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