Death by Black Hole: And Other Cosmic Quandaries (26 page)

G
alaxies are phenomenal objects in every way. They are the fundamental organization of visible matter in the universe. The universe contains as many as a hundred billion of them. They each commonly pack hundreds of billions of stars. They can be spiral, elliptical, or irregular in shape. Most are photogenic. Most fly solo in space, while others orbit in gravitationally linked pairs, familial groups, clusters, and superclusters.

The morphological diversity of galaxies has prompted all manner of classification schemes that supply a conversational vocabulary for astrophysicists. One variety, the “active” galaxy, emits an unusual amount of energy in one or more bands of light from the galaxy’s center. The center is where you will find a galactic engine. The center is where you will find a supermassive black hole.

The zoo of active galaxies reads like the manifest for a carnival grab-bag: Starburst galaxies, BL Lacertae galaxies, Seyfert galaxies (types I and II), blazars, N-galaxies, LINERS, infrared galaxies, radio galaxies, and of course, the royalty of active galaxies—quasars. The extraordinary luminosities of these elite galaxies derive from mysterious activity within a small region buried deep within their nucleus.

Quasars, discovered in the early 1960s, are the most exotic of them all. Some are a thousand times as luminous as our own Milky Way galaxy, yet their energy hails from a region that would fit comfortably within the planetary orbits of our solar system. Curiously, none are nearby. The closest one sits about 1.5 billion light-years away—its light has been traveling for 1.5 billion years to reach us. And most quasars hail from beyond 10 billion light-years. Possessed of small size and extreme distance, on photographs one can hardly distinguish them from the pointlike images left by local stars in our own Milky Way, leaving visible-light telescopes quite useless as tools of discovery. The earliest quasars were, in fact, discovered using radio telescopes. Since stars do not emit copious amounts of radio waves, these radio-bright objects were a new class of something or other, masquerading as a star. In the we-call-them-as-we-see-them tradition among astrophysicists, these objects were dubbed Quasi-Stellar Radio Sources, or more affectionately, “quasars.”

What manner of beast are they?

One’s ability to describe and understand a new phenomenon is always limited by the contents of the prevailing scientific and technological toolbox. An eighteenth-century person who was briefly, but unwittingly, thrust into the twentieth century would return and describe a car as a horse-drawn carriage without the horse and a lightbulb as a candle without the flame. With no knowledge of internal combustion engines or electricity, a true understanding would be remote indeed. With that as a disclaimer, allow me to declare that we think we understand the basic principles of what drives a quasar. In what has come to be known as the “standard model,” black holes have been implicated as the engine of quasars and of all active galaxies. Within a black hole’s boundary of space and time—its event horizon—the concentration of matter is so great that the velocity needed to escape exceeds the speed of light. Since the speed of light is a universal limit, when you fall into a black hole, you fall in for good, even if you’re made of light.

HOW, MIGHT YOU ASK
, can something that emits no light power something that emits more light than anything else in the universe? In the late 1960s and 1970s, it didn’t take long for astrophysicists to discover that the exotic properties of black holes made remarkable additions to the theorists’ toolbox. According to some well-known laws of gravitational physics, as gaseous matter funnels toward a black hole, the matter must heat up and radiate profusely before it descends through the event horizon. The energy comes from the efficient conversion of gravity’s potential energy into heat.

While not a household notion, we have all seen gravitational potential energy get converted at some time in our terrestrial lives. If you have ever dropped a dish to the floor and broken it, or if you have ever nudged something out the window that splattered on the ground below, then you understand the power of gravitational potential energy. It’s simply untapped energy endowed by an object’s distance from wherever it might hit if it fell. When objects fall, they normally gain speed. But if something stops the fall, all the energy the object had gained converts to the kind of energy that breaks or splatters things. Therein is the real reason why you are more likely to die if you jump off a tall building instead of a short building.

If something prevents the object from gaining speed yet the object continues to fall, then the converted potential energy reveals itself some other way—usually in the form of heat. Good examples include space vehicles and meteors when they heat up while descending through Earth’s atmosphere: they want to speed up, but air resistance prevents it. In a now-famous experiment, the nineteenth-century English physicist James Joule created a device that stirred a jar of water with rotating paddles by the action of falling weights. The potential energy of the weights was transferred into the water and successfully raised its temperature. Joule describes his effort:

The paddle moved with great resistance in the can of water, so that the weights (each of four pounds) descended at the slow rate of about one foot per second. The height of the pulleys from the ground was twelve yards, and consequently, when the weights had descended through that distance, they had to be wound up again in order to renew the motion of the paddle. After this operation had been repeated sixteen times, the increase of the temperature of the water was ascertained by means of a very sensible and accurate thermometer…. I may therefore conclude that the existence of an equivalent relation between heat and the ordinary forms of mechanical power is proved…. If my views are correct, the temperature of the river Niagara will be raised about one fifth of a degree by its fall of 160 feet.
(Shamos 1959, p. 170)

 

Joule’s thought-experiment refers, of course, to the great Niagara Falls. But had he known of black holes, he might have said instead, “If my views are correct, the temperature of gas funneled toward a black hole will be raised a million degrees by its fall of a billion miles.”

