A Short History of Nearly Everything Part 1

A Short History of Nearly Everything.

by Bryson, Bill.

PART I

LOST IN THE COSMOS

They're all in the same plane.They're all going around in thesame direction. . . . It's perfect,you know. It's gorgeous. It'salmost uncanny.-Astronomer Geoffrey Marcydescribing the solar system

CHAPTER 1: HOW TO BUILD A UNIVERSE

NO MATTER HOW hard you try you will never be able to grasp just how tiny, how spatially una.s.suming, is a proton. It is just way too small.A proton is an infinitesimal part of an atom, which is itself of course an insubstantial thing. Protons are so small that a little dib of ink like the dot on thisi can hold something in the region of 500,000,000,000 of them, rather more than the number of seconds contained in half a million years. So protons are exceedingly microscopic, to say the very least.Now imagine if you can (and of course you can't) shrinking one of those protons down to a billionth of its normal size into a s.p.a.ce so small that it would make a proton look enormous. Now pack into that tiny, tiny s.p.a.ce about an ounce of matter. Excellent. You are ready to start a universe.I'm a.s.suming of course that you wish to build an inflationary universe. If you'd prefer instead to build a more old-fashioned, standard Big Bang universe, you'll need additional materials. In fact, you will need to gather up everything there is every last mote and particle of matter between here and the edge of creation and squeeze it into a spot so infinitesimally compact that it has no dimensions at all. It is known as a singularity.In either case, get ready for a really big bang. Naturally, you will wish to retire to a safe place to observe the spectacle. Unfortunately, there is nowhere to retire to because outside the singularity there is nowhere . When the universe begins to expand, it won't be spreading out to fill a larger emptiness. The only s.p.a.ce that exists is the s.p.a.ce it creates as it goes.It is natural but wrong to visualize the singularity as a kind of pregnant dot hanging in a dark, boundless void. But there is no s.p.a.ce, no darkness. The singularity has no around around it. There is no s.p.a.ce for it to occupy, no place for it to be. We can't even ask how long it has been there-whether it has just lately popped into being, like a good idea, or whether it has been there forever, quietly awaiting the right moment. Time doesn't exist. There is no past for it to emerge from.And so, from nothing, our universe begins.In a single blinding pulse, a moment of glory much too swift and expansive for any form of words, the singularity a.s.sumes heavenly dimensions, s.p.a.ce beyond conception. In the first lively second (a second that many cosmologists will devote careers to shaving into ever-finer wafers) is produced gravity and the other forces that govern physics. In less than a minute the universe is a million billion miles across and growing fast. There is a lot of heat now, ten billion degrees of it, enough to begin the nuclear reactions that create the lighter elements-princ.i.p.ally hydrogen and helium, with a dash (about one atom in a hundred million) of lithium. In three minutes, 98 percent of all the matter there is or will ever be has been produced. We have a universe. It is a place of the most wondrous and gratifying possibility, and beautiful, too. And it was all done in about the time it takes to make a sandwich.When this moment happened is a matter of some debate. Cosmologists have long argued over whether the moment of creation was 10 billion years ago or twice that or something in between. The consensus seems to be heading for a figure of about 13.7 billion years, but these things are notoriously difficult to measure, as we shall see further on. All that can really be said is that at some indeterminate point in the very distant past, for reasons unknown, there came the moment known to science ast = 0. We were on our way.There is of course a great deal we don't know, and much of what we think we know we haven't known, or thought we've known, for long. Even the notion of the Big Bang is quite a recent one. The idea had been kicking around since the 1920s, when Georges Lemaitre, a Belgian priest-scholar, first tentatively proposed it, but it didn't really become an active notion in cosmology until the mid-1960s when two young radio astronomers made an extraordinary and inadvertent discovery.Their names were Arno Penzias and Robert Wilson. In 1965, they were trying to make use of a large communications antenna owned by Bell Laboratories at Holmdel, New Jersey, but they were troubled by a persistent background noise-a steady, steamy hiss that made any experimental work impossible. The noise was unrelenting and unfocused. It came from every point in the sky, day and night, through every season. For a year the young astronomers did everything they could think of to track down and eliminate the noise. They tested every electrical system. They rebuilt instruments, checked circuits, wiggled wires, dusted plugs. They climbed into the dish and placed duct tape over every seam and rivet. They climbed back into the dish with brooms and scrubbing brushes and carefully swept it clean of what they referred to in a later paper as white dielectric material, or what is known more commonly as bird s.h.i.t. Nothing they tried worked.Unknown to them, just thirty miles away at Princeton University, a team of scientists led by Robert d.i.c.ke was working on how to find the very thing they were trying so diligently to get rid of. The Princeton researchers were pursuing an idea that had been suggested in the 1940s by the Russian-born astrophysicist George Gamow that if you looked deep enough into s.p.a.ce you should find some cosmic background radiation left over from the Big Bang. Gamow calculated that by the time it crossed the vastness of the cosmos, the radiation would reach Earth in the form of microwaves. In a more recent paper he had even suggested an instrument that might do the job: the Bell antenna at Holmdel. Unfortunately, neither Penzias and Wilson, nor any of the Princeton team, had read Gamow's paper.The noise that Penzias and Wilson were hearing was, of course, the noise that Gamow had postulated. They had found the edge of the universe, or at least the visible part of it, 90 billion trillion miles away. They were seeing the first photons-the most ancient light in the universe-though time and distance had converted them to microwaves, just as Gamow had predicted. In his bookThe Inflationary Universe , Alan Guth provides an a.n.a.logy that helps to put this finding in perspective. If you think of peering into the depths of the universe as like looking down from the hundredth floor of the Empire State Building (with the hundredth floor representing now and street level representing the moment of the Big Bang), at the time of Wilson and Penzias's discovery the most distant galaxies anyone had ever detected were on about the sixtieth floor, and the most distant things-quasars-were on about the twentieth. Penzias and Wilson's finding pushed our acquaintance with the visible universe to within half an inch of the sidewalk.Still unaware of what caused the noise, Wilson and Penzias phoned d.i.c.ke at Princeton and described their problem to him in the hope that he might suggest a solution. d.i.c.ke realized at once what the two young men had found. Well, boys, we've just been scooped, he told his colleagues as he hung up the phone.Soon afterward theAstrophysical Journal published two articles: one by Penzias and Wilson describing their experience with the hiss, the other by d.i.c.ke's team explaining its nature. Although Penzias and Wilson had not been looking for cosmic background radiation, didn't know what it was when they had found it, and hadn't described or interpreted its character in any paper, they received the 1978 n.o.bel Prize in physics. The Princeton researchers got only sympathy. According to Dennis Overbye inLonely Hearts of the Cosmos , neither Penzias nor Wilson altogether understood the significance of what they had found until they read about it in theNew York Times .Incidentally, disturbance from cosmic background radiation is something we have all experienced. Tune your television to any channel it doesn't receive, and about 1 percent of the dancing static you see is accounted for by this ancient remnant of the Big Bang. The next time you complain that there is nothing on, remember that you can always watch the birth of the universe.Although everyone calls it the Big Bang, many books caution us not to think of it as an explosion in the conventional sense. It was, rather, a vast, sudden expansion on a whopping scale. So what caused it?One notion is that perhaps the singularity was the relic of an earlier, collapsed universe-that we're just one of an eternal cycle of expanding and collapsing universes, like the bladder on an oxygen machine. Others attribute the Big Bang to what they call a false vacuum or a scalar field or vacuum energy-some quality or thing, at any rate, that introduced a measure of instability into the nothingness that was. It seems impossible that you could get something from nothing, but the fact that once there was nothing and now there is a universe is evident proof that you can. It may be that our universe is merely part of many larger universes, some in different dimensions, and that Big Bangs are going on all the time all over the place. Or it may be that s.p.a.ce and time had some other forms altogether before the Big Bang-forms too alien for us to imagine-and that the Big Bang represents some sort of transition phase, where the universe went from a form we can't understand to one we almost can. These are very close to religious questions, Dr. Andrei Linde, a cosmologist at Stanford, told theNew York Times in 2001.The Big Bang theory isn't about the bang itself but about what happened after the bang. Not long after, mind you. By doing a lot of math and watching carefully what goes on in particle accelerators, scientists believe they can look back to 10-43seconds after the moment of creation, when the universe was still so small that you would have needed a microscope to find it. We mustn't swoon over every extraordinary number that comes before us, but it is perhaps worth latching on to one from time to time just to be reminded of their ungraspable and amazing breadth. Thus 10-43is 0.0000000000000000000000000000000000000000001, or one 10 million trillion trillion trillionths of a second.