He spent the war in a division that never actually went into combat, and after the armistice he returned to his astronomical work. This time he managed to get a place at the Mount Wilson Observatory in California—an observatory that boasted a brand-new 100-inch telescope, the largest in the world. Here, for the first time, he was able to see into a nebula.
In 1923 he identified a very specific type of object inside the nebula known as M31: a Cepheid, a star that brightens and dims in a regular cycle. The astronomers Henrietta Leavitt (a Radcliffe graduate, and one of the few women pursuing astronomy in the early twentieth century) and Harlow Shapley (a natural enemy of Hubble’s; the two men were professional rivals, and constitutionally inclined to loathe each other) had pioneered a method of measuring the distance to Cepheid stars.‡ Using their yardstick, Hubble figured the distance to the Cepheid in M31 to be at least 930,000 light-years from the earth§—far, far larger than any previous estimate of the size of the Milky Way.
The implication was clear. Either the Milky Way was impossibly large, or else the M31 nebula lay outside of our galaxy.3
Hubble’s continuing observations of nebulae over the next ten years led to his study The Realm of the Nebulae (1937), which presented his carefully researched and documented conclusions: not only M31, but forty-four thousand other galaxies (“island universes”) were scattered throughout the cosmos. This was, in the words of astronomers Jay Pasachoff and Alex Filippenko, a sort of Copernican revolution. Copernicus had startled us all by showing that the earth is simply a planet orbiting a star, like many others; Hubble showed that the Milky Way is “just one galaxy among the myriads in the Universe.” Never again would our galaxy be equated with what there is. In the grand scheme of things, we had become that much smaller.4
Which was not even the most startling conclusion Hubble came to.
“Light which reaches us from the nebulae,” he wrote in 1937, using the term he continued to apply to galaxies, “is reddened in proportion to the distance it has traveled. This phenomenon is . . . evidence that the nebulae are all rushing away from our stellar system, with velocities that increase directly with distances.” The redshifts did not necessarily confirm Einstein’s general theory of relativity (that confirmation, laboratory produced and reproducible, would not come until 1959), but they revealed a somewhat unexpected phenomenon. Those forty-four thousand galaxies were all moving—something that Hubble could measure, thanks to the redshift in their light.5
Hubble’s precise observations allowed him to come up with a formula for this expansion: Hubble’s Constant, or Ho. Multiplying Hubble’s Constant by the distance (D) of a galaxy produces the velocity (V) at which the galaxy is moving away:
V = Ho × D
In other words, the farther away a galaxy has traveled, the faster it is moving.6
“This explanation,” Hubble writes, “interprets red-shifts as Doppler effects, that is to say, as velocity-shifts, indicating actual motion of recession.” He adds, cautiously, that further investigation is necessary:
Nebular red-shifts . . . on a very large scale [are] quite new in our experience, and empirical confirmation of their provisional interpretation as familiar velocity-shifts, is highly desirable. Critical tests are possible at least in principle, since rapidly receding nebulae should appear fainter than stationary nebulae at the same distance. . . . The interpretation of red-shifts is [thus] at least partially within the range of empirical investigation.7
In other words, this was not the same sort of science that quantum physicists were doing; the redshift was visible, and better telescopes would make it even more visible. The “island universes” were, demonstrably, sailing through the cosmos.
The slightly more theoretical question was, Why?
On this point, Hubble was more cautious. “The explorations of space end on a note of uncertainty,” he concludes in The Realm of the Nebulae. “With increasing distance [from the earth], our knowledge fades, and fades rapidly. Eventually, we reach the dim boundary—the utmost limits of our telescopes. There, we measure shadows, and we search among ghostly errors of measurement for landmarks that are scarcely more substantial.” But, tentatively, he holds out an explanation: Galaxies are moving, because the universe itself is expanding.8
This theory was not original with Hubble. In 1922 the young Russian physicist and mathematician Alexander Friedmann had suggested that Einstein’s general theory of relativity required the universe to be expanding regularly outward.¶ Three years later, Friedmann died of typhoid, leaving his work unfinished; but in 1927 the Belgian astronomer Georges Lemaître came to the same conclusion.
