Why Does the World Exist?: An Existential Detective Story (19 page)

9

WAITING FOR THE FINAL THEORY

“
S
o you didn’t like the Shoreline Grill? I thought the food there was reasonably good. Pricey for Austin, but not by New York standards. By the way, I’ve completely forgotten why we’re having this chat.”

It was Steven Weinberg, speaking over the phone in his deeply sonorous, ironically gruff voice.

I reminded him that I was writing about why there is Something rather than Nothing.

“That’s a nice idea for a book,” he said, his tone rising on the word “nice.”

The compliment was gratifying. But did he feel the same way that Wittgenstein and so many others did about the question? Was he awed at the very fact of existence? Did he find it extraordinary that there should be a world at all?

“For me,” Weinberg said, “it’s part of a larger question, which is ‘Why are things the way they are?’ That’s what we scientists try to find out, in terms of deep laws. We don’t yet have what I call a final theory. When we do, it might shed some light on the question of why there is anything at all. The laws of nature might dictate that there
has
to be something. For example, those laws might not allow for empty space as a stable state. But that wouldn’t take away the wonder. You’d still have to ask, ‘Why are the laws
that
way, rather than some
other
way?’ I think we’re permanently doomed to that sense of mystery. And I don’t think belief in God helps. I’ve said it before, and I’ll repeat it. If by ‘God’ you have something definite in mind—a being that is loving, or jealous, or whatever—then you’re faced with the question of why God’s that way and not another way. And if you don’t have anything very definite in mind when you talk about ‘God’ being behind the existence of the universe, then why even use the word? So I think religion doesn’t help. It’s part of the human tragedy: we’re faced with a mystery we can’t understand.”

And Weinberg didn’t seem to think that his fellow physicists could shed much light on the ultimate origin of the universe either. “I’m very skeptical,” he said, “because we don’t really understand the physics. General relativity breaks down when you go back to extreme conditions of density and temperature near the Big Bang. I’m also skeptical of anyone who quotes theorems about inevitable singularities—Hawking theorems and so on. Those theorems are valuable because they imply that at a certain point in, say, the collapse of a star our theories don’t apply any more. But beyond that you can’t say anything. We’re just too ignorant at the moment.”

This epistemic modesty was refreshing after all the wild speculation I’d been hearing over the past year. I felt I was talking to a latter-day Montaigne, or Socrates. But what did Weinberg think of the efforts of some of his more adventurous peers to explain existence itself? I mentioned Alex Vilenkin’s notion that the present universe might have inflated out of a little nugget of “false vacuum” that itself “quantum-tunneled” into being from sheer nothingness. Physics or metaphysics?

“Vilenkin is a really clever guy, and these are fascinating conjectures,” Weinberg said. “The problem is that we have no way, at present, of deciding whether they’re
true
or not. It’s not just that we don’t have the observational data—we don’t even have the
theory
.”

When we do have the theory—the final theory of physics—that would furnish the last word, scientifically speaking, on how the universe came into existence. But would it also explain
why
the universe exists?

“We don’t know,” Weinberg said. “It depends what the final theory ends up looking like. Suppose it looks like Newton’s theory. In Newton’s theory, there’s a clear separation between
laws
and
initial conditions
. For instance, Newtonian physics offers no hint about the initial conditions of the solar system. Newton himself was aware of this—he thought the initial conditions were laid down by God.”

If the final theory allows for unexplained initial conditions—sometimes called “boundary conditions”—then even if it can fully account for the evolution of the universe, it will still leave the origins of the universe cloaked in mystery. Who or what decreed
those
initial conditions? I thought of one of the “messages from the unseen” that the great Alan Turing left behind at his death:
Science is a differential equation. Religion is a boundary condition.

“If the final theory turned out to be like that, I’d be disappointed,” Weinberg continued. “Hawking and others hope that the final theory will fix all the initial conditions, that it will leave no freedom to the universe as to how it began. But we just don’t know yet.”

