Hubble went on to identify not only Cepheids but novae and giant stars in Andromeda and other galaxies. These studies helped allay his fear that the laws of physics might break down beyond our home galaxy, rendering his distance measurements invalid. Newton, too, had wondered whether “God is able … to vary the Laws of Nature, and make Worlds of several sorts in several Parts of the Universe.”15 Hubble, in his short paper announcing the finding of Cepheids in M31, took time to caution that his results depended upon the assumption that “the nature of Cepheid variation is uniform throughout the observable portion of the universe.” When he found Cepheids and other familiar stars in the galaxy NGC 6822, he wrote with evident relief that “the principle of the uniformity of nature thus seems to rule undisturbed in this remote region of space.”16
Some astronomers have a gift for making lovely, sharp photographs of galaxies with large telescopes. Hubble was not one of them, though he was adept at extracting essential data from the generally flawed plates he did obtain. Nor was he especially skillful at taking spectra, but in this he was soon aided by one Milton Humason, a resourceful young man with an inquiring mind who started out on Mount Wilson as a muleteer and observatory janitor, began assisting the astronomers in their work at the telescope, and eventually became an expert observational astronomer in his own right. Throughout the 1930s and 1940s, Hubble and Humason pushed back the frontiers of the observable universe, charting and cataloging ever more distant galaxies. Eventually, Hubble was taking photographs that were strewn with the images of more remote galaxies than foreground stars.
In 1952, the year before Hubble died, Walter Baade announced at a meeting of the International Astronomical Union in Rome that he had discovered an error in the calibration of the Cepheid period-luminosity value, the correction of which doubled the cosmic distance scale. Further refinements in the distance scale were attained by Hubble’s former assistant Allan Sandage, later in collaboration with the Swiss astronomer Gustav Tammann, and it became possible for astronomers to measure, with some confidence, the distance to galaxies hundreds of millions to billions of light-years away.
At these distances, time commands a significance equal to that of space. Inasmuch as it takes time for the light from a distant galaxy to pass through space, we see the galaxy as it was long ago: The galaxies of the Coma cluster, for instance, appear to us as they looked seven hundred million years ago, when the first jellyfish were just appearing on Earth. Owing to this phenomenon, called lookback time, telescopes probe not only out into space but back into the past. It should, therefore, be possible to determine, by looking far into deep space, whether the universe was once different than it is today. Evidence that this is indeed the case came in the 1960s, when Sandage and radio astronomer Thomas Matthews discovered quasars, and Maarten Schmidt determined that they were extraordinarily far away. Quasars appear to be the nuclei of young galaxies, at distances of a billion light-years and more. There is nothing quite like them in the universe today. And so the exploration of space opened the pages of cosmic history.
The work of charting our place in the universe goes on, and today we can say with some confidence that the sun is a typical yellow star that lies in the disk of a major spiral galaxy, about two thirds of the way out from the galactic center. The disk contains not only stars and their planets but also vast, rarefied lakes of hydrogen and helium gas, denser knots of gas where atoms have been able to find one another and bind together as molecules, and giant thunderheads of soot given off by smoky stars. Waves generated by harmonics in the gravitational interaction of the myriad stars move across the disk in a graceful, spiral pattern, plowing the interstellar material into globules dense enough to collapse under the attraction of their own gravitational force. In this way new stars are formed, and it is the light of the most massive and shortest-lasting of the young stars that illuminates the spiral arms, making them visible. The spiral arms, then, are not objects but processes—as transitory, by the bounteous spatiotemporal standards of the Milky Way, as the back-blowing veils of froth that whitecap the waves of earthly oceans.
Beyond the Milky Way lie more galaxies. Some, like the Large Magellanic Cloud and the Andromeda galaxy, are spirals. Others are ellipticals, their stars hung in pristine, cloudless space. Others are dim dwarfs, some not much larger than globular clusters. Most belong, in turn, to clusters of galaxies. The Milky Way is one of a few dozen galaxies comprizing a gravitationally bound association that astronomers call the Local Group. That group in turn lies near one extremity of a lanky archipelago of galaxies called the Virgo Supercluster. If we could fly the sixty million or so light-years from here to the center of the supercluster, we would encounter many sights worth seeing along our way—the giant cannibal galaxy Centauras A, an elliptical busily gobbling up a spiral that blundered into it; the distended spiral M51, with its one outflung arm stretching after a departing companion galaxy; the furiously glowing spiral M106, with its bright yellow nucleus and its shoals of blue-white stars; and, at the supercluster core, the giant elliptical Virgo A, wreathed in thousands of globular star clusters, harboring some three trillion stars, and adorned by a blue-white plasma jet that has been spat from its core with the velocity of a bolt of lightning.
Beyond Virgo lie the Perseus, Coma, and Hercules clusters, and beyond them so many more clusters and superclusters of galaxies that it takes volumes just to catalog them. There is structure even on these enormous scales; the superclusters appear to be arrayed into gigantic cosmic domains that resemble the cells of a sponge. Beyond that, light from faraway galaxies, riding the contours of curved space, becomes as dappled as the moon’s reflection on a pond in a gentle breeze. Out there, awaiting some future Hubble or Herschel, lie many a tale of things past, or passing, or to come.
