ETHERIAL DENSITY
This leads us to enter upon the question of whether it is possible to
determine with any approach to accuracy the actual density or
massiveness of the ether of space, compared with those forms of matter
to which our senses have made us accustomed.
The arguments on which an estimate may be made of the density or
massiveness of the ether as compared with that of matter depend on the
following considerations, the validity of which again is dependent
upon an electrical theory of matter. In this theory, or working
hypothesis, an assumption has to be made: but it is one for which
there is a large amount of justification, and the reasons for it are
given in many books,--among others in my book on _Electrons_, and
likewise at the end of the new edition of _Modern Views of
Electricity_, also in my _Romanes Lecture_, published by the Clarendon
Press in 1903. Put briefly, the assumption is that matter is composed,
in some way or other, of electrons; which again must be considered to
be essentially peculiarities, or singularities, or definite
structures, in the ether itself. Indeed, a consideration of electrons
alone is sufficient for the argument, provided it be admitted that
they have the mass which experiment shows them to possess, and the
size which electrical theory deduces for them: the basis of the
idea--which, indeed, is now experimentally proved--being that their
inertia is due to their self-induction,--i.e. to the magnetic field
with which they must be surrounded as long as they are in motion.
The mass, or inertia, of an electron is comparable to the thousandth
part of that of the atom of hydrogen. Its linear dimension, let us say
its diameter, is comparable to the one-hundred-thousandth part of what
is commonly known as molecular or atomic dimension; which itself is
the ten-millionth part of a millimetre.
Hence, the mass and the bulk of an electron being known, its density
is determined, provided we can assume that its mass is all dependent
on what is contained within its periphery. But that last assumption is
one that quite definitely cannot be made: its mass is for the most
part outside itself, and has to be calculated by magnetic
considerations. (See Appendix 2.)
These details are gone into in my paper in the _Philosophical
Magazine_ for April, 1907, and in Chapter XVII of _Modern Views of
Electricity_. But without repeating arguments here, it will suffice
to say that although the estimates may be made in various ways,
differing entirely from each other, yet the resulting differences are
only slight; the calculated densities come out all of the same order
of magnitude, namely, something comparable to 10¹² C.G.S.
units,--that is to say, a million million grammes per cubic
centimetre, or, in other words, a thousand tons to the cubic
millimetre.
But, throughout, we have seen reason to assert that the ether is
incompressible; arguments for this are given in _Modern Views of
Electricity_, Chapter I. And, indeed, the fundamental medium filling
all space, if there be such, _must_, in my judgment, be ultimately
incompressible; otherwise it would be composed of parts, and we should
have to seek for something still more fundamental to fill the
interstices.
The ether being incompressible, and an electron being supposed
composed simply and solely of ether, it follows that it cannot be
either a condensation or a rarefaction of that material, but must be
some singularity of structure, or some portion otherwise
differentiated. It might, for instance, be something analogous to a
vortex ring, differentiated kinetically, i.e. by reason of its
rotational motion, from the remainder of the ether; or it might be
differentiated statically, and be something which would have to be
called a strain-centre or a region of twist, or something which cannot
be very clearly at present imagined with any security; though various
suggestions have been made in that direction.
The simplest plan for us is to think of it somewhat as we think of a
knot on a piece of string. The knot differs in no respect from the
rest of the string, except in its tied-up structure; it is of the same
density with the rest, and yet it is differentiated from the rest;
and, in order to cease to be a knot, would have to be untied--a
process which as yet we have not learned how to apply to an electron.
If ever such a procedure becomes possible, then electrons will thereby
be resolved into the general body of the undifferentiated ether of
space,--that part which is independent of what we call "matter."
The important notion for present purposes is merely this: that the
density of the undifferentiated or simple ether, and the density of
the tied-up or be-knotted or otherwise modified ether constituting an
electron, are one and the same. Hence the argument above given, at
least when properly worked out, tends to establish the etherial
density as of the order 10¹² times that of water.
There ought to be nothing surprising (though I admit that there is
something very surprising) in such an estimate; inasmuch as many
converging lines of argument tend to show that ordinary matter is a
very porous or gossamer-like substance, with interspaces great as
compared with the spaces actually occupied by the nuclei which
constitute it. Our conception of matter, if it is to be composed of
electrons, is necessarily rather like the conception of a solar
system, or rather of a milky way; where there are innumerable dots
here and there, with great interspaces between. So that the average
density of the whole of the dots or material particles taken
together,--that is to say, their aggregate mass compared with the
space they occupy,--is excessively small.
In the vast extent of the Cosmos, as a whole, the small bulk of actual
matter, compared with the volume of empty space, is striking--as we
shall show directly; and now on the small scale, among the atoms of
matter, we find the conditions to be similar. Even what we call the
densest material is of extraordinarily insignificant massiveness as
compared with the unmodified ether which occupies by far the greater
proportion of its bulk.
When we speak of the density of _matter_, we are really though not
consciously expressing the group-density of the modified ether which
constitutes matter,--not estimated per unit, but per aggregate; just
as we might estimate the group or average density of a cloud or mist.
Reckoned per unit, a cloud has the density of water; reckoned per
aggregate, it is an impalpable filmy structure of hardly any density
at all. So it is with a cobweb, so perhaps it is with a comet's tail,
so also with the Milky Way, with the cosmos,--and, as it now turns
out, with ordinary matter itself.
For consider the average density of the material cosmos. It comes out
almost incredibly small. In other words, the amount of matter in
space, compared with the volume of space it occupies, is almost
infinitesimal. Lord Kelvin argues that ultimately it must be really
infinitesimal (_Philosophical Magazine_, Aug., 1901, and Jan., 1902),
that is to say that the volume of space is infinitely greater than the
total bulk of matter which it contains. Otherwise the combined force
of gravity--or at least the aggregate gravitational potential--on
which the velocity generated in material bodies ultimately depends,
would be far greater than observation shows it to be.
The whole visible universe, within a parallax of 1/1000 second of arc,
is estimated by Lord Kelvin as the equivalent of a thousand million of
our suns; and this amount of matter, distributed as it is, would have
an average density of 1·6 × 10⁻²³ grammes per c.c. It is noteworthy
how exceedingly small is this average or aggregate density of matter
in the visible region of space. The estimated density of 10⁻²³
c.g.s. means that the visible cosmos is as much rarer than a "vacuum"
of a hundred millionths of an atmosphere, as that vacuum is itself
rarer than lead.
It is because we have reason to assert that any ordinary mass of
matter consists, like the cosmos, of separated particles, with great
intervening distances in proportion to their size, that we are able to
maintain that the aggregate density of ordinary stuff, such as water
or lead, is very small compared with the continuous medium in which
they exist, and of which all particles are supposed to be really
composed. So that lead is to the ether, as regards density, very much
as the "vacuum" above spoken of is to lead. The fundamental medium
itself must be of uniform density everywhere, whether materialised or
free.