Well, then, perhaps it _is_ stagnant. The experiments I have quoted do not prove that it is so. They are equally consistent with its perfect freedom and with its absolute stagnation; though they are not consistent with any intermediate position. Certainly, if the ether were stagnant nothing could be simpler than their explanation.
The only phenomena then difficult to explain would be those depending on light coming from distant regions through all the layers of more or less dragged ether. The theory of astronomical aberration would be seriously complicated; in its present form it would be upset (p. 45).
But it is never wise to control facts by a theory; it is better to invent some experiment that will give a different result in stagnant and in free ether. None of those experiments so far described are really discriminative. They are, as I say, consistent with either hypothesis, though not very obviously so.
[Ill.u.s.tration: FIG. 10. The course of the light and of the two half-beams in Michelson"s most famous experiment. The light is split at A, one half sent towards B and back, the other half to C and back.
Compare with Fig. 7.]
_Michelson Experiment._
Mr. Michelson, however, of the United States, invented a plan that looked as if it really would discriminate; and, after overcoming many difficulties, he carried it out. It is described in the _Philosophical Magazine_ for 1887.
Michelson"s famous experiment consists in looking for interference between two half-beams of light, of which one has been sent to and fro _across_ the line of ether drift, and the other has been sent to and fro _along_ the line of ether drift.
A semi-transparent mirror set at 45 is employed to split the beam, and a pair of normal and ordinary mirrors, set perpendicular to the two half-beams, are employed to return them back whence they came, so that they can enter the eye through an observing telescope.
It differs essentially from the interference kaleidoscope, Fig. 7, inasmuch as there is now no luminous path B C, and no contour enclosed by the light. Each half-beam goes to and fro on its own path, and these paths, instead of being coincident, are widely separate,--one North and South, for instance, and the other East and West.
Under these conditions the bands are much more tremulous than they were in the arrangement of Fig. 7, and are subject to every kind of disturbance. The apparatus has to be excessively steady, and no fluctuation even of temperature must be permitted in the path of either beam. To secure this, the source, the mirrors, and the observing telescope, were all mounted upon a ma.s.sive stone slab; and this was floated in a bath of mercury.
The slab could then be slowly turned round, so that sometimes the path A B and sometimes the path A C lay approximately along or athwart the direction of the earth"s motion in s.p.a.ce.
And inasmuch as the motion along would take a little longer than the motion across, though everything else was accurately the same, some shift of the interference bands might be expected as the slab rotated.
But whereas in all the experiments previously described the effect looked for was a first-order effect, of magnitude one in ten or twenty thousand,--depending, that is to say, on the first power of the ratio of speed of earth to speed of light,--the effect now to be expected depends on the _square_ of that same ratio, and therefore cannot be greater, even in the most favourable circ.u.mstances, than 1 part in a hundred million.
It is easy to realise therefore that it is an exceptionally difficult experiment, and that it required both skill and pertinacity to perform it successfully.
That it is an exceptionally difficult experiment will be realised when I say that it would fail in conclusiveness unless one part in 400 millions could be clearly detected.
Mr. Michelson reckons that by his latest arrangement he could see 1 in 4000 millions if it existed (which is equivalent to detecting an error of 1/1000 of an inch in a length of 60 miles); but he saw nothing.
Everything behaved precisely as if the ether was stagnant; as if the earth carried with it all the ether in its immediate neighbourhood.
And that was his conclusion.
_Theory of Michelson Experiment._
The theory of the Michelson experiment can be expressed thus: its optical diagram being the same as is expressed geometrically in Fig.
6.
If a relatively fixed source and receiver move through the ether with velocity _u_, such that u/v=a the aberration constant; then the time of any to and fro journey S M, inclined at angle ? to the direction of the drift, is increased, above what it would be if there were no drift, in the ratio
v(1-asin?) / (1-a)
This follows from merely geometrical considerations.
Hence if a ray is split, and half sent so that ?=0 while the other half is sent so that ?=90 (as in Fig. 10), the one will lag behind the other by a distance a times the distance travelled; which, though very small, may be a perceptible fraction of a wave-length, and therefore may cause a perceptible shift of the bands.
But when the experiment is properly performed, no such shift is observed.
The experiment thus seems to prove that there is no motion through the ether at all, that there is no etherial drift past the earth, that the ether immediately in contact with the earth is stagnant--or that the earth to that extent carries all neighbouring ether with it.
If we wish to evade this conclusion, there is no easy way of doing so.
For it depends on no doubtful properties of transparent substances, but on the straightforward fundamental principle underlying all such simple facts as that--It takes longer to row a certain distance and back, up and down stream, than it does to row the same distance in still water; or that it takes longer to run up and down a hill, than to run the same distance laid out flat; or that it costs more to buy a certain number of oranges at three a penny and an equal number at two a penny than it does to buy the whole lot at five for twopence.