*Equally unpunctual in his social commitments, Ramsden once arrived for a party at Buckingham Palace at the hour and day inscribed on an invitation sent him by the king, but one year late.
*By the 1980s, theoretical astrophysicists using computer models had derived a general theory of the origin of the solar system that, though more sophisticated than those of Kant, Laplace, or Jeans, resembles them at least superficially. The new model envisions the sun congealing from a nebular cloud, the remnants of which formed a flattened disk that condensed, as it cooled, into a multitude of little chunks of material, or “planetesimals,” which in turn collided to form the planets. Indirect confirmation of the theory came when an orbiting infrared telescope detected cold, planetesimallike disks around Vega and several other bright, young stars. The details of the theory, however, are quantitatively difficult, and still have not been worked out. It is one of the humbling truths of contemporary science that, while we theorize about the origin of the entire universe, we do not yet fully understand how our own little planetary system began.
*Modern estimates put the diameter of the Milky Way disk at seventy to one hundred thousand light-years. There probably are dim stars much farther out, however, as well as stray halo stars and “tramp” globular clusters orbiting the galaxy at distances of over three hundred thousand light-years from the galactic center.
10
EINSTEIN’S SKY
I want to know how God created this world. I am not interested in this or that phenomenon, in the spectrum of this or that element. I want to know His thoughts, the rest are details.
—Einstein
Once the validity of this mode of thought has been recognized, the final results appear almost simple; any intelligent undergraduate can understand them without much trouble. But the years of searching in the dark for a truth that one feels, but cannot express; the intense desire and the alternations of confidence and misgiving, until one breaks through to clarity and understanding, are only known to him who has himself experienced them.
—Einstein
In much the same way that Newton’s account of gravitation and inertia advanced physics to the point that it could embrace a moving Earth in a heliocentric solar system, Einstein
’s relativity enabled physics to deal with the much higher velocities, greater distances and more furious energies encountered in the wider universe of the galaxies. If Newton’s domain was that of the stars and planets, Einstein’s extended from the centers of stars to the geometry of the cosmos as a whole.
To bring about so great an expansion of the scope of science, Einstein was obliged to abandon Newton’s conceptions of space and time. Newtonian space and time were inflexible and inalterable; they formed a changeless proscenium arch within which all events took place and against which all could be unambiguously measured. “Absolute space, in its own nature, without relation to anything external, remains always similar and immovable,” Newton wrote. “… Absolute, true and mathematical time, of itself, and from its own nature, flows equably without relation to anything external.”1 Einstein determined that this assumption was both superfluous and misleading. The special theory of relativity revealed that the rate at which time flows and the length of distances gauged across space vary, according to the relative velocities of those measuring them. The general theory of relativity went on to portray space as curved, and derived from spatial curvature the phenomena that Newtonian dynamics had attributed to the force of gravity.
Einstein grew up in an age when the classical conception of space, if not of time, was already coming unraveled. In order to explain how “absolute” space could have any reality—and, more to the point, how light and gravitational force could be conveyed across the empty space separating the stars and planets—Newton and his followers had postulated that space is pervaded by an invisible substance, an aether. The word was borrowed from Aristotle’s term for the celestial element of which the stars and planets were made, and like its namesake this new, updated aether was wonderful stuff. Lucid and friction-free, static and unchanging, it not only permitted the unimpeded motion of the planets and stars but actually wafted right through them—like a breeze through a grove of trees, as the English physicist Thomas Young put it.*
The appealing idea that space is pervaded by an aether began to run into trouble once it became possible to make precise measurements of the velocity of light. That light travels at a finite velocity had been appreciated since the 1670s, when the Danish astronomer Olaus Römer detected periodic variations in the time when Io, innermost of the four bright moons of Jupiter, went into eclipse: The eclipses came earlier than expected when Jupiter was relatively close to the earth and later when Jupiter was farther away. Römer realized that the discrepancy must be caused by the time it takes light to travel across the changing distance from Jupiter to Earth. From what was then known of the absolute distance of Jupiter, he was able to calculate the velocity of light to within about 30 percent of the accurate value (which is 186,272 miles per second).
Galileo had once tried to determine the velocity of light. He stationed two men with shuttered lanterns on hilltops about one mile apart, then timed the interval that elapsed between the instant when the first man opened his shutter and the second, responding to this signal, opened his shutter, sending a light beam back to the first. Römer’s finding made it clear why Galileo had failed; the interval he had attempted to measure (without a clock!) was less than a hundred thousandth of a second.
