Everything You Ever Wanted to Know About Black Holes

FAECIASP/NASA/Conicet of Argentina/Getty Images
FAECIASP/NASA/Conicet of Argentina/Getty Images

Black holes always seem to be in the news—especially when scientists reveal the first-ever photo of one, or when an Israeli researcher created an artificial black hole (sort of) in his laboratory.

Black holes are 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). 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 is a black hole created?

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.

Can black holes be seen?

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.

Can a black hole kill you?

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.

Can you escape a black hole?

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.)

When do black holes die?

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 reacted cautiously. Silke Weinfurter of the University of Nottingham in the 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.

How are black holes being studied?

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 of the (event) horizon?

On April 10, 2019, we got a glimpse of a black hole event horizon, thanks to the Event Horizon Telescope. With the combined power of the entire globe-spanning array of radio telescopes, astronomers produced a detailed picture of radiation emitted by gas and dust just before it crosses a black hole’s event horizon in the galaxy Messier 87, about 55 million light years from Earth.

The Event Horizon Telescope's next 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. 

Does the Full Moon Really Make People Act Crazy?

iStock.com/voraorn
iStock.com/voraorn

Along with Mercury in retrograde, the full moon is a pretty popular scapegoat for bad luck and bizarre behavior. Encounter someone acting strangely? Blame it on the lunar phases! It's said that crime rates increase and emergency rooms are much busier during the full moon (though a 2004 study debunked this claim). Plus, there's that whole werewolf thing. Why would this be? The reasoning is that the moon, which affects the ocean's tides, probably exerts a similar effect on us, because the human body is made mostly of water.

This belief that the moon influences behavior is so widely held—reportedly, even 80 percent of nurses and 64 percent of doctors think it's true, according to a 1987 paper published in the Journal of Emergency Medicine [PDF]—that in 2012 a team of researchers at Université Laval's School of Psychology in Canada decided to find out if mental illness and the phases of the moon are linked [PDF].

To test the theory, the researchers evaluated 771 patients who visited emergency rooms at two hospitals in Montreal between March 2005 and April 2008. The patients chosen complained of chest pains, which doctors could not determine a medical cause for the pains. Many of the patients suffered from panic attacks, anxiety and mood disorders, or suicidal thoughts.

When the researchers compared the time of the visits to the phases of the moon, they found that there was no link between the incidence of psychological problems and the four lunar phases, with one exception: in the last lunar quarter, anxiety disorders were 32 percent less frequent. "This may be coincidental or due to factors we did not take into account," Dr. Geneviève Belleville, who directed the team of researchers, said. "But one thing is certain: we observed no full-moon or new-moon effect on psychological problems."

So rest easy (or maybe not): If people seem to act crazy during the full moon, their behavior is likely pretty similar during the rest of the lunar cycle as well.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

Rare Harvest ‘Micromoon’ Will Appear on Friday the 13th

pattier/iStock via Getty Images
pattier/iStock via Getty Images

The first Friday the 13th of 2019 is coming this September, coinciding with a spooky full moon—and that unlucky event will also be a harvest micromoon, Newsweek reports. Here's everything you need to know about the lunar spectacle.

What is a harvest micromoon?

Harvest moon describes the full moon that appears in September. You may have heard that the harvest moon is larger and deeper in color than full moons that appear at different times of the year, but this isn't the case. The name harvest moon has nothing to do with its size or appearance. Many people observe the harvest moon just as it surfaces above the horizon—the time when it looks biggest due to the moon illusion, and reddish or orange-y through the filter of Earth's atmosphere. But as the moon climbs higher in the sky throughout the night, these characteristics fade away—just as they would at any other time of year.

This year, the harvest moon will actually look smaller compared to other full moons. On Friday, September 13, the celestial body reaches its apogee, or the point in its orbit where it's farthest from Earth. It has been dubbed a micromoon, which is the opposite of a supermoon.

When to see the harvest micromoon

Besides its scaled-down appearance, Friday's moon won't look any different from a regular full moon. But its rare conjunction with Friday the 13th makes it an event that anyone with a superstitious side won't want to miss. The moon will achieve maximum fullness at 12:33 a.m. the morning of Saturday, September 14 in the Eastern time zone (earlier the further west you go), but it will appear full and bright the previous and following nights. To catch the mini-moon on the 13th, look up late Friday night in a place with minimal light pollution. And if you want the full harvest moon effect, look to the horizon just after moonrise at 7:33 p.m.

[h/t Newsweek]

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