Why Does the World Exist?: An Existential Detective Story (16 page)

Deutsch paced back and forth as he spoke. I remained seated on the sofa next to Lulie, who had finished her plate of macaroni and cheese.

“This point about the fluid nature of explanation is a real hobby horse of mine,” he continued, his voice gaining in intensity. “I think we’re going to need a different style of explanation to solve problems like free will and consciousness. These are fundamentally philosophical problems, not technical problems. I don’t think artificial intelligence will be achieved until philosophical progress is made in understanding what consciousness
is
. We couldn’t make artificial life without the concept of a replicator, and we don’t have the equivalent concept yet for consciousness. You can’t program what you can’t specify.”

This struck me as refreshingly at odds with the prevailing orthodoxy of the artificial-intelligence community, whose members seemed to think that the mystery of consciousness would wither away with the advent of superintelligent computers, which was supposedly just around the corner.

But back to the multiverse. Where did it come from? Why is there a “fabric of reality” at all?

“To my way of thinking,” Deutsch said, “that question could only be answered by finding a more encompassing fabric of which the physical multiverse was a part. But there is no ultimate answer.”

Could he then see what form that larger fabric of reality might take?

“I would start with the principle of comprehensibility,” he said. “Look, there’s a quasar out there in space, billions of light-years away. And in our brain there’s a model of the quasar, a model that has remarkable properties. There’s not just an image of the quasar in our brain, there’s a structural model with the same causal and mathematical relationships. So here you have two objects that are physically as dissimilar as they could possibly be—a quasar, which is this black hole with jets, and our brain, which is chemical scum—and yet they embody the same mathematical relationships!”

Interesting point, I interjected, but the relevance of it escaped me.

“For that to happen, the laws of physics must have a very special property. They permit—they
mandate
—their own comprehensibility. And you can take this further. If it’s true that the world is comprehensible, that we’re capable of understanding it, then in order to understand the behavior of humans, you need to understand everything! Since the structure of quasars is represented in the brains of human scientists, the behavior of scientists depends on the behavior of quasars. To predict what papers a physicist will write next year, you have to know something about quasars. By the same argument, it follows that to know all truths about humans, you need to know all the truths there are.”

Deutsch paused, as if to regather his thoughts. “We’re bootstrapping our way toward better and better explanations. And that’s why we can never have an ultimate explanation. Anything pretending to be an ‘ultimate’ explanation would be a
bad
explanation, because there would be nothing left over to explain why it was the right one—to explain why reality was that way and not another way.”

Deutsch had long maintained that quantum theory was a key to understanding the fabric of reality. And in quantum theory, I observed, you can seemingly get Something from Nothing. A particle and its antiparticle, for instance, can spontaneously appear out of the vacuum. Some physicists have conjectured that the universe itself began as a vacuum fluctuation—that it “tunneled” into existence out of nothingness. Might quantum theory explain why there was a world at all?

“Not the least!” he replied. “Quantum theory is too parochial to address the question of existence. When you talk about a particle and an antiparticle appearing in the vacuum, that’s not at all like coming into existence out of nothing. The quantum vacuum is a highly structured thing that obeys deep and complex laws of physics. It’s not ‘nothingness’ in the philosophical sense at all. It’s not even as little as the kind of nothing you have in your bank account when there’s no money in it. I mean, there’s still the bank account! A quantum vacuum is much more even than an empty bank account, because it’s got
structure
. There’s stuff
happening
in it.”

So the laws governing the quantum multiverse can’t tell us anything at all about why the multiverse exists?

“No, none of our laws of physics can possibly answer the question of why the multiverse is there,” he said. “Laws don’t do that kind of work.” He recalled an image from the great John Archibald Wheeler, his onetime mentor. “Wheeler used to say, take all the best laws of physics and write them down on bits of paper and put those bits of paper on the floor. Then stand back and look at them and say, ‘Fly!’ They won’t fly. They just sit there. Quantum theory may explain why the Big Bang happened, but it can’t answer the question you’re interested in, the question of existence. The very concept of existence is a complex one that needs to be unpacked. And the question
Why is there something rather than nothing?
is a layered one, I expect. Even if you succeeded in answering it at some level, you’d still have the next level to worry about.”

Click! My tape recorder switched off. Somewhat depressingly, it had reached the end of side B of the microcassette without registering a single genuine advance toward a resolution to the mystery of existence.

