Death by Black Hole: And Other Cosmic Quandaries - Neil deGrasse Tyson (2014)


Chapter 32. KNOCK ’EM DEAD

Ever since people discovered the bones of extinct dinosaurs, scientists have proffered no end of explanations for the disappearance of the hapless beasts. Maybe a torrid climate dried up the available sources of water, some say. Maybe volcanoes covered the land in lava and poisoned the air. Maybe Earth’s orbit and axis tilt brought on a relentless ice age. Maybe too many early mammals dined on too many dino eggs. Or maybe the meat-eating dinosaurs ate up all the vegetarian ones. Maybe the need to find water led to massive migrations that rapidly spread diseases. Maybe the real problem was a reconfiguration of landmasses, caused by tectonic shifts.

All these crises have one thing in common: the scientists who came up with them were well trained in the art of looking down.

Other scientists, however, trained in the art of looking up, began to make connections between Earth’s surface features and the visits of vagabonds from outer space. Maybe meteor impacts generated some of those features, such as Barringer Crater, that famous, mile-wide, bowl-shaped depression in the Arizona desert. In the 1950s, the American geologist Eugene M. Shoemaker and his associates discovered a kind of rock that forms only under short-lived, but extremely high, pressure—just what a fast-moving meteor would do. Geologists could finally agree that an impact caused the bowl (now sensibly called Meteor Crater), and Shoemaker’s discovery resurrected the nineteenth-century concept of catastrophism—the idea that changes to our planet’s skin can be caused by brief, powerful, destructive events.

Once the gates of speculation opened, people began to wonder whether the dinosaurs might have disappeared at the hands of a similar, but bigger, assault. Meet iridium: a metal rare on Earth but common in metallic meteorites and conspicuous in a 65-million-year-old layer of clay that appears at scores of sites around the world. That clay, dating to about the same time as the dinos checked out, marks the crime scene: the end of the Cretaceous. Now meet Chicxulub Crater, a 200-kilometer-wide depression at the edge of Mexico’s Yucatan Peninsula. It, too, is about 65 million years old. Computer simulations of climate change make it clear that any impact that could make that crater would thrust enough of Earth’s crust in the stratosphere that global climatic catastrophe would ensue. Who could ask for anything more? We’ve got the perpetrator, the smoking gun, and a confession.

Case closed.

Or is it?

Scientific inquiry shouldn’t stop just because a reasonable explanation has apparently been found. Some paleontologists and geologists remain skeptical about assigning Chicxulub the lion’s share—or even a substantial share—of responsibility for the dinos’ departure. Some think Chicxulub may have significantly predated the extinction. Furthermore, Earth was volcanically busy at about that time. Plus, other waves of extinction have swept across Earth without leaving craters and rare cosmic metals as calling cards. And not all bad things that arrive from space leave a crater. Some explode in midair and never make it to Earth’s surface.

So, besides impacts, what else might a restless cosmos have in store for us? What else could the universe send our way that might swiftly unravel the patterns of life on Earth?

SEVERAL SWEEPING EPISODES of mass extinction have punctuated the past half-billion years on Earth. The biggest are the Ordovician, about 440 million years ago; the Devonian, about 370 million; the Permian, about 250 million; the Triassic, about 210 million; and, of course, the Cretaceous, about 65 million. Lesser extinction episodes have taken place as well, at timescales of tens of millions of years.

Some investigators pointed out that, on average, an episode of note takes place every 25 million years or so. People who spend most of their time looking up are comfortable with phenomena that repeat at long intervals, and so astrophysicists decided it was our turn to name some killers.

Let’s give the Sun a dim and distant companion star, a few up-lookers said in the 1980s. Let’s declare its orbital period to be about 25 million years and its orbit to be extremely elongated, so that it spends most of its time too far from Earth to be detected. This companion would discombobulate the Sun’s distant reservoir of comets whenever it passed through their neighborhood. Legions of comets would jiggle loose from their stately orbits in the outer solar system, and the rate of impacts on Earth’s surface would vastly increase.

