prototype at length, he expanded the system to a surprisingly complex system of conveyer belts (powered by a home treadmill), various pieces of cabinetry, and "copious quantities of crazy glue."

Here's a video showing the current system running at low speed:

The key part of the system was running the bricks past a camera paired with a computer running a neural net-based image classifier. That allows the computer (when sufficiently trained on brick images) to recognize bricks and thus categorize them by color, shape, or other parameters. Remember that as bricks pass by, they can be in any orientation, can be dirty, can even be stuck to other pieces. So having a flexible software system is key to recognizing—in a fraction of a second—what a given brick is, in order to sort it out. When a match is found, a jet of compressed air pops the piece off the conveyer belt and into a waiting bin.

After much experimentation, Mattheij rewrote the software (several times in fact) to accomplish a variety of basic tasks. At its core, the system takes images from a webcam and feeds them to a neural network to do the classification. Of course, the neural net needs to be "trained" by showing it lots of images, and telling it what those images represent. Mattheij's breakthrough was allowing the machine to effectively train itself, with guidance: Running pieces through allows the system to take its own photos, make a guess, and build on that guess. As long as Mattheij corrects the incorrect guesses, he ends up with a decent (and self-reinforcing) corpus of training data. As the machine continues running, it can rack up more training, allowing it to recognize a broad variety of pieces on the fly.

Here's another video, focusing on how the pieces move on conveyer belts (running at slow speed so puny humans can follow). You can also see the air jets in action:

In an email interview, Mattheij told Mental Floss that the system currently sorts LEGO bricks into more than 50 categories. It can also be run in a color-sorting mode to bin the parts across 12 color groups. (Thus at present you'd likely do a two-pass sort on the bricks: once for shape, then a separate pass for color.) He continues to refine the system, with a focus on making its recognition abilities faster. At some point down the line, he plans to make the software portion open source. You're on your own as far as building conveyer belts, bins, and so forth.

Check out Mattheij's writeup in two parts for more information. It starts with an overview of the story, followed up with a deep dive on the software. He's also tweeting about the project (among other things). And if you look around a bit, you'll find bulk LEGO brick auctions online—it's definitely a thing!

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Cs California, Wikimedia Commons // CC BY-SA 3.0
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science
How Experts Say We Should Stop a 'Zombie' Infection: Kill It With Fire
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Cs California, Wikimedia Commons // CC BY-SA 3.0

Scientists are known for being pretty cautious people. But sometimes, even the most careful of us need to burn some things to the ground. Immunologists have proposed a plan to burn large swaths of parkland in an attempt to wipe out disease, as The New York Times reports. They described the problem in the journal Microbiology and Molecular Biology Reviews.

Chronic wasting disease (CWD) is a gruesome infection that’s been destroying deer and elk herds across North America. Like bovine spongiform encephalopathy (BSE, better known as mad cow disease) and Creutzfeldt-Jakob disease, CWD is caused by damaged, contagious little proteins called prions. Although it's been half a century since CWD was first discovered, scientists are still scratching their heads about how it works, how it spreads, and if, like BSE, it could someday infect humans.

Paper co-author Mark Zabel, of the Prion Research Center at Colorado State University, says animals with CWD fade away slowly at first, losing weight and starting to act kind of spacey. But "they’re not hard to pick out at the end stage," he told The New York Times. "They have a vacant stare, they have a stumbling gait, their heads are drooping, their ears are down, you can see thick saliva dripping from their mouths. It’s like a true zombie disease."

CWD has already been spotted in 24 U.S. states. Some herds are already 50 percent infected, and that number is only growing.

Prion illnesses often travel from one infected individual to another, but CWD’s expansion was so rapid that scientists began to suspect it had more than one way of finding new animals to attack.

Sure enough, it did. As it turns out, the CWD prion doesn’t go down with its host-animal ship. Infected animals shed the prion in their urine, feces, and drool. Long after the sick deer has died, others can still contract CWD from the leaves they eat and the grass in which they stand.

As if that’s not bad enough, CWD has another trick up its sleeve: spontaneous generation. That is, it doesn’t take much damage to twist a healthy prion into a zombifying pathogen. The illness just pops up.

There are some treatments, including immersing infected tissue in an ozone bath. But that won't help when the problem is literally smeared across the landscape. "You cannot treat half of the continental United States with ozone," Zabel said.

And so, to combat this many-pronged assault on our wildlife, Zabel and his colleagues are getting aggressive. They recommend a controlled burn of infected areas of national parks in Colorado and Arkansas—a pilot study to determine if fire will be enough.

"If you eliminate the plants that have prions on the surface, that would be a huge step forward," he said. "I really don’t think it’s that crazy."

[h/t The New York Times]

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