[*]Most of what we know, or believe we know, about the early moments of the universe is thanks to an idea called inflation theory first propounded in 1979 by a junior particle physicist, then at Stanford, now at MIT, named Alan Guth. He was thirty-two years old and, by his own admission, had never done anything much before. He would probably never have had his great theory except that he happened to attend a lecture on the Big Bang given by none other than Robert d.i.c.ke. The lecture inspired Guth to take an interest in cosmology, and in particular in the birth of the universe.The eventual result was the inflation theory, which holds that a fraction of a moment after the dawn of creation, the universe underwent a sudden dramatic expansion. It inflated-in effect ran away with itself, doubling in size every 10-34seconds. The whole episode may have lasted no more than 10-30seconds-that's one million million million million millionths of a second-but it changed the universe from something you could hold in your hand to something at least 10,000,000,000,000,000,000,000,000 times bigger. Inflation theory explains the ripples and eddies that make our universe possible. Without it, there would be no clumps of matter and thus no stars, just drifting gas and everlasting darkness.According to Guth's theory, at one ten-millionth of a trillionth of a trillionth of a trillionth of a second, gravity emerged. After another ludicrously brief interval it was joined by electromagnetism and the strong and weak nuclear forces-the stuff of physics. These were joined an instant later by swarms of elementary particles-the stuff of stuff. From nothing at all, suddenly there were swarms of photons, protons, electrons, neutrons, and much else-between 1079and 1089of each, according to the standard Big Bang theory.Such quant.i.ties are of course ungraspable. It is enough to know that in a single cracking instant we were endowed with a universe that was vast-at least a hundred billion light-years across, according to the theory, but possibly any size up to infinite-and perfectly arrayed for the creation of stars, galaxies, and other complex systems.What is extraordinary from our point of view is how well it turned out for us. If the universe had formed just a tiny bit differently-if gravity were fractionally stronger or weaker, if the expansion had proceeded just a little more slowly or swiftly-then there might never have been stable elements to make you and me and the ground we stand on. Had gravity been a trifle stronger, the universe itself might have collapsed like a badly erected tent, without precisely the right values to give it the right dimensions and density and component parts. Had it been weaker, however, nothing would have coalesced. The universe would have remained forever a dull, scattered void.This is one reason that some experts believe there may have been many other big bangs, perhaps trillions and trillions of them, spread through the mighty span of eternity, and that the reason we exist in this particular one is that this is one wecould exist in. As Edward P. Tryon of Columbia University once put it: In answer to the question of why it happened, I offer the modest proposal that our Universe is simply one of those things which happen from time to time. To which adds Guth: Although the creation of a universe might be very unlikely, Tryon emphasized that no one had counted the failed attempts.Martin Rees, Britain's astronomer royal, believes that there are many universes, possibly an infinite number, each with different attributes, in different combinations, and that we simply live in one that combines things in the way that allows us to exist. He makes an a.n.a.logy with a very large clothing store: If there is a large stock of clothing, you're not surprised to find a suit that fits. If there are many universes, each governed by a differing set of numbers, there will be one where there is a particular set of numbers suitable to life. We are in that one.Rees maintains that six numbers in particular govern our universe, and that if any of these values were changed even very slightly things could not be as they are. For example, for the universe to exist as it does requires that hydrogen be converted to helium in a precise but comparatively stately manner-specifically, in a way that converts seven one-thousandths of its ma.s.s to energy. Lower that value very slightly-from 0.007 percent to 0.006 percent, say-and no transformation could take place: the universe would consist of hydrogen and nothing else. Raise the value very slightly-to 0.008 percent-and bonding would be so wildly prolific that the hydrogen would long since have been exhausted. In either case, with the slightest tweaking of the numbers the universe as we know and need it would not be here.I should say that everything is just rightso far . In the long term, gravity may turn out to be a little too strong, and one day it may halt the expansion of the universe and bring it collapsing in upon itself, till it crushes itself down into another singularity, possibly to start the whole process over again. On the other hand it may be too weak and the universe will keep racing away forever until everything is so far apart that there is no chance of material interactions, so that the universe becomes a place that is inert and dead, but very roomy. The third option is that gravity is just right-critical density is the cosmologists' term for it-and that it will hold the universe together at just the right dimensions to allow things to go on indefinitely. Cosmologists in their lighter moments sometimes call this the Goldilocks effect-that everything is just right. (For the record, these three possible universes are known respectively as closed, open, and flat.)Now the question that has occurred to all of us at some point is: what would happen if you traveled out to the edge of the universe and, as it were, put your head through the curtains? Where would your headbe if it were no longer in the universe? What would you find beyond? The answer, disappointingly, is that you can never get to the edge of the universe. That's not because it would take too long to get there-though of course it would-but because even if you traveled outward and outward in a straight line, indefinitely and pugnaciously, you would never arrive at an outer boundary. Instead, you would come back to where you began (at which point, presumably, you would rather lose heart in the exercise and give up). The reason for this is that the universe bends, in a way we can't adequately imagine, in conformance with Einstein's theory of relativity (which we will get to in due course). For the moment it is enough to know that we are not adrift in some large, ever-expanding bubble. Rather, s.p.a.ce curves, in a way that allows it to be boundless but finite. s.p.a.ce cannot even properly be said to be expanding because, as the physicist and n.o.bel laureate Steven Weinberg notes, solar systems and galaxies are not expanding, and s.p.a.ce itself is not expanding. Rather, the galaxies are rushing apart. It is all something of a challenge to intuition. Or as the biologist J. B. S. Haldane once famously observed: The universe is not only queerer than we suppose; it is queerer than we can suppose.The a.n.a.logy that is usually given for explaining the curvature of s.p.a.ce is to try to imagine someone from a universe of flat surfaces, who had never seen a sphere, being brought to Earth. No matter how far he roamed across the planet's surface, he would never find an edge. He might eventually return to the spot where he had started, and would of course be utterly confounded to explain how that had happened. Well, we are in the same position in s.p.a.ce as our puzzled flatlander, only we are flummoxed by a higher dimension.Just as there is no place where you can find the edge of the universe, so there is no place where you can stand at the center and say: This is where it all began. This is the centermost point of it all. We areall at the center of it all. Actually, we don't know that for sure; we can't prove it mathematically. Scientists just a.s.sume that we can't really be the center of the universe-think what that would imply-but that the phenomenon must be the same for all observers in all places. Still, we don't actually know.For us, the universe goes only as far as light has traveled in the billions of years since the universe was formed. This visible universe-the universe we know and can talk about-is a million million million million (that's 1,000,000,000,000,000,000,000,000) miles across. But according to most theories the universe at large-the meta-universe, as it is sometimes called-is vastly roomier still. According to Rees, the number of light-years to the edge of this larger, unseen universe would be written not with ten zeroes, not even with a hundred, but with millions. In short, there's more s.p.a.ce than you can imagine already without going to the trouble of trying to envision some additional beyond.For a long time the Big Bang theory had one gaping hole that troubled a lot of people-namely that it couldn't begin to explain how we got here. Although 98 percent of all the matter that exists was created with the Big Bang, that matter consisted exclusively of light gases: the helium, hydrogen, and lithium that we mentioned earlier. Not one particle of the heavy stuff so vital to our own being-carbon, nitrogen, oxygen, and all the rest-emerged from the gaseous brew of creation. But-and here's the troubling point-to forge these heavy elements, you need the kind of heat and energy of a Big Bang. Yet there has been only one Big Bang and it didn't produce them.So where did they come from?Interestingly, the man who found the answer to that question was a cosmologist who heartily despised the Big Bang as a theory and coined the term Big Bang sarcastically, as a way of mocking it. We'll get to him shortly, but before we turn to the question of how we got here, it might be worth taking a few minutes to consider just where exactly here is.

