Stephen Hawking is talking about God again.

His new book, due in America in September 7, has the champions of atheism all excited. Jerry Coyne is ecstatic that the new book will put to rest the claim that the heroic cosmologist is "religious, even in a deistic sense."

In the précis for the book, Hawking says" In The Grand Design we explain why, according to quantum theory, the cosmos does not have just a single existence, or history, but rather that every possible history of the universe exists simultaneously. We question the conventional concept of reality, posing instead a "model-dependent" theory of reality. We discuss how the laws of our particular universe are extraordinarily finely tuned so as to allow for our existence, and show why quantum theory predicts the multiverse-the idea that ours is just one of many universes that appeared spontaneously out of nothing, each with different laws of nature."

Hawking's grand claims are indeed based on science -- sort of. Quantum mechanics is a real scientific theory, established beyond all doubt to be the way the world works. But quantum mechanics, like everything in fundamental physics, is deeply mathematical and not everything mathematical has a counter part in the real world. Not every solution to an equation corresponds to something that is actually happening.

Physicists have had lots of practice in solving equations with many solutions and then examining the solutions to see which ones, if any, correspond to the real world. Even something so simple as the equation for the path of a baseball has two solutions. One solution will correspond to, say, the path that the baseball took as it rocketed off the bat of David Ortiz. But there will be another solution to the equation that will describe a very different baseball -- one that is clearly not coming off the bat of David Ortiz. In freshman physics, students learn to toss these "non-physical" solutions aside.

Physics is the art of looking for equations that somehow describe the real world, finding the solutions to these equations, and then matching the solution against the real world. The importance of connecting the mathematical descriptions to the physical reality is essential. When Hawking says that "every possible history of the universe exists simultaneously," he is making an assumption that his freshman professor probably told him not to make -- namely that every solution to one's equation describes a real physical reality.

"Every possible history of the universe" is a remarkable set of narratives. "Possible" in this case, would loosely mean "not-self contradictory." So we have universes more or less like this one but with interesting differences -- the French won in North America instead of the English; Hitler was successfully assassinated. Or the earth is at the center of the solar system. Reality TV was never developed. And, of course, there will be all the universes where the laws of nature are just different enough to prevent life from existing. And universes that never really got going because gravity was too strong and they collapsed immediately after they were born. We are being asked to believe that all these universe actually exist.

If all these many universes exist, then one mystery of our universe is dispelled -- the so-called "fine-tuning" puzzle. Our universe is unexpectedly hospitable to life, a fact that many have suggested provides support for belief in a purposeful Creator. But, if all possible universes exist, then of course one will be like this one. We do know that this one is "possible" since it is "actual."

It is entirely possible that these histories are real. If we have learned anything from physics, it is that lots of crazy things are true. But physicists have always anchored their more speculative theories in careful observation. When Newton was developing his theory of gravity he waited anxiously for new observational data so he could be sure he was on the right track. Unfortunately, the multi-verse enthusiasts have no such caution. It appears that the impossibility of actually finding real evidence for alternative "histories," has become a license to make confident and grandiose claims without such evidence.