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Everything You Ever Wanted to Know About Black Holes

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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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15 Subatomic Word Origins
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In July 2017, researchers at the European Organization for Nuclear Research (CERN) found evidence for a new fundamental particle of the universe: Ξcc++, a special kind of Xi baryon that may help scientists better understand how quarks are held together. Is that Greek to you? Well, it should be. The names for many of the particles that make up the universe—as well as a few that are still purely theoretical—come from ancient Greek. Here’s a look at 15 subatomic etymologies.

1. ION

An ion is any atom or molecule with an overall electric charge. English polymath William Whewell suggested the name in an 1834 letter to Michael Faraday, who made major discoveries in electromagnetism. Whewell based ion on the ancient Greek verb for “go” (ienai), as ions move towards opposite charges. Faraday and Whewell had previously considered zetode and stechion.

2. ELECTRON

George Stoney, an Anglo-Irish physicist, introduced the term electron in 1891 as a word for the fundamental unit of charge carried by an ion. It was later applied to the negative, nucleus-orbiting particle discovered by J. J. Thomson in 1897. Electron nabs the -on from ion, kicking off the convention of using -on as an ending for all particles, and fuses it with electric. Electric, in turn, comes from the Greek for “amber,” in which the property was first observed. Earlier in the 19th century, electron was the name for an alloy of gold and silver.

3. PROTON

The electron’s counterpart, the positively charged proton in the nuclei of all atoms, was named by its discoverer, Ernest Rutherford. He suggested either prouton or proton in honor of William Prout, a 19th-century chemist. Prout speculated that hydrogen was a part of all other elements and called its atom protyle, a Greek coinage joining protos ("first") and hule ("timber" or "material") [PDF]. Though the word had been previously used in biology and astronomy, the scientific community went with proton.

4. NEUTRON

Joining the proton in the nucleus is the neutron, which is neither positive nor negative: It’s neutral, from the Latin neuter, “neither.” Rutherford used neutron in 1921 when he hypothesized the particle, which James Chadwick didn’t confirm until 1932. American chemist William Harkins independently used neutron in 1921 for a hydrogen atom and a proton-electron pair. Harkins’s latter application calls up the oldest instance of neutron, William Sutherland’s 1899 name for a hypothetical combination of a hydrogen nucleus and an electron.

5. QUARK

Protons and neutrons are composed of yet tinier particles called quarks. For their distinctive name, American physicist Murray Gell-Mann was inspired in 1963 by a line from James Joyce’s Finnegan’s Wake: “Three quarks for Muster Mark.” Originally, Gell-Mann thought there were three types of quarks. We now know, though, there are six, which go by names that are just as colorful: up, down, charm, strange, top, and bottom.

6. MESON

Made up of a quark and an antiquark, which has identical mass but opposite charge, the meson is a short-lived particle whose mass is between that of a proton and an electron. Due to this intermediate size, the meson is named for the ancient Greek mesos, “middle.” Indian physicist Homi Bhabha suggested meson in 1939 instead of its original name, mesotron: “It is felt that the ‘tr’ in this word is redundant, since it does not belong to the Greek root ‘meso’ for middle; the ‘tr’ in neutron and electron belong, of course, to the roots ‘neutr’ and ‘electra’.”

7., 8., AND 9. BOSON, PHOTON, AND GLUON

Mesons are a kind of boson, named by English physicist Paul Dirac in 1947 for another Indian physicist, Satyendra Nath Bose, who first theorized them. Bosons demonstrate a particular type of spin, or intrinsic angular momentum, and carry fundamental forces. The photon (1926, from the ancient Greek for “light”) carries the electromagnetic force, for instance, while the gluon carries the so-called strong force. The strong force holds quarks together, acting like a glue, hence gluon.

10. HADRON

In 2012, CERN’s Large Hadron Collider (LHC) discovered a very important kind of boson: the Higgs boson, which generates mass. The hadrons the LHC smashes together at super-high speeds refer to a class of particles, including mesons, that are held together by the strong force. Russian physicist Lev Okun alluded to this strength by naming the particles after the ancient Greek hadros, “large” or “bulky,” in 1962.

11. LEPTON

Hadrons are opposite, in both makeup and etymology, to leptons. These have extremely tiny masses and don’t interact via the strong force, hence their root in the ancient Greek leptos, “small” or “slender.” The name was first suggested by the Danish chemist Christian Møller and Dutch-American physicist Abraham Pais in the late 1940s. Electrons are classified as leptons.

12. BARYON

Another subtype of hadron is the baryon, which also bears the stamp of Abraham Pais. Baryons, which include the more familiar protons and neutrons, are far more massive, relatively speaking, than the likes of leptons. On account of their mass, Pais put forth the name baryon in 1953, based on the ancient Greek barys, “heavy” [PDF].

13. AXION

Quirky Murray Gell-Mann isn't the only brain with a sense of humor. In his 2004 Nobel Prize lecture, American physicist Frank Wilczek said he named a “very light, very weakly interacting” hypothetical particle the axion back in 1978 “after a laundry detergent [brand], since they clean up a problem with an axial current” [PDF].

14. TACHYON

In ancient Greek, takhys meant “swift,” a fitting name for the tachyon, which American physicist Gerald Feinberg concocted in 1967 for a hypothetical particle that can travel faster than the speed of light. Not so fast, though, say most physicists, as the tachyon would break the fundamental laws of physics as we know them.

15. CHAMELEON

In 2003, the American physicist Justin Khoury and South African-American theoretical physicist Amanda Weltman hypothesized that the elusive dark energy may come in the form of a particle, which they cleverly called the chameleon. Just as chameleons can change color to suit their surroundings, so the physical characteristics of the chameleon particle change “depending on its environment,” explains Symmetry, the online magazine dedicated to particle physics. Chameleon itself derives from the ancient Greek khamaileon, literally “on-the-ground lion.”

For more particle names, see Symmetry’s “A Brief Etymology of Particle Physics,” which helped provide some of the information in this list.

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Space
Look Up! The Orionid Meteor Shower Peaks This Weekend
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October is always a great month for skywatching. If you missed the Draconids, the first meteor shower of the month, don't despair: the Orionids peak this weekend. It should be an especially stunning show this year, as the Moon will offer virtually no interference. If you've ever wanted to get into skywatching, this is your chance.

The Orionids is the second of two meteor showers caused by the debris field left by the comet Halley. (The other is the Eta Aquarids, which appear in May.) The showers are named for the constellation Orion, from which they seem to originate.

All the stars are lining up (so to speak) for this show. First, it's on the weekend, which means you can stay up late without feeling the burn at work the next day. Tonight, October 20, you'll be able to spot many meteors, and the shower peaks just after midnight tomorrow, October 21, leading into Sunday morning. Make a late-night picnic of the occasion, because it takes about an hour for your eyes to adjust to the darkness. Bring a blanket and a bottle of wine, lay out and take in the open skies, and let nature do the rest.

Second, the Moon, which was new only yesterday, is but a sliver in the evening sky, lacking the wattage to wash out the sky or conceal the faintest of meteors. If your skies are clear and light pollution low, this year you should be able to catch about 20 meteors an hour, which isn't a bad way to spend a date night.

If clouds interfere with your Orionids experience, don't fret. There will be two more meteor showers in November and the greatest of them all in December: the Geminids.

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