Physics changes so rapidly that a new survey of its landmarks is necessary every few years; here’s an update from a popular British science writer. The focus in this book (a spinoff from the author’s earlier The Search for Black Holes) is the nature of matter. Our modern conception of matter dates from the realization roughly a century ago that the atom, in theory the tiniest particle of matter, was itself a compound entity. The discovery of radioactivity opened up a window to peer inside atoms, revealing some of the structural units (electrons, the nucleus) of which these “indivisible” atoms are made. Ernest Rutherford’s “classical” picture of the atom as a small but solid nucleus surrounded by a whirling cloud of electrons, had taken shape by the 1920s. But this easily understood model was rapidly modified, as quantum theory began to blur the intuitive divisions between matter and energy. The number of subatomic particles also underwent a population explosion, leading to quantum chromodynamics, in which the ultimate divisions of matter are invisible quarks and gluons, from which other particles are built. But even this “standard model” fails to unite the various forces and particles into a coherent structure; thus the search for a Grand Unified Theory, the holy grail of modern physics. This takes us into the rarefied territory of supersymmetry, superstrings, and gauge theory in a ten-dimensional matrix. Gribbin gives the reader a good overview of the progress to date of this research, describing its key experiments and noting the contributions of various scientists, and making the theory itself as clear as possible for readers not prepared to tackle serious math. Perhaps in ten years’ time the key questions will be answered—always assuming that some new discovery doesn’t send everyone back to the starting line again. A clear and comprehensive popular treatment of the cutting edge of physics.