Why Does the World Exist?: An Existential Detective Story (20 page)

The best that physicists can do to satisfy this sense of wonder, Weinberg seemed to believe, is to discover their holy grail, the final theory. “
This may happen
in a century or two,” he has written, “and if it does then I think that physicists will be at the extreme limits of their powers of explanation.”

The final theory envisaged by Weinberg promises to go far beyond current physics in clarifying the origins of the universe. It might, for instance, show how space and time emerged from still more fundamental entities that we as yet have no conception of. But it is hard to see how even a final theory could explain why there is a universe instead of nothing at all. Are the laws of physics somehow to inform the Abyss that it is pregnant with Being? If so, where do the laws themselves live? Do they hover over the world like the mind of God, commanding to exist? Or do they inhere within the world, amounting to a mere summary of what goes on inside it?

Cosmologists like Stephen Hawking and Alex Vilenkin sometimes entertain the first possibility, only to be perplexed by it. Here, for example, is Vilenkin on the “quantum tunneling” by which, he submits, the universe might have been born from nothing at all: “
The tunneling process
is governed by the same fundamental laws that describe the subsequent evolution of the universe. It follows that the laws should be ‘there’ even prior to the universe itself. Does this mean that the laws are not mere descriptions of reality and can have an independent existence of their own? In the absence of space, time, and matter, what tablets could they be written upon? The laws are expressed in the form of mathematical equations. If the medium of mathematics is the mind, does this mean that mind should predate the universe?” As to
whose
mind this might be, Vilenkin passes over that question in silence.

Hawking, too, has admitted to bafflement over the ontological status, and seeming potency, of the laws of physics: “
What is it that breathes
fire into the equations and makes a universe for them to govern? Is the ultimate unified theory so compelling that it brings about its own existence?”

If the ultimate laws of physics, like Plato’s eternal and transcendent Forms, did have a reality of their own, that would only raise a new mystery—two mysteries, in fact. The first is the one that bothered Hawking. What gives these laws their ontic clout, their “fire”? How do they reach out and make a world? How do they force events to
obey
them? Even Plato needed a divine craftsman, a “demiurge,” to do the actual work of fashioning the world according to the blueprint that the Forms provided.

The second mystery that arises if the laws of physics have their own transcendent reality is even more basic: Why should those laws exist? Why not some other set of laws or, even simpler, no laws at all? If the laws of physics are Something, then they cannot explain why there is Something rather than Nothing, since they are a part of the Something to be explained.

So consider the other possibility—that the laws of physics have
no
ontological status of their own. On this view, these laws do not hover over the world or exist prior to it in any way. Rather, they are merely the most general possible summary of patterns of events within the world. On this view, the planets don’t orbit the sun because they “obey” the law of gravity; instead, the law of gravity (or rather, the general theory of relativity, which superseded it) summarizes a regular pattern in nature, a pattern that includes the planetary orbits.

Suppose the laws of physics—even the deepest laws, those that will make up the hoped-for final theory—are indeed just summaries of what goes on in the world. Then how can those laws
explain
anything? Perhaps they can’t. That was what Ludwig Wittgenstein thought. “
The whole modern conception
of the world,” Wittgenstein wrote in his
Tractatus
, “is founded on the illusion that the so-called laws of nature are the explanations of natural phenomena. Thus people today stop at the laws of nature, treating them as something inviolable, just as God and Fate were treated in past ages.”

Weinberg clearly does not share this Wittgensteinian skepticism. Physicists aren’t like priests or oracles. They really do explain things. Explanation is what has happened when they are moved to say, “Aha!” To explain an event scientifically, Weinberg argues, is to show how it fits into the pattern of regularities encoded in some physical principle. And to explain that principle, in turn, is to show that it can be deduced from a more fundamental principle. (Thus, for example, the chemical properties of many molecules can be deduced from—and hence are explained by—the deeper principles of quantum mechanics and electrostatic attraction.) Eventually, according to Weinberg’s scheme, all such arrows of scientific explanation will converge at a single bedrock level, the deepest and most comprehensive of all—that of the final theory.

