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. 

Neil Armstrong’s Spacesuit Will Go Back on Display for Apollo 11's 50th Anniversary

Phil Plait, Wikimedia Commons // CC BY-SA 2.0
Phil Plait, Wikimedia Commons // CC BY-SA 2.0

Neil Armstrong made history when he became the first person to walk on the Moon 50 years ago. Space exploration has changed since then, but the white space suit with the American flag patch that Armstrong wore on that first walk is still what many people think of when they picture an astronaut. Now, after sitting in storage for a decade, that iconic suit is ready to go on display, according to Smithsonian.

NASA donated Neil Armstrong's suit to the Smithsonian shortly after the Apollo 11 mission. For about 30 years, it was displayed at the National Air and Space Museum in Washington, D.C. Then, in 2006, the museum moved the artifact to storage to minimize damage.

Even away from the exhibit halls, the suit was deteriorating, and the Smithsonian knew it would need to be better preserved if it was to be shown to the public again. In 2015, the institution launched its first-ever Kickstarter campaign and raised more than $700,000 for conservation efforts.

After a multi-year preservation project, the suit will finally return to the museum floor on July 16, 2019—the date that marks 50 years since Apollo 11 launched. This time around, the suit will be displayed on a structure that was custom built to support its interior, protecting it from the weight of gravity. Climate-controlled air will flow through the gear to recreate the stable environment of a storage unit.

Even if you can't make it to the National Air and Space Museum to see Armstrong's space suit in person, soon you'll be able to appreciate it from home in a whole new way. The museum used various scanning techniques to create an intricate 3D model of the artifact. Once the scans are reconfigured for home computers, the Smithsonian's digitization team plans to make an interactive version of the digital model freely available on its website.

[h/t Smithsonian]

What Is the Kitchen Like on the International Space Station?

iStock/Elen11
iStock/Elen11

Clayton C. Anderson:

The International Space Station (ISS) does not really have a "kitchen" as many of us here on Earth might relate to. But, there is an area called the "galley" which serves the purpose of allowing for food preparation and consumption. I believe the term "galley" comes from the military, and it was used specifically in the space shuttle program. I guess it carried over to the ISS.

The Russian segment had the ONLY galley when I flew in 2007. There was a table for three, and the galley consisted of a water system—allowing us to hydrate our food packages (as needed) with warm (tepid) or hot (extremely) water—and a food warmer. The food warmer designed by the Russians was strictly used for their cans of food (about the size of a can of cat food in America). The U.S. developed a second food warmer (shaped like a briefcase) that we could use to heat the more "flexibly packaged" foodstuffs (packets) sent from America.

Later in the ISS lifetime, a second galley area was provided in the U.S. segment. It is positioned in Node 1 (Unity) and a table is also available there for the astronauts' dining pleasures. Apparently, it was added because of the increasing crew size experienced these days (6), to have more options. During my brief visit to ISS in 2010 (12 days or so) as a Discovery crewmember, I found the mealtimes to be much more segregated than when I spent five months on board. The Russians ate in the Russian segment. The shuttle astronauts ate in the shuttle. The U.S. ISS astronauts ate in Node 1, but often at totally different times. While we did have a combined dinner in Node 1 during STS-131 (with the Expedition 23 crew), this is one of the perceived negatives of the "multiple-galley" scenario. My long duration stint on ISS was highlighted by the fact that Fyodor Yurchikhin, Oleg Kotov, and I had every single meal together. The fellowship we—or at least I—experienced during those meals is something I will never, ever forget. We laughed, we argued, we celebrated, we mourned …, all around our zero-gravity "dinner table." Awesome stuff!

This post originally appeared on Quora. Click here to view.

Clayton "Astro Clay" Anderson is an astronaut, motivational speaker, author, and STEAM education advocate.

His award-winning book The Ordinary Spaceman, Astronaut Edition Fisher Space Pen, and new children's books A is for Astronaut; Blasting Through the Alphabet and It's a Question of Space: An Ordinary Astronaut's Answers to Sometimes Extraordinary Questions are available at www.AstroClay.com. For speaking events www.AstronautClayAnderson.com. Follow @Astro_Clay #WeBelieveInAstronauts

SECTIONS

arrow
LIVE SMARTER