CHAPTER 2: WELCOME TO THE SOLAR SYSTEM

ASTRONOMERS THESE DAYS can do the most amazing things. If someone struck a match on the Moon, they could spot the flare. From the tiniest throbs and wobbles of distant stars they can infer the size and character and even potential habitability of planets much too remote to be seen-planets so distant that it would take us half a million years in a s.p.a.ceship to get there. With their radio telescopes they can capture wisps of radiation so preposterously faint that thetotal amount of energy collected from outside the solar system by all of them together since collecting began (in 1951) is less than the energy of a single snowflake striking the ground, in the words of Carl Sagan.In short, there isn't a great deal that goes on in the universe that astronomers can't find when they have a mind to. Which is why it is all the more remarkable to reflect that until 1978 no one had ever noticed that Pluto has a moon. In the summer of that year, a young astronomer named James Christy at the U.S. Naval Observatory in Flagstaff, Arizona, was making a routine examination of photographic images of Pluto when he saw that there was something there-something blurry and uncertain but definitely other than Pluto. Consulting a colleague named Robert Harrington, he concluded that what he was looking at was a moon. And it wasn't just any moon. Relative to the planet, it was the biggest moon in the solar system.This was actually something of a blow to Pluto's status as a planet, which had never been terribly robust anyway. Since previously the s.p.a.ce occupied by the moon and the s.p.a.ce occupied by Pluto were thought to be one and the same, it meant that Pluto was much smaller than anyone had supposed-smaller even than Mercury. Indeed, seven moons in the solar system, including our own, are larger.Now a natural question is why it took so long for anyone to find a moon in our own solar system. The answer is that it is partly a matter of where astronomers point their instruments and partly a matter of what their instruments are designed to detect, and partly it's just Pluto. Mostly it's where they point their instruments. In the words of the astronomer Clark Chapman: Most people think that astronomers get out at night in observatories and scan the skies. That's not true. Almost all the telescopes we have in the world are designed to peer at very tiny little pieces of the sky way off in the distance to see a quasar or hunt for black holes or look at a distant galaxy. The only real network of telescopes that scans the skies has been designed and built by the military.We have been spoiled by artists' renderings into imagining a clarity of resolution that doesn't exist in actual astronomy. Pluto in Christy's photograph is faint and fuzzy-a piece of cosmic lint-and its moon is not the romantically backlit, crisply delineated companion orb you would get in aNational Geographic painting, but rather just a tiny and extremely indistinct hint of additional fuzziness. Such was the fuzziness, in fact, that it took seven years for anyone to spot the moon again and thus independently confirm its existence.One nice touch about Christy's discovery was that it happened in Flagstaff, for it was there in 1930 that Pluto had been found in the first place. That seminal event in astronomy was largely to the credit of the astronomer Percival Lowell. Lowell, who came from one of the oldest and wealthiest Boston families (the one in the famous ditty about Boston being the home of the bean and the cod, where Lowells spoke only to Cabots, while Cabots spoke only to G.o.d), endowed the famous observatory that bears his name, but is most indelibly remembered for his belief that Mars was covered with ca.n.a.ls built by industrious Martians for purposes of conveying water from polar regions to the dry but productive lands nearer the equator.Lowell's other abiding conviction was that there existed, somewhere out beyond Neptune, an undiscovered ninth planet, dubbed Planet X. Lowell based this belief on irregularities he detected in the orbits of Ura.n.u.s and Neptune, and devoted the last years of his life to trying to find the ga.s.sy giant he was certain was out there. Unfortunately, he died suddenly in 1916, at least partly exhausted by his quest, and the search fell into abeyance while Lowell's heirs squabbled over his estate. However, in 1929, partly as a way of deflecting attention away from the Mars ca.n.a.l saga (which by now had become a serious embarra.s.sment), the Lowell Observatory directors decided to resume the search and to that end hired a young man from Kansas named Clyde Tombaugh.Tombaugh had no formal training as an astronomer, but he was diligent and he was astute, and after a year's patient searching he somehow spotted Pluto, a faint point of light in a glittery firmament. It was a miraculous find, and what made it all the more striking was that the observations on which Lowell had predicted the existence of a planet beyond Neptune proved to be comprehensively erroneous. Tombaugh could see at once that the new planet was nothing like the ma.s.sive gasball Lowell had postulated, but any reservations he or anyone else had about the character of the new planet were soon swept aside in the delirium that attended almost any big news story in that easily excited age. This was the first American-discovered planet, and no one was going to be distracted by the thought that it was really just a distant icy dot. It was named Pluto at least partly because the first two letters made a monogram from Lowell's initials. Lowell was posthumously hailed everywhere as a genius of the first order, and Tombaugh was largely forgotten, except among planetary astronomers, who tend to revere him.A few astronomers continue to think there may be a Planet X out there-a real whopper, perhaps as much as ten times the size of Jupiter, but so far out as to be invisible to us. (It would receive so little sunlight that it would have almost none to reflect.) The idea is that it wouldn't be a conventional planet like Jupiter or Saturn-it's much too far away for that; we're talking perhaps 4.5 trillion miles-but more like a sun that never quite made it. Most star systems in the cosmos are binary (double-starred), which makes our solitary sun a slight oddity.As for Pluto itself, n.o.body is quite sure how big it is, or what it is made of, what kind of atmosphere it has, or even what it really is. A lot of astronomers believe it isn't a planet at all, but merely the largest object so far found in a zone of galactic debris known as the Kuiper belt. The Kuiper belt was actually theorized by an astronomer named F. C. Leonard in 1930, but the name honors Gerard Kuiper, a Dutch native working in America, who expanded the idea. The Kuiper belt is the source of what are known as short-period comets-those that come past pretty regularly-of which the most famous is Halley's comet. The more reclusive long-period comets (among them the recent visitors Hale-Bopp