by Bill Bryson
Einstein photographed shortly before his death in 1955. Unable to reconcile himself with the new quantum theories of the 1920s, he became increasingly marginalized. He spent most of the second half of his career working unsuccessfully on a “Grand Unified Theory.” (credit 8.7)
So if the ideas of relativity seem weird, it is only because we don’t experience these sorts of interactions in normal life. However, to turn to Bodanis again, we all commonly encounter other kinds of relativity—for instance, with regard to sound. If you are in a park and someone is playing annoying music, you know that if you move to a more distant spot the music will seem quieter. That’s not because the music is quieter, of course, but simply that your position relative to it has changed. To something too small or sluggish to duplicate this experience—a snail, say—the idea that a boom box could seem to two observers to produce two different volumes of music simultaneously might seem incredible.
The most challenging and non-intuitive of all the concepts in the General Theory of Relativity is the idea that time is part of space. Our instinct is to regard time as eternal, absolute, immutable; to believe that nothing can disturb its steady tick. In fact, according to Einstein, time is variable and ever-changing. It even has shape. It is bound up—“inextricably interconnected,” in Stephen Hawking’s expression—with the three dimensions of space in a curious dimension known as spacetime.
Spacetime is usually explained by asking you to imagine something flat but pliant—a mattress, say, or a sheet of stretched rubber—on which is resting a heavy round object, such as an iron ball. The weight of the iron ball causes the material on which it is sitting to stretch and sag slightly. This is roughly analogous to the effect that a massive object such as the Sun (the iron ball) has on spacetime (the material): it stretches and curves and warps it. Now, if you roll a smaller ball across the sheet, it tries to go in a straight line as required by Newton’s laws of motion, but as it nears the massive object and the slope of the sagging fabric, it rolls downwards, ineluctably drawn to the more massive object. This is gravity—a product of the bending of spacetime.
Every object that has mass creates a little depression in the fabric of the cosmos. Thus the universe, as Dennis Overbye has put it, is “the ultimate sagging mattress.” Gravity on this view is no longer so much a thing as an outcome—“not a ‘force’ but a byproduct of the warping of spacetime,” in the words of the physicist Michio Kaku, who goes on: “In some sense, gravity does not exist; what moves the planets and stars is the distortion of space and time.”
Of course, the sagging mattress analogy can take us only so far, because it doesn’t incorporate the effect of time. But then, our brains can take us only so far, because it is so nearly impossible to envision a dimension comprising three parts space to one part time, all interwoven like the threads in a plaid fabric. At all events, I think we can agree that this was an awfully big thought for a young man staring out of the window of a patent office in the capital of Switzerland.
Among much else, Einstein’s General Theory of Relativity suggested that the universe must be either expanding or contracting. But Einstein was not a cosmologist and he accepted the prevailing wisdom that the universe was fixed and eternal. More or less reflexively, he dropped into his equations something called the cosmological constant, which arbitrarily counterbalanced the effects of gravity, serving as a kind of mathematical pause button. Books on the history of science always forgive Einstein this lapse, but it was actually a fairly appalling piece of science and he knew it. He called it “the biggest blunder of my life.”
Coincidentally at about the time that Einstein was affixing a cosmological constant to his theory, at the Lowell Observatory in Arizona an astronomer with the cheerily intergalactic name of Vesto Slipher (who was in fact from Indiana) was taking spectrographic readings of distant stars and discovering that they appeared to be moving away from us. The universe wasn’t static. The stars Slipher looked at showed unmistakable signs of a Doppler shift—the same mechanism behind that distinctive stretched-out yee–yummm sound cars make as they flash past on a racetrack.5 The phenomenon also applies to light, and in the case of receding galaxies it is known as a red shift (because light moving away from us shifts towards the red end of the spectrum; approaching light shifts to blue).
