Turn Right At Orion (31 page)

All the questions I had repressed, when I stood opposite the Milky Way's great black hole on that first, disappointing leg of my journey, came rushing back into my head. How did this black hole come to rest here? Did it grow from a much smaller seed by swallowing stars and clouds of gas? Could it have started out as the collapsed remnant of a single star of only a few times the mass of the Sun, like the one that had collapsed to become the black hole Cygnus X-1? I remembered daydreaming about a star being torn apart and swallowed by the Milky Way's black hole. Had I waited 10,000 years or so, I really would have seen it happen. But even if the Milky Way's black hole were fed in this way every few thousand years, it is doubtful whether the hole could have grown to its 2.5 million solar masses during the time since the Milky Way formed.

The problem was even more acute in M87. Here, 3
billion
stars like the Sun would have been required to bring the black hole up to its current mass. The central star cluster contained nowhere near that much matter, and the disk of gas I had seen on my way to the nucleus was dribbling in much too slowly to take up the slack.

Even if the requisite amount of matter were available near the hole, that doesn't mean the hole would grab it. I had seen how
many impediments there were to the growth of black holes, the most important being the motion—the angular momentum—of the matter that might be swallowed. The ability of a black hole to grab and swallow stars does not increase very rapidly as the mass of the hole increases, and it starts out shaky when the hole has little mass. If the M87 or Milky Way black hole had started out small, how many stars would have been within its reach? Stars would have had to venture much closer to the hole before being torn apart. Instead of doubling its mass in 5 billion years, say, might it not have taken the hole 10 billion, 15 billion, 100 billion years or more, to swallow enough stars—a time so long that it would have exceeded the age of the Universe, to say nothing of that of the Milky Way or the Virgo Cluster? And even if the black hole could swallow stars at an adequate rate, would the supply of stars have remained adequate over the entire lifetime of the galaxy? As stars were depleted from the danger zone near the black hole, would they have been replaced quickly enough to keep the black hole growing apace?

My natural optimism soon asserted itself. Of course, I had been assuming that the environment at the Milky Way's center or the nucleus of M87 was always similar to its present state. But who was to say that these places had always been so sparse, so relatively gas-free? Hadn't there been a time when the tumbling bar in the Milky Way had not yet assumed such proportions as a gatekeeper against too much inbound gas? During an earlier epoch in the Virgo Cluster, mightn't the flow of the cluster atmosphere into M87 have supplied 100 or 1000 solar masses of gas each year instead of just 10? Perhaps the central black hole, here and in the center of virtually every other galaxy, is merely the dumping ground for much of the debris left over from the galaxy's creation. Or do external events routinely overwhelm the ability of a bar, or a cavity blown by a pair of jets, to keep abundant gas from reaching the black hole. The crucial event might have occurred long after the galaxy formed—the collision of the Milky Way with another galaxy, perhaps, or some hapless (and nameless) spiral torn apart and absorbed by
M87. In either case, the nucleus could have become a dramatic beacon indeed—ultimately as bright as a million or a billion Cygnus X-1's—and the black hole could have grown to its present size in merely a few hundred million years.

I could not be sure that this black hole—or the Milky Way's black hole, for that matter—was homegrown. My visit to Virgo, following on the heels of my stay in the doomed Magellanic Clouds, had shown me that whole galaxies do collide and merge from time to time. If the cluster atmosphere, which, after all, contained much more matter than all of Virgo's galaxies combined, was flowing into M87, then why not spice it up with the occasional nucleus of some small galaxy that happened to contain a black hole. Maybe the featured black hole of M87 or the Milky Way came originally from a galaxy where conditions for growth were more favorable. An alien black hole captured by the Milky Way, like a pebble thrown into a whirlpool, would spin with the swirling disk for a little while but then quickly sink into the center. And if a modest black hole had already been waiting there, then repeated black-hole mergers could have built up the monster I saw, more rapidly than any steady feeding by absorption of gas clouds or stars.

Or perhaps these central black holes antedate their galaxies altogether. Perhaps they are primordial beings from the era before there were any stars at all, when the Universe consisted of lumpy gaseous soup. Could some especially dense clod of this undifferentiated stuff simply have collapsed to form the black hole, and could the galaxy have collected around it later? Which came first, the black hole or the galaxy?

I could not answer these questions, even after all I had seen, but I was much better prepared to think about them than I had been during the early days of my journey. I no longer believed there were simple answers to any of these questions. Galaxies and their huge, central black holes were probably interrelated in as
many complex ways as stars were with galaxies, galaxies with clusters of galaxies, and dark interstellar clouds with glittering young star clusters like the Trapezium. Here, on this vast stage of M87's nucleus, were so many of the same attributes—the same jets; the same swirling motions of the disk, engendered by gravity; the same intermediary action of the magnetic field, stretching and snapping with the transmission of energy from one form to another—that I had seen in so many places before and on so many scales. There was also the black hole, the same engine that drove only modest activities at the center of the Milky Way but here was expanded a thousand times.

The important hierarchy was not just one of objects. It was a symphony of geometric arrangements, patterns of motion, and sequences of events, repeated all over the Universe, over a range of scales that was still difficult for me to comprehend, even though I had seen it firsthand.

I hovered near the brink of M87's huge black hole, wondering what to do next. Did I dare to throw myself into the hole, for science's sake or to resolve my state of uncertainty? The answer in either case was clear: No, out of cowardice if for no other reason. My sickening encounters with tidal forces years ago had left me with a phobia that made it impossible to take such a plunge. Falling into a black hole this large would buy me an hour or two inside before the inevitable stretching forces . . . I could not even bear to think about it. It would be pointless, anyway. There would be no hope of recording what I saw.

