Why Does the World Exist?: An Existential Detective Story (17 page)

8

THE ULTIMATE FREE LUNCH?

Science cannot answer the deepest questions. As soon as you ask why there is something instead of nothing, you have gone beyond science.

—ALLAN SANDAGE,
the father of modern astronomy

S
cience is impotent to address the mystery of existence—so, at least, it is often claimed. The point was put forcefully by the secular humanist (and evolutionary biologist) Julian Huxley. “
The clear light of science
, we are often told, has abolished mystery, leaving only logic and reason,” Huxley wrote. “This is quite untrue. Science has removed the obscuring veil of mystery from many phenomena, much to the benefit of the human race: but it confronts us with a basic and universal mystery—the mystery of existence… . Why does the world exist? Why is the world-stuff what it is? Why does it have mental or subjective aspects as well as material or objective ones? We do not know… . But we must learn to accept it, and to accept its and our existence as the one basic mystery.”

The question
Why is there something rather than nothing?
is supposed to be “too big” for science to explain. Scientists can account for the organization of the physical universe. They can trace how the individual things and forces within it causally interact. They can shed light on how the universe as a whole has, in the course of its history, evolved from one state into another. But when it comes to the ultimate origin of reality, they have nothing to say. That is an enigma best left to metaphysics, or to theology, or to poetic wonderment, or to silence.

As long as the universe was thought to be eternal, its existence did not greatly vex scientists anyway. Einstein in this theorizing simply assumed the eternity of the universe, and he fudged his relativity equations accordingly. With the discovery of the Big Bang, however, everything changed. We are evidently living in the dilute, expanding, cooled-down remnants of a great cosmic explosion that occurred some 14 billion years ago. What could have caused this primal explosion? And what, if anything, preceded it? These certainly sound like scientific questions. But any attempt by science to answer them faces a seemingly insuperable obstacle, known as the
singularity
.

Suppose we take the laws of general relativity, which govern cosmic evolution on the largest scale, and extrapolate them backward in time toward the beginning of the universe. As we watch the evolution of our expanding and cooling cosmos in reverse, we would see its contents contracting and growing hotter. At
t
= 0—the moment of the Big Bang—the temperature, density, and curvature of the universe all go to infinity. Here the equations of relativity break down, become meaningless. We have reached a singularity, a boundary or edge to spacetime itself, a point at which all causal lines converge. If there
is
a cause for this event, it must transcend spacetime and hence escape the reach of science.

The conceptual breakdown of science at the Big Bang was disturbing to cosmologists, so disturbing that they searched for scenarios in which the initial singularity was somehow avoided. But in 1970, the physicists Stephen Hawking and Roger Penrose showed that these efforts were futile. Hawking and Penrose began by assuming, quite reasonably, that gravity is always attractive, and that the density of matter in the universe is roughly what it has been measured to be. Given this pair of assumptions, they proceeded to prove, with mathematical certainty, that there
must
have been a singularity at the beginning of the universe.

Did this mean that the ultimate origin of the universe is forever shrouded in unknowability? Not necessarily. It merely means that the Big Bang cannot be completely understood by “classical” cosmology—that is, the kind of cosmology that is based on Einstein’s general relativity alone. Other theoretical resources would be needed.

As a clue to what kind of resources, consider that, a fraction of a second after its birth, the entire observable universe was no bigger than an atom. At that size scale, classical physics no longer applies. It is quantum theory that governs the realm of the very tiny. So cosmologists—Stephen Hawking prominently among them—began to ask, What if quantum theory, previously used to describe subatomic phenomena, were applied to the universe as a whole? Thus was born the field of
quantum cosmology
, which has been described (by the physicist John Gribbin) as “
the most profound development
in science since Isaac Newton.”

Quantum cosmology seemed to offer a way around the singularity problem. Classical cosmologists had supposed that the singularity lurking behind the Big Bang was a pointlike thing, with zero volume. But quantum theory forbids such a sharply defined state of affairs. It decrees that nature, at the most fundamental level, is irredeemably fuzzy. It rules out the possibility of a precise temporal origin to the universe, a time
t
= 0.