AS YOU MIGHT
suspect, black holes enjoy a prodigious appetite for stars that wander too close. A paradox of galactic engines is that their black holes must eat to radiate. The secret to powering the galactic engine lies in a black hole’s ability to ruthlessly and gleefully rip apart stars before they cross the event horizon. The tidal forces of gravity for a black hole elongate the otherwise spherical stars in much the same way that the Moon’s tidal forces elongate Earth’s oceans to create high and low oceanic tides. Gas that was formerly part of stars (and possibly ordinary gas clouds) cannot simply gain speed and fall in; the gas of previously shredded stars impedes wanton free fall down the hole. The result? A star’s gravitational potential energy gets converted to prodigious levels of heat and radiation. And the higher the gravity of your target, the more gravitational potential energy was available to convert.

Faced with the proliferation of words to describe oddball galaxies, the late Gerard de Vaucouleurs (1983), a consummate morphologist, was quick to remind the astronomical community that a car that has been wrecked does not all of a sudden become a different kind of car. This car-wreck philosophy has led to a standard model of active galaxies that largely unifies the zoo. The model is endowed with enough tweakable parts to explain most of the basic, observed features. For example, the funneling gas often forms an opaque rotating disk before it descends through the event horizon. If the outward flow of radiation cannot penetrate the disk of accreted gas, then radiation will fly out from above and below the disk to create titanic jets of matter and energy. The observed properties of the galaxy will be different if the galaxy’s jet happens to be pointing toward you or sideways to you—or if the ejected material moves slowly or at speeds close to the speed of light. The thickness and chemical composition of the disk will also influence its appearance as well as the rate at which stars are consumed.

To feed a healthy quasar requires that its black hole eat up to ten stars per year. Other less-active galaxies from our carnival shred many fewer stars per year. For many quasars, their luminosity varies on time scales of days and even hours. Allow me to impress you with how extraordinary this is. If the active part of a quasar were the size of the Milky Way (100,000 light-years across) and if it all decided to brighten at once, then you would first learn about it from the side of the galaxy that was closest to you, and then 100,000 years later the last part of the galaxy’s light would reach you. In other words, it would take 100,000 years for you to observe the quasar brightening fully. For a quasar to brighten within hours means that the dimensions of the engine cannot be greater than light-hours across. How big is that? About the size of the solar system.

With a careful analysis of the light fluctuations in all bands, a crude, but informative three-dimensional structure can be deduced for the surrounding material. For example, the luminosity in x-rays might vary over a time scale of hours but the red light might vary over weeks. The comparison allows you to conclude that the red light-emitting part of the active galaxy is much larger than the x-ray emitting part. This exercise can be invoked for many bands of light to derive a remarkably complete picture of the system.

If most of this action takes place during the early universe in distant quasars, then why does it no longer happen? Why are there no local quasars? Do dead quasars lurk under our noses?

Good explanations are available. The most obvious is that the core of local galaxies ran out of stars to feed the engine, having vacuumed up all stars whose orbits came too close to the black hole. No more food, no more prodigious regurgitations.

A more interesting shut-off mechanism comes from what happens to the tidal forces as the black hole’s mass (and event horizon) grows and grows. As we will see later in this section, tidal forces have nothing to do with the total gravity felt by an object—what matters is the difference in gravity across it, which increases dramatically as you near an object’s center. So large, high-mass black holes actually exert lower tidal forces than the smaller, low-mass black holes. No mystery here. The Sun’s gravity on Earth dwarfs that of the Moon’s on Earth yet the proximity of the Moon enables it to exert considerably higher tidal forces at our location, a mere 240,000 miles away.

It’s possible, then, for a black hole to eat so much that its event horizon grows so large that its tidal forces are no longer sufficient to shred a star. When this happens, all of the star’s gravitational potential energy converts to the star’s speed and the star gets eaten whole as it plunges past the event horizon. No more conversion to heat and radiation. This shut-off valve kicks in for a black hole of about a billion times the mass of the Sun.

These are powerful ideas that do indeed offer a rich assortment of explanatory tools. The unified picture predicts that quasars and other active galaxies are just early chapters in the life of a galaxy’s nucleus. For this to be true, specially exposed images of quasars should reveal the surrounding fuzz of a host galaxy. The observational challenge is similar to that faced by solar system hunters who try to detect planets hidden in the glare of their host star. The quasar is so much brighter than the surrounding galaxy that special masking techniques must be used to detect anything other than the quasar itself. Sure enough, nearly all high-resolution images of quasars reveal surrounding galaxy fuzz. The several exceptions of uncloaked quasars continue to confound the expectations of the standard model. Or is it that the host galaxies simply fall below the detection limits?

The unified picture also predicts that quasars would eventually shut themselves off. Actually, the unified picture must predict this because the absence of nearby quasars requires it. But it also means that black holes in galactic nuclei should be common, whether or not the galaxy has an active nucleus. Indeed, the list of nearby galaxies that contain dormant supermassive black holes in their nuclei is growing longer by the month and includes the Milky Way. Their existence is betrayed through the astronomical speeds that stars achieve as they orbit close (but not too close) to the black hole itself.

Fertile scientific models are always seductive, but one should occasionally ask whether the model is fertile because it captures some deep truths about the universe or because it was constructed with so many tunable variables that you can explain anything at all. Have we been sufficiently clever today, or are we missing a tool that will be invented or discovered tomorrow? The English physicist Dennis Sciama knew this dilemma well when he noted:

Since we find it difficult to make a suitable model of a certain type, Nature must find it difficult too. This argument neglects the possibility that Nature may be cleverer than we are. It even neglects the possibility that we may be cleverer to-morrow than we are to-day.
(1971, p. 80)

 

E
ver 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 (O
₃
) normally absorbs damaging ultraviolet radiation from the Sun. In so doing, the radiation breaks the ozone molecule apart into oxygen (O) and molecular oxygen (O
₂
). 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.