Fundamental Cosmological Understanding Eludes Us
Brian Greene’s new book, The Fabric of the Cosmos, reveals him to be a scientific romantic in the tradition of Albert Einstein and Stephen Hawking. “God does not play dice with the universe,” Einstein famously quipped, disdaining the vision of irreducible uncertainty enshrined in the equations of quantum mechanics. When we search for a final theory, Hawking proclaimed in the stirring conclusion of A Brief History of Time, we seek to glimpse nothing less than the mind of God.
Greene writes in a similarly optimistic spirit, hoping to convince us that his field of physics (M-theory, previously known as superstring theory) has the potential to reveal the ultimate secrets of nature and to unify our best theory of the very large (relativity) with our most successful theory of the very small (quantum mechanics).
But is his argument convincing? This skeptic is forced to admit harboring serious doubts. While the conventional beef about M-theory among non-string theorists is that it is not falsifiable (and thus not genuine science), a more profound critique—curiously omitted from Greene’s discussion—is that it does not appear to be “brittle” in the sense of yielding a single unique solution that corresponds to the universe we inhabit.
The closest Greene comes to acknowledging this problem is the following passage:
Over the last century, scientists have realized that the universe has the familiar features of common experience only because the [subatomic] particles . . . have precisely the properties they do. Even fairly minor changes to the masses or electrical charges of some of the particles would, for example, make them unable to engage in the nuclear processes that power stars. And without stars, the universe would be a completely different place. Thus, the detailed features of the elementary particles are entwined with what many view as the deepest question in all of science: Why do the elementary particles have just the right properties to allow nuclear processes to happen, stars to light up, planets to form around stars, and on at least one such planet, life to exist?
In fact, the universe unveiled by the hellishly complex mathematics of superstring theory is not even remotely close to what string theorists anticipated. Instead of a unique solution that corresponds to the world we inhabit, the theory has revealed instead a terrifyingly vast landscape of theoretically possible universes, the overwhelming majority of which would be utterly alien and devoid of any possibility of birthing life and intelligence. The equations of superstring theory provide no insights about how our particular universe, with all of its stunningly life-friendly physical characteristics, just happened to pop out of the Big Bang, seemingly pre-engineered to speed down the pathway of cosmological evolution toward the emergence of those very results.
Just how big is this landscape of possible alternative models of particle physics that’s allowed by M-theory? According to Stanford physicist and superstring pioneer Leonard Susskind, the mathematical landscape is horrifyingly gigantic, permitting 10
500 different and distinct environments, none of which appear to be mathematically favored, let alone foreordained, by the theory. And in virtually none of those other mathematically permissible environments would matter and energy have possessed the qualities that are necessary for stars, galaxies, and carbon-based living creatures to have emerged from the primordial chaos.
This is, as Susskind says, an intellectual cataclysm of the first magnitude, because it seems to deprive our most promising new theory of fundamental physics—M-theory—of the power to uniquely predict the emergence of anything remotely resembling our universe. As Susskind puts it, the picture of the universe that is emerging from the deep, mathematical recesses of M-theory is not an “elegant universe” at all. It’s a Rube Goldberg device, cobbled together by some unknown process in a supremely improbable manner that just happens to render the whole ensemble miraculously fit for life. In the words of Steve Giddings, a theoretical physicist at the University of California, “No longer can we follow the dream of discovering the unique equations that predict everything we see, and writing them on a single page. Predicting the constants of nature becomes a messy environmental problem. It has the complications of biology.”