Slipher was the first to notice this effect with light and to realize its potential importance for understanding the motions of the cosmos. Unfortunately, no-one much noticed him. The Lowell Observatory, as you will recall, was a bit of an oddity thanks to Percival Lowell’s obsession with Martian canals, which in the 1910s made it, in every sense, an outpost of astronomical endeavour. Slipher was unaware of Einstein’s theory of relativity and the world was equally unaware of Slipher. So his finding had no impact.
Glory instead would pass to a large mass of ego named Edwin Hubble. Hubble was born in 1889, ten years after Einstein, in a small Missouri town on the edge of the Ozarks, and grew up there and in Wheaton, Illinois, a suburb of Chicago. His father was a successful insurance executive, so life was always comfortable, and Edwin enjoyed a wealth of physical endowments, too. He was a strong and gifted athlete, charming, smart and immensely good-looking—“handsome almost to a fault,” in the description of William H. Cropper, “an Adonis” in the words of another admirer. According to his own accounts, he also managed to fit into his life more or less constant acts of valour—rescuing drowning swimmers, leading frightened men to safety across the battlefields of France, embarrassing world-champion boxers with knockdown punches in exhibition bouts. It all seemed too good to be true. It was. For all his gifts, Hubble was also an inveterate liar.
Warped grid lines illustrate how space behaves like a sagging mattress, its fabric distorted by gravity as predicted by Einstein’s General Theory of Relativity. The illustration shows the relative warping effects of (from left to right) the Sun, a neutron star and a black hole. (credit 8.8a)
This was more than a little odd, for Hubble’s life was filled from an early age with a level of genuine distinction that was at times almost ludicrously golden. At a single high-school track meeting in 1906, he won the pole vault, shot-put, discus, hammer throw, standing high jump and running high jump, and was on the winning mile relay team—that is, seven first places in one meeting—and came third in the long jump. In the same year, he set a state record for the high jump in Illinois.
Vesto Slipher, of the Lowell Observatory in Arizona, was the first person to notice that distant galaxies appeared to be moving away from us—evidence that the universe was not, as everyone had long assumed, static. (credit 8.8b)
As a scholar he was equally proficient, and had no trouble gaining admission to study physics and astronomy at the University of Chicago (where, coincidentally, the head of the department was now Albert Michelson). There he was selected to be one of the first Rhodes Scholars at Oxford. Three years of English life evidently turned his head, for he returned to Wheaton in 1913 wearing an Inverness cape, smoking a pipe and talking with a peculiarly orotund accent—not quite British but not quite not—that would remain with him for life. Though he later claimed to have passed most of the second decade of the century practising law in Kentucky, in fact he worked as a high-school teacher and basketball coach in New Albany, Indiana, before belatedly attaining his doctorate and passing briefly through the Army. (He arrived in France one month before the armistice and almost certainly never heard a shot fired in anger.)
In 1919, now aged thirty, he moved to California and took up a position at the Mount Wilson Observatory near Los Angeles. Swiftly, and more than a little unexpectedly, he became the most outstanding astronomer of the twentieth century.
It is worth pausing for a moment to consider just how little was known of the cosmos at this time. Astronomers today believe there are perhaps 140 billion galaxies in the visible universe. That’s a huge number, much bigger than merely saying it would lead you to suppose. If galaxies were frozen peas, it would be enough to fill a large auditori
um—the old Boston Garden, say, or the Royal Albert Hall. (An astrophysicist named Bruce Gregory has actually computed this.) In 1919, when Hubble first put his head to the eyepiece, the number of these galaxies that were known to us was exactly one: the Milky Way. Everything else was thought to be either part of the Milky Way itself or one of many distant, peripheral puffs of gas. Hubble quickly demonstrated how wrong that belief was.
