"In the world of physics, the fundamental constituents of reality, such as protons, electrons, and other subatomic particles, are not hard and indivisible.  They behave like both waves and particles. "They can appear out of nothing - a pure void - and disappear again.  Physicists have even managed to teleport atoms, to move them from one place to another without passing through any intervening space.  On the quantum scale, objects seem blurred and indistinct, as if created by a "be-sorted god", states physicist David Deutsch.  "If the theory weren't valid, no one would be walking around with cell phones or Palm Pilots."

To grapple with the contradictions, most physicists have chosen an easy way out restricting the validity of quantum theory to the subatomic world.  But Deutsch argues that the theory's laws must hold at every level of reality.  Everything in the world, including ourselves, is made of these particles, and since quantum theory has proved infallible in every conceivable experiment, the same weird quantum rules must apply to us!

Deutsch argues that physicists who use quantum mechanics in a utilitarian way suffer from loss of nerve.  They simply can't accept the strangeness of quantum reality.  It is probably the first time in history that they have refused to believe what their reigning theory says about the world.  It is like Galileo refusing to believe that Earth orbits around the Sun and using the heliocentric model of the solar system only as a convenient way to predict  the positions of stars and planets in the sky.  Like modern physicists who speak of photons as being both wave and particle, here and there at once, Galileo could have argued that Earth is both moving and stationary at the same time and ridiculed impertinent graduate students for questioning what that could possibly mean. 

"This dilemma of whether you should accept that the world is the way a theory says it is or whether you should just think of the theory as a manner of speaking, has occurred with every fundamentally new scientific theory right back to Copernicus," says Deutsch.  "I'm not quite sure why physicists should move ready to believe in planets in distant galaxies that  believe in Everett's other worlds".  "Of course the number of parallel universes is really huge.  I like to say that some physicists are comfortable with little huge numbers but not with big huge numbers."

In the many worlds view, time travel is no more paradoxical - although it may prove a bit more difficult - than any other form of transportation.  If you got particularly angry with yourself or something you once did, or might do, you could even travel to the past - or the future - and change your actions.  Different times are nothing less than different universes.  "The universes we can affect we call the future.  Those that can affect us, we call the past."

Deutsch knows the idea takes some getting use to, especially when one pauses to consider what it means on an everyday level.  For starters - it solves once and for all the ancient question of whether we have free will.  "The bottom line is that the universe is open," Deutsch says.  "In the relevant sense of the word, we have free will."  We also have every possible option we've ever encountered acted out somewhere in some universe by at least one of our other selves.  We take all the roads in our lives.

Driving a car, for example, becomes extremely hazardous, because it is almost certain that somewhere in some other universe the driver will accidentally hit and kill a child.  So should we never drive?  Deutsch thinks it is impossible to control the fate of our other selves in the multiverse.  But, if we are cautious, other copies of us may decide to be cautious. 

Coming from a physicist of lesser statue, such startling views might be dismissed.  But Deutsch has impeccable credentials.  While still in his early thirties he created the theoretical framework for an entirely new discipline called quantum computation. 

Deutsch is not the originator of the multiverse concept.  That credit goes to Hugh Everett, whose 1957 Princeton doctoral thesis first presented what has come to be called the "many worlds" interpretation of quantum mechanics.  He was trying to solve the problem of why we see only one of the multiple states in which a particle can exist.  In the conventional view, the very act of our observation causes all the possible states of a particle to "collapse" abruptly into a single value, which specifies the position or energy of the particle.  To understand how this works, imagine that the particle is an e-mail message.  When the message is sent, there are multiple possible outcomes.  The e-mail could reach its intended destination; any number of people could get it be mistake; or the sender might receive a notice that the message could not be delivered.  But when one outcome is observed, all other possibilities with regard to the e-mail delivery collapse into one reality.

Other physicists greeted Everett's theory with resounding indifference.  The article appeared, and that was the end of it - just total silence.  This did not faze Everett and he remained convinced until his death in 1982 that he was correct about quantum mechanics.  And if the many worlds theory is true, Deutsch believes that  other copies of Everett might remain alive somewhere in the multiverse.