Quantum Legacies: Dispatches From an Uncertain World
Page 20
Since the 1980s, physicists have recognized that they need more than just knowledge of the governing laws of the theory in order to make specific predictions about how our universe should behave. They also need to know how the extra dimensions are arranged: are they curled up like tiny soda straws or twisted in some more complicated shapes? Every quantitative prediction from string theory depends on the (unknown) topology of the extra dimensions. For two decades, physicists thought the number of topologically distinct possibilities numbered in the hundreds of thousands.18 The situation became exacerbated in 2000 when leading string theorists Joseph Polchinski (at the University of California at Santa Barbara) and Raphael Bousso (then a postdoctoral fellow at Stanford, now a professor at Berkeley) recognized that other structures—dubbed “fluxes” and “membranes”—could wrap around these extra dimensions. Instead of 105 possibilities, there appear to be upward of 10500 distinct low-energy states in string theory, any one of which (or none) might describe our observable universe. Every single observable quantity in our universe, from the masses of elementary particles, to the strengths of the fundamental forces, to the expansion rate of our universe, and more, would depend on precisely which of these stringy states our universe happened to be in. And yet string theorists to date have found no way to explain why our universe is in any particular one of these many possibilities.19
Pause for a moment to consider that number: 10500. It is utterly removed from our everyday experience, all out of proportion to other numbers that scientists usually encounter. In fact, it is difficult to generate a number that large using familiar quantities. Let us start with the earthly and mundane: the ratio of billionaire Jeff Bezos’s personal wealth (if internet accounts are to be believed) to my own is a measly 105—and whether that number seems encouraging or depressing, it is nowhere near 10500.20 Cosmic numbers likewise fail to come close. The age of our observable universe is about 1017 seconds; the ratio of the mass of the Milky Way galaxy to the mass of a single electron is roughly 1071.
The story gets still more bizarre. Building on the now-standard supplement to the big-bang model—inflationary cosmology, which posits a brief burst of exponential expansion early in our universe’s history—some string theorists began to argue that these 10500 states are not just theoretical possibilities but are actually out there, real “island universes” all their own. Central to the argument is that once inflation begins somewhere, it will continue forever. (This has been dubbed “eternal inflation.”) In any given region of spacetime, the exponential expansion will halt after a characteristic period of time, much like the half-life of radioactive substances here on Earth. But in most inflationary models, this half-life is longer than the time it takes for a volume of space to double in size. So the volume of space that is inflating will always win out over the pockets that happen to stop inflating. On this view, we live within one “pocket” or “island” universe within a much larger “multiverse.”21 As theorists like Stanford University’s Leonard Susskind and Tufts University’s Alex Vilenkin see it, by combining the “landscape” of string possibilities with eternal inflation, the relevant question becomes not why one unique state got picked out of the huge number, but why we happen to live in the particular island universe that we do.
To tackle this question, Susskind, Vilenkin, and growing numbers of their colleagues have turned to something called the “anthropic principle.” The natural constants in our observable universe—all those particle masses, force strengths, expansion rates, and so on that depend on which string state our universe occupies—must fall within rather narrow ranges in order for life as we know it to exist. Presumably, these constants would not be conducive to life (at least not life like us) in the vast majority of the other string states and hence in the vast majority of island universes out there. With each of these 10500 states realized in an infinite number of island universes, pure random chance might be enough to “explain” why we happened to evolve where we did.22
Three-quarters of this argument is of quite venerable vintage. Back in the seventeenth century, natural philosophers like Bernard Le Bovier de Fontenelle in France and Isaac Newton in England argued that the constants of nature had to be finely tuned in order to support life as we know it. Fontenelle, Newton, and their contemporaries considered such fine-tuning to be scientific proof that God must exist. That is, they took evidence of fine-tuning to be proof of design: of an omnipotent Master Architect at work, designing our universe just for us. Fontenelle and Newton thus were charter members of the “intelligent design” club.23
On this last point, Susskind parts company with Fontenelle and Newton. The epigraph to his popular book The Cosmic Landscape: String Theory and the Illusion of Intelligent Design (2005) taunts today’s adherents of intelligent design. He quotes the response by that famously secular Enlightenment physicist Pierre-Simon de Laplace to Emperor Napoleon, who had asked Laplace where God fit into his view of the cosmos. “Your Highness,” Laplace is said to have replied, “I have no need of this hypothesis.” Susskind and many of his colleagues have planted their staff firmly in the Darwinians’ turf: given enough time and ample possibilities, natural evolution took its cosmic course, and here we are.24
Other physicists have reacted to these dramatic claims much as Einstein rebuffed Friedmann’s calculations or as Hoyle decried Gamow’s creation-talk. Several outspoken critics, including Nobel laureates like Santa Barbara’s David Gross, have described the string landscape and anthropic musings to be “dangerous,” “disappointing,” even an “abdication” of how physicists should approach their craft.25
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While physicists debate string theory and the landscape, many nonscientists have drawn their own conclusions. One response has been a resurgent biblical literalism. Unlike earlier creationists, however, today’s advocates no longer excuse physics and cosmology from their purview. For example, despite a raft of high-precision astrophysical observations that have verified a central tenet of the big bang—that our observable universe is 13.8 billion years old—newly emboldened creationists readily dismiss such timescales. “I wasn’t there [at the big bang], and neither were they [cosmologists],” exclaimed accountant and Kansas State Board of Education member John W. Bacon in 1999. Bacon was explaining to journalists why he had voted with a majority of board members to remove the big bang as well as biological evolution from statewide high school curricula. “Based on that,” he went on, “whatever explanation they may arrive at is a theory and it should be taught that way.” Several other states followed Kansas’s lead in the late 1990s.26 Since then, the teaching bans have ebbed and flowed with various election cycles; the issue is far from finished.
