_THE MYSTERY OF GRAVITY._
The law of gravity, or of the mutual attraction of masses of matter upon
each other, accounts so perfectly for all the observed motions of the
heavenly bodies, that we are apt to regard Newton's discovery of the
great law as though it had finally solved the mystery of these motions.
Many accept the verdict given by the poet Pope in the famous epitaph
which he suggested for Newton,--
"Nature and Nature's laws lay hid in night:
God said, _Let Newton be!_ and all was Light."
But Newton, who probably knew as much about his work as Pope, was of
another opinion. Every one knows how he compared himself to a child who
had picked up a few shells on the shore, while the ocean of truth lay
unexplored before him. He has, however, spoken definitely of the great
discovery which has rendered his name illustrious, in terms which show
that _he_ did not find that all was light. Among the questions which he
specially would have had answered, amongst the secrets of nature
concealed beneath the ocean of truth, the mystery of gravity was
probably the chief. When Newton asked of the Ocean of Truth what Mrs.
Hemans later said, and in another sense, of the natural sea--
"What hidest thou in thy treasure-caves and cells,
Thou hollow-sounding and mysterious main?"
he had in his thoughts the very power which he is commonly supposed to
have explained, but which was in truth for him, more than for any man
that had ever lived, the mystery of mysteries.
It may be well to consider the very words of the great philosopher, so
far at least as our more diffuse language can present the concise
expressions of the original Latin:
"Hitherto we have explained," he says, "the phenomena of the heavens and
of our sea by the power of gravity, but have not yet assigned the cause
of this power. This is certain" (we must hearken attentively here, for
when a man like Newton speaks of aught as certain, we have sure ground
to go upon),--"this is certain, that it must proceed from a cause that
penetrates to the very centres of the sun and planets, without suffering
the least diminution of its forces; that operates, not according to the
quantity of surfaces of particles on which it acts (as mechanical causes
usually do), but according to the quantity of the solid matter which
they contain, and propagates its virtue on all sides to immense
distances, decreasing always as the squares of the distances.
Gravitation towards the sun is made up of the gravitations towards the
several particles of which the body of the sun is composed, and in
receding from the sun decreases accurately as the square of the
distances as far as the path of Saturn..., nay, and even to the
remotest parts of the paths of comets.... But hitherto I have not been
able to discover the cause of those properties of gravity from
phenomena; and I frame no hypotheses:[4] for, whatever is not deduced
from phenomena is to be called an hypothesis; and hypotheses, whether
metaphysical or physical, whether of occult qualities or mechanical,
have no place in experimental philosophy.... To us it is enough that
gravity does really exist, and act according to the laws which we have
explained, and abundantly serves to account for all the motions of the
celestial bodies and of our sea."
"Hitherto I have not been able to discover the cause of the properties
of gravity." Such is the simple statement of the man who discovered
those properties.
And now let us inquire a little into this law of gravity, not with the
hope of explaining this great mystery of nature,--though, for my own
part, I believe that the time is not far distant when the progress of
discovery will enable man to make this approach towards the mystery of
mysteries,--but in order to recognise the real nature of the mystery,
which is a very different thing from explaining it.
In the first place the study of gravity brings us at once to the
consideration of the infinitely minute,--at least of what is for us
practically infinite in its minuteness. If we consider the above
quotation attentively, we perceive that this quality of gravity was
recognised by Newton. "It is not the quantity of the surfaces of
_particles_," he says, "but the quantity of solid matter which they
contain," that gives to gravity its power. Gravity resides in the
ultimate particles of matter. We cannot conceive of matter so divided,
no matter how finely, that non-gravitating particles could be separated
from gravitating particles. Without entering into the question what
atoms are, we perceive that these ultimate constituents of matter must
contain, each according to the quantity of matter in it, the gravitating
energy. Only, observe how incongruously we are compelled to speak. (It
is always so when we deal with the infinite, whether the infinitely
great or the infinitely minute.) We are speaking of atoms as the
ultimate constituents of matter, and yet we are compelled, in describing
their gravitating energy, to speak of the quantity of matter contained
in each atom,--in other words, we speak in the same breath of an atom as
not admitting of being divided or diminished, and of its containing
matter by quantity, that is, by more or less. May we not, however,
reasonably accept both views? The reasoning is sound by which science
has proved that, so far as our material universe is concerned, there is
a limit beyond which the division of matter cannot be supposed to
go,--insomuch that Sir W. Thomson has indicated the actual limits of
size of the atoms composing matter. Yet, passing in imagination beyond
the bounds of our visible universe, and so entering into the next order
of universe below it (in scale of construction),--the ether of
space,--the atoms of our universe may be infinitely divisible in that
universe, may be, in fact, compared with _its_ particles, as the suns
and worlds of our universe are to our atoms and molecules.