It is conceivable that future physicists will bring the existence of the universe itself into this grand deductive scheme. Perhaps, employing the final theory, they will be able to calculate that the seed of an inflationary multiverse was bound to quantum-tunnel out of nothingness. But what would be the meaning of such a calculation? Would it explain why there is Something rather than Nothing? No. It would merely show that the laws describing the regularities
inside
the world are incompatible with the
nonexistence
of that world. (If, for example, Heisenberg’s uncertainty principle says that the value of a field and its rate of change cannot both be precisely zero, then the world as a whole can’t very well consist of unchanging nothingness.) To the metaphysical optimist, that might not look like such a bad result. It would mean that the world is, in a sense, self-subsuming, since its existence is entailed, or at least rendered probable, by regularities within it. To the cynic, though, it looks like a vicious circle. Since the world is logically prior to the patterns within it, those internal patterns can’t be called on to explain the existence of the world.

My encounter with Weinberg had deepened my understanding of how scientific explanation works. But it had also left me in agreement with him that no such explanation could dispel the mystery of existence. The question
Why is there something rather than nothing?
lies outside the ambit even of the final theory. Notwithstanding the clever imaginative leaps of cosmologists like Stephen Hawking, Ed Tryon, and Alex Vilenkin, a satisfying answer, if there was one, would have to be sought elsewhere, beyond the precincts of theoretical physics.

Would the search prove futile? Perhaps. But that made it all the more noble, in a Sisyphean kind of way. After all, as Weinberg wrote at the very end of
The First Three Minutes
, “The effort to understand the universe is one of the very few things that lifts human life above the level of farce, and gives it some of the grace of tragedy.”

Interlude

A Word on Many Worlds

T
he existence of one world is mysterious enough. But what about the existence of
many
worlds? Such a wanton profusion of being would appear to make the quest for an ultimate explanation all the more hopeless. To the already intractable questions
Why anything?
and
Why this?
, it seems to adjoin a third:
Why so much?

Yet the hypothesis of many worlds was evidently quite congenial to some of the thinkers I’d encountered. Steven Weinberg, despite his generally skeptical turn of mind, had not been shy about embracing it. Nor had (the rather less skeptical) David Deutsch. Both thought that the existence of multiple universes would render less mysterious certain deep features of our own universe: its otherwise inexplicable quantum behavior (Deutsch), and its improbable suitability for life (Weinberg).

Richard Swinburne, by contrast, had denounced the postulation of “
a trillion trillion other universes
” as “the height of irrationality.” And he is not alone in taking this dim view. The great science-popularizer and fraud-debunker Martin Gardner insisted that “
there is not a shred
of evidence that there is any universe other than the one we are in.” Theories of multiple universes, Gardner said, are “all frivolous fantasies.” And the physicist Paul Davies, carrying the debate to the op-ed page of the
New York Times
, declared that “
invoking an infinity
of unseen universes to explain the unusual features of the one we see is just as ad hoc as invoking an unseen Creator.” Each, Davies said, requires a “leap of faith.”

Should we or should we not believe in multiple universes? And does our decision have any bearing on the deeper question of why there is Something rather than Nothing?

Before getting to these matters, there’s a semantic point to be dealt with. If the universe is “everything there is,” then isn’t it true by definition that there is only one of these things? Well, yes. But when physicists and philosophers talk about two different regions of spacetime being “two universes,” what they generally mean is that those regions are (1) very, very large; (2) causally isolated from each other; and hence (3) mutually unknowable by direct observation. The case for saying the two regions are separate universes is strengthened if (4) they have very different characters—if, for instance, one of them has three spatial dimensions whereas the other has seventeen dimensions. Finally—and here is the existentially titillating possibility—two regions of spacetime might be called separate universes if (5) they are “parallel,” meaning that they contain somewhat different versions of the same entities. They might, for instance, contain various alter egos of yourself. Thinkers who entertain the possibility that there are lots of universes in one or another combination of these senses use the term “multiverse” (or sometimes “megaverse”) for the entire ensemble of them.

So why believe in the multiverse?

Since other universes are, by definition, not directly observable from our own, the burden of proof is clearly on those who claim they exist. The pro-multiverse camp has essentially two kinds of arguments.

One kind of “pro” argument—the good kind—is that the existence of other universes is entailed by features of our own universe and the theories that best explain those features. For example, measurements of the cosmic background radiation—the echo left over from the Big Bang—indicate that the space we live in is infinite, and that matter is spread randomly throughout it. Therefore, all possible arrangements of matter must exist out there somewhere—including exact and inexact replicas of our own world and the beings in it. A back-of-the-envelope calculation shows that there should be an exact copy of you some 10 to the 10 to the twenty-eighth (10
10 28
) meters (or miles or angstroms or light-years—it doesn’t really matter what the unit is when the numbers are this large) away. Because of the finite speed of light, however, these parallel worlds—and our doppelgängers within them—are inaccessible to us, and will forever remain so if the expansion of the universe continues to accelerate.

Another, more extravagant kind of multiverse is entailed by the theory of “chaotic inflation.” Proposed in the 1980s by the Russian physicist Andrei Linde to explain why our universe looks the way it does—big, uniform, flat, low in entropy—this theory also predicts that Big Bangs should be a fairly routine occurrence. In the inflationary picture, the multiverse is a ferment of innumerable, mutually isolated “bubble universes.” These bubble universes do not spring into being out of nothingness; rather, they are held to arise from a sort of preexisting chaos.

The inflationary multiverse thus sheds no light on the mystery of why there is Something rather than Nothing. But it does, as Steven Weinberg observed in our conversation, furnish a neat resolution to another mystery—the mystery of
our
existence. In inflationary cosmology, the laws of nature take the same general form throughout the entire multiverse. However, the details of those laws—the precise strength of the forces, the relative masses of the particles, the number of spatial dimensions, and so on—vary at random from universe to universe. (This randomness is due to quantum fluctuations at the birth of the different bubble universes.) If our own universe is but one among a vast ensemble of universes with randomly varying physical details, it is only to be expected that a few of these universes should have just the right conditions to foster intelligent life. Add to this the truism that, if we exist at all, we are bound to find ourselves in a universe with such life-fostering conditions—the so-called anthropic principle—and the supposed fine-tuning of our universe for life seems wholly unremarkable. No need to invoke the God hypothesis to answer the question
Why are we here?

So if scientific observations give us reason to think there are other universes, certain mysteries about our own universe go away, almost as a fringe benefit. That was the point that Weinberg had made. But some thinkers want to turn this reasoning around. They insist that other universes
must
exist precisely to make certain mysteries go away. This is the second kind of argument for the multiverse—the bad kind, since it has nothing to do with empirical observation.

One version of this argument derives from attempts to make sense of quantum theory. Take the famous paradox of Schrödinger’s cat—the unfortunate feline in a box who, because of the quantum superposition of possibilities, is simultaneously alive and dead. According to the “many-worlds” interpretation of quantum theory, Schrödinger’s thought experiment splits the universe into two parallel copies, one with a live cat, the other with a dead one (and each of them with a version of you). Physicists who look favorably on this interpretation—and many distinguished ones have, among them Richard Feynman, Murray Gell-Mann, and Stephen Hawking—claim that each universe splits into copies every second numbering something like 10 followed by one hundred zeros, all of them equally real. Yet, since quantum theory forbids these parallel worlds from interacting in any but the ghostliest of ways, their reality cannot be experimentally observed.

Another version of this backward argument for multiple universes was championed by the late Princeton philosopher David K. Lewis. Lewis shocked his fellow philosophers by claiming that all logically possible worlds are real—just as real as the one we call the “actual” world. Why did he think such a thing? Because, he said, their reality would crisply solve a wide range of philosophical problems. Take the problem of counterfactuals. What does it mean to say, “If JFK hadn’t gone to Dallas, the Vietnam War would have ended earlier”? According to Lewis, the counterfactual statement is true only if there is a possible world very similar to the actual world in which JFK
didn’t
go to Dallas and the Vietnam War
did
end earlier. Lewis’s possible worlds are also useful for making sense of propositions beginning, “If pigs could fly …”

Such dubious arguments in support of the multiverse idea have evoked equally dubious arguments against it—such as these three:

(1)
It’s not science.
Both Paul Davies and Martin Gardner submit that the proposition “the multiverse exists” has no empirical content, and hence amounts to empty metaphysics. But some of the theories that
imply
the existence of a multiverse—like the theory of chaotic inflation—
do
lead to testable predictions; moreover, these predictions have been borne out by the evidence collected so far. And improved measurements over the next decade of the microwave background radiation and of the large-scale distribution of matter may further confirm these theories—or overturn them. That looks like real science.

