THE GRAND DESIGN

By Stephen Hawking and

Leonard Mlodinow

Bantam. 198 pp. $28

In "The Hitchhiker's Guide to the Galaxy," Douglas Adams famously had his characters ask a computer to provide the ultimate answer to "Life, the Universe, and Everything." As Stephen Hawking and Leonard Mlodinow point out in their book "The Grand Design," the computer's response -- 42 -- was less than helpful. Hawking, who needs no introduction, and Mlodinow, a Caltech physicist with a string of excellent books to his credit, have taken on that ultimate question in a somewhat more rigorous form by asking three related ones:

Why is there something instead of nothing?

Why do we exist?

Why does this particular set of laws govern our universe and not some other set?

Deep stuff, indeed. In the first chapter, Hawking and Mlodinow launch into an accessible and elegant history of the progression of scientific knowledge from the Greeks to modern cosmology. As is customary in such treatments, the authors point out the significance of certain milestones. The first of these, the realization by the Ionian Greeks that nature could be explained by laws rather than by the whims of the gods, is really the start of modern science. The second, the discovery by Copernicus that the Earth is not at the center of the universe, opened the door for a realistic exploration of our solar system and, later, our galaxy and universe.

So far, so good, but the 20th century was not kind to scientific orthodoxy. Two new fields -- quantum mechanics, which deals with the behavior of things at the atomic and sub-atomic level, and relativity, which is our best explanation of gravity -- came on the scene, changing our perspective on the laws of classical physics. (I have to add, as do the authors, that those laws are still valid in our everyday lives.)

The authors describe quantum mechanics in clear, non-technical language using a formulation devised by the late Richard Feynman and called the "sum over histories" approach. (Typical of the breezy style of the book, when the authors first mention Feynman, they ignore his Nobel Prize and point out that he liked to play the bongo drums at a strip club near Caltech.) By Feynman's method of quantum mechanics, the probability of an event -- an electron moving from where you are to the door of your room, for example -- is calculated by adding up the probabilities of all the ways it could happen. It could move in a straight line, circle the room a couple of times or even (with very small probability) visit Mars on its way to the door.

With that background, Hawking and Mlodinow get to the real meat of their book: the way theories about quantum mechanics and relativity came together to shape our understanding of how our universe (and possibly others) formed out of nothing. Our current best description of the physics of this event, they explain, is the so-called "M-theories," which predict that there is not a single universe (the one we live in) but a huge number of universes. In other words, not only is the Earth just one of several planets in our solar system and the Milky Way one of billions of galaxies, but our known universe itself is just one among uncounted billions of universes. It's a startling replay of the Copernican Revolution.

The conclusions that follow are groundbreaking. Of all the possible universes, some must have laws that allow the appearance of life. The fact that we are here already tells us that we are in that corner of the multiverse. In this way, all origin questions are answered by pointing to the huge number of possible universes and saying that some of them have the properties that allow the existence of life, just by chance.

I've waited a long time for this book. It gets into the deepest questions of modern cosmology without a single equation. The reader will be able to get through it without bogging down in a lot of technical detail and will, I hope, have his or her appetite whetted for books with a deeper technical content. And who knows? Maybe in the end the whole multiverse idea will actually turn out to be right!

James Trefil is a professor of physics at George Mason University. His next book will be an illustrated tour of the multiverse.