If these education board members need any additional encouragement, they have dozens of new “authoritative” texts to turn to. Books like D. Russell Humphreys’s Starlight and Time: Solving the Puzzle of Distant Starlight in a Young Universe (1994) have been joined by a raft of recent publications, including Donald DeYoung’s Thousands, Not Billions (2005), Alex Williams and John Hartnett Dismantling the Big Bang (2005), and Jason Lisle’s Taking Astronomy Back: The Heavens Declare Creation (2006). Several of these authors sport advanced degrees in the physical sciences and are supported by a robust institutional network, including the “Answers in Genesis” ministry, complete with its own lecture circuit and educational museum. Much like the island universes dotting Susskind’s string landscape, in other words, today’s creationists have carved out a parallel universe all their own. Most of their books have a sales rank on Amazon.com an order of magnitude better than mine.
Alongside biblical literalism, a second response has come from devotees of “intelligent design.” While most news coverage of intelligent design has concentrated on fights over biological evolution in the classroom—such as the headline-grabbing legal showdown in Dover, Pennsylvania, during 2005—the issue has cropped up in other surprising places as well. In February 2006, for example, the story broke of a young public-affairs off
icer at NASA named George Deutsch, who had circulated an internal memorandum stipulating that the word “theory” be appended to “big bang” in all NASA documents, especially educational websites. “The big bang is not proven fact; it is opinion,” began Deutsch’s memo, a copy of which was leaked to the New York Times. “It is not NASA’s place, nor should it be, to make a declaration such as this about the existence of the universe that discounts intelligent design by a creator.” Although the twenty-four-year-old political appointee was forced out of NASA soon after the memo was leaked, the episode makes plain just how well placed today’s advocates of intelligent design have become.27
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Why have the evolution debates played out so differently in biology and cosmology? Looking back over the past century, two features seem especially salient: pedagogy and prestige.
Unlike biological evolution, the big bang has never been a central part of high school curricula. Modern cosmology draws on material—such as general relativity, let alone string theory—that lies well beyond the scope of secondary school instruction. Thus, whereas Darwinian natural selection has long provided an obvious target for critics of evolution in the classroom, until recently cosmic evolution has been something of a nonissue.
The recent bans on teaching the big bang might not disrupt many lesson plans, but they remain potent symbolically. They signal a sea change in relative prestige. Physicists emerged from the Second World War as national heroes. Their wartime projects had “delivered the goods,” and they found themselves fêted like no other group of academics before or since. Biologists enjoyed no such culminating moment at midcentury. Indeed, some historians have argued that American biologists’ zeal to use the centennial of Darwin’s Origin of Species in 1959 to reassert their own cultural importance might have backfired, wakening the sleeping giant of anti-evolution creationists.28
Since the end of the Cold War, physicists’ cultural standing has changed dramatically. The era of limitless funding came to a sudden halt in the early 1990s. Congress provided a clear indication of the change when it canceled the Superconducting Supercollider in October 1993. For the rest of the decade, federal funding for basic physics continued to slide. The changing fortunes for physics, combined with physicists’ own internal divisions and the obviously speculative nature of recent work, have opened the door for concerted criticism and pushback.
Today’s critics of cosmology have learned to leverage the power of the internet. I stumbled onto this thriving, wired network a few years ago, after my colleague Alan Guth and I published a review of recent cosmological research in Science. About a week after our article appeared, Alan received an email directing him to a rebuttal of our piece, posted on a creationist website. Curious, I checked out the site; one page linked to others, and still more beyond that. I followed dozens of hyperlinks to like-minded “refutations” of the big bang, inflationary cosmology, string theory, and the rest. The sites boasted good graphics and high production values. Single clicks brought me zooming to the homepages of the “Bible-Science Association,” the “Creation Science Association,” the “Center for Scientific Creation,” the “Institute for Creation Research,” the “Answers in Genesis” ministry, and dozens of similar groups. I found plenty of sites eager to sell the recent anti-big-bang books, along with DVDs such as The Privileged Planet, proffering “evidence” of supernatural intelligent design. Separate links revealed detailed “alternative” science lesson plans available for download and offered special nature tours to places like the Grand Canyon, to go sightseeing with specially trained creationist tour guides. (The websites were certainly ecumenical, featuring equal parts biblical literalism and intelligent design.)
