Quantum by Manjit Kumar

Subtitled Einstein, Bohr and the Great Debate About the Nature of Reality, Kumar's Quantum might equally well have been subtitled If a Tree Falls in the Forest and No One Hears It Did It Really Fall?

Such is the debate at the core of Kumar's history of the development of quantum theory and quantum mechanics.  Starting in 1859 with Kirchhoff's work on the radiation of a perfect blackbody and ending in 2007 with the publication of Aspelmeyer and Zeilinger's measurements on entangled photons, Quantum walks the reader through the major developments, both theoretical and experimental, and the personalities in the evolution of quantum mechanics.

The title's Great Debate is this: Bohr's subatomic world does not exist unless observed while Einstein favored an observation independent reality.  Bohr considered quantum mechanics to be a complete theory while Einstein thought it incomplete.  Note that the debate does not necessarily center around Einstein's adamant claim that God does not play dice.  According to Quantum, Einstein accepted the role of probability in the theory.

Einstein was quoted as asking rhetorically "Does the moon only exist when we look at it?"  Yet despite this obvious absurdity, Bohr's view is that there is no reason to believe that rules of classical (Newtonian) physics apply at the subatomic level.  At that level, where quantum mechanics has over decades proved successful at describing phenomenon after phenomenon, different rules apply in which not only can we not know certain things deterministically, it doesn't even make sense to try.

Quantum includes some tantalizing tidbits in its closing pages.  For one is Everett's theory that it's possible to have each quantum outcome bifurcate into its own universe.  For those familiar with Schrodinger's cat, when the box is opened the world splits into to separate worlds, one in which the cat is alive and the other in which it is dead.  To me, this is just a tease.

I suppose that in the end I'll side with Einstein despite the lack of theory or experiment to support his claims of quantum mechanics' lack of completeness.  It just seems that a universe where truth is dependent on observation requires too much chutzpah.  On the other hand, as Bohr said, "Stop telling God what to do."