I tallied up the matter flying away from this object, in all its forms—hot wind, debris, fast jets—and came to a startling conclusion. SS 433’s intense gravitation was extracting from its companion 100 times more matter every second than was disappearing into Cygnus X-1’s maw. But instead of swallowing this matter, SS 433 was shooting most of it back out into space. Why?
I saw how such an imbalance might come about. As matter sinks deep into the gravitational field, it acquires a lot of energy. First comes motion, as the matter is accelerated by the gravitational pull, then perhaps heat, and then radiation. I had already worked out this chain of energy flow for Cygnus X-1, But I hadn’t taken into account the many ways in which energy can be transported from one place to another. Heat is conducted from high temperature to low; radiation leaks out into dark space; stretchy coils of magnetic field set widely separated gaseous filaments in motion; hot cells of turbulent fluid boil up through a cooler atmosphere. In each case, energy is deposited somewhere other than the place in which it was liberated, and deposition of energy can lead to motion where it is least expected. The pressure of radiation forcing its way through from below was perpetually blowing apart the shroud of SS 433, puffing it up from within until it released its wind and debris. I was less certain what mechanism propelled the jets, because their origin was hidden—but I now understood that means, opportunity, and perhaps motive could readily exist for this seemingly illegal escape from the clutches of a black hole. Whatever the mechanism, it wouldn’t take much for a small amount of matter, skimming just outside the black hole, to unleash enough power to blow away a much larger amount of matter that never even got close.
This all seemed to make sense, but did it require that there actually be a black hole at the center of SS 433? I had read somewhere that a debate still raged over whether the enshrouded collapsed star was a neutron star or a black hole, because attempts to estimate its mass had been inconclusive. I suppose I could have solved that problem on the spot with a few measurements of speed and distance for my freely falling craft or with precise orbital measurements of the two stellar companions. But I was too preoccupied by what I had just learned to worry about such refinements. Black hole or neutron star, I doubted that it would make much difference to the outcome. A neutron star’s gravity had to be nearly as strong as that of a black hole, and it was the competition between gravity and motion, not any special properties of the gravitating body, that was in question.
What I now understood was that gravity and motion were not always so finely tuned. I had seen some examples of an exquisite balance—were they the exception? My thoughts returned to the steady march of the stars orbiting the Galaxy, in nearly perfect circular orbits. Even the slight imbalances that led to spiral arms—in which gravity wins a little bit each time a star slows down as it encounters one of those interstellar traffic jams—had a certain grace and delicacy about them. The tidal disruption of a star by a black hole was more heavy-handed and really quite violent, but at least it was a fleeting event that occurred and then was over. Except for a modest escape of some of the accreting gas, Cygnus X-1 made a clean trade of matter for energy. Yet the violent disequilibrium of SS 433 seemed to be a chronic condition. Did the black hole (or neutron star) not know its own strength? Did it grasp the substance it sought to incorporate with such vehemence that it overshot and flung most of it away?
I knew that gravity and motion were not really out of control. All of this had to be predictable. It was just shocking to see matter flout such a strong gravitational field. And SS 433 was not unique. There were episodic jet-emitting black holes that produced surges every so often, for reasons unknown. In these cases, the entire insides of a disk would suddenly drain toward the black hole (like the extremely low tide that precedes a tsunami), only to resurge in the form of jets shooting outward along the rotational axis at speeds even closer to the speed of light than SS 433’s jets. Apparently, black holes did not exist simply to attract and devour, as popular literature would have it. Surely this would be the impulse of gravity, left to its own devices. But I saw that when combined with the curious and often contrary properties of matter—its momentum, pressure, radiation, and magnetic fields—gravity could often repel matter or expel it, or even compel it to whirl with wild abandon. Yet in the disk of the Milky Way, the departures from regularity were subtle. Somehow gravity was able to play with motion to achieve a nearly perfect balance. I knew, of course, that the same thing, on a microscopic scale, prevented the Sun and all stars from collapsing in on themselves, at least for a while.
I had a choice to make. Should I seek out examples of ever more violent and contrary phenomena driven by gravitation? I felt naturally attracted to the dramatic, the surprising, and the exotic, and at first this seemed to argue for undertaking the quest after super-fast jets. Probably I was also attracted to the idea that jets mounted a heroic resistance to authority: the victory of the underdog in standing up to gravity. But there was also a kind of tension and high drama in the challenge I finally selected—the quest for equilibrium. For I knew that equilibrium was not just stasis; it was a standoff in the fierce competition between huge forces of attraction and equally huge forces of resistance. When expressed in these terms, even the benign equilibrium of the smiling Sun seemed to survive in a kind of “balance of terror.” One uncompensated deviation on either side of the fulcrum could lead to disaster.