But while gravity thus draws us to the contemplation of the infinitely
minute, it also leads us to the consideration of what is for us the
infinitely vast.
Newton was only able to speak confidently of the action of gravity at
the distance of Saturn, the remotest planet known in his day. He did,
indeed, refer to the comets as probably obeying, even in the remotest
parts of their paths, the force of the sun's gravity; but he could not
be certain on that point, because in his time no comet had been proved
to travel back to the sun after receding to the remotest portion of its
track. We now know not only that the sun's attraction extends to the
farthest parts of the solar system, having thus a domain in space nearly
thirty times larger than the sphere of Saturn, but we perceive that many
among the stars exert a similar force; for around them travel other
stars even as the planets travel around the sun. Thus we know that
gravity is exerted in regions lying hundreds of thousands of times
farther from the sun than Saturn is. We have, indeed, every reason to
believe, not only that star unto star extendeth this mysterious
attractive influence, but that the least particle in the inmost depths
of sun or world exerts in full force on each particle, even of suns
lying millions of times beyond the range of the most powerful telescope
yet constructed by man, the full energy corresponding (i.) to the
quantity of matter in itself and such particle, and (ii.) to the
distance separating each from each.
This is amazing enough; but there is something more perplexing and
mysterious in gravity even than this. Not only does gravity lead us to
consider the infinitely minute in space on the one hand, and the
infinitely vast in space on the other, but also it leads us to consider
the infinitely minute and the infinitely vast in time also, and this in
such a way as to suggest a difficulty which, as yet, no man has been
able to solve.
Light travels, as we know, with a velocity so enormous, that, by
comparison with it, all the velocities we are familiar with seem
absolutely as rest. But gravity acts so quickly that even the velocity
of light becomes as rest by comparison with the velocity of the
propagation of gravity. Laplace had occasion, now nearly a century ago,
to inquire whether a certain change in the moon's motion, by which she
seemed to be gradually hastening her motion round the earth, might not
be caused by the circumstance that gravity requires time for its action
to be propagated over great distances. He found that if the whole of
that change had to be explained in this way, which would be giving to
gravity the slowest admissible rate of transmission, the velocity with
which gravity is propagated would be _eight million_ times greater than
the velocity of light. If, on the other hand, that change in the moon's
motion could be satisfactorily explained in some other way, then the
velocity of gravity must be at least 16,000,000 times greater than the
velocity of light. He himself soon after discovered what was in his day
regarded as a complete explanation of the hastening of the moon's
motion; and though in our own time Adams of Cambridge has shewn that
only half the hastening can be accounted for by Laplace's reasoning, the
general explanation of the remaining half is that it is not a real
hastening of the moon, but is only an apparent hastening caused by the
gradual slowing of the earth's rate of turning on her axis. This makes
the _day_ by which we measure the moon's motion seem longer (very
slightly, however).[5] Supposing, however, half the moon's hastening
were left unexplained, and that the non-instantaneous transmission of
gravity were the only way of accounting for it, even then it would be
certain that gravity is propagated at a rate exceeding 12,000,000 times
the velocity of light.
Indeed, at present, owing to the more exact observations available, and
the greater range of time over which they extend, it may safely be said
that the rate of propagation of gravity is far greater than this. It is
even held by some that gravity acts instantaneously over any distance,
however vast.
[Illustration: Fig. 6.]
