N. 3 - 21 aprile 2001 - Giano Bifronte

affected by the medium in which it propagates [...] it acts on bodies" (p.
139), so its unique status is not a priori plausible. But the relativists
would reply either that it is only a posteriori, i. e. by experimental
evidence, that we are led to a notion of light having a special status, or
that what is essential to special relativity is not so much that the limiting
signal speed be identified with the speed of light, as that a finite
limiting signal speed exists at all and is close to c (this option is
favoured by J.-P. Vigier and J.-M. Lévy-Leblond, for instance).
Moreover, to Assis the idea that there can be a length contraction
without an aether is unacceptable (p. 146), but this kind of
philosophical opinions should be argued much more in depth in order to
bear the burden of justifying rejection of a physical theory apparently in
good agreement with experiments (if one admits that this is the case).
But are, vice versa, the philosophical grounds of relational mechanics
very sound? One can have doubts, not for any fault in Assis's approach,
but for general reasons which affect the whole Machian programme.
For instance, special and general relativity have accustomed us not to
lend an absolute meaning to simultaneity and distance between
simultaneous events; however, while formally denying absolute space
and time, Assis has no qualms whatsoever in endorsing absolute
simultaneity and distances. What is the rationale for this different
treatment of absoluteness? A second general objection pertains to action
at a distance. We know that for Leibniz (and Galileo as well) this
concept was the height of philosophical absurdity, and from his first
letter to Bentley we know that also Newton disliked it. As far as I can
see, it is not clear why a Machian should not find anything
objectionable about it. Finally, why should the constant c enter into the
general expression for the force between two particles if the velocity of
light has nothing to do with interactions, these being instantaneous?
These remarks, of course, are not meant as a refutation of the Machian
programme. They only show that a contemporary Machian physics
today cannot avoid to come to terms again with certain philosophical
issues which could be dismissed out of hand more easily in late
Nineteenth century than it is desirable, or even legitimate, today. A new
philosophy of nature has to be built in which our theoretical preferences
would be given as ample, explicit, and argued foundations as possible.
Notwithstanding these critical remarks, Relational Mechanics is an
interesting book, rich in information, and with abundant quotations
from the classical works of Berkeley, Leibniz, Newton and, of course,
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