Turn Right At Orion (7 page)

Did I dare to immerse
Rocinante
in the disk? It looked formidable, completely opaque, and not at all calm. Arches of luminescent gas kept erupting from beneath a roiling gaseous sea. Plugs of vapor would shoot out of the disk to various heights, spread out in striations along an arch, and then drain back toward the surface. Sometimes the arches would twist, merge, separate, or suddenly open up to the sky, lobbing their contents into space. The flitting network of arches looked like a pattern of iron filings created by a convention of drunken magnets, and even more like the prominences a colleague once showed me—through a specially designed telescope—erupting from the surface of the Sun. The latter flares were in fact magnetic in origin,
as my colleague had found by measuring the polarized light (a sign of the helical gyration of electrons along the magnetic field).

The flares were obviously pumping out a lot of energy, because it was getting hot even before I reached the surface of the disk. The disk, it seemed, had an atmosphere of hot gas, presumably evaporated from the surface by the flares. I held my breath and descended through the surface. There was less resistance than I expected—the “surface” was more an impediment to vision than a physical boundary, although I could feel increasing discomfort as
Rocinante
was buffeted by the highly turbulent gas in the denser layers of the disk. The magnetic field was in evidence everywhere and stronger now than ever; lines of force and the striations they cause seemed to dominate the organization of the gas. Here they formed no arches, however, and they did not seem to be seeking the surface of the disk and freedom. They were tightly confined, combed out by the swirling orbital motion of the disk, much like marsh grass bent over flat in a raging flood.

The symbiosis between the magnetic field and the orbital motion—
that
was the key to how matter was able to spiral toward the black hole. I struggled to remember the theory that described the complex behavior of hot matter in the presence of magnetism. This subject had been my nemesis as a student. Magnetism, it turned out, gave hot matter an added elasticity and an odd kind of tension. It imbued ordinary gas with certain qualities of both rubber bands and the mainsprings of old-fashioned watches. Conversely, hot matter was able to grab onto magnetic lines of force and to stretch, twist, and bend them as though they were candy stripes frozen into toffee. This stretching was what combed out the field lines here, and the symbiotic back-reaction of the magnetic force was what was dragging on the gas, allowing it to spiral in.

Confident now that matter would flow all the way in, I surfed along the disk toward the black hole. I dipped in and out of the obscured disk interior, enjoying increased confidence in my mastery of
Rocinante's
controls. The speed of the swirling gas increased,
as predicted by the theory of gravity, and so did the temperature, the luminous glow inside the disk, and the violence of the stretching and churning magnetic fields both inside and out. The disk's radiance grew so strong that it seemed to be bucking the dips in my inward motion, pushing me away from the disk's central plane. This was probably an illusion, but it reminded me that radiation itself, when intense enough, can actually levitate matter against the pull of gravity.

I fully intended to reach the place in the center of the disk where matter departed from its gradual spiral path and made its final plunge into the hole—the “orbit of instability” that I had so carefully avoided in the center of the Milky Way. That place, I calculated, should be about 100 kilometers from the horizon of the black hole, which turned out to have a mass equal to that of 15 Suns. I was now so adept at navigating
Rocinante
that I would have felt comfortable going in nearly that close. Noting that the stream flowing in from the companion had merged into the disk at a distance of about 1 million kilometers from the hole, I relaxed and watched the kilometers tick off as I sped inward. Then, suddenly, I became violently ill. I had never been so sick in my life. I checked my distance from the hope: 10,000 kilometers. What could be happening? It felt as though my torso and upper body were being pulled away from my lower extremities. I was riding with my head toward the black hole and was able to achieve a minor increase in comfort by turning my body sideways. But the respite proved temporary. My craft was on autopilot, heading straight for the hole, and I soon felt as though my head were being pulled apart, front from back. And as for my gut. . . well, I will spare you the details.

I'm sure that I was about to pass out, and it was sheer luck that I managed—with. a great effort of will—to locate and activate the reverse lever. I pulled back out to 100,000 kilometers, up and away from the disk, and collapsed in a cold sweat. The disk formed an immense floor in my field of view, its center a barely discernible spot in the distance. As I regained my composure, I immediately realized what had happened to me. At first, the
black hole's gravity had not posed a problem, because I had been allowing
Rocinante
to fall almost freely toward the hole. Like any astronaut orbiting Earth, I had felt virtually weightless. But my head, being slightly closer to the black hole than my feet, had actually been subjected to a slightly stronger gravitational pull, and that
difference
is what had gotten me. When I came within 10,000 kilometers of the hole, the difference in gravitational pull had approached and then exceeded the normal gravity of Earth, under which a human body is designed to operate. I was indeed being pulled apart. It was as though I had somehow discovered a way to stand upright and upside down
simultaneously!
The blood (and every other fluid in my body) began to rush away from my middle and to pool in both extremities. A force comparable to my weight on Earth was stretching every tissue in my body. You can imagine the discomfort. When I turned sideways, the effect was diminished: Because I am thinner than I am long, the difference in the gravitational pull was correspondingly reduced. But as I approached still closer to the black hole, even the difference across my torso became intolerable. In a sense, I had suffered a milder version of the fate that befell Cygnus X-1's companion, whose midsection had been extruded into a pointy bulge from which the accretion stream was pulled toward the black hole. I thanked the deity of mass transfer in binary stars that my encounter with extreme tidal forces had not proceeded to the gruesome outcome that was keeping Cygnus X-1 fed.

Apparently, I was not to close in on the black hole after all. I was up against physical laws that I could neither change nor circumvent. Fortunately I am a stoic at heart, and I knew that remote observation via telescope, though not the ideal way to collect data, sometimes has to do. One simply accepts that limitation as an astronomer. My ego needed to be reminded that, after all, I was closer to a black hole than anyone else had ever been. I could live with the disappointment of not getting quite as close as 100 kilometers.

Having come to terms with this unforeseen restriction on my movement, I surveyed the disk below and ahead of me. From
this height the magnetic flares seemed less significant, little more than minor viscous eruptions like the bubbles and cheese-spouts that erupt from the surface of a simmering fondue pot or, given the disk's thinness, a bubbling pizza (my harrowing encounter with the black hole's gravity had unaccountably given me an appetite). I could see the underlying large-scale structure of the disk, stretching away toward the black hole. The disk was luminous everywhere, and the more so the closer one got to the hole. With increased radiance came the expected increase in temperature, manifested in the progression of “colors.” I have to put this word in quotation marks, because the progression far surpassed the sequence from red to yellow to blue-white that a human eye perceives when a metal bar is heated to thousands of degrees. Here the colors went off the human scale at violet proceeding through the ultraviolet, far ultraviolet, extreme ultraviolet, and on into the X-rays. Apart from these gradations of temperature and radiance, the disk was overwhelming in its flatness and featurelessness.

To get my bearings, I focused on the hottest and most luminous region of all, the area surrounding the black hole. I could see the blackness in the center and the disk continuing beyond. There the flatness appeared to be broken by a curious asymmetry. Just the other side of the hole, the disk seemed to warp upward, forming a distorted band that folded over on top of the gap that signified the location of the black hole. I moved around toward the far side to get a better look, but the distorted region of the disk seemed to compensate for my motions, always remaining exactly on the opposite side of the hole. It dawned on me that I was witnessing an illusion caused by the wrapping of light rays around the black hole as they crossed the intense gravitational field on their way to my instruments. I should have expected this. I knew that radiation, as well as matter, responds to gravity, though it was shocking to see the effect so forcefully displayed. There was another asymmetry as well, but the origin of this one was easier to visualize. I noticed that the disk was a little bit brighter, the X-ray luminescence just a little bit harsher, to
the right of the hole than to the left. This, I immediately recognized, must be due to the “headlight effect,” the forward beaming of the radiation emitted by moving matter. In this case, the light would be beamed toward the direction of the disk's spinning motion, and the brighter side had to be the side that was coming toward me. According to the theory of relativity, an asymmetry this pronounced could mean only one thing: The matter that I was viewing swirled around the black hole at nearly the speed of light. These were all well-known theoretical predictions, but by this point I was so awed by the spectacle that I was disinclined to trust the intuition I had developed through years of painstaking study. As if to pinch myself mentally, I double-checked that the disk was spinning in a clockwise direction, as it would have to be if light from the right-hand side were really boosted by its oncoming motion. It was.

I tried to follow the gas as it disappeared through the black hole's horizon. The horizon was a sphere with a radius of 45 kilometers. Was I right that the matter in the disk would break its slow inward spiral and plummet into the black hole from 100 kilometers farther out? I saw what looked like evidence of a dramatic change in the disk's state of motion near the predicted place, but I couldn't be sure; the hot atmosphere above the innermost parts of the disk became so thick that it all but obscured my view. At the other end of the death-plunge I could barely make out the gas fading just outside the horizon. The textbooks appeared to be right. The X-ray radiation regressed through the colors as it fought its way out through an increasingly debilitating gravitational field. As the signal faded, the color passed back through the varieties of ultraviolet, blue, yellow, and red, and then it was too faint to tell. What happened in the intervening space between the horizon and the orbit of instability? The artists' impressions in my musty old textbooks had represented the disk as having a distinct inner edge, but this seemed unlikely. This disk was so rich with matter, right to the center, that it could not have thinned out enough to become transparent. I sighed with relief at this confirmation of my “theorist's
instincts.” Still, I felt one last pang of regret that I would be forever denied a closer view.

It could be that I was fortunate to have been forced to take a long view of the disk, for I began to perceive certain causal connections that had eluded me before. The black hole's gravity caused motion, first the open spiral of the narrow accretion stream, pulled from the companion star into the black hole's sphere of influence. I looked back, away from the black hole, and saw the distant panorama of the stream swinging one full turn around the system and crashing into itself, that big splash now dwarfed by the immensity of the disk. I saw the incipient disk spread away from the crashing stream, both inward and outward. Most of the gas spiraled inward and, as it did so, drifted ever deeper into the gravitational pull of the black hole, where it swirled faster and faster while its increasingly ferocious magnetic fields (stretched and amplified by the motion) tugged at it chaotically until it heated up. Thus motion begat heat, and heat begat luminosity. Four trillion tons of its companion's substance was disappearing into the black hole of Cygnus X-1 every second, but 100,000 times the luminosity of the Sun was coming out. It seemed a fair trade. This black hole
was
an efficient engine for turning matter into energy, an intense flickering source of X-rays that finally reached Earth.

I pulled out my measurements of the Milky Way's central black hole, expecting to find that it was starved of nearly all matter. That would surely explain why its environs emitted just the barest of glows rather than blazing away like Cygnus X-1. But the Milky Way's black hole seemed not to be playing by the same rules. Despite its sparse surroundings, that huge black hole was actually swallowing matter more rapidly than Cygnus X-1 was gobbling its companion. Somehow the chain—matter plus gravity goes to motion to heat to radiation—had been broken, the trade of matter for luminosity not consummated. The bigger black hole was greedy, swallowing most of the heat along with the matter, before the heat could turn into luminosity and radiate into space. I pondered an old theoretical idea in the hope
that it could explain such a difference. The Milky Way's central black hole was 100,000 times heavier than Cygnus X-1,
Its
horizon was millions of kilometers across. I focused on this idea of relative size and what it might signify. The amount of matter flowing toward the black hole in the Galactic Center was indeed modestly larger, but the dimensions over which this matter was forced to spread were
vastly
larger. Could this be the clue I was looking for? Spread out so thinly, perhaps the matter never reached the concentrations at which heat could be generated and released efficiently.