But what is more interesting than what it
forbids
is what quantum theory
permits.
It permits particles to pop into existence spontaneously, if briefly, out of a vacuum. This scenario of creation
ex nihilo
led quantum cosmologists to entertain an arresting possibility: that the universe itself, through the laws of quantum mechanics, bounded into existence out of nothing. The reason there is Something rather than Nothing is, as they fancifully put it, that
nothingness is unstable.

The physicist’s statement “nothingness is unstable” is sometimes mocked by philosophers as an abuse of language. “Nothingness” does not name an object, they say; therefore, it is meaningless to ascribe a property, like instability, to it. But there is another way of thinking of nothingness: not as a thing, but as a description of a state of affairs. For a physicist, “nothingness” describes a state of affairs where there are no particles and where all the mathematical fields have the value zero.

Now we can ask, Is such a state of nothingness possible? That is, is it logically consistent with physical principles? One of the deepest of these principles, lying at the very basis of our quantum understanding of nature, is Heisenberg’s uncertainty principle. This principle says that certain pairs of properties—called “canonically conjugate variables”—are linked in such a way that they cannot both be measured precisely. One such pair is position and momentum: the more precisely you locate the position of a particle, the less you know about its momentum, and vice versa. Another pair of conjugate properties is time and energy: the more precisely you know the time span in which something occurred, the less you know about the energy involved, and vice versa.

Quantum uncertainty also forbids the precise determination of the
value
of a field and the
rate
at which that field value changes. (That’s like saying you can’t know the exact price of a stock and how quickly that price is changing.) And, when you think about it, this pretty much rules out nothingness. Nothingness is, by definition, a state in which all field values are timelessly equal to zero. But Heisenberg’s principle tells us that if the value of a field is precisely known, its rate of change is completely random. In other words, that rate of change can’t be precisely zero. So a mathematical description of changeless emptiness is incompatible with quantum mechanics. To put the point more pithily, nothingness is unstable.

Could this have something to do with cosmogenesis? The thought that it might seems first to have occurred back in 1969 to a New York City physicist named Ed Tryon. Doing a bit of wool-gathering during a talk by a visiting celebrity physicist at Columbia University, Tryon suddenly blurted out, “
Maybe the universe
is a quantum fluctuation!” The remark was reportedly greeted by the several Nobel laureates present with derisive laughter.

But Tryon was on to something. It may seem implausible that a universe containing so much sheer stuff—there are a hundred billion galaxies just in the little region of it we can observe, each with a hundred billion stars—could have arisen from
nothing
. As we know from Einstein, all of this mass is frozen energy. But against the vast amount of
positive
energy locked up in the stars and galaxies must be set the
negative
energy of the gravitational attraction among them. In fact, in a “closed” universe—one that will eventually collapse back on itself—these positive and negative energies precisely cancel each other. In other words, the net energy of such a universe is zero.

The possibility that the entire universe could be made out of no energy at all is an astonishing one. It certainly astonished Einstein: when the idea was explained to him by a fellow physicist, George Gamow, while the two were walking in Princeton, a stunned Einstein “
stopped in his tracks
,” Gamow recalled, “and, since we were crossing a street, several cars had to stop to avoid running us down.”

From the quantum point of view, a zero-energy universe presents an interesting possibility, which Tryon seized upon. Suppose the total energy of the universe is indeed exactly zero. Then, owing to the trade-off in uncertainty between energy and time (as decreed by the Heisenberg principle), the indeterminacy in its time span becomes infinite. In other words, such a universe, once it popped into existence out of the void, could run away with itself and last forever. It would be like a loan of pure being that need never be repaid. As for what “caused” such a universe to pop into existence, that is simply a matter of quantum chance. “
In answer to the question
of why it happened,” Tryon later commented, “I offer the modest proposal that our universe is simply one of those things which happen from time to time.”