Well, I said, let’s be optimistic. Let’s assume the final theory
will
account for everything about the universe, including its initial conditions. That would still leave open the question of why that final theory takes the particular form it does. Why should it describe a world of quantum particles interacting through certain forces? Or a world of vibrating strings of energy? Or any world at all? Clearly, the final theory won’t be dictated by logic alone. There is more than one logically consistent way that reality could have turned out to be. But maybe there is only one logically consistent final theory that describes a reality rich enough to include conscious observers like us.

“That would be really interesting,” Weinberg said. “Would it be cause for wonder? I just had a correspondence with a philosopher at Cornell about the so-called anthropic principle. This philosopher thought, if I understood him correctly, that the universe
had
to be such as to allow for observers to evolve within it—in other words, that a universe without conscious observers would be logically inconsistent. So he wasn’t surprised that this universe seemed improbably fine-tuned for life. In me, this apparent fine-tuning arouses wonder. The only explanation for it, other than a theological explanation, is in terms of a
multiverse
—I mean a universe consisting of many parts, each with different laws of nature and different values for its constants, like the ‘cosmological constant’ which governs cosmic expansion. If there
is
a multiverse consisting of many universes, most of them hostile to life but a few favorable to it, then it’s not surprising that we find ourselves in one where conditions are in the fortunate range.”

Still, I observed, that would leave open the question of why this huge ensemble of universes should exist.

“I’m not saying that the multiverse would resolve
all
philosophical issues. It would eliminate the sense of wonder surrounding the fact that conditions in our universe are just right for life and consciousness. But we would still be faced with the mystery of why the laws of nature are such as to produce the multiverse of which our universe is a part. And I don’t see any way out of
that
mystery. Believing that a theory can bring a world into existence is a little like believing in Saint Anselm’s ontological proof of the existence of God. Anselm asks, can you think of something than which nothing more perfect can be conceived? If you’re stupid enough to say yes, then Anselm goes on to show you that since existence is a perfection, it follows that the being you’re thinking of must exist, because if it didn’t, you could conceive of something more perfect: the same being, only existent! The ontological proof has been shot down and resurrected many times. There’s a modern theologian at Notre Dame named Alvin Plantinga who claims to have a version of it which is watertight. I think it’s all nonsense myself. It seems obvious to me that you can’t go from thinking about something to concluding that it exists. And it also seems obvious to me that the laws of nature cannot
require
that they describe something real. No theory can tell you that the things it talks about exist.”

Maybe then, I said, quantum theory holds out the best hope for an explanation of existence. It not only explains events
in
the world. It also—unlike the classical physics that it overturned—offers to explain the event of that world’s coming into existence in the first place. By quantum uncertainty, it says, a seed for the cosmos is bound to pop out of the void. So the same theory that works inside the world might also buttress the world’s existence from the outside.

“Yes, that might be a plus in its favor,” Weinberg said. “But there’s something that I’m not very happy about here. Quantum mechanics is really an empty stage. It doesn’t tell you anything by itself. That’s why I think Karl Popper was wrong to say that a scientific theory must be open to falsification. You can’t falsify quantum mechanics, since it doesn’t make predictions. It’s a very general framework, one within which you can formulate theories that
do
make predictions. Newtonian physics isn’t formulated in quantum mechanics, but all our modern theories are. And quantum mechanics by itself does not say anything about the universe spontaneously coming into existence. For that sort of thing, you need quantum mechanics with other theories married to it.”

So where does that leave us?

“In a fairly unsatisfactory place, I’d say. In the long run, one would like to have a truly unified theory—not just quantum mechanics plus stuff, but a theory that combines everything into an indissoluble union. And nothing we’ve seen so far is at all like that. I mean, you can have a quantum theory of gravity, or quantum electrodynamics, or the standard model, but that’s just adding players to the quantum stage. We still seem to be far from the final theory.”

