Turn Right At Orion (10 page)

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!

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 shield 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.

The Crab had to be my next destination, for one reason. Near its center lay a neutron star, the remnant of a star whose explosive demise had been witnessed on Earth in A.D. 1054. In theory, a neutron star resembles black hole as closely as a body can without actually becoming one. Somehow its collapsing precursor knew how to stop gravity in its tracks, to create the tightest possible form of equilibrium. It weighed more than the Sun, but
it was only 20 kilometers across. I needed to see what it could teach me.

Despite my misgivings about the visit, I took some consolation from the fact that the Crab Nebula, or its precursor, had once known true glory. Before A.D. 1054, no Earthly astronomer could have seen the Crab Nebula (even if telescopes had been invented) because there was no such object in the sky. But for a few weeks during the summer of 1054, this shattered orb outshone any of its compatriots and would have surpassed Venus as the beacon of the predawn horizon. Unbeknownst to our medieval ancestors, a star had already exploded some 6000 years earlier, in the direction of Taurus. Because the explosion had taken place 6000 light-years away, the evidence of this star's demise did not burst forth upon earthly skies until the evening of the fourth of July 1054, whereupon the Chinese court astronomers duly took note. The Chinese had provided such detailed records that by 1921, astronomers believed they knew the location of this “guest star” and noted its proximity to the well-known nebula. That same year, it was discovered that the Crab Nebula was expanding and apparently had been doing so for nearly a millennium. It didn't take long for astronomers to put 2 and 2 together.

I was so fascinated by this rare convergence of historiography with astrophysics that I didn't notice my entrance into the nebula itself. This surprised me, because I had expected a strong jolt as I crossed from the undisturbed surroundings into the zone occupied by the outward-rushing debris. As I had learned during my very first traversal of the Milky Way, no star lives in a vacuum, and when a star is audacious enough to explode, it must push its surroundings out of the way. At typical explosive speeds (covering 1500 kilometers every second, if not more), the quiet gas that envelops the star cannot anticipate what is about to befall; the blast overruns it without warning. Like the famous horror film in which decent citizens are overcome by the lumbering swarm of zombies and then become zombies themselves, the swept-up atmosphere is shocked into sudden motion and inexorably becomes
part of the blast, plowing in turn into its surroundings and infecting
them
with unstoppable motion. The signature of such a shock wave is a sharp increase in pressure—this is exactly what a sonic boom is—and, often, the radiance that accompanies flash-heated or disturbed gas. I
must
have crossed the shock, I thought; the fact that I had felt nothing perplexed me.

Not that the existence of a shock had actually been demonstrated by my observer-colleagues on Earth. The environment of the Crab Nebula had been regarded as one of its deeper mysteries. Astronomers were always looking for its “shell,” and not merely for the purpose of justifying its name. Optical, radio, and X-ray observations—any of these should have given evidence of the shock—all came up empty. When the nebula's mass had been toted up in terms of the gas that one could easily “see,” it fell short by at least four times the mass of the Sun. Only stars more massive than a certain threshold were supposed to explode like this, and the four (some said six) missing solar masses constituted the difference between what was seen and what the theorists expected. Some conjectured that the “shell” was not really part of the explosion but, rather, consisted of slowly moving gas that had wafted off the star's surface during the eons before it blew up, when it was not a hot star but a cool red giant. They contorted their reasoning to find ways in which this effluence could somehow shield itself from disturbance to the point where it was invisible. Others said that the shell was there, and was actually rushing out at speeds several times greater than the expansion speed of the observable nebula, but that it was too faint to see. Why this should be so was anybody's guess. To me, the most plausible explanation was that the Crab lived in very sparse surroundings, like the interior of one of those hot stellar bubbles, superbubbles, or chimneys I had seen during my trip to the Galaxy's center. Such bubbles could be blown, over millions of years, by just the sort of star that the Crab once was. Maybe the Crab-star had evacuated its own neighborhood, and now there was little for its debris to run into.

I was confident that I would soon encounter the familiar nebula, but as I continued onward without finding a trace of the exploded star, I became worried. The space around me seemed eerily empty. Debris in any form—if compacted into dense enough clouds—would have been hard to detect (assuming I didn't run into it!), but not so the luminous features that were easily visible from Earth. As I searched in vain for a landmark, it dawned on me that the reason for this desolation was profound. I was not in the Crab Nebula I remembered from Earth; I was in its ruin. I was disheartened, though I should have known. Ninety thousand years of continuous expansion had sapped the nebula's vitality. The debris from the star was now well mixed with ordinary interstellar matter, and its explosive energy had spread over a volume thousands of times larger than the historical Crab Nebula.

I knew there would still be a neutron star to study, if only I could locate it. I looked around—there were hundreds of faint stellar-looking objects that could have been candidates. At this age, the neutron star should have been very faint at all wavelengths except perhaps the radio band, where it might have shown up as a slowly winking pulsar.

I was debating whether to commence a half-hearted radio search or to give up entirely when I remembered an event that
had made an impression on me as
Rocinante
approached the location of the Crab. I had spotted a new stellar explosions, off to starboard, and had noted in my journal that it exhibited many features in common with the sort of explosion thought to have produced the Crab. As I moved through the Galaxy, I could tell from its changing displacement against the background of more distant stars that it was less than a thousand light-years away. The remarkable coincidence had pleased me. The chance of a second such, explosion, this close in time and space to the first, was miniscule. Now I viewed it as a godsend. What excellent luck! I could be there in less than a thousand years of Galaxy time and, if it did prove to be another Crab, experience the familiar nebula as though it had been reborn. I set course immediately for the nebula I dubbed Crab II.