Therein was the genesis of Nemesis, the name given to this hypothetical killer star. Subsequent analyses of the extinction episodes have convinced most experts that the average time between catastrophes varies too greatly to signify anything truly periodic. But for a few years the idea was big news.

Periodicity wasn’t the only intriguing idea about death from outer space. Pandemics were another. The late English astrophysicist Sir Fred Hoyle and his longtime collaborator Chandra Wickramasinghe, now at Cardiff University in Wales, pondered whether Earth might occasionally pass through an interstellar cloud laden with microorganisms or be on the receiving end of similarly endowed dust from a passing comet. Such an encounter might give rise to a fast-spreading illness, they suggested. Worse yet, some of the giant clouds or dust trails might be real killers—bearing viruses with the power to infect and destroy a wide range of species. Of the many challenges to making this idea work, nobody knows how an interstellar cloud could manufacture and carry something as complex as a virus.

You want more? Astrophysicists have imagined a nearly endless spectrum of awesome catastrophes. Right now, for instance, the Milky Way galaxy and the Andromeda galaxy, a near twin of ours 2.4 million light-years up the road, are falling toward each other. As discussed earlier, in about 7 billion years they may collide, causing the cosmic equivalent of a train wreck. Gas clouds would slam into one another; stars would be cast hither and yon. If another star swung close enough to confound our gravitational allegiance to the Sun, our planet could get flung out of the solar system, leaving us homeless in the dark.

That would be bad.

Two billion years before that happens, however, the Sun itself will swell up and die of natural causes, engulfing the inner planets—including Earth—and vaporizing all their material contents.

That would be worse.

And if an interloping black hole comes too close to us, it will dine on the entire planet, first crumbling the solid Earth into a rubble pile by virtue of its unstoppable tidal forces. The remains would then be extruded though the fabric of space-time, descending as a long string of atoms through the black hole’s event horizon, down to its singularity.

But Earth’s geologic record never mentions any early close encounters with a black hole—no crumbling, no eating. And given that we expect a vanishingly low number of neighborhood black holes, I’d say we have more pressing issues of survival before us.

HOW ABOUT GETTING fried by waves of high-energy electromagnetic radiation and particles, spewed across space by an exploding star?

Most stars die a peaceful death, gently shedding their outer gases into interstellar space. But one in a thousand—the star whose mass is greater than about seven or eight times that of the Sun—dies in a violent, dazzling explosion called a supernova. If we found ourselves within 30 light-years of one of those, a lethal dose of cosmic rays—high-energy particles that shoot across space at almost the speed of light—would come our way.

The first casualties would be ozone molecules. Stratospheric ozone (O3) normally absorbs damaging ultraviolet radiation from the Sun. In so doing, the radiation breaks the ozone molecule apart into oxygen (O) and molecular oxygen (O2). The newly freed oxygen atoms can then join forces with other oxygen molecules, yielding ozone once again. On a normal day, solar ultraviolet rays destroy Earth’s ozone at the same rate as the ozone gets replenished. But an overwhelming high-energy assault on our stratosphere would destroy the ozone too fast, leaving us all in desperate need of sunblock.

Once the first wave of cosmic rays took out our defensive ozone, the Sun’s ultraviolet would sail clear down to Earth’s surface, splitting oxygen and nitrogen molecules as it went. For the birds, mammals, and other residents of Earth’s surface and airspace, that would be unpleasant news indeed. Free oxygen atoms and free nitrogen atoms would readily combine. One product would be nitrogen dioxide, a component of smog, which would darken the atmosphere and cause the temperature to plummet. A new ice age might dawn even as the ultraviolet rays slowly sterilized Earth’s surface.

BUT THE ULTRAVIOLET blasted in every direction by a supernova is just a mosquito bite compared to the gamma rays let loose from a hypernova.

