But the black hole offered the means to gain a new perspective on this problem. It occurred when a few young up-and-comers in physics extended their analysis of a black hole’s properties. Stephen Hawking was one of them. Diagnosed at the age of twenty-one with amyotrophic lateral sclerosis, also known as Lou Gehrig’s disease, he had been expected to live no longer than two or three more years. He has now beat the odds by half a century. As an undergraduate at Oxford, Hawking was recognized as brilliant but underchallenged. He himself admits that it was his ensuing illness—the possibility of an early death—that put an end to his academic laziness.
Moving to Cambridge University for his doctoral studies, Hawking chose to specialize in cosmology at a time when it was more speculation than science, a risky choice. Yet, on obtaining his PhD in 1966, success was swift. First, he proved not only that the Big Bang didn’t just appear to emerge from an infinitely dense point of mass-energy but that there was no other way. Then he discovered a vital link between gravity and quantum mechanics, two fields completely incompatible before then. As he reported in his best-seller A Brief History of Time, this particular insight was initiated as he was turning in for the night. “One evening in November [1970] … I started to think about black holes as I was getting into bed. My disability makes this rather a slow process, so I had plenty of time.”
Continuing these mental ruminations, he eventually proved that a black hole’s event horizon must always increase—never decrease—when matter falls into it. This might seem obvious, given that a black hole by definition allegedly never gives anything back, but until then it was mathematically fuzzy. As Hawking put it, “There was no precise definition of which points in space-time lay inside a black hole and which lay outside.” Hawking defined it. He announced his discovery the following month at the Fifth Texas Symposium of Relativistic Astrophysics, held that year in Austin. The session devoted to black hole research was unexpectedly popular, attracting so many people that the organizers had to move it to a bigger auditorium.
The fact that a black hole’s surface area must always increase looked a lot like the law of entropy in classical physics. Entropy is the measure of a system’s disorderliness, how jumbled up it is. The higher the entropy, the greater the disarray. If things are left to themselves, entropy always increases. A rigid ice cube will melt and form a messy puddle, but without a refrigerator on hand to provide the energy, the untidy water can’t reorder itself into ice on its own. It remains a patchy pool of water. Similarly, a black hole can only increase the size of its event horizon as it gulps down more matter. It can never decrease its girth. But Hawking and his colleagues took this resemblance between entropy and the size of a black hole—the fact that both properties are constrained to increase only—as merely a nice analogy. It didn’t mean they were actually related in any way.
But one of John Wheeler’s students, Jacob Bekenstein, boldly decided that the connection was real—that the area of an event horizon was indeed a direct measure of the black hole’s entropy. On the face of it, this seemed like an outlandish proposition. From a classical perspective, a black hole is highly ordered. It gravitationally attracts everything within its reach and never gives its back. In fact, some wondered whether a black hole’s entropy was zero, meaning it was the highest organized state possible. Wasn’t all that matter getting compressed into an infinitesimally small point?
Because of this understanding, some warned Bekenstein that his research was headed in the wrong direction. But, as he later recalled, “I drew some comfort from Wheeler’s opinion that ‘black hole thermodynamics is crazy, perhaps crazy enough to work.’” It certainly attracted a lot of attention. There was a packed house when Bekenstein gave a seminar on his ongoing work at the University of Texas. “I suspect … ,” he wrote Wheeler, “that the excellent attendance was due not so much to the particular topic, but rather to the great glamour of black hole physics in general. Yes, black holes are the hottest things in physics (and astronomy), not least thanks to your own early efforts in spreading the idea.”
As Bekenstein continued to work out his calculations, the young student eventually came to see that the black hole would also have a temperature. But this is where Bekenstein drew the line. Even he tip-toed away from that comparison at first. It was universally accepted that a black hole holds on to everything it swallows. It emits nothing. So, it couldn’t have a “temperature.” Not a real temperature. That would imply that it was releasing radiation that could be measured by us as heat. “Such an identification can easily lead to all sorts of paradoxes, and is thus not useful,” concluded Bekenstein in his 1973 published paper. So, all the top theorists declared that the temperature of a black hole was “unambiguously zero.”
Stephen Hawking thought so, too. He was highly skeptical of Bekenstein’s black-hole-has-entropy scheme and planned to publish a paper proving its conclusion was wrong. “I was motivated partly by irritation with Bekenstein,” said Hawking in A Brief History of Time. Hawking felt that Bekenstein was misusing the earlier paper Hawking had written on the increasing area of an event horizon. “However,” admitted Hawking, “it turned out in the end that he was basically correct.”
Hawking was initially doubtful because anything with a large entropy should also be radiating. But a black hole, by its very definition, doesn’t let anything out of its grip. Or does it? The more Hawking looked into this problem, the more he was intrigued, leading him to one of his greatest theoretical triumphs.
Hawking’s outlook changed when he began looking at the black hole from a different perspective: from the viewpoint of an atom. His musings along this line were sparked during a visit to Moscow in the fall of 1973, where he talked with Yakov Zel’dovich and the Russian’s graduate student Alexei Starobinsky. These two men suggested that under special circumstances—that is, when a black hole rotates—it should convert that rotational energy into radiation, thus creating particles. This emission would continue until the spinning black hole wound down and stopped turning.