and Hyakutake) come from the much more distant Oort cloud, about which more presently.It is certainly true that Pluto doesn't act much like the other planets. Not only is it runty and obscure, but it is so variable in its motions that no one can tell you exactly where Pluto will be a century hence. Whereas the other planets...o...b..t on more or less the same plane, Pluto's...o...b..tal path is tipped (as it were) out of alignment at an angle of seventeen degrees, like the brim of a hat tilted rakishly on someone's head. Its...o...b..t is so irregular that for substantial periods on each of its lonely circuits around the Sun it is closer to us than Neptune is. For most of the 1980s and 1990s, Neptune was in fact the solar system's most far-flung planet. Only on February 11, 1999, did Pluto return to the outside lane, there to remain for the next 228 years.So if Pluto really is a planet, it is certainly an odd one. It is very tiny: just one-quarter of 1 percent as ma.s.sive as Earth. If you set it down on top of the United States, it would cover not quite half the lower forty-eight states. This alone makes it extremely anomalous; it means that our planetary system consists of four rocky inner planets, four ga.s.sy outer giants, and a tiny, solitary iceball. Moreover, there is every reason to suppose that we may soon begin to find other even larger icy spheres in the same portion of s.p.a.ce. Then wewill have problems. After Christy spotted Pluto's moon, astronomers began to regard that section of the cosmos more attentively and as of early December 2002 had found over six hundred additional Trans-Neptunian Objects, or Plutinos as they are alternatively called. One, dubbed Varuna, is nearly as big as Pluto's moon. Astronomers now think there may be billions of these objects. The difficulty is that many of them are awfully dark. Typically they have an albedo, or reflectiveness, of just 4 percent, about the same as a lump of charcoal-and of course these lumps of charcoal are about four billion miles away.And how far is that exactly? It's almost beyond imagining. s.p.a.ce, you see, is just enormous-just enormous. Let's imagine, for purposes of edification and entertainment, that we are about to go on a journey by rocketship. We won't go terribly far-just to the edge of our own solar system-but we need to get a fix on how big a place s.p.a.ce is and what a small part of it we occupy.Now the bad news, I'm afraid, is that we won't be home for supper. Even at the speed of light, it would take seven hours to get to Pluto. But of course we can't travel at anything like that speed. We'll have to go at the speed of a s.p.a.ceship, and these are rather more lumbering. The best speeds yet achieved by any human object are those of theVoyager 1 and2s.p.a.cecraft, which are now flying away from us at about thirty-five thousand miles an hour.The reason theVoyager craft were launched when they were (in August and September 1977) was that Jupiter, Saturn, Ura.n.u.s, and Neptune were aligned in a way that happens only once every 175 years. This enabled the twoVoyagers to use a gravity a.s.sist technique in which the craft were successively flung from one ga.s.sy giant to the next in a kind of cosmic version of crack the whip. Even so, it took them nine years to reach Ura.n.u.s and a dozen to cross the orbit of Pluto. The good news is that if we wait until January 2006 (which is when NASA'sNew Horizons s.p.a.cecraft is tentatively scheduled to depart for Pluto) we can take advantage of favorable Jovian positioning, plus some advances in technology, and get there in only a decade or so-though getting home again will take rather longer, I'm afraid. At all events, it's going to be a long trip.Now the first thing you are likely to realize is that s.p.a.ce is extremely well named and rather dismayingly uneventful. Our solar system may be the liveliest thing for trillions of miles, but all the visible stuff in it-the Sun, the planets and their moons, the billion or so tumbling rocks of the asteroid belt, comets, and other miscellaneous drifting detritus-fills less than a trillionth of the available s.p.a.ce. You also quickly realize that none of the maps you have ever seen of the solar system were remotely drawn to scale. Most schoolroom charts show the planets coming one after the other at neighborly intervals-the outer giants actually cast shadows over each other in many ill.u.s.trations-but this is a necessary deceit to get them all on the same piece of paper. Neptune in reality isn't just a little bit beyond Jupiter, it's way beyond Jupiter-five times farther from Jupiter than Jupiter is from us, so far out that it receives only 3 percent as much sunlight as Jupiter.Such are the distances, in fact, that it isn't possible, in any practical terms, to draw the solar system to scale. Even if you added lots of fold-out pages to your textbooks or used a really long sheet of poster paper, you wouldn't come close. On a diagram of the solar system to scale, with Earth reduced to about the diameter of a pea, Jupiter would be over a thousand feet away and Pluto would be a mile and a half distant (and about the size of a bacterium, so you wouldn't be able to see it anyway). On the same scale, Proxima Centauri, our nearest star, would be almost ten thousand miles away. Even if you shrank down everything so that Jupiter was as small as the period at the end of this sentence, and Pluto was no bigger than a molecule, Pluto would still be over thirty-five feet away.So the solar system is really quite enormous. By the time we reach Pluto, we have come so far that the Sun-our dear, warm, skin-tanning, life-giving Sun-has shrunk to the size of a pinhead. It is little more than a bright star. In such a lonely void you can begin to understand how even the most significant objects-Pluto's moon, for example-have escaped attention. In this respect, Pluto has hardly been alone. Until theVoyager expeditions, Neptune was thought to have two moons;Voyager found six more. When I was a boy, the solar system was thought to contain thirty moons. The total now is at least ninety, about a third of which have been found in just the last ten years.The point to remember, of course, is that when considering the universe at large we don't actually know what is in our own solar system.Now the other thing you will notice as we speed past Pluto is that we are speeding past Pluto. If you check your itinerary, you will see that this is a trip to the edge of our solar system, and I'm afraid we're not there yet. Pluto may be the last object marked on schoolroom charts, but the system doesn't end there. In fact, it isn't even close to ending there. We won't get to the solar system's edge until we have pa.s.sed through the Oort cloud, a vast celestial realm of drifting comets, and we won't reach the Oort cloud for another-I'm so sorry about this-ten thousand years. Far from marking the outer edge of the solar system, as those schoolroom maps so cavalierly imply, Pluto is barely one-fifty-thousandth of the way.Of course we have no prospect of such a journey. A trip of 240,000 miles to the Moon still represents a very big undertaking for us. A manned mission to Mars, called for by the first President Bush in a moment of pa.s.sing giddiness, was quietly dropped when someone worked out that it would cost $450 billion and probably result in the deaths of all the crew (their DNA torn to tatters by high-energy solar particles from which they could not be shielded).Based on what we know now and can reasonably imagine, there is absolutely no prospect that any human being will ever visit the edge of our own solar system-ever. It is just too far. As it is, even with the Hubble telescope, we can't see even into the Oort cloud, so we don't actually know that it is there. Its existence is probable but entirely hypothetical.