Hence, although there may be _some_ way of getting round Mr.
Michelson"s experiment, there is no obvious way; and if the true conclusion be not that the ether near the earth is stagnant, it must lead to some other important and unknown fact.
That fact has now come clearly to light. It was first suggested by the late Professor G.F. FitzGerald, of Trinity College Dublin, while sitting in my study at Liverpool and discussing the matter with me.
The suggestion bore the impress of truth from the first. It independently occurred also to Professor H.A. Lorentz, of Leiden, into whose theory it completely fits, and who has brilliantly worked it into his system. It may be explained briefly thus:--
Electric charges in motion const.i.tute an electric current.
Similar charges repel each other, but currents in the same direction attract. Consequently two similar charges moving in parallel lines will repel each other less than if stationary,--less also than if moving one after the other in the same line. Likewise two opposite charges, a fixed distance apart, attract each other less when moving side by side, than when chasing each other. The modification of the static force, thus caused, depends on the squared ratio of their joint speed to the velocity of light.
Atoms of matter are charged; and cohesion is a residual electric attraction (see end of Appendix 1). So when a block of matter is moving through the ether of s.p.a.ce its cohesive forces across the line of motion are diminished, and consequently in that direction it expands, by an amount proportioned to the square of aberration magnitude.
A light journey, to and fro, across the path of a relatively moving medium is slightly quicker than the same journey, to and fro, along (see p. 64). But if the journeys are planned or set out on a block of matter, they do not remain quite the same when it is conveyed through s.p.a.ce: the journey across the direction of motion becomes longer than the other journey, as we have just seen. And the extra distance compensates or neutralises the extra speed; so that light takes the same time for both.
FOOTNOTES:
[4] The word "stationary" is ambiguous. I propose to use "stagnant,"
as meaning stationary with respect to the earth, i.e. as opposed to stationary in _s.p.a.ce_.
[5] Lord Rayleigh, _Nature_, March 25, 1892.
CHAPTER V
SPECIAL EXPERIMENT ON ETHERIAL VISCOSITY
The balance of evidence at this stage seems to incline in the sense that there is no ether drift, that the ether near the earth is stagnant, that the earth carries all or the greater part of the neighbouring ether with it,--a view which, if true, must singularly complicate the theory of ordinary astronomical aberration: as is explained at the beginning of the last chapter.
But now put the question another way. _Can_ matter carry neighbouring ether with it when it moves? Abandon the earth altogether; its motion is very quick, but too uncontrollable, and it always gives negative results. Take a lump of matter that you can deal with, and see if it pulls any ether along.
That is the experiment which I set myself to perform, and which in the course of the years 1891-97 I performed. It may be thus described in essence:--
Take a steel disk, or rather a couple of large steel disks a yard in diameter clamped together with a s.p.a.ce between. Mount the system on a vertical axis, and spin it like a teetotum as fast as it will stand without flying to pieces. Then take a parallel beam of light, split it into two by a semi-transparent mirror, M, a piece of gla.s.s silvered so thinly that it lets half the light through and reflects the other half, somewhat as in Fig. 7; and send the two halves of this split beam round and round in opposite directions in the s.p.a.ce between the disks. They may thus travel a distance of 20 or 30 or 40 feet.
Ultimately they are allowed to meet and enter a telescope. If they have gone quite identical distances they need not interfere, but usually the distances will differ by a hundred-thousandth of an inch or so, which is quite enough to bring about interference.
The mirrors which reflect the light round and round between the disks are shown in Fig. 11. If they form an accurate square the last two images will coincide, but if the mirrors are the least inclined to one another at any unaliquot part of 360 the last image splits into two, as in the kaleidoscope is well known, and the interference bands may be regarded as resulting from those two sources. The central white band bisects normally the distance between them, and their amount of separation determines the width of the bands. There are many interesting optical details here, but I shall not go into them.
[Ill.u.s.tration: FIG. 11. Diagrammatic Plan of Optical Frame for Ether Machine; with Steel Disks, one yard in diameter, inside the frame. The actual apparatus is shown in Figs. 13 and 14 and Fig. 12.
M is a semi-transparent mirror, reflecting half an incident beam and transmitting the other half. The two half-beams each go three times round the square contour, in opposite directions, and then reunite. It is an extension of the idea of Fig. 7.]
The thing to observe is whether the motion of the disks is able to replace a bright band by a dark one, or vice versa. If it does, it means that one of the half-beams, viz. that which is travelling in the same direction as the disks, is helped on a trifle, equivalent to a shortening of journey by some quarter millionth of an inch or so in the whole length of 30 feet; while the other half-beam, viz. that travelling against the motion of the disks, is r.e.t.a.r.ded, or its path virtually lengthened, by the same amount.