Kumar writes a conventional narrative history, focusing on the long-running debate between Niels Bohr and Albert Einstein, which took place from the mid-1920s through to the mid-1950s, over the adequacy of the quantum theory as a framework for fundamental physics.

Manjit Kumar's book is an exhaustive and brilliant account of decades of emotionally charged discovery and argument, friendship and rivalry spanning two world wars. In what also has to operate as a kind of group biography of Planck, Einstein, Bohr, Heisenberg, Pauli, Dirac et al, the quasi-novelistic character sketches occasionally have a comic quality ("The son of a tax collector, Ludwig Boltzmann was short and stout with an impressive late 19th-century beard"); but the real meat of the book is the explanations of science and philosophical interpretation, which are pitched with an ideal clarity for the general reader. Perhaps most interestingly, although the author is admirably even-handed, it is difficult not to think of Quantum, by the end, as a resounding rehabilitation of Albert Einstein.