Should I have been surprised? In the opening pages of
The Fabric of Reality
, after all, Deutsch had written, “
I do not believe
that we are now, or ever shall be, close to understanding
everything there is
.” Still, he had managed to impress one positive lesson on me: that there is a lot more to reality than we might imagine. The part of it that we inhabit not only is tiny, but also may be grossly unrepresentative of the whole, giving us a partial and distorted view. We are like the prisoners chained inside the cave of illusion in Plato’s famous allegory. It may even be—although Deutsch told me that he thought it improbable—that we exist within a
simulated
reality, one created by higher beings—beings who, like Descartes’s evil genie, have deliberately programmed it with the wrong laws of physics. Yet even if we were inmates confined to such a partial and distorted reality, our quest for understanding would eventually take us beyond its virtual walls.

“
It is not enough
that the inmates be prevented from observing the outside,” he had written in
The Fabric of Reality
. “The rendered environment would also have to be such that no explanations of anything inside would ever require one to postulate an outside. The environment, in other words, would have to be self-contained as regards explanations. But I doubt that any part of reality, short of the whole thing, has that property.”

But if the whole of reality
were
explanatorily self-contained, then it would presumably have to contain the explanation of its own existence, the reason for its triumph over sheer nothingness. So perhaps there was hope after all.

I WAS A
little sad to leave Deutsch. Despite the gelid beginnings of our acquaintance, he had revealed a real sweetness of character and intellectual generosity. And Lulie, sitting next to me on the couch with her plate of macaroni, auditing our conversation with keen interest, her adoring eyes fixed on Deutsch, seemed a very angel. I had even grown comfortable in the towering chaos of junk that surrounded me, coming to see it as an adventure in high-entropy housekeeping.

As I made my solitary way along the highway back down to Oxford, a pinkish-orange ray of sunlight broke over the clouded horizon. The bells of the colleges were ringing in the distance again. I tried to picture myself as a denizen of Deutsch’s multiverse. In innumerable parallel worlds, my quantum counterparts were also descending such a hill, were also hearing such bells, were also rejoicing in such a brilliant display of sunlight as the late-winter day waned. And, like me, they were pondering the mystery of why the multiverse exists. Their thoughts—
my
thoughts—were embodied in a physical structure that extended, like a higher-dimensional crystal, across parallel universes. Surely one of these quantum counterparts, shadowing me somewhere in Deutsch’s vast fabric of reality, had made more progress than I had toward ultimate enlightenment. What thoughts could be running through
his
head? Or was the resolution to the mystery of existence somehow encoded in that crystalline structure as a whole, transcending the denizens of any particular quantum world?

Just then a passing bus startled me by hooting its horn, and my vision of this unsubstantial pageant faded, leaving not a wrack behind.

Interlude

The End of Explanation

B
ertrand Russell, according to the philosophical lore, was once in the course of giving a public lecture on cosmology when he was interrupted by an old lady in the audience. “Everything you’ve been telling us is rubbish,” the lady vociferously objected. “The world is actually flat, and it’s supported by a giant elephant that is standing on the back of a turtle.” Russell, humoring her, asked what might support the turtle. The old lady replied, “It’s turtles all the way down!”

When it comes to understanding reality, David Deutsch turned out to be something of a “turtles all the way down” man. Our explanatory quest will be unending, he maintained. There is no bedrock principle that explains absolutely everything (including the principle itself). There is no self-supporting “superturtle” holding up the tower of turtles above.

But suppose Deutsch is wrong. Suppose there
is
an ultimate explanation for everything. What could such a principle look like? How would we know when we had reached it?

It was Aristotle, in his logical work,
Posterior Analytics
, who first addressed this matter. There are three ways an explanatory chain might go, Aristotle observed.

First, it might go in a circle:
A
is true because
B
, and
B
is true because
A
. (The circle might be widened by lots of intermediate explanatory truths:
A
because
B
,
B
because
C
, …
Y
because
Z
,
Z
because
A
.) But a circular explanation is no good. Saying “
A
because
B
because
A
” is a roundabout way of saying “
A
because
A
.” And no truth explains itself.

Second, the explanatory chain might go on forever:
A1
is true because
A2
,
A2
is true because
A3
,
A3
is true because
A4
, and so on, to infinity. But that’s no good either. Such an endless regress, Aristotle observed, supplies no ultimate explanatory foundation for knowledge.

That leaves the third kind of explanatory chain, one that terminates in a finite number of steps:
A1
because
A2
,
A2
because
A3
, and so on, down to some final truth
X
. And what sort of truth could
X
be?

