N. 3 - 21 aprile 2001 - Giano Bifronte

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understand what the stakes are, let us remember Newton's viewpoint. As
is well known Newton postulated, in the "Scholium" to the
"Definitions" of the 1st book of the Principia, both "absolute space"
and "absolute time", but he did not conceive them just as metaphysical
entities. In fact, in order to justify "absolute space", Newton offered
what became a very famous physical argument. Take a bucket filled
with water, and set it into rotation (for instance, by attaching it by a
rope to the ceiling, twisting the rope and then letting it unwind); you
shall see that as soon as the motion of the bucket is communicated to
the water, the surface of the liquid will become curved (as a
paraboloid), and curved it will remain if the bucket is stopped all of a
sudden. This means that the water 'feels' the rotation independently of
its relative motion with respect to the bucket. So this rotation must be
regarded, in Newton's opinion, as relative to absolute space. Mach's
objection, as it had been more or less Berkeley's, was that the fact that
the relative motion of the water and the bucket is not causally
responsible for the surface's curvature, does not imply that the motion
with respect to other bodies cannot be so. In particular, one may
conjecture that it is motion with respect to the distribution of matter at
large in the universe (shortly described as the 'heaven of fixed stars' -
we should say 'galaxies' instead of 'stars') which causes the deviation
from flatness of the water's surface.
To give a mathematical formulation of the idea that inertial effects,
such as the one just described, do not prove the existence of absolute
space, but only the interaction of matter with other matter has remained
a challenge for positivistic-minded physicists. Einstein accepted the
challenge and tried to construct a gravitational theory which would
satisfy this requirement, which he named Mach's principle, though
ultimately he became rather more sceptical both on the importance of
the principle and on the degree it was embodied in his theory. The book
under review follows a different path, reaching back to W. Weber's
theory of action at a distance, to which the author had already devoted,
among other writings, also a book in English (Weber's Electrodynamics,
Kluwer 1994).
In a short review such as this one I cannot deal adequately with the
rich content of this book. I shall limit myself to describing very briefly
its main features and make some general comments. The book is
divided in two parts, according to the following scheme: