UNIVERSAL FORCES
We find it equally impossible to conceive of matter without force,
as of force without matter. The two modes of action, or of being,
are inseparable. Yet our minds strongly distinguish between them;
they impart a dual aspect to the world. Phenomena are not simple
manifestations of disembodied energy, if such a thing could be; they
have a substantial basis which, nevertheless, eludes apprehension,
and seems to slip away into nothingness if we try to empty it of its
immaterial contents. Nor do these energize in the void. They and the
bodies they animate are knowable only in combination, and exist, to our
apprehension, only on the condition of mutual dependence. All we can do
towards discriminating them is to fix our attention predominantly on
one or the other side of things, and so facilitate thought by drawing
ideal lines of demarcation.
Just as there are many forms of matter--all springing, we are led to
believe, from an undifferentiated, fundamental world-stuff--so there
are various kinds of force, reducible, possibly without exception, to
one universal principle. Their correlation, indeed, has been already
in large measure demonstrated; heat, light, electricity, and kinetic
power are known to be equivalent and interchangeable; but there
are outstanding activities, which resist assimilation, and seem to
originate under different conditions from the rest. Forces manifest
themselves chiefly through attractive and repulsive effects, varying
in accordance with their natures and the modification of attendant
circumstances. The minute particles of matter, for instance, cohere;
they cling together tenaciously; yet no pressure avails to bring them
into actual contact; at a certain point of mutual approach, they
develop an invincible power of resistance to any further encroachments
upon their separate molecular domains. And it is this faculty which
gives to matter its distinctive property of hardness. It is rendered
tangible to sense just by its recalcitrance to constraint.
Neither the mode of operation nor the nature of the forces by which
molecules are organized into masses is known; while the power acting on
the masses thus organized, and regulating by its action the mechanism
of the universe, is fully as baffling to comprehension. Wonder at
its results is blunted by familiarity; presented to us as novelties,
they should be pronounced to outrage reason. The relations of gravity
are of the utmost simplicity; and they are, on that very account,
supremely perplexing. They are governed by one steadfast law, the same
everywhere, and under all varieties of conditions within the range of
experiment or precise observation. It governs impartially every kind
and quality of matter, taking no notice of its states or combinations,
ignoring its subjection to chemical, thermal, magnetic, or electrical
influences. Gravity is not only indifferent, but inevitable and
inexorable; there is no resisting its sway; no screen serves as a
shelter against its persuasions; it spreads equably in all directions,
becoming enfeebled, like wave-motion, in the strict proportion of its
diffusion from a centre over successive spherical surfaces. Its most
singular peculiarity, however, is its apparent unconcern with time. The
gravitational pull is virtually instantaneous; its transmission--if
it be transmitted--takes place millions of times faster than that of
light; the finest tests have, so far, failed to elicit symptoms of
delay. These would be found in minute discrepancies between calculated
and observed perturbational effects in the heavens.
The action of gravity, if propagated with finite velocity, should
differ for bodies preserving an invariable distance from its source,
and for bodies travelling towards or away from it. Their movement would
modify the law of attraction.[62] Yet, up to the present, it has proved
impossible to detect the slightest deviation from the plain rule of
inverse squares. Again, the penetrative faculty of this strange force
seems absolute and unlimited. We know by ordinary experience that we
cannot diminish the weight of an object by interposing any kind of
shield between it and the earth; and no refinement in experimentation
avails to alter this result. That it is so, is a fortunate circumstance
for the harmony of the world. We can dimly imagine the riot of
confusion that would ensue if a transiting globe could intercept the
attraction, as it does the light, of a central governing mass. But
from the wonderfully adjusted universal order gravitational eclipses
are excluded; nor does the densest body throw even the faintest
gravitational shadow.