There would seem to be two possibilities. First,
X
might be a brute fact, lacking any explanation of its own. But if
X
itself has no explanatory support, Aristotle remarked, it can hardly provide support for other truths. The second possibility is that
X
is a logically necessary truth, one that could not have been otherwise. And, for Aristotle, this was the only satisfactory way for an explanatory chain to end—the only alternative to circularity, infinite regress, and unjustified explanatory danglers.

But—with due respect to Aristotle—how could a logically necessary truth really explain anything? In particular, how could it explain anything that is
logically contingent
—like the fact that there is a world? If the existence of a world could be deduced from a logically necessary truth, then it too would be logically necessary. But it isn’t. Although there is a world, there might not have been. Nothingness cannot be dismissed as a logical possibility. Even the most promising attempt to derive being from pure logic—the ontological argument for the existence of God—in the end comes to nothing.

So, in our quest for total understanding, we cannot complete our explanatory chain with a logically necessary truth. We are therefore driven back to a choice among three evils: circularity, infinite regress, and brute fact. Of this trio, brute fact would appear to be the least objectionable. But is there any way the brute-fact dangler at the end of an explanatory chain can be made to seem less arbitrary? Can it be rendered less brutal?

The Harvard philosopher Robert Nozick had an interesting proposal along these lines. The only way an explanation could leave nothing at all unexplained, Nozick began by observing, is if the final truth in the series were somehow self-explanatory. But how could a truth explain itself? “
X
because
X
” is an evasion of explanation rather than the real thing. No child is satisfied if you answer the question “Why is the sky blue?” by saying “Because it is.” We are back to the evil of circularity again. That is why philosophers from Aristotle to Richard Swinburne have staunchly maintained that nothing explains itself—that the explanatory relation is, to use the technical term, “irreflexive.”

Nozick, however, saw more to the matter. He conceded that “
X
because
X
” is no good as an explanatory paradigm. But there is another way, he observed, that a truth might be deduced from itself. Let’s say our deepest principle—the one that explained all the laws of nature—turned out to have this form:

Any law having characteristic
C
is true.

Let’s call this deepest-of-all-principles
P
. The principle
P
explains why other laws hold true: because they have characteristic
C
. But what explains why
P
is true? Well, suppose that
P
turned out to have characteristic
C
. Then the truth of
P
would logically follow from
P
itself! In that case, principle
P
would be
self-subsuming
, to use Nozick’s term.

“
Self-subsumption is
a way a principle turns back on itself, yields itself, applies to itself, refers to itself,” Nozick wrote. He admitted that explanatory self-subsumption is “quite weird—a feat of legerdemain.” However, compared to the alternatives—circularity, infinite regress, and brute-fact danglers—it doesn’t look so bad.

Of course, showing that a principle is self-subsuming is no proof that the principle in question is valid. Consider the sentence “Every sentence of exactly eight words is true.” Call this sentence
S
. Since
S
has exactly eight words, the truth of
S
is derivable from
S
itself, making it self-subsuming. But
S
is clearly false. (I leave this as an exercise for the reader.) Another statement that is self-subsuming yet false is “All generalizations are true.”

When a self-subsuming principle
is
true, however, it does in a sense
explain
why it is true. (What is explanation, after all, but subsumption under a law?) “
The ultimate principle
which is true will, I have suggested, explain itself by subsuming itself,” Nozick wrote. “Being a deep fact, deep enough to subsume itself and to yield itself, the principle will not be left dangling without any explanation.” So as the terminus of an explanatory chain, a self-subsuming principle is certainly preferable to a brute fact.

Still, self-subsumption does not by itself eliminate all explanatory loose ends. Consider again the self-subsuming sentence
S
: “All sentences of exactly eight words are true.” Although
S
is false, it is possible to imagine a world that makes it true. Even in this world, however, we would not be satisfied with
S
as an ultimate explanation. For one thing, it looks arbitrary. Why should
S
be true and not some rival self-subsuming sentence—say, “All sentences that contain exactly nine words are true”? For another thing, S does not have the look of ultimacy. If it were true, we would seek some deeper explanation of why it was the case—of why the world and language were arranged just that way.

Even though self-subsumption is not a guarantee of ultimacy, it may at least be a
mark
of ultimacy. Suppose, Nozick said, we were to find “
a self-subsuming statement
that is deep enough to yield everything else in an area or realm, while repeated efforts fail to find a further truth that yields it.” Then, he contended, it would be “a reasonable conjecture, tentatively held and overturnable, that an ultimate truth has been reached.” In other words, we may have found our superturtle.