At least once a day, a brief burst of gamma rays—the highest of high-energy radiation—unleashes the energy of a thousand supernovas somewhere in the cosmos. Gamma-ray bursts were accidentally discovered in the 1960s by U.S. Air Force satellites, launched to detect radiation from any clandestine nuclear-weapons tests the Soviet Union might have conducted in violation of the 1963 Limited Test Ban Treaty. What the satellites found instead were signals from the universe itself.

At first nobody knew what the bursts were or how far away they took place. Instead of clustering along the plane of the Milky Way’s main disk of stars and gas, they came from every direction on the sky—in other words, from the entire cosmos. Yet surely they had to be happening nearby, at least within a galactic diameter or so from us. Otherwise, how was it possible to account for all the energy they registered here on Earth?

In 1997 an observation made by an orbiting Italian x-ray telescope settled the argument: gamma-ray bursts are extremely distant extra-galactic events, perhaps signaling the explosion of a single supermassive star and the attendant birth of a black hole. The telescope had picked up the x-ray “afterglow” of a now-famous burst, GRB 970228. But the x-rays were “redshifted.” Turns out, this handy feature of light and the expanding universe enables astrophysicists to make a fairly accurate determination of distance. The afterglow of GRB 970228, which reached Earth on February 28, 1997, was clearly coming from halfway across the universe, billions of light-years away. The following year Bohdan Paczynski, a Princeton astrophysicist, coined the term “hypernova” to describe the source of such bursts. Personally, I would have voted for “super-duper supernova.”

A hypernova is the one supernova in 100,000 that produces a gamma-ray burst, generating in a matter of moments the same amount of energy as our Sun would emit if it shone at its present output for a trillion years. Barring the influence of some undiscovered law of physics, the only way to achieve the measured energy is to beam the total output of the explosion in a narrow ray—much the way all the light from a flashlight bulb gets channeled by the flashlight’s parabolic mirror into one strong, forward-pointing beam. Pump a supernova’s power through a narrow beam, and anything in the beam’s path will get the full brunt of the explosive energy. Meanwhile, whoever does not fall in the beam’s path remains oblivious. The narrower the beam, the more intense the flux of its energy and the fewer the cosmic occupants who will see it.

What gives rise to these laserlike beams of gamma rays? Consider the original supermassive star. Not long before its death from fuel starvation, the star jettisons its outer layers. It becomes cloaked in a vast, cloudy shell, possibly augmented by pockets of gas left over from the cloud that originally spawned the star. When the star finally collapses and explodes, it releases stupendous quantities of matter and prodigious quantities of energy. The first assault of matter and energy punches through weak points in the shell of gas, enabling the succeeding matter and energy to funnel through that same point. Computer models of this complicated scenario suggest that the weak points are typically just above the north and south poles of the original star. When seen from beyond the shell, two powerful beams travel in opposite directions, headed toward all gamma-ray detectors (test-ban-treaty detectors or otherwise) that happen to lie in their path.

Adrian Melott, an astronomer at the University of Kansas, and an interdisciplinary crew of colleagues assert that the Ordovician extinction may well have been caused by a face-to-face encounter with a nearby gamma-ray burst. A quarter of Earth’s families of organisms perished at that time. And nobody has turned up evidence of a meteor impact contemporary with the event.

WHEN YOU’RE A hammer (as the saying goes), all your problems look like nails. If you’re a meteorite expert pondering the sudden extinction of boatloads of species, you’ll want to say an impact did it. If you’re an igneous petrologist, volcanoes did it. If you’re into spaceborne bioclouds, an interstellar virus did it. If you’re a hypernova expert, gamma rays did it.

No matter who is right, one thing is certain: whole branches in the tree of life can go extinct almost instantly.

Who survives these assaults? It helps to be small and meek. Microorganisms tend to do well in the face of adversity. More important, it helps if you live where the Sun don’t shine—on the bottom of the ocean, in the crevices of buried rocks, in the clays and soils of farms and forests. The vast underground biomass survives. It is they who inherit the Earth again, and again, and again.