Stephen Hawking (American Institute of Physics Emilio Segrè Visual Archives, Physics Today Collection)
Devising his own mathematical attack on the problem, Hawking was surprised to discover that all black holes—spinning or not—would be radiating. As Hawking later put it, in one of the chapter titles of his popular book, “Black holes ain’t so black.”
Hawking announced his discovery in February 1974 at a symposium on quantum gravity, held at the Rutherford Laboratory near Oxford. His report was soon published in the journal Nature on March 1. Both his talk and paper were titled with the intriguing question, “Black Hole Explosions?” There was a reason that he mentioned explosions. In applying the laws of quantum mechanics to a black hole, Hawking found that black holes create and emit particles as if they were hot bodies. As a consequence, the black hole slowly decreases in mass and eventually disappears in a final blast! Such a finding turned black-hole physics upside down; a black hole, by definition, holds on to everything it swallows. It’s supposed to emit nothing and never go away.
Hawking estimated it would take far longer than the age of the universe for a regular black hole, weighing a few stellar masses, to evaporate entirely. For a stellar-mass black hole (or larger), such a decay would take more than 1066 years. But what if extremely small holes were created in the turbulence of the Big Bang, each containing 1015 grams or so, about the mass of a small mountain? They could be popping off right now. Hawking estimated that in its final breath—its last tenth of a second of life—such a “tiny” object would release the energy of one million megaton hydrogen bombs.
Needless to say, this idea did not enthrall his fellow physicists. Relativist Werner Israel says that it “aroused strong opposition almost as soon as it was in print. … Skepticism was prolonged and virtually unanimous.” When Hawking first announced his result at the February conference, it was greeted with total disbelief. At the end of the talk the chairman of the session, John Taylor from Kings College, Lo
ndon, claimed it was all nonsense. “Sorry, Stephen,” he said, “but this is absolute rubbish.”
But gradually, over the following two years, it came to be recognized that Hawking had made a startling breakthrough. “I was probably the most pleased,” said Bekenstein, “for it provided the missing pieces of black-hole thermodynamics.” The temperature of a black hole was not zero at all; it was the temperature of the radiation coming out of the hole, now known as “Hawking radiation.”
Hawking arrived at his conclusion by asking how a black hole might affect its surroundings on the submicroscopic scale. He concluded that space-time gets so twisted near a black hole that it enables pairs of particles (a nuclear particle and its antimatter mate) to pop into existence just outside the black hole. You could think of it as energy being extracted from the black hole’s intense gravitational field and then converted into matter.
But because we’re talking about physics on the tiniest of levels, the exact boundary line of the event horizon is now quite vague. So at times, one of the newly created particles can disappear into the black hole, never to return, while the remaining one remains outside and flies off. As a result, the hole’s total mass-energy is reduced a smidgen. This means the black hole is actually evaporating! Ever so slowly, particle by particle, the black hole is losing mass.
For stellar-size holes, this bizarre quantum-mechanical process is just about meaningless. As noted earlier, it would take trillions upon trillions of years for a regular black hole to shrink away to nothingness. Its temperature, gauged by the radiation coming out of it, would be less than a millionth of a degree above absolute zero. But Hawking suggested that the early universe, in the first turbulent moments of the Big Bang, might have manufactured a multitude of tiny black holes: mini–black holes. Like a ball rolling down a hill, the evaporation of such a mini–black hole would accelerate as time progresses. The more mass this tiny primordial object loses, the easier it is for the particles to escape. The hole ends up fizzling away faster and faster, until it reaches a cataclysmic end.
If the Big Bang did forge some mini–black holes, the smallest would have vanished before their dying light could catch our attention, but objects containing that mass of a mountain, yet compressed to the size of a proton, would be shedding the last of their mass at this very moment in a short and spectacular burst of gamma rays. No signals from such tiny holes have been detected with absolute certainty as yet, but astronomers continue to be on the lookout for that distinctive pop.
There is more to this story. Hawking’s revelation initiated an entirely new examination of the black hole and opened up questions about the known laws of physics. Bizarre as its behavior may seem, the black hole originated within classical equations of physics. Einstein’s general theory of relativity used the math of the nineteenth century, with space-time as his fundamental quantity. From that viewpoint, the black hole is a smooth and unbroken pit in the fabric of space-time. The event horizon is a point of no return, but there is no visible change in space-time at the transition. But Hawking showed that the black hole had an entirely different personality when examined on the submicroscopic scale. The event horizon is no longer smooth but more fuzzy and indistinct, as particles boil out of the vacuum—even violently as the black hole ages. It made physicists take a real, hard look at the black hole. Which was the real black hole? The version that arises out of Einstein’s theory or the quantum mechanical one? How could these two completely different views be reconciled?
For a while, some wondered whether the rules of quantum mechanics are altered within a black hole: that quantum effects, inside the event horizon, are somehow different from those we measure on the outside. But the physicists who are examining this cutting edge of black-hole science are coming to suspect that it’s general relativity that breaks down at the event horizon, much the way Newton’s laws faced a crisis when dealing with strong fields of gravity, such as those near the Sun or a massive neutron star. Einstein amended Newton, and now Einstein’s theory will likely need revision to reveal a black hole’s full character. Answers will arrive when physicists are able to join general relativity with quantum mechanics in that all-encompassing theory of quantum gravity.