Back I clicked to the original website. After quoting extensively from our article, the commentator switched gears. “We had to show you in their own words what these MIT eggheads are saying,” it began. (I was impressed: the “egghead” line, at least, suggested keen powers of observation.) “Guth and Kaiser need to take up truck driving. That would get them out of their ivory towers at MIT and into the real world, where they would be forced to look at trees, mountains, weather, ecology, and all the other observable things on our privileged planet that are inexplicable by chance: realities that proclaim design, purpose, intention.”29
Well, I consoled myself: at least someone is still reading Science. As for the rest of us in the cosmic evolution business, we’ll just have to keep on truckin’ . . .
17
No More Lonely Hearts
Back in 1991, science writer Dennis Overbye published a marvelous book, Lonely Hearts of the Cosmos. The book traced the development of cosmology—the scientific study of the universe as a whole—during the second half of the twentieth century. The cosmologists in Overbye’s book were lonely for two reasons. They included the last remnants of a generation of astronomers who used to sit up all night, alone, under unheated domes, squinting through huge telescopes to catch the faintest glimpses of light from faraway galaxies—a time before large groups of collaborators and automated data collection had become routine. For much of the period that Overbye covered, meanwhile, the field of cosmology hung on the margins of respectability among physicists, a kind of neglected stepchild in the shadow of more flashy fields like high-energy particle physics, with its hulking accelerators and skyrocketing budgets.1
Overbye captured the cosmologists’ struggles to measure basic features of our universe. Usually their answers could be trusted only to within a factor of two—that is, each measurement carried roughly 100 percent uncertainty. Were galaxies receding from each other at such-and-such a speed, or twice that fast? The answer bore a direct impact on how old our observable universe could be—another key feature that could be pinned down only to within a factor of two. (On the first day of one of my undergraduate courses in the subject, the instructor merrily informed us that we could use the equation “1 = 10,” since most quantities of interest had comparable uncertainties. We were not, however, allowed to square that equation.) No wonder cosmologists suffered lonely hearts for so long: those huge uncertainties appeared downright amateurish when compared with triumphs of precision in other branches of physics. For the energy levels of a hydrogen atom, for example, theory and experiment had long since converged, agreeing all the way out to eleven decimal places.
With the basic pace of the universe’s expansion so difficult to discern, cosmologists often threw up their hands (or argued at length) over follow-up questions, such as whether the universe’s expansion was speeding up or slowing down. The answer to that question would reveal how much stuff the universe contained. A densely packed universe, with lots of matter and energy per cubic meter, should eventually halt its expansion and collapse back onto itself, a big-crunch end to mirror a big-bang beginning. A universe with less matter and energy stuffed into each unit of volume would expand forever, becoming progressively more dilute. Balanced right between, Einstein’s equations predicted a Goldilocks solution: some critical amount of stuff per volume for which the rate of expansion would slowly fade but the universe would never recollapse; a gentle, quiet coasting into oblivion. The fate of the entire universe hung on numbers like the present-day expansion rate and the amount of stuff per volume. Yet try as they might—and their efforts were often extraordinarily clever—cosmologists simply could not measure the universe’s basic features with requisite confidence or precision.
That began to change, and to change fast, soon after Overbye’s Lonely Hearts appeared. Indeed, we cosmologists feel a lot less lonely these days. The field is booming, attracting new recruits, fantastic new instruments, and no shortage of exciting new ideas. One often hears talk nowadays of a “golden era” of precision cosmology. In the autumn of 1992—just one year after Overbye’s book was published—my fellow undergraduate physics students and I enjoyed a champagne study break with several professors to mark the first release of data from the Cosmic Background Explorer (COBE) satellite. From its orbit high above the atmosphere, th
e satellite had measured the first light ever released after the big bang: photons that had been streaming freely throughout the universe since the moment when electrons had begun to combine with protons to form stable, neutral hydrogen atoms, about 380,000 years after our universe began. (Before that moment, ambient temperatures were too high to allow stable hydrogen to form.) From the subtle bumps and wiggles in the distribution of those photons, cosmologists could discern that the temperature of outer space today is just 2.725 degrees above absolute zero and is consistent across the entire sky to about 1 part in 100,000.2
The following year, spacewalking astronauts repaired the Hubble Space Telescope, paving the way for further astronomical inquiry unhindered by Earth’s atmosphere. Two independent teams used the refurbished Hubble (as well as several large, ground-based telescopes) to study supernovae, cataclysmic explosions from self-destructive stars that can temporarily outshine entire galaxies. Their data, first announced in 1998, suggested the shocking conclusion that our universe’s rate of expansion is speeding up. The universe is not just getting bigger; it’s getting bigger faster. To reconcile the robust observations with Einstein’s relativity, cosmologists were forced to consider some residual energy of empty space—dubbed “dark energy,” to signal our ignorance of its origin—whose tendency to stretch space overwhelms the competing gravitational tendency of matter to clump together. Five years after that came the first data from the Wilkinson Microwave Anisotropy Probe (WMAP), a satellite whose instruments sported resolution thirty times greater than those on COBE. Measuring totally different phenomena than the supernova studies, the WMAP data confirmed that nearly three-quarters of the energy content of the universe consists of dark energy. In 2013, a different team, using the European Space Agency’s Planck satellite, reported comparable results, with even greater resolution.