The nature of a power so singularly conditioned is almost
inconceivable; yet attempts have not been wanting to fathom the mystery
that surrounds it. Professor Osborne Reynolds, in the Rede Lecture for
1902, claimed to have arrived at 'a complete, quantitative, purely
mechanical explanation of the cause of gravitation,' based on the
'dilatancy' of a granular medium in close piling. But his working model
of the universe will probably be remembered only as a lesson in the
'inversion of ideas,' showing that with skill and ingenuity a fairly
concordant outcome of phenomena may be derived from antagonistic
hypotheses. In this author's view matter is equivalent to a deficiency
of mass, the spaces where his cosmic grains are relatively few,
because their arrangement is out of gear, being driven towards one
another by the pressure of the surrounding medium, in which they are
compactly stowed, and therefore numerous. Thus, the acting forces in
nature are made to depend upon the compression by the denser medium
of interspatial tracts of rarer consistence, forming what we call
matter. The theory is difficult, if not impossible of acceptance, not
because it involves the overthrow of conceptions which may be rooted in
habitual modes of thought, rather than in absolute truth, but because
of its startling postulates and large vacuities. To be valid, it should
be complete; and there are obvious chasms in the vast expanse of ground
which it covers with surprising, though only partial success.
The _multa renascentur_ of the poet is verified by the revolutions no
less of thought than of speech. Flights of minute material particles
have served the turn of theorists often, and in more ways than one,
and have as frequently been consigned to discredited oblivion; but
they are in vogue once more. George Louis Lesage, of Geneva, devoted
sixty-three of the seventy-nine years of his life, which came to
an end in 1803, to the elaboration of a mechanical rationale of
gravity, first given to the world in the _Transactions_ of the Berlin
Academy of Sciences for 1782, and with details of amplification in
his _Traité de Physique Mécanique_, edited by Pierre Prévost in
1818.[63] The explanation it offered of molar attractions was by the
supposed unceasing impacts of 'ultramundane corpuscles,' speeding in
countless numbers and at fabulous velocities, from nowhere everywhere,
and thus enforcing the mutual approach of masses of gross matter.
This involved the supposition of an infinitesimal screening effect,
producing a small inequality in the strength of the bombardment on
the sheltered and unsheltered sides of the bodies exposed to it. This
inequality, in fact, was taken to be the _causa causans_ of gravity.
Yet its production encountered a difficulty. There was required for
it a trifling degree of opacity in every kind of matter, while
perfect gravitational transparency is asserted by the most delicate
observations. Lesage, then, reduced the arrests laid upon his particles
to a minimum; one in ten thousand, for instance, might at the utmost
be intercepted by the terrestrial globe.[64] Even this insignificant
minority, however, would suffice, through the surrender of their
momentum to the impeding bodies, to endow them with the noted property
of gravitation.
Clerk Maxwell urged the objection that the accompanying loss of kinetic
energy by the corpuscles should, if transformed into heat, render all
gravitating bodies white-hot. But Professor J. J. Thomson holds that
the transferred corpuscular energy might, instead of reappearing in
thermal form, be converted into some highly penetrative species of
radiation capable of escaping unperceived into surrounding space.[65]
'A simple calculation,' he adds, 'will show that the amount of kinetic
energy transformed per second in each gramme of the gravitating body
must be enormously greater than that given out in the same time by
one gramme of radium.' This consequence of Lesage's theory takes
one's breath away; the 'fables of the Talmud' seem, by comparison,
easy of belief; nevertheless, Lord Kelvin[66] declared it in 1873 to
be more complete in the expository sense, and not more arduous in its
assumptions, than the kinetic theory of gases.
Its fundamental postulate, at any rate, has been curiously verified in
the course of recent researches into the arcana of physics. Entities
in some degree corresponding to the ultramundane corpuscles of the
Genevese philosopher do actually exist. Electrons are being continually
expelled from bodies in all parts of the universe; they issue forth
under all conceivable conditions and in unlimited numbers. Space is
perhaps thronged with them; no material object can be exempt from their
multitudinous buffetings, which are beginning to be taken account of
in many cosmical speculations, and cannot certainly be ignored in
efforts to solve the most obvious to superficial apprehension, the most
intricate on a profound consideration, of all cosmical problems. But
there is one fatal objection to an electronic theory of gravitation.