[]About all that can be said with confidence about the Oort cloud is that it starts somewhere beyond Pluto and stretches some two light-years out into the cosmos. The basic unit of measure in the solar system is the Astronomical Unit, or AU, representing the distance from the Sun to the Earth. Pluto is about forty AUs from us, the heart of the Oort cloud about fifty thousand. In a word, it is remote.But let's pretend again that we have made it to the Oort cloud. The first thing you might notice is how very peaceful it is out here. We're a long way from anywhere now-so far from our own Sun that it's not even the brightest star in the sky. It is a remarkable thought that that distant tiny twinkle has enough gravity to hold all these comets in orbit. It's not a very strong bond, so the comets drift in a stately manner, moving at only about 220 miles an hour. From time to time some of these lonely comets are nudged out of their normal orbit by some slight gravitational perturbation-a pa.s.sing star perhaps. Sometimes they are ejected into the emptiness of s.p.a.ce, never to be seen again, but sometimes they fall into a long orbit around the Sun. About three or four of these a year, known as long-period comets, pa.s.s through the inner solar system. Just occasionally these stray visitors smack into something solid, like Earth. That's why we've come out here now-because the comet we have come to see has just begun a long fall toward the center of the solar system. It is headed for, of all places, Manson, Iowa. It is going to take a long time to get there-three or four million years at least-so we'll leave it for now, and return to it much later in the story.So that's your solar system. And what else is out there, beyond the solar system? Well, nothing and a great deal, depending on how you look at it.In the short term, it's nothing. The most perfect vacuum ever created by humans is not as empty as the emptiness of interstellar s.p.a.ce. And there is a great deal of this nothingness until you get to the next bit of something. Our nearest neighbor in the cosmos, Proxima Centauri, which is part of the three-star cl.u.s.ter known as Alpha Centauri, is 4.3 light-years away, a sissy skip in galactic terms, but that is still a hundred million times farther than a trip to the Moon. To reach it by s.p.a.ceship would take at least twenty-five thousand years, and even if you made the trip you still wouldn't be anywhere except at a lonely clutch of stars in the middle of a vast nowhere. To reach the next landmark of consequence, Sirius, would involve another 4.6 light-years of travel. And so it would go if you tried to star-hop your way across the cosmos. Just reaching the center of our own galaxy would take far longer than we have existed as beings.s.p.a.ce, let me repeat, is enormous. The average distance between stars out there is 20 million million miles. Even at speeds approaching those of light, these are fantastically challenging distances for any traveling individual. Of course, it ispossible that alien beings travel billions of miles to amuse themselves by planting crop circles in Wiltshire or frightening the daylights out of some poor guy in a pickup truck on a lonely road in Arizona (they must have teenagers, after all), but it does seem unlikely.Still, statistically the probability that there are other thinking beings out there is good. n.o.body knows how many stars there are in the Milky Way-estimates range from 100 billion or so to perhaps 400 billion-and the Milky Way is just one of 140 billion or so other galaxies, many of them even larger than ours. In the 1960s, a professor at Cornell named Frank Drake, excited by such whopping numbers, worked out a famous equation designed to calculate the chances of advanced life in the cosmos based on a series of diminishing probabilities.Under Drake's equation you divide the number of stars in a selected portion of the universe by the number of stars that are likely to have planetary systems; divide that by the number of planetary systems that could theoretically support life; divide that by the number on which life, having arisen, advances to a state of intelligence; and so on. At each such division, the number shrinks colossally-yet even with the most conservative inputs the number of advanced civilizations just in the Milky Way always works out to be somewhere in the millions.What an interesting and exciting thought. We may be only one of millions of advanced civilizations. Unfortunately, s.p.a.ce being s.p.a.cious, the average distance between any two of these civilizations is reckoned to be at least two hundred light-years, which is a great deal more than merely saying it makes it sound. It means for a start that even if these beings know we are here and are somehow able to see us in their telescopes, they're watching light that left Earth two hundred years ago. So they're not seeing you and me. They're watching the French Revolution and Thomas Jefferson and people in silk stockings and powdered wigs-people who don't know what an atom is, or a gene, and who make their electricity by rubbing a rod of amber with a piece of fur and think that's quite a trick. Any message we receive from them is likely to begin Dear Sire, and congratulate us on the handsomeness of our horses and our mastery of whale oil. Two hundred light-years is a distance so far beyond us as to be, well, just beyond us.So even if we are not really alone, in all practical terms we are. Carl Sagan calculated the number of probable planets in the universe at large at 10 billion trillion-a number vastly beyond imagining. But what is equally beyond imagining is the amount of s.p.a.ce through which they are lightly scattered. If we were randomly inserted into the universe, Sagan wrote, the chances that you would be on or near a planet would be less than one in a billion trillion trillion. (That's 1033, or a one followed by thirty-three zeroes.) Worlds are precious.Which is why perhaps it is good news that in February 1999 the International Astronomical Union ruled officially that Pluto is a planet. The universe is a big and lonely place. We can do with all the neighbors we can get.

