by Bill Bryson
Now, the question that has occurred to all of us at some point is: what would happen if you travelled out to the edge of the universe and, as it were, put your head through the curtains? Where would your head be 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 travelled 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, space curves, in a way that allows it to be boundless but finite. Space cannot even properly be said to be expanding because, as the physicist and Nobel laureate Steven Weinberg notes, “solar systems and galaxies are not expanding, and space 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 analogy that is usually given for explaining the curvature of space 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 space 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 centre and say: “This is where it all began. This is the centremost point of it all.” We are all at the centre of it all. Actually, we don’t know that for sure; we can’t prove it mathematically Scientists just assume that we can’t really be the centre 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 travelled 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 space than you can imagine already without going to the trouble of trying to envision some additional beyond.
This nineteenth-century woodcut depicts man’s boundless curiosity about what lies beyond the boundaries of our universe, a question to which we may never find the answer. (credit 1.6)
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 per cent 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 thrown off by 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.
1 A word on scientific notation. Since very large numbers are cumbersome to write and nearly impossible to read, scientists use a shorthand involving powers (or multiples) of ten in which, for instance, 10,000,000,000 is written 1010 and 6,500,000 becomes 6.5 × 106. The principle is based very simply on multiples of ten: 10 × 10 (or 100) becomes 102; 10 × 10 × 10 (or 1,000) is 103; and so on, obviously and indefinitely. The little superscript number signifies the number of zeroes following the larger principal number. Negative notations provide essentially a mirror image, with the superscript number indicating the number of spaces to the right of the decimal point (so 10−4 means 0.0001). Though I salute the principle, it remains an amazement to me that anyone seeing “1.4 × 109 km3” would see at once that that signifies 1.4 billion cubic kilometres, and no less a wonder that they would choose the former over the latter in print (especially in a book designed for the general reader, where the example was found). On the assumption that many readers are as unmathematical as I am, I will use notations sparingly, though they are occasionally unavoidable, not least in a chapter dealing with things on a cosmic scale.
The astronomer Percival Lowell at work at the observatory he founded in Flagstaff, Arizona, in the early 1920s. Convinced that there existed an undiscovered ninth planet, Lowell spent his last years searching unsuccessfully for the gassy giant “Planet X.” Thirteen years after his death, the search was resumed by young enthusiast Clyde Tombaugh. (credit 2.1)
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 spaceship to get there. With their radio telescopes they can capture wisps of radiation so preposterously faint that the total 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 Lowell 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 space occupied by the moon and the space 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 onl
y 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 a National 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 God), endowed the famous observatory that bears his name, but is most indelibly remembered for his belief that Mars was covered with canals 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 Uranus and Neptune, and devoted the last years of his life to trying to find the gassy 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 canal saga (which by now had become a serious embarrassment) the Lowell Observatory directors decided to resume the search and to that end hired a young man from Kansas named Clyde Tombaugh.
Clyde Tombaugh at work on a piece of equipment of his own invention known as a blink comparator, which helped him to discern the dull glint of Pluto, over 6 billion kilometres away, from the great mass of background stars through which it drifted. (credit 2.2)
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 massive 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.
The idea that Mars was laced by canals built by another race of beings was vigorously pursued by Lowell until his death in 1916. Though there was never any real evidence to support the notion, it remained popular among science writers (and even a few scientists) up to the middle of the century. (credit 2.3)
A few astronomers continue to think there may yet be a Planet × 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, nobody is quite sure how big it is, 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 honours 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.
The moon Charon as it might appear from the surface of its mother planet, Pluto. The discovery of Charon in 1978 meant that Pluto was much smaller than previously supposed—smaller even than our own Moon—and arguably not a planet at all, but merely the largest of a body of orbiting dark objects in a zone known as the Kuiper belt. (credit 2.4)
It is certainly true that Pluto doesn’t act much like the other planets. Not only is it runty and obscure, 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 orbit on more or less the same plane, Pluto’s orbital path is tipped (as it were) out of alignment at an angle of 17 degrees, like the brim of a hat tilted rakishly on someone’s head. Its orbit 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 11 February 1999 did Pluto return to the outside lane, there to remain for the next 228 years.
Halley’s comet as it appeared to a fifteenth-century German observer. The most famous of visitors from the outer solar system, it has been noted on Earth every seventy-six years since 240 BC, though it wasn’t until 1682 that the astronomer Edmond Halley realized it was the same object being seen repeatedly. (credit 2.5)
So if Pluto really is a planet, it is certainly an odd one. It is very tiny: just one quarter of 1 per cent as massive 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 gassy 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 space. Then we will 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 per cent, about the same as a lump of charcoal—and of course these lumps of charcoal are over six billion kilometres away.
And how far is that, exactly? It’s almost beyond imagining. Space, 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 space 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 (300,000 kilometres per second) 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 spaceship, and these are rather more lumbering. The best speeds yet achieved by any human object are those of the Voyager 1 and 2 spacecrafts, which are now flying away from us at about 56,000 kilometres an hour.
The reason the Voy
ager craft were launched when they were (in August and September 1977) was that Jupiter, Saturn, Uranus and Neptune were aligned in a way that happens only once every 175 years. This enabled the two Voyagers to use a “gravity assist” technique in which the craft were successively flung from one gassy giant to the next in a kind of cosmic version of crack the whip. Even so, it took them nine years to reach Uranus and a dozen to cross the orbit of Pluto. The good news is that if we take off in January 2006 when NASA’s New Horizon’s spacecraft is scheduled to depart for Pluto and points beyond, we can take advantage of favourable 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.