40 
NATURE 
[SEPTEMBER II, 1913 
we fail too. So long as transparent matter is moving 
relatively to us, light can be affected inside that 
matter; but when matter is relatively stationary to 
matter nothing observable takes place, however fast 
things may be moving, so long as they move together. 
Hence arises the idea that motion with respect 
to gwther is meaningless; and the fact that only 
relative motion of pieces of matter with respect to 
each other has so far been observed is the foundation 
of the principle of relativity. It sounds simple 
enough as thus stated, but in its developments it is 
an ingenious and complicated doctrine embodying 
surprising consequences which have been worked 
out by Prof. Einstein and his disciples with con- 
summate ingenuity. 
What have I to urge against it? Well, in the first 
place, it is only in accordance with common sense 
that no effect of the first order can be observed with- 
out relative motion of matter. An zther-stream 
through our laboratories is optically and electrically un- 
detectable, at least as regards first-order observation ; 
this is clearly explained for general readers in my 
book, ‘‘The Ether of Space,” chapter iv. But the 
principle of relativity says more than that; it says 
that no effect of any order of magnitude can ever be 
observed without the relative motion of matter. 
The truth underlying this doctrine is that absolute 
motion without reference to anything is unmeaning. 
But the narrowing down of ‘‘anything”’ to mean any 
piece of matter is illegitimate. The nearest approach 
to absolute motion that we can physically imagine is 
motion through or with respect to the ather of space. 
It is natural to assume that the zther is on the whole 
stationary, and to use it as a standard of rest; in 
that sense motion with reference to it may be called 
- absolute, but in no other sense. 
The principle of relativity claims that we can never 
ascertain such motion: in other words it practically 
or pragmatically denies the existence of the ether. 
Every one of our scientifically observed motions, it 
says, are of the same nature as our popularly observed 
ones, viz. motion of pieces of matter relatively to 
each other; and that is all that we can ever know. 
Everything goes on—says the principle of relativity— 
as if the ether did not exist. 
Now the facts are that no motion with reference to 
the zther alone has ever yet been observed: there 
are always curious compensating effects which just 
cancel out the movement-terms and destroy or effec- 
tively mask any phenomenon that might otherwise 
be expected. When matter moves past matter 
observation can be made; but, even so, no consequent 
locomotion of zther, outside the actually moving par- 
ticles, can be detected. 
(It is sometimes urged that rotation is a kind of 
absolute motion that can be detected, even in isola- 
tion. It can so be detected, as Newton pointed out; 
but in cases of rotation matter on one side the axis is 
moving in the opposite direction to matter on the 
other side of the axis; hence rotation involves relative 
material motion, and therefore can be observed.) 
To detect motion through ather we must use an 
zthereal process. We may use radiation, and try 
to compare the speeds of light along or across the 
motion, or we might try to measure the speed, first 
with the motion and then against it. But how are 
we to make the comparison? If the time of emis- 
sion from a distant source is given by a distant clock, 
that clock must be observed through a telescope, that 
is by a beam of light; which is plainly a compensating 
process. Or the light from a neighbouring source 
can be sent back to us by a distant mirror; when 
again there will be compensation. Or the starting 
of light from a distant terrestrial source may be 
telegraphed to us, either with a wire or without; 
NO. 2289, VOL. 92| 
but it is the ether that conveys the message in either 
case, so again there will be compensation. Elec- 
tricity, magnetism, and light, are all effects of the 
zether. . i 
Use cohesion, then; have a rod stretching from 
one place to another, and measure that. But cohesion 
is transmitted by the zther too, if, as believed, it is 
the universal binding medium. Compensation is 
likely; compensation can, on the electrical theory of 
matter, be predicted. 
Use some action not dependent on ether, then. 
Very well, where shall we find it? 
To illustrate the difficulty I will quote a sentence 
from Sir Joseph Larmor’s paper before the Inter- 
national Congress of Mathematicians at Cambridge 
last year :— 
“Tf it is correct to say with Maxwell that all radia- 
tion is an electrodynamic phenomenon, it is equally 
correct to say with him that all electrodynamic rela- 
tions between material bodies are established by the 
operation, on the molecules of those bodies, of fields 
of force which are propagated in free space as radia- 
tion and in accordance with the laws of radiation, 
from one body to the other.” é 
The fact is, we are living in an epoch of some 
very comprehensive generalisations. The physical 
discovery of the twentieth century, so far, is the 
electrical theory of matter. This is the great new 
theory of our time; it was referred to, in its philo- 
sophical aspect, by Mr. Balfour in his presidential 
address at Cambridge in 1904. We are too near it 
to be able to contemplate it properly; it has still to 
establish itself and to develop in detail, but I antici- 
pate that in some form or other it will prove true.* 
Here is a briefest possible summary of the first 
chapter (so to speak) of the electrical theory of 
matter :-— 
(1) Atoms of matter are composed of electrons—of — 
positive and negative electric charges. : 
(2) Atoms are bound together into molecules by 
chemical affinity which is intense electrical attraction 
at ultra-minute distances. 
(3) Molecules are held together by cohesion, which 
I for one regard as residual or differential chemical 
affinity over molecular distances. 
(4) Magnetism is due to the locomotion of elec- 
trons. There is no magnetism without an electric 
current, atomic or otherwise. There is no electric 
current without a moving electron. 
(5) Radiation is generated by every accelerated 
electron, in amount proportional to the square of its 
acceleration; and there is no other kind of radiation, 
except indeed a corpuscular kind; but this depends 
on the velocity of electrons and therefore again can 
only be generated by their acceleration. 
The theory is bound to have curious consequences ; 
and already it has contributed to some of the up- 
rooting and uncertainty that I speak of. For, if it 
be true, every material interaction will be electrical, 
i.e. ethereal; and hence arises our difficulty. Every 
kind of force is transmitted by the ether, and hence, 
so long as all our apparatus is travelling together at 
one and the same pace, we have no chance of detect- 
ing the motion. That is the strength of the principle 
of relativity. The changes are not zero, but they 
cancel each other out of observation (NATURE, 
vol. xlvi., p. 165, 1892). 
Many forms of statement of the famous Michelson- 
Morley experiment are misleading. It is said to 
prove that the time taken by light to go with the 
zether stream is the same as that taken to go against 
or across it. It does not show that. What it shows 
is that the time taken by light to travel to and fro 
4 Fora general introductory account of the electrical theory of matter my 
Romanes lecture for 1903 (Clarendon Press) may be referred to. 