CHAPTER 3: THE REVEREND EVANS'S UNIVERSE

WHEN THE SKIES are clear and the Moon is not too bright, the Reverend Robert Evans, a quiet and cheerful man, lugs a bulky telescope onto the back deck of his home in the Blue Mountains of Australia, about fifty miles west of Sydney, and does an extraordinary thing. He looks deep into the past and finds dying stars.Looking into the past is of course the easy part. Glance at the night sky and what you see is history and lots of it-the stars not as they are now but as they were when their light left them. For all we know, the North Star, our faithful companion, might actually have burned out last January or in 1854 or at any time since the early fourteenth century and news of it just hasn't reached us yet. The best we can say-can ever say-is that it was still burning on this date 680 years ago. Stars die all the time. What Bob Evans does better than anyone else who has ever tried is spot these moments of celestial farewell.By day, Evans is a kindly and now semiretired minister in the Uniting Church in Australia, who does a bit of freelance work and researches the history of nineteenth-century religious movements. But by night he is, in his una.s.suming way, a t.i.tan of the skies. He hunts supernovae.Supernovae occur when a giant star, one much bigger than our own Sun, collapses and then spectacularly explodes, releasing in an instant the energy of a hundred billion suns, burning for a time brighter than all the stars in its galaxy. It's like a trillion hydrogen bombs going off at once, says Evans. If a supernova explosion happened within five hundred light-years of us, we would be goners, according to Evans-it would wreck the show, as he cheerfully puts it. But the universe is vast, and supernovae are normally much too far away to harm us. In fact, most are so unimaginably distant that their light reaches us as no more than the faintest twinkle. For the month or so that they are visible, all that distinguishes them from the other stars in the sky is that they occupy a point of s.p.a.ce that wasn't filled before. It is these anomalous, very occasional p.r.i.c.ks in the crowded dome of the night sky that the Reverend Evans finds.To understand what a feat this is, imagine a standard dining room table covered in a black tablecloth and someone throwing a handful of salt across it. The scattered grains can be thought of as a galaxy. Now imagine fifteen hundred more tables like the first one-enough to fill a Wal-Mart parking lot, say, or to make a single line two miles long-each with a random array of salt across it. Now add one grain of salt to any table and let Bob Evans walk among them. At a glance he will spot it. That grain of salt is the supernova.Evans's is a talent so exceptional that Oliver Sacks, inAn Anthropologist on Mars , devotes a pa.s.sage to him in a chapter on autistic savants-quickly adding that there is no suggestion that he is autistic. Evans, who has not met Sacks, laughs at the suggestion that he might be either autistic or a savant, but he is powerless to explain quite where his talent comes from.I just seem to have a knack for memorizing star fields, he told me, with a frankly apologetic look, when I visited him and his wife, Elaine, in their picture-book bungalow on a tranquil edge of the village of Hazelbrook, out where Sydney finally ends and the boundless Australian bush begins. I'm not particularly good at other things, he added. I don't remember names well.Or where he's put things, called Elaine from the kitchen.He nodded frankly again and grinned, then asked me if I'd like to see his telescope. I had imagined that Evans would have a proper observatory in his backyard-a scaled-down version of a Mount Wilson or Palomar, with a sliding domed roof and a mechanized chair that would be a pleasure to maneuver. In fact, he led me not outside but to a crowded storeroom off the kitchen where he keeps his books and papers and where his telescope-a white cylinder that is about the size and shape of a household hot-water tank-rests in a homemade, swiveling plywood mount. When he wishes to observe, he carries them in two trips to a small deck off the kitchen. Between the overhang of the roof and the feathery tops of eucalyptus trees growing up from the slope below, he has only a letter-box view of the sky, but he says it is more than good enough for his purposes. And there, when the skies are clear and the Moon not too bright, he finds his supernovae.The termsupernova was coined in the 1930s by a memorably odd astrophysicist named Fritz Zwicky. Born in Bulgaria and raised in Switzerland, Zwicky came to the California Inst.i.tute of Technology in the 1920s and there at once distinguished himself by his abrasive personality and erratic talents. He didn't seem to be outstandingly bright, and many of his colleagues considered him little more than an irritating buffoon. A fitness buff, he would often drop to the floor of the Caltech dining hall or other public areas and do one-armed pushups to demonstrate his virility to anyone who seemed inclined to doubt it. He was notoriously aggressive, his manner eventually becoming so intimidating that his closest collaborator, a gentle man named Walter Baade, refused to be left alone with him. Among other things, Zwicky accused Baade, who was German, of being a n.a.z.i, which he was not. On at least one occasion Zwicky threatened to kill Baade, who worked up the hill at the Mount Wilson Observatory, if he saw him on the Caltech campus.But Zwicky was also capable of insights of the most startling brilliance. In the early 1930s, he turned his attention to a question that had long troubled astronomers: the appearance in the sky of occasional unexplained points of light, new stars. Improbably he wondered if the neutron-the subatomic particle that had just been discovered in England by James Chadwick, and was thus both novel and rather fashionable-might be at the heart of things. It occurred to him that if a star collapsed to the sort of densities found in the core of atoms, the result would be an unimaginably compacted core. Atoms would literally be crushed together, their electrons forced into the nucleus, forming neutrons. You would have a neutron star. Imagine a million really weighty cannonb.a.l.l.s squeezed down to the size of a marble and-well, you're still not even close. The core of a neutron star is so dense that a single spoonful of matter from it would weigh 200 billion pounds. A spoonful! But there was more. Zwicky realized that after the collapse of such a star there would be a huge amount of energy left over-enough to make the biggest bang in the universe. He called these resultant explosions supernovae. They would be-they are-the biggest events in creation.On January 15, 1934, the journalPhysical Review published a very concise abstract of a presentation that had been conducted by Zwicky and Baade the previous month at Stanford University. Despite its extreme brevity-one paragraph of twenty-four lines-the abstract contained an enormous amount of new science: it provided the first reference to supernovae and to neutron stars; convincingly explained their method of formation; correctly calculated the scale of their explosiveness; and, as a kind of concluding bonus, connected supernova explosions to the production of a mysterious new phenomenon called cosmic rays, which had recently been found swarming through the universe. These ideas were revolutionary to say the least. Neutron stars wouldn't be confirmed for thirty-four years. The cosmic rays notion, though considered plausible, hasn't been verified yet. Altogether, the abstract was, in the words of Caltech astrophysicist Kip S. Thorne, one of the most prescient doc.u.ments in the history of physics and astronomy.Interestingly, Zwicky had almost no understanding of why any of this would happen. According to Thorne, he did not understand the laws of physics well enough to