Everything You Ever Wanted to Know About Black Holes

FAECIASP/NASA/Conicet of Argentina/Getty Images
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.


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.


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.


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.


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.


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.)


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.


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.


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.

Can You Tell an Author’s Identity By Looking at Punctuation Alone? A Study Just Found Out.


In 2016, neuroscientist Adam J Calhoun wondered what his favorite books would look like if he removed the words and left nothing but the punctuation. The result was a stunning—and surprisingly beautiful—visual stream of commas, question marks, semicolons, em-dashes, and periods.

Recently, Calhoun’s inquiry piqued the interest of researchers in the United Kingdom, who wondered if it was possible to identify an author from his or her punctuation alone.

For decades, linguists have been able to use the quirks of written texts to pinpoint the author. The process, called stylometric analysis or stylometry, has dozens of legal and academic applications, helping researchers authenticate anonymous works of literature and even nab criminals like the Unabomber. But it usually focuses on an author's word choices and grammar or the length of his or her sentences. Until now, punctuation has been largely ignored.

But according to a recent paper led by Alexandra N. M. Darmon of the Oxford Centre for Industrial and Applied Mathematics, an author’s use of punctuation can be extremely revealing. Darmon’s team assembled nearly 15,000 documents from 651 different authors and “de-worded” each text. “Is it possible to distinguish literary genres based on their punctuation sequences?” the researchers asked. “Do the punctuation styles of authors evolve over time?”

Apparently, yes. The researchers crafted mathematical formulas that could identify individual authors with 72 percent accuracy. Their ability to detect a specific genre—from horror to philosophy to detective fiction—was accurate more than half the time, clocking in at a 65 percent success rate.

The results, published on the preprint server SocArXiv, also revealed how punctuation style has evolved. The researchers found that “the use of quotation marks and periods has increased over time (at least in our [sample]) but that the use of commas has decreased over time. Less noticeably, the use of semicolons has also decreased over time.”

You probably don’t need to develop a powerful algorithm to figure that last bit out—you just have to crack open something by Dickens.

What Happens to Your Body If You Die in Space?


The coming decades should bring about a number of developments when it comes to blasting people into orbit and beyond. Private space travel continues to progress, with Elon Musk and Richard Branson championing civilian exploration. Professional astronauts continue to dock at the International Space Station (ISS) for scientific research. By the 2040s, human colonists could be making the grueling journey to Mars.

With increased opportunities comes the increased potential for misadventure. Though only 18 people have died since the emergence of intragalactic travel in the 20th century, taking more frequent risks may mean that coroners will have to list "space" as the site of death in the future. But since it's rare to find a working astronaut in compromised health or of an advanced age, how will most potential casualties in space meet their maker?

Popular Science posed this question to Chris Hadfield, the former commander of the ISS. According to Hadfield, spacewalks—a slight misnomer for the gravity-free floating that astronauts engage in outside of spacecraft—might be one potential danger. Tiny meteorites could slice through their protective suits, which provide oxygen and shelter from extreme temperatures. Within 10 seconds, water in their skin and blood would vaporize and their body would fill with air: Dissolved nitrogen near the skin would form bubbles, blowing them up like a dollar-store balloon to twice their normal size. Within 15 seconds, they would lose consciousness. Within 30 seconds, their lungs would collapse and they'd be paralyzed. The good news? Death by asphyxiation or decompression would happen before their body freezes, since heat leaves the body slowly in a vacuum.

This morbid scene would then have to be dealt with by the accompanying crew. According to Popular Science, NASA has no official policy for handling a corpse, but Hadfield said ISS training does touch on the possibility. As he explained it, astronauts would have to handle the the body as a biohazard and figure out their storage options, since there's really no prepared area for that. To cope with both problems, a commander would likely recommend the body be kept inside a pressurized suit and taken someplace cold—like where garbage is stored to minimize the smell.

If that sounds less than regal, NASA agrees. The company has explored the business of space body disposal before, and one proposition involves freeze-drying the stiff with liquid nitrogen (or simply the cold vacuum of space) so it can be broken up into tiny pieces of frozen tissue, which would occupy only a fraction of the real estate that a full-sized body would.

Why not eject a body, like Captain Kirk and his crew were forced to do with the allegedly dead Spock in 1982's Star Trek II: The Wrath of Khan? Bodies jettisoned into space without a rocket to change their trajectory would likely fall into the wake of the spacecraft. If enough people died on a long trip, it would create a kind of inverted funeral procession.

Even if safely landed on another planet, an astronaut's options don't necessarily improve. On Mars, cremation would likely be necessary to destroy any Earth-borne bacteria that would flourish on a buried body.

Like most everything we take for granted on Earth—eating, moving, and even pooping—it may be a long time before dying in space becomes dignified.

[h/t Popular Science]

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