(2)
Alternative universes should be shaved away by Occam’s razor.
Both Davies and Gardner complain that the multiverse notion is too extravagant. “
Surely the conjecture
that there is just one universe and its Creator is infinitely simpler and easier to believe than that there are countless billions upon billions of worlds,” Gardner writes. Is it? Our universe came into being with the Big Bang, and (as the Canadian philosopher John Leslie has observed) it would be exceedingly odd if the mechanism behind this world-engendering event bore the label “THIS MECHANISM OPERATED ONLY ONCE.” A computer program that prints out the entire sequence of numbers is simpler than one that prints out only a single, very long number.

(3)
The multiverse, if real, would reduce our own world to a
Matrix
-like simulation.
This objection, voiced by Davies, is surely the most bizarre of the lot. If there really were myriad universes, Davies argues, then some of them would contain advanced technological civilizations that could use computers to simulate endless
virtual
worlds. These virtual worlds would vastly outnumber the actual universes that made up the multiverse. So, he continues, taking the multiverse theory at face value, it is much more likely that we ourselves are creatures in a virtual world than in an actual physical universe. If the multiverse theory is true, Davies says, “
there is no reason
to expect our world—the one in which you are reading this right now—to be real as opposed to a simulation.” He takes this to be a
reductio ad absurdum
of the multiverse idea. But Davies’s argument is a poor one, for at least two reasons. If it were valid, it would rule out the existence of technologically advanced civilizations in
this
universe, since they too would presumably create simulated worlds in great abundance. And the hypothesis that we are living in a simulation itself has no empirical content. How could it be verified or falsified? We cannot even talk about it coherently, as Hilary Putnam has pointed out, since our words could refer only to things “inside” the alleged simulation.

Among those who take the idea of the multiverse seriously, perhaps the greatest disagreement is over how many distinct versions of it there are. Is, for instance, the “quantum multiverse” the same as the “inflationary multiverse”? The quantum multiverse, as I mentioned earlier, is the version that is invoked to make sense of quantum weirdness. First put forth in the 1950s by the physicist Hugh Everett III in the form of his many-worlds interpretation, it says that the different possible outcomes of a quantum measurement correspond to parallel universes, all coexisting in some sort of larger reality. The inflationary multiverse, by contrast, was suggested by cosmological considerations. It encompasses an infinity of bubble universes, each arising with its own Big Bang out of a primordial chaos.

The worlds making up the inflationary multiverse are separated from one another by regions of space that, since they are expanding faster than the speed of light, cannot be traversed. By contrast, the worlds making up the quantum multiverse are separated from one another by … well, no one can quite say. The image of quantum worlds “branching off” from one another suggests that they are in some sense close together; so does the notion of such parallel worlds jostling one another ever so slightly (as in the double-slit experiment).

Given such differences, one might think that we are talking about two distinct species of multiverse here. Surprisingly, though, there are distinguished physicists who happily conflate the two. One such is Leonard Susskind, a coinventor of string theory. “
The many-worlds
[multiverse] of Everett seems, at first sight, to be quite a different conception than the eternally-inflating universe,” Susskind has observed. “However, I think the two may really be the same thing.”

Susskind’s belief in the identity of these two seemingly distinct versions of the multiverse puzzled me, so I made sure to mention it to Steven Weinberg. “I found it puzzling too,” he said. “I’ve spoken to other people about it, and they don’t understand it either.” Although Weinberg is himself sympathetic to the many-worlds interpretation of quantum mechanics, he finds it “completely perpendicular” to the issue of the inflationary multiverse. In other words, Weinberg could see no reason to equate the two multiverses, as Susskind did. “I don’t agree with Susskind on that,” Weinberg told me, “and I don’t know why he said it.”

Whether the multiverses posited by physicists are one or many, they are certainly
contingent
, not necessary. There is nothing within them that explains
why
they exist. And the individual worlds a multiverse comprises, while randomly varying in their features, nevertheless obey the same laws of nature—laws that inexplicably take one particular form rather than another. So even the most extravagant multiverse, conceived in merely physical terms, leaves unresolved a pair of fundamental questions: Why
these
laws? And why should there be a multiverse that embodies them, rather than nothing at all?

“
It is probable
that there is some secret here which remains to be discovered,” observed the great nineteenth-century American pragmatist philosopher C. S. Peirce—the same thinker, as it happens, who mockingly regretted that universes are not “as plentiful as blackberries.” Physics alone seems impotent to discover this secret. And that has driven some physicists to flirt with—and even embrace—a mystical way of thinking about reality which harkens back to Plato, if not to Pythagoras.