Einstein had reviewed both papers, but he remained unconvinced. He believed that his theories made the most sense in a universe that contained a finite amount of matter and was static, remaining still and unchanged. If the universe appeared to be infinite, this was an illusion caused by the curvature of space and time. (“Your calculations are correct,” he wrote to Lemaître, “but your physics is abominable.”)9
But now, for the first time, visible proof could be brought to bear on the question. Einstein traveled to the Mount Wilson Observatory, met with Hubble, reviewed Hubble’s results, peered through the 100-inch telescope—and changed his mind. “The redshift of distant nebulae has smashed my old construction like a hammer blow,” he told a California audience that had gathered to hear him lecture.10
By the end of the 1930s, most astronomers and physicists agreed with Einstein: the universe was not static, but expanding. This had implications for its past form. Lemaître, following his calculations backward, had come to the logical conclusion that, if the universe is steadily expanding outward, at some point it must have been much smaller. In fact, far back at the beginning of the process, there must have been a “zero” hour when all matter in the universe was packed closely together into one point.
There was no way, under current physical laws, to explain this point, or to predict how it would behave, or even to understand what it was. So Lemaître borrowed a term from mathematics: it was a singularity, a place where the rules governing the world we know break down.11
This was not so much an explanation, as the absence of one: reaching the point where he could no longer explain the universe, Lemaître had substituted the lack of explanation in its place, and given the blank spot a name. In 1931 he attempted to borrow from quantum theory to fill the gap. “If the world has begun with a single quantum,” he wrote in the journal Nature,
the notions of space and time would altogether fail to have any meaning at the beginning; they would only begin to have a sensible meaning when the original quantum had been divided into a sufficient number of quanta. If this suggestion is correct, the beginning of the world happened a little before the beginning of space and time.12
This got rid of the rabbit-in-the-hat singularity, but Lemaître’s quantum, a “primeval atom” that somehow contained all matter now in the universe and produced space and time by beginning to expand outward, wasn’t much easier to explain: “It may be difficult to follow up the idea in detail,” Lemaître added, somewhat apologetically.
This proved to be an understatement. Nor was the singularity the only problem that needed solving. If the universe is expanding steadily outward (and has been doing so for quite a long time), shouldn’t it be getting thinner at the center? Wouldn’t a far-future observer find himself standing in the middle of empty space, since all of the receding galaxies would eventually be so far away that they could no longer be observed?
In 1948 the Yorkshire-born astronomer Fred Hoyle tackled the “thinning universe” problem and disposed of Lemaître’s primeval atom at the same time. In ten billion years, he suggested, a theoretical observer will actually see just about the same number of galaxies that we see now, because
new galaxies will have condensed out of the background material at just about the rate necessary to compensate for those that are being lost as a consequence of their passing beyond our observable univer
se. At first sight it might be thought that this could not go on indefinitely because the material forming the background would ultimately become exhausted. The reason why this is not so, is that new material appears to compensate for the background material that is constantly being condensed into galaxies. . . . I find myself forced to assume that the nature of the Universe requires continuous creation—the perpetual bringing into being of new background material.13
Hoyle argued that this “continuous creation” model was more scientific than Lemaître’s explanation. “The hypothesis that all matter in the universe was created in one big bang at a particular time in the remote past . . . [is] in conflict with the observational requirements,” he insisted, in a widely heard 1949 radio talk. “This big bang hypothesis is . . . an irrational process that cannot be described in scientific terms.”14
Hoyle—thirty-four years old, four years into his first academic post as a Cambridge lecturer in astrophysics—had a gift for rhetoric. Adherents to Lemaître’s primeval-atom hypothesis were quick to point out that the proposed expansion was neither big (it started out very small, in fact) nor a bang (there was no explosion, just the steady expansion of space). But the term “Big Bang” stuck fast.