The American astronomer Edwin Hubble, a man of curiously erratic character whose remarkable insights in the 1920s began to show that the universe was far more enormous than anyone had ever supposed. (credit 8.9)
Over the next decade, Hubble tackled two of the most fundamental questions of the universe: how old is it, and how big? To answer both it is necessary to know two things—how far away certain galaxies are and how fast they are flying away from us (what is known as their recessional velocity). The red shift gives the speed at which galaxies are retiring, but doesn’t tell us how far away they are to begin with. For that you need what are known as “standard candles”—stars whose brightness can be reliably calculated and used as a benchmark to measure the brightness (and hence relative distance) of other stars.
Hubble’s luck was to come along soon after an ingenious woman named Henrietta Swan Leavitt had figured out a way to find these stars. Leavitt worked at the Harvard College Observatory as a computer, as they were known. Computers spent their lives studying photographic plates of stars and making computations—hence the name. It was little more than drudgery by another name, but it was as close as women could get to real astronomy at Harvard—or, indeed, pretty much anywhere—in those days. The system, however unfair, did have certain unexpected benefits: it meant that half the finest minds available were directed to work that would otherwise have attracted little reflective attention and it ensured that women ended up with an appreciation of the fine structure of the cosmos that often eluded their male counterparts.
One Harvard computer, Annie Jump Cannon, used her repetitive acquaintance with the stars to devise a system of stellar classifications so practical that it is still in use today Leavitt’s contribution was even more profound. She noticed that a type of star known as a Cepheid variable (after the constellation Cepheus, where the first was identified) pulsated with a regular rhythm—a kind of stellar heartbeat. Cepheids are quite rare, but at least one of them is well known to most of us. Polaris, the Pole Star, is a Cepheid.
Annie Jump Cannon (left) and Henrietta Leavitt, whose unsung labours and incisive deductions made Hubble’s breakthroughs possible. (credit 8.10)
We now know that Cepheids throb as they do because they are elderly stars that have moved past their “main sequence phase,” in the parlance of astronomers, and become red giants. The chemistry of red giants is a little weighty for our purposes here (it requires an appreciation for the properties of singly ionized helium atoms, among quite a lot else), but put simply it means that they burn their remaining fuel in a way that produces a very rhythmic, very reliable brightening and dimming. Leavitt’s genius was to realize that by comparing the relative magnitudes of Cepheids at different points in the sky you could work out where they were in relation to each other. They could be used as standard candles—a term she coined and still in universal use. The method provided only relative distances, not absolute distances, but even so it was the first time that anyone had come up with a usable way to measure the large-scale universe.
(Just to put these insights into perspective, it is perhaps worth noting that at the time Leavitt and Cannon were inferring fundamental properties of the cosmos from dim smudges of distant stars on photographic plates, the Harvard astronomer William H. Pickering, who could of course peer into a first-class telescope as often as he wanted, was developing his seminal theory that dark patches on the Moon were caused by swarms of seasonally migrating insects.)
Combining Leavitt’s cosmic yardstick with Vesto Slipher’s handy red shifts, Hubble began to measure selected points in space with a fresh eye. In 1923 he showed that a puff of distant gossamer in the Andromeda constellation known as M31 wasn’t a gas cloud at all, but a blaze of stars, a galaxy in its own right, a hundred thousand light years across and at least nine hundred thousand light years away. The universe was vaster—vastly vaster—than anyone had ever supposed. In 1924 Hubble produced a landmark paper, “Cepheids in Spiral Nebulae” (nebulae, from the Latin for “clouds,” was his word for galaxies), showing that the universe consisted not just of the Milky Way but of lots of independent galaxies—“island universes”—many of them bigger than the Milky Way and much more distant.
This finding alone would have ensured Hubble’s reputation, but he now turned to the question of working out just how much vaster the universe was, and made an even more striking discovery. Hubble began to measure the spectra of distant galaxies—the business that Slipher had begun in Arizona. Using Mount Wilson’s new 100-inch Hooker telescope and some clever inferences, by the early 1930s he had worked out that all the galaxies in the sky (except for our own local cluster) are moving away from us. Moreover their speed and distance were neatly proportional: the further away the galaxy, the faster it was moving.
