At the March meeting that year of the American Astronomical Society, then being held in Baton Rouge, Louisiana, Giacconi boldly suggested Cygnus X-1 might be a black hole. With so many neutron stars being found with more confidence, the thought of black holes existing became easier to contemplate. The day after Giacconi’s announcement, a headline in the New York Times, splashed across the top of page 20, proclaimed, “An X-Ray Scanning Satellite May Have Discovered a ‘Black Hole’ in Space.” Notice that quotation marks were bracketing the term as late as 1971. The object still seemed too strange to be true.
Follow-up work by both radio and optical astronomers at last pinpointed the source. Observers determined that Cygnus X-1’s powerful X-rays were coming from a double-star system, in which a giant blue star (with the rather mundane tag HDE 226868, for its number in the Henry Draper extended star catalog) is coupled with a dark, invisible companion. The blue supergiant closely orbits its unseen partner once every 5.6 days, a tempo that allowed astronomers to apply Newton’s laws and determine that the unseen companion must have a mass beyond that of our Sun. By late 1972 orbital measurements suggested that the mass was at least ten times greater than our Sun’s—too massive for a neutron star and so a prime candidate to be a black hole. (Current estimates put its weight at around fifteen Suns.) Invisibility plus sizable mass—coupled with the rapidity of its X-ray fluctuations suggesting a tiny size—added up to black hole with far more surety. It became astronomy’s first prime suspect.
If you could somehow hover above Cygnus X-1, situated about six thousand light-years away, you would witness a gaseous whirlpool of enormous dimensions. Observations suggest that the black hole is pulling matter off of its generously endowed companion and forming a disk of gas around itself. This disk is flattened due to both centrifugal and gravitational forces. Like satellites orbiting the Earth, this material does not fall straight into the hole. Instead, it orbits the space-time drain in tighter and tighter spirals. Wheeler once compared it to traffic converging on a sports stadium from all directions and becoming more and more closely packed as the cars approach their destination.
And this has definite consequences. As the gas gets squeezed further and further, its temperature rises tremendously. Heated to tens of millions of degrees, the hot gases start to emit copious amounts of X-ray energy. This is the radiation that X-ray telescopes spy before the matter is drawn into the gravitational abyss and lost to our view. It may take weeks or even months for any one blob of gas to travel the few million miles from the outer edges of the disk to the point of no return. But in its last moments, the gas swirls around the hole thousands of times each second, possibly causing those rapid X-ray fluctuations.
An illustration of the black hole Cygnus X-1 stealing gas from the atmosphere of its companion star. This gas first orbits the space-time drain, emitting in the process large amounts of energy, before it is eventually swallowed by the black hole. (NASA/CXC/M. Weiss)
Of course, this scenario took time to formulate and prove. The first claim that Cygnus X-1 might be a black hole was fraught with peril. “Show me the evidence” was the cry on nearly every astronomer’s lips. Much of the proof was circumstantial, more a game of connect-the-dots than irrefutable confirmation.
This made the prospect both exciting and controversial, so much so that Stephen Hawking and Kip Thorne made an infamous bet at Caltech in December 1974 on whether Cygnus X-1 was truly a black hole. Hawking bet against, Thorne for. The agreement, handwritten on one page of stationery and with racy US and British magazines at stake, declares:
Whereas Stephen Hawking has such a large investment in General Relativity and Black Holes and desires an insurance policy, and whereas Kip Thorne likes to live dangerously without an insurance policy,
Therefore be it resolved that Stephen Hawking bets 1 year’s subscription to “Penthouse” as against Kip Thorne’s wager of a 4-year subscription to “Private Eye”, that Cygnus X 1 does not contain a black hole of mass above the Chandra-sekhar limit.
Given his more generous bet, Thorne appeared to be four times more confident.
Verification was slow, but advances in X-ray astronomy helped. The Uhuru satellite was succeeded by astronomy’s first true X-ray telescope in 1978. Instead of radiation counters that simply registered signal strengths, the spaceborne Einstein observatory housed a set of nested mirrors that actually focused the X-rays, allowing the radiation to be recorded as images as clear as any ground-based optical telescope. By 1990, according to Thorne, Cygnus X-1 was looking more and more like a black hole, with 95 percent confidence. That was a high-enough threshold for Hawking to cry uncle. “Late one night in June 1990, while I was in Moscow working on research with Soviet colleagues,” recounted Thorne, “Stephen and an entourage of family, nurses, and friends broke into my office at Caltech, found the framed bet, and wrote a concessionary note on it with validation by Stephen’s thumbprint.” To the dismay of his wife, Carolee Winstein, Thorne won the Penthouse subscription.
For some, even stronger evidence for black holes had arrived from the far universe, as astronomers continued their examination of both quasars and radio galaxies with every spectral weapon in their arsenal: optical, X-ray, and especially radio.
In optical photographs, radio galaxies can appear quite boring. But radio telescopes, as mentioned earlier, revealed them to have a bewildering architecture. Astronomers saw that the visible galaxy is but a smudge caught between two sizable lobes of radio emission. Looking like a pair of water wings, these lobes stretch out for hundreds of thousands of light-years beyond the edges of the visible galaxy. By the early 1970s, a number of British theorists, including Martin Rees and Roger Blandford, concluded that some kind of plasma beams, monstrous ones at that, had to be responsible for pumping energy out into the lobes.