In 1950, Hoyle published his own “continuous creation” theory in a well-written, engaging book called The Nature of the Universe. It offered a coherent alternative to the Big Bang: Rather than emerging from a singularity or a (still undefined) quantum, the universe was in a “steady state.” The universe had no beginning, and would have no end; tiny quantities of matter (only a few atoms per cubic mile) were constantly coming into being; the expansion of the universe, and the creation of matter between its existing particles, would continue forever.15
Clearly, this theory had its own difficulties. But, as Hoyle himself pointed out, they were no more fatal than the difficulties with Lemaître’s hypothesis. It was no less likely that matter was being constantly created than that all matter had come into being at one time.
The Nature of the Universe garnered both an enormous popular following and widespread scientific support. More physicists and astronomers threw their weight behind the primeval-atom theory, but it was a smallish majority. Both proposals explained certain phenomena and failed to account for others; and in the end, both pulled an unexpected rabbit from an invisible hat. Matter appeared. Somehow.
But the steady-state theory had one great advantage: it didn’t require the abandonment of all known physical laws at any point in the past.
•
In the absence of actual proofs, both primeval-atom supporters and steady-state thinkers inevitably ended up doing metaphysics: arguing origins and first principles, with no chapter and verse to cite.
The search for validation continued, though. Two years before The Nature of the Universe presented the steady-state theory to the general public, the Russian physicist George Gamow—a firm believer in the “big bang”—had coauthored a brief but well-regarded academic paper, proposing that the distribution of chemical elements in the observable universe could be accounted for only if the universe had expanded steadily outward from a primordial, superdense, molten singularity. Shortly afterward, one of Gamow’s junior coauthors, Ralph Alpher, had taken the conclusions further and suggested that the singularity’s enormous heat would have dissipated as the superdense starting point expanded.# But some of that heat would still be radiating around the universe—residual microwave radiation, a steady background presence in the cosmos, a detectable remnant of the “primeval fireball” (as a later researcher termed it, apparently having learned something from Hoyle’s rhetorical flourishes).16
This cosmic background buzz would be a particular type of radiation. It would be on the “Planck spectrum”; its character would be determined by temperature alone, and the electromagnetic rays would be isotropic (the same in all directions), not emitted by any particular body. A steady-state system could not account for this type of radiation.
If it even existed.
Cosmic background radiation remained theoretical until fifteen years after The Nature of the Universe appeared. In 1965 the Princeton astrophysicist Robert Dicke and his team were attempting to build a receiver that would be sensitive enough to locate cosmic background radiation; at the same time, 30 miles away, two physicists working for Bell Laboratories, Arno Penzias and Robert Wilson, kept picking up unexplained static in their state-of-the-art microwave antenna. Connected (fortuitously) by a mutual acquaintance, the Princeton and Bell Labs scientists analyzed the static. It was the right spectrum to be Gamow’s leftover cosmic radiation; it was, as Gamow’s theory implied, isotropic; it was the first pebble on the scale that would tilt cosmological theory toward the Big Bang.17
In the next few years, multiple measurements of this cosmic background radiation confirmed its existence, verified its temperature, and clarified its spectrum. The steady-state theory wilted faster than a morning glory. “It has always seemed remarkable to me,” says the distinguished physicist and astronomer Woodruff Sullivan, who was finishing up his first science degree at the time, “that, for the great majority of the astronomical community in the 1960s, the steady-state theory was killed off . . . by a serendipitous discovery. . . . It died because of the fulfilment of a little-known, never emphasised prediction by its rival theory.”18
Fred Hoyle continued to defend his steady-state universe. The existence of cosmic background radiation made him no more comfortable with that pesky singularity, a stubborn and violent violation of all observable laws of physics. He was not alone. Dennis Sciama, chair of physics at the University of Maryland, lamented the presence of cosmic background radiation in 1967: “I must add,” he told a class of graduate students, “that for me the loss of the steady-state theory has been a cause of great sadness. The steady-state theory has a sweep and beauty that for some unaccountable reason the architect of the universe appears to have overlooked. The universe in fact is a botched job, but I suppose we shall have to make the best of it.”19
By the end of the 1960s, physicists and astronomers had converted en masse (with the exception of Hoyle, who, twenty-five years later, was still adjusting the steady-state theory in an attempt to make it work). The Big Bang had triumphed: neither big nor a bang, sitting stubbornly at the beginning of the universe, refusing to obey its laws.