This was truly startling. The universe was expanding, swiftly and evenly in all directions. It didn’t take a huge amount of imagination to read backwards from this and realize that it must therefore have started from some central point. Far from being the stable, fixed, eternal void that everyone had always assumed, this was a universe that had a beginning. It might therefore also have an end.
The wonder, as Stephen Hawking has noted, is that no-one had hit on the idea of the expanding universe before. A static universe, as should have been obvious to Newton and every thinking astronomer since, would collapse in upon itself. There was also the problem that if stars had been burning indefinitely in a static universe they’d have made the whole intolerably hot—certainly much too hot for the likes of us. An expanding universe resolved much of this at a stroke.
The Hubble Space Telescope, which was launched in 1990, photographed here after being captured and serviced for the third time in 1999. The white streak in the upper right corner of this photograph shows the outline of the Earth’s atmosphere. (credit 8.11)
Hubble was a much better observer than a thinker and didn’t immediately appreciate the full implications of what he had found. Partly this was because he was woefully ignorant of Einstein’s General Theory of Relativity This was quite remarkable because, for one thing, Einstein and his theory were world-famous by now. Moreover, in 1929 Albert Michelson—now in his twilight years but still one of the world’s most alert and esteemed scientists—accepted a position at Mount Wilson to measure the velocity of light with his trusty interferometer, and must surely have at least mentioned to Hubble the applicability of Einstein’s theory to his own findings.
At all events, Hubble failed to make theoretical hay when the chance was there. Instead, it was left to a Belgian priest-scholar (with a PhD from MIT) named Georges Lemaître to bring together the two strands in his own “fireworks theory,” which suggested that the universe began as a geometrical point, a “primeval atom,” which burst into glory and had been moving apart ever since. It was an idea that very neatly anticipated the modern conception of the Big Bang, but was so far ahead of its time that Lemaître seldom gets more than the sentence or two that we have given him here. The world would need additional decades, and the inadvertent discovery of cosmic background radiation by Penzias and Wilson at their hissing antenna in New Jersey, before the Big Bang would begin to move from interesting idea to established theory.
Neither Hubble nor Einstein would be much of a part of that big story. Though no-one would have guessed it at the time, both men had done about as much as they were ever going to do.
In 1936 Hubble produced a popular book called The Realm of the Nebulae, which explained in flattering style his own considerable achievements. Here at last he showed that he had acquainted himself with Einstein’s theory—up to a poi
nt, anyway: he gave it four pages out of about two hundred.
Hubble died of a heart attack in 1953. One last small oddity awaited him. For reasons cloaked in mystery, his wife declined to have a funeral and never revealed what she did with his body. Half a century later the whereabouts of the century’s greatest astronomer remain unknown. For a memorial you must look to the sky and the Hubble Space Telescope, launched in 1990 and named in his honour.
1 Specifically, it is a measure of randomness or disorder in a system. Darrell Ebbing, in the textbook General Chemistry, very usefully suggests thinking of a deck of cards. A new pack fresh out of the box, arranged by suit and in sequence from ace to king, can be said to be in its ordered state. Shuffle the cards and you put them in a disordered state. Entropy is a way of measuring just how disordered that state is and of determining the likelihood of particular outcomes with further shuffles. To grasp entropy fully, it is also necessary to understand concepts such as thermal non-uniformities, lattice distances and stoichiometric relationships, but that’s the general idea.
2 Planck was often unlucky in life. His beloved first wife died early, in 1909, and the younger of his two sons was killed in the First World War. He also had twin daughters whom he adored. One died giving birth. The surviving twin went to look after the baby and fell in love with her sister’s husband. They married and two years later she died in childbirth. In 1944, when Planck was eighty-five, an Allied bomb fell on his house and he lost everything—papers, diaries, a lifetime of accumulations. The following year his surviving son was caught in a conspiracy to assassinate Hitler and executed.
