The New Physics and Its Evolution | Page 7

the great thought clearly expressed by the Encyclopædists of
the eighteenth century; and if the necessity of interpreting the
phenomena of electricity or light led the physicists of last century to
imagine particular fluids which seemed to obey with some difficulty
the ordinary rules of mechanics, these physicists still continued to
retain their hope in the future, and to treat the idea of Descartes as an

ideal to be reached sooner or later.
Certain scholars--particularly those of the English School--outrunning
experiment, and pushing things to extremes, took pleasure in proposing
very curious mechanical models which were often strange images of
reality. The most illustrious of them, Lord Kelvin, may be considered
as their representative type, and he has himself said: "It seems to me
that the true sense of the question, Do we or do we not understand a
particular subject in physics? is--Can we make a mechanical model
which corresponds to it? I am never satisfied so long as I have been
unable to make a mechanical model of the object. If I am able to do so,
I understand it. If I cannot make such a model, I do not understand it."
But it must be acknowledged that some of the models thus devised
have become excessively complicated, and this complication has for a
long time discouraged all but very bold minds. In addition, when it
became a question of penetrating into the mechanism of molecules, and
we were no longer satisfied to look at matter as a mass, the mechanical
solutions seemed undetermined and the stability of the edifices thus
constructed was insufficiently demonstrated.
Returning then to our starting-point, many contemporary physicists
wish to subject Descartes' idea to strict criticism. From the
philosophical point of view, they first enquire whether it is really
demonstrated that there exists nothing else in the knowable than matter
and movement. They ask themselves whether it is not habit and
tradition in particular which lead us to ascribe to mechanics the origin
of phenomena. Perhaps also a question of sense here comes in. Our
senses, which are, after all, the only windows open towards external
reality, give us a view of one side of the world only; evidently we only
know the universe by the relations which exist between it and our
organisms, and these organisms are peculiarly sensitive to movement.
Nothing, however, proves that those acquisitions which are the most
ancient in historical order ought, in the development of science, to
remain the basis of our knowledge. Nor does any theory prove that our
perceptions are an exact indication of reality. Many reasons, on the
contrary, might be invoked which tend to compel us to see in nature

phenomena which cannot be reduced to movement.
Mechanics as ordinarily understood is the study of reversible
phenomena. If there be given to the parameter which represents time,[1]
and which has assumed increasing values during the duration of the
phenomena, decreasing values which make it go the opposite way, the
whole system will again pass through exactly the same stages as before,
and all the phenomena will unfold themselves in reversed order. In
physics, the contrary rule appears very general, and reversibility
generally does not exist. It is an ideal and limited case, which may be
sometimes approached, but can never, strictly speaking, be met with in
its entirety. No physical phenomenon ever recommences in an identical
manner if its direction be altered. It is true that certain mathematicians
warn us that a mechanics can be devised in which reversibility would
no longer be the rule, but the bold attempts made in this direction are
not wholly satisfactory.
[Footnote 1: I.e., the time-curve.--ED.]
On the other hand, it is established that if a mechanical explanation of a
phenomenon can be given, we can find an infinity of others which
likewise account for all the peculiarities revealed by experiment. But,
as a matter of fact, no one has ever succeeded in giving an indisputable
mechanical representation of the whole physical world. Even were we
disposed to admit the strangest solutions of the problem; to consent, for
example, to be satisfied with the hidden systems devised by Helmholtz,
whereby we ought to divide variable things into two classes, some
accessible, and the others now and for ever unknown, we should never
manage to construct an edifice to contain all the known facts. Even the
very comprehensive mechanics of a Hertz fails where the classical
mechanics has not succeeded.
Deeming this check irremediable, many contemporary physicists give
up attempts which they look upon as condemned beforehand, and adopt,
to guide them in their researches, a method which at first sight appears
much more modest, and also much more sure. They make up their
minds not to see at once to the bottom of things; they no longer seek to