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.
Acknowledgements
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, Marti
n 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.
My editors—Amanda Cook, Sean Abbott, and Connie Day—helped to improve the book immeasurably, and I also thank Jeffrey Robbins for his early enthusiasm and support of the project. Dr. Ka Chun Yu found time to complete his ingenious illustration of Rocinante’s path the very week he was preparing to defend his Ph.D. thesis on the Orion Nebula. And my wife, Claire Hay, provided encouragement, sound advice, and thought-provoking discussion throughout.
Finally, I cannot overemphasize the role that public funding plays in making progress in astrophysics possible. Without research support from organizations like the National Science Foundation and the National Aeronautics and Space Administration, there would have been little to write about.
Glossary
accretion disk: disk of gas orbiting in the gravitational field of a body. Internal friction in the gas causes it to spiral toward the body, resulting in accretion.
atomic hydrogen: gas in which hydrogen exists in the form of individual atoms, neither ionized nor paired off into molecules. Much of the interstellar matter in the Milky Way’s disk takes this form.
Betelgeuse: a red supergiant located in the constellation Orion, about 500 light-years from Earth.
black hole: body whose gravitational field is so strong that northing that falls in, not even light, can escape. Two populations exist: Stellar-mass black holes are formed by the collapse of massive stars; supermassive black holes are of uncertain origin and exist at the centers of most, if not all, galaxies.
bulge: central stellar component of a spiral galaxy, consisting of stars on chaotic orbits.
Copernican Principle: guiding principle of astrophysics, according to which no special advantage is accorded to our viewpoint on the Universe. Thus any phenomena we observe are assumed to be commonplace.
Crab Nebula: remnant of a supernova explosion observed in A.D. 1054. A pulsar that spins 30 times a second powers this compact nebula in Taurus. By the time the narrator reaches it, the nebula has dispersed into interstellar space. M1 in Messier’s catalogue.
Crab II: an imaginary supernova remnant, similar to today’s Crab, visited by the narrator 90,000 years after his departure from Earth.
Cygnus X-1: massive X-ray–emitting object in a binary system. Believed to be a black hole accreting matter from a disk.
degeneracy pressure: resistance to compression exhibited by dense gases consisting of elementary particles, such as electrons or neutrons, regardless of temperature. Arises from random motions of tightly packed particles predicted by quantum mechanics. Prevents white dwarfs and neutron stars from collapsing.
Dumbbell Nebula: prominent planetary nebula located in the constellation Vulpecula. Its shape, resembling two luminous masses connected by a bar, gave rise to its name. M27 in Messier’s catalogue.
Einstein, Albert (1879–1955): German born, Swiss-American physicist who formulated the special and general theories of relativity. Also demonstrated the particulate nature of light, a key step in the development of quantum mechanics.
elliptical galaxy: roughly spherical galaxy consisting of stars moving on chaotic orbits under their mutual gravitational attractions.
general theory of relativity: theory of gravitation propounded by Albert Einstein in 1915, according to which gravity is a manifestation of the curvature of spacetime. Builds on the foundation laid 10 years earlier by Einstein’s special theory of relativity.
globular cluster: compact, spherical cluster of 100,000 to a few million stars, a few light-years across, orbiting a galaxy. Several hundred globular clusters orbit the Milky Way; huge galaxies like M87 contain thousands.
halo: extended region of stars and gas enveloping a galaxy.
Herschel, William (1738–1822): English astronomer who (among other discoveries) deduced the shape of the Milky Way through star counts, catalogued and classified numerous binary stars, star clusters and nebulae (including planetary nebulae), and discovered the planet Uranus.
horizon: surface surrounding a black hole from within which nothing can escape.
hot star: star with a high surface temperature and blue-white color. Any star that is a few times more massive than the Sun and is still burning hydrogen in its core. Also, an evolved (non-hydrogen burning) star with a surface temperature considerably higher than that of the Sun.
interstellar cloud: region of interstellar space where the density of gas is relatively high and the temperature is relatively low, compared to the surroundings.
interstellar dust: extremely fine granular material that coexists with gas in most regions of interstellar space.
ion: an atom stripped of one or more of its electrons.
jet: fast-moving, narrow stream of plasma that shoots out of the center of an accretion disk or other rotating system. Associated especially with black holes and protostars.
Kepler, Johannes (1571–1630): German mathematician and astronomer who deduced three fundamental laws of planetary motion. His Somnium, sive Astronomia lunaris (“Dream, or Lunar Astronomy”) was written in 1611 and published posthumously in 1634.
light, speed of: a universal constant, 300,000 kilometers per second. A light-year, the distance light travels in a year, equals 9.5 trillion kilometers.
Local Group: a loose grouping of several dozen galaxies that includes the Milky Way.
M87: an enormous elliptical galaxy in the Virgo Cluster— notable for its X-ray–emitting atmosphere, its rich cloud of globular clusters, and the high-speed jets shooting out of its center—that contains a three-billion-Solar-mass black hole. Catalogued by Messier.
Magellanic Clouds: two prominent satellite galaxies of the Milky Way.
Messier, Charles (1730–1817): French astronomer who compiled an early catalogue of astronomical objects, including many prominent nebulae, star clusters, and galaxies. Objects in his catalogue are denoted by the prefix M.
Milky Way: large spiral galaxy that contains the Solar System and most destinations described in this book. When capitalized, the word Galaxy refers specifically to the Milky Way.
molecular cloud: relatively cool, dense region of interstellar space in which a large proportion of the atoms have combined to form molecules.
nebula: illuminated patch of interstellar gas.
neutron star: superdense body with a mass similar to that of the Sun but a size of only 10–20 kilometers. Believed to form from the collapsing core of a massive star. Neutron stars have the strongest gravitational fields of all known objects except for black holes.
Orion: region of vigorous star formation located about 1500 light-years from Earth. Its appearance from Earth is dominated by the Orion Nebula (M42 in Messier’s catalogue), which is illuminated by the Trapezium. The Orion star-forming region is located in the constellation Orion.
planetary nebula: expanding envelope of gas released by a dying, low-mass star and illuminated by the still-hot stellar core.
plasma: an ionized gas—that is, one in which electrons have been stripped from their atomic nuclei. The hot gases that make up stars, accretion disks, and most other systems discussed in this book are plasmas. They are extremely good electrical conductors, which enables them to trap magnetic fields.
protostar: star in the process of formation, consisting of a central core, an accretion disk, and, often, jets.
pulsar: magnetized, spinning neutron star producing beams of radiation that rotate with the star.
quantum mechanics: laws of physics, formulated during the first third of the twentieth century, that describe atoms, molecules, and other small-scale phenomena. This system posits that all forms of energy and matter exhibit characteristics of both particles and waves.
red giant: star that has exhausted the hydrogen in its core and is burning hydrogen in a shell surrounding the core. Characterized by an enormous, cool envelope.
red supergiant: star with an inert core that is burning fuels heavier than hydrogen in shells surrounding the core. Larger and more luminous than a red giant.
Shangri-La effect: factor by which time is slowed down for the narrator, relative to other objects in the Galaxy, as a result of the narrator’s extremely high speed.
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