The agency appealed to travels too slowly to be available for the
required purpose. The velocity of light, there is reason to believe,
sets a limit impossible to be surpassed or even attained by the
velocity of electrons; yet it is incomparably smaller than the rate of
gravitational transmission.
Tisserand estimated at six million times the quickness of luminous
travel the minimum speed at which the sun's attraction must be
propagated in view of the imperceptibility in planetary observations
of effects corresponding to a time-inequality;[67] and this value
may be taken as authentic. So colossal a discrepancy excludes any
kind of impact-rationale of the mutual pull of heavy masses; Lesage's
corpuscles remain 'ultramundane'; their identification with known
atoms or sub-atoms appears to be precluded; no products of ionic
disintegration possess the qualities necessarily to be ascribed to them.
We turn, then, inevitably to the menstruum of mysteries, the bank of
the insolvent in speculation, to the all-serviceable ether. Ethereal
radiations exercise an impulsive power; light-pressure has secured
a recognised status among cosmic agencies; and every vibrational
system of the luminous type undoubtedly shares the faculty by which
light tends to drive minute particles forward along the lines of its
propagation. Professor J. J. Thomson, accordingly, considered that but
for the drawback of their insufficient velocity, 'very penetrating
Röntgen rays' might with advantage be substituted for corpuscular
streams as the cause of gravity.[68] They would, in some slight
degree, be absorbed by encountered masses, to which they would impart
a proportionate amount of momentum. Two bodies mutually shadowing one
another would, under such circumstances, be drawn together with a force
varying as the inverse square of distance; and if further they absorbed
the impinging rays strictly in the measure of their density (as
observation shows to be approximately the case), the attraction would
increase in the same ratio as the product of their masses. But Röntgen
rays travel with the precise velocity of light; they are, in truth,
ultra-invisible light; and they must hence be regarded as hopelessly
incompetent to explain an influence transmitted at least six million
times more rapidly.
This was fully admitted by Dr. H. A. Lorentz,[69] who, five years
ago, weighed the vibrational hypothesis of gravity in the balance of
rigorous calculation, and found it wanting. His equations yielded the
unexpected result that, if its postulates were granted, the noted
attractions between massive bodies could subsist only on the terms
of an incessant waste of electro-magnetic energy. But this is of
course inadmissible. The theory involving such a consequence stands
self-condemned, to say nothing of the wholly inadequate rate of
propagation afforded by it.
Impulsion hypotheses, whether by corpuscles or rays, being hopelessly
discredited, Dr. Lorentz reverted to a half-forgotten speculation
by Mosotti, which, though sixty years old, struck him as capable of
being adapted to modern requirements. It was of an electrical nature,
and, in the novel shape given to it, supposed gravitational action to
depend upon strains of the ether due to the disturbing effects of the
positive and negative ions constituting ordinary matter. These 'states'
of the medium are distinct in kind; they cannot neutralize one another;
and the familiar law of attraction represents their resultant effect.
To bring it about much has to be taken for granted; yet the hypothesis
can lay claim to one singular prerogative. Although the disturbances
invoked by it traverse the ether with no more than the standard speed
of light, it appears from Dr. Lorentz's investigation that, owing to
certain modifications in the properties of the medium produced by
moving matter, the planetary perturbations betraying loss of time in
gravitational transmission would, on the electrical theory, be so
small as to evade detection. As regards this crucial point, the Dutch
physicist has hit upon a felicity of explanation entirely original,
and, as it were, unsought.
An 'undulatory theory' of gravity, adumbrated, rather than advanced
by Mr. Whittaker in 1902,[70] excited hopes that the ideal aim of
science--a complete unification of the forces of Nature--might at
last be within reach. Based upon a striking mathematical research,
it exhibited the attraction between masses as, in a manner, the
integration of innumerable wave disturbances, propagated at a rate
not strictly definable, but perhaps immensely surpassing that of
gravity. No suggestion was made as to the primary mode or cause of
agitation, yet it seemed much to learn that the medium we are cognizant
of in space might be capable of transmitting the pull of gravity.