be able to substantiate his ideas. Zwicky's talent was for big ideas. Others-Baade mostly-were left to do the mathematical sweeping up.Zwicky also was the first to recognize that there wasn't nearly enough visible ma.s.s in the universe to hold galaxies together and that there must be some other gravitational influence-what we now call dark matter. One thing he failed to see was that if a neutron star shrank enough it would become so dense that even light couldn't escape its immense gravitational pull. You would have a black hole. Unfortunately, Zwicky was held in such disdain by most of his colleagues that his ideas attracted almost no notice. When, five years later, the great Robert Oppenheimer turned his attention to neutron stars in a landmark paper, he made not a single reference to any of Zwicky's work even though Zwicky had been working for years on the same problem in an office just down the hall. Zwicky's deductions concerning dark matter wouldn't attract serious attention for nearly four decades. We can only a.s.sume that he did a lot of pushups in this period.Surprisingly little of the universe is visible to us when we incline our heads to the sky. Only about 6,000 stars are visible to the naked eye from Earth, and only about 2,000 can be seen from any one spot. With binoculars the number of stars you can see from a single location rises to about 50,000, and with a small two-inch telescope it leaps to 300,000. With a sixteen-inch telescope, such as Evans uses, you begin to count not in stars but in galaxies. From his deck, Evans supposes he can see between 50,000 and 100,000 galaxies, each containing tens of billions of stars. These are of course respectable numbers, but even with so much to take in, supernovae are extremely rare. A star can burn for billions of years, but it dies just once and quickly, and only a few dying stars explode. Most expire quietly, like a campfire at dawn. In a typical galaxy, consisting of a hundred billion stars, a supernova will occur on average once every two or three hundred years. Finding a supernova therefore was a little bit like standing on the observation platform of the Empire State Building with a telescope and searching windows around Manhattan in the hope of finding, let us say, someone lighting a twenty-first-birthday cake.So when a hopeful and softspoken minister got in touch to ask if they had any usable field charts for hunting supernovae, the astronomical community thought he was out of his mind. At the time Evans had a ten-inch telescope-a very respectable size for amateur stargazing but hardly the sort of thing with which to do serious cosmology-and he was proposing to find one of the universe's rarer phenomena. In the whole of astronomical history before Evans started looking in 1980, fewer than sixty supernovae had been found. (At the time I visited him, in August of 2001, he had just recorded his thirty-fourth visual discovery; a thirty-fifth followed three months later and a thirty-sixth in early 2003.)Evans, however, had certain advantages. Most observers, like most people generally, are in the northern hemisphere, so he had a lot of sky largely to himself, especially at first. He also had speed and his uncanny memory. Large telescopes are c.u.mbersome things, and much of their operational time is consumed with being maneuvered into position. Evans could swing his little sixteen-inch telescope around like a tail gunner in a dogfight, spending no more than a couple of seconds on any particular point in the sky. In consequence, he could observe perhaps four hundred galaxies in an evening while a large professional telescope would be lucky to do fifty or sixty.Looking for supernovae is mostly a matter of not finding them. From 1980 to 1996 he averaged two discoveries a year-not a huge payoff for hundreds of nights of peering and peering. Once he found three in fifteen days, but another time he went three years without finding any at all.There is actually a certain value in not finding anything, he said. It helps cosmologists to work out the rate at which galaxies are evolving. It's one of those rare areas where the absence of evidenceisevidence.On a table beside the telescope were stacks of photos and papers relevant to his pursuits, and he showed me some of them now. If you have ever looked through popular astronomical publications, and at some time you must have, you will know that they are generally full of richly luminous color photos of distant nebulae and the like-fairy-lit clouds of celestial light of the most delicate and moving splendor. Evans's working images are nothing like that. They are just blurry black-and-white photos with little points of haloed brightness. One he showed me depicted a swarm of stars with a trifling flare that I had to put close to my face to see. This, Evans told me, was a star in a constellation called Fornax from a galaxy known to astronomy as NGC1365. (NGC stands for New General Catalogue, where these things are recorded. Once it was a heavy book on someone's desk in Dublin; today, needless to say, it's a database.) For sixty million silent years, the light from the star's spectacular demise traveled unceasingly through s.p.a.ce until one night in August of 2001 it arrived at Earth in the form of a puff of radiance, the tiniest brightening, in the night sky. It was of course Robert Evans on his eucalypt-scented hillside who spotted it.There's something satisfying, I think, Evans said, about the idea of light traveling for millions of years through s.p.a.ce andjust at the right moment as it reaches Earth someone looks at the right bit of sky and sees it. It just seems right that an event of that magnitude should be witnessed.Supernovae do much more than simply impart a sense of wonder. They come in several types (one of them discovered by Evans) and of these one in particular, known as a Ia supernova, is important to astronomy because it always explodes in the same way, with the same critical ma.s.s. For this reason it can be used as a standard candle to measure the expansion rate of the universe.In 1987 Saul Perlmutter at the Lawrence Berkeley lab in California, needing more Ia supernovae than visual sightings were providing, set out to find a more systematic method of searching for them. Perlmutter devised a nifty system using sophisticated computers and charge-coupled devices-in essence, really good digital cameras. It automated supernova hunting. Telescopes could now take thousands of pictures and let a computer detect the telltale bright spots that marked a supernova explosion. In five years, with the new technique, Perlmutter and his colleagues at Berkeley found forty-two supernovae. Now even amateurs are finding supernovae with charge-coupled devices. With CCDs you can aim a telescope at the sky and go watch television, Evans said with a touch of dismay. It took all the romance out of it.I asked him if he was tempted to adopt the new technology. Oh, no, he said, I enjoy my way too much. Besides-he gave a nod at the photo of his latest supernova and smiled-I can still beat them sometimes.The question that naturally occurs is What would it be like if a star exploded nearby? Our nearest stellar neighbor, as we have seen, is Alpha Centauri, 4.3 light-years away. I had imagined that if there were an explosion there we would have 4.3 years to watch the light of this magnificent event spreading across the sky, as if tipped from a giant can. What would it be like if we had four years and four months to watch an inescapable doom advancing toward us, knowing that when it finally arrived it would blow the skin right off our bones? Would people still go to work? Would farmers plant crops? Would anyone deliver them to the stores?Weeks later, back in the town in New Hampshire where I live, I put these questions to John Thorstensen, an astronomer at Dartmouth College. Oh no, he said, laughing. The news of such an event travels out