Römer’s result also suggested a way of measuring the velocity of the earth relative to absolute space: If light were propagated by a stationary aether, the absolute motion of the earth relative to the aether could be detected by measuring variations in the observed velocity of light. Imagine that the earth were a sailboat on an aether lake, and think of the light coming from two stars on opposite sides of the sky as ripples spreading from two stones dropped in the lake, one ahead of the boat and one behind. If we were standing on the deck of the boat and we measured the velocity of each set of ripples, we would find that those radiating from the stone dropped ahead would appear to be moving faster than those coming from behind. By measuring the difference in the observed velocity of the ripples coming from ahead and behind, we could calculate the speed of the boat. Similarly, it was assumed that the velocity of the earth’s motion could be determined by observing differences in the velocity of light waves coming through the stationary aether from stars ahead and behind.*
To measure this “aether drift”—as it was called, though what was thought to be drifting was not the aether but the earth—would of course be a delicate matter, since the velocity of the earth amounts to but a tiny fraction of the velocity of light. But by the latter part of the nineteenth century, technology had advanced to a sufficient degree of precision to make the task feasible. The critical experiment was conducted in the 1880s by the physicist Albert Michelson (who devoted his career to the study of light, he said, “because it’s so much fun”) and the chemist Edward Morley.
Aether drift theory held that if the velocity of light was constant relative to a stationary, all-pervading aether, then when the earth in its orbit was moving away from star A and toward star B, the observed speed of the light coming from star ? would be higher than that of the light coming from star A.
The Michelson-Morley apparatus, set up in a basement laboratory at Western Reserve University in Cleveland, Ohio, was based on the principle of interferometry. A beam of light was split and the two resulting light beams were reflected at right angles, then recombined and brought to a focus at an eyepiece. The idea was that the earth’s motion through the stationary aether would show up as a change in the interference pattern produced when one of the light beams, the one that had to travel into the aether wind, was retarded relative to the other beam. As Michelson explained the principle to his young daughter Dorothy, “Two beams of light race against each other, like two swimmers, one struggling upstream and back, while the other, covering the same distance, just crosses and returns. The second swimmer will always win, if there is any current in the river.”3 Since we know the earth is moving, there had to be some current—provided that, as Michelson and most other physicists then believed, there was such a thing as an aether that delineated the frame of reference of absolute Newtonian space.
To minimize exterior vibrations, the interferometer floated on a pool of mercury. To alter its orientation relative to the motion of the earth, it rotated on its mercury pool. Michelson spent days peering through the slowly moving eyepiece of the interferometer, looking for the telltale change in the interference patterns that would betray the earth’s motion through the aether. To his intense disappointment, he saw no such change at all. The conclusion was as inescapable as it was repugnant to Michelson: There was no detectable “aether drift.”
At first, few theorists were prepared to abandon the aether hypothesis, and several tried to reconcile it with the null outcome of the Michelson-Morley experiment. Their efforts gave rise to the bizarre idea that the experimental apparatus—and, indeed, the entire earth—contracted in the direction of its motion by just enough to cancel the effects of their velocity through the aether. “The only way out of it that I can see,” said the Irish physicist George FitzGerald, “is that the equality of [light] paths must be inaccurate.”4 In other words, the two beams of light seemed to be of equal length, because their length was distorted by the very motion of the earth they were intended to detect. As FitzGerald put it, “The block of stone [holding the apparatus] must be distorted, put out of shape by its motion … the stone would have to shorten in the direction of motion and swell out in the other two directions.”5 The Dutch physicist Hendrik Antoon Lorentz independently arrived at the same hypothesis, and worked it out in mathematical detail.
This, the “Lorentz contraction,” was to emerge in a different form as a key element in the special theory of relativity. The French physicist Henri Poincaré, one of the few leading scientists to take the Lorentz contraction seriously, came close to developing it into a form that was mathematically equivalent to Einstein’s theory; Poincaré spoke presciently of “a principle of relativity” that would prescribe that no object could exceed the velocity of light.6 But most resear
chers found it odd to the point of desperation to suggest that the velocity of the earth causes the entire planet to contract, like an orange squashed between a titan’s hands, and Lorentz himself soon set the idea aside. “I think he must have been held back by fears,” the physicist Paul Dirac speculated, years later. “… I do not suppose that one can ever have great hopes without their being combined with great fears.”7
Enter Einstein. He was born in 1879, in Ulm, where Kepler had once wandered in search of a printer, the manuscript of the Rudolphine Tables under his arm. A strong-willed but dreamy boy, Einstein did not begin speaking until he was three years old, and he forever retained something of the brooding intensity owned by the silent child. Intuitively antiauthoritarian, he rebelled against outside discipline, a habit that infuriated many of his teachers. (Years later he would joke that “to punish me for my contempt for authority, Fate made me an authority myself.”)8
At the age of sixteen Einstein escaped from the confines of the Luitpold Gymnasium in Munich—where his Greek instructor told him, “You will never amount to anything,” thus unwittingly earning himself a place in history—by persuading a doctor to write a note stating that the school regimen was pushing him to the brink of a nervous breakdown.9 He failed his college entrance examination, spent a year in preparatory school, and was graduated from the Federal Polytechnic Institute in Zurich in 1900 with respectable but unexceptional marks, having habitually cut classes to play the violin, languish in the cafés, and idle on Lake Zurich aboard rented sailboats with his fiancée, Mileva Marie, one of the few female students at the Polytechnic.
Coming of Age in the Milky Way Page 18
