Mach’s Principle: From Newton’s Bucket to Quantum Gravity

P Saulson

When was the last time a conference proceedings volume made good bedtime reading? Mach’s Principle: From Newton’s Bucket to Quantum Gravity, is such a book. Editors Julian Barbour and Herbert Pfister have crafted a nutritious stew from the strangest variety of ingredients: well-written papers not only by physicists but by historians and philosophers of science, brief quotes from famous physicists of the past and full translations of more obscure papers from 100 years ago, extensively transcribed and beautifully edited discussions of the papers, an index of 21 different formulations of Mach’s principle, and the results of several `straw polls’ of the participants at the July 1993 Tubingen conference on which this volume is based. Readers will be tempted again and again to dip into the riches provided here. The bedside table, or the coffee table, is indeed the best home for this book.




Paul Gaugin is supposed to have said that `No mean woman can cook well, for it calls for a light head, a generous spirit, and a large heart’. The generous spirit and large heart, at least, are evident throughout this volume. The transcribed discussions are full of evidence for the mutual respect and vigorous interchange of ideas among the very mixed audience of historians, philosophers, and physicists of both orthodox and heterodox tendencies. Dieter Brill’s contribution `Comments on Dragging Effects: Response to Pfister’, for example, not only pays tribute to the work of Pfister and of Robert Dicke, but reserves strong praise for the insight of philosopher of science Abner Shimony. One of the two epigraphs of the book quotes the last paragraph of Sir Fred Hoyle’s contribution, where he magnanimously says `I think it was Hermann Bondi who once said that physics is such a consistent and connected logical structure that if one starts to investigate it at any point and if one pursues correctly every issue that branches away from one’s starting point, in the outcome one will be led to understand the whole of physics. With Mach’s principle it seems something like that’.

This statement may perhaps call to mind the opening sentences of Karl Marx’s essay, `The Eighteenth Brumaire of Louis Bonaparte’: `Hegel remarks somewhere that all facts and personages of great importance in world history occur, as it were, twice. He forgot to add: the first time as tragedy, the second as farce.’ Undoubtedly, Einstein’s reading (or misreading, as von Borsezkowski and Wahsner point out) of Mach played a central role in the development of one of the most beautiful and successful theories of physics, the general theory of relativity. The status of Mach’s principle within the theory that Einstein bequeathed to us is the subject of much dispute, as is documented in the straw poll results on page 106. The speakers in the experimental section, in particular, are divided rather sharply on the question of whether general relativity contains any Machian effects at all. Ciufolini argues that the effect of `dragging of inertial frames’ by a rotating body, to be searched for by the experiments GP-B and LAGEOS III, is a physical manifestation of the general relativistic formulation of Mach’s principle. Will, on the other hand, attributes the dragging effect to a gravitomagnetism completely analogous to the electrodynamic case; other effects that Will would class as `Machian’ are equivalent to violations of principles that most physicists hold dear: the equivalence principle, local Lorentz invariance, and local position invariance. Since sensitive tests for violations of these principles have only yielded null results, Will is moved to wonder, `whether Mach’s principle has any meaningful content at all’. Nortvedt, too, comments that `the near perfection of local laboratory physical law in the presence of the perceived imperfections in cosmological structure is, in my view, a key mystery or clue. It either casts serious doubt on the Machian program or, more likely, confirms the modern `field paradigm’ of physics—matter acts to influence other matter at a distance only through the intermediation of fields—and the cosmological field(s) must be dominated by the role played by the gravitational metric field, with participation by any other cosmological fields being strongly suppressed, if not totally absent.’

The prominence the book gives to such disputes is, far from being a weakness, actually its greatest strength. Typical science writing, conference proceedings included, concentrates on well-established `results’, nearly effacing the traces of many paths that were tested before the right one became apparent. Here, on the other hand, thrust to the forefront are all of the confusion, lack of clarity, and honest intellectual disagreements that characterize science as it is actually done. This is one benefit that readers receive from the immense work the editors have put into preparing the transcripts of the discussions. (Several participants also offered post-conference addenda to their comments, complete with references.) The spirit of free-wheeling speculation without any emerging consensus is nowhere more evident than in the long discussion that concludes the book, on `Time, General Relativity, and Quantum Gravity’. I cannot think of another text that better illustrates how science functions as a social process for groping, however uncertainly, toward the truth.

So, whether in search of a dollop of historical wisdom, a bite of physical insight, or a dose of sociological understanding, the reader is unlikely to be disappointed. There is ample nourishment here.

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