Clifford M. Will

An excellent popular account of the various experimental tests of general relativity (GR) conducted up to the mid 1980s. Will traces the origins of these "modern era" experiments to the 12 months beginning in September 1959, which saw the first attempt to measure the Earth-Venus separation using radar ranging; the Pound-Rebka experiment which was the first successful laboratory measurement of the gravitational red shift; theoretical developments such as Roger Penrose’s spinor algebra, and the Brans-Dicke theory which offered a possible alternative to general relativity; and the detection of the unusual radio source 3C48 which would subsequently become known as the first quasar to be discovered.

A strong point of the book, occupying two early chapters, is an excellent discussion of the distinction between Einstein’s principle of equivalence and the full theory of GR. Will emphasises that the principle of equivalence – the idea that an observer in a sufficiently localised frame of reference cannot differentiate between the effects of acceleration and gravitation – is, on its own, enough to demand that spacetime must be curved. Several physical phenomena, including the classic Eötvös experiment at the turn of the last century and the gravitational red shift measured by Pound and Rebka, establish only that different materials "fall" at the same rate. This result is, in fact, predicted by the principle of equivalence without any reference to GR.

But predicting the actual amount of curvature requires GR – or one of its competitors, such as the Brans-Dicke theory. The remainder of the book describes modern approaches to the other two classical tests proposed by Einstein, i.e., the perihelion precession of Mercury and the gravitational deflection of light by the sun. Will also describes several modern tests which were not envisaged until the 1960s and became experimentally feasible only when the Apollo and Viking landers made it possible to anchor laser and radar reflectors to the moon and Mars, enabling solar system distances to be measured with centimetre accuracy. The most recent tests covered in the book are those derived from the Hulse-Taylor binary pulsar discovered in 1974, providing the first evidence (albeit indirect) of gravitational radiation.

The book is now inevitably somewhat dated, with little mention of phenomena such as gravitational lensing (let alone micro-lensing), and predating the re-introduction of Einstein’s cosmological constant as a more or less standard component of GR. However, this should not deter readers from seeking it out.

The only gripe I have – which I hasten to add is directed at the publisher rather than the author – is the unnecessary (and potentially counter-productive) attempt to sensationalise the title. “Was Einstein Right?” stories may be standard fare for popular science magazines hoping to sell more copies at the newsstand, but have little place otherwise. For a theory as successful and well-tested as GR, the question is not whether it is right or wrong, but to what extent it approximates the truth. On one hand, we know from physicists such as Will that GR makes predictions which are in excellent agreement with what we are able to measure; but we also know that reconciling it with quantum mechanics will require fundamental changes to one or both theories. Yet whatever replaces GR must reduce to GR in a region of spacetime where the curvature is acceptably low, just as GR reduces to special relativity in flat spacetime, and special relativity reduces to Newtonian mechanics at low velocities.… (més)