Did I dare return to Earth? For me, only a few decades had passed, but the relativity of time, I knew, would preclude a comfortable homecoming. By my reckoning, Earth had aged at least 60 million years since I left. It would be another 60 million years, Earth's time, before I could return to my home planet's vicinity. Did any creatures resembling humans still exist? It seemed unlikely. Was there still a breathable atmosphere? Perhaps my descendants had decamped for other worlds—how was I to find them? I did not know and there was no way to find out. Any signals I could receive from Earth now would be tens of
millions of years old. Still, I wagered that the home planet was still there; its lure was almost irresistible. I could not quite let go of the thought of returning.

The alternative? To continue onward, probing other nearby galaxies or venturing more distantly. Another 40 years of travel with the Shangri-La effect could bring me as far as I wanted to go across the Universe—billions of light-years from home if I so chose. Now that I was aware of the great hierarchy of structures, the repetition of themes on ever-widening scales, I began to perceive new possibilities as I stared out into space. There, in the direction that, had I been on Earth, would have framed a beautiful telescopic sight behind the constellation Coma Bernices, was a cluster of galaxies even richer than Virgo's. Six times farther away than Virgo, I could perceive the symmetrical grouping of thousands of galaxies, centered on an elliptical of comparably gargantuan proportions. What could it tell me about the development of cosmic structure that I hadn't already seen? I could see, in my imagination, the swarms of galaxies even greater than those that I had seen in Virgo, merging, blending, smoothing out their structures. The Virgo Cluster and the Local Group might someday come together and perhaps then merge with some even larger cluster of galaxies. Would the Milky Way retain its identity then, or would it have been subsumed into some larger galaxy, just as (by then) it will have swallowed the Magellanic Clouds? Where would this hierarchy of processes, this extension of scales, end? By traveling farther, would I encounter black holes 10 billion and 100 billion times the mass of the Sun, even grander evolutionary cycles, and longer and more powerful jets? I already knew that the answer to at least some of these questions, perhaps all of them, was undoubtedly yes. But I wasn't sure that this knowledge was sufficient justification to spur me onward. The more important question: would the patterns merely repeat, scaled up or scaled down? Or were there great new organizing principles waiting to be discovered in the galaxies beyond? I wanted to think so, but I wasn't sure.

Certain things I might never be able to explore. Now, I could look out into the far reaches of the Universe, dimly, and see things as they were billions of years ago. I had seen firsthand that the Universe was evolving, in the constant production of heavy elements, the streams of gas falling into or escaping galaxies, the merging and disruption of whole galaxies themselves. The Universe had developed from a very different sort of place. There was an era when the sky shone bright with quasars, huge black holes in their first flush of glory. I can see them out there, their light having been emitted 3, 5, 10 billion years ago. I could be at their locations in 3, 5, 10 billion years from now, but by then they would almost certainly be gone.

Only one choice is easy. The urge to communicate is as strong as ever. I do not know to whom, if anyone, I am addressing this memoir. But not to have recorded my impressions of this voyage and its progress, so far, would have been unthinkable. I therefore cast this memoir into space near the center of this wonderful, enormous galaxy M87, in the hope that it may someday be deciphered.

This book owes its existence largely to my fellow astrophysicists, whose collective efforts have led to the images and ideas I try to portray. I have been asked which portions of the story are based on “fact” and which are speculative. This is a difficult question to answer. We astrophysicists hone our cosmic perspectives through constant debate over what is plausible and what is not, given the constraints of observation and logic. Little in astrophysics is ever proven in the sense that a mathematical theorem can be proven. What the narrator presents as facts or observations are my best guesses, based on our current level of understanding. Where an issue is really up in the air, I have made sure that the narrator has not been able to discover the answer, either!

I have tried not to take too many liberties with the principle of causality. Astrophysical objects change over time. I present such objects as SS 433 and the jet in M87 as they are seen today, even though they are likely to be quite different when the narrator reaches them 65,000 and 60 million years, respectively, in the future. I could not ignore this problem in the case of the Crab Nebula, which will have changed beyond recognition by the time the narrator arrives. Therefore, I decided to invent a clone that happens to go off, just in time, a thousand light-years from the old Crab. The chance of this happening in reality is exceedingly small (probably no more than. 1 chance in 1000, if that); I hope you will indulge me this artifice.

The narrator's method of travel, which exploits the effect known technically as “relativistic time dilation,” is physically sound if not very feasible. It really is possible to travel arbitrarily
far across the Universe in a human lifespan (as perceived by the traveler) without violating any laws of nature. I try to be realistic in estimating the fuel and shielding requirements as demanded by physical law, but have little familiarity with the immense literature that exists on methods of space travel. I do not attempt to discuss the exotic sensors that the narrator would surely need to view the scene outside his spacecraft's windows, given the distortions that would result from his motion.

I am grateful to the many friends and colleagues who read and criticized drafts in various stages of completion. I'd like especially to thank Jill Banwell, Caroline Bugler, Annalisa Celotti, Peta Dunstan, Betty Fingold, Michael Nowak, Martin Rees, and Marek Sikora. Most of this book was written while I was on sabbatical at the Institute of Astronomy, University of Cambridge, and the Institute for Theoretical Physics, University of California, Santa Barbara. In addition to the colleagues who provided stimulating environments at these two institutions, I especially thank my landlords and neighbors: Jim and Pat Hennessy in Cambridge, Saral Burdette and David Wieger in Santa Barbara, who did much to make my Earthbound travels so enjoyable. I am also indebted to the John Simon Guggenheim Memorial Foundation and the University of Colorado Council on Research and Creative Work for financial support during my sabbatical.