Is this
creatio ex nihilo
? Not quite. It is true that Tryon’s genesis scenario has a zero cost in terms of energy and matter; in that sense, it does seem to get “something from nothing.” But the state out of which Tryon’s cosmos spontaneously materializes, called the “quantum vacuum,” is very far from the philosopher’s conception of nothingness. For one thing, it is a kind of empty
space
, and space is not nothing. Nor is the space of the quantum vacuum really empty. It has a complicated mathematical structure; it bends and flexes like rubber; it is saturated with energy fields and seethes with virtual-particle activity. The quantum vacuum is a physical object; indeed, it is a little proto-cosmos unto itself. Why should such a thing as a quantum vacuum ever have existed? As the physicist Alan Guth has observed, “
A proposal that
the universe was created from empty space seems no more fundamental than a proposal that the universe was spawned by a piece of rubber. It might be true, but one would still want to ask where the piece of rubber came from.”

The man who seems to have come the closest to solving the “rubber problem” is Alex Vilenkin. Vilenkin was born in Ukraine, in the former Soviet Union, where, after obtaining an undergraduate degree in physics, he held a job as a night watchman in a zoo. In 1976 he immigrated to the United States, and in little more than a year he managed to earn a Ph.D. in physics. Vilenkin now teaches at Tufts University near Boston, where he is also director of the Tufts Institute of Cosmology. He is known for wearing dark glasses during seminars, Anna Wintour–like, supposedly because of the sensitivity of his eyes to light.

When Vilenkin talks about the universe arising from “nothing,” he means it quite literally, as I learned from chatting with him a few years ago. “Nothing is nothing!” he insisted to me, with some vehemence. “Not just no matter. It’s no space. No time.
Nothing
.”

But how could a physicist even define a state of sheer nothingness? Here is where Vilenkin showed ingenuity. Imagine spacetime as the surface of a sphere. (Such a spacetime is called “closed,” since it curves back on itself; it is finite, even though it has no boundaries.) Now suppose that this sphere is shrinking, like a balloon that is losing its air. The radius grows smaller and smaller. Eventually—try to imagine this—the radius goes all the way to zero. The surface of the sphere disappears completely, and with it spacetime itself. We have arrived at nothingness. We have also arrived at a precise
definition
of nothingness: a closed spacetime of zero radius. This is the most complete and utter nothingness that scientific concepts can capture. It is mathematically devoid not only of stuff but also of location and duration.

With this characterization in hand, Vilenkin was able to do an interesting calculation. Using the principles of quantum theory, he showed that, out of such an initial state of nothingness, a tiny bit of energy-filled vacuum could spontaneously “tunnel” into existence. How tiny would this bit of vacuum be? Perhaps as little as one hundred-trillionth of a centimeter. But that, it turns out, is good enough for cosmogonic purposes. Driven by the negative pressure of “inflation,” this bit of energetic vacuum would undergo a runaway expansion. In a couple of microseconds it would attain cosmic proportions, issuing in a cascading fireball of light and matter—the Big Bang!

So the transition from Nothingness to Being, as imagined in the Vilenkin scenario, is a two-stage affair. In the first stage, a tiny bit of vacuum appears out of nothing at all. In the second stage, this bit of vacuum blows up into a matter-filled precursor of the universe we see around us today. The whole scheme would appear to be scientifically irreproachable. The principles of quantum mechanics, which govern the first stage, have so far proved to be the most reliable principles in all of science. And the theory of inflation, which describes the second stage, not only has been a conceptual success since it was introduced in the early 1980s, but also has been triumphantly confirmed by empirical observations—notably, by the patterns of background radiation left over from the Big Bang that have been observed by the COBE satellite.

So Vilenkin’s calculations appeared to be sound. Yet, in chatting with him, I had to confess that my imagination bridled at his scenario of creation from nothing. Surely, the bubble of false vacuum out of which the cosmos was born had to come from
somewhere
. So, rather impishly, he told me to picture the bubble forming in a glass of champagne—and then to subtract the champagne.