The desire to locate this river of plasma spurred countries to build ever bigger radio telescope networks, such as the Very Large Array (now known as the Jansky Very Large Array), twenty-seven radio dishes aligned in the shape of a Y over the plains of New Mexico. Together they can simulate a single radio telescope as large as the city of Dallas. With the increased power and resolution of such arrays, astronomers confirmed what the British theorists had suspected: the radio images displayed a sort of umbilical cord running from the nucleus of a radio galaxy out to its lobes: two thin beams of energetic, charged particles, each shooting out of the galactic core in opposite directions, at speeds of tens of thousands of miles per second.
A powerful jet of electrons and subatomic particles streaming from the supermassive black hole situated in the center of the giant elliptical galaxy M 87, situated some fifty million light-years from Earth. (NASA and the Hubble Heritage Team at the Space Telescope Science Institute and AURA)
Like the fierce stream of a fire hose, these cosmic jets can bore through the thin gases found in intergalactic space, until they ram into a denser region of gas, as if the stream had come up against a brick wall. The particles in the jets then fly off, filling up the gigantic lobe regions.
The next question was a natural: What could possibly keep these cosmic jets flowing? Theorists agreed the engine had to be pretty special. First of all, the power source had to be fairly stable, in order for the jets to maintain their orientation over millions of years. And radio “pictures” were getting so good that they were able to zoom into the very heart of the galaxy’s core, showing a tiny spot whose brightness could fluctuate over days or weeks, which suggested that the engine was as small as our solar system. Moreover, this power source had to somehow eject its energy into two, oppositely directed beams.
There was only one power source that fulfilled all the design specifications: a spinning black hole formed from the collapse of millions, even up to billions, of suns. These suns may have first huddled together as an extraordinarily dense herd of stars, a type of cluster that could easily have developed early on in a crowded galactic center. Moreover, these first-generation stars, formed out of the pristine hydrogen
and helium forged in the Big Bang and devoid of the heavier elements made later, were likely very big: so massive that they lived fast and died young as black holes. Driven by the inward force of gravity, these many holes could have ultimately coalesced into one giant black hole, which continued to grow over the eons as it “ate” any available stars or gas that got too close.
Or maybe a host of “baby” galaxies, building blocks in essence, coalesced into a bigger galaxy and during the turbulent chaos of this merger directed huge amounts of gas toward the center, which accumulated to incredibly high densities—so dense that the gas didn’t turn into stars but directly collapsed into a massive black hole, serving as the seed for the supermassive black hole as it continued to grow and grow. The ultimate size of the supermassive black hole appears to depend on the mass of the galaxy’s central bulge. Astronomers have found a direct correlation: the higher the bulge mass, the more massive the central black hole.
Whatever the giant hole’s origins, theorists soon recognized that such an object was the most efficient energy generator for an active galaxy. When matter is thrown down a deep gravity well, particles are accelerated to velocities near the speed of light. Such gravity-driven motors can generate up to a hundred times more energy than nuclear-fired engines.
You might ask, “But doesn’t a black hole completely eat up everything that enters it? How does any energy survive to get out?” The answer lies in picturing the environment surrounding the black hole. Throughout the 1960s, as noted earlier, a number of theorists, including Yakov Zel’dovich, Igor Novikov, Edwin Salpeter, and the British astrophysicist Donald Lynden-Bell, realized that as stars and gas are drawn in by the powerful gravitational pull of the black hole, it will form a doughnutlike ring around itself. Like water building up and swirling around a drain, the plummeting gas forms an “accretion disk” around the supermassive black hole, like the one described earlier circling Cygnus X-1. This disk rotates around, and in the same direction as, the spinning black hole. Enormous amounts of energy can then be released as this maelstrom of matter spirals inward toward the black abyss and is ripped apart by the gravitational tug-of-war. Excess gas that has not yet reached the hole’s infamous point of no return might get magnetically deflected—funneled out like a cream filling from the top and bottom of the doughnut. That could be one source of the jets.
But there’s another way as well. It’s likely that an appreciable amount of the power is tapped from the spin energy of the galaxy’s supermassive black hole. The black hole, in this case, is an electrical dynamo, but one of cosmic proportions. In this scenario magnetic lines of force, originating from the disk’s gas, thread through the spinning hole’s outer surface and whirl around with its rotation. Because of the hole’s incredible spin, the magnetic field lines come out of the north and south poles of the hole coiled like streamers around a maypole. This forms two narrow yet powerful channels. Like a gigantic turbine in a galactic power plant, these spinning fields produce a huge electrical potential, which generates beams of particles that shoot out along each channel at near the speed of light (a model originated in 1977 by the British-born theorist Roger Blandford, now at Stanford University, and Roman Znajek). In this way energy is extracted from the rapid spin of the hole. It’s the most efficient mechanism now known in the universe for converting matter into energy.