Many have been trying for decades but are still far from success, yet there are hints on where it might lead. Many explorers into quantum gravity are coming to the conclusion that space-time itself—the central and core unit in Einstein’s theory—may not be fundamental at all. It could be that space-time emerges (as physicists like to put it) out of other sorts of “bits,” some kind of quantum grains that would be identified once a full theory of quantum gravity is in place. From this perspective, both space and time on the smallest of scales would have no meaning, just as a pointillist painting, built up from dabs of paint, cannot be fathomed close up. At that range, the painting looks like nothing more than a random array of dots. But as you move back, the dots begin to blend together and a recognizable picture slowly comes into focus. Likewise, space-time, the entity so familiar to us, might take form and reveal itself only when we scrutinize larger and larger scales. Space-time could be simply a matter of perception, present on the large scale but not on the smallest scale imaginable. You could think of space-time as “congealing” or “crystallizing” out of the chaotic quantum jumble that lies deep in the heart of the vacuum.
And it is at the black hole’s event horizon where this new vision might be revealed. For decades, only astrophysicists or general relativists looking for the ultimate test case for Einstein’s theory studied black holes. But now quantum physicists are mightily interested in black holes as well. They believe that clues to a unified theory of all the forces of nature may be found right there at the event horizon, that vital boundary where the microcosm of quantum mechanics directly meets the macrocosm of general relativity. Some recent models even suggest that any astronaut who dares to enter a black hole would not get the smooth entry past the event horizon, as described in chapter 10. Rather than an uneventful passage past the horizon and then a quick plummet to the singularity, they’d instead dramatically slam into a “firewall,” where space and time are breaking up into its fundamental units. A singularity would no longer be featured. Saved from a plunge to a singularity, the astronauts would still be recycled into quantum bits.
But no one knows for sure. Physicists searching for the “theory of everything,” such as string theory or loop quantum gravity, have no idea as yet what the final answer will reveal. Rather than a firewall, some other change (or no change at all) could arise as an event horizon is crossed. Toward the end of his life, John Wheeler held out the hope that the center of a black hole has a finite structure. He imagined that “the core of a black hole will prove to have some structure, albeit tiny beyond all imagining.”
Explorers of this puzzle are much like the astrophysicists in the 1960s who were struggling to understand how traditional methods of power generation could possibly fuel a quasar’s tremendous output, before realizing that the new kid on the block—the black hole that was hardly accepted as real—was operating as a dynamo, a real surprise to many.
Hawking in the 1970s began a conversation about the ultimate nature of black holes that continues to this day. Following in the footsteps of the pioneering quantum gravity theorists, he allowed more of his colleagues to see the profound connections between gravitation and quantum mechanics. Even though these two disparate laws of nature have yet to be officially joined, there are now fundamental signs that unity—the holy grail of physics—may well be achievable someday. And serving as physicists’ prime guide in this aspiration is the black hole.
Epilogue
The Hanford Site, a prime repository for nuclear waste in the United States, sprawls over hundreds of square miles of scrub desert in south-central Washington State. There resides the Laser Interferometer Gravitational-Wave Observatory operated by the California Institute of Technology and the Massachusetts Institute of Technology. It’s simply known as LIGO (pronounced LI
E-go) for short.
Standing alone on the vast plain, a landscape long ago carved flat by the immense outflow of an ancient glacial lake, the complex resembles a modern art museum inexplicably placed in the middle of nowhere. An exact duplicate, painted in the same hues of cream, blue, and silver gray, can be found in the pine forests of Livingston Parish in Louisiana, outside Baton Rouge. Together, with similar observatories set up (or setting up) in Italy, Japan, and India, they form one of the most advanced astronomical tools of the twenty-first century.
The signals all these observatories are seeking are waves of gravitational radiation, or more simply gravity waves, as they are better known in the popular media. Einstein first wrote on their possible existence in 1916 and 1918, shortly after he had introduced his general theory of relativity. He had recognized that just as electromagnetic waves, such as radio waves, are generated when electrical charges travel up and down an antenna, waves of gravitational radiation are produced when masses move about. Electromagnetic waves—be they visible light, infrared, or radio waves—generally reveal a celestial object’s physical condition, such as how hot it is, how old, and what it is made of. That’s what standard astronomical observatories have been doing for decades. Gravity waves, on the other hand, will convey an entirely different story. They will tell us about the titanic movements of massive celestial objects.
Gravity waves are literally quakes in the very fabric of space-time, rumbles that emanate from the most violent events the universe has to offer—a once-blazing star burning out and going supernova, the dizzying spin of neutron stars, or the cagey dance of two black holes whirling around each other, approaching closer and closer until they collide in a spectacular merger. It is in this way that astronomers hope to obtain direct proof of a black hole’s existence, the actual capture of a signal generated by the black hole itself. In this way, the black hole would obtain its ultimate affirmation.
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