I've always thought of Stephen Hawking—whose new book The Grand Design (Bantam 2010), co-written with Leonard Mlodinow, has become an instant bestseller—less as a scientist than as a cosmic, comic performance artist, who loves goofing on his fellow physicists and the rest of us.

This penchant was already apparent in 1980, when the University of Cambridge named Hawking Lucasian Professor of Mathematics at Cambridge, the chair held three centuries earlier by Isaac Newton. Many would have been cowed into caution by such an honor. But in his inaugural lecture, "Is the End in Sight for Theoretical Physics?", Hawking predicted that physics was on the verge of a unified theory so potent and complete that it would bring the field to a close. The theory would not only unite relativity and quantum mechanics into one tidy package and "describe all possible observations." It would also tell us why the big bang banged and spawned our weird world rather than something entirely different.

At the end of his speech Hawking slyly suggested that, given the "rapid rate of development" of computers, they might soon become so smart that they "take over altogether" in physics. "So maybe the end is in sight for theoretical physicists," he said, "if not for theoretical physics." This line was clearly intended as a poke in his colleagues' ribs. Wouldn't it be ironic if our mindless machines usurped our place as discoverers of Cosmic Truth? Hilarious!

The famous last line of Hawking's monumental bestseller A Brief History of Time (Bantam 1988) was also a joke, although many people didn't get it at the time. A final theory of physics, Hawking declared, "would be the ultimate triumph of human reason—for then we should know the mind of God." Hawking seemed to imply that physics was going to come full circle back to its spiritual roots, yielding a mystical revelation that tells us not just what the universe is but why it is. Science and religion are compatible after all! Yay!

But Hawking ain't one of these New Agey, feel-good physicist–deists like John Barrow, Paul Davies, Freeman Dyson or other winners of the Templeton Prize for Progress Toward Research or Discoveries about Spiritual Realities. Deep inside Brief History Hawking showed his true colors when he discussed the no-boundary proposal, which holds that the entire history of the universe, all of space and time, forms a kind of four-dimensional sphere. The proposal implies that speculation about the beginning or end of the universe is as meaningless as talking about the beginning or end of a sphere.

In the same way a unified theory of physics might be so seamless, perfect and complete that it even explains itself. "What place, then, for a creator?" Hawking asked. There is no place, he replied. Or rather, a final theory would eliminate the need for a God, a creator, a designer. Hawking's first wife, a devout Christian, knew what he was up to. After she and Hawking divorced in the early 1990s she revealed that one of the reasons was his scorn for religion.

Hawking's atheism is front and center in Grand Design. In an excerpt Hawking and Mlodinow declare, "There is a sound scientific explanation for the making of our world—no Gods required." But Hawking is, must be, kidding once again. The "sound scientific explanation" is M-theory, which Hawking calls (in a blurb for Amazon) "the only viable candidate for a complete 'theory of everything'."

Actually M-theory is just the latest iteration of string theory, with membranes (hence the M) substituted for strings. For more than two decades string theory has been the most popular candidate for the unified theory that Hawking envisioned 30 years ago. Yet this popularity stems not from the theory's actual merits but rather from the lack of decent alternatives and the stubborn refusal of enthusiasts to abandon their faith.

M-theory suffers from the same flaws that string theories did. First is the problem of empirical accessibility. Membranes, like strings, are supposedly very, very tiny—as small compared with a proton as a proton is compared with the solar system. This is the so-called Planck scale, 10^–33 centimeters. Gaining the kind of experimental confirmation of membranes or strings that we have for, say, quarks would require a particle accelerator 1,000 light-years around, scaling up from our current technology. Our entire solar system is only one light-day around, and the Large Hadron Collider, the world's most powerful accelerator, is 27 kilometers in circumference.

Hawking recognized long ago that a final theory—because it would probably involve particles at the Planck scale—might never be experimentally confirmable. "It is not likely that we shall have accelerators powerful enough" to test a unified theory "within the foreseeable future—or indeed, ever," he said in his 1980 speech at Cambridge. He nonetheless hoped that in lieu of empirical evidence physicists would discover a theory so logically inevitable that it excluded all alternatives.

Quite the opposite has happened. M-theory, theorists now realize, comes in an almost infinite number of versions, which "predict" an almost infinite number of possible universes. Critics call this the "Alice's restaurant problem," a reference to the refrain of the old Arlo Guthrie folk song: "You can get anything you want at Alice's restaurant." Of course, a theory that predicts everything really doesn't predict anything, and hence isn't a theory at all. Proponents, including Hawking, have tried to turn this bug into a feature, proclaiming that all the universes "predicted" by M-theory actually exist. "Our universe seems to be one of many," Hawking and Mlodinow assert.

Why do we find ourselves in this particular universe rather than in one with, say, no gravity or only two dimensions, or a Bizarro world in which Glenn Beck is a left-wing rather than right-wing nut? To answer this question, Hawking invokes the anthropic principle, a phrase coined by physicist Brandon Carter in the 1970s. The anthropic principle comes in two versions. The weak anthropic principle, or WAP, holds merely that any cosmic observer will observe conditions, at least locally, that make the observer's existence possible. The strong version, SAP, says that the universe must be constructed so as to make observers possible.

The anthropic principle has always struck me as so dumb that I can't understand why anyone takes it seriously. It's cosmology's version of creationism. WAP is tautological and SAP is teleological. The physicist Tony Rothman, with whom I worked at Scientific American in the 1990s, liked to say that the anthropic principle in any form is completely ridiculous and hence should be called CRAP.

In his 1980 speech in Cambridge Hawking mentioned the anthropic principle—which he paraphrased as "Things are as they are because we are"—as a possible explanation for the fact that our cosmos seems to be fine-tuned for our existence. But he added that "one cannot help feeling that there is some deeper explanation."

Like millions of other people I admire Hawking's brilliance, wit, courage and imagination. His prophecy of the end of physics inspired me to write The End of Science (which he called "garbage"). Hawking also played a central role in one of the highlights of my career. It dates back to the summer of 1990, when I attended a symposium in a remote Swedish resort on "The Birth and Early Evolution of Our Universe." The meeting was attended by 30 of the world's most prominent cosmologists, including Hawking.

Toward the end of the meeting, everyone piled into a bus and drove to a nearby village to hear a concert in a Lutheran church. When the scientists entered the church, it was already packed. The orchestra, a motley assortment of blond-haired youths and wizened, bald elders clutching violins, clarinets and other instruments, was seated at the front of the church. Their neighbors jammed the balconies and seats at the rear of the building.

The scientists filed down the center aisle to pews reserved for them at the front of the church. Hawking led the way in his motorized wheelchair. The townspeople started to clap, tentatively at first, then passionately. These religious folk seemed to be encouraging the scientists, and especially Hawking, in their quest to solve the riddle of existence.

Now, Hawking is telling us that unconfirmable M-theory plus the anthropic tautology represents the end of that quest. If we believe him, the joke's on us.