Unfortunately, however, the physical foundation of this reassuring
congruity proves to be weak or unsound. The mathematical mill works
magnificently, but the grist put into it is of dubious quality.
Stripping Mr. Whittaker's result of its purely analytical form, Dr.
Johnstone Stoney showed that an assumption of extreme improbability lay
concealed in his equations, which could not, he concluded, be seriously
taken to correspond with the reality of things.[71]
There would then seem to be no alternative but to accept _ad interim_
the electro-dynamical view of the nature of gravity. If not true, it
is at least not obviously false. Through its subtlety it escapes direct
confutation. And the method of exclusions, by eliminating competitors,
has left it in virtual possession of the field.
Nothing is more curious in the history of recent science than the
continual and irresistible growth which it records in the importance
of electrical phenomena. All others tend to become merged in them; the
most varied data of experience claim to be translated into electrical
terminology. They are not, assuredly, rendered more intelligible by
the process, but it at any rate abolishes the confusion incidental
to multitudinous points of view. Thus, in the last resort, we find
electrical forces (if they may be so designated) swaying the world.
What they essentially consist in, we cannot tell; the utmost that may
reasonably be hoped for is to arrive at a clear conception of modes
of action reduced to antagonistic stresses, by which the play and
counterplay of the universe may be kept up. And to this extent we find
it possible to understand how electricity works the ethereal machinery.
It is strongly dualistic. The nearer we get to the foundations of
nature, the more sharply positive and negative charges appear to be
differentiated.
The opinion is nevertheless held by some inquirers that negative
electricity is the only substantive kind, and that its complement is
ordinary matter deprived of some of its negative particles. This, in
fact, revives Franklin's 'one fluid theory,' only with the substitution
of negative for positive electricity as the active principle.[72] But
we are met by the doubt whether 'ordinary matter' can be said to exist
in and by itself. If it do, the mode of its existence becomes more and
more baffling to comprehension, as the association of mass with charge
makes its way into the foreground of thought.[73] Moreover, a charge
is, or produces, a 'state of the ether' (to use the unsatisfactory
current phrase); and the ether being capable of opposite distortions,
the effects upon it of opposite charges are contrary and similar,
though perhaps not equivalent; whence the 'two fluid theory' obtains
a _primâ facie_ recommendation as the simplest, though a crude
interpretation of electrical phenomena. These are ubiquitous; destitute
though we are of sense-organs for their perception, we still indirectly
recognise their presence on every side. If the unification of the
forces in nature be attainable, the unifying formula will doubtless
be derived from them. Electricity is the _mot de l'énigme_; yet it is
itself the most inscrutable of enigmas.
FOOTNOTES:
[Footnote 62: Lorentz, _Proceedings Amsterdam Academy_, 1900, p. 565.]
[Footnote 63: Sir W. Thomson, _Philosophical Magazine_, vol. xlv.,
fourth series, 1873. Many hints have been taken, in what is above
written, from this valuable paper.]
[Footnote 64: Sir W. Thomson (Lord Kelvin), _loc. cit._, p. 323.]
[Footnote 65: _Electricity and Matter_, p. 159.]
[Footnote 66: _Loc. cit._, p. 331.]
[Footnote 67: _Traité de Mécanique Céleste_, tome. iv., p. 495.]
[Footnote 68: _Electricity and Matter_, p. 160.]
[Footnote 69: _Proceedings Amsterdam Academy of Sciences_, 1900, p.
559.]
[Footnote 70: _Monthly Notices_, vol. lxii., p. 619; vol. lxiii., p.
258.]
[Footnote 71: _Monthly Notices_, vol. lxiii., p. 424.]
[Footnote 72: J. J. Thomson, _Electricity and Matter_, p. 88.]
[Footnote 73: _Ibid._, p. 47.]