Life is an enduring mystery. Yet, science tells us that living beings are merely sophisticated structures of lifeless molecules. If this view is correct, where do the seemingly purposeful motions of cells and organisms originate? In Life's Ratchet, physicist Peter M. Hoffmann locates the answer to this age-old question at the nanoscale.

Below the calm, ordered exterior of a living organism lies microscopic chaos, or what Hoffmann calls the molecular storm--specialized molecules immersed in a whirlwind of colliding water molecules. Our cells are filled with molecular machines,...