The spin also allows the black hole to act like a gyroscope, a device that has the ability to maintain a fixed orientation. That’s how the cosmic jets can be steadfastly pointed in the same direction over long stretches of time, never varying. While this model has been tweaked and adjusted by many theorists over the decades, it had its roots in Roy Kerr’s solution to Einstein’s equations of general relativity, which was the first to demonstrate the behavior of a rotating object on the fabric of space-time.
Astronomers now see an evolutionary link between the quasars (those active supermassive holes) of yesteryear and the galaxies of today. As observers gaze farther and farther back in time, they count more and more quasars with very high luminosities. That’s because the universe was spanking new and vibrant, constructing galaxies full of newly formed stars that were surrounded by lots and lots of gas. Under such conditions, the supermassive black hole building up within each young galaxy was able to gobble up its rations like a chowhound at an all-you-can-eat buffet.
But such a food supply is finite, and the black hole can haul in material only within a certain distance. As astronomer Richard Green once put it, “The gas gauge says ‘empty,’ and there’s no gas station in sight.” So, after some ten million to a hundred million years have gone by, a blip in cosmic history, the quasar’s fireworks eventually taper off or simmer down to a less active state. It turns into a rather normal-looking galaxy. It was once thought that quasar activity was somewhat rare, but astronomers now believe that every sizable galaxy with a central bulge has a supermassive black hole at its center, an ancient quasar that could be triggered once again. A fairly commonplace galaxy, for instance, can transform into a bright active galaxy or strong radio galaxy, loudly broadcasting its presence, after it collides with another galaxy, which causes new supplies of gas to feed the drowsing monster that had been sitting restfully in the galaxy’s center.
The heart of our own Milky Way galaxy harbors a former quasar, a dormant supermassive black hole. A global team of radio astronomers is now readying a massive effort to image the “shadow” this hole casts against the brighter gas emissions surrounding it. This black hole, estimated to contain the mass of around four million Suns (on the small side, compared to galaxies with billion-solar-mass black holes), is now idling at low gear. Its engine does rev up occasionally, whenever it can grab some nearby fuel (say, a gas cloud falling into it), but that activity is peanuts compared to what may happen some four billion years from now. That’s when the behemoth could fully reawaken and roar loudly as our spiraling home slowly collides with our close neighbor, the Andromeda galaxy, whose central black hole is roughly ten times more massive. By the end of this fateful meeting, the two galaxies will combine to form a giant elliptical galaxy. Their holes will merge as well, putting on a breathtaking show of activity as the newly combined hole gulps down fresh sources of gas, released in the collision, and continues to grow to at least one hundred million solar masses.
12
Black Holes Ain’t So Black
The portrayal of the black hole described in these pages so far is not complete. The behaviors depicted—strange as they may sound—have been rooted in a very classical mathematical scheme, general relativity. What hasn’t been taken into account is quantum mechanics. What would a black hole look like from the perspective of an atom? And therein lies the rub: no one has successfully formulated a quantum theory of gravity. It’s been done for all the other forces: electromagnetism, as well as the weak and strong nuclear forces. Gravity so far has been left out. It is the last, great task of theoretical physics—to fully merge general relativity with quantum theory.
There’s a reason gravity’s been the odd man out. Where all the other forces involve particles that follow the probabilistic rules of the quantum world—allowing these forces to be united into one grand mathematical scheme—general relativity’s key parameter (at least in the way Einstein formulated it) is geometrical: curvatures in space-time. It’s as if nature has set up two different sets of rules—one for gravity, and another for all the other forces. The tools that work so well in one realm are difficult to apply in the other. Gravity and quantum mechanics don’t easily share the same mathematical vocabulary.
Despite these difficulties, several researchers in the 1950s and 1960s felt that the best way to energize the field as general relativity was waking back up from its decades-long slumber was to restart an effort that had begun in the 1930s—to bring quantum effects into general relativity. A number of notables, including Paul Dirac, Richard Feynman, and Bryce DeWitt, pioneered this effort, showing how gravity could be described in another way. Everything in the qu
antum mechanical universe—energy, motion, spin, and so forth—comes in indivisible bits. Forces fit naturally into this framework. Instead of viewing magnetism, say, as the result of invisible lines of force emanating from a magnet, the quantum world transforms the notion of force into an exchange of force particles—a subatomic tennis game. In electromagnetism this diminutive tennis ball is the photon, a particle that constantly bounces between charged particles, generating a force of either attraction or repulsion. By applying this same principle to gravity, the force of attraction between masses is conveyed by the continual transmission and absorption of “gravitons,” particles that exist, for now, only hypothetically; they have not been detected.
But there’s a big problem in recasting gravity in this way. Theories that treat forces as particles assume that every event in the subatomic world takes place on a fixed, unchanging background of space and time. Space-time is the stage upon which the actors, particles such as photons, flit to and fro. Space-time is not a participant. But in general relativity the distinction between stage and actor doesn’t exist. According to Einstein, gravity is the very geometry of space-time. Thus the graviton becomes both actor and stage simultaneously. A graviton enters onto the stage of space-time, but by doing so ends up bending and warping the stage as if it were so much Jell-O. Theorists involved in solving this conundrum are far from arriving at a complete and unified solution.
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