Could some self-subsuming principle of the kind envisioned by Nozick furnish the answer to the question
Why is there something rather than nothing?
David Deutsch thought there could be no such answer, no end to explanation. Richard Swinburne thought the best we could do was to find the right explanatory “stopping point,” a hypothesis of maximum simplicity and power, which for him was the existence of God. Yet Swinburne conceded that God’s own existence had no explanation, “
for surely never
does anything explain itself.” Nozick, by contrast, saw a way that a principle
could
explain itself without being blatantly circular. His ideal of self-subsumption would thus seem to mark an explanatory advance over Swinburne’s ideal of simplicity.

But what sort of self-subsuming principle could explain why there is Something rather than Nothing?

Nozick thought he might have the answer. He proposed what he called “the principle of fecundity.” This is the most liberal of all ontological principles. It states that
all possible worlds are real
. The principle of fecundity was not invented by Nozick. In essence, the idea—which is also known as the “principle of plenitude”—goes back to Plato. Versions of it have been entertained throughout the history of thought. What is novel with Nozick is the claim that the principle of fecundity, being self-subsuming, furnishes its own justification. “
If it is a very deep fact
that all possibilities obtain,” he wrote, “then that fact, being a possibility, obtains in virtue of the deep fact that all possibilities do.”

A reality governed by the principle of fecundity would be the richest and most expansive reality conceivable. But it would have a rather odd structure. All possible worlds would exist, but they would exist as “parallel universes,” in logical isolation from one another. Some of these worlds would be very large and complicated. The largest of them, which we might call the
maximal
world, would itself contain every possibility, mirroring the richness of the entire ensemble of possible worlds that made up reality as a whole. At the other end of the range of possibilities would be the
minimal
or
null
world, representing the possibility that nothing at all existed. In between would come all possibilities of intermediate size and complexity: worlds containing a single electron and positron orbiting each other, worlds looking much like our own universe, worlds containing the Greek gods, worlds made of cream cheese, and so on.

The principle of fecundity, if true, would mean that reality is infinitely more encompassing than we had imagined. It would make our little universe look provincial in the extreme. And such a reality would have the virtue of eliminating the mystery of existence—so, at any rate, contended Nozick. The minimal world, one of the separate possibilities realized according to the principle of fecundity, is just our old friend nothingness. So why is there something rather than nothing? “
There isn’t
,” Nozick replied. “There’s both.”

But wait—the logic seems to have gone askew here. There can’t be
both
something
and
nothing. If you have a reality that consists of bits of something and you add a bit of nothing to it, you still have something. And the absurdity does not stop there. The principle of fecundity says that all possibilities are realized. Now, one possibility is

R
: Everything is red.

Another possibility is:

not-R
: There is at least one thing that isn’t red.

So the principle of fecundity implies
R
and not-
R
—a contradiction. And anything that implies a contradiction must be false.

Nozick had a response to this objection. Although the two possibilities
R
and
not-R
are both realized, he said, “
they exist in independent
noninteracting realms.” We might think of them as two different planets, “Planet Red” and “Planet not-Red.” That’s one way out of the contradiction. But it’s not a good way. For even if
R
and
not-R
prevail on separate planets, there can be no planet where both possibilities are realized together. In other words, there can be no “Planet Fecundity” among the possible planets. Even if all possible planets are realized, there is no planet where all possibilities are realized. So fecundity is not self-subsuming after all. It’s a cruel dilemma for Nozick: either his ultimate explanatory principle leads to contradiction, or it fails to be self-subsuming.

A self-subsuming ultimate principle is like a barber who shaves all the men in the village and himself too. There’s nothing logically wrong with that. It’s the principle of fecundity that’s the problem. It countenances
too many
possibilities—including the paradoxical one of a barber who shaves all and only those men who
don’t
shave themselves. Given this fatal logical defect, the principle of fecundity is clearly not fit to serve as the ultimate explanation.

Is the search for a self-subsuming principle of reality then hopeless? Unhappily, Nozick himself had nothing else to offer. (He died in 2002, of stomach cancer, at the age of sixty-three.) Perhaps his ontological speculations, wild as they seemed to many of his fellow philosophers, were not quite wild enough. If philosophy, like theology before it, had so far failed to come up with the goods, maybe it was time for me to look elsewhere, in the still-wilder reaches of contemporary physics. I might not find the sought-for explanatory “superturtle” there. But I had heard theoretical physicists talking about the universe as a “free lunch,” and that sounded almost as good.