It took the general public a few more years to sign on. Cosmic background radiation was not an easy thing to understand. It was detectable only by highly specialized instruments, explicable only through advanced and obscure mathematics. The reasons for its presence were difficult to grasp. Steady-state theory had been so convincing, in part, because Hoyle was able to present it so well. The expansion of the universe from that superdense, hot singularity needed an equally able popularizer.
He appeared in 1977: Steven Weinberg, a theoretical physicist from New York who won the Nobel Prize two years later. Weinberg wrote The First Three Minutes at the same time that he was doing intensive and highly technical work on cosmic background radiation. But he was able to shift successfully from an academic to a popular voice. Like Hoyle, he had a gift for metaphor (“if some ill-advised giant were to wiggle the sun back and forth, we on earth would not feel the effect for eight minutes, the time required for a wave to travel at the speed of light from the sun to the earth”), and he was able to recast the science of the early universe as a gripping story. The First Three Minutes clearly lays out background information about the expansion of the universe, runs through the historical development of various explanations (including steady-state theory), and shows the necessity of cosmic background radiation.
It was the first widely read explanation of the Big Bang, and the catalyst for an explosion of books for lay readers on cosmology and theoretical physics over the next decade. The popular appetite for an origins story, created by Weinberg and fed by scores of other titles (John Gribbin’s In Search of the Big Bang: Quantum Physics and Cosmology, Heinz Pagels’s Perfect Symmetry: The Search for the
Beginning of Time, James Trefil’s The Moment of Creation: Big Bang Physics from Before the First Millisecond to the Present Universe, and many, many, many more) carved a path directly to Stephen Hawking’s A Brief History of Time, which rode the swelling wave to outsell them all. (“Surely not another book on the Big Bang and all that stuff,” physicist Paul Davies remembers thinking when he first saw Hawking’s tome.)20
Yet, as groundbreaking as it was, The First Three Minutes shared the drawbacks of all origin stories. It demanded a leap of faith about the beginning of the universe; and it led, inevitably, to speculation about its end.
“There is an embarrassing vagueness about the very beginning,” Weinberg wrote, in his introduction, “the first hundredth of a second or so.” A later chapter, devoted to the problem of that first fraction of time, doesn’t do much better:
With the aid of a good deal of highly speculative theory, we have been able to extrapolate the history of the universe back in time to a moment of infinite density. But this leaves us unsatisfied. We naturally want to know what there was before this moment, before the universe began to expand and cool. . . . We may have to get used to the idea of an absolute zero of time—a moment in the past beyond which it is in principle impossible to trace any chain of cause and effect.21
Baconian science both had its limitations and was reluctant to admit them. Weinberg concedes that some aspects of the Big Bang resist explanation and at the same time struggles against them. “Embarrassing” implies that he should be able to answer all questions (Bacon would have agreed). It carves out little space for uncertainty.
So The First Three Minutes ends on a very certain note, even as it moves into speculation. In the absence of the continuously created material proposed by Hoyle, Weinberg writes, the universe must, ultimately, stop expanding; it will either simply cease, fading away into cold and darkness, or else “experience a kind of cosmic ‘bounce,’ and begin to re-expand. . . . We can imagine an endless cycle of expansion and contraction stretching into the infinite past, with no beginning whatever.”
The Story of Western Science Page 25