Although I cannot here indicate the exact nature of the reasoning by
which the enormous rapidity of the action of gravity is inferred, I must
briefly indicate the general argument, that the reader may not suppose
the matter to be merely speculative. Suppose that the action of gravity
were propagated at the same rate as light. Then the earth would feel the
pull of the sun eight minutes or so after she had been in the place
where the sun began to exert that particular pull. The direction of the
pull then would not be that of the straight line connecting the earth
and sun at the moment when the pull was felt, but that of the straight
line connecting the sun and the earth eight minutes or so before. For
instance, when the earth is at E_{1}, fig. 6, the sun at S would begin
to exert a pull in the line E_{1} S, but the earth would only feel this
pull when she got to E_{2}, her place eight minutes later, when it would
act upon her in the direction E_{2} F, parallel to E_{1} S. Now this
pull, E_{2} F, may be divided into two parts, one along E_{2} S, pulling
the earth towards the sun S, the other along E_{2} T in the earth's
course, _hastening her therefore_. But the maintenance by the earth of
the same constant track depends entirely on the action of gravity
sunwards. If there is any action in addition, hastening the earth, then
she will not keep her course,[6] but will travel in a constantly
widening path,--or, in a sort of spiral, very slowly retreating from the
sun, but retreating constantly. The change of distance would not be
measurable in millions of years; but the increase in the length of the
year _would_, before long, be observable. Because there is no such
increase, astronomers feel well assured that gravity is not only
propagated more swiftly than light, but many times, even, as we have
seen, many millions of times, more swiftly.
It is then in an infinitely minute time that the action of gravity
traverses all ordinary distances. The earth's pull on the moon takes
less than the 50,000,000th part of a second in reaching the moon,--and
the particles constituting the mass of the earth act on ourselves, and
on all the objects which lie near the earth's surface, in far less than
the 10,000,000th part even of this utterly minute time-interval.
Yet age after age has passed during which this infinitely active force
has been at work without diminution, and age after age will continue to
pass without any change in its activity. For millions of millions of
æons it has lasted and will last, so permanent is it; while its
operation is felt simultaneously at points millions of millions of
star-distances apart. What infinities of distance has this wonderful
attractive force traversed!
But even these considerations do not present the greatest of the marvels
of gravity. It is wonderful, indeed, to consider a form of attraction
possessed by the infinitely minute, and exerted over the infinitely
vast, operating in portions of time immeasurably small, and extending
its operations throughout time infinite. But the mystery of mysteries is
not here. The marvel of marvels is this, that, so far as we can
perceive, the force of gravity is exerted without any material
connection with the objects moved by it. Matter seems to act where it is
not, to use the phraseology of the schools. Of this "action at a
distance," Newton himself said, that it is inconceivable, that in point
of fact it is impossible. "No man," he said, "who has, in philosophical
matters, a competent faculty of thinking," can "for a moment believe
that a body can act through a vacuum, without the intervention of
anything else by or through which the force may be conveyed from one
body to another." Yet this is precisely what gravity seems to do. The
ether occupies, indeed, all space; but there is nothing at present known
to us by which we can understand how the ether can transmit the force of
gravity. The power of the ether in the rapid transmission of undulations
seems to attain its limit in the propagation of light and heat and
electricity at the rate of nearly 200,000 miles per second. How the
ether can act so as to serve as a medium of communication between bodies
at all distances, transmitting impressions 10,000,000 times faster, at
least, than light travels, nothing at present known to us enables us to
say. I have, in a lecture which I gave in America upon the mysteries of
the universe, indicated a way in which gravity may be conceived to be
generated and transmitted; and I may hereafter describe the conception
(based partly on the views of Le Sage). But it is only a conception.
There is no phenomenon (except the very form of attraction which has to
be explained) tending to show that the conception is correct And even
if it be accepted, it brings us face to face with only greater marvels.
At present, however, let this simply be said in conclusion--that the
apparent action of gravity at a distance is, of all physical wonders,
the greatest yet known to man. If we accept the opinion of Newton,
which, indeed, seems to me indisputable, that matter cannot act through
a vacuum, then we must admit the existence of properties, as yet
unthought of, in the ether of space, or in some still more subtle
universe permeating that ether. If, on the other hand, we accept the
belief that matter can act at a distance, then is there no miracle,
either of those believed in by mankind generally, or of those more
generally rejected, which exceeds in marvellousness this wonder of all
the wonders of physical science.