at the speed of light, but so does the destructiveness, so you'd learn about it and die from it in the same instant. But don't worry because it's not going to happen.For the blast of a supernova explosion to kill you, he explained, you would have to be ridiculously close-probably within ten light-years or so. The danger would be various types of radiation-cosmic rays and so on. These would produce fabulous auroras, shimmering curtains of spooky light that would fill the whole sky. This would not be a good thing. Anything potent enough to put on such a show could well blow away the magnetosphere, the magnetic zone high above the Earth that normally protects us from ultraviolet rays and other cosmic a.s.saults. Without the magnetosphere anyone unfortunate enough to step into sunlight would pretty quickly take on the appearance of, let us say, an overcooked pizza.The reason we can be reasonably confident that such an event won't happen in our corner of the galaxy, Thorstensen said, is that it takes a particular kind of star to make a supernova in the first place. A candidate star must be ten to twenty times as ma.s.sive as our own Sun and we don't have anything of the requisite size that's that close. The universe is a mercifully big place. The nearest likely candidate he added, is Betelgeuse, whose various sputterings have for years suggested that something interestingly unstable is going on there. But Betelgeuse is fifty thousand light-years away.Only half a dozen times in recorded history have supernovae been close enough to be visible to the naked eye. One was a blast in 1054 that created the Crab Nebula. Another, in 1604, made a star bright enough to be seen during the day for over three weeks. The most recent was in 1987, when a supernova flared in a zone of the cosmos known as the Large Magellanic Cloud, but that was only barely visible and only in the southern hemisphere-and it was a comfortably safe 169,000 light-years away.Supernovae are significant to us in one other decidedly central way. Without them we wouldn't be here. You will recall the cosmological conundrum with which we ended the first chapter-that the Big Bang created lots of light gases but no heavy elements. Those came later, but for a very long time n.o.body could figure out howthey came later. The problem was that you needed something really hot-hotter even than the middle of the hottest stars-to forge carbon and iron and the other elements without which we would be distressingly immaterial. Supernovae provided the explanation, and it was an English cosmologist almost as singular in manner as Fritz Zwicky who figured it out.He was a Yorkshireman named Fred Hoyle. Hoyle, who died in 2001, was described in an obituary inNature as a cosmologist and controversialist and both of those he most certainly was. He was, according toNature 's obituary, embroiled in controversy for most of his life and put his name to much rubbish. He claimed, for instance, and without evidence, that the Natural History Museum's treasured fossil of an Archaeopteryx was a forgery along the lines of the Piltdown hoax, causing much exasperation to the museum's paleontologists, who had to spend days fielding phone calls from journalists from all over the world. He also believed that Earth was not only seeded by life from s.p.a.ce but also by many of its diseases, such as influenza and bubonic plague, and suggested at one point that humans evolved projecting noses with the nostrils underneath as a way of keeping cosmic pathogens from falling into them.It was he who coined the term Big Bang, in a moment of facetiousness, for a radio broadcast in 1952. He pointed out that nothing in our understanding of physics could account for why everything, gathered to a point, would suddenly and dramatically begin to expand. Hoyle favored a steady-state theory in which the universe was constantly expanding and continually creating new matter as it went. Hoyle also realized that if stars imploded they would liberate huge amounts of heat-100 million degrees or more, enough to begin to generate the heavier elements in a process known as nucleosynthesis. In 1957, working with others, Hoyle showed how the heavier elements were formed in supernova explosions. For this work, W. A. Fowler, one of his collaborators, received a n.o.bel Prize. Hoyle, shamefully, did not.According to Hoyle's theory, an exploding star would generate enough heat to create all the new elements and spray them into the cosmos where they would form gaseous clouds-the interstellar medium as it is known-that could eventually coalesce into new solar systems. With the new theories it became possible at last to construct plausible scenarios for how we got here. What we now think we know is this:About 4.6 billion years ago, a great swirl of gas and dust some 15 billion miles across acc.u.mulated in s.p.a.ce where we are now and began to aggregate. Virtually all of it-99.9 percent of the ma.s.s of the solar system-went to make the Sun. Out of the floating material that was left over, two microscopic grains floated close enough together to be joined by electrostatic forces. This was the moment of conception for our planet. All over the inchoate solar system, the same was happening. Colliding dust grains formed larger and larger clumps. Eventually the clumps grew large enough to be called planetesimals. As these endlessly b.u.mped and collided, they fractured or split or recombined in endless random permutations, but in every encounter there was a winner, and some of the winners grew big enough to dominate the orbit around which they traveled.It all happened remarkably quickly. To grow from a tiny cl.u.s.ter of grains to a baby planet some hundreds of miles across is thought to have taken only a few tens of thousands of years. In just 200 million years, possibly less, the Earth was essentially formed, though still molten and subject to constant bombardment from all the debris that remained floating about.At this point, about 4.5 billion years ago, an object the size of Mars crashed into Earth, blowing out enough material to form a companion sphere, the Moon. Within weeks, it is thought, the flung material had rea.s.sembled itself into a single clump, and within a year it had formed into the spherical rock that companions us yet. Most of the lunar material, it is thought, came from the Earth's crust, not its core, which is why the Moon has so little iron while we have a lot. The theory, incidentally, is almost always presented as a recent one, but in fact it was first proposed in the 1940s by Reginald Daly of Harvard. The only recent thing about it is people paying any attention to it.When Earth was only about a third of its eventual size, it was probably already beginning to form an atmosphere, mostly of carbon dioxide, nitrogen, methane, and sulfur. Hardly the sort of stuff that we would a.s.sociate with life, and yet from this noxious stew life formed. Carbon dioxide is a powerful greenhouse gas. This was a good thing because the Sun was significantly dimmer back then. Had we not had the benefit of a greenhouse effect, the Earth might well have frozen over permanently, and life might never have gotten a toehold. But somehow life did.For the next 500 million years the young Earth continued to be pelted relentlessly by comets, meteorites, and other galactic debris, which brought water to fill the oceans and the components necessary for the successful formation of life. It was a singularly hostile environment and yet somehow life got going. Some tiny bag of chemicals twitched and became animate. We were on our way.Four billion years later people began to wonder how it had all happened. And it is there that our story next takes us.

PART IITHE SIZE OF THE EARTH

Nature and Nature's laws lay hid innight;G.o.d said, Let Newton be! And allwas light.-Alexander Pope

CHAPTER 4: THE MEASURE OF THINGS

IF YOU HAD to select the least convivial scien