When I brought up string theory, a melancholy strain became detectable in Weinberg’s voice.

“I was hoping that with string theory things would fall into place much more rapidly than they have,” he said. “But it’s been rather disappointing. I’m not one of those people who bad-mouth string theory. I still think it’s the best effort we’ve made to step beyond what we already know, but it hasn’t worked out the way we were expecting it would. There are an enormous number of different solutions to the equations of string theory, something like 10 to the five-hundredth power. If each of these solutions is somehow realized in nature, then string theory would provide a natural multiverse, and a pretty big one—big enough for the anthropic principle to work very nicely.”

Weinberg was referring to what string theorists call “the Landscape”: an inconceivably vast ensemble of “pocket universes,” each embodying a different possible solution to the equations of string theory. These pocket universes would vary in the most basic ways: in the number of spatial dimensions they had, in the kind of particles of which their matter consisted, in the strengths of their forces, and so on. Most of them would be bio-unfriendly “dead universes,” devoid of life or consciousness. But a few among this enormous multiplicity would be bound to have just the right features for intelligent observers to emerge—observers who would then be amazed to find themselves in a world that seemed miraculously fine-tuned for their comfort. Some physicists find this string-theoretic vision of the Landscape thrilling. Others contemptuously view it as a
reductio ad absurdum.

“By the way,” Weinberg added, “there’s another approach to the multiverse, one that’s purely philosophical. Robert Nozick, a philosopher at Harvard—he’s dead now—came up with it. Nozick thought it was a philosophical principle that everything you can imagine existing actually does exist.”

Right, I said—the “principle of fecundity.”

“Exactly. So, on Nozick’s picture, there are all of these different possible worlds, all causally disconnected from one another, each subject to entirely different laws. There’s a world in which Newton’s mechanics apply, and another where there are just two particles orbiting around each other forever, and still another that is totally empty. You can justify the principle of fecundity, as Nozick did, by pointing out that it has a certain pleasing self-consistency. The principle says that all possibilities are realized, but the principle itself is just one of those possibilities, so by its own lights it must be realized.”

I objected that the principle of fecundity, far from being self-consistent, might be so ontologically prodigal that it actually leads to contradiction. It’s like the set of all sets—which, being a set, has to contain itself. But if some sets contain themselves, one can also consider the set of all sets that
don’t
contain themselves. Call this set
R
. Now ask, Does
R
contain itself? If it does, then by definition it doesn’t; and if it doesn’t, then by definition it does. Contradiction! (Weinberg, of course, immediately recognized this as Russell’s paradox.) The fecundity principle, I claimed, suffered from a similarly fatal logical defect. If all possibilities are realized, and some possibilities include themselves, whereas others don’t, then the possibility that all self-excluding possibilities are realized must itself be realized. And that possibility is as self-contradictory as the set of all sets that do not contain themselves.

This led to an extended argument between Weinberg and me over just what it means for one possibility to exclude another. The argument ended somewhat inconclusively when we both agreed that it amounted to no more than “metaphysical fun.” After a little light chat about life in New York—Weinberg was born here to immigrant parents in 1933 and attended the Bronx High School of Science, but confessed that he hadn’t been back to the city “for years”—my conversation with the father of the standard model of physics was over.

HAD IT DEEPENED
my insight into the mystery of existence? Well, I was surprised that Weinberg, so bracingly skeptical and scientifically tough-minded, had declared himself open to a metaphysically extravagant notion like the principle of fecundity. I went back to his
Dreams of a Final Theory
to see what he might have to say there about the matter. The principle of fecundity, he had written, “
supposes that there
are entirely different universes, subject to entirely different laws. But, if these other universes are totally inaccessible and unknowable, then the statement that they exist would seem to have no consequences, except to avoid the question of why they do not exist. The problem seems to be that we are trying to be logical about a question that is not really susceptible to logical argument: the question of what should or should not engage our sense of wonder.”