Confronted with this mystery, advocates of M-theory-as-final-theory take two principal approaches. The first (apparently favored by Greene) is to continue searching patiently for a unique final theory—something that you could write on your tee-shirt like E=mc
2—which might yet, against the odds, emerge from M-theory or one of its competitors (like loop quantum gravity) aspiring to the status of a so-called “theory of everything.” This is the fond hope of virtually every professional theoretical physicist, including those who have been driven to desperation by the horrendously messy and complex landscape of theoretically possible, M-theory-allowed universes that distresses Susskind and other superstring theorists. Perhaps the laws and constants of nature—an ensemble the late New York Academy of Sciences president and physicist Heinz Pagels dubbed the cosmic code—will, in the end, turn out to be uniquely specified by mathematics and thus subject to no conceivable variation. Perhaps the ultimate equations will someday slide out of the mind of a new colossus of physics as slickly and beautifully as E=mc
2 emerged from Einstein’s brain. Perhaps. But that prospect appears increasingly unlikely.
A second approach, born of desperation on the part of Susskind and others, is to overlay a refinement of Big Bang inflation theory called eternal chaotic inflation with an explanatory approach that has been traditionally reviled by most scientists, known as the weak anthropic principle. The weak anthropic principle merely states in tautological fashion that since human observers inhabit this particular universe, it must perforce be life-friendly or it would not contain any observers resembling ourselves. Eternal chaotic inflation, invented by Russian-born physicist Andrei Linde, asserts that instead of just one Big Bang, there are, always have been, and always will be, zillions of Big Bangs going off in inaccessible regions all the time. These Big Bangs create zillions of new universes constantly, and the whole ensemble constitutes a multiverse.
Now here’s what happens when these two ideas—eternal chaotic inflation and the weak anthropic principle—are joined together. In each Big Bang, the laws, constants, and the physical dimensionality of nature come out differently. In some, dark energy is stronger. In others, dark energy is weaker. In some, gravity is stronger. In others, gravity is weaker. This happens, according to M-theory-based cosmology, because the ten-dimensional physical shapes in which superstrings vibrate—known as Calabi-Yau shapes—evolve randomly and chaotically at the moment of each new Big Bang. The laws and constants of nature are constantly reshuffled by this process, like a cosmic deck of cards.
And here’s the crucial part. Once in a blue moon, this random process of eternal chaotic inflation will yield a winning hand, as judged from the perspective of whether a particular new universe is life-friendly. That outcome will be pure chance—one lucky roll of the dice in an unimaginably vast cosmic crap shoot with 10,500 unfavorable outcomes for every winning turn.
Our universe was a big winner, of course, in the cosmic lottery. Our cosmos was dealt a royal flush. Here is how the eminent Nobel laureate Steve Weinberg explained this scenario in a New York Review of Books essay a couple of years ago: “The expanding cloud of billions of galaxies that we call the Big Bang may be just one fragment of a much larger universe in which Big Bangs go off all the time, each one with different values for the fundamental constants.” It is no more a mystery that our particular branch of the multiverse exhibits life-friendly characteristics, according to Weinberg, than that life evolved on the hospitable Earth “rather than some horrid place, like Mercury or Pluto.”
If you find this scenario unsatisfactory—the weak anthropic principle superimposed on Andrei Linde’s theory of eternal chaotic inflation—I can assure you that you are not alone. To most scientists, offering the tautological explanation that since human observers inhabit this particular universe, it must necessarily be life-friendly or else it would not contain any observers resembling ourselves is anathema. It just sounds like giving up.
One thing it certain: there is a profound mystery here, captured eloquently by the great Princeton physicist Freeman Dyson: “Mind and intelligence are woven into the fabric of our universe in a way that altogether surpasses our understanding.” True enough, but the fundamental credo of science is that deep mysteries like these will someday, if only in the distant future, succumb to rational explanation.
M-theory, contrary to the faith expressed by Brian Greene and other superstring enthusiasts, may not turn out to be the final explanation of what Greene aptly calls “the deepest question in all of science”—why the universe is life-friendly. But M-theory’s inherent shortcomings—its messiness and the fact that it seems to exhibit all the complications of biology—may offer valuable clues about the existence of some fundamental cosmological process or principle that lies just beyond the reach of current scientific theory.