As I debated with myself, I began to pull away from the SS 433 system, not really sure of my next destination. One more feature of SS 433 nagged at me. What caused the disk to warp? This really didn’t seem to make sense. The matter flowing across from SS 433’s companion star arrived imprinted with the spin of the binary’s orbit. If anything should have been a gyroscope, retaining its orientation as the world around it turned topsy-turvy, it is that disk. Could this too be a symptom of the general failure to attain equilibrium, with a portion of that redeposited energy—some radiative, magnetic, or thermal torque—twisting the disk away from its preferred direction? Remarkably, even that seemed possible. And it also seemed possible in Cygnus X-1, whose disk I had perceived as being so flat. Hadn’t I read somewhere that it, too, showed some evidence of a wobble? Maybe, in my naïveté, I had wished it flat. Knowing now that the interactions between gravity and matter could be more complex than I had ever imagined, I turned my attention to the next phase of my exploration, the quest for a rock-steady truce between gravity and matter.
Part Three
EQUILIBRIUM
8
Shangri-La
It struck me, as I accelerated away from SS 433 and prepared for hibernation, that more than 65,000 years had passed on Earth since my departure. In all likelihood, the scientific conundrums I was confronting—so far, with mixed success—had long ago been resolved by more ingenious if less direct methods . . . assuming that science was still practiced at home. The goals, norms, and technologies of civilization must have been altered far beyond recognition by now. How many other travelers, or colonies of travelers, from Earth might now be sharing interstellar space with me? Or had physical space travel been a transient fashion, quickly superseded by something more efficient, perhaps the transmission of some digital “essence” of intelligence and personality without the need for an accompanying organic receptacle?
The passage of time might be quite irrelevant to such a virtual being. I, on the other hand, had to worry about aging. As I intimated earlier, I address this problem by manipulating the passage of my time. There is nothing mysterious about this. I exploit the most elementary consequence of Einstein’s special theory of relativity, the effect known as time dilation. As viewed from the Earth or from nearly any other observing platform in the Galaxy, time flows more slowly for me, simply by virtue of my high speed. The astute reader might have noticed my allusion to the phenomenon of time dilation when I explained how SS 433’s motions had been deduced using Doppler shifts. To show why time dilation occurs and how to exploit it, I first need to explain a
little bit more about the principles of relativity.
People had known since the seventeenth century that light travels with a finite speed, about 300,000 kilometers per second. It had seemed self-evident that if you could travel at, say, 150,000 kilometers per second toward the source of a light beam, then you should measure its speed as 300,000 + 150,000 = 450,000 kilometers per second. After all, in normal experience speed is relative: When you pass a car on the freeway, it actually seems to be going backwards, relative to you. However, in 1905 Albert Einstein realized that speeds close to the speed of light behave differently. For example, if a truck is barreling toward you at 200,000 kilometers per second (two-thirds the speed of light) and you are suicidal and head toward it at 100,000 km/s (one-third the speed of slight), then the two speeds do not add up to the speed of light, as a simple sum would suggest. Instead you will find yourself closing in on the truck at only , or 82 percent, of the speed of light. This is because speeds become “less relative”—they do not simply add up—as they approach light speed. The speed of a light beam is not relative at all; it has exactly the same value, no matter how fast you are going and in what direction. What Einstein showed is that space and time, not speed, are the fundamentally relative quantities. Space and time become distorted—stretched or compressed—according to your state of motion, in such a way that all observers will agree on the value of the speed of light.
Time dilation is one of the more dramatic consequences of the principle of relativity, and it is entirely responsible for my ability to travel across the Galaxy within my natural lifetime. After my encounter with the black hole’s tidal forces, I was especially sensitized to my own physiology, so let me try to illustrate this effect by using the example of my beating heart. My heart possesses a little clump of nerves to one side that sends an electrical signal wrapping around the entire muscle every second, telling it to beat. The chemical processes that set this timer are complex and involve molecular interactions that crisscross this natural pacemaker in every direction and at a variety of speeds. But I will simplify for the sake of argument and suppose that the billions of molecular reactions can be visualized as a single pinball that bounces around inside the pacemaker at the speed of light. The trajectory can be as fiendishly complicated as you like, provided only that the pinball hits the same bull’s-eye once every second, triggering my heartbeat. You will have to trust Einstein’s velocity transformation law to take care of deviations from the speed of light in the real pacemaker, and the law of averages to correct for my substitution of a single projectile for multiple concurrent interactions.
First look at the operation of this little synaptic device from my point of view. Between heartbeats the pinball, traveling at the speed of light, covers a distance totaling one light-second, or 300,000 kilometers. It must take a tortuous path indeed! Now let me revert to an observer watching my craft zoom past, from left to right. According to the principle of relativity, this observer must also see the pinball as moving at the speed of light, but now the heart is moving as well. When the pinball happens to be heading rightward, it has to chase the heart, which is moving along at nearly the same speed. These legs of the pinball’s trajectory are therefore stretched and must take longer. The opposite is true when the pinball is moving toward the left: The heart then overtakes the pinball, and these legs are compressed. When the pinball is moving in any other direction, one gets a result between these extremes. What is amazing is that the total length of the ball’s trajectory between beats, as viewed by the outside observer, is completely independent of the trajectory taken and is always longer than the trajectory as measured inside the spacecraft. Because light-seconds of distance translate into seconds of time, the outside observer sees my heart beating more slowly, and because any of my physiological processes may be represented by the same pinball analogy, to the outside universe I am aging more slowly.
How much more slowly depends on how close my speed is to the speed of light. When I am within 1 part in 100 of the speed of light, for example, I age 7 times more slowly. I call this slowdown in the passage of time my “Shangri-La factor.” At 1 part in 1000 less than the speed of light, my Shangri-La factor is 22, and so forth. What I do in order to cover these vast distances is simply to accelerate, until I have reached the halfway point, at the comfortable (some would say leisurely) rate of 1g, the acceleration of any object dropped off the top of a building on Earth. Then I decelerate, at the same rate, for the remaining half of the distance. This has the advantage of giving me a sensation identical to the normal force of gravity (except when I choose to experience weightlessness during brief periods of free fall, as when I attempted to approach Cygnus X-1 ). I could have withstood somewhat higher accelerations, but one never quite gets used to them and they really are not necessary. At my leisurely rate and starting from rest, it takes just 2 ½ years of my time to reach 99 percent of the speed of light. To reach the Galactic Center, I had to accelerate for 10 of my years (most of which were spent in hibernation) and then to decelerate for an equal time. My peak speed on this leg was within 1 part in 300 million of the speed of light, and (when I was awake) I was aging 13,000 times more slowly than everyone on Earth. In the 65,000 years that had passed on Earth since my travels began, little more than 60 years had elapsed inside Rocinante. Given my rigorous hibernation schedule, I had aged less than a decade!
9
The Soft-Shell Crab
Now I needed all the speed I could muster, not because I sensed an impending midlife crisis but rather because I wanted to get the next leg of my journey over with. For the first time, I was to travel beyond the circle of the Sun’s orbit around the center of the Milky Way. Cygnus X-1, lay at roughly the same distance from the Galactic Center as the Solar System, but my detour to SS 433 had forced me to backtrack halfway to the center. Thus, although my next destination lay only 6000 light-years outbound along a continuation of the line connecting the Solar System to the center of the Galaxy, it was nearly 22,000 light-years distant from SS 433. With the time compression of the Shangri-La effect in force, I settled in for another 20 years en route.
Although I was hurtling toward a famous target—the Crab Nebula—I was not entirely thrilled with the prospect of visiting it. For one thing, I like my nebulae bold and extravagant. Orion, the Trifid, the Lagoon, Eta Carinae—those are my ideas of nebulae, sprawling fluorescent spectacles illuminated by newly formed hot stars. The compact, prolate fuzzball that I remembered as the Crab from my observing days on Earth was shot through with luminous red and green filaments, and a diffuse bluish background glow added a unique, if slightly garish, touch, but there were no bright stars to be seen. And with dimensions of barely 9 by 12 light-years across, this nebula was clearly a wimp in comparison to the others. In 1784 Charles Messier had placed it first on his list of objects that could fool comet hunters, a designation that seemed apropos—from a distance it resembled a comet that was still so far from the Sun that it had not yet sprouted a tail.
Moreover, to me the Crab Nebula looked nothing like a crab. It was Lord Rosse, constructor of the world’s largest telescope in the 1840s, who had given it that label. It makes one wonder whether he had ever seen such a creature. I pulled up a copy of Rosse’s earliest pencil sketch in my database, and to be honest, his own drawing looks more like a pineapple than a crab. (Curiously, his version of the nebula does have a tail, which reminded me of the pineapple’s crown.) He later repudiated that early impression (made with a smaller telescope, not his “Leviathan of Parsonstown”), replacing it with a more familiar-looking mottled lozenge. Still no resemblance to a crab, despite the cage of thin filaments that was later made visible with the advent of photography. The nebula didn’t even have an association of place, lying as it does (from Earth’s perspective) near the tip of one of the Bull’s horns, one full zodiacal notch away (clear the other side of Gemini!) from the arthropodous constellation of Cancer. And so far as I could remember, it had nothing resembling what I would call a shell—a shie
ld of luminous or compressed gas, for instance. At best, it might be able to pass for “The Soft-Shell Crab Nebula.” Or maybe the term crab refers to the curmudgeonly attitude with which I was approaching this system. Perhaps it had inexplicably